co2 refrigerant for industrial refrigeration

8/9/2019 CO2 Refrigerant for Industrial Refrigeration

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REFRIGERATION AND

AIR CONDITIONING Article

CO2 refrigerantfor industrial refrigeration



0 202 Pro uce y Dan oss RC-IS. 04.2004.mwa

rt c e re r gerant or n ustr a re r gerat on

ontents age

Introduction 3

Characteristics of CO ........................................ .................................................... ...................................................... ........... 4

CO2 as a refrigerant............................................. .................................................... ...................................................... ........... 5

CO2 as a refrigerant in industrial systems ............................................... .................................................... ..................... 6

es gn pressure 8

Safety 9

Effi ciency .............................................. .................................................... ...................................................... ...........................10

Oil in O systems ............................................... .................................................... ...................................................... .........10

Comparison of component requirements in O , ammonia and R134a systems ..........................................11

et return nes n rec rcu at on systems 13

uct on nes n ry expans on systems: 13

Liquid lines ................................................. .................................................... ...................................................... .........13

ater n CO Systems ................................................. .................................................... ..................................................... .13

Chemical reactions .....................................................................................................................................................15

ater n apor ase

u r cant 15

PAO lubricant ............................................. .................................................... ...................................................... .........15

emov ng water .................................................. .................................................... ...................................................... .........16

How does water enter a CO2 system? .............................................................................................................................17

sce aneous eatures to e ta ng nto cons erat on n re r gerat on systems 8

Safety valve 18

Filter cleaning ...............................................................................................................................................................19

Trapped liquid .................................................... .................................................... ..................................................... .19Charging CO2 ................................................................................................................................................................19

ea s n 2- 3 casca e systems 9

ater a compat ty 20

Conclusion 20

References ................................................................................................................................................................................20



Pro uce y Dan oss RC-IS. 04.2004.mwa 0 202 3

rticle CO2 refrigerant for industrial refrigeration

e app cat on o car on ox e n

refrigeration systems is not new. Carbon dioxidewas first proposed as a refrigerant by Alexander Twining (ref. [1]), who mentioned it in his Britishpatent in 1850. Thaddeus S.C. Lowe experimentedwith CO

2 military balloons, but he also designed an

ce mac ne w t n 1867. owe a so eve opea mac ne on oar a s p or transportat on ofrozen meat.

From reading the literature it can be seen that COrefrigerant systems were developed during thefollowing years and they were at their peak in the1920s an ear y 1930s. was genera y t epre erre c o ce or use n t e s pp ng n ustr esbecause it was neither toxic nor flammable, whilstammonia (NH or R717) was more common in

industrial applications (ref. [2]). CO disappearedfrom the market, mainly because the new “wonderworking refrigerant” “Freon” had come on themar et, an was very success u n mar et ng t s.

Ammonia has continued to be the dominantrefrigerant for industrial refrigeration applicationsover the years. In the 1990’s there was renewedfocus of the advantages offered by using CO dueto ODP (Ozone Depletion Potential) and GWP

o a arm n g otent a , w c as restr ctet e use o s an s an restr ct ons on t erefrigerant charge in large ammonia systems.

CO belongs to the so-called “Natural” refrigerants,together with e.g. ammonia, hydrocarbons such as

propane and butane, and water. All of theserefrigerants have their respective disadvantages

ntro uct on mmon a s tox c, y rocar ons are amma e,

nd water has limited application possibilities. Inomparison, CO is non-toxic and non-flammable.

differs from other common refrigerants inmany aspects, and has some unique properties.ec n ca eve opments s nce 1920 ave remove

many o t e arr ers to us ng , ut users musttill be highly aware of its unique properties, and

take the necessary precautions to avoid problemsin their refrigeration systems.

The chart in figure 1 shows the pressure/temperature relationship for CO

2, 134a an

mmon a. g g ts o s propertes re at ve tot e ot er re r gerants ncu e:

Higher operating pressure for a given

em eraturearrower ran e o o era n em era ures

r e o nt at a muc er ressure

r ca o n a a ver ow em era ure.

While the triple point and critical point arenormally not important for common refrigerants,

is different. The triple point is high: 5.2 bar [75.1psi], but more importantly, it is higher than thenorma atmosp er c pressure.

s c rcumstance can create some pro ems,unless the proper precautions are taken. Also,

O ’s critical point for is very low: 31.1°C [88.0°F],which greatly affects the design requirements.

In table 1, the different properties of CO2 reompare w t 134a an ammon a.

igure 1

Pressure - Temperature

0,001

0,01

0,1

1

10

100

1000

-120 -60 0 60 120 180

Temperature

P r e s s u r e

[ b a r ]

Pressure[psi] [bar]

14500 1000

145 10

1.45 0.1

0.015 0.001

-120 -60 0 60 120 180 [oC]

-184 -76 32 140 248 356 [oF]

Temperature

CO2 R717

R134a

Triple point

Critical point

propert es compare w t var ous re r gerants

Refr gerant 134a 3 2

atura su stance

zone ep eton otent a 0

o a arm ng otent a 1300 1

r t ca pont ar [psi]

[°F]

40.7 [590]

101.2 [214]

113 [1640]

132.4 [270]

73.6 [1067]

31.1 [87.9]

r p e pont ar [psi]

°C [°F]

0.004 [0.06]

–103 [–153]

0.06 [0.87]

–77.7 [–108]

