c02 application

8/3/2019 C02 Application

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CO2Refrigerant for Industrial Refrigeration

Article

Refrigeration and Air Conditioning Controls

R E F R I G E R A T I O N A N D A I R C O N D I T I O N I N G


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2 RZ0ZR102 Danfoss A/S (RC-CM / MWA), 12- 2002

Article CO2 - Refrigerant for Industrial Refrigeration

Contents Page

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

CO2

as a refrigerant ..................................................................................................................... 4

CO2 as a refrigerant in industrial systems ................................................................................... 6Design pressure .......................................................................................................................... 7

Efficiency ..................................................................................................................................... 8

Oil in CO2

systems ....................................................................................................................... 8

Component size .......................................................................................................................... 8

Refrigerant charge in CO2-NH

3cascade systems ...................................................................... 9

Material compatibility ................................................................................................................. 10

Water in CO2

systems ................................................................................................................ 10

Leaks in CO2-NH

3cascade systems ......................................................................................... 12

Safety valves in CO2 systems .................................................................................................... 12Safety ......................................................................................................................................... 13

Conclusion ................................................................................................................................ 13

Danfoss valves and controls for CO2......................................................................................... 14


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Danfoss A/S (RC-CM / MWA), 12 - 2002 RZ0ZR102 3


The application of carbon dioxide (CO2)

refrigeration systems is not new. Carbondioxide was first proposed as a refrigerant byAlexander Twining [1], who mentioned it in hisBritish patent in 1850. Thaddeus S.C. Loweexperimented with CO2 military balloons, buthe also designed an ice machine with CO2 in1867. Lowe also developed a machineonboard a ship for transportation of frozenmeat.

From reading the literature it can be seen thatCO2 refrigerant systems were developedduring the following years and they were attheir peak in the 1920's and early 1930's. CO2was generally the preferred choice for use on

board ships whilst ammonia (NH3 or R717)was more common in land applications [2].With the advent of the "Freon" refrigerantsinitially R12, the application of CO2 lessened.The main reason for its decline was certainlythe rapid loss of capacity and need forpressure increase at high temperatures.

Ammonia has continued to be the dominantrefrigerant for industrial refrigerationapplications over the years.

Introduction

In the 1990's there was renewed focus of the

advantages offered by using CO2, due toODP (Ozone Depletion Potential) and GWP(Global Warming Potential), which hasrestricted the use of CFC's and HFC's andrestrictions on the refrigerant charge in largeammonia systems.

CO2 belongs to the so-called "Natural"refrigerants, together with e.g. ammonia,hydrocarbons such as propane and butane,and water. All of these refrigerants have theirrespective disadvantages: Ammonia is toxic,hydrocarbons are flammable, and water haslimited application possibilities. Incomparison, CO2 is non-toxic and non-

flammable, but has a double role in theenvironment. CO2 is necessary for all life onearth, but is also a green house gas, whichcan change the environment if itsconcentration in the atmosphere changes.

[1] Bondinus, William S ASHRAE Journal April 1999

[2] Lorentzen, Gustav Reprint from IIR Conference 1994 Proceedings "New Applications of Natural

Working Fluids in Refrigeration and Air Conditioning"


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Characteristics of CO2The pressure-enthalpy diagram for CO2 isshown in fig 1. The pressure-temperaturediagram for CO2 is shown in fig 2.

Thermodynamic properties of CO2 seem tobe similar to many other commonrefrigerants, but there are some exceptions:The triple point of CO2 occurs at higherpressure and temperature than all othercommon refrigerants.The triple point for CO2 is much higher thanfor all other common refrigerants(see table 1). In the pressure-enthalpydiagram (see fig 1), the triple point is actuallya line at a pressure of 5.18 bar, and atemperature of 56.6C. In the triple point, anequilibrium of CO2 vapour, liquid and solidexists.

CO2 as a refrigerant The critical pressure of CO2 is 73.6 bar, andthe critical temperature is + 31C. In thesupercritical phase, CO2 has propertieswhich are almost similar to a high-densityvapour.

Note:CO2 is a unique substance usedfor many different purposes inother industries. E.g.- By increasing pressure, CO2

becomes a solvent. This isutilized in extraction of onesubstance from another, andfor cleaning processes.

