co2-ashrae

Heat Exchangers in Carbon Dioxide Cascade Systems.

Part I. A Comparison with Different Refrigerants.

The Use of Halogenated Hydrocarbons and Carbon Dioxide in Commercial Refrigeration.

1950 1960 1970 1980 1990 2000 2010

HCFCs

Carbon dioxide

The Use of Halogenated Hydrocarbons and Carbon Dioxide in Commercial Refrigeration.

1950 1960 1970 1980 1990 2000 2010

HCFCs

Supermarkets

Carbon dioxide

This Study Concern the Following Uses of Carbon Dioxide?

MT. Carbon dioxide condenses in the cascade condenser and is pumped to display cabinets, where it evaporates at the same pressure. It the recon-denses in the cascade condenser. Temperature level around -10 °C/14 °F.

LT. It expands and evaporates in the deep freezers. After compression it joins the MT-CO2 and recondenses. Temperature level around -40 °C/-40 °F.

MTLT

A Cascade System for a Supermarket

Properties of carbon dioxide.

Chemical formula CO2

Molecular weight 44.01

Critical temperature 30.98 C/87.76F

Critical pressure 73.77 bar/1070 psi

Triple point -56.56 C/-69.8 F

Normal sublimation point -78.4 C/-109.1 F

Below the triple point, carbon dioxide is a solid and the triple point then gives the absolute lowest temperature in a refrigeration system.

The following refrigerants have been studied, A.

-125

-75

-25

25

75

125

175

225

275

CO

2

NH

3

R22

R23

R32

R41

R11

6

R12

5

R14

3a

R40

4A

R40

7C

R41

0A

R50

7A

R50

8A

R29

0

R12

70

R21

8

Tcrit, F T (at 1 bar/14.5 psi), F

T (at 41 bar/595 psi), F >Tcrit

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

CO

2

NH

3

R22

R23

R32

R41

R11

6

R12

5

R14

3a

R40

4A

R40

7C

R41

0A

R50

7A

R50

8A

R29

0

R12

70

R21

8

30

50

70

90

110

130

150

170

190

210

230

250

COP Vol-40°C/V-10°C/14°F Capacity kW/(lit/s) Tdischarge °F

The following refrigerants have been studied, B.

Unit coolers, Plate freezers,Brine coolers, etc.

Cascade condenser/evaporator

TsubC = 3 K/5F

Tevap = -40 °C/-40 °F Tcond = -10 °C/14 °FTsupH = 5 K/9 F

COP, compression ratio, volumetric capacity and discharge temperatures.

Pipe diameters for a capacity of 100 kW (28 RT).

The diameter, which gives a pressure drop corresponding to 0.5 K/0.9F for a 5 m/ 16ft long pipe.

0

50

100

CO

2

NH

3

R22

R23

R32

R41

R11

6

R12

5

R14

3a

R40

4A

R40

7C

R41

0A

R50

7A

R50

8A

R29

0

R12

70

R21

8

Ø Suction mm Ø Discharge, mm

Ø Condensate, mm Ø Two-phase, mm

0

50

100

150

200

250

300

350

400

450

CO

2

NH

3

R22

R23

R32

R41

R11

6

R12

5

R14

3a

R40

4A

R40

7C

R41

0A

R50

7A

R50

8A

R29

0

R12

70

R21

8

Condenser heat transfer Evaporator heat transfer

Condenser and evaporator heat transfer coefficients relative to R507A.

Comparison between carbon dioxide and some brines

Cascade condenser/ evaporator100 kW

Brine cooler/ evaporator100 kWNote! Danger of maldistribu-tion in high viscosity brines.