5.18 [75.1]

–56.6 [–69.9]

Flammable or explosive NO (YES) NO

ox c

Table 1 pr 378- 1 20 03

Author: Niels P. Vestergaard R & D Manager Danfoss Industrial Refrigeration





-40 -20 0 20 40 [oC]

-40 -4 32 68 104 [

o

F]Saturated temperature

-40 -20 0 20 40 [oC]

-40 -4 32 68 104 [oF]Saturated temperature

Density

93.6 1500

62.4 1000

31.2 500

0 0

Liquid

Vapour

Critical point:+31oC [87.9oF]

73.6 bar [1067 psi]

[Lb/ft3] [kg/m3]

Density - CO Liquid / Vapour 2

Figure 2 shows the temperature-pressure phaseagram o pure

.e areas etween t e

urves e ne t e m ts o temperature anpressure at which different phases can exist:olid, liquid, vapor and supercritical.

Points on these curves indicate the pressure andorresponding temperatures under which twoifferent phases can exist in equilibrium, e.g.,o an vapor, qu an vapor, so an qu .t atmospheric pressure CO can exist only as aolid or vapor.

haracteristics of CO2

At this pressure, it has no ability to form a liquid;e ow –78.4 –109.1 , t s a so ry ce ;

a ove t s temperature, t su mates rect y to avapor phase.

At 5.2 bar [75.1 psi] and –56.6°C [–69.9°F], COreaches a unique state called the triple point. Att s po nt a 3 p ases .e., so , qu an vapor,ex st s mu taneous y n equ r um.

1

10

100

1000

-80 -60 -40 -20 0 20 40 60 80 100

Temperature (Deg.C)

P r e s s u r e

( b a r - a )

Liquid

Solid

Vapour

Supercritical

CO2 Phase diagramPressure[psi] [bar]

14500 1000

1450 100

145 10

14.5 1

-80 -40 0 40 80 [oC]

-112 -40 32 104 176 [oF]

Temperature

- o

Triple point: –56.6°C [–69.9°F]

5.2 bar [75.1 psi]

Critical point:]

Critical point:+31 oC [87.9 oF]

73.6 bar [1067 psi]

gure 2

gure 3

O reaches its critical . . .° °t this temperature, the density of liquid andapor states is equal (figure 3). Consequently, theistinction between the two phases disappears,n t s new p ase, t e supercr t ca p ase, ex sts.

mmonly-used for refrigeration purposes. The diagramis extended to show the solid and supercriticalphases (figure 4). The marked areas indicate thedifferent phases.





2 may be employed as a refrigerant in a

num er o erent system types, nc u ng otsu cr t ca an supercr t ca . or any type osystem, both the critical point and the triplepoint must be considered.

The classic refrigeration cycle we are all familiarwith is subcritical, i.e., the entire range oftemperatures an pressures are e ow t e cr t capoint and above the triple point.A single stage subcritical CO system is simple,but it also has disadvantages because of itslimited temperature range and high pressure(figure 5).

2as a refrigerant Transcritical CO

2 systems are at present only of

nterest or sma an commerc a app cat ons,.g., mo e a r con t on ng, sma eat pumps,nd supermarket refrigeration, not for industrialystems (figure 6). Transcritical systems will not

be described further in this handbook.

perating pressures for subcritical cycles areusua y n t e range 5.7 to 35 ar 83 to 507 ps

orresponding to –55 to 0°C [–67 to 32°F]. If thevaporators are defrosted using hot gas, then theperating pressure is approximately 10 bar

[145 psi] higher.

1

10

100

-

P r e s s u r e

( b a r - a )

1

10

100

-

P r e s s u r e

( b a r - a )

1

10

100

-

1

10

100

-

Solid - Vapour

Liquid - Vapour

Solid -Liquid

Log p,h-Diagram of CO2

Solid

Liquid

Supercritical

Vapour

–78.4 oC [–109.1 oF]

Enthalpy

Pressure[psi] [bar]

1450 100

145 10

14.5 1

[87.9 oF

Critical point:+31oC F]

73.6 bar [1067 psi]

[69.9

Triple point (line):-56.6 oC [–69.9oF]

5.2 bar [75.1 psi]

101570

43530

72550

130590

735

58040

29020

14510

87060

116080

1450100

101570

43530

72550

130590

735

58040

29020

14510

87060

116080

1450100

bar psi

Enthalpy

Subcritical

Subcritical refrigeration processPressure

–5.5°C [22°F]

–40°C [–40°F]

gure 4

gure 5





101570

43530

72550

130590

735

58040

29020

14510

87060

116080

1450100

101570

43530

72550

130590

735

58040

29020

14510

87060

116080

1450100

bar psi

Enthalpy

Transcritical refrigeration processPressure

–12°C [10°F]

Gas coolingGas cooling35°C

[95°F]95°C

[203°F]

Figure 6

O is most commonly applied in cascade orhybrid system designs in industrial refrigeration,because its pressure can be limited to such

xtent t at commerc a y ava a e componentse compressors, contro s an va ves can e

used.

Figure 7 shows a low temperature refrigeratingystem –40 –40 usng as a phase change

refrigerant in a cascade system with ammonia ont e g -pressure s e.

CO cascade systems can be designed in differentways, e.g., direct expansion systems, pumpcirculating systems, or CO in volatile secondary“brine” systems, or combinations of these.

