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

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

Introduction

Compressor controls

Condenser controls

Introduction

Compressor controls

Condenser controls

1.1 Refrigerants

1.2 Refrigerant feed to the evaporator

1.3 One-stage systems

1.4 Two-stage/multistage systems

1.5 Cascade systems

1.6 Transcritical systems

2.1 Reverse flow control

2.2 Suction pressure control

2.3 Compressor capacity control

2.4 Discharge Temperature Control with Liquid Injection

2.5 Economizer damper

3.1 High-pressure float valve operation

1.1 Refrigerants

1.2 Refrigerant feed to the evaporator

1.3 One-stage systems

1.4 Two-stage/multistage systems

1.5 Cascade systems

1.6 Transcritical systems

2.1 Reverse flow control

2.2 Suction pressure control

2.3 Compressor capacity control

2.4 Discharge Temperature Control with Liquid Injection

2.5 Economizer damper

2.6 Summary

3.1 High-pressure float valve operation

1.1.1 Ammonia - R717

1.1.2 Carbon dioxide (CO2) – R744

1.1.3 Halocarbons – CFCs, HCFCs and HFCs

1.1.4 Hydrocarbons – HCs

1.2.1 Direct expansion system (DX)

1.2.2 Circulated system (flooded system)

2.3.1 Step control

2.3.2 Slide valve control

2.3.3 Variable speed control

2.3.4 Hot gas bypass

1.1.1 Ammonia - R717

1.1.2 Carbon dioxide (CO2) – R744

1.1.3 Halocarbons – CFCs, HCFCs and HFCs

1.1.4 Hydrocarbons – HCs

1.2.1 Direct expansion system (DX)

1.2.2 Circulated system (flooded system)

2.3.1 Step control

2.3.2 Slide valve control

2.3.3 Variable speed control

2.3.4 Hot gas bypass

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

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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.

AB13778641621700-000702

Application Handbook	Industrial Refrigeration ammonia and CO2 applications

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

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

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

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.

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

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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)) 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.

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:

Ammonia – R717

Carbon dioxide (CO2) – R744

Halocarbons – CFCs, HCFCs and HFCs

Hydrocarbons – HCs

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

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.

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

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.

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.

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.

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.

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Application Handbook

Industrial Refrigeration ammonia and CO2 applications

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.

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 being fed to the compressor.

An expansion valve is controlled by the superheat. This is either done by a thermostatic expansion valve or an electronic expansion valve.

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.

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).

Figure 1.2: Direct expansion evaporator

DX Evaporator	To Compressor
	To Compressor
TC	From Receiver

NOTE: A thermostatic expansion valve can only keep a constant superheat, rather than a constant evaporating temperature.

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Application Handbook	Industrial Refrigeration ammonia and CO2 applications

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

A circulation system is therefore more efficient than a corresponding DX system.

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 evaporator driven by gravity.

Figure 1.3: Pumped flooded evaporator

To Compressor

From Receiver

Flooded

Evaporator

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Application Handbook

Industrial Refrigeration ammonia and CO2 applications

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

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.

Figure 1.4: System diagram for a one-stage system

	Oil Cooler
1		Oil Separator	2
	Compressor
Evaporator			Condenser
			Condenser
4	Expansion valve		3

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Application Handbook

Industrial Refrigeration ammonia and CO2 applications

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 operating conditions. Additionally, the desire for another temperature level or efficiency considerations may result in a two-stage system being chosen over a single 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 operation 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 bypassing and flashing some refrigerant. The bypassed refrigerant is evaporated and fed to an economiser port in the high-pressure compressor which can increase the capacity and/or the efficiency of the system. The LP compressor may also have an economiser, but it is not shown here.

Figure 1.5: System diagram for two-stage system

	Oil Cooler
LP Compressor
1	3	Oil Separator
	2
	HP Compressor	4
		4
Evaporator	Inter-stage cooler	Condenser
	Inter-stage cooler	Condenser
	6
8	5
Expansion valve	7	Economizer
© Danfoss A/S (RC-MDP/MWA), 2020-10	AB13778641621700-000702	9

	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- ter-stage cooler helps to keep a low discharge gas

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).

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

		(4)
(5)		(4)
(5)

(7)	(2)
(6)	(3)

(8)	(1)

AB13778641621700-000702

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 the compressor without affecting the suction flow.

This is utilised with the economiser, which is a heat exchanger that further cools the liquid from the condenser. 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:

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-super- heating 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 at (6). The evaporative capacity is used to sub-cool

the remaining liquid from (7) to (9). It is noteworthy that the liquid flashed to the intermediate temperature is further to the left than it would have been without the economizer, thus the enthalpy difference from (10) to (3) is bigger and that increases the compressor’s cooling capacity. The power consumption is increased as well, because there is an additional mass flow that needs to be compressed, but the overall effect – depending on the operating conditions – is a higher cooling capacity and, usually, a higher COP.

