Danfoss Industrial Refrigeration Ammonia and CO2 Applications Application guide [ar]

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Application Handbook

Industrial Refrigeration

Ammonia and CO2 Applications

Application Handbook Industrial Refrigeration ammonia and CO2 applications

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

Content Page

Introduction ................................................................................................................................................................................... 3

1.1 Refrigerants ............................................................................................................................................................................. 4

1.2 Refrigerant feed to the evaporator ............................................................................................................................................ 6

1.3 One-stage systems ................................................................................................................................................................... 8

1.4 Two-stage/multistage systems .................................................................................................................................................. 9

1.5 Cascade systems .................................................................................................................................................................... 12

1.6 Transcritical systems .............................................................................................................................................................. 13

Compressor controls ..................................................................................................................................................................... 18

2.1 Reverse flow control ............................................................................................................................................................... 19

2.2 Suction pressure control ......................................................................................................................................................... 20

2.3 Compressor capacity control ................................................................................................................................................... 21

2.4 Discharge Temperature Control with Liquid Injection ................................................................................................................ 24

2.5 Economizer damper ................................................................................................................................................................ 26

2.6 Summary ............................................................................................................................................................................... 27

Condenser controls ....................................................................................................................................................................... 28

3.1 High-pressure float valve operation ......................................................................................................................................... 28

3.2 Low-pressure float valve operation .......................................................................................................................................... 29

3.3 Air-cooled condensers............................................................................................................................................................. 29

3.4 Evaporative condensers .......................................................................................................................................................... 33

3.5 Water-cooled condensers ........................................................................................................................................................ 36

3.6 Summary ............................................................................................................................................................................... 38

Liquid level regulation ................................................................................................................................................................... 39

4.1 High-pressure liquid level regulation system (HP LLRS) ............................................................................................................. 41

4.2 Low-pressure liquid level regulation system (LP LLRS) .............................................................................................................. 47

4.3 Summary ............................................................................................................................................................................... 52

Evaporator controls ...................................................................................................................................................................... 53

5.1 Temperature controls ............................................................................................................................................................. 54

5.2 Liquid supply control............................................................................................................................................................... 58

5.3 Injection with a solenoid valve (EVRA) ..................................................................................................................................... 65

5.4 Injection with a pulse width modulation AKV(A) valve............................................................................................................... 65

5.5 Risers .................................................................................................................................................................................... 68

5.6 Defrost methods .................................................................................................................................................................... 73

5.7 Summary ............................................................................................................................................................................... 90

Oil systems .................................................................................................................................................................................. 92

6.1 Oil cooling controls ................................................................................................................................................................. 93

6.2 Oil differential pressure controls .............................................................................................................................................. 98

6.3 Oil recovery system .............................................................................................................................................................. 102

6.4 Summary ............................................................................................................................................................................. 106

Safety systems ........................................................................................................................................................................... 107

7.1 Pressure relief devices .......................................................................................................................................................... 107

7.2 Pressure and temperature limiting devices ............................................................................................................................. 110

7.3 Liquid level safety devices ..................................................................................................................................................... 111

7.4 Refrigerant detection ............................................................................................................................................................ 112

Refrigerant pump controls........................................................................................................................................................... 116

8.1 Pump protection with differential pressure control .................................................................................................................. 117

8.2 Pump bypass flow control ..................................................................................................................................................... 120

8.3 Pump pressure control .......................................................................................................................................................... 122

8.4 Summary ............................................................................................................................................................................. 123

Others ....................................................................................................................................................................................... 124

9.1 Filter driers in fluorinated systems ......................................................................................................................................... 124

9.2 Water removal for CO2 systems ............................................................................................................................................ 126

9.3 Water removal for ammonia systems ..................................................................................................................................... 128

9.4 Air purging systems .............................................................................................................................................................. 130

9.5 Heat recovery systems.......................................................................................................................................................... 135

Using CO2 in refrigeration systems ............................................................................................................................................ 136

10.1 CO2 as a refrigerant ........................................................................................................................................................... 137

10.2 Comparision of line sizings in CO2 systems ........................................................................................................................... 138

10.3 -Subcritical CO2 systems ..................................................................................................................................................... 145

10.4 Special considerations for CO2 refrigeration systems ............................................................................................................. 153

10.5 Conclusion ......................................................................................................................................................................... 157

10.6 Safety and gas detection ..................................................................................................................................................... 159

10.7 Gas detection ..................................................................................................................................................................... 159

10.8 Pressure control CO2 systems ............................................................................................................................................. 161

10.9 Cascade system controls ..................................................................................................................................................... 163

10.10 Control methods for hot gas defrosting .............................................................................................................................. 164

10.11 Oil in CO2 systems ............................................................................................................................................................ 171

10.12 Water in CO2 Systems ....................................................................................................................................................... 174

Transcritical CO2 systems ......................................................................................................................................................... 179

11.1 Explanation of the transcritical cycle. ................................................................................................................................... 179

11.2 Typical CO2 transcritical system types .................................................................................................................................. 184

11.3 CO2 in Industrial refrigeration ............................................................................................................................................. 190

11.4 Industrial CO2 transcritical systems. .................................................................................................................................... 193

11.5 Defrosting of industrial pump-circulated transcritical systems ................................................................................................ 209

11.6 Oil management ................................................................................................................................................................. 211

11.7 Oil return ........................................................................................................................................................................... 212

11.8 Heat recovery ..................................................................................................................................................................... 217

11.9 Design pressure and safety ................................................................................................................................................. 223

Heat exchangers ...................................................................................................................................................................... 225

12.1 Heat transfer fundamentals ................................................................................................................................................. 230

12.2 Plate heat exchangers ......................................................................................................................................................... 237

12.3 Installation of heat exchangers ............................................................................................................................................ 242

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Foreword:

Some of the solutions presented here might be subject to special requirements in local laws and legislation and Danfoss has made no verification of such and expressly disclaims any compliance there with. A licensed and skilled professional engineer should always be consulted when designing, using, making or selling any device or equipment to ensure compliance with applicable laws and standards.

Refrigeration systems and methods of operating the same may be subject to intellectual property rights in some jurisdictions. A licensed attorney should always be consulted when designing, using, making or selling any such system, methods or equipment to ensure freedom to operate.

Danfoss shall have no liability or obligation to indemnify or hold harmless for any claims, legal proceedings, losses, actions, damages, suits, judgments, liabilities, and expenses, including attorneys’ fees, arising from any information provided in this document, or arising out of the modification or combination of any Danfoss product or system with any third party equipment, materials or intellectual property.

All diagrams and drawings are included as principle sketches and for illustration purposes only. All Danfoss products and pipe lines etc. should be dimensioned according to the actual capacity and temperature range they are to be used at.

Any information, including, but not limited to information on selection of product, its application or use, product design, weight, dimensions, capacity or any other technical data in this document shall be considered informative, and is only binding if and to the extent, explicit reference is made in a quotation or order confirmation by Danfoss. Danfoss reserves the right to alter its products without notice.

This document itself, all text, diagrams. drawings and all trademarks in this material are property of Danfoss A/S or Danfoss group companies. Danfoss and the Danfoss logo are trademarks of Danfoss A/S. All rights reserved.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

Introduction

The refrigeration systems and their applications described in this guide are all vapour compression refrigeration systems.

the refrigerant to condense and release heat. When the refrigerant is fully condensed, it is expanded to lower the pressure, the evaporation pressure, corresponding to a saturation temperature that is low

Vapour compression refrigeration systems utilise the heat absorbed or released in a phase change, the latent heat, of refrigerants. The boiling temperature

enough for the refrigerant to absorb heat from the cooled environment, thus evaporating, and the processes repeat themselves.

changes with the pressure of the refrigerant.

The refrigerant is pressurised at different pressure levels, evaporation pressure levels and condensation pressure levels, to absorb heat from the heat source (cold room) and release heat to the heat sink (ambient), thus the refrigerant acts as a heat transport media. The refrigerant is evaporated and absorbs heat from the environment that is to be cooled. Once the refrigerant is evaporated, it is compressed to a higher pressure level, where the saturation tem-

To visualise the processes, a log(p)-h diagram is shown in Figure 1.1. The log(p)-h diagram shows the thermodynamic properties of a refrigerant. It is used for representing refrigeration cycles by plotting in state points from the refrigeration cycle. It can also be used for reading values for the state points in a refrigeration cycle. The log(p)-h diagram has been created with the Danfoss calculation tool Coolselector2.

perature is higher than the ambient temperature for

Figure 1.1: Log(p)-h diagram for one-stage ammonia system

Application Handbook Industrial Refrigeration ammonia and CO2 applications

A simple one-stage ammonia refrigeration cycle is shown in the log(p)-h diagram above. In this example, the evaporation temperature is -10°C and the condensation temperature is 35°C.

The bell shape seen in the diagram is the two-phase area of the refrigeration and the horizontal lines inside the bell shape represent the evaporation ((4) to

1.1 Refrigerants

The refrigerant is the medium that transports energy from the evaporator to the condenser. Different refrigerants have different thermodynamic properties, such as latent heat, density, critical point etc. Different refrigerants also have different safety precautions, environmental impacts and regulations to be taken into account when designing a refrigeration system.

The most common refrigerants used in industrial systems are:

(1)) and the condensation ((2) to (3)). The area to the left of the bell shape is the liquid phase, while the area to the right of the bell shape is the vapour/gas phase. In the gas phase, the pressure is increased from (1) to (2) - this is the vapour compression. From (3) to (4), the refrigerant liquid is expanded to the evaporation pressure level.

Ammonia is toxic and flammable (in certain concentrations), so proper safety precautions must be taken when handling large amounts of ammonia.

1.1.2 Carbon dioxide (CO

) – R744

CO2 is a non-toxic (in small concentrations), nonflammable substance that is present in the atmosphere. CO2 has become more popular in recent years as CFC refrigerants have been phased out, especially in smaller systems and as a secondary refrigerant in commercial applications.

 Ammonia – R717  Carbon dioxide (CO  Halocarbons – CFCs, HCFCs and HFCs  Hydrocarbons – HCs

) – R744

Safety systems, such as safety relief valves and gas detectors are described in chapter 7.

1.1.1 Ammonia - R717

Ammonia is a natural refrigerant with the chemical composition NH3. Ammonia is inexpensive, easy to manufacture and has a high amount of latent heat, which is why it is a very commonly used refrigerant in industrial applications. At ambient conditions, 20°C and 1 atm, ammonia is lighter than air. Additionally, ammonia has no ozone depletion potential (ODP) and no global warming potential (GWP).

Ammonia’s operational area as a refrigerant ranges from around -40°C of evaporation temperature and up. Ammonia is not compatible with copper, so steel components should be used.

The challenges regarding CO2 are the high pressure for condensation/recuperating and the low critical temperature, requiring transcritical operation with warm ambient temperatures. CO2 can be dangerous when leaking, as it is not self-alarming like ammonia. Large concentrations of CO2 can cause dizziness and, in the most extreme cases, death. The other side of CO2’s high pressure is that it is very efficient at low temperatures, making it a good refrigerant for freezing applications. However, CO2’s triple point at -

56.6°C/5.1 bar abs is the lower limit for its use.

1.1.3 Halocarbons – CFCs, HCFCs and HFCs

Halocarbons are made by replacing one or more hydrogen atoms with halogens in methane and ethane molecules.

Three classes of these exists: CFCs (chlorofluorocarbons), HCFCs (hydrochlorofluorocarbons) and HFCs (hydrofluorocarbons). These refrigerant classes are known for:

CFCs: All hydrogen atoms are substituted by either chloride or fluoride, e.g. R11. These are very stable

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

compounds with a long lifetime. CFCs are non-toxic and non-flammable refrigerants. However, these refrigerants have been phased out because they contribute to the breakdown of the ozone layers in the Earth’s stratosphere when they are leaked.

HCFCs: At least one hydrogen atom is present in these molecules, e.g. R22, which is used to replace R12. These components are less stable and have shorter lifetimes, thus they are less harmful regarding ozone layer depletion and global warming potential. HCFCs are being phased out by some countries.

HFCs: These halocarbons do not contain either chlorine or bromine, e.g. R134a, and are therefore not ozone depleting. However, they do still have a high GWP and are therefore on a phasedown. The properties of R134a are close to R12, and therefore

R134a can normally replace R12 in new and existing systems. Many new blends are entering the market at the present time though. However, due to the relatively high GWP compared to natural refrigerants, there is a risk that these will be phased out in the future.

1.1.4 Hydrocarbons – HCs

Hydrocarbons have become very common in domestic refrigeration applications. Hydrocarbons are flammable and explosive, but in small systems the charge is typically too low to pose any risk, and these systems are typically hermetic. Isobutane - R600a is a hydrocarbon refrigerant which is used in domestic applications. R290 – propane – is used in some larger chillers but is yet to be seen in large-scale industrial applications.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

1.2 Refrigerant feed to the evaporator

There are two common ways that refrigerant is fed to the evaporator in a vapour compression refrigeration system, which are presented in the following paragraphs.

1.2.1 Direct expansion system (DX)

After expansion, the liquid/vapour mixture of refrigerant is fed directly to the evaporator. The refrigerant is then fully evaporated, so that only the vapour phase is present at the evaporator outlet. To ensure that no liquid is present in the suction line, the refrigerant vapour is superheated (the actual temperature is above saturation) in the evaporator before

A sketch of a direct expansion evaporator is shown in Figure 1.2. The evaporator is depicted as a plate heat exchanger, with the cooled product on the left side and the refrigerant on the right side. The thermostatic expansion valve is regulated based on the superheat measured on the suction line (line to compressor).

being fed to the compressor.

Figure 1.2: Direct expansion evaporator An expansion valve is controlled by the superheat. This is either done by a thermostatic expansion valve

DX Evaporator

or an electronic expansion valve.

To Compressor

By keeping a constant level of superheat at the evaporator outlet, the expansion valve feeds the right flow of refrigerant to the evaporator according to the cooling load.

A certain level of superheat should ensure that only refrigerant vapour is fed to the compressor suction. Liquid droplets in the suction will cause liquid hammering in the compressor.

From Receiver

NOTE: A thermostatic expansion valve can only keep

a constant superheat, rather than a constant evapo-

rating temperature.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

1.2.2 Circulated system (flooded system)

In circulated systems, the liquid/vapour mixture of refrigerant is separated after the expansion in a sep-

A circulation system is therefore more efficient than

a corresponding DX system. aration vessel. The saturated liquid phase of the refrigerant is pumped, or driven by gravity, into the evaporator and is partially evaporated, so that a liquid/vapour mixture of the refrigerant is present at the evaporator outlet. In the separator vessel, only the saturated vapour will be fed to the compressor, while the saturated liquid is circulated through the

A sketch of a flooded evaporator is shown in Figure

1.3. The evaporator type is the same as in Figure 1.2.

For the flooded evaporator, the pump is shown, but

it would be left out if it were a thermosyphon evapo-

rator driven by gravity.

Figure 1.3: Pumped flooded evaporator evaporator.

As the evaporator does not need to superheat the suction vapour, the entire surface is used for evaporation, making the flooded evaporator much more efficient. Furthermore, creating superheat in a DX

Flooded

Evaporator

evaporator requires a higher temperature difference and thus the evaporating temperature in a flooded evaporator can be closer to the product temperature, allowing for a more efficient refrigeration system

To Compressor

From Receiver

Application Handbook Industrial Refrigeration ammonia and CO2 applications

1.3 One-stage systems

One-stage or single-stage refrigeration systems are refrigeration systems where the compression from low pressure to high pressure is done in one stage. A one-stage refrigeration system with the most basic components is shown in

Figure 1.4.

Neither the evaporator nor the condenser is specified as a specific type, but rather represent the general concept of evaporation and condensation. Both

Figure 1.4: System diagram for a one-stage system

Oil Cooler

1 2

can be constructed and controlled in a number of ways. A log(p)-h diagram for a one-stage system can be seen in Figure 1.1. The numbers shown in

Figure 1.4 corresponds to the numbers shown for the state points in the log(p)-h diagram.

Oil Separator

Compressor

Evaporator

Expansion valve

Condenser

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

1.4 Two-stage/multistage systems

A two-stage or multistage refrigeration systems are refrigeration systems with two or more stages of vapour compression.

Industrial two-stage refrigeration systems usually have inter-stage cooling between compression stages to cool the discharge gas from the first compression stage.

By cooling the discharge gas from the low-pressure compressor, excessively high discharge temperatures from the high-pressure compressor are avoided. The cooling in the inter-stage cooler is supplied by expanding some of the liquid refrigerant from the condenser to the intermediate pressure, where some of it evaporates in the process of cooling the LP compressor discharge gas. The gas mixed from cooled LP compressor discharge gas, refrigerant flashed and refrigerant evaporated is passed to the HP compressor.

Two or more stages of vapour compression are used when there is a large difference between the evaporating and condensing temperature, which often results in compression ratios that are too high for the

compressors or results in other undesirable operat-

ing conditions. Additionally, the desire for another

temperature level or efficiency considerations may

result in a two-stage system being chosen over a sin-

gle stage.

It is therefore suitable to have two or more stages of

compression to have the compressors operating at

their optimal compression ratio, and within their op-

eration limits.

