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Friterm Technical Data Page: CO2 Cooling Systems,

CO2 Unit Coolers/Evaporators, CO2 Gas Coolers Rev.0.0 01.09.2010 1 / 10

CO2COOLING SYSTEMS,

CO2UNIT COOLERS/EVAPORATORS, CO2GAS COOLERS

Hasan ACL Hatice CANBAZ Fatih KASAP Selim ERBLMechanical Engineer Mechanical Engineer Mechanical Engineer Mechanical Engineer

FRTERM A.. R&D Department

General Description

Because of its good environmental properties there is renewed and widespread interest in

carbon dioxide (CO2 or R-744) as a refrigerant. Because of its low critical-point temperature

(31,06 C) and high pressure (73,8 bar), CO2 presents some unusual technological

requirements compared to conventional refrigerants. Another constraint in applying CO2 is its

relatively high triple point (solid, liquid, gas) at -56,6 C and coincident pressure of 5,1 bar.

CO2 was used in the early stages of the refrigeration industry, but it lost the competition withhalocarbon refrigerants because of its high operating pressure and the loss of capacity and

coefficient of performance when rejecting heat near or above the critical point. Because the

negative effect of halocarbon refrigerants on environment, that CO2 started to be used

recently. New machine and heat exchanger technology and system components allow CO 2

to reach competitive efficiency levels for transcritic cycle especially in northern countries and

for sub-critic cascade cycle in southern countries.

Recently, CO2 has been intensely studied for application as the primary refrigerant in

transcritical mobile air conditioners, vending machines, supermarkets, cold rooms, food

production and process industry, industrial ice cream machineries and heat pumps.[1][2]

Environmental Properties

It has no ozone depletion potential (ODP=0), and negligible direct global warming potential

(GWP=1). (Reference Table 1)

Refrigerant (direct effect) and CO2 emissions from energy supply to refrigerating systems

(indirect effect) both contribute to greenhouse gas emissions expressed by using Total

Equivalent Warming Impact (TEWI). Therefore, refrigeration systems with a high degree of

emission are preferred application areas for CO2 as alternative refrigerant, as long as the

energy efficiency, defined as Coefficient of Performance (COP), can be kept at the same

level [3].

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Safety and Reliability

Toxicity. CO2 is in Class A which signifies refrigerants for which toxicity has not been

identified at concentrations less than or equal to 400 ppm.

Flammability. CO2 is non-flammable class 1 refrigerant. Class 1 indicates refrigerants that do

not show flame propagation when tested in air at 21C and 101 kPa (Reference T able 2)

Inhalation safety. Although CO2 is usually regarded as non-toxic there are physiological

effects from breathing air with a CO2-concentration above a few percent. The IDLH

(Immediate Danger to Life and Health) concentration a maximum allowable concentration of

about 5% by volume seems to be a reasonable limit.[4]

Thermophysical Properties

CO2 has a number of attractive thermophysical properties and other characteristics.Compared to counterpart halocarbon refrigerants, it has low viscosity, high volumetric

capacity, high thermal conductivity, and high vapor density. (Reference Table 3)

Cost

Unit price of CO2 is relatively cheaper than the conventional refrigerants. It has low level of

price and low level of operation cost. (Reference Table 4)

[1] Ashrae Handbook, 2006

[2] Friterm A. R&D Technical Documents

[3] IIR, February 2000

[4] M. H. Kim et al, 2004

Table 1. Environmental Properties of VariousWidely Known Refrigerants

Refrigerant ODP [4] GWP [4]

R 11 1 4600

R 12 0,82 10600

R 22 0,034 1700

R 134a 0 1300

R 410a 0 1980

R 404a 0 3780

R 407c 0 1650

R 507a 0 3850

R 744-CO2 0 1

R 717-NH3 0

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Table 3. Thermophisical Properties of Various Widely Used Refrigerants [6]

Tcrit, (C)Pcrit,

(bar)

Liquid PhaseDensity

f, (kg/m3) [c]

Gas PhaseDensity g,

(kg/m3) [d]

Spesific HeatCapacity cp,

(kj/kg) [c]

VolumetricCapacity(kj/m3) [c]

ThermalConductivity k,

(W/m.K) [c]

DynamicViscosity

, (mPa.s) [c]