2as a refrigerant in

industrial systems

R717

CO2CO2

+30°C

+86°F

(12 bar)

(171 psi)

–20°C –4°F

(1.9 bar)

(28 psi)

+30°C [+86°F]

–20°C [–4°F]

–15°C [+5°F]

–40°C [–40°F]

P r e s s u r e

Enthalpy

P r e s s u r e

Enthalpy

CO2

P r e s s u r e

Enthalpy

CO2

R717

–40°C [–40°F]

CO2 evaporator

CO2 compressor

CO2-receiver

CO2-R717 Heat exchanger

–15°C

+5°F

(23 bar)

(333 psi)

–40°C

–40°F

(10 bar)

(135 psi)

gure 7

r nc pa agram717 - casca e system

2as a refrigerant

ont nue





R717 Heat exchanger

R717

COCO P

r e s s u r e

P r e s s u r e

Enthalpy

Enthalpy

CO2

R717

+8°C [+46°F]

CO2-

CO2 compressor

CO2 defrostcompressor

CO2-receiver

CO2 evaporator

2

+30°C [+86°F]

–20°C [–4°F]

–15°C [+5°F]

–40°C [–40°F]

–40°C [–40°F]

+30°C

+86°F

(12 bar)

(171 psi)

–20°C

–4°F

(1.9 bar)

(28 psi)

–15°C

+5°F

(23 bar)

(333 psi)

–40°C

–40°F

(10 bar)

(135 psi)

+8°C

+46°F

(43 bar)

(633 psi)

Figure 8

The CO system is a pump circulating systemwhere the liquid CO is pumped from the receiverto the evaporator, where it is partly evaporated,before it returns to the receiver. The evaporated

s t en compresse n a ,an con ense n t e eat exc anger. The heat exchanger acts as an evaporator in the

system. Compared to a traditional ammoniaystem, the ammonia charge in the above

mentioned cascade system can be reduced topprox. 1 10.

gure. 8 s ows t e same system as n gure 9,but includes a CO hot gas defrosting system.

R717 Heat exchanger

R717

COCO P

r e s s u r e

P r e s s u r e

Enthalpy

Enthalpy

CO2

R717

CO2-

CO2-receiver

CO2 evaporator

2

+30°C [+86°F]

–45°C [–49°F]

–40°C [–40°F]

–40°C [–40°F]

–40°C [–40°F]

+30°C

+86°F

(12 bar)

(171 psi)

–45°C

–49°F

(0.5 bar)

(7 psi)

–40°C

–40°F

(10 bar)

(135 psi)

Figure 9

Principal diagram717 - cascade system with CO hot gas defrosting

r nc pa agram717 - r ne system


n ustr a systemsont nue





R717,

R 404A,

R 134a, ......

CO2

Pump circulating system

DX system

+30°C [+86°F]

–12°C [+10°F]

–7°C [+19°F]

–7°C[+19°F]

–20°C[–4°F]

gure 10

Principal diagram cascade system with 2 temperature levels

(e.g. supermarket refrigeration)

Figure 9 shows a low temperature refrigeratingystem –40 –40 us ng

2 as a “brine”

ystem w t ammon a on t e g -pressure s e.

e system s a pump c rcu at ng system,here the liquid CO is pumped from the receivero the evaporator. Here it is partly evaporated,

before it returns to the receiver.

e evaporate s t en con ense n t eCO - NH heat exchanger. The heat exchangeracts as an evaporator in the NH system.

Figure 10 shows a mixed system with floodedand DX-system, e.g. for a refrigeration system ina supermarket, where 2 temperature levels arerequ re

hen determining the design pressure for COystems, the two most important factors toons er are:

ressure ur n stan st

ressure re u re ur n e rost n

Importantly, without any pressure control, attand still , i.e., when the system is turned off, theystem pressure will increase due to heat gain

rom the ambient air. If the temperature were toreac 0 32 , t e pressure wou e 34.9 ar505 psi] or 57.2 bar [830 psi] @ 20°C [68°F]. For

industrial refrigeration systems, it would be quitexpensive to design a system that can withstandhe equalizing pressure (i.e., saturation pressureorrespon ng to t e am ent temperatureur ng tan st . ere ore, nsta ng a smauxiliary condensing unit is a common way to

limit the maximum pressure during stand still to areasonable level, e.g., 30 bar [435 psi].

Design pressure With CO , many different ways of defrosting canbe applied (e.g., natural, water, electrical, hotgas). Hot gas defrosting is the most effi cient,espec a y at ow temperatures, ut a so eman sthe highest pressure. With a design pressureof 52 bar-g [754 psig], it is possible to reach adefrosting temperature of approx. 10°C [50°F].