A system sketch of a two-stage refrigeration system

is shown in Figure 1.5. The figure is shown with an

inter-stage cooler and economiser. An economiser

like the one shown in the system diagram is used for

subcooling the refrigerant after the condenser by by-

passing and flashing some refrigerant. The bypassed

refrigerant is evaporated and fed to an economiser

port in the high-pressure compressor which can in-

crease the capacity and/or the efficiency of the sys-

tem. The LP compressor may also have an econo-

miser, but it is not shown here.

Figure 1.5: System diagram for two-stage system

LP Compressor

Evaporator

Expansion valve

HP Compressor

Inter-stage coole r

Oil Cooler

Oil Separator

Condenser

Economizer

Application Handbook Industrial Refrigeration ammonia and CO2 applications

A log(p)-h diagram for the two-stage system is shown in Figure 1.6. The numbering in Figure 1.5 follows the numbering of the state points shown in the log(p)-h diagram. It should be noted that the evaporation temperature is -30°C and the condensation temperature is kept at 35°C, as for the example for the one-stage system. It should be noted how the in-

temperature from the high-pressure compression stage. If no inter-stage cooling was used, the line (1) to (2) would be extended and end on the same horizontal level as (4), but shifted to the right, thus increasing the discharge temperature to around 160°C, which is not acceptable (primarily for oil considerations).

ter-stage cooler helps to keep a low discharge gas

Figure 1.6: Log(p)-h diagram for two-stage ammonia system

(5)

(4)

(7)

(8)

(2)

(6)

(3)

(1)

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

In the above Log(P)-H diagram, the economiser is not shown. The economiser port is a port in screw compressors that allows access to the compression chamber after the suction port has closed. As such, if a high enough pressure is applied to the economiser port, an additional refrigerant flow can be added to

This is utilised with the economiser, which is a heat

exchanger that further cools the liquid from the con-

denser. To do this, a little of the condenser liquid

flow is flashed to a lower pressure/temperature,

where it is evaporated to cool the remaining liquid.

That modifies the Log(P)-H diagram a little: the compressor without affecting the suction flow.

Figure 1.7: Log(p)-h diagram for two-stage ammonia system with economizer on the high stage

Low-stage compression from (1) to (2), de-superheating in the inter-stage cooler from (2) to (3) and initial high-stage compression from (3) to (4) is the same as in the non-economized system. The discharge from the high-stage compressor (6) is condensed from (6) to (7) as before. After the condenser (7), a small amount of refrigerant is flashed to a lower temperature (8), where it is evaporated and added to the partially compressed suction gas of the high-stage compressor. The resulting mixture is (5), which is compressed together to the final discharge

the remaining liquid from (7) to (9). It is noteworthy

that the liquid flashed to the intermediate tempera-

ture is further to the left than it would have been

without the economizer, thus the enthalpy differ-

ence from (10) to (3) is bigger and that increases the

compressor’s cooling capacity. The power consump-

tion is increased as well, because there is an addi-

tional mass flow that needs to be compressed, but

the overall effect – depending on the operating con-

ditions – is a higher cooling capacity and, usually, a

higher COP. at (6). The evaporative capacity is used to sub-cool

Application Handbook Industrial Refrigeration ammonia and CO2 applications

1.5 Cascade systems

A cascade system consists of two separate refrigeration circuits. The separate refrigeration circuits are connected by a heat exchanger, which acts as a condenser for the low-temperature circuit and an evaporator for the high-temperature circuit.

The refrigerants for the two circuits can be different and optimised for each circuit. For example, the refrigerant for the high-temperature circuit could be ammonia, and the refrigerant for the low-temperature circuit could be CO2.

A CO2/ammonia system needs a smaller charge of ammonia and proves to be more efficient in lowtemperature refrigeration than a similar two-stage ammonia system.

Figure 1.8: Cascade system diagram

A system diagram for a cascade system is shown in Figure 1.8. The cascade system diagram is somewhat similar to the two-stage system, where the interstage cooler is replaced by a cascade cooler, thus making two closed cycles. A cascade system is usually more complex than a two-stage system, but it offers some benefits. CO2 is very efficient down to very low evaporating temperatures where ammonia‘s efficiency drops, while ammonia can condense at relatively low pressure against warm ambient temperatures, where CO2 has to go transcritical where the efficiency drops. So, choosing CO2 in the LP circuit and ammonia in the HP circuit allows for the best of both worlds.

Oil Cooler

LP Compressor

Evaporator

Expansi on valve

HP Compressor

Cascade cooler

Oil Separator

Condenser

Economizer

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

1.6 Transcritical systems

The properties of CO2 make it necessary to operate the system in a different way. The ‘critical point’ – the top of the bell-shaped two-phase area – is at 31°C / 72.8 bara. In areas/periods where the ambient temperature is low, CO2 can be operated with condensing below the critical point. In that case the cycle is, in principle, the same as for other refrigerants (different pressures and enthalpies), but if the ambient temperature is relatively high, it is no longer possible to condense in the traditional way. Figure 1.9: Log(p)-h diagram for transcritical CO2 system

Above the critical point, the refrigerant is called a

transcritical fluid. The transcritical fluid does not

condense, but rather displays a gradual change of

density as it is cooled. In a traditional condensing

process, the condensing temperature defines the

pressure, but in the transcritical fluid there is no

such connection. Thus, the gas cooling pressure

(equal to compressor discharge pressure) needs to

be controlled.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Evaporation takes place from (4) to (1) and compression from (1) to (2) as in other refrigeration systems. From (2) to (3) is the ‘gas cooling’ process that replaces the condensing process. Point (3) is defined by the outlet temperature of the gas cooler – here set to 40°C – and the pressure that is

controlled by the expansion device that expands to (4). The actual pressure in the gas cooler is based on an analysis of the cycle. Consider a case where the gas cooler outlet is at 40°C, but different pressures are controlled. The corresponding cycles are as shown below:

Figure 1.10: Log(p)-h diagram for a transcritical CO2 system at different gas cooler pressures

The cooling capacity of the system is the enthalpy difference between points (1) and (4) multiplied by the mass flow. The necessary compression power is the enthalpy difference between points (2) and (1) (horizontally) multiplied by the mass flow. The COP of the system is thus (H1-H4)/(H2-H1), which is independent of the mass flow. It can be seen that if the pressure is varied up (red) or down (blue), both the cooling capacity and power consumption change. Calculating the COP along the (red) temperature curve, it can be found to have a maximum at a specific pressure, which for 40°C works out to be 102.4 bar. Mapping this maximum COP across different gas cooler exit temperatures has resulted in a curve that expresses the best pressure for a given gas cooler exit temperature. This curve has been implemented in the Danfoss EKC 326 controller.

Controlling the gas cooler pressure with the expansion device results in a problem. Since it is intended that the low-pressure side be supplied with the right

amount of liquid – especially with DX operation of the low-pressure evaporators – it is simply not possible to have an expansion valve serve both purposes at once.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

The solution is to make an intermediate stop in the form of a receiver. The transcritical fluid is flashed from the gas cooler exit through the high-pressure valve into the receiver. In the receiver, the gas and

the evaporators as necessary, while the gas part is

reduced in pressure and fed to the compressor’s suc-

tion line, thereby controlling the pressure in the re-

ceiver. liquid part is separated, the liquid part is flashed to

Figure 1.11: Transcritical CO2 system with intermediate pressure receiver

Receiver

Gas cooler

Figure 1.12: Log(p)-h diagram for a transcritical CO2 system with intermediate pressure receiver

Application Handbook Industrial Refrigeration ammonia and CO2 applications

This process can be optimised. The gas from the receiver is reduced in pressure to the compressor suction, however it is possible to take advantage of the

compressor to compress this gas to gas cooler pressure with a higher efficiency due to the higher suc-

tion pressure. This is called parallel compression. higher pressure in the receiver to employ another Figure 1.13: Transcritical CO2 system with parallel compression

Receiver

Gas cooler

Figure 1.14: log(P)-h diagram for a transcritical CO2 system with parallel compression

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

One further optimisation for CO2 transcritical systems is the use of ejectors. Ejectors utilise the power in the expansion from gas cooler pressure to lift a gas flow to a higher pressure. One of several differ-

ent ways of employing this is to lift part of the suction gas from the low-stage compressor to the suction of the parallel compressor, resulting in an efficiency gain.

Figure 1.15: Transcritical CO2 system with parallel compression and gas ejectors

Receiver

Gas cooler

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Compressor controls

Oil Cooler

Oil Separator

Compressor

Evaporator

Expansion valve

The compressor is the “heart” of the refrigeration system. It has two basic functions:

Maintain the pressure in the evaporator so that the liquid refrigerant can evaporate at the required temperature.

Compress the refrigerant so that it can be condensed at a higher temperature.

Condenser

If the compressor capacity is greater than the de-

mand, the evaporating pressure and temperature

will be lower than that required, and vice versa.

Additionally, the compressor should not be allowed

to operate outside of the approved operation enve-

lope for the mechanical safety of the compressor

and operational safety of the entire system.

The basic function of compressor control, therefore, is to adjust the capacity of the compressor to the actual demand of the refrigeration system so that the required evaporating temperature can be maintained.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

2.1 Reverse flow control

Reverse flow and condensation of refrigerant during stand-still from the condenser to the oil separator and the compressor should be avoided at all times. For piston compressors, reverse flow can result in liquid hammering. For screw compressors, reverse

Application Example 2.1.1: Reverse flow control

RT 1A RT 5A

Compressor

From evaporator/ liquid separator

SVA

FIA

flow can cause reversed rotation and damage to the compressor bearings. To avoid this reverse flow, it is necessary to install a check valve on the outlet of the oil separator.

SCA

To condenser

Oil Separator

ICFS

ICFE

The stop check valve SCA can function as a check valve when the system is running and can also shut off the discharge line for service as a stop valve. This combined stop/check valve solution is easier to install and has lower flow resistance compared to a normal stop valve plus check valve installation.

When selecting a stop check valve, it is important to:

Select a valve according to the capacity and not the pipe size.

Consider both the nominal and part-load working conditions. The velocity in the nominal condition should be close to the recommended value, while at the same time, the velocity in the part-load condition should be higher than the minimum recommended velocity.

ICFF

ICFO

ICF

Pressure switches RT 1A and RT 5A are shown in the suction line and discharge line of the compressor. It is important to have pressure switches to cut-out or cut-in operation if the pressure fluctuates outside the operational range in the suction and discharge line. Not all compressors are delivered with built-in controls and safety switches, therefore the RT 1A and RT 5A are shown. The pressure switches are not shown for all application examples, but they should always be present in the system. Often these switches are part of the compressor package.

For details on how to select valves, please refer to the product catalogue and use the Danfoss software tool Coolselector2 for sizing the valve according to the system capacity.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

2.2 Suction pressure control

During start-up or after defrosting, the suction pressure needs to be controlled, otherwise it may be too high, and the compressor motor will be overloaded.

The electric motor for the compressor may be damaged by this overloading.

There are two ways to overcome this problem:

Start the compressor at part load. The capacity control methods can be used to start the compressor at part load, e.g. unload part of the pistons for multi-

Application example 2.2.1: Crankcase pressure control

CVC

piston reciprocating compressors, or bypass some

suction gas for screw compressors with slide valves,

etc.

Control the suction pressure for reciprocating com-

pressors. By installing a back pressure-controlled

regulating valve in the suction line, which will not

open until the pressure in the suction line drops be-

low the set value, suction pressure can be kept un-

der a certain level.

SCA

To condenser

Compressor

From evaporator/ liquid separator

ICS

SVA

In order to control the suction pressure during startup, after defrosting, or in other cases when the suction pressure may be too high, the pilot-operated servo valve (ICS) with the back pressure controlled pilot valve (CVC) is installed in the suction line. The

Oil Separator

FIA

ICFS

ICFE

ICFF

ICFO

ICF

ICS will not open until the downstream suction pres-

sure falls below the set value of the pilot valve (CVC).

In this way, the high-pressure vapour in the suction

line can be released into the compressor gradually,

which ensures a manageable capacity for the com-

pressor motor.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

2.3 Compressor capacity control

The compressor in a refrigeration system is normally selected to be able to satisfy the highest possible cooling load. However, the cooling load during normal operation is usually lower than the design cooling load. This means that it is always necessary to control the compressor capacity so that it matches the actual cooling load. There are several common ways to control the compressor capacity:

2.3.1 Step control

This means to unload cylinders in a multi-cylinder compressor, to open and close the suction ports of a screw compressor, or to start and stop some compressors in a multi-compressor system. This system is simple and convenient. Furthermore, efficiency decreases very little during part-load by step control, although this regulation is by nature coarser than many other regulation types. It is especially applicable to systems with several multi-cylinder reciprocating compressors.

2.3.2 Slide valve control

The most common device used to control the capacity of a screw compressor is the slide valve. The action of the oil-driven slide valve allows part of the suction gas to avoid being compressed. The slide valve permits a smooth and continuous modulation of capacity from 100% down to 10%, but the efficiency drops at part load, usually quite significantly.

2.3.3 Variable speed control

This solution is applicable to all kinds of compressors and is efficient. A two-speed electric motor or a frequency converter can be used to vary the speed of the compressor. The two-speed electric motor regulates the compressor capacity by running at the high speed when the cooling load is high (e.g. cooling down period) and at the low speed when the cooling load is low (e.g. storage period). The frequency converter can vary the rotation speed continuously to satisfy the actual demand. The frequency converter observes limits for min. and max. speed, temperature and pressure control, protection of compressor motor as well as current and torque limits. Frequency converters offer a low start-up current. Variable speed control usually has little impact on the efficiency of the compressor and is thus a very popular control. In some cases, the range of allowed RPM of the compressor is not enough to cover the desired range of capacities, so often variable speed control is combined with step control.

2.3.4 Hot gas bypass

This solution is applicable to compressors with fixed capacities and more typical for commercial refrigeration. In industrial refrigeration systems it is used as a safety measure to avoid an excessively low suction pressure when there is a sudden drop in cooling load. To control the refrigeration capacity, part of the hot gas flow in the discharge line is bypassed into the low-pressure circuit. This works as a fast response to a sudden drop in cooling load, thus adding extra cooling load to the system when the compressor is ramping down. This generally decreases the efficiency of the system.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application example 2.3.1: Step control of compressor capacity

Compressor

pack

EKC 331

AKS 33

FIA

From evaporator/ liquid separator

SVA

A step control solution for compressor capacity can be achieved by using a step controller, e.g. Danfoss EKC 331. EKC 331 is a four-step controller with up to four relay outputs. It controls the loading/unloading of the compressors/pistons or the electric motor of the compressor according to the suction pressure

SCA

To condenser

Oil Separator

ICFO

ICFE

SCA

ICFS

ICFF

ICF

signal from a pressure transmitter, e.g. Danfoss AKS

33. Based on a neutral zone control, EKC 331 can

control a pack system with up to four equally sized

compressor steps or, alternatively, two capacity-con-

trolled compressors (each with one unload valve).

Application example 2.3.2: Variable speed capacity control

VLT

Controller

AKS 33

FIA

From evaporator/ liquid separator

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SVA

Compressor

SVA

To oil separator

Application Handbook Industrial Refrigeration ammonia and CO2 applications

The VLT frequency converter is controlled by a controller module that adjusts the control signal to the converter from an input signal from a pressure transmitter, e.g. AKS 33. The controller module can be either a Danfoss AK-CC or a PLC/OEM module. The VLT frequency converter can also receive the signal from the pressure transmitter and work without a separate controller.

Application example 2.3.3: Hot gas bypass

CVC

ICS

SVA

FIA

Frequency converter control offer the following advantages:

 Energy savings  Improved control and product quality  Noise reduction  Longer lifetime  Simplified installation  Easy-to-use complete control of the system

SCA

Compressor

ICFS

ICFE

CVC

EVM

ICFO

ICFF

ICF

To condenser

Oil Separator

SVA

Evaporator

SVA

TEA

ICFE

ICFO

Hot gas bypass can be used in specific cases to compensate for a suddenly reduced cooling load. Normally in industrial refrigeration plants, the compressors will be speed controlled or step controlled. These controls can, however, be too slow in reacting in cases where there is a sudden drop in cooling load. This sudden drop will initially cause the compressor to decrease the suction pressure. Hot gas bypass can be used as a fast response to a sudden decrease in cooling load, which will prevent the suction pressure from dropping too low. The hot gas is bypassed to the evaporator until the compressor has ramped down to match the reduced cooling load.

ICS SVA

ICFS

From receiver

ICFF

ICF

Hot gas bypass is energy inefficient and should not be used as a primary solution to “control” the compressor, but rather as a safety measure to avoid excessively low suction pressure.

The pilot-operated servo valve ICS (1) with a CVC pilot valve is used to control the hot gas bypass flow according to the pressure on the suction line. The CVC is a back-pressure controlled pilot valve which opens the ICS (1) and increases the flow of hot gas when the suction pressure is below the set value. In this way, the suction pressure ahead of the compressor is kept constant, and thus the refrigeration capacity satisfies the actual cooling load.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

2.4 Discharge Temperature Control with Liquid Injection

Compressor manufacturers generally recommend limiting the discharge temperature below a certain value to prevent overheating of the compressor and its parts, prolonging their life and preventing the breakdown of oil at high temperatures.

