Danfoss Industrial Refrigeration Ammonia and CO2 Applications Application guide [ar]
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Application Handbook
Industrial Refrigeration
Ammonia and CO2 Applications
© Danfoss A/S (RC-MDP/MWA), 2020-10 AB13778641621700-000702 1
Application Handbook Industrial Refrigeration ammonia and CO2 applications
2 AB13778641621700-000702 © Danfoss A/S (RC-MDP/MWA), 2020-10
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
© Danfoss A/S (RC-MDP/MWA), 2020-10 AB13778641621700-000702 1
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
© Danfoss A/S (RC-MDP/MWA), 2020-10 AB13778641621700-000702 3
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
2
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
2
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.
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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.
TC
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
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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
4
Expansion valve
3
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
1
2
Evaporator
8
Expansion valve
7
3
HP Compressor
Inter-stage coole r
6
5
Oil Cooler
Oil Separator
4
Condenser
Economizer
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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
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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
1
2
Evaporator
4
Expansi on valve
3
5
HP Compressor
Cascade cooler
8
7
Oil Separator
6
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.
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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
3
4
5
Receiver
Gas cooler
2
6
1
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 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
3
4
5
Receiver
6
Gas cooler
2
1
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
5
Receiver
Gas cooler
2
6
1
TC
7
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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.
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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.
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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
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
22 AB13778641621700-000702 © Danfoss A/S (RC-MDP/MWA), 2020-10
SVA
M
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
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
1
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.
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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
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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
PC
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
M
PC
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
TC
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
TC
M
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.
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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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