The saturated pressure at 10°C [50°F] is 45 bar

[652 psi]. By adding 10% for the safety valvesan approx mate y 5% or pressure pea s, t eindicated maximum allowable working pressurewould be ~ 52 barg [~754 psig] (figure 11 & 12).


n ustr a systemsont nue





es gn pressureont nue

gure 11

Practical limit: PS Psaturated +15%

Design pressure

Pressure peaks 5%

Saturated pressure10%Safety valve

≥

Figure 12

20 290

30 435

40 580

50 725

60 870

–30

–22

–20

–4

–10

14

0

32

10

50

20 °C

68 °F

Design temperature

52 bar

754 psi

25 bar 363 psi

”Saturated”

pressure”p”(bara / psia)

”p” + 10%(barg / psig)

Design pressure”p” + 15%

(barg / psig)

Design pressure / temperature for CO2

bar psi

D e s i g n

p r e s s u r e

40 bar 580 psi

is an odourless, colourless substancelassified as a non-flammable and non-toxic

refrigerant, but even though all the propertieseem very pos t ve, lso has somesa vantages.

ue to t e act t at s o our ess, t s not se -larming, if leaks occur, (ref. [6]).

is heavier than air, which means that it fallso the floor. This can create dangerous situations,specially in pits or confined spaces. COsp ace oxygen to a po nt w en t s ata . e

relative density of CO is 1.529 (air=1 @ 0°C32°F]). This risk requires special attention duringesign and operation. Leak detection and / ormergency ventilation are obvious equipment.ompared to ammonia, CO

2 is a safer refrigerant.

e t res o m t va ue s t e max mum

Safety concentration of vapour CO in air, which can betolerated over an eight-hour shift for 40 hoursa week. The TLV safety limit is for Ammonia 25[ppm] and for CO

2 5000 ppm 0.5% .

pprox. 0.04% s present n t e r.t g er concentrat on, some a verse

reactions are reported:

% 50% increase in breath rate% 100% increase in breath rate5% 300% increase in breath rate8-10% e natura o y s resp rat on s

srupte , an reat ng ecomesalmost impossible. Headache,dizziness, sweating and disorientation.

> 10% Can lead to loss of consciousness anddeath.

> 30% Quickly leads to death.



0 0 202 Pro uce y Dan oss RC-IS. 04.2004.mwa


n cascade systems it is necessary touse a eat exc anger. ntro uc ng exc angers

reates a oss n t e system e c ency, ue to t enecessity of having a temperature differencebetween the fluids. However, compressors

Effi ciency running with CO2 have a better effi ciency and

eat trans er s greater. e overa e c ency oa - casca e system s not re uce w encompared to a traditional NH system (figure 13& ref. [3]).

COP-coefficient of refrigerant system performanc

1,75

1,921,77

1,86

2,18

1,09

1,381,22

1,42

1,78

0

0,5

1

1,5

2

2,5

Ammonia,

single stage

Ammonia,

two stages

R22, single

stage

R22, two

stage

Ammonia/CO2cascade

system

C O P

Source: IIAR - Albuquerque, New Mexico 2003, P.S Nielsen & T.Lund

Introducing a New Ammonia/CO2 Cascade Concept for Large Fishing Vessels

–40 / +25°C (–40 / +77°F)

–50 / +25°C (–58 / +77°F)

gure 13

Example:

n systems w t t ra t ona re r gerat oncompressors, ot m sc e an mm s c e otypes are used (table 2).

For immiscible lubricants, such as polyalphaolefin

(PAO), the lubricant management system isrelatively complicated. The density of PAO isower t an t e ens ty o t e qu . us t e

lubricant floats on top of the refrigerant, makingit more diffi cult to remove than in ammoniaystems. Also, to avoid fouling evaporators, the

compressor oil separation with non- miscible oilsmust be highly effective; basically, a virtually oil-free system is desirable.

n systems t m sc e u r cants, suc as po yo e ster, t e o management system can e muc

impler. POE oils have high affi nity with water, sothe challenge when using POE is to ensure thetability of the lubricant.

n vo at e r n e ystems us ng as a secon aryre r gerant, an n recrcu at ng systems w t

il free compressors, no oil is present in theirculated CO . From an effi ciency point of view,

this is optimum because it results in good heattransfer coeffi cients in the evaporators. However,it requires that all valves, controls and other

omponents can operate ry.

CO2 and oil

il type PAOly- lpha- lefin oil

(Synthetic Mineral oil)

POElyol- ter oil

(Ester oil)

Solobility ow (immiscible) High (miscible)

Hydrolysis Low High affi nity to water

separat on system pec a eman : High filtration demanded

Multistage coalescing filters Active carbon filter

o spec a requ rements(System requirements like HCFC/HFC)

il return system Special demand: Oil drain from low temperature

receiver (oil density lower than

2 - oppos te 3

Simple(System requirements like HCFC/HFC)

hallenge Oil separation and return system ong term o accumu at on n

e.g. evaporators

High affi nity to water Long term stability of oil ean re r gerant system requ re

Table 2





ompare to ammona anR134a, CO differs in manyrespects. The following

omparison illustrates this fact;o allow an “true” comparison,peratona con t ons, .e.,vaporat ng temperature,ondensing temperature, are

kept constant.

omparison of componentrequ rements n ,

mmon a an 134a systems

omparison of pipe cross section areaWet return / Liquid lines

e re u

u

Re r gerant R 134a R 717 CO2

Capacity W [TR] 250 [71] 250 [71] 250 [71]

Wet return” line ∆ T K [F] 0.8 [1.4] 0.8 [1.4] 0.8 [1.4]

∆p bar [psi] 0.0212 [0.308] 0.0303 [0.439] 0.2930 [4.249]

Velocity m/s [ft/s] 11.0 [36.2] 20.2 [66.2] 8.2 [26.9]

Diameter mm [inch] 215 [8.5] 133 [5.2] 69 [2.7]

Area “Wet return” mm [inch2] 36385 [56.40] 13894 [21.54] 3774 [5.85]

Liquid” line Velocity m/s [ft/s] 0.8 [2.6] 0.8 [2.6] 0.8 [2.6]