There are several ways to reduce the discharge tem-

perature. One way is to install water-cooled heads in

reciprocating compressors. Another method is liquid

injection, which involves liquid refrigerant from the

outlet of the condenser or receiver being injected

into the suction line, the intermediate cooler beReferring to the log p-h diagram in chapter 1, it can

be seen that the discharge temperature may be high

tween compressors, or the side port of the screw

compressor. when:

 the compressor runs with high pressure differential.  the compressor receives highly superheated suction

vapour.

 the compressor runs with capacity control by hot gas

bypass.

Application example 2.4.1: Liquid injection with thermostatic injection valve

Screw compressor

RT 107

From evaporator/ liquid separator

SVA

FIA

Oil injection

When the discharge temperature rises above the set value of the thermostat RT 107, RT 107 will energize the solenoid valve ICFE which will start liquid injection into the side port of the screw compressor. The

To oil Separator

ICFS

ICFE

SVA

TEAT

ICFO

ICFF

From receiver

ICF

thermostatic injection valve (TEAT) controls the in-

jected liquid flow according to the discharge temper-

ature, which prevents the discharge temperature

from rising further.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application example 2.4.2: Liquid injection with motor valve

Screw compressor

To oil Separator

From evaporator/ liquid separator

SVA

FIA

AKS 21

EKC 361

ICAD

Oil injection

ICM

An electronic solution for liquid injection control can be achieved with the motorized valve ICM. An AKS 21 temperature sensor will register the discharge temperature and transmit the signal to the temperature controller EKC 361.

ICFS

ICFO

ICFE

ICF

ICFS

ICFF

From receiver

SVA

ICAD

ICM

Single component solution

EVRA

The EKC 361 controls the ICAD actuator which adjusts the opening degree of the ICM motor valve in order to limit and maintain the required discharge temperature.

SVA

FIA

Application Handbook Industrial Refrigeration ammonia and CO2 applications

2.5 Economizer damper

An economizer is used to increase system capacity and efficiency.

Pulsations need to be dampened to avoid compo-

nent damage. Correct and well considered dimen-

sioning of economizer lines is necessary to reduce However, pulsations from the compressor are trans-

ferred into the economizer line which damages components and leads to valve chattering. The pulsations will lead to increased maintenance costs and downtime if they are not taken care of.

pulsations. The Danfoss Eco-Damper combines three

components into a high-impact solution which re-

duces pulsations significantly and ensures high oper-

ational reliability thanks to its solid design. The

Danfoss Eco-Damper is based on the Danfoss ICV

platform.

Application example 2.5.1: Economizer line pulsation damping solution

To oil Separator

Screw compressor

Max 150 mm

Oil injection

Max 150 mm Max 150 mm

ICD ICC

EVMCVP

ICS

From economizer vessel

From evaporator/ liquid separator

SVA

FIA

The Danfoss Eco-damper solution consists of 3 components. The Eco-damper (ICD), the Eco-check valve (ICC) and a standard control valve, e.g. an ICS servo valve with CVP, constant pressure pilot valve, and the EVM solenoid valve. The combination shown in the application example can dampen the pulsations by up to 80% and prevent premature failure of economizer components and reduce system downtime. The Eco-damper solution unit has a unique broadband damping effect in the 100-500 Hz working range.

Support on pipe segments is essential to reduce the pulsations and thus reduce vibrations induced by the pulsations. Therefore, each pipe segment between the components in the Eco-damper solution must be fixed to a pipe support element which can absorb the pulsations. The tube on the Eco-damper (ICD) must also be fixed to a pipe support element that can absorb the pulsations.

The ICD Eco-damper comes with an integrated

strainer, and is applicable for ammonia systems,

along with R134a and R407C systems.

The ICC Eco-check valve has a solid design and is fully

opening at low differential pressure to prevent oscil-

lating movements. The ICC Eco-check valve is appli-

cable for ammonia, CO2 and HFC systems.

For less severe or occasional pulsation issues, the ro-

bust Eco-check valve (ICC) and the control valve can

be used without an Eco-damper (ICD). Note! In this

case, the pulsations from the compressor eco-port

will not be damped.

It is recommended to have a maximum pipe length

of 150 mm between the individual components and

between the ICD and the economizer port in the

compressor.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

2.6 Summary

Solution Application Benefit Limitations

Reverse Flow Control – see section 0

Reverse flow control with SCA

Applicable to all refrigeration plants.

Crankcase Pressure Control – see section 2.2

Crankcase pressure control with ICS and CVC

Applicable to reciprocating compressors, normally used for small and medium systems.

Compressor Capacity Control – see section 2.3

Step control of compressor capacity with EKC 331 and AKS 33

Compressor variable speed capacity control

Hot gas bypass using ICS and CVC

Controller

Applicable to multicylinder compressor, screw compressor with multiple suction ports, and systems with several compressors running in parallel.

Applicable to all compressors with the ability to run at reduced speed.

Applicable to compressors with fixed capacities. Used as safety measure against excessively low suction pressure

Discharge Temperature Control with Liquid Injection – see section 0

Simple. Easy to install. Low flow resistance.

Simple and reliable. Effective in protecting reciprocating compressors on start-up or after hot gas defrost.

Simple. Almost as efficient at part load as at full load.

Low start-up current Energy savings Lower noise Longer lifetime Simplified installation

Fast response to sudden drops in cooling load. The hot gas can help the oil return from the evaporator.

Causes constant pressure drop in the discharge line.

Causes constant pressure drop in the suction line.

The control is not continuous, especially when there are only a few steps. Fluctuations in the suction pressure.

Compressor must be suited for reduced speed operation.

Should not be used as primary compressor capacity control. Energy inefficient.

Mechanical solution for liquid injection with TEAT, EVRA(T) and RT

Electronic solution for liquid injection control with EKC 361 and ICM/ICF

Applicable to systems

TSHL

where the discharge temperatures may run too high.

Simple and effective.

Injection of liquid refrigerant may be dangerous to the compressor. Not as efficient as intermediate cooler.

Applicable to systems

where the discharge temperatures may run too high.

Flexible and compact. Possible to monitor and control remotely.

Not applicable to flammable refrigerants. Injection of liquid refrigerant may be dangerous to the compressor. Not as efficient as intermediate cooler.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Condenser controls

Oil Cooler

Oil Separator

Compressor

Evaporat or

Expansi on valve

In areas where there are large variations in ambient air temperatures and/or load conditions, it is necessary to control the condensing pressure to prevent it from falling too low. Excessively low condensing pressures result in there being insufficient pressure differential across the expansion device and the evaporator not being supplied with enough refrigerant. It means that condenser capacity control is mainly used in the temperate climate zones and to a lesser degree in subtropical and tropical zones.

The basic idea of condensing capacity control is to control the condenser capacity when the ambient temperature is low, so that the condensing pressure is maintained above the minimum acceptable level and to keep the condensing pressure as low as possible for optimal efficiency. This condensing capacity control is achieved either by regulating the flow of circulating air or water through the condenser, or by reducing the effective heat exchanger surface area.

Different solutions can be designed for different types of condensers:

 Air cooled condensers

Condenser

Condenser installation depends on the way injection

to the low-pressure side of the refrigeration system

is controlled. Note that the terms ‘high-pressure

float valve operation’ and ‘low-pressure float valve

operation’ refer to the mode of operation, not spe-

cifically the use of a float valve. Both operation

modes can be achieved by using level

switches/transmitters and a normal expansion valve.

3.1 High-pressure float valve operation

High-pressure float valve operation is expansion of

the liquid immediately after the condenser. Any vari-

ation in the charge volume due to variations in ca-

pacity must be handled at the low-pressure side, e.g.

in a liquid separator. Since the flow after the expan-

sion valve is two-phase, it is not suitable for distribu-

tion to more than one location and thus it is primar-

ily used in systems with only one low-pressure sepa-

rator, such as a chiller unit.

High-pressure float valves are usually mounted im-

mediately after the condenser and, as such, pose no

special problems with regard to condenser installa-

tion.

 Evaporative condensers  Water cooled condensers

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

3.2 Low-pressure float valve operation

Low-pressure float valve operation controls the expansion of the condensed liquid to keep a given level in one or more low-pressure separators. Any varia-

3.3 Air-cooled condensers

An air-cooled condenser consists of tubes mounted within a fin block. The condenser can be horizontal, vertical or V-shaped. The ambient air is drawn across the heat exchanger surface with axial or centrifugal fans.

3.3.1 Step control of air-cooled condensers

This method utilizes a step controller to control the air flow in the air-cooled condenser by switching the fans on or off according to a condensing pressure signal.

tion of the charge volume due to variations in capacity must be handled on the high-pressure side, e.g. in a receiver.

To ensure proper functioning of a condenser in a low-pressure float valve system, the installation must be correct.

Air-cooled condensers are used on industrial refrigeration systems where the relative air humidity is high. Air-cooled condenser application examples are depicted below as low-pressure float valve operation.

Controlling the condensing pressure for air-cooled condensers can be achieved in the following ways:

A pressure transmitter, e.g. Danfoss AKS 33, measures the condensing pressure and sends a signal to a step controller, e.g. Danfoss EKC 331, which controls the switching of the fans according to the pressure signal.

3.3.2 Fan speed control of air-cooled condensers

This method of condenser fan control is mainly used when a reduction in noise level is desired due to environmental concerns.

3.3.3 Area control of air-cooled condenser

For area control of air-cooled condensers, a receiver is required. This receiver must have sufficient volume to be able to accommodate the variations in the amount of refrigerant in the condenser.

There are two ways this condenser area control can be done:

The main valve ICS combined with the constant pressure pilot CVP-H (high-pressure) mounted in the discharge line on the inlet side to the condenser and ICS combined with a differential pressure pilot CVPP mounted in the pressure equalization line between the discharge line and the receiver. In the pipe between the condenser and the receiver, a stop-check valve SCA is mounted to prevent liquid migration from the receiver to the condenser.

For this type of installation, Danfoss frequency converter VLT can be used.

Main valve ICS combined with the constant pressure pilot CVP-H mounted in the pipe between the condenser and the receiver and an ICS combined with a differential pressure pilot CVPP mounted in the pressure equalization line between the discharge line and the receiver. This method is mainly used in commercial refrigeration.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 3.3.1: Air flow control of air-cooled condenser with step controller EKC 331

EKC 331

AKS 33

From

discharge line

SVA

LLG

SVA

To oil cooler

EKC 331 is a four-step controller with up to four relay outputs. It controls the switching of the fans according to the condensing pressure signal from a pressure transmitter AKS 33.

Based on neutral zone control, EKC 331 can control the condensing capacity so that the condensing pressure is maintained above the required minimum level.

The bypass pipe (thin red line) where SVA is installed is an equalizing pipe, which helps balance the pressure in the receiver with the inlet pressure of the condenser so that the liquid refrigerant in the condenser can be drained into the receiver.

Condenser

SVA

SNV

SFASFA

DSV

Receiver

SCA

To expansion device

Liquid trap

Priority for oil cooler

receiver with a liquid trap at the bottom will allow

the build-up of liquid in the drop leg. The liquid col-

umn in the drop leg will provide a positive pressure

to counter the pressure drop in the condenser. The

height of the drop leg must be larger than the pres-

sure loss in the condenser, expressed in metres of

liquid. In some installations, EKC 331T is used. In this

case, the input signal could be from a PT 1000 tem-

perature sensor, e.g. AKS 21. The temperature sen-

sor is usually installed in the outlet of the condenser.

Note! The EKC 331T + PT1000 temperature sensor

solution is not as accurate as the EKC 331 + pressure

transmitter solution, because the condenser outlet

temperature may not entirely reflect the actual con-

densing pressure due to the liquid subcooling or the

presence of incondensable gasses in the refrigera-

tion system. If the subcooling is too low, flash gas

may occur when the fans start.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 3.3.2: Fan speed control of air-cooled condenser

VLT

From

discharge line

AKS

SVA

LLG

SVA

To oil cooler

The VLT frequency converter is controlled by the condensing pressure signal from a pressure transmitter, e.g. Danfoss AKS 33, on the discharge line. The VLT frequency converter adjusts the speed of the fans in the air-cooled condenser according to the signal from the pressure transmitter.

Frequency converter control offers the following advantages:

 Energy savings  Improved control and product quality  Noise reduction  Longer lifetime  Simplified installation

Condenser

SVA

SNV

Receiver

To expansion device

SFASFA

DSV

SCA

Liquid trap

Priority for oil cooler

It is important to account for the pressure drop in the condenser when designing the piping from the condenser to the receiver. The pressure at the outlet of the condenser can be lower than the pressure in the receiver, which will restrict the flow to the receiver. Using a drop leg between the condenser and receiver, with a liquid trap at the bottom, will allow the build-up of liquid in the drop leg. The liquid column in the drop leg will provide a positive pressure to counter the pressure drop in the condenser. The height of the drop leg must be greater than the pressure loss in the condenser, expressed in metres of liquid.

 Easy-to-use complete control of the system

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 3.3.3: Area control of air-cooled condenser (for cold climates)

CVP

Suction

line

SVA

LLG

To oil cooler

ICS

SVA

ICS

CVPP

SVA

SFASFA

SNV

DSV

Receiver

To expansion device

Condenser

SCA

Liquid trap

Priority for oil cooler

This regulating solution maintains the pressure in the receiver at a sufficiently high level during low ambient temperatures.

The ICS pilot-operated servo valve in the discharge line opens when the discharge pressure reaches the set pressure on the CVP pilot valve and closes when the pressure drops below the set pressure of the CVP pilot valve.

The ICS pilot-operated servo valve with the CVPP constant differential pressure pilot in the pressure equalization line (thin red line) maintains sufficient pressure in the receiver.

The SCA stop-check valve ensures increased condenser pressure by liquid back-up within the condenser. This requires a sufficiently large receiver. The SCA stop-check valve also prevents liquid flow from the receiver back into the condenser when the latter is colder during compressor shut-down periods.

It is important to account for the pressure drop in

the condenser when designing the piping from the

condenser to the receiver. The pressure at the outlet

of the condenser can be lower than the pressure in

the receiver, which will restrict the flow to the re-

ceiver. Using a drop leg between the condenser and

receiver, with a liquid trap at the bottom, will allow

the build-up of liquid in the drop leg. The liquid col-

umn in the drop leg will provide a positive pressure

to counter the pressure drop in the condenser. The

height of the drop leg must be greater than the pres-

sure loss in the condenser, expressed in metres of

liquid.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

3.4 Evaporative condensers

An evaporative condenser is a condenser cooled by ambient air combined with water sprayed through orifices and air baffles in counterflow with the air. The water evaporates, and the evaporation effect of the water drops adds considerably to the condenser capacity, as the air is cooled to the ‘wet bulb’ temperature.

Today’s evaporative condensers are enclosed in a steel or plastic enclosure with axial or centrifugal fans at the bottom or at the top of the condenser.

The heat exchanger surface in the wet air stream consists of steel pipes.

Above the water spray orifices (in the dry air) it is common to have a de-superheater made of steel pipes with fins to reduce the discharge gas temperature before it reaches the heat exchanger in the wet air stream. In this way, the build-up of limescale on

the surface of the main heat exchanger pipes is reduced.

This type reduces the water consumption considerably compared to a normal water-cooled condenser. Capacity control of an evaporative condenser can be achieved by either a two-speed fan or variable speed control of the fan and, at very low ambient temperature conditions, switching off the water circulation pump.

The use of evaporative condensers is limited in areas with high relative humidity. In cold surroundings (ambient temperatures < 0°C), frost damage prevention must be carried out by removing the water in the evaporative condenser.

The applications shown in this section are depicted as low-pressure float valve operated.

3.4.1 Control of evaporative condensers

Controlling the evaporative condensers’ condensing pressure or the condenser capacity can be achieved in different ways:

 RT or KP pressure controls for fan and water pump

control.

 RT-L neutral zone pressure control for fan and water

pump control.

 Step controller for controlling two-speed fans and the

water pump.

 Frequency converters for fan speed control and water

pump control.

 Saginomiya flow-switch for alarm if water circulation

fails.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 3.4.1: Step control of evaporative condenser with pressure controller RT

Suction line

Compressor

SCA

RT 5A

LLG

To oil cooler

RT 5A

SVA

SNV

Condenser

SFASFA

DSV

Receiver

To expansion device

Water pump

SCA

Liquid trap

Priority for oil cooler

This solution maintains the condensing pressure, as well as the pressure in the receiver at a sufficiently high level in low ambient temperatures.

When the inlet pressure of the condenser drops below the setting of the pressure controller RT 5A, the controller will switch off the fan to decrease the condensing capacity.

In extremely low ambient temperatures, when the

condensing pressure drops below the setting of RT

5A after all the fans have been switched off, RT 5A

will stop the water pump.

When the pump is stopped, the condenser and the

water pipes should be drained to avoid scaling and

freezing.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 3.4.2: Step control of evaporative condenser with step controller EKC 331

EKC 331

AKS 33

Suction line

Compressor

SCA

To oil cooler

LLG

SVA

SNV

Receiver

To expansion device

SFASFA

DSV

Condenser

SCA

Water pump

Liquid trap

Priority for oil cooler

This solution works in the same way as Application Example 3.4.1, but operated via step controller EKC

331.

A capacity regulation solution for evaporative con-

EKC 331T version can accept a signal from a PT 1000 temperature sensor, which may be necessary for secondary systems.

densers can be achieved by using an EKC 331 power regulator and an AKS 33 pressure transmitter. Sequential control for the water pump must be selected as the last step. Sequential control means that the steps will always cut in and out in the same order.