R 11 198 44,1 1536,9 2,36 0,85 450,76 0,09 0,5

R 12 112 42,2 1400,1 17,185 0,93 2636,52 0,62 0,25

R 22 96,2 49,9 1285,7 20,41 1,16 4205,28 0,09 0,22

R 134a 101,1 40,6 1298,9 13,9 1,3 2773,75 0,09 0,27

R 410a 72,13 49,3 1175 28,82 1,5 6566,35 0,1 0,16

R 404a 72 37,3 1154,8 29,91 1,3 4953,99 0,07 0,18

R 407c 86,74 46,2 1240,8 18,86 1,4 3973,24 0,01 0,21

R 507a 70,6 37,05 1161,1 30,98 1,37 5055,32 0,072 0,18

R 744-CO2 31 73,7 934,26 94,148 2,5 22089,00 0,11 0,101

R 717-NH3 132,3 113,3 640,28 3,31 4,41 4192,51 0,56 0,172[c] For saturated liquid at -1,1C[d] For saturated vapor at -1,1C[6] Lemmon et al. NIST, 2007.

Transcritical CO2 Cycle

Compared with other refrigerants commonly used in the refrigeration industry CO 2 has very

low critical temperature: 31,06 C (73,8 bar), so that heat discharge into the ambient

atmosphere above this temperature is impossible through condensation as is the case in the

usual vapor-compression cycle; CO2 can only be used in this cycle when the heat discharge

temperature is lower than the critical temperature. For heat rejection at supercritical

pressure, the refrigerant can only be cooled in a gaseous state, without being condensed.

This cycle is known as the transcritical cycle. [9]

R22 R134a R404a R407c R410a R507 R744-CO2 R717-NH3

3,54 /kg 9,38 /kg 8,85 /kg 10,68 /kg 9,58 /kg 19,53 /kg 0,52 /kg 1,3 /kg

[8] DuPont - Canta A, Gne Gaz Ltd. ti. May, 2010

Figure 1. CO2 Phase Diagram Figure 2. CO2 log p - h Diagram

[7] Food Retail CO2RefrigerationSystems,Danfoss,

Table 4. Comparative Prices of Various Widely Known Refrigerants [8]

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Subcritical CO2 Cycle

The classic refrigeration cycle, we are all familiar with, is subcritical i.e. the entire range of

temperatures and pressures are below the critical point and above the triple point.

A single stage subcritical CO2 system is simple but it also has some disadvantages because

of its limited temperature range and high pressure. So a cascade system can be designed.

In a cascade refrigeration system there can be more than one refrigerant depending on the

application or requirement of the plant. In such a cascade system, each refrigerant circuit is

separate. [10, 11]

System Components

The main components of a transcritical CO2 refrigeration cycle are the gas cooler,

compressor, evaporator and expansion valve.

The Method

Simple one stage transcritical CO2 cycle:

Saturated vapor at state 6 is superheated to state 1 in the internal heat exchanger and then

compressed in the compressor to state 2. The carbon dioxide at state 2, which is above the

critical point, is cooled in the gas cooler to state 3 by rejecting heat to the atmosphere. Unlike

in a condenser, in the gas cooler the heat rejection takes place with a gliding temperature.

Carbon dioxide at high pressure is further cooled from 3 to 4 in the internal heat exchanger.

After the heat exchanger, the carbon dioxide is expanded through the expansion device to

state 5, which is the inlet to the evaporator. The state of the refrigerant changes from 5 to 6

as it evaporates in the evaporator by extracting heat from the external fluid (cooling effect).[4]

One stage with gas bypass transcritical cycle:

The difference of this cycle and simple one stage transcritical cycle is, after expansion from

state 4 to 5 the mixture of liquid and gas CO2 is separated. After separation saturated liquid

at state 6 is expanded through the second expansion device to state 7 which is the inlet to

the evaporator. After evaporation CO2 leaves from the evaporator at state 8. Meanwhile the

saturated vapor at state 9 is expanded to evaporation pressure. And a mixture is occurred at

state 11. Finally saturated vapor at state 11 is superheated to state 1 in the internal heatexchanger.

CO2 subcritic cascade cycle:

For the sample application CO2 is used as a refrigerant for the low temperature circuit and

NH3 is used for the high temperature circuit. The condenser of the CO2 circuit acts as the

evaporator of the other refrigerant circuit [3]. For better understanding please refer Figure 3

which shows a schematic arrangement for a CO2 / NH3 cascade system.