Diameter mm [inch] 61 [2.4] 36 [1.4] 58 [2.3]

Area “liquid” mm [inch2] 2968 [4.6] 998 [1.55] 2609 [4.04]

Tota pipe cross

ection area

Area “Wet return” mm2 [inch2] 39353 [61.0] 14892 [23.08] 6382 [9.89]

Liquid cross section area % 8 41

Leqv = 50 [m] / 194 [ft] - Pump circ.: ncirc = 3 - Evaporating temp.: TE = –40[°C] / –40[°F]

Table 3

omparison of pipe cross section areaDry suction / Liquid lines

Dry suction

Li uid

Refr gerant R 134a R 717 CO2

Capacity kW [TR] 250 [71] 250 [71] 250 [71]

Dry suction” ine ∆ T K [F] 0.8 [1.4] 0.8 [1.4] 0.8 [1.4]

∆p ar [psi] 0.0212 [0.308] 0.0303 [0.439] 0.2930 [4.249]

Ve ocity m/s [ft/s] 20.4 [67] 37.5 [123] 15.4 [51]

Diameter mm [inch] 168 [6.6] 102 [4.0] 53 [2.1]

Area “Dry suction” mm2 [inch2] 22134 [34.31] 8097 [12.55] 2242 [3.48]

Liqui ” ine Ve ocity m/s [ft/s] 0.8 [2.6] 0.8 [2.6] 0.8 [2.6]

Diameter mm [inch] 37 [1.5] 21 [0.8] 35 [1.4]

Area “liquid” mm2 [inch2] 1089 [1.69] 353 [0.55] 975 [1.51]

Total pipe cross

ection area

Area “Dry suction

+ liquid”

mm2 [inch2] 23223 [36.00] 8450 [13.10] 3217 [4.99]

Liqui cross section area % 30

Leqv = 50 [m] / 194 [ft] - Evaporating temp.: TE = –40[°C] / –40[°F] - Condensing temp.: TE = –15[°C] / –5[°F]

Table 4





omparison of pipe cross section areaDry suction / Liquid lines

ry suc on

Li uid

Refr gerant R 134a R 717 COCapacity W [TR] 250 [71] 250 [71] 250 [71]

Dry suction” ine Area “Dry suction” mm [inch2] 22134 [34.31] 8097 [12.55] 2242 [3.48]

Liquid” line Area “liquid” mm2 [inch2] 1089 [1.69] 353 [0.55] 975 [1.51]

Total pipe cross

ection area

Area “Dry

suction + liquid

mm2 [inch2] 23223 [36.00] 8450 [13.10] 3217 [4.99]

Re ative cross section area .2 .6 1.0

Liquid cross section area % 4 0

Vapour cross section area % 95 96 0

Leqv = 50 [m] / 194 [ft] - Evaporating temp.: TE = –40[°C] / –40[°F] - Condensing temp.: TE = –15[°C] / –5[°F]

0

1

2

3

4

56

7

8

R134a CO2R717

5%

95%

4%

96%30%

70%

Liquid

Suction

0

1

2

3

4

56

7

8

2

5%

95%

4%96%

30%70%

Liquid

Suction

Table 5

Comparison of componentrequ rements n ,ammon a an 134a systems(Continued)

Comparison of compressor displacement Compressor

Refr gerant R 134a R 717 CO2

Re rigerant capacity W [TR] 250 [71] 250 [71] 250 [71]

Required compressor displacement m3 /h [ft3 /h] 1628 [57489] 1092 [38578] 124 [4387]

Re ative isp acement - 13.1 .8 1.0

Evaporating temp.: TE = –40[°C] / –40[°F] - Condensing temp.: TE = –15[°C] / –5[°F]

Table 6

omparison of pressure / subcoolingpro uce n qu r sers

Refr gerant R 134a R 717 COHight of liquid riser “H” m [ft] 3 [9.8] 3 [9.8] 3 [9.8]

Pressure produced in liquid riser “∆p” bar [psi] 0.418 [6.06] 0.213 [2.95] 0.329 [4.77]

u coo i ng pro uce in iqui riser “ t” K [°F] 14.91 [26.8] 5.21 [9.4] 0.88 [1.6]

Evaporating temp.: TE = –40[°C] / –40[°F]

H

CO2 reciever

H

CO2 - reciever

∆p ∆t

Table 7





A comparison of pump circulating systems showst at or wet return nes, systems requ remuch smaller pipes than ammonia or R134a(table 3). In CO “wet return” lines, the allowablepressure drop for an equivalent temperature

rop is approximately 10 times higher than

et return lines in recirculation ystems:

or ot rec rcu at ng an ry expans on systems,ca cuate s zes or qu nes are muc argerthan those for ammonia, but only slightly largerthan those for R134a (table 3 and 4). This can beexplained by ammonia’s much larger latent heatrelative to CO

2 and R134a.

Refer to the tables showing the relative liquid and

vapor cross-sect ona areas or t e t ree re r gerantsta e 5 . e tota cross-sect on area or t e

system is approximately 2.5 times smaller than thatof an ammonia system and approximately seventimes smaller than that of R134a. This result hasinteresting implications for the relative installationcosts for the three refrigerants.

ue to t e re atve sma vapor voume o t esystem and large volumetric refrigeration capacity,the CO system is relatively sensitive to capacityfluctuations. It is therefore important to designthe liquid separator with suffi cient volume tocompensate for the small vapor volume in the pipes.

for ammonia or R134a wet return lines. Thisp enomenon s a resu t o t e re at ve y g

ens ty o t e vapor. e a ove compar sonis based on a circulating rate of 3. The resultwould be slightly different if the circulating rate is

ptimized for each refrigerant.