Alternatively, a frequency converter can control the condensing pressure by controlling the fan RPM, possibly with a step function on individual fans, as in the air cooled condenser version, with an RT 5A cutting out the pump at low condensing temperatures.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

3.5 Water-cooled condensers

In the past, the water-cooled condenser was typically a shell and tube heat exchanger, but today it is very often a plate heat exchanger in a modern design.

Control of the condensing pressure can be achieved

using a pressure-controlled water valve, or a motor-

ized water valve controlled by an electronic control-

ler to control the flow of the cooling water according

to the condensing pressure. Water-cooled condensers with open water loops are

not commonly used, because in many places it is not permitted to use the large amount of water these types consume (water shortage and/or high prices for water). However, such restrictions usually do not

The applications shown in the following section are

depicted as high-pressure float valve operation.

Thus, no receiver is shown in the following applica-

tions. apply in maritime applications.

Today, water-cooled condensers are popular in chillers, with the cooling water cooled by a cooling tower and re-circulated. It can also be used as a heat recovery condenser to supply hot water.

Application Example 3.5.1: Water flow control of water-cooled condensers with a water valve

SCA

SVA

Suction line

Compressor

To expansion

SVA

device

This solution maintains the condensing pressure at a constant level. The refrigerant condensing pressure is directed through a capillary tube to the top of the water valve WVS and adjusts the opening of WVS accordingly. The water valve (WVS) is a P-regulator.

SFA

SNV

DSV

Cooling water out

WVS

Cooling water in

Danfoss SW-PHE

The condenser is shown as a Danfoss semi-welded

plate heat exchanger.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 3.5.2: Water flow control of water-cooled condenser with a motor valve

Controller

AMV 20

SFA

AKS 33

SNV

DSV

SCA

SVA

Suction line

Compressor

To expansion

device

SVA

The controller receives the condensing pressure signal from the pressure transmitter AKS 33 and sends out a corresponding modulating signal to actuator AMV 20 of the motor valve VM 2. In this way, the flow of cooling water is adjusted, and the condensing pressure is kept constant.

In this solution, PI or PID control can be configured in the controller.

VM2

Cooling water in

Danfoss SW-PHE

Cooling water out

VM 2 and VFG 2 are motor valves designed for district heating and can also be used for water flow control in refrigeration plants.

Alternatively, an AB-QM pressure-independent balancing and control valve can be used. The AB-QM valve consists of 2 functions: a differential pressure controller and a control valve. The differential pressure controller maintains a constant differential pressure across the control valve, while the control valve adjusts the flow through the valve.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

3.6 Summary

Solution Application Benefit Limitations

Error! Reference source not found. – see section Error! Reference source not found.

Step control of fans with step controller EKC 331.

Condenser

Receiver

Fan speed control of aircooled condensers.

Condenser

Receiver

Evaporative condensers– see section 3.4

Step control of evaporative condenser with pressure controller

PS PS

Condenser

Receiver

Step control of evaporative condensers with step controller EKC

331

Condenser

Receiver

Water-cooled condensers– see section 3.5

Used mainly in industrial refrigeration in hot climates and to a lesser extent in colder climates.

Applicable to all condensers with the ability to run at reduced speed.

Industrial refrigeration with very large capacity requirement

Control of air volume flow in steps. No use of water.

Low start-up current Energy savings Lower noise Longer lifetime Simplified installation

Large reduction in water consumption compared to water-cooled condensers and relatively easy to capacity control

Very low ambient temperatures. Fan step control can be noisy.

Very low ambient temperatures.

Not applicable in countries with high relative humidity. In cold climates, special precautions must be taken to ensure the water pipe is drained of water during pump off-periods.

Liquid flow control with a water valve

Liquid flow control with a motior valve

Compressor

Condenser

Compressor

Condenser

Water out

Water in

Water out

Water in

Chillers, heat recovery condensers

It is easy to capacity control.

It is easy to capacity control the condenser and the heat recovery. Possible to control remotely.

Not applicable when availability is a problem

This type of installation is more expensive than a normal set up. Not applicable when water availability is a problem.

38 AB13778641621700-000702 © Danfoss A/S (RC-MDP/MWA), 2020-10

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Liquid level regulation

Liquid level regulation is an important element in the design of industrial refrigeration systems. It controls the liquid injection to maintain a constant liquid level.

Two main different principles may be used when designing a liquid level regulation system:

 High-pressure liquid level regulation system (HP LLRS)  Low-pressure liquid level regulation system (LP LLRS)

Both principles can be achieved using mechanical and electronic components. Float valves offer a simple control, but as the float valve needs to be mounted in the desired level, a change necessitates a physical relocation of the valve. Systems employing a level transmitter are more complex but enable an easy level change.

High-pressure liquid level regulation systems are typically characterized by:

 The sensing device (sensor/float) is placed on the

high-pressure side of the system.

 An increase in liquid level will open the expansion de-

vice and pass high-pressure liquid to the low-pressure side of the system, e.g. the regulation attempts to keep the high-pressure side level constant.

 Critical refrigerant charge  Small HP receiver or even no HP receiver  Mainly applied where the HP liquid is distributed to

only one low-pressure application.

 Variations in refrigerant volume must be accommo-

dated on the low-pressure side

Figure 4.1: HP liquid level regulation diagram

HP Liquid level control

LP seperator

Evaporator

Compressor

Expansion

valve

HP Receiver

Condenser

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Low-pressure liquid level regulation systems are typically characterized by:

 The sensing device (sensor/float) is placed on the low-

pressure side of the system

 A decrease in liquid level will open the expansion de-

vice and pass high-pressure liquid to the low-pressure side of the system

 Receiver is generally large in size

Figure 4.2: LP liquid level regulation diagram

LP Liquid level control

LP seperator

Evaporator

Expansion

valve

 (Fairly) large charge of refrigerant  Mainly applied to decentralized systems where there

can be several low-pressure applications

 Variations in refrigerant volume must be accommo-

dated on the high-pressure side

Compressor

HP Receiver

Condenser

In conclusion, HP LLRS are suitable for compact systems like chillers; the advantage is the reduced cost (small receiver or no receiver). Whereas LP LLRS are very suitable for decentralised systems with more than one LP separator and long piping, for instance a large cold storage; the advantage being the increased safety and reliability.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

4.1 High-pressure liquid level regulation system (HP LLRS)

When designing a HP LLRS, the following points must be taken into consideration:

As soon as liquid is “formed” in the condenser/condenser outlet, the liquid is fed to the low-pressure side (the evaporator).

The liquid leaving the condenser will have little or no sub-cooling. This is important to consider when the liquid flows to the low-pressure side. If there is pressure loss in the piping or the components, flash-gas may occur and cause the capacity of the expansion device to be reduced.

If it is a critical charged system, the refrigerant charge must be precisely calculated in order to ensure that there is adequate refrigerant in the system.

An overcharge increases the risk of flooding the

evaporator or the liquid separator, causing liquid carryover into the compressor, as well as liquid hammering. If the system is undercharged, the evaporator will be starved. The size of the low-pressure vessel (liquid separator/evaporator) must be carefully designed so that it can accommodate the refrigerant in all conditions without causing liquid carryover.

For the reasons mentioned above, HP LLRS are especially suitable for systems that require a small refrigerant charge, such as chiller units or small freezers. Liquid chiller units usually do not need receivers, however, if a receiver is necessary in order to install a liquid sensing device and/or provide refrigerant to an oil cooler, the receiver (priority vessel) could be physically small.

Application Example 4.1.1: Mechanical solution for HP liquid level regulation

From condenser

Pressure equalization line

LLG

To oil cooler

SVA

To separator

SNV

SVA

EVM

DA NFO S S

PMFH

SVA

SNV

SVA

Receiver

FIA

SFA

SVA

DSV

Priority for oil

cooler

SVA

SV1

On large HP LLRS, the SV1 or SV3 float valve is used as a pilot valve for a PMFH, main valve. As illustrated above, when the liquid level in the receiver rises

signal to the PMFH main valve to open.The receiver’s function here is to provide a more stable signal for the SV1 float to work with.

above the set level, the SV1 float valve provides a

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 4.1.2: Electronic solution for HP liquid level regulation

SVA

From condenser

SVA

AKS

4100

Pressure equalization line

SFA

SVA

SNV

DSV

SNV

LLG

SNV

To oil cooler

ICM

To separator

ICFS

SVA

ICAD

ICFS

ICFF

ICF

The system illustrated is an AKS 4100/4100U level transmitter which sends a level signal to an EKE 347 liquid level controller. The ICM motor valve acts as an expansion valve.

When designing an electronic LLRS solution, the liquid level signal can be given either by an LLS 4000/AKS 38, which is a level switch (ON/OFF), or an AKS 4100/4100U, which is a level transmitter (4-20 mA).

The electronic signal is sent to an EKE 347 electronic controller which controls the injection valve.

Receiver

Priority for oil

cooler

SVA

EKE 347

The liquid injection can be regulated in several dif-

ferent ways:

With a modulating motor valve type ICM with an

ICAD actuator (as shown in the application example

above).

With a pulse-width-modulating expansion valve type

AKVA. The ICFA (in an ICF valve station)/AKVA valve

should be used only where the pulsation from the

valve is acceptable. Line size should be sized to the

full capacity of the ICFA/AKVA valve.

With a regulating valve REG acting as an expansion

valve and an ICS/EVRA solenoid valve to implement

ON/OFF regulation.

42 AB13778641621700-000702 © Danfoss A/S (RC-MDP/MWA), 2020-10

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 4.1.3: Electronic solution for HP liquid level regulation with small/no HP receiver. Same principle for other evaporative condensers or other condenser types.

From discharge line Cooling water out

Pressure

equalization line

AKS

4100

SVA

EKE 347

Condenser

SNV

In this example, the expansion valve which should be connected to the liquid level controller, EKE 347, is not shown.

The system illustrated is an AKS 4100/4100U level transmitter which sends a level signal to an EKE 347 liquid level controller.

SVA

Cooling water in

PHE

To receiver/

expansion device

The electronic signal is sent to an EKE 347 electronic controller which controls the injection valve.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 4.1.4: Mechanical solution for HP liquid level regulation with HFI

From discharge line Cooling water out

Pressure equalization line

SNV

HFI

To separator

The HFI is a direct-acting high-pressure float valve; therefore, no differential pressure is required to activate the valve.

If the condenser is a plate heat exchanger, the mechanical float valve HFI can be mounted directly on the heat exchanger header plate.

It may be necessary to connect an equalization line to either the HP or LP side to remove refrigerant vapour from the float housing, as this may prevent the

Cooling water in

PHE

Condenser

liquid from entering the float housing and thereby

prevent the HFI valve from opening.

During standstill of the plant system, the pressure

will slowly equalize, allowing the entire refrigerant

charge to transfer to the coldest part of the system.

(During wintertime, this can be the condenser).

If pressure equalization is not desired, a solenoid

valve and/or check valve must be fitted.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

If the HFI is not mounted directly on the PHE/condensers:

To enable any liquid condensate to flow to the HFI by gravity, the HFI must be installed underneath the condenser.

To ensure trouble-free operation, in most cases a bypass orifice must be installed.

The by-pass orifice connects the gas space in the HFI housing with the outlet connection of the HFI.

Due to the pressure difference between the highand low-pressure side, the gas is drawn to the lowpressure side, resulting in a slight underpressure in the housing.

This effect allows gas/vapour to be drawn off.

In addition, this allows the small amount of flash gas that can form in the liquid feed line or during plant standstill to be bled away.

Application Example 4.1.5: Installation example for HFI fitted away from condenser with standard HFI housing:

If the HFI is installed close to the condenser and the pipe design allows vapour/gas to return freely and easily to the condenser, then the bypass orifice might not be needed/need to be in operation.

We generally recommend installing/preparing the bypass orifice.

Guidelines for size of by-pass orifice:

By-pass orifice

(washer with hole) in

this line

SNV

SVA

To LP separator

HP liquid from

condenser

HFI

SVA

REG

For manual operation/

expansion

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 4.1.6: Installation example for HFI fitted away from condenser with HFI housing with two extra connections:

SNV

HP liquid from

By-pass orifice

condenser

(washer with hole) in

this line

SNV

HFI

SVA

For drain of

liquid in float

SVA

REG

SNV

To LP separator

For manual operation/

expansion

46 AB13778641621700-000702 © Danfoss A/S (RC-MDP/MWA), 2020-10

Application Handbook Industrial Refrigeration ammonia and CO2 applications

4.2 Low-pressure liquid level regulation system (LP LLRS)

When designing an LP LLRS, the following points must be taken into consideration:

The liquid level in the low-pressure vessel (liquid separator/ shell-tube evaporator) is maintained at a constant level. This is safe for the system, since an

The HP receiver must be large enough to accumulate the liquid refrigerant coming from the evaporators. When the content of refrigerant in some evaporators varies with the cooling load, some evaporators are shut off for service, or part of the evaporators are drained for defrosting.

excessively high liquid level in the liquid separator may cause liquid carryover to the compressor, and an excessively low level may lead to cavitation of the refrigerant pumps in a pump circulation system.

Often, LLS 4000/AKS 38s are employed as high- and low-level alarms, although when a level transmitter

As a result of the above, LP LLRS are especially suitable for decentralised systems in which there are many evaporators, and there is a large refrigerant charge, e.g. cold stores. With LP LLRS, these systems can run safely even though it is impossible to precisely calculate the refrigerant charge.

is used, the low-level alarm is usually taken from the level transmitter.

Application Example 4.2.1: Mechanical solution for LP liquid level regulation

SVA

Suction line

SVA

SV4

Wet return line

LLG

SVA

SNV

Liquid

separator

SFA

LLS 4000/

AKS 38

DSV

AKS 38

ICS

EVM

SNV

FIA

SVA

From

receiverLLS 4000/

SNV

SVA

Liquid feed line/

pump suction

line

SV float valves “monitor” the liquid level in low-pressure vessels.

If the system capacity is small, the SV4, valves can act directly as an expansion valve in the low-pressure vessel as shown.

SVA

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 4.2.2: Mechanical solution for LP liquid level regulation

SVA

SNV

SFA

Wet return line

DSV

SNV

LLS 4000/

AKS 38

LLS 4000/

AKS 38

LLG

Liquid

separator

SNV

SVA

Liquid feed line/

pump suction

line

If the system capacity is large, the float valve SV4 is used as a pilot valve for the PMFL main valve.

SVA

Suction line

SVA

EVM

DANFOSS

PMFL

SNV

FIA

SVA

SV4

SVA

As illustrated above, when the liquid level in the liq-

uid separator falls below the set level, the float valve

SV4 provides a signal to the PMFL value to open.

From

receiver

48 AB13778641621700-000702 © Danfoss A/S (RC-MDP/MWA), 2020-10

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 4.2.3: Electronic solution for LP liquid level regulation

Wet return line

SVA

SNV

LLG

SNV

Liquid feed line/

pump suction

line

SNV

Liquid

separator

SVA

SFA

DSV

LLS 4000/

AKS 38

SVA

AKS

4100

SVA

Suction line

EKE 347

ICM

ICFS

ICFE

ICFO

ICF

ICFS

From

receiver

ICFF

The level transmitter AKS 4100/4100U monitors the liquid level in the separator and sends a level signal to the liquid level controller EKE 347, which sends a modulating signal to the ICAD actuator of the motor valve ICM on the ICF valve station. The ICM motor valve acts as an expansion valve.

The solenoid valve ICFE on the ICF valve station is being used as an additional solenoid valve to ensure 100% closure during “off” cycles.

The liquid level controller EKE 347 also provides relay outputs for upper and lower limits and for alarm levels. However, it is recommended that a level switch LLS4000/AKS38 is fitted as a mechanical highlevel alarm.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 4.2.4: Electronic solution for LP liquid level regulation

Wet return line

SVA

SNV

LLG

SNV

Liquid feed line/

pump suction

line

SNV

Liquid

separator

SVA

SFA

DSV

LLS 4000/

AKS 38

SVA

AKS

4100

SVA

Suction line

ICFA

EKE 347

ICFS

ICFE

ICFO

ICF

ICFS

From

receiver

ICFF

This solution is similar to the solution shown in Application Example 4.2.3. However, with this example the motor valve ICM is replaced by a pulse-width electronically operated expansion valve ICFA on the ICF valve station. The solenoid valve ICFE, which is also mounted on the ICF valve station, is being used as an additional solenoid valve to ensure 100% closure during “off” cycles.

The line should be sized to the full capacity of the

ICFA valve, or AKVA valve if serially connected.

The liquid level controller EKE 347 also provides re-

lay outputs for upper and lower limits and for alarm

level. However, it is recommended that a level

switch LLS4000/AKS38 be fitted as a mechanical

high-level alarm.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 4.2.5: Electronic solution for LP liquid level regulation

Wet return line

SVA

SNV

LLG

SNV

Liquid feed line/

pump suction

line

SNV

Liquid

separator

SVA

SFA

LLS 4000/

AKS 38

DSV

LLS 4000/

AKS 38

SVA

Suction line

LLS 4000/

AKS 38

ICFR

ICFS

ICFE

ICFO

ICFS

From

receiver

ICFF

ICF

This solution controls the liquid injection using on/off regulation. The level switch LLS 4000/AKS 38 on the right side of the level sensor controls the switching of the solenoid valve ICFE in accordance with the liquid level in the separator. The hand regulating valve ICFR acts as the expansion valve.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

4.3 Summary

Solution Application Benefit Limitations

High-pressure liquid level regulation system (HP LLRS)– see section 4.1

High-pressure mechanical solution: SV1/3 + PMFH

Receiver

Applicable, especially to critical charged systems.