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Application Handbook

Industrial Refrigeration ammonia and CO2 applications

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-tempera- ture 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.

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.

Figure 1.8: Cascade system diagram

			Oil Cooler
	LP Compressor
	1		Oil Separator
	2	5
			HP Compressor
			6
	Evaporator	Cascade cooler	Condenser
		Cascade cooler	Condenser
4		8
4	Expansion valve	3	Economizer
	Expansion valve	3	Economizer
			7

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Application Handbook

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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.

AB13778641621700-000702

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

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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 liquid part is separated, the liquid part is flashed to

the evaporators as necessary, while the gas part is reduced in pressure and fed to the compressor’s suction line, thereby controlling the pressure in the receiver.

Figure 1.11: Transcritical CO2 system with intermediate pressure receiver

Gas cooler

Receiver

TC 7

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

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Application Handbook	Industrial Refrigeration ammonia and CO2 applications

This process can be optimised. The gas from the re-		compressor to compress this gas to gas cooler pres-
ceiver is reduced in pressure to the compressor suc-		sure with a higher efficiency due to the higher suc-
tion, however it is possible to take advantage of the		tion pressure. This is called parallel compression.

higher pressure in the receiver to employ another

Figure 1.13: Transcritical CO2 system with parallel compression

Gas cooler

Receiver

TC 7

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

	3
4	2
	Gas cooler
5
Receiver
6
	1
TC	7

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Application Handbook

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	Oil Cooler
	Oil Separator
	Compressor
Evaporator	Condenser
	Condenser
	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.

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.

If the compressor capacity is greater than the demand, 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 envelope for the mechanical safety of the compressor and operational safety of the entire system.

AB13778641621700-000702

Application Handbook	Industrial Refrigeration ammonia and CO2 applications

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

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.

	RT 1A	RT 5A	SCA
			To condenser
		Compressor
From evaporator/	SVA		Oil
From evaporator/			Separator
liquid separator			Separator
liquid separator
	FIA
		ICFS
		ICFE
		ICFF
		ICFO
		ICF

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.

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.

AB13778641621700-000702

Application Handbook	Industrial Refrigeration ammonia and CO2 applications

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

piston reciprocating compressors, or bypass some suction gas for screw compressors with slide valves, etc.

Control the suction pressure for reciprocating compressors. By installing a back pressure-controlled regulating valve in the suction line, which will not open until the pressure in the suction line drops below the set value, suction pressure can be kept under a certain level.

			SCA
	CVC		To condenser
			Compressor
From evaporator/		SVA	Oil
liquid separator	ICS	SVA	Separator
			FIA
			ICFS
			ICFE
			ICFF
			ICFO
			ICF

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

ICS will not open until the downstream suction pressure 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 compressor motor.

AB13778641621700-000702

Application Handbook	Industrial Refrigeration ammonia and CO2 applications

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:

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.

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.

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.

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.

AB13778641621700-000702

Application Handbook	Industrial Refrigeration ammonia and CO2 applications

Application example 2.3.1: Step control of compressor capacity

Compressor

pack

	SCA
EKC 331	SCA

To condenser

AKS 33
		FIA
From evaporator/	SVA	Oil
From evaporator/		Separator
liquid separator		Separator
liquid separator
		SCA
		ICFS
		ICFE
		ICFF
		ICFO
		ICF

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

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/	SVA
	liquid separator	SVA
	liquid separator

SVA	To oil separator

Compressor

AB13778641621700-000702

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

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

<![if ! IE]>

<![endif]>SVA

CVC		SCA	To condenser
CVC			To condenser
		Compressor
	SVA	Oil
ICS	SVA	Separator
	FIA	ICFS
		ICFE
		CVC

	EVM	ICFF
		ICFF
		ICFO
	1	ICF
	ICS	SVA
	SVA
Evaporator	ICFS
	ICFS
	ICFE
		From receiver
	SVA TEA
	ICFF
	ICFO
	ICF

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.

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.

AB13778641621700-000702

Application Handbook	Industrial Refrigeration ammonia and CO2 applications

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.

Referring to the log p-h diagram in chapter 1, it can be seen that the discharge temperature may be high 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.

There are several ways to reduce the discharge temperature. 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 between compressors, or the side port of the screw compressor.