In the comparison of “dry suction” lines, theresu ts are very near y t e same as n t e prev ouscompar son, n terms o ot pressure rop anline size (table 4).

uct on nes n ry expans on ystems:

qu nes: The required compressor capacity for identicalre r gerat on oa s s ca cu ate or t e t reerefrigerants (table 6). As illustrated, the CO systemrequires a much smaller compressor than the

mmona or 134a systems.

For compressors of identical displacements, the

apacity of the compressor using CO s 8.8 t meshigher than using ammonia, and 13 times higherthan that using R134a.

e su coo ng pro uce n a qu r s er ogiven height “H” is calculated for the three

refrigerants (table 7). The subcooling for theO liquid riser is much smaller than that formmonia and R134a. This characteristic must be

noted when designing CO systems to preventav tat ons an ot er pro ems w t qu

.

In ammonia systems, oil is changed and non-ondensables are purged frequently to minimizehe oil, oxygen, water and solid contaminantshat can cause problems.

ompare to ammon a systems, s essensitive, but if water is present, problems mayccur. Some early CO installations reported

problems with control equipment, among otheromponents. Investigations revealed that manyf these problems are caused by water freezing

n t e system. o ern systems use ter r ers

ater in CO Systems to maintain water content in the system at an

acceptable level.

The acceptable level of water in CO systemss muc ower t an w t ot er common

re r gerants. e agram n gure 14 s s ow ngthe solubility of water in both liquid and vaporphases of the CO liquid and vapor as functionof temperature. The solubility in the liquid phaseis much higher than in the vapor phase. Theso u ty n t e vapor p ase s a so nown as t e

ew po nt

0

200

400

600

800

1000

1200

-60 -40 -20 0 20 40 60

Temperature

W e i g h t * 1 0 - 6 o f w a t e r / w e i g h t o f

r e f r i g e r a n t [ p p m ]

Liquid CO2

Vapour CO2

[oC]

[oF]-40 32 104-76 -4 68 140

Water solubility in liquid / vapour CO2

Figure 14





gure 15

0

500

1000

1500

2000

-60 -40 -20 0 20 40 60

R717

CO2

R134a

-40 32 104-76 -4 68 140

Water solubility in vapour phase

[°C]

[°F]



Temperature

ater n2

ystemsont nue

Figure 16

0

10

20

30

40

50

60

70

80

90

100

–40

–40

–20

–4

0

32

CO2 + H2O

CO2 +

ICE

CO2 +

Water

vapour phase

Temperature

20 °C

68 ºF

Water solubility in vapour CO2



gure 17





It is important to notice, that the belowmentioned reactions with water don’t take place

in a well-maintained CO2 system, where the waterontents s e ow t e max mum so u ty m t.

In a closed system such as a refrigerationystem, CO can react with oil, oxygen, andater, especially at elevated temperatures and

pressures. For example, if the water content isllowed to rise above the maximum solubilitym t, can form carbonic acid, as followsre . 4 an 5 .

O + H H CO

(CO + water carbonic acid)

Chemical reactions In CO production systems, where waterconcentrations can rise to high levels, it is well

known that carbonic acid can be quite corrosiveto several kinds of metals, but this reaction doesnot ta e p ace n a we -ma ntane system,because the water content in the system is keptbelow the maximum solubility limit.

t e water concentrat on s re at ve y g ,nd water in vapor phase can react to form a COas hydrate.

+ 8 (H )

2 + water hydrated CO

2

ater in Vapor Phase e gas y rate s a arge mo ecu e an canexist above 0°C [32°F]. It can create problemsin control equipment and filters, similar to theproblems that ice can make.

enerally, esters such as POE react with water asollows:

+2

+

ester + water a co o + organ c ac

OE lubricant As shown, if water is present, POE will reactwith water to form alcohol and an organic acid(carboxylic acid), which is relatively strong andmay corrode the metals in the system. Thus, it isvery mportant to m t t e water concentrat onn systems u r cants are use .

+ 3 2 0 + 2

(oil + oxygen water + acid)

AO lubricant PAO lubricant is also called synthetic mineraloil. Ordinarily, PAO is very stable. However, ifsu c ent ree oxygen s present, suc as m g tbe available from corrosion in pipes, the oxygenwill react with the lubricant, and form carboxylicacid.

The diagram in figure 15 is showing that thewater so u ty n s muc ower t anor 134a or ammon a. t –20 –4 , water

solubility in the liquid phase is:O 20.8 m

a m mmon a m

Be ow ese eve s, wa er rema ns sso ve nthe refrigerant and does not harm the system.

gure 16 ustrates ow water mo ecu esare dissolved if the concentration is lower thanthe maximum solubility limit, and how the

O molecules precipitate out of solution intodroplets if the water concentration is higher thant e max mum so u ty m t .

If the water is allowed to exceed this limit insystem, pro ems may occur, espec a y

t e temperature s e ow 0 . n t s case,the water will freeze, and the ice crystals canblock control valves, solenoid valves, filters

nd other equipment (figure 17). This problemis in particular critical in flooded and directxpans on systems, ut not so muc n

vo at e secon ary systems ecause ess sens t vequipment is used.





ontrolling the water content in a refrigerationystem s a very e c ent met o e to prevent t e

ove-mentone c em ca react ons.