Purely mechanical. Wide capacity range.

High-pressure electronic solution: AKS 4100 + EKE 347 +ICM

High-pressure mechanical solution: HFI

Receiver

PHE

condenser

Applicable, especially to critical charged systems.

Flexible and compact. Possible to monitor and control remotely. Covers wide range of capacity.

Purely mechanical. Simple solution. Especially suitable for plate heat exchangers.

Low-pressure liquid level regulation system (LP LLRS)– see section 4.2

Low-pressure mechanical solution: SV4-6

Low-pressure mechanical solution: SV4-6+PMFL

Low-pressure electronic solution: AKS 4100 + EKE 347 +ICM

Low-pressure electronic solution: AKS 4100 + EKE 347 +AKVA

Low-pressure solution: LLS 4000/AKS 38 +ICFE+ICFR

Liquid

separator

Liquid

separator

Liquid separator

AKVA

Applicable to small systems

Particularly applicable to decentral systems like cold storages.

Purely mechanical. Simple, low cost solution.

Purely mechanical. Wide capacity range.

Flexible and compact. Possible to monitor and control remotely. Covers wide range of capacity.

Flexible and compact. Possible to monitor and control remotely. Covers a wide range of capacities. Easy to install.

Simple. Inexpensive.

Unable to control remotely. The distance between SV and PMFH is limited to several metres. A bit slow in response. Complicated to change setpoint if not correct. Not permitted for flammable refrigerants.

Limited capacity. Complicated to change setpoint if not correct.

Unable to control remotely, the distance between SV and PMFL is limited to several metres. A little bit slow in response. Complicated to change setpoint if not correct. Not permitted for flammable refrigerants.

Not permitted for flammable refrigerants.

Just 40 mm for level adjustment. Very dependent on the adjustment of the REG valve. Not suitable for systems with big capacity fluctuations. Complicated to change setpoint if not correct.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

Evaporator controls

Oil Cooler

Compressor

Evaporator

Expansion valve

The evaporator is the part of the refrigeration system where the effective heat is transferred from the media you want to cool down (e.g. air, brine, or the product directly) to the refrigerant.

Oil Separator

Condenser

Liquid supply controls (Section 5.2): Describes methods for controlling liquid injection for a direct expansion (DX) evaporator and liquid supply for a pumped circulation evaporator.

Therefore, the primary function of an evaporator control system is to achieve the desired media temperature. Furthermore, the control system should also keep the evaporator operating efficiently and trouble-free at all times.

Specifically, the following control methods may be necessary for evaporators:

Temperature controls (Section 5.1): Describes methods for controlling media temperature within tight thresholds with high accuracy, as well as for operating evaporators at different temperature levels.

Defrost Methods (Section 5.6): Briefly describes the different methods for defrosting evaporators.

Hot Gas Defrosting Controls (Section 5.6.3): Describes the steps of the hot gas defrost sequence and general considerations. Hot gas defrosting application examples are covered separately for DX evaporators and pumped circulation evaporators, since they differ, for example, in the draining methods.

Not all valves are shown.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

5.1 Temperature controls

5.1.1 Media temperature control

Solutions are provided for where there are stringent requirements for accurate temperature control in connection with refrigeration, for example:

 Cold room for fruits and food products  Work premises in the food industry  Process cooling of liquids

Application Example 5.1.1: Media temperature control using either pilot-operated valve or motor-operated valve

ICAD

CVP

Option 1

CVE

SVA

To liquid separator

Option 2

ICAD

ICMICS

ICFE

ICFF

REG

ICFS

EVRA

ICF solution

Serial solution

FIA

ICFR

Evaporator

ICFO

ICFS

SVA

Application Example 5.1.1 shows a solution for accurate media temperature control. Furthermore, there is a need to protect the evaporator against an excessively low pressure to prevent the products from freezing up in the application.

This design can be applied for DX or pumped liquid circulation evaporators with any type of defrost system.

From liquid separator

SVA

The servo valve (ICS) is controlled by the two serially

connected pilot valves (CVE) in the S2 port, con-

trolled by a media temperature controller, e.g.

Danfoss EKC 361, and CVP in the S1 port.

The CVP is adjusted according to the lowest pressure

permitted for the application.

The functionality of the pilot-operated servo valve

solution (ICS) is described on the next page.

Not to be used for construction purposes

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

The media temperature controller will control the temperature in the application at the desired level by controlling the opening of the CVE pilot valve, and thereby controlling the evaporating pressure to match the required cooling load and temperature.

The second control option is the motor valve ICM, where a media temperature controller controls the opening degree of the ICM motor valve.

This solution will control the temperature with an accuracy of +/- 0.25°C. If the temperature falls below this range, the EKC controller can close the solenoid valve in the liquid line.

The media temperature controller EKC 361 will control all functions of the evaporator, including thermostat and alarms.

For more details, please refer to the manual of the EKC 361 controller.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

5.1.2 Multi-temperature changeover

In the process industry, it is very common to use an evaporator for different temperature settings.

When the operation of an evaporator is required at two different fixed evaporating pressures, this can be achieved by using one servo valve ICS with two constant pressure pilots.

Application Example 5.1.2: Evaporator pressure control - changeover between two pressure levels

P: CVP

Evaporator

S1: EVM

ICFR

ICFS

ICS

SVA

S2: CVP

ICFE

ICFO

ICFF

REG

ICFS

EVRA

To liquid separator

SVA

ICF solution

From liquid separator

Serial solution

SVA

FIA

Application Example 5.1.2 shows a solution for controlling two evaporating pressures in evaporators. This solution can be used for DX or pumped liquid circulation evaporators with any type of defrost system.

CVP pilot in the S2 port. When the solenoid is de-en-

ergized, the evaporator pressure will follow the set-

ting of the CVP pilot in the P port.

The Danfoss ICS pilot-operated servo valve solution

has options for being controlled by either 1 pilot or 3 The servo valve ICS is equipped with one EVM (NC) solenoid valve pilot in the S1 (serial 1) port and two CVP constant pressure pilots in ports S2 and P respectively.

pilots. The 1-pilot system works by having a pilot

valve to the opening of the servo valve based on the

pilot’s set conditions. The 3-pilot system has two

lines of pilots, one line with 1 pilot (the parallel line)

and another line with 2 pilots serially connected (S1 The CVP in the S2 (serial 2) port is adjusted to the

lower operating pressure and the CVP in the P port is adjusted to the higher operating pressure.

When the solenoid in the S1 port is energized, the

and S2 ports). Having 2 pilots serially connected al-

lows more options to be set for the operation of the

servo valve. The pilot in the parallel line (P port) can

open the servo valve independently of the pilots in

evaporator pressure will follow the setting of the

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I II

Outlet air tempera-

+3°C

+8°C

Evaporating tem-

–

2°C +2°C

Temperature

5K 6K

Refrigerant

R 717

Evaporating pres-

3.0 bar

3.6 bar

Application Handbook Industrial Refrigeration ammonia and CO2 applications

the serial ports, which is shown in the example below.

P port P port

ICS ICS

1 pilot 3 pilots

Example:

ture

perature

change

S1 S2

Serial

Parallel

sure

S2: CVP is preset to 3.0 bar, and

P: CVP is preset to 3.6 bar.

I: EVM pilot opens. The evaporating pressure is therefore controlled by S2: CVP.

II: EVM pilot closes. The evaporating pressure is therefore controlled by P: CVP.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

5.2 Liquid supply control

Liquid supply to the evaporator is controlled differently depending on the system complexity, refrigerant and application.

There are 3 different liquid supplies to the evaporator: Direct expansion (DX), pump circulation and natural circulation.

For direct expansion, high-pressure liquid refrigerant is supplied to the evaporator through an expansion valve. In the evaporator it is fully evaporated and superheated to avoid liquid hammering in the compressor. The liquid supply is controlled by a superheat-controlled expansion valve.

For pump circulation, a liquid refrigerant is pumped to the evaporator from a separator vessel. The refrigerant is partially evaporated in the evaporator and returned to the separator, where the vapour is fed to the compressor.

For natural circulation, a liquid refrigerant is driven

by gravity to the evaporator. Since the refrigerant

flow is driven by gravity, and not a pump, special

consideration must be given to the distance from

separator to evaporator and the height of risers in

the return lines. The refrigerant is partially evapo-

rated, like for the pump-circulated system, and re-

turned to the separator. Controls for natural circula-

tion are similar to those for a pump-circulated sys-

tem, therefore they are covered in the section with

pump-circulated controls.

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Application Handbook Industrial Refrigeration ammonia and CO2 applications

5.2.1 Direct expansion control

When designing liquid supply for direct expansion evaporators, the following requirements should be met:

The liquid injection is controlled by a superheat-controlled expansion valve, which maintains the superheat at the outlet of the evaporator within a desired range. This expansion valve can be either a thermo-

The liquid refrigerant supplied to the evaporator is completely evaporated. This is necessary to protect

static expansion valve, or an electronic expansion valve.

the compressor against liquid hammer.

The media “off” temperature from the evaporator is maintained within the desired range.

The temperature control is normally achieved by ON/OFF control, which starts and stops the liquid supply to the evaporator based on the media temperature.

Application Example 5.2.1: Direct expansion (DX) evaporator with thermostatic expansion

SVA

To suction line

Evaporator

SVA

Application Example 5.2.1 shows a typical installation for a DX evaporator without hot gas defrosting.

The liquid injection is controlled by the thermostatic expansion valve, TEA, which maintains the refrigerant superheat at the outlet of the evaporator at a constant level. TEA is designed for ammonia. Danfoss also supply thermostatic expansion valves for fluorinated refrigerants. For CO2 DX systems, electronic expansion valves should be used, such as an ICM motor valve.

The media temperature should be controlled by a digital thermostat (Danfoss EKC 202), which will control the on/off switching of the solenoid valve, EVRA, according to the temperature signals from the temperature sensors. Danfoss can provide digital thermostats and temperature sensors.

TEA

From receiver

SVA

EVRA

This solution can also be applied to DX evaporators with natural or electric defrost.

Natural defrost is only possible if the cold room temperature is well above 0°C. It is achieved by stopping the refrigerant flow to the evaporator, and keeping the fans running. Electric defrost is achieved by stopping the fans and the refrigerant flow to the evaporator, and at the same time switching on an electric heater inside the evaporator fin block.

FIA

Not all valves are shown.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Not to be used for construction purposes

60 AB13778641621700-000702 © Danfoss A/S (RC-MDP/MWA), 2020-10

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 5.2.2: Direct expansion (DX) evaporator with electronic expansion by a motor valve (ICM)

To suction line

SVA

Evaporator

ICM

ICFS

SVA

ICFE

ICFO

ICFS

ICFF

ICAD

ICM

EVRA

ICF solution

Serial solution

FIA

SVA

From receiver

Application Example 5.2.2 shows a typical installation for an electronically controlled DX evaporator without hot gas defrost.

The ICF control solution is shown in the liquid supply line, but the serial solution could be used instead. The ICF will accommodate up to six different modules assembled in the same housing, offering a compact and easy-to-install control solution.

The liquid injection is controlled by the motor valve, ICM, which is controlled by an evaporator controller, e.g. EKC 315A (not shown). The evaporator controller will measure the superheat by means of a pressure transmitter and a temperature sensor on the outlet of the evaporator. The evaporator controller controls the opening degree of the ICM motor valve

in order to maintain the superheat at the optimum level.

At the same time, the evaporator controller operates as a digital thermostat which controls the on/off switching of the solenoid valve, ICFE/EVRA, depending on the media temperature signal from a temperature sensor on the evaporator.

Compared with the solution in Application Example

5.2.1, this solution will constantly adapt the opening degree of the injection valve to ensure maximum capacity and efficiency. Furthermore, this solution offers more accurate control of media temperature.

This solution can also be applied to DX evaporators with natural or electric defrost.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 5.2.3: Direct expansion (DX) evaporator with electronic expansion by a PWM AKVA valve

To suction line

SVA

ICFS

ICFA

Evaporator

ICFF

ICFS

SVA

AKVA

Application Example 5.2.3 shows an installation for an electronically controlled DX evaporator without hot gas defrost.

This application example shows an ICF control solution for an electronically controlled DX evaporator without hot gas defrost.

The ICF control solution is shown in the liquid supply line, but the serial solution could be used instead. The ICF can accommodate up to six different models in the same housing, offering a compact and easy-toinstall control solution.

The liquid injection is controlled by the pulse width modulation (PWM) electronic expansion valve, ICFA, which is controlled by an evaporator controller, e.g.

ICF solution

From receiver

Serial solution

FIA

EKC 315A (not shown). The evaporator controller will

measure the superheat by means of a pressure

transmitter and a temperature sensor on the outlet

of the evaporator. The evaporator controller con-

trols the opening degree of the ICFA valve in order to

maintain the superheat at the optimum level.

Compared with the solution in Application Example

5.2.1, this solution will constantly adapt the opening

degree of the injection valve to ensure maximum ca-

pacity and efficiency. Furthermore, this solution of-

fers more accurate control of media temperature.

This solution can also be applied to DX evaporators

with natural or electric defrost.

SVA

62 AB13778641621700-000702 © Danfoss A/S (RC-MDP/MWA), 2020-10

Application Handbook Industrial Refrigeration ammonia and CO2 applications

5.2.2 Flooded evaporator circulation control

Traditional industrial refrigeration systems are flooded systems. In a flooded system, the evaporators are injected with more liquid than is needed for full evaporation, which means that the refrigerant is not superheated in the evaporator. A separator vessel is used for supplying liquid refrigerant to the evaporator, to collect the wet return flow and sepa-

Circulation rate

The amount of liquid supplied to the evaporators is defined by the "circulation rate". The circulation rate is 1 when exactly enough liquid is supplied to be fully evaporated in the cooler. If, however, twice as much liquid is injected, the circulation rate is 2. See Table

5.1 below.

rate the phases of the refrigerant. From the separator, the gas phase of the refrigerant is fed to the compressor suction line and the liquid phase is fed to the evaporator supply line.

For flooded evaporators, the heat exchanger area is not used for superheating the refrigerant. In terms of heat transfer, superheating refrigerant is inefficient compared to evaporation, due to a lower heat transfer for superheating compared to evaporation. Therefore, a flooded evaporator can be operated at a higher evaporation temperature than if it was running as DX with superheat. A detailed explanation of heat transfer principles for evaporators can be found in chapter 12.

The benefit of liquid overfeed is increased efficiency of the coolers, due to better utilization of evaporator surface area, and better heat transfer, due to a higher heat transfer coefficient. In addition, flooded systems are relatively easy to control.

Table 5.1: Examples of refrigerant phases’ fractions at different circulation rates

Circulation rate, n Liquid mass flow sup-

plied 1 100% 100% 0% 2 100% 50% 50% 4 100% 25% 75% 8 100% 12.5% 87.5%

Gas mass flow out Liquid mass flow out

Natural circulation flooded evaporators

For natural circulation flooded evaporators, the refrigerant flow is driven by gravity. It is essential to the refrigerant flow that pressure losses are kept as low as possible, thus fewer flow controlling components are typically installed in the supply and return

The refrigerant flow is less controllable compared to a pumped circulation flooded evaporator and a natural circulation evaporator is placed closer to the liquid separator since it is mainly used as a plate heat exchanger for water or glycol chillers.

lines.

Pumped circulation flooded evaporators

When compared to ammonia DX systems, ammonia pump circulation systems control becomes simpler

Application Handbook Industrial Refrigeration ammonia and CO2 applications

as a well-dimensioned pump separator protects compressors against hydraulic shock. The pump separator ensures that only “dry” refrigerant vapour is returned to the compressors. The evaporation control is also simplified as only a basic on/off liquid control to the evaporators is required.

The injected liquid at the correct temperature is pumped from a separator to the evaporators. When liquid is needed, a solenoid valve in front of the evaporator is opened. A manual regulating valve is usually fitted after the solenoid valve to allow the required circulation rate to be set and hydraulic balance to be achieved in the system.

Temperature control in evaporators can be managed as follows:

 Regulating valve for distribution control + ON/OFF so-

lenoid valve for temperature control

 Regulating valve for distribution control + pulse-width

modulated solenoid valve for temperature control

 AKV valves for both distribution control (orifice size)

and PWM temperature control.

64 AB13778641621700-000702 © Danfoss A/S (RC-MDP/MWA), 2020-10

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Figure 5.2.1: Liquid injection valves for pumped circulated systems

To separator

SVA

AKVA

Option 1

From separator

SVA

Option 2

5.3 Injection with a solenoid valve (EVRA)

In a traditional flooded system, liquid injection is controlled by a thermostat which constantly measures the air temperature.

The solenoid valve is opened for several minutes or longer until the air temperature has reached the setpoint. During injection, the mass of the refrigerant flow is constant.

This is a very simple way to control the air temperature. However, the temperature fluctuation caused by the differential of the thermostat may cause unwanted side effects in some applications, e.g. dehumidification and inaccurate control.