MT refrigerant

Unit coolers100 kW

Unit coolers100 kW

Pumps0.75 kWξ= 0.764

Δt = 5 KPumps0.75 kWξ= 0.764Circulation = 2

Two-phase pipe, 100 m

Carbon dioxide circuit

Liquid pipe, 100 m

Pipe, 100 m

Brine circuit

Pipe, 100 m

CO2 liquid line

CO2 2-phase line

P.Glycol39 / 57%

CaCl218 / 54 %

T = -10 °CPipe Ø, mm

NH3 sol.11 / 20 %

T = -40 °C160

140

120

100

80

60

40

20

0

Reference condition:

Calcium chloride: 18 %

Mean temperature: -10 °C

Brine Δt: 5 K

Capacity: 100 kW

Pipe, length: 100 m

“ diameter: 0.1 m

These conditions give in each leg of the circuita net pumping power of 0.573 kW

The diameters for the other brines are calculated to give the same pumping power.

1. The cooled liquid is evenly distributed over the channels.

Maldistribution of a viscous liquid between parallel channels.

Suddenly the viscosity increases in one channel for whatever reason, e.g. a body of higher viscosity.


The ΔP increases and the velocity decreases.

The slower liquid is better cooled and the viscosity increases


The velocity is further decreased.


Lower velocity means:

Better cooling.

Increased viscosity.

Increased ΔP.

Decreased velocity.


Etc., etc., and the entire channel is blocked.


4. Another channel gets a lump of high viscosity liquid and the process repeats.

The result will be a severe maldistribution (on both sides).


The pressure increase of an enclosed body of a liquid when the temperature increases.

0

10

20

30

40

50

60

CO

2

NH

3

R22

R23

R32

R41

R11

6

R12

5

R14

3a

R40

4A

R40

7C

R41

0A

R50

7A

R50

8A

R29

0

R12

70

R21

8

1. ΔT to 200 bar, K 2. Psat (200 bar temp.), bar 3. Heat to 200 bar, kJ

Tc

ri

Tc

ri

20 °C

The necessary temperature change to increase the pressure of an enclosed liquid refrigerant body.

Initial state: Sat. liquid at -10 °C.Contained liquid volume: 0.53 litre.Final state: 200 barExpansion of the vessel: 0.4 %

0

1

2

3

4

5

6

CO2

NH3

R22

R23

R32

R41

R116

R125

R143

a

R404

A

R407

C

R410

A

R507

A

R508

A

R290

R127

0

R218

D e p t h a t - 1 0 ° C , m D e p t h a t - 4 0 ° C , m

Flashing depth.

In a separator, the liquid surface is at the saturation point. At lower depth the temperature remains but as the pressure is higher, the liquid is subcooled.

In case of a sudden pressure decrease in the separator, e.g. start of an additional compressor, the pressure be

comes suddenly less than the saturation pressure down to a certain depth, H m.

Flashing then starts in the liquid down to the depth H.

The graph shows the depth where the flashing starts for two temperatures and a pressure decrease of 2 %.

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

CO

2

NH

3

R22

R23

R32

R41

R11

6

R12

5

R14

3a

R40

4A

R40

7C

R41

0A

R50

7A

R50

8A

R29

0

R12

70

R21

8

ΔtSubC ( -10°C), K ΔtSubC (-40°C), K

ΔP = 5 m liquid column

Subcooling for some refrigerants

Conclusion

Anybody planning the low temperature circuit in a com-mercial refrigeration plant should seriously consider the use of carbon dioxide.

It is an excellent refrigerant in its own right. To this come the low price, the availability and the lack of negative environmental effects.

Evidently, all refrigerants have advantages and disad-vantages and so has carbon dioxide, but in properly designed system, neither the high triple point nor the low critical temperature has any larger importance.

Some practical aspects on the use of carbon dioxide. Material, A.

Metals. When CO2 is mixed with water, the carbonic acid, H2CO3, is formed. This is corrosive, especially if oxygen is present. Stainless steels are not affected but carbon steel, brass, copper and copper alloys are.

The corrosive behaviour is worse by the addition of corrosive breakdown products of oil. Stainless steel PHEs have not had any problem with corrosion due to CO2, but there are cases of compressor breakdowns due to too high water content.

Thus some precaution should be taken of the carbon dioxide quality, see below.

Some practical aspects on the use of carbon dioxide. Material, B.