In Freon systems, filter driers are commonly used

o remove water, usually the type with a zeoliteore. The zeolite has extremely small pores, andcts like a molecular sieve (figure 18).

Removing water ater mo ecu es are sma enoug to penetratet e s eve, an e ng very po ar, are a sor ens e t e zeo te mo ecu es. 134a mo ecu es

are too large to penetrate the sieve. When thereplaceable core is removed, the water goes with

t.

gure 18

Principle diagram: CO2-NH3 cascade system

CO2

Evaporator

Liquid

CO2 reciever

CO2

Compressor Dry suction

CO2 - NH3heat exchanger

Filter drier

Moisture indicator

Filter driers installed in:

• bypass lines or

• main liquid line

Liquid

Filter drier

Moisture indicator

gure 19

is a non-polar molecule, so the removalprocess is different. Like water molecules, COmo ecu es are sma enoug to penetrate t emo ecu ar s eve. owever, t e water mo ecu esa sor e onto t e mo ecu ar s eve act n suc asway as to “kick out” the CO molecule, due to thedifference in polarity. Zeolite filter driers cannotbe used in ammonia systems, because bothwater and ammonia are very polar. Even thoughthe driers function differently in this respect in

systems, t e e c ency s a r y goo . ewater retention capacity is approximately the

ame as in R134a systems.

The most effective location to detect and removewater is where the concentration is high. Theolubility of vapor-phase water in CO s mucower t an n t e qu p ase. ere ore, areater amount o water can e transporte n

liquid lines. Taking advantage of this principle,moisture indicators and filter driers are typicallyinstalled in a liquid line or liquid bypass linefrom the receiver (figure 19). The moisture leveln cate y t ese ev ces var es accor ng to

temperature an a so y type o n cator. nfigure 20, the indication level of a Danfoss SGN

indicator is shown for liquid CO .





gure 20

emov ng wateront nue

Unlike in some ammonia systems, the pressuren

2 systems is always above atmospheric.

However, water can still find its way into COystems.

ater may contaminate a CO system throughve different mechanisms:

. Diffusion

. Maintenance and repair practices

. ncomp ete water remova ur ng nsta at oncommissioning

. Water-contaminated lubricant charged intothe system

5. Water-contaminated CO2 charged into the

system

bviously, all these mechanisms should bevoided/minimized.

How does water enter a CO

ystem To illustrate a scenario in which water maycontaminate a system, think of a contractor, who,believing CO

2 is a very safe refrigerant, thinks that

it may be handled without following the normalammon a sa ety requ rements. e m g t openup the system to perform a repair. Once thesystem is opened up, air enters, and the moisturein the air condenses inside the piping. If he doesnot evacuate the system very thoroughly, somewater may we e reta ne .

n anot er scenar o, our contractor orgets t atthe lubricant used in the system, POE, has ahigh affi nity for water, and leaves the cap offthe container. After charging the POE into thesystem, the water may begin to cause mischiefw t n t e system.





50% solid CO2 at

the triple point

Liquid

20 bar [290 psi]

–78.4°C [–109.1°F]

–56.6°C [–69.9°F]

–5.2 bar-a [75.1 psi-a]

+31°C [87.9°F] 1450 100

145 10

14.5 1

psi bar

Pressure

Enthalpy (J)gure 22

If the set pressure of a safety valve in the vaporphase is 50 bar [725 psi], e.g., the centerline,the relief line pressure will pass the triple pointand 3% of the CO

2 will change into solid as it

continues to relieve. In a worst-case scenario(e.g., a long relief line with many bends), solid

may oc t s ne. e most e c entolution to this problem would be to mount theafety valve without an outlet line, and relieve

the system directly to the atmosphere. The

phase change of the CO does not take place inthe valve, but just after the valve, in this case, int e atmosp ere.

If a pressure relief valve is set to relieve liquid at0 bar [290 psi], the relief products would pass

through the triple point, whereupon 50% ofthe CO

2 would change into solid upon further

relief, subjecting the relief line to a high risk ofblockage. Thus, to safely protect liquid lines

ga nst ormat on o ry ce, connect sa ety re evalves to a point in the system at a pressurehigher than the triple point pressure of 5.2 bar75.1 ps .

s part cu ar y g tr p e po nt can cause soCO to form under certain conditions. Figure

1 shows the expansion processes occurring inpressure relief valves starting at three different

conditions. If the set pressure of a pressure relief

Miscellaneous features to bea ng nto cons erat on n

re r gerat on systems

Safety valve

a ve n t e vapor p ase s 35 ar 507 ps or ess,.g., the rightmost line, the pressure in the relief

line will pass through the triple point at 5.2 bar75.1 psi]. Once below the triple point, the CO

ill be pure vapor.

50% solid CO2 at

the triple point

Liquid

20 bar [290 psi]

–78.4°C [–109.1°F]

–56.6°C [–69.9°F] –5.2 bar-a [75.1 psi-a]

+31°C [87.9°F]

Vapour 50 bar [725 psi]

Vapour 35 bar [507 psi]

0% solid CO2 at

the triple point

3% solid CO2 at

the triple point

1450 100

145 10

14.5 1

psi bar

Pressure

Enthalpy (J)

gure 21

2 expansion - phase changes

Safety valves

CO expansion - phase changesCleaning filers / charging CO





It is important to start up with CO in the vaporp ase, an cont nue, unt t e pressure asreac e 5.2 ar 75.1 ps . us, t s strong yrecommended to write a procedure for charging

CO system. One must be aware when charging

refrigerant system that until the pressurereaches the triple point, the CO2 can only exist

Charging CO as a solid or vapor inside the refrigerationsystem. so, t e system w ex t very owtemperatures unt t e pressure s su c ent yraised (figure 22). For example, at 1 bar [14.5 psi],the sublimation temperature will be –78.4°C

–109 .

ter c ean ng e same p enomenon app es a so w enean ng qu stra ners ters. ven t ougO is non-toxic, one cannot just drain the liquidutside the system. Once the liquid CO contacts

he atmosphere, the liquid phase will partlyhange into the solid phase, and the temperature

rop ramat ca y, as n t e examp e

escr e a ove. us su en temperature rops a t erma s oc to t e system mater a s, an

can cause mechanical defects in the materials.Such a procedure would be considered to be acode violation because this equipment is notnormally designed for such low temperatures.

Trapped liquid is a potential safety risk inrefrigerant systems, and must always be avoided.

This risk is even higher for CO2 systems thanfor ammonia or R134a systems. The diagram in

gure 23 are s ow ng t e re at ve qu vo ume

Trapped liquid hange for the three refrigerants. As shown,liquid CO expands much more than ammonia

nd R134a, especially when the temperaturepproac es s cr t ca pont.

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

-20 0 20 40

Temperature

V o l u m e c h a n g

e

[ % ]

CO2

R134a

R717

Relativ liquid volume

Reference: –40 [oC] / [

oF]

[oC]

32 104-4 68 [oF]

-20 0 20 40 [oC]

–40 -4 32 68 104 [oF]

Temperature

–40

gure 23

The most critical leak in a CO2-

3 cascade

system is in the heat exchangers between CO2

an . The pressure of the CO w e g ert an t e , so t e ea w occur nto t esystem, which will become contaminated.

+ 2

2 ammonia ammonium carbamate

Leaks in CO2-

3

cascade systems The solid substance ammonium carbamate is

ormed immediately when CO2 is in contact with

. Ammonium carbamate is corrosive (ref. [5]).




2 is compatible with almost all common

meta c mater a s, un e . ere are norestr ct ons rom a compat ty pont o v ew,when using copper or brass.

The compatibility of CO and polymers is much

more complex. Because CO s a very nertand stable substance, the chemical reactionw t po ymers s not cr t ca . e ma n concernw t s t e p ys oc em ca e ects, sucas permeation, swelling and the generation ofcavities and internal fractures. These effects areconnected with the solubility and diffusivity of

2 in the actual material.

Danfoss has carried out a number of tests toensure t at components re ease or use w tCO can withstand the impact of CO in allaspects.

aterial compatibility The tests have shown that CO2 is different,

n mo cat ons ave to e ma e on somepro ucts. e arge amount o , w c

an dissolve in polymers, has to be taken intoonsideration. Some commonly used polymers

re not compatible with CO , and others requireifferent fixing methods e.g. sealing materials.en t e pressure s c ose to t e cr t ca pressure

n t e temperature s g , t e mpact onpolymers is much more extreme. However,those conditions are not important for industrialrefrigeration, as pressure and temperatures arelower for these systems.

has good properties, in particular at lowtemperature, but it is not a substitution forammon a. e most common n ustr are r gerat on systems, s y r systems w tammonia on the high temperature side of theystem.

2 is in many aspects a very uncomplicated

refrigerant, but it is important to realize that CO2

has some unique features compared with othercommon re r gerants. now ng t e erences,and taking these into account during design,installation, commissioning and operation, willhelp avoid problems.

onclusion The availability of components for industrialrefrigeration systems with pressuresup to approx mate y 40 ar s goo . everamanu acturers o equ pment or tra t onarefrigerants can also supply some componentsfor CO systems. The availability of componentsfor the higher pressure industrial COrefrigeration systems is limited, and the

vailability of critical components is an importantfactor in the growth rate of CO app cat on.

1] Bondinus, William S ASHRAE Journal April 1999

2] Lorentzen, Gustav, Reprint from IIR Conference 1994 Proceedings “New Applications of atura or ng u s n e r gerat on an r on t on

3] P.S Nielsen & T.Lund IIAR - Albuquerque, New Mexico 2003, Introducing a Ne Ammonia/CO Cascade Concept for Large Fishing Vessels

4 roes y- sen, nn a oratory o ys ca em sty, an ossInternational Symposium on HCFC Alternative Refrigerants. Kobe 1998

– omm ss on 1, 2 an 2, ur ue n vers ty

5] Broesby-Olsen, Finn Laboratory of Physical Chemisty, Danfoss A/S IIF – IIR Commissions B1, B2, E1 and E2 – Aarhus Denmark 1996

6] IoR. Safety Code for Refrigeration Systems Utilizing Carbon Dioxide The Institute of Refrigeration. 2003.

7] Vestergaard N.P. IIAR – Orlando 2004. CO2 in subcritical Refrigeration Systems

8] Vestergaard N.P. RAC – refrigeration and air condition magazine, January 2004.Getting to grips with carbon dioxide.

References

co2 refrigerant for industrial refrigeration

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