SVAREG

EVRA

FIA

5.4 Injection with a pulse width modulation

AKV(A) valve

Instead of injecting periodically, as described above, one can also constantly adapt the liquid injection to the actual demand. This can be done by means of a pulse-width-modulated (PWM) AKV(A) valve type controlled by an AK-CC 450.

The air temperature is constantly measured and compared to the reference temperature. When the air temperature reaches the setpoint, the opening of the AKV(S) is reduced, giving it a smaller opening angle during a cycle, resulting in less capacity and vice versa. The duration of a cycle is adjustable between 30 sec. and 900 sec.

In principle, the regulation in this system is performed with a PI function. This results in reduced fluctuation of the regulated air temperature with stable loads, giving a more constant air humidity. The function offers constant temperature regulation with a temperature value that lies half way between the on and off values of the thermostat.

The operating parameters of the PI regulation are automatically optimized via the preset on and off values and the degree of opening of the valve. The

Application Handbook Industrial Refrigeration ammonia and CO2 applications

differential affects the amplification of the regulator and can therefore not be set to less than 2K to ensure regulation stability. In a flooded system, this means that the average refrigerant flow is constantly controlled and adapted to the demand, with the circulation rate decreasing when less refrigerant is injected.

This approach to liquid injection in a flooded system is very versatile. The amount of injected liquid can be precisely controlled.

A direct effect of this is a lower average surface tem-

perature of the air cooler, resulting in a smaller ΔT

between the refrigerant and the air. This increases the accuracy and the energy efficiency of the system.

Not all valves are shown.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 5.4.1: Pumped liquid circulation evaporator with solenoid valve for liquid injection

To liquid separator

SVA

ICFR

ICFS

ICFE

Evaporator

ICFF

ICFO

ICFS

SVA

REG

Application Example 5.4.1 shows a typical installation for a pumped liquid circulation evaporator without hot gas defrost and can also be applied to pumped liquid circulation evaporators with natural or electric defrost.

The media temperature is maintained at the desired level by a digital thermostat, e.g. EKC 202, which controls the on/off switching of the solenoid valve, EVRA, according to a media temperature signal from a temperature sensor, e.g. AKS 21.

ICF solution

From liquid separator

Serial solution

SVA

EVRA

FIA

The amount of liquid injected into the evaporator is controlled by the opening of the manual regulating valve, ICFR/REG. It is important to set this regulating valve at the right opening degree. Too high an opening degree will lead to frequent operation of the solenoid valve with resultant wear. Too low an opening degree will starve the evaporator of liquid refrigerant.

Not to be used for construction purposes

Not all valves are shown.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 5.4.2: Pumped liquid circulation evaporator with pulsed width modulated (PWM) AKV valve for liquid injection

To liquid separator

SVA

ICFS

Evaporator

ICFA

ICFF

ICFS

SVA

AKVA

Instead of injecting periodically, as described above, one can also constantly adapt the liquid injection to the actual demand. This can be done with a PWM AKVA valve or an ICF with an ICFA solenoid module.

The air temperature is constantly measured and compared to the reference temperature with a digital thermostat, e.g. AK-CC 450. When the air temperature reaches the setpoint, the AKVA valve opening is reduced. This decreases the degree of opening during the cycle, resulting in less capacity. The duration of a cycle is adjustable between 30 sec. and 900 sec.

ICF solution

From liquid separator

Serial solution

FIA

SVA

In a flooded system, this means that the average refrigerant flow is constantly controlled and adapted to demand.

This approach to liquid injection in a flooded system is very versatile. The amount of injected liquid can be precisely controlled, which increases the accuracy and the energy efficiency of the system.

Two-phase flow – a flow that is a mixture of vapour and liquid – presents some special problems with regard to the design of piping. Two-phase flow is usu-

Not to be used for construction purposes

5.5 Risers

from flooded evaporators, which are evaporators where more refrigerant is circulated than is evaporated. The amount evaporated is proportional to the

ally in the return line

Application Handbook Industrial Refrigeration ammonia and CO2 applications

capacity of the heat exchanger and usually the situation is described with a capacity and a ‘circulation rate’.

There are two key considerations for two-phase lines:

The pressure loss: The pressure loss of a two-phase flow is much higher than a single phase (vapour) flow at the same speed, so the speed in two-phase lines is usually kept lower than in vapour-only lines. For low temperature applications, pressure loss presents a large performance penalty, while for thermosyphon (self-circulation) systems, the pressure loss must be small to ensure proper circulation.

Figure 5.5.1: Two-phase flow patterns in horizontal pipe

The flow pattern: The flow pattern describes how the two phases flow, mixed or separately. Depending on the composition (% of each phase) and overall speed, the combined flow behaves very differently, and the pressure loss is a result of the combined behaviour.

The topic of flow pattern is very complex, so only a simple explanation will be given here. Looking at the sketched flow patterns in horizontal pipes in Figure

5.5.1 below, it can generally be seen that from top to bottom and left to right there is an increasing amount of vapour and with that a higher overall velocity.

Note how a higher vapour speed disturbs the liquid through friction on the surface, inducing waves that eventually reach the top of the pipe to enclose gas ‘slugs’. Eventually the speed becomes so high that the friction forces are stronger than the gravitational forces, and the ‘annular’ flow pattern is reached. In the annular flow, the liquid flows in a ‘film’ on the inner surface of the pipe with the high-speed vapour flowing in the centre of the pipe.

In a horizontal pipe, any of these flow patterns are usually not a problem, provided the pressure loss is accounted for. It is common practice to install twophase ‘horizontal’ lines with a small downward slope towards the destination to ensure that the liquid is safely transported to the destination.

In a vertical pipe, however, it is very different. Downward flow presents its own problems, mainly relating

to liquid falling and possibly inducing pressure fluctuations, but transport of the liquid to the destination is almost guaranteed. With upward flow, however, the liquid transport is not guaranteed and that can give rise to problems in refrigeration systems.

In Figure 5.5.2 below, a different flow pattern in upwards flow is shown. The vapour speed generally increases from left to right. With very low velocity, the pipe fills with liquid and the vapour bubbles through it. Increased velocity collects the vapour in larger ‘slugs’ that, with a further increase, start to fill the centre of the pipe. Once the slugs collect to form a channel in the middle, the flow is said to be annular. The remarkable thing about annular flow is that the friction forces on the vapour/liquid surface can drag the liquid on the pipe wall upwards, thereby providing an upward liquid transport.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Figure 5.5.2: Two-phase flow patterns in vertical pipe

Since the amount of vapour is determined by the evaporating capacity, it is not usually an option to increase vapour flow. However, the same vapour flow can yield higher velocities in a smaller pipe and thus the Figure 5.5.2 above can be seen as the same vapour flow with pipe size decreasing left to right.

m/s2 = 0.19 bar. The pressure loss is most likely somewhat lower since, because of the vapour bubbles, the average density is lower than saturated liquid, though not much lower.

Reducing the pipe size increases the speed. Below in

Figure 5.5.3 is a Coolselector2 calculation of a 3m Most technicians would intuitively think that a higher velocity means greater pressure loss, but that is not the case with vertical upward two-phase flow.

DN50 pipe with 100kW evaporator capacity at -10°C

R717, circulation rate 3. It is remarkable that the

speed is relatively high at 14 m/s, but the pressure

loss is only 0.022 bar. The explanation is that the In the ‘bubbly’ flow pattern, the liquid fills up most

of the pipe from top to bottom before it starts to overflow the top. Thus, the pressure loss associated with this flow is the height of the pipe, multiplied by the liquid density and the gravitational constant. In an example with R717 at -10°C in a large pipe with a height of 3 meters, this is 652 kg/m3 * 3 m * 9.81

pressure loss relates to (almost) pure gas running in

a slightly smaller ‘pipe’ (the channel in the centre of

the pipe) and the friction of this pulls up the liquid.

The gravitational pressure loss is no longer the major

part of the pressure loss, but rather the frictional

pressure loss dominates.

Figure 5.5.3: Pressure loss in riser at appropriate circulation rate

Application Handbook Industrial Refrigeration ammonia and CO2 applications

The above 100kW at -10°C R717, circulation rate 3, equates to 833.9 kg/hr. Using the same flow but with a much larger circulation rate simulates a very low capacity and thus a low vapour flow as shown in

Figure 5.5.4. The speed is now 0.48 m/s – 1/28th that of before – but the pressure loss is now 0.14 bar – approximately 6 times higher. This is due to the pipe filling up with liquid.

Figure 5.5.4: Pressure loss in riser at too high circulation rate

An excessively high pressure loss has, as mentioned above, some serious penalties. In low temperature applications, a pressure loss of 0.15 bar could have a significant impact on the efficiency and capacity of the system. For thermosyphon systems, the combined pressure loss in the return pipe and the heat exchanger must be smaller than the driving pressure provided by the liquid column in the inlet of the evaporator. Similar to the pressure loss calculation above, the driving pressure is the product of liquid density, height and gravitation. The vertical part of the return pipe is, more or less, the same as the vertical pipe providing the driving height, so a liquidfilled return pipe provides a pressure loss that is more or less the same as the driving pressure – leaving very little pressure drop available for the evaporator. In effect, that means that there will be no circulation in the system, the evaporator performance will drop and it will not deliver what is expected of it.

The return pipe therefore needs to be designed carefully to avoid too great a pressure drop. The top Coolselector2 calculation in Figure 5.5.3 represents a good solution. Pressure loss is relatively low, and it shows that it is possible to reduce capacity on the heat exchanger without passing into the area where

the return pipe starts to fill up with liquid and provide a greater pressure loss.

The resulting pressure loss should be added to the pressure loss in the evaporator with the pressure loss in the valves and compared to the driving pressure. A good margin is recommended for this.

It is also important to consider the circulation rate at part-load operation of the evaporators.

A properly designed vertical return pipe, utilizing the annular flow, is called a ‘riser’. The Danfoss calculation software Coolselector2 can be used to calculate an appropriate circulation rate for a riser.

Pressure drop in risers can pose challenges when operating an evaporator in part-load conditions. The evaporator is typically dimensioned for a certain circulation rate at full load. A rule of thumb is that a circulation rate of 3 results in reasonable operation of the evaporator and pressure drop. However, it is extremely challenging to control the circulation rate, hence the mass-% of evaporated refrigerant under part-load conditions.

Figure 5.5.5 shows the pressure drop in a riser for a flooded evaporator with ammonia and DN 65 piping.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

The blue lines intersect at 67 kW, which is the reference value condition at full load and a circulation rate of 3. The solid blue line shows the pressure drop at constant mass flow, hence the circulation rate varies with part-load conditions. The broken blue line shows the pressure drop at a constant circulation rate, hence the mass flow varies. It can clearly be seen that the pressure drop in the riser is significantly lower during part-load operation when the

Figure 5.5.5: Pressure drop in riser in part-load operation

circulation rate is kept constant. The solid green line

represents a situation with 1.5 in constant circula-

tion rate under part-load conditions, which yields an

even lower pressure drop. This clearly demonstrates

that being able to control the circulation rate of a

flooded evaporator can result in a better part-load

system performance.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

5.6 Defrost methods

In applications where the evaporator operates at evaporating temperatures below 0°C, frost will form on the heat exchanger surface, with its thickness increasing with time. The frost build-up leads to a drop in evaporator performance by reducing the heat transfer coefficient and blocking air circulation at the same time. These evaporators should therefore be defrosted periodically to keep their performance at the desired level.

The various types of defrost commonly used in industrial refrigeration are:

 Natural defrost  Electric defrost  Hot gas defrost  Natural defrost

Natural defrost is achieved by stopping the refrigerant flow to the evaporator and keeping the fan running. This can only be used for room temperatures well above 0°C. The resulting defrosting time is long.

5.6.1 Electric defrost

Electric defrost is achieved by stopping the fan and the refrigerant flow to the evaporator and, at the same time, switching on an electric heater inside the evaporator fin block. With a timer function and/or a defrost termination thermostat, the defrosting can be terminated when the heat exchange surface is completely free of ice. While this solution is easy to install and low in initial investment, the operating costs (electricity) are considerably higher than for other solutions. Furthermore, there are considerable losses of heat from the electric defrost to the cold room. This adds an additional cooling load.

5.6.2 Hot gas defrost

For hot gas defrost systems, hot gas will be injected into the evaporator from the suction line to defrost the surface. This solution requires more automatic controls than other systems but has the lowest operating cost over time. A positive effect of hot gas injection into the evaporator is the removal and return of oil. To ensure enough hot gas capacity, this solution must only be used in refrigeration systems with three or more evaporators. As a rule of thumb, only a third of the total evaporator capacity can be under defrost at a given time.

5.6.3 Hot gas defrosting controls

The hot gas defrost method is an internal heat source defrost system that is designed in the main refrigeration system. The liquid feed to the evaporators is stopped and discharge gas from the compressors is directed to the evaporators. Hot gas defrosting systems are faster and more energy efficient than their alternatives, natural defrost and electric defrost.

Figure 5.6.1: Hot gas defrost principle diagram

To achieve the expected operation, behaviour and result, hot gas defrost systems must be designed and controlled correctly.

A principle diagram for a hot gas defrosting system is shown below in Figure 5.6.1.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Evaporator

Drip Tray

Wet Return Line

Liquid Drain Line

Liquid Feed Line

Hot Gas Supply Line

Application Handbook Industrial Refrigeration ammonia and CO2 applications

5.6.4 Hot gas defrosting sequence

To perform and operate hot gas defrost systems in a safe and efficient way, it is important that valves are opened and closed in a certain sequence and, most importantly, slowly and with great consideration. This is due to the large pressure difference between the hot gas and the evaporator, and because both gas and liquid phases are present.

A hot gas defrosting sequence is shown schematically below in Figure 5.6.2. The horizontal bars at the top of the figure show whether a valve or fan is open/on or closed/off. When a bar is coloured black, the valve is open, and closed when the bar is grey. The graph at the bottom of the figure shows the pressure in the evaporator during the defrosting sequence. The numbers on the valves in the principle diagram in Figure 5.6.1 correspond to the numbers in the defrost sequence figure below. Each step in the hot gas defrosting sequence is described in detail below the figure.

Figure 5.6.2: Hot gas defrost sequence

Defrost sequence

1 Liquid feed

2 Wet return

3 Hot gas

4 Defros t valve

5 Fan

A B C D E F G H I

Freeze

mode

“Soft opening”

defros t val ve

A: Freezing mode

In this mode, the freezing process is enabled and both the liquid line (valve 1) and wet suction line (valve 2) are in the open position.

The hot gas supply line valves (3) for the individual evaporator, and the condensate drain line valve (4) are all closed.

B: Draining phase

Prior to the actual hot gas injection into the evaporator, as much liquid CO2 as possible must be boiled out. This phase is a must in the defrost sequence because it reduces safety risks.

The purpose of the draining phase is to reduce the following 2 phenomena:

Defros t press ure in evapor ator

Defrosti ng

 Liquid possibly being propelled  Gas pockets imploding

Equal izing pres sure in

evapora tor

Freeze

mode

Both are well-known contributors to liquid hammer. Liquid hammer causes extreme pressure shocks in the system, which in a worst case scenario could lead to fractures in components, pipes etc.

The liquid line is closed (valve 1), preventing CO2 from entering the evaporator. The wet suction line (valve 2) remains open and the evaporator fans are kept running to enable a fast and efficient boil-out of the liquid CO2. The duration of this phase depends on the temperature of the CO2, the volume of the evaporator and the air flow across the evaporator. Typically, it takes a few minutes. Despite the possibility of using other methods to release the liquid

Application Handbook Industrial Refrigeration ammonia and CO2 applications

CO2 in this phase, the method described above is generally accepted to be the safest.

C: Stabilizing phase

The wet suction line closes (Valve 2) and the fans stop. Now all valves around the evaporator are closed and any remaining liquid can collect at the bottom of the evaporator and enable a smooth start to defrost.

D & E: Soft opening phase

Hot gas injection, step 1 and 2. It is generally not recommended to inject hot gas without any control. Remaining liquid in transport lines or evaporators must be prevented from causing liquid hammer. Furthermore, the pressure differences in the evaporator should be considered. This depends on the refrigerant used in the system. The pressure difference in CO2 systems is much higher compared to NH3 or Freon, so this further recommends the need for a controlled way of opening valves.

As an example, a generally accepted defrost temperature of 10°C in CO2 systems is equal to a pressure of 47.23 bar. The pressure of an evaporator at -40°C is 10 bar. The pressure difference is 37.23 bar. A similar situation with NH3 or R 404a only results in a pressure difference of 5.87 bar and 7.33 bar, respectively.

The pressure of the evaporator must be increased slowly. There are several ways to do this with Danfoss valve solutions:

ICSH: Dual position solenoid valve. Step 1 is 20% of the kV value of step 2 and allows a smooth pressure build-up in the evaporator. Steps 1 and 2 are controlled by an EVM pilot valve and, for maximum operating freedom, step 2 can be made dependent or independent of step 1. The steps can be controlled by a PLC where the delay between steps 1 and 2 can be freely set.