- CO2 solid can be used as asubstitute for sand in"sandblasting".

- Fire fighting systems

Table 1: Triple points for CO2 and NH3

Fig. 2 Pressure-temperature diagram for CO2

Fig. 1 The pressure-enthalpy diagram for CO2

Pressure Temperature Pressure Temperature

Triple point 5.18 bar 56.6C 0.06 bar 77.7C

Critical

pressure73.6 bar (31C) 113 bar (132C)

CO2 NH3

1

10

100

1000

-80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100

Temperature (C)

Pres

sure(bar)

Liquid

Solid

Vapour

Supercritical

CO2 Phases

Triple point

CO2

1

10

100

-200 -100 0 100 200 300 400 500

Enthalpy(J)

Pressure(bar)

Solid - Vapour

Liquid - Vapour

Liquid

Solid

Solid - Liquid

73.6

-5.2

- 78.4C

+31C

1

10

100

-200 -100 0 100 200 300

Solid - Vapour

Liquid -

Liquid

Solid

Solid -

-

Vapour

56.6C

Supercritical


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The high-saturated pressure at ambienttemperature is often the first barrier thatneeds to be considered when proposing CO2as a refrigerant. At a temperature of 20C, thesaturated pressure is 57.2 bar. The design of

CO2 refrigeration systems depends verymuch on the application.

There are a number of different ways thatCO2 can be utilised. A single stage subcriticalCO2 system is simple but it also hasdisadvantages due to the limitation intemperature and high pressure.

The transcritical (supercritical) CO2 systemsare only interesting for small systems, wheresystem pressure is not an important designfeature. A number of research programs arerunning in the automotive industry for air-conditioning, but also for residential air-

conditioning (e.g. Japan).

CO2 as a refrigerant(cont.)

CO2 in hybrid systems is the most commonsystem design in industrial refrigeration,because the pressure can be limited to alevel where the requirements for componentslike compressors, controls and valves only

differ slightly compared to traditionallyindustrial refrigeration plants.

CO2 systems can be designed in differentways e.g. direct expansion systems, pumpcirculating systems or CO2 in secondary"brine" systems. Some common industrialsystems are described on pages 6 and 7.

Fig. 3: Subcritical 10/35 bar

Fig. 4: Supercritical 10/90 bar


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CO2 as a refrigerant inindustrial systems

Figure. 5 shows a low temperaturerefrigerating system (40C) using CO2 as aphase change refrigerant in a cascadesystem with ammonia on the high-pressureside.

The CO2 system is a pump circulatingsystem where the liquid CO2 is pumped fromthe receiver to the evaporator, where it ispartly evaporated, before it returns to thereceiver.

The evaporated CO2 is then compressed in aCO2 compressor, and condensed in the CO2-NH3 heat exchanger. The heat exchangeracts as an evaporator in the NH3 system.

Figure. 6 shows the same system as in fig 5,but includes a CO2 hot gas defrosting system.

Fig. 5: Diagram of R717/CO2 cascade system

Fig. 6: Diagram of R717/CO2 cascade system with hot gas defrosting

R717

CO2CO2

+30C (12 bar)

20C (1.9 bar)

+30C

20C

15C

40C

Pressure

Enthalpy

15C (23 bar)

40C (10 bar)P

ressure

Enthalpy

CO2

Pressure

Enthalpy

CO2

R717

40C

CO2

evaporator

CO2 compressor

CO2-receiver

CO2-R717 Heat exchanger

R717 Heat exchanger

R717

COCO

+30C (12 bar)

20C (1.9 bar)

15C (23 bar)

40C (10 bar)

+30C

20C

-40C

40C

Pressure

Pressure

Enthalpy

Enthalpy

CO2

R717

+8C

+8C (43 bar)

15C

CO2-

CO2 compressor

CO2 defrostcompressor

CO2-receiver

CO2evaporator

2

R717 - CO2 cascade system

R717 - CO2 cascade system with CO2 hot gas defrosting


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Figure. 7 shows a low temperaturerefrigerating system (40C) using CO2 as a"brine" system with ammonia on the high-pressure side.The CO2 system is a pump circulating

system, where the liquid CO2 is pumped

from the receiver to the evaporator. Here it ispartly evaporated, before it returns to thereceiver. The evaporated CO2 is thencondensed in the CO2-NH3 heat exchanger.The heat exchanger acts as an evaporator in

the NH3 system.