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Sample Simple One Stage Transcritical Cycle

1-2 Isentrophic compession in Compressor

2-3 Isobaric heat removal in Gas Cooler

3-4 Cooling in Internal Heat Exchanger

4-5 Isenthalpic expansion in Expansion Valve

5-6 Isobaric evaporation in Transcritical Evaporator

6-1 Super heating in Internal Heat Exchanger

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Sample One Stage with Gas Bypass Transcritical Cycle

1-2 Isentrophic compression in Compressor

2-3 Isobaric heat removal in Gas Cooler

3-4 Cooling in Internal Heat Exchanger

4-5 Isenthalpic expansion in Expansion Valve

5- Seperation of phases in Reciever

6-7 Isenthalpic expansion of liquid in Expansion Valve

7-8 Isobaric evaporation in Transcritical Evaporator

9-10 Isenthalpic expansion of flash gas in Expansion Valve

11-1 Super heating in Internal Heat Exchanger

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Sample CO2 Subcritic Cascade Cycle

1-2 Isentrophic compression in CO2 Compressor

2-3 Isobaric condensation of CO2 in Cascade Condenser (shell and tube)

3-4 Isenthalpic expansion of liquid CO2 in Expansion Valve4-1 Isobaric evaporation in Subcritic Evaporator

5-6 Isentrophic compression in NH3 Compressor

6-7 Isobaric condensation in NH3 Condenser

7-8Isenthalpic expansion in NH3 Expansion Valve

8-5 Isobaric evaporation in of NH3 in Cascade Condenser (shell and tube)

[9] Thermophsical Properties R744, International Institute of Refrigeration, 2003

[10] Food Retail CO2 Refrigeration Systems, Danfoss, 2009

[11] Air condition and refrigeration Journal, Oct-Dec 2002

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Friterm CO2 Projects

A cascade system has been designed with the refrigerant R744 and R404a for the CO2

Project.

There is a Heat Exchanger Performance TestFacility in Friterm A R&D department whichhas an ability of testing R744 products workingtranscritical and subcritical conditions. The datasof R&D tests are used to create. CO2 module inFRTCOILS program which is developed byFriterm R&D. Product design and capacity

calculations are made by using this program.

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FRTERM CO2 UNIT COOLERS

TECHNICAL SPECIFICATIONS

FEATURES AND APPLICATION

CO2 Evaporators are specially designed for small and medium cold room and frozen

storage room applications which are working with subcritic and transcritic cycles. The

capacity range is 1,1 kW 80,7 kW.

5 different fin spacing as 4-6-8-10-12 mm.

Compact and highly efficient coils have these features:

Aluminum fins,

3/8" and 1/2" copper or special alloy of copper tubes

Maximum working pressure 45 bar.

Inlet and outlet connections are copper and steel Refrigerant distributor,

Delivered under positive pressure

CASING

Robust and all-round powder coated (RAL 9016) galvanized steel casing parts, providedecorative, high corrosion resistance and smooth surface finish. Proper for foodprocessing applications.

Intermediate drain pan prevents air by-pass. Double skin drain pan with insulation is optional.

Hinged/Folding drain tray is standard for all models. This application provides easycleaning and maintenance

Stainless steel casing is optional. Side panels are removable as standard for easy maintenance

FANS 230 V, 50 Hz, 1400 rpm. Motor protection IP44 and IP 54; Insulation class B and F.

Recommended working conditions between -30C, -40C and +55C.

Lubrication-free closed type motors. Fan guards according to safety standards.

Different kinds of motors available as optional.

DEFROSTStandard electrical defrost system: E1 type (Light duty)defrost system for 0C / +5C cold room applications.Defrost heatersare applied on heat exchanger coil.

E2 type (Heavy duty) defrost system for -35C / 0C frozen room applications. Defrostheaters are applied on both heat exchanger coil and drain tray.

Drain line heaters, fan hausing heaters, hot gas defrost system and water defrost systemare optional.

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CO2 Unit Coolers/Evaporators, CO2 Gas Coolers Rev.0.0 01.09.2010 10 /

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FRTERM CO2 GAS COOLERS

TECHNICAL SPECIFICATIONS

FEATURES AND APPLICATION

Casing of CHS series gas coolers are made of electrostatically powder coated galvanizedsteel or aluminum (RAL 7044) that provides excellent corrosion protection

COILS

Heat Exchangers are manufactured with V type Corrugated aluminum fins, 5/16 copper orspecial alloy copper tubes. Inlet and outlet connections are copper and steel. The systemdelivered under positive pressure. Maximum working pressure 120 bar.

FANS

Properties of highly efficient axial type fans are: Standard (S) and Low (L) noise level fans are 230 V, 50 Hz, 1 PHz; 1400 rpm

Motor insulation class "B" and "F" protection IP44 and IP5 and recommended workingconditions -30C and -40C /+55C