ICM: Proportional motor valve type. The valve is controlled by an actuator, ICAD. The ICAD motor is connected to the ICM valve with a magnetic coupling, enabling easy service and maintenance activities without the need to open the ICM valve. The valve is designed to be controlled in 2 ways:

Analogue. The opening degree is controlled, and the

lifting height responds proportionally with an ana-

logue input signal (i.e. 0-20/4-20mA and 0-10/2-10V)

On/Off. The valve responds to a digital input contact

and the opening speed can be set to the requested

demand. The valve is fully closed or fully open or

moving towards one of these positions.

ICS+EVM: An ICS servo valve with an EVM solenoid

valve as pilot valve acting as a solenoid valve step 2

with a small EVRS solenoid as bypass valve acting as

step 1.

F: Defrosting phase

During the actual defrost phase, the main purpose is

to defrost as efficiently as possible. In this phase, the

liquid supply valve (1), wet suction valve (2), and a

condensate drain valve for the main hot gas supply

line (not shown) are closed. The hot gas supply line

valves (3) are open.

At the start of defrosting, the warm hot gas will con-

dense in the evaporator.

During the defrosting cycle, the pressure in the evap-

orator will gradually rise. This phase must be con-

trolled, otherwise large amounts of uncondensed

hot gas (blow-by gas) return to the liquid separator

and must be re-compressed by the compressors, de-

creasing system efficiency.

5.6.5 Controlling the defrost phase:

Pressure-controlled defrost (valve 4)

The pressure control method is the most common

method for controlling the defrost pressure in evap-

orators. The pressure control method is a simple and

reliable method, but it is not the most effective. The

defrost pressure is controlled at a set pressure corre-

sponding to 7-12°C saturated pressure. Valve 4 can

be an ICS main control servo valve with a constant

pressure CVP-H pilot to control the required defrost

pressure. Hot gas will condense in the evaporator

and pressure will gradually increase. When the set-

point is reached, the control valve starts to open and

control the pressure. During this process, the

amount of flash-gas gradually increases, decreasing

the system efficiency.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

To limit efficiency losses, it is recommended to connect the liquid drain line to the liquid separator with the highest temperature. This liquid separator pressure must be lower than the defrosting control pressure. A check valve must be mounted downstream of the ICS valve to avoid liquid streaming back into the evaporators during a cooling cycle.

Liquid drain control (valve 4)

The liquid drain control method is known as an effective defrosting method. However, it is less common, mainly due to the lack of optimum drain valve solutions, complex installation and rather high cost. The defrost is NOT controlled by pressure, but rather by the presence of liquid condensate. Only when liquid condensate is formed will it be drained by a float valve from the bottom of the evaporator. This solution reduces the possible blow-by gas by approximately 95%.

To make this type of defrost commercially and technically more attractive, Danfoss has designed a float valve function (ICFD) incorporated in the ICF valve. This solution offers a cost effective and very efficient drain solution and simplifies the installation compared with traditional float solutions.

G: Defrost end signal – stabilizing phase

In this phase, all valves are closed and any remaining liquid in the evaporator can drip down and collect at the bottom of the evaporator.

H: Pressure equalization of the evaporator

At this point, the evaporator pressure is still at defrost pressure. And this pressure must be gradually equalized to the pressure of the liquid separator. For a CO2 system, suppose the defrost temperature at the end of defrost is 12°C, which equals a saturated pressure of 47.23 bar. The pressure of the liquid separator is -40°C, which equals a saturated pressure of 10 bar. The pressure difference is comparable with the hot gas supply line at the start of defrost, still considerably high, at around 37.25 bar. Soft opening valves are strongly recommended here. Several solutions are possible.

ICLX: A 2-step solenoid valve. Steps 1 and 2 are controlled by EVM pilot valves. When step 1 is activated (kV value is at 10% of step 2), the evaporator pressure slowly decreases. Only when the pressure is decreased enough does the valve fully open (step 2).

ICM proportional motor valves can also be used.

I: Droplet freezing phase

Any water droplets between the evaporator fins can freeze to the evaporator to prevent them being blown into room when the fans start again. When droplets have frozen to the evaporator, the fans are started.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

5.6.6 General hot gas defrosting considerations

Hot gas defrost pressure

A popular misconception is that the higher the defrost temperature, the better. In reality, a number of studies (Stoecker, 1983) indicate that a source of lower pressure and temperature gas could obtain good results as well. There is most likely an optimal pressure / temperature (Hoffenbecker, 2005) that would achieve the highest efficiency.

Hot gas defrost time

In industrial refrigeration, it is very typical to set up defrost based on a fixed time adjusted during the startup of the installation. The problem with this approach is that in many cases this time would be on the “safe side” to ensure a fully clean evaporator. But when the defrost is finished earlier, a consequence is that the efficiency of defrost drops significantly.

Pressure-controlled versus liquid drain control defrost

The liquid drain method is very important for the energy and time spent on defrosting. The liquid drain method determines how much of the energy bound in the hot gas is used for defrosting the evaporator.

Figure 5.6.3: Quality of the drained liquid

Figure 5.6.3 shows the hot gas defrosting process in a log(p)-h diagram. Comparing Figure 5.6.3 to Figure

5.6.1, the process (A) to (B) shows pressure reduction from the compressor’s discharge pressure to the hot gas supply pressure set by the main hot gas supply valve (3) in Figure 5.6.1. The pressure in point (C) is not defined but depends on the components in the hot gas defrost line. Point (D) corresponds to the defrost pressure in the evaporator, however the horizontal position of (D), the dimensioning quality, depends on the liquid drain method. (E) is the evaporator pressure in cooling mode.

Liquid drain controlled defrost: The dimensioning quality of a liquid drain controlled defrost should be 0.0, as the hot gas will be fully condensed to point (D*). The purpose of a float valve in the defrost drain line is to prevent gas passing through.

Pressure-controlled defrost: Initially all the hot gas supplied to the evaporator will condense, and the drain valve will only see liquid at its inlet. When the temperature in the evaporator has been increased, some gas will not be condensed in the evaporator, and a mixture of liquid and gas will be at the valve inlet. This is the process seen from (D*) to (D) in Figure 5.6.3. Selecting the right dimensioning quality for a pressure-controlled drain is important for selecting the right valve size. A dimensioning quality that is too low will result in a smaller valve, which will prolong the defrost time. If the dimensioning quality is set too high, the valve could be too large, resulting in a lot of gas being bypassed, which results in higher energy consumption for the defrost sequence. Consult Coolselector2 when dimensioning your system’s drain valve. Generally, a relatively low dimensioning quality (around 0.05) is recommended when sizing valves for the drain line.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Defrost capacity

The needed amount of hot gas for an efficient defrost depends on the evaporator size, the requested defrost time, and the liquid drain method. As a rule of thumb, the design mass flow (hot gas) for each evaporator requires 2-3 times the required mass flow during the cooling (based on complete evaporation (1:1). The lower the evaporating temperature, the more the ratio goes in the direction of 3. Max. 1/3 of the total number of evaporators in the same installation can be defrosted at the same time (the “two‐to‐one rule”).

When determining the mass flow needed for the defrost sequence, one should be aware of the capacity of the liquid drain valves and the pressure in the hot gas supply line. They create the defrost pressure graph, which is shown in

Figure 5.6.4. On the x-axis, the mass flow of hot gas is shown, which corresponds to the defrost capacity

of the hot gas defrosting system, and on the y-axis the pressure is shown.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Figure 5.6.4: Defrost pressure graph obtained from Coolselector2

Figure 5.6.4 shows 5 different lines. The vertical broken line is the specified mass flow of hot gas to the evaporator. The green horizontal line is the reduced hot gas pressure (B) from Figure 5.6.3, where the solid blue line is the hot gas defrost pressure at the evaporator inlet (C), the red line is the pressure in the defrost drain line/evaporator outlet (D), and the purple line at the bottom is the relieving pressure (E) corresponding to the evaporator pressure in cooling mode.

The green area between the evaporator inlet and the evaporator outlet shows the pressure drop available for the evaporator during defrost, if no components have been added to the end of the hot gas line to simulate the pressure drop in the evaporator.

For the hot gas system to function properly, it is important that the hot gas mass flow is kept within the green area. If the mass flow should go beyond that area, the following steps can be taken:

Increase the reduced pressure – and check it against the condensation pressure in the system

A thorough guide to dimensioning the mass flow hot

gas defrost lines can be found in the Danfoss calcula-

tion tool Coolselector2.

Injecting hot gas

All examples in this document are shown with hot

gas injection into evaporators in the top of the evap-

orator. This method is generally seen as a safe solu-

tion with a very low risk for “liquid hammer”.

Other hot gas injection methods can be used safely,

but generally they will require more detailed docu-

mentations to ensure safe operation.

Liquid hammer

This is the “nickname” given to various phenomena

that result in high-pressure impact in the system.

Two of these are important when designing hot gas

defrost systems:

Pressure impact caused by vapour-propelled liquid in

gas lines where liquid pockets are present. It can typ-

ically occur in hot gas supply and wet return lines.

The design should be such that liquid pockets cannot

occur and that valves open slowly.

Decrease the hot gas mass flow – at the cost of a slower defrost

Decrease the defrost pressure, and thus the defrost temperature – at the cost of a slower defrost

Pressure impact caused by collapsing gas pockets in

liquid lines trapped by moving liquid. The impact can

be reduced by removing as much liquid as possible

from the evaporator prior to defrosting, having a

Application Handbook Industrial Refrigeration ammonia and CO2 applications

smaller pipe design where possible, slow opening valve procedures, and limited hot gas supply pressure.

Draining condensate from main hot gas lines to avoid liquid hammer

When a hot gas line is not operated, the remaining gases easily condense. It is good practice to install the hot gas lines with slopes, and then install drain facilities at the lowest point. This could be a float valve, i.e. ICF with ICFD, or a periodically activated expansion valve to drain the hot gas pipes and thus avoid liquid hammer.

Maximum operating pressure difference (MOPD)

Automatic control valves like solenoid valves or motor valves require a certain force to enable a smooth opening. The required force depends on the design and system parameters. One important system parameter is the pressure difference across the valve. The greater this is, the more force is required. For solenoid valves, this force depends on the given coil power. For motor valves, this is the available motor power. So, for all valves, the MOPD is a known factor. In CO2 systems, the pressure difference can be quite considerable. Especially for hot gas supply lines or wet suction, this must be checked. Danfoss solenoid valves, such as ICS with EVM, have a max. MOPD of 40 bar with a 20W AC coil. The MOPD of ICM/ICAD depends on the type chosen.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

5.6.7 Hot gas defrost for DX evaporators

Application Example 5.6.5: DX evaporator with hot gas defrost system

The Application Example 5.6.5 above is a DX evaporator system with hot gas defrost. The same application is shown on the next page for large-scale systems with the larger sizes of the Danfoss ICF valve station range. The numbering of the lines in this application example follows the numbering of the lines in Figure 5.6.1, hence the lines shown there correspond to the lines shown in the application examples in this chapter. Whilst this method of defrosting is not common, it is even less so for ammonia DX evaporator systems and is more applicable to fluorinated and CO2 systems. The serial solution for components in the hot gas line and liquid supply line is only applicable for ammonia systems.

Refrigeration Cycle

The solenoid valve, ICFE, in the liquid line is kept open. The liquid injection is controlled by the motor valve ICM.

The two-step solenoid valve ICLX/motor valve ICM in

the suction line is kept open, and the defrosting sole-

noid valve ICFE in the hot gas supply line is kept

closed.

The servo valve ICS in the discharge line is kept open

by its solenoid valve pilot EVM.

Defrost Cycle

After initiation of the defrost cycle, the liquid injec-

tion solenoid valve ICFE/EVRA in line (1) is closed.

The fan is kept running for 120 to 600 seconds, de-

pending on the evaporator size, to pump down the

evaporator of liquid.

The fans are stopped and the pilots on the ICLX are

de-energized. The servo piston is kept open by the

hot gas pressure. However, in cooling mode the hot

gas condenses in the cold ICLX valve, so that the

servo piston chamber is filled with liquid at hot gas

pressure. The pressure between the hot gas and the

suction line is equalized by the NO-pilot, when it is

de-energized, on the ICLX and the servo piston

Application Handbook Industrial Refrigeration ammonia and CO2 applications

chamber is slowly drained by the bleed line in the NO-pilot. The equalized pressure between the piston chamber and the suction line lets the main spring push the servo piston down to close the valve.

The exact time taken from when the pilot valves changes position to complete closing of the valve depends on the temperature, pressure, refrigerant and valve size. It is therefore not possible to state an exact closing time for the valves, but lower pressures generally result in longer closing times.

It is very important to take the closing times into consideration when hot gas defrost is used in evaporators.

A further delay of 10 to 20 seconds is required for the liquid in the evaporator to settle down in the bottom without vapour bubbles. The solenoid valve ICFE in the hot gas supply line is then opened and supplies hot gas to the evaporator. As an alternative to the ICFE solenoid valve, an ICSH dual-position solenoid valve can be used to supply hot gas. The benefit of the ICSH dual-position solenoid valve is that it is opened in two steps, which allows a smooth buildup of pressure in the evaporator.

noid valve will have a capacity of just 10% at high differential pressure, allowing the pressure to be equalized before opening fully to ensure smooth operation and avoid liquid slugging in the suction line.

As an alternative to the ICLX two-step solenoid valve, an ICM motor valve with an ICAD actuator could be used for the soft opening of the suction line. This is recommended for CO2 systems for more accurate control.

After the ICLX/ICM fully opens, ICFE in the liquid line is opened to restart the refrigeration cycle. The fan is started after a delay in order to freeze remaining liquid droplets on the surface of the evaporator.

During the defrost cycle, the solenoid valve pilot EVM for the servo valve ICS is closed so that ICS is controlled by the differential pressure pilot CVPP.

The CVPP pilot for the ICS in the discharge line then

creates a differential pressure, Δp, between hot gas

pressure and the receiver pressure. This pressure drop ensures that the liquid which is condensed during defrosting is forced out into the liquid line through a check valve, CHV.

When the temperature in the evaporator (measured by a temperature sensor, e.g. Danfoss AKS 21) reaches the set value, defrost is terminated. The solenoid valve ICFE in the hot gas supply line is closed, the solenoid valve EVM for ICS in the discharge line is opened, and the two-step solenoid valve ICLX is opened.

Because of the high differential pressure between the evaporator and the suction line after defrosting, it is necessary to use a two-step solenoid valve like the Danfoss ICLX solution. The ICLX two-step sole-

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 5.6.6: DX evaporator with hot gas defrost system for large-scale systems

Application Handbook Industrial Refrigeration ammonia and CO2 applications

5.6.8 Hot gas defrost for pumped liquid circulation evaporators

Application Example 5.6.7: Pumped liquid circulation evaporator with hot gas defrost system

Soft-opening

Option 1

Evaporator

Drip Tray

SVA

CHV

NC NO

ICLX

Option 2

ICAD

ICM

ICFR

ICFC

ICFW

ICFE

ICFS

ICFE

ICFF

ICFS

ICFF

Option 1

ICFS

ICFD

SVA

Option 2

CVP

ICFE

ICS

ICFS

To liquid

separator

Liquid drain

From liquid

separator

Hot gas

supply line

line

REG

CHV

Single component solution

SVA

EVRA

Soft opening solution

Single component solution

EVRA

FIA

SVA

FIA

SVA

Application Example 5.6.7 shows the typical installation options for pumped liquid circulation evaporators with hot gas defrost systems. This application example is shown with ICF valve stations which can accommodate up to 6 modules in the same housing. The same application is shown on the next page for large-scale systems with the larger sizes of the Danfoss ICF valve station range. The numbering of the lines in this application example follows the numbering of the lines in Figure 5.6.1, hence the lines shown there correspond to the lines shown in the application examples in this chapter. The application can also be made with components in a serial connection instead, which is only applicable for ammonia systems.

Refrigeration Cycle

The solenoid valve ICFE/EVRA on the liquid line (line (1)) is kept open. The liquid injection is controlled by the manual regulating valve ICFR/REG.

The two-step solenoid valve ICLX in the suction line

is kept open (alternatively, an ICM motor valve can

SVA

ICSH

FIA

SVA

be used instead of an ICLX), and the defrosting solenoid valve ICFE/EVRA is kept closed. It is recommended that an ICM motor valve be used for softopening in the wet suction line for CO2 systems.

Defrost Cycle

After initiation of the defrost cycle, the liquid supply

solenoid ICFE is closed. The fan is kept running for

120 to 600 seconds, depending on the evaporator size, to pump down the evaporator of liquid.

The fans are stopped and the pilots on the ICLX are de-energized. The servo piston is kept open by the hot gas pressure. However, in cooling mode the hot gas condenses in the cold ICLX valve, so that the servo piston chamber is filled with liquid at hot gas pressure. The pressure between the hot gas and the suction line is equalized by the NO-pilot, when it is de-energized, on the ICLX and the servo piston chamber is slowly drained by the bleed line in the NO-pilot. The equalized pressure between the piston chamber and the suction line lets the main spring push the servo piston down to close the valve.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

The exact time taken from when the pilot valves change position to complete closing of the valve depends on the temperature, pressure, refrigerant and valve size. It is therefore not possible to state an exact closing time for the valves, but lower pressures generally result in longer closing times.

It is very important to take the closing times into consideration when hot gas defrost is used in evaporators.