CO2 as a refrigerant inindustrial systems(cont.)

Design pressure There are 2 important factors to take intoconsideration when determining the designpressure.

Fig. 7: Diagram of R717/CO2 brine system

R717 Heat exchanger

R717

COCO

+30C (12 bar)

45C (0.5 bar)

40C (10 bar)

+30C

45C

40C

40C

Pressure

Pressure

Enthalpy

Enthalpy

CO2

R717

40C

CO2-

CO2-receiver

CO2 evaporator

2

1.The pressure during stand still.The pressure during stand still can be veryhigh and this has to be taken intoconsideration:

A small separate refrigeration systemcan be used to keep the liquidtemperature at a level, where thesaturated pressure is lower than thedesign pressure

Designing the system with an expansionvessel of a size that prevents thepressure from exceeding the designpressure.

Designing the plant so that it canwithstand the saturated pressure at thedesign temperature (approx. 80 bar)

From Danfoss' experience, it would appearthat the most common solution forindustrial refrigeration applications, is touse a small separate refrigeration systemto cool down the liquid CO2.

2.Defrosting pressure by CO2 hot gasdefrosting:

Depending on the actual design, differentways of defrosting can be applied (natural,water, electrical or CO2 hot gas defrosting).The CO2 hot gas defrosting is the mostefficient, especially at low temperatures,but it also has the highest pressuredemand. With a design pressure of PS=50bar, it is possible to reach a defrosting

temperature of approx. 9-10C.The saturated pressure at 9C is43.9 bar-g. By adding 10% for the safetyvalves and approx. 5% for pressure peaks,the requirement is for pressure PS ~50 bar-g. (See figure 8 and 9).

There is not one common method toperform defrosting. All methods, asdescribed above, are used, depending onthe system, but also on the availability ofsuitable compressors and othercomponents.

R717 - CO2 brine system


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Practical limit: PS Psaturated +15%

Design pressure

Pressure peaks 5%

Saturated pressure10%Safety valve

CO2

20

25

30

35

40

45

50

55

60

-30 -20 -10 0 10 20

Design temperature (C)

Pressure(bar)

Design pressure (bar-g): PS + 15 %

"Saturated"pressure (bar-a)

PS + 10% (bar-g)

PS 50

PS 40

PS 25

Fig. 9

Fig. 8

Efficiency In CO2-NH3 cascade systems it is necessaryto use a heat exchanger. Introducingexchangers creates a loss in the systemefficiency, due to the necessity of having atemperature difference between the fluids.

Design pressure(cont.)

Oil in CO2 systems In CO2 "brine" systems, and in pumpcirculating systems with oil freecompressors, there is no oil present in thecirculated CO2. From an efficiency point of

Component size Due to the thermodynamic properties of CO2,in particular the relative high pressure level,the compressor capacity is significantlyhigher for CO2 than it is for NH3. The pipe

[3] Stoecker, Will IIAR Ammonia Refrigeration Conference Nashville TN 2002

However, compressors running with CO2have a better efficiency and heat transfer isgreater. The overall efficiency of a CO2-NH3cascade system is not reduced whencompared to a traditional NH3 system [3].

view, this is an optimum solution due to goodheat transfer coefficients in the evaporators.However, it requires that all valves, controlsand other components can operate "dry".

dimensions in the vapour lines are smaller,but in liquid lines are larger.(See figure 10and 11).


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Refrigerant charge inCO2-NH3 cascade systems

EN 378 classifies CO2 as a L1 non-toxic,non-flammable refrigerant and ammonia asa L2 toxic refrigerant. Even though NH3 hasbeen used for many years, the requirementshave become more restricted in recent years,in particular in some European countries.Therefore, there is a great interest in

Example:

Return line CO2 and a NH3 pump circulating

system with a capacity of 100 kW, circulatingrate of 3, is designed to have a temperature

drop from the evaporator to the receiver on 1K.

Example:

Liquid line CO2 and a NH3 pump circulating

system with a capacity of 100 kW, circulating

rate of 3, is designed to have a velocity on

0.8 m/s.

Component size(cont.)