Application example 2.4.1: Liquid injection with thermostatic injection valve

RT 107

Screw compressor

To oil

Separator

From evaporator/ SVA

liquid separator

FIA

ICFS

Oil injection

SVA TEAT

ICFE

ICFF ICFO

From receiver

ICF

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

thermostatic injection valve (TEAT) controls the injected liquid flow according to the discharge temperature, which prevents the discharge temperature from rising further.

AB13778641621700-000702

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/	SVA		AKS 21
liquid separator			AKS 21

			EKC 361
		FIA	EKC 361
		FIA
			ICAD

	ICFS		ICAD	Single component solution
			ICAD	Single component solution
ICFE					SVA
Oil injection					SVA
ICM
		SVA	ICM	EVRA	FIA
			ICM

ICFO	ICFF
ICF		From
ICFS		receiver
		receiver

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.

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.

AB13778641621700-000702

Application Handbook	Industrial Refrigeration ammonia and CO2 applications

An economizer is used to increase system capacity and efficiency.

However, pulsations from the compressor are transferred 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 need to be dampened to avoid component damage. Correct and well considered dimensioning of economizer lines is necessary to reduce pulsations. The Danfoss Eco-Damper combines three components into a high-impact solution which reduces pulsations significantly and ensures high operational 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

CVP EVM

Screw compressor

Max 150 mm ICD Max 150 mm ICC Max 150 mm ICS

Oil injection

From economizer vessel

From evaporator/ SVA

liquid separator

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 oscillating movements. The ICC Eco-check valve is applicable for ammonia, CO2 and HFC systems.

For less severe or occasional pulsation issues, the robust 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.

AB13778641621700-000702

Reverse Flow Control – see section 0

Reverse flow control with	Applicable to all	Simple.	Causes constant pressure
SCA	refrigeration plants.	Easy to install.	drop in the discharge
		Low flow resistance.	line.

Crankcase Pressure Control – see section 2.2

Crankcase pressure control with ICS and CVC

Applicable to	Simple and reliable.	Causes constant pressure
reciprocating	Effective in protecting	drop in the suction line.
compressors, normally	reciprocating
used for small and	compressors on start-up
medium systems.	or after hot gas defrost.

Compressor Capacity Control – see section 2.3

Step control of			Applicable to multi-	Simple.	The control is not
compressor capacity with			cylinder compressor,	Almost as efficient at part	continuous, especially
EKC 331 and AKS 33			screw compressor with	load as at full load.	when there are only a few
			multiple suction ports,		steps. Fluctuations in the
			and systems with several		suction pressure.
			compressors running in
			parallel.
Compressor variable		M	Applicable to all	Low start-up current	Compressor must be
speed capacity control	Controller	M	compressors with the	Energy savings	suited for reduced speed
			ability to run at reduced	Lower noise	operation.
			speed.	Longer lifetime
				Simplified installation

Hot gas bypass using ICS	Applicable to	Fast response to sudden	Should not be used as
and CVC	compressors with fixed	drops in cooling load. The	primary compressor
PC	capacities. Used as safety	hot gas can help the oil	capacity control. Energy
	measure against	return from the	inefficient.
	excessively low suction	evaporator.
	pressure

Discharge Temperature Control with Liquid Injection – see section 0

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

Simple and effective.

Injection of liquid

where the discharge

refrigerant may be

TSHL

temperatures may run

dangerous to the

too high.

compressor. Not as

efficient as intermediate

cooler.

Applicable to systems

Flexible and compact.

Not applicable to

where the discharge

Possible to monitor and

flammable refrigerants.

temperatures may run

control remotely.

Injection of liquid

too high.

refrigerant may be

dangerous to the

compressor. Not as

efficient as intermediate

cooler.

AB13778641621700-000702

Application Handbook

Industrial Refrigeration ammonia and CO2 applications

	Oil Cooler
	Oil Separator
	Compressor
Evaporator	Condenser
	Condenser
	Expansion 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

Evaporative condensers

Water cooled condensers

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 specifically the use of a float valve. Both operation modes can be achieved by using level switches/transmitters and a normal expansion valve.

High-pressure float valve operation is expansion of the liquid immediately after the condenser. Any variation in the charge volume due to variations in capacity must be handled at the low-pressure side, e.g. in a liquid separator. Since the flow after the expansion valve is two-phase, it is not suitable for distribution to more than one location and thus it is primarily used in systems with only one low-pressure separator, such as a chiller unit.

High-pressure float valves are usually mounted immediately after the condenser and, as such, pose no special problems with regard to condenser installation.

AB13778641621700-000702

+ 226 hidden pages

Application Handbook

Industrial Refrigeration ammonia and CO2 applications

2.6 Summary

Application

Limitations