A further delay of 10 to 20 seconds is required for the liquid in the evaporator to settle down in the bottom without vapour bubbles. The solenoid valve ICFE/EVRA in the hot gas supply line is then opened and supplies hot gas to the evaporator. As an alternative to the ICFE solenoid valve, an ICSH dual-position solenoid valve can be used to supply hot gas. The benefit of the ICSH dual-position solenoid valve is that it is opened in 2 steps, which allows a smooth build-up of pressure in the evaporator.

delay to freeze remaining liquid droplets on the sur-

face of the evaporator.

During the defrost cycle, the opening degree of the high-pressure float valve ICFD in the liquid drain line is controlled by the level of condensed hot gas in the ICFD module. The ICFD module only drains liquid to the low-pressure side (wet return line). Uncondensed gas can bypass the ICFD through a small gasbypass orifice. The ICFD solution can reduce the blow-by gas by 90%. The ICFD solution can also be replaced with a back-pressure controlled servo valve, ICS+CVP.

When the temperature in the evaporator (measured by temperature sensor, e.g. Danfoss AKS 21) reaches the set value, defrost is terminated, the solenoid valve ICFE in the hot gas supply line is closed, and the two-step solenoid valve ICLX/ICM motor valve is opened.

Because of the high differential pressure between the evaporator and the suction line, it is necessary to relieve the pressure slowly, allowing the pressure to be equalized before opening fully to ensure smooth operation and avoid liquid slugging in the suction line.

After the ICLX/ICM fully opens, the liquid supply solenoid valve ICFE in the liquid feed line is opened to start the refrigeration cycle. The fan is started after a

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 5.6.8: Pumped liquid circulation evaporator with hot gas defrost system

ICAD

Option 2

SVA

ICM

ICF-65

ICLX

SVA

ICF-65

Soft-opening options

SVA

Option 1

SVA

To liquid

separator

Evaporator

Drip Tray

CHV

SVA

ICFR

ICFE

ICFW

ICFC

ICSH

FIA

ICF-50

Soft-opening

ICFF

ICFS

Option 1

ICFS

ICFD

SVA

ICFE

ICFS

Option 2

CVP

ICS

Liquid drain

From liquid

separator

Hot gas

supply line

line

REG CHV

Single component solution

SVA

EVRA

FIA

Application Handbook Industrial Refrigeration ammonia and CO2 applications

5.6.9 Special application: Defrosting of plate freezers

Hot gas defrosting of plate freezers differs somewhat from defrosting an air cooler. The defrosting of a plate freezer is primarily necessary to remove the frozen product from the freezer and thus the defrosting is part of the production cycle. For this reason, the defrosting of a plate freezer should be as fast as possible. Furthermore, the product is in contact with the plate freezer and a fast and well-con-

ity. However, it should be noted that, as the defrost-

ing capacity of a plate freezer is very high, the ICFD is

not always sufficiently large. Alternatively, an ICM

main valve controlled by an AKS 4100 level transmit-

ter can function as a float valve. It is a more complex

and expensive solution. However, many plate freez-

ers can be serviced by a single ICM with AKS 4100

combination. trolled defrost is essential for product quality. A bad defrost can partially thaw the product, which reduces the quality significantly.

Usually the defrosting should be performed in less than 3 minutes, which is the main difference from air coolers where the defrost time is significantly longer.

The defrosting cycle is slightly different to the de-

frosting cycle for air coolers. In particular, the time

spent to ‘pump down’ the evaporator is simply not

available. Instead, the hot gas is applied (with a soft

opening) and the pressure is used to push the liquid

out of the plate freezer. In this function, the float

valve-controlled defrost is an advantage. The actual As with air coolers, the defrosting of a plate freezer

can be controlled by a float valve or by back pressure, where the float valve control is recommended as it provides defrosting with the lowest possible plate temperature, ensuring the best product qual-

defrosting starts once the freezer has been emptied.

The defrosting is usually ended by a timer, but in

some cases the defrosting pressure in the freezer is

used to determine when the plate is above the

freezing point.

Application Example 5.6.7: Pumped liquid circulation evaporator with hot gas defrost system

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 5.6.9: Pumped plate freezer with hot gas defrost system

Soft-opening

SVA

ICF-50

Soft-opening options

SVA

ICM

ICF-65

ICLX

SVA

ICSH

FIA

Option 1

ICAD

Option 2

SVA

Hot gas

supply line

To liquid

separator

ICF-65

Plate freezers

Liquid drain line

ICFSICFR

ICFE

ICFW

ICFC

ICFF

The solutions shown in the broken-line boxes are options that depend on the cooling capacity of the plate freezer and the refrigerant. In the wet return line (2), ICF-65 with ICM (option 1) is recommended for ammonia and CO2 systems, since the alternative, ICLX, has a long closing time, which must be considered when defrosting a plate freezer. The extended closing time of the ICLX can lead to bypassing hot gas to the wet suction line, which increases the energy consumption for the defrosting sequence. The ICF65 with ICLX (option 2) can be used if the defrosting time of the plate freezer is less critical for the quality of the frozen product in the plate freezer. In the liquid drain line (4), use of the ICFD module (option 1) is limited by the defrost capacity/demand of the plate freezer and the refrigerant. The ICFD module is only available for ammonia systems. Consult Coolselector2 to examine the limits of the ICFD solution.

Option 1

ICFS

ICFE

AKS

4100

Option 2

Gas

Liquid

ICFD

ICFS

ICM

From liquid

separator

Option 2 for the liquid drain line (4) is sending the condensate to a vessel with a liquid level transmitter, AKS 4100. A regulating valve allows gas to bypass to avoid increasing the pressure in the vessel, while a motor valve, ICM, is controlled by the level transmitter to drain liquid from the vessel. This solution would be the alternative for CO2 systems, until the ICFD is approved for CO2.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

5.7 Summary

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Solution Application Benefit Limitations

Temperature Controls – see section 5.1

Media temperature control with ICS, CVE and CVP

Media temperature control with motor valve ICM

Multi-temperature control with ICS and CVP

PCE

CVE

CVP

ICS

ICM

CVP

EVM

CVP

ICS

Very precise temperature control combined with minimum pressure (frost) protection. Options of running at different temperatures

Very precise temperature control. Option of running at different temperatures.

Applicable to all compressors with the ability to run at reduced speed.

The CVE/ICAD will precisely control the temperature. CVP can keep the pressure above the required lowest level.

The ICM will control the temperature very accurately, by adjusting the opening degree.

Low start-up current Energy savings Lower noise Longer lifetime Simplified installation

Pressure drop in suction line.

Maximum capacity is ICM

150.

Pressure drop in suction line.

Liquid Supply Controls – see section 5.2

DX evaporator with thermostatic expansion

All DX systems

control

DX evaporator with electronic expansion

All DX systems

control with ICM/ICF solution

Pumped liquid circulation

Pump circulating systems High capacity and evaporator with solenoid valve or pulse-widthmodulated AKVA valve

Hot Gas Defrosting Controls – see section 5.6.3

DX evaporator with hot gas defrost

Pumped liquid circulation evaporator with hot gas defrost with ICFD (back-pressure control is alternative as

CVPP

EVM

ICLX

EVM

All DX systems

ICS

EVM

ICLX

CVP

ICFD

All pumped circulated systems

ICS

shown in the broken-line box)

Simple installation without separator and pump system.

Optimized superheat, quick response, possible to control remotely, wide capacity range.

efficient evaporator

Quick defrost. The hot gas can bring out the oil left in the lowtemperature evaporator.

Quick defrost. The hot gas can bring out the oil left in the lowtemperature evaporator. The float valve is efficient and stable in regulating the hot gas flow. Blow-by gas is reduced.

Lower capacity and efficiency than circulated systems. Not suitable for flammable refrigerants.

Not suitable for flammable refrigerants.

Fluctuations, and high refrigerant charge. Fewer fluctuations when using the AKVA solution.

Not applicable for systems with fewer than 3 evaporators.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Oil systems

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Oil Cooler

Oil Sepa rator

Compresso r

Evaporat or

Expansion v alve

Generally, industrial refrigeration compressors are lubricated with oil, which is forced by the oil pump or by the pressure difference between the high and the low-pressure sides to the moving parts of the compressors (bearings, rotors, cylinder walls etc.). To guarantee reliable and efficient operation of the compressor, the following oil parameters should be controlled:

Oil temperature. This should be kept within the limits specified by the manufacturer to avoid degradation of the oil. The oil should have the correct viscosity and the discharge temperature from the compressor should be kept below the temperatures at which the oil starts to decompose.

Oil pressure. Oil pressure difference should be kept above the minimum acceptable level specified by the compressor manufacturer.

Condenser

There are generally some supporting components and equipment within refrigeration systems for oil cleaning, oil separation from the refrigerant, oil return from the low-pressure side, equalization of the oil level in systems with several piston compressors and oil drain-off points. Most of these are supplied by the compressor manufacturer.

The oil system design of an industrial refrigeration plant depends on the type of compressor (screw or piston) and on the refrigerant (ammonia, HFC or CO2). An immiscible oil type is generally used for ammonia, and a miscible one for fluorinated refrigerants and CO2. As oil systems are very closely related to compressors, some of the above-mentioned points have been described in Compressor controls (section 2) and Safety systems (section 7).

6.1 Oil cooling controls

Refrigeration compressors (including all screw compressors and some piston compressors) generally re-

also necessary to control it. The oil temperature is

usually specified by the compressor manufacturer. quire oil cooling. Discharge temperatures that are too high can destroy oil, which leads to damage of the compressor. It is also important for the oil to have the right viscosity, which is heavily dependent on the temperature level. It is not enough just to keep the temperature below the critical limit, it is

There are a few different types of oil cooling systems

used in refrigeration. The most common types are:

 water cooling  air cooling  refrigerant cooling

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Oil can also be cooled by means of injection of the liquid refrigerant directly into the intermediate compressor port. For piston compressors, it is quite common not to have any special oil cooling systems at

all, as temperature is less critical than for screw compressors, with the oil being cooled in the crankcase of the piston compressor.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 6.1.1: Oil cooling with water or brine

Cold oil

SVA WVTS

Cold oil

SVA

Hot oil

SVA

Oil cooler

Hot oil

ORV

SVA

Oil cooler

SNV

Cooling water out

Cooling water in

Cooling water out

Cooling water in

These types of systems are normally used in plants where it is possible to get a cheap water source. Otherwise, it is necessary to install a cooling tower to cool down the water. Water-cooled oil coolers are quite common for marine refrigeration plants.

The water flow is controlled by the water valve type WVTS, which controls the water flow according to the oil temperature.

An alternative is to equip the hot oil line with a three-way oil regulating valve, ORV. The ORV threeway valve can be used as a diverting valve or a mixing valve. The ORV is used to bypass hot oil from the separator to adjust the oil return temperature, without regulating the water flow.

The SNV safety relief valves are mounted with blanking plugs on their outlets and must always remain so.

Please contact your local Danfoss sales company to

check the suitability of components to be used with

sea water as the cooling medium.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 6.1.2: Oil cooling with air

Hot oil

ORV

Cold oil

FIA

Oil cooler

It is quite common to use air-cooled oil coolers on the compressor units with semi-hermetic screw compressor refrigeration packs. Typical oil return temperatures are in the range of 50-60°C – well above ambient temperatures in most parts of the world.

The ORV is used as a diverting three-way valve to bypass hot oil (uncooled oil from separator) to compensate for low ambient temperatures that could result in excessively low oil return temperatures.

The oil temperature valve is controlled by the oil regulating valve ORV.

In this case, the ORV divides the flow from the oil separator and regulates based on the change in oil return temperature.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

Application Example 6.1.3: Refrigerant oil cooling

From liquid

separator

SVA

Sight glass

FIA

SCA

RT 1A

RT 5A

Oil separator

Compressor

ORV

Oil cooler

SVA

Overfeed riser

Sight glass

These types of systems are very convenient, as oil is cooled inside the system. It is only necessary to oversize the condenser for the amount of heat taken from the oil cooler. Conversely, Refrigerant oil cooling requires additional piping on site and it is sometimes also necessary to install an additional priority vessel (in cases where the HP liquid receiver is placed too low or not installed). In the case above, the feed pipe to the liquid separator is placed at a certain height above the bottom of the receiver vessel to create a priority for the oil cooler that is drained from the bottom of the receiver.

SVA

Condenser

SVA

LLG

SVA

REG

SVA

SNV DSV

Receiver

QDV

SFA

To liquid separator

SVA

The oil temperature is maintained at the correct

level by the oil regulating valve (ORV), a three-way

valve. The ORV keeps the oil temperature within the

limits defined by its thermostatic element. If the oil

temperature rises too high, then all the oil will re-

turn to the oil cooler. If it is too low, then all the oil

flow is bypassed around the oil cooler.

High-pressure liquid refrigerant flows from the receiver due to gravity into the oil cooler, where it evaporates and cools the oil. It is therefore important to dimension the overfeed risers properly according to the refrigerant flow needed for the oil cooler and pressure losses in the risers. Otherwise the refrigerant will not return from the oil cooler and the system will not function correctly. Only a minimal number of SVA stop valves should be installed. No pressure-dependent solenoid valves are permitted. On the return pipe, installation of a sight glass is recommended.

Application Handbook Industrial Refrigeration ammonia and CO2 applications

6.2 Oil differential pressure controls

During normal running of the refrigeration compressor, oil is circulated by the oil pump and/or pressure difference between the HP and LP sides. The most critical phase is during start-up.

It is vital to have a quick build-up of oil pressure, otherwise the compressor may be damaged.

There are two basic ways to quickly build up oil dif-

The first is to use an external oil pump, and the second is to install a control valve on the compressor discharge line after the oil separator.

For the latter method, it is necessary to check if the compressor manufacturer allows a few seconds of dry operation. Normally, this is possible for screw compressors with ball bearings, but not possible for those with slide bearings

ferential pressure in the refrigeration compressor.

Application Example 6.2.1: Oil differential pressure control with ICS and CVPP

CVPP

SCA

RT 1A

RT 5A

From liquid

separator

Oil separator

SVA

To condenser

ICS

Compressor

From oil cooler

To oil cooler

In this application, a servo-operated valve, ICS, complete with differential pilot, CVPP, should be used. The pilot line from the CVPP valve is connected to the suction line. ICS is closed when the compressor is started up.

As the piping between the compressor and the valve is very short, the discharge pressure increases rapidly. It requires very little time before the valve fully opens and the compressor runs at normal conditions.

The main advantage of this solution is its flexibility, as differential pressure can be readjusted on site and, using other pilots, the ICS can also serve some other functions.

In areas with a very low ambient temperature, an excessively high setpoint of the differential pressure pilot valve (CVPP) will result in no benefit from the lower condensation temperature, since the compressor will need to provide the required discharge pressure to open the pilot valve.

Danfoss Industrial Refrigeration Ammonia and CO2 Applications Application guide [ar]

Specifications and Main Features

Frequently Asked Questions

User Manual

Introduction

1.1 Refrigerants

1.1.1 Ammonia - R717

1.1.3 Halocarbons – CFCs, HCFCs and HFCs

1.1.4 Hydrocarbons – HCs

1.2 Refrigerant feed to the evaporator

1.2.1 Direct expansion system (DX)

1.2.2 Circulated system (flooded system)

1.3 One-stage systems

1.4 Two-stage/multistage systems

1.5 Cascade systems

1.6 Transcritical systems

Compressor controls

2.1 Reverse flow control

2.2 Suction pressure control

2.3 Compressor capacity control

2.3.1 Step control

2.3.2 Slide valve control

2.3.3 Variable speed control

2.3.4 Hot gas bypass

2.4 Discharge Temperature Control with Liquid Injection

2.5 Economizer damper

2.6 Summary

Condenser controls

3.1 High-pressure float valve operation

3.2 Low-pressure float valve operation

3.3 Air-cooled condensers

3.3.1 Step control of air-cooled condensers

3.3.2 Fan speed control of air-cooled condensers

3.3.3 Area control of air-cooled condenser

3.4 Evaporative condensers

3.4.1 Control of evaporative condensers

3.5 Water-cooled condensers

3.6 Summary

Liquid level regulation

4.1 High-pressure liquid level regulation system (HP LLRS)

4.2 Low-pressure liquid level regulation system (LP LLRS)

4.3 Summary

Evaporator controls

5.1 Temperature controls

5.1.1 Media temperature control

5.1.2 Multi-temperature changeover

5.2 Liquid supply control

5.2.1 Direct expansion control

5.2.2 Flooded evaporator circulation control

Circulation rate

Natural circulation flooded evaporators

Pumped circulation flooded evaporators

5.3 Injection with a solenoid valve (EVRA)

5.5 Risers

5.6 Defrost methods

5.6.1 Electric defrost

5.6.2 Hot gas defrost

5.6.3 Hot gas defrosting controls

5.6.4 Hot gas defrosting sequence

5.6.5 Controlling the defrost phase:

Pressure-controlled defrost (valve 4)

Liquid drain control (valve 4)

5.6.6 General hot gas defrosting considerations

Hot gas defrost pressure

Hot gas defrost time

Defrost capacity

Injecting hot gas

Liquid hammer

Maximum operating pressure difference (MOPD)

5.6.7 Hot gas defrost for DX evaporators

5.6.8 Hot gas defrost for pumped liquid circulation evaporators

5.6.9 Special application: Defrosting of plate freezers

5.7 Summary

Oil systems

6.1 Oil cooling controls

6.2 Oil differential pressure controls