Wet suction line

Wet suction line

020

40

60

80

100

R717 CO2

DN

0

5

10

15

20

25

R717 CO2

0

0.1

0.2

0.3

0.4

R717 CO2

p(bar)

Wet suction line

Wet suction line

Velocity(m/s)

p(bar)

Liquid line

Liquid line

DN

Liquid lineLiquid line

05

101520253035

R717 CO2

0

0.05

R717 CO2

Example:100 kW, ncirc = 3Velocity ~ 0,8 (m/s)

Example:100 kW, ncirc = 3DT - 1(K)

minimizing the NH3 charge. A CO2-NH3cascade system is a perfect solution with theNH3 being limited to a small charge, whichcan be contained in a special machineryroom having the necessary safetyarrangements.The CO2 is then distributed toall coolers.

Fig. 10

Fig. 11


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Material compatibility CO2 is compatible with almost all commonmetallic materials, unlike NH3. There are norestrictions from a compatibility point of view,when using copper or brass.The compatibility of CO2 and polymers is

much more complex. Because CO2 is a veryinert and stable substance, the chemical

reaction with polymers is not critical. Themain concern with CO2 is the physiochemicaleffects, such as permeation, swelling andthe generation of cavities and internalfractures. These effects are connected with

the solubility and diffusivity of CO2 in theactual material.

Approximation of relative permeation, diffusion and solubility of different gases in polymers [4].

Explanation

The permeation "Q" indicates the amount of gas

penetrating into the material.

The permeation depends on pressure and in

particular the temperature.

The diffusion "D" indicates the amount of gas

passing through the material.

Danfoss has carried out a number of tests toensure that components released for usewith CO2 can withstand the impact of CO2 inall aspects.The tests have shown that CO2 is different,and modifications have to be made on some

products. The large amount of CO2, whichcan dissolve in polymers, has to be takeninto consideration. Some commonly used

polymers are not compatable with CO2, andothers require different fixing methods e.g.sealing materials.When the pressure is close to the criticalpressure and the temperature is high, theimpact on polymers is much more extreme.

However, those conditions are not importantfor industrial refrigeration as pressure andtemperatures are lower for these systems.

Water in CO2 systems In NH3 systems it is well known that there arereaction problems with oil, oxygen, water andsolid contaminations, but these are allhandled today by frequent oil changes, anduse of air purges. Compared to NH3, CO2 isless sensitive, but if water is present,problems may occur.

The solubility "S" indicates the amount of gas

dissolved (accumulated in) the material.

S/D indicates the sensitivity of creating blisters and

fractures in polymers. The high value of S/D for

CO2 indicates that CO2 is one of the strongest

promoters.

Relative permeation

coefficient

Relative diffusion

coefficient

Relative solubility

coefficient

Q D S

N2 - Nitrogen 1 1 1 1

CO2 Carbon Dioxide 24 1 24 24

CH4 - Methane 3.4 0.7 4.9 7

He - Helium 15 60 0.25 0.004

O2 - oxygen 3.8 1.7 2.2 1.29

S/D

How can water penetrate into a CO2 system? The pressure of CO2 systems is always

above the atmospheric pressure; thereforethere are no risks that leaks may causepenetration of H2O into the system.

When charging CO2, there are differentspecifications of CO2. Some of them allowrelative high amounts of water.

CO2 is treated as a very safe refrigerant,and is therefore handled without followingthe normal safety requirements. If a systemis opened up, air can penetrate into it, andthe moisture can condense inside thetubes. If the system is not evacuatedproperly, some water may well be retained.

[4] Leisenheimer, Bert and Thomas Fritz , Eaton Corporation. Aeroquip Groupe

IIF - IIR Commission B1,B2 and E2, Purdue University


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0

200

400

600

800

1000

1200

1400

1600

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60

Temperature [C]

mgofwater/kgofrefrigerant[ppm]

CO2

NH3R134a

R22

R404A

0

10

20

30

40

50

60

70

80

90

100

-50 -40 -30 -20 -10 0 10 20

Temperature [C]

gofwater/kgofrefrigerant[ppm]

CO2 + H2O gasPhase

CO2 +

ICE

CO2 +

WaterCO2

Water Solubility in Refrigerants.