Oil. The oil is not chemically affected by CO2 but CO2 dis solves in some oils and at a pressure decrease there will be foaming. Oil can deteriorate by wear and tear and high temperature and form corrosive products. Water and oxygen form corrosive organic acids with oil decomposition products.

Some practical aspects on the use of carbon dioxide. Material, C.

Elastomers. CO2 will not corrode or affect these chemically but if high pressure CO2 diffuses into an elastomer it can sometimes break this – as can practically all refrigerants - if the pressure is released too rapidly.

Some practical aspects on the use of carbon dioxide. Material, D.

Moisture. Thus water should thus be kept out of the system as much as possible, see below.

One observation in Europe is that installers trained for HCFs are better than ammonia trained installers. Ammonia is more forgiving to water than other refrigerants.

Some practical aspects on the use of carbon dioxide. Material, E.

Carbon dioxide quality.

A compressor in the CO2 cycle. R744, Refrigerant quality 4.0 with < 10 ppm O2 and < 10 ppm H2O should be used. This is ex pensive, though. The final word should however the compressor manufacturer have.

Note that some oils, e.g. ester oils are hygroscopic and their use is thus somewhat questionable

Pump circulation (without compressor). Practically any CO2 can be used. PHEs has been used for many decades in treating all type of CO2 qualities, including with a high water content without any problem. Other components,e.g. valve & pumps, could be more sensitive. As for compressors: check with makers for a suitable CO2 quality.

Some practical aspects on the use of carbon dioxide. Material, F.

Pipe material wen using CO2 as a brine replacement in an ice rink.

The cooling pipes in an ice rink are U-tubes – about 100 to 150tubes - with a total length of some 400 ft. In case of a brine like calcium chloride, the U-tubes are welded togther from pipes lengths of some 30 ft. That means a lot of welding.

In case of carbon dioxide, copper tubes in coils can be used. The entire U-tube for a rink can be made from one tube from the coil. Special, plastic clad, tubes are developed for use in ice rinks. These tubes allows for a certain movement against the concrete where the tubes are embedded.

Some practical aspects on the use of carbon dioxide. Leakage, A.

Initial liquid level

Some practical aspects on the use of carbon dioxide. Leakage, B.

0

10

20

30

40

50

60

70

80

90

100

CO

2

NH

3

R22

R23

R32

R41

R11

6

R12

5

R14

3a

R40

4A

R40

7C

R41

0A

R50

7A

R50

8A

R29

0

R12

70

R21

8

Initial evaporation, % Evaporation rate, % of R508A%

Some practical aspects on the use of carbon dioxide. Leakage, C.

Initial liquid level

However, carbon dioxide solidifies and the remaining leakage is through evaporation only.

Note that the density of the solid is larger than the liquid. Carbon dioxide will burst vessels as water does.

Some practical aspects on the use of carbon dioxide. Leakage, D.

If the pressure decreases below the triple point, dry ice lumps can enter piping and destroy or block valves and pumps.

Some practical aspects on the use of carbon dioxide. Leakage, E.

A better design is shown to the left.

Some practical aspects on the use of carbon dioxide. Leakage, E.

Leakage of carbon dioxide into the ammonia circuit in carbon/ammonia dioxide cascade condenser/evaporator.

If carbon dioxide enters the ammonia circuit solid ammonium carbamate will form.

It can destroy a compreesor, especially in dry expansion system, less in a flooded system as the carbamate dissolves in the ammonia.

It can be washed out with water, a tedious but not very difficult job.

If water cannot be admitted into the evaporator, carbamate can be decomposed by heating and venting with air of at least 140 °F.

Not that carbon dioxide is not worse in this respect than other refrigerants, on the contrary. A leakage of one type of a refrigerant into another different, could lead to difficult separation and operation problems.

Some practical aspects on the use of carbon dioxide. Compressor shut down.

In case of a compressor shut down, planned or by accident, the temperature and the pressure start to increase. There are various responses to this.

Some practical aspects on the use of carbon dioxide. Response, A.

A. No response, the emergency valves release CO2 when the set pressure is reached. The CO2 is then replaced.

A

Some practical aspects on the use of carbon dioxide. Response, B.

B