Liquid Phase(Y-Axis Linea r)

0

500

1000

1500

2000

2500

-60 - 50 - 40 - 30 - 20 - 10 0 10 20 30 40 50 60

Temperature [ oC]

mgofwater/kgrefrigerant[ppm]

Water Solubility in Refrigerants.

Gas Phase(Y-Axis Linea r)

0

200

400

600

800

1000

1200

1400

1600

-60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60

Temperature [oC]

mgofwater/kgofrefrigerant[ppm]

CO2

CO2

R134aR134a

Fig. 12: Water solubility

Fig. 13: Water solubility

Water in CO2 systems(cont.)

The acceptable amount of water in CO2systems is much lower than in systems withother common refrigerants.

If the water content exceeds the dew-point,and the temperature is below 0C, the waterwill freeze, creating a risk of problems withequipment in the system e.g. blocking controlvalves.

The water can very easily be removed bymounting a drier in the system.

Driers in CO2 are very efficient, and arenormally mounted in the liquid line to avoidany unnecessary pressure drops beingcreated.

Water solubility in Refrigerants

Gas Phase

(Y-Axis Linear)


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0

10

20

30

40

50

60

70

80

90

100

-50 -40 -30 -20 -10 0 10 20

Temperature [C]

mgofwater/kgrefrigerant[ppm] "Wet"

"Dry"

Fig. 15: CO2 systems with filter drier and indicator

Leaks in CO2-NH3 cascade

systems

The most critical leak in a CO2-NH3 cascade

system is in the heat exchangers betweenCO2 and NH3. The pressure of the CO2 will behigher than the NH3, so the leak will occurinto the NH3 system, which will become

Water in CO2 systems(cont.)

contaminated. The solid substance

ammonium carbonate is formed immediatelywhen CO2 is in contact with NH3. Ammoniumcarbonate is corrosive [5].

Safety valves in CO2systems

Due to the Thermodynamic properties ofCO2, in particular the triple point, which islocated at much higher pressure than for allother common refrigerants, the formation ofsolid CO2 can occur.If a safety valve is mounted in a CO2 systemat e.g. 50 bar, the pressure in thedownstream (outlet) line from the safety valvewill pass the triple point at 5.2 bar. Below the

triple point, CO2 will change from a mixture of

liquid and vapour into a mixture of solid andvapour .The formation of solid CO2 in thedownstream line can, in the worst case,block this line. The most efficient solution tothis problem is to mount the safety valvewithout an outlet line, thus blowing directlyinto the atmosphere. The phase change ofthe CO2 does not take place directly in thevalve, but just after the valve, and in this case

in the atmosphere

Fig. 14: SGN moisture indication for CO2

CondenserEvaporator

CO2 system with filter drier and indicator

Water in CO2

Danfoss Sight Glass SGN in CO2

Filter drier Moisture indicator

[5] Broesby-Olsen, Finn Laboratory of Physical Chemisty, Danfoss A/S

IIF - IIR Commissions B1, B2, E1 and E2 - Aarhus Denmark 1996


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Fig. 16: Flow in downstream lines from safety valves

As previously mentioned, CO2 is classifiedas a non-toxic refrigerant, but unlike NH3, CO2has no distinctive smell.

The availability of components for CO2industrial refrigeration systems havingreduced pressure to approx. 30 bar is good.Several manufacturers of equipment for thetraditional refrigerants can also supply somecomponents for CO2 systems, but theavailability of higher-pressure components forCO2 industrial refrigeration systems is limited.An important factor in the speed of

introducing CO2 systems, is very muchdependent on the availability of criticalcomponents for high pressure CO2.Within the area of industrial refrigeration, CO2will not replace ammonia. The industrial CO2systems are all hybrid systems, which alsorequire ammonia on the high temperatureside of the system, but only with a smallammonia charge.

Safety valves in CO2systems(cont.)

Safety

Conclusion

Safety Aspects of CO2 [6]

Carbon dioxide replaces air, and causes lackof oxygen. In the presence of sufficientoxygen, CO2 has a narcotic effect at stronger

concentrations. With smaller amounts, CO2has a stimulating effect on the respiratory

centre. Due to the acidic characteristics ofCO2, a certain local irritation may appear,particularly on the mucous membrane of the

nose, throat and eyes, and it may inducecoughing, as well.

The symptoms associated with theinhalation of air containing carbon dioxideare, with increasing carbon dioxideconcentrations.

0.04% Concentration in the atmospheric air2% 50% increase in breathing rate3% 10 minutes short term exposure limit;

100% increase in breathing rate5% 300% increase in breathing rate,

headache and sweating may begin

after about an hourCom.: This is tolerated by mostpersons, but it is physicallyburdening

8% Short term exposure limit

8-10% Headache after 10 or 15 minutes.Dizziness, buzzing in the ears, bloodpressure increase, high pulse rate,excitation, and nausea.

10-18% After a few minutes, cramps similarto epileptic fits, loss of consciousness,

and shock(i.e. a sharp drop in bloodpressure) The victims recover veryquickly in fresh air.

18-20% Symptoms similar to those of astroke.

Note: The data, valued for adults with good health

[6] Ahlberg, Kersti AGA Gas Handbook 1985 ISBN 91-970061-1-4

CO2

1

10

100

-200 -100 0 100 200 300 400 500Enthalpy (J)

Pressure(bar)

Solid - Vapour

Liquid - Vapour

Liquid

Solid

Solid - Liquid

73.6

-5.2

- 78.4C

+31C

1

10

100

Solid - Vapour

Liquid -

Liquid

Solid

Solid -

-

56.6C

Supercritical 50 bar

at the

Safety valve 35 bar

Safety valve

Safety valve 35 bar

0% solid CO

triple point

Vapour

2

at the

Safety valve 50 bar

5% solid CO

triple point2


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Danfoss industrial refrigeration valves havebeen used for CO2 systems for more than 15years. The CO2 systems have beenrefrigeration, process and fire fightingsystems where the pressure has been limited

to 25 bar. During the last few years, Danfosshas supplied an increasing number of valvesfor several CO2 systems at different pressurestages up to 50 bar.

Danfoss valves andcontrols for CO2

Today, Danfoss is offering a broadprogramme of industrial products applicablefor CO2, however this is dependent ondemands regarding the design pressure. Thetable below includes the components

available for the different pressure stages.

High Pressure Components - CO2

All marked components areapplicable for CO2 in thestandard execution.These components are

applicable for CO2 40 bar or50 bar.

High pressure componentsDN PS

25

bar

PS40

bar

PS50

bar

Modulating liquid level regulators PMFL PMFH all 25-65

Main Valves, Solenoid Valves PM1 PM3 PML PMLX all 25-125

Pilot Valves for PM Main Valves EVM

Other pilots

Motor Regulating Valves MRV all 25-65

Motor Expansion Valves MEV all 25-65

Float Valves HFI 100, 150 100 - 150

SV 1-3 & 4,5,6

Stop Valves SVA 15-200 15-200

SVA 250, 300 250-300

Regulating Valves REG all 15 - 40

Stop Check Valves SCA all 20-125

Gas Powered Stop Valve GPLX 80, 100, 125 80-125

SFV 15, 20, 25 15 - 25

BSV 8 8DSV 32 32 **) 1

POV 40, 50, 80 40-80

Filters FIA all 32-200

NRVA all 15 - 65

NRVS 20, 25, 32 20-32

CHV all 20 - 40

Solenoid Valves EVRA all 15-40 **) 1

EVRS all 15-25 **) 1 **) 1

Liquid Injection AKVA all 15-40 **) 1

Level Indicator AKS 45 all

The product can

be used in

standard version.

All products are

CE approved

The product must

be manufactured

in a special

version (design

modification,

higher test

pressure, marking

All products are

CE approved

and documentation).

Safety Valves and Change Over Valves

Check Valves

The product must

be manufactured

in a special

version (higher

test pressure,

marking and

documentation).

All products are

CE approved

Danfoss industrial refrigeration

In connection with the CE approval of thesevalves, Danfoss has implemented productmodifications and approved them for themaximum pressure. The products are

subject to specific testing and marking,but all of them are CE approved.

CO2

20

25

30

35

40

45

50

55

60

-30 -20 -10 0 10 20

Design temperature (C)

Pressure(bar) Design pressure (bar-g): PS + 15 %

"Saturated"pressure (bar-a)

PS + 10% (bar-g)

PS 50

PS 40

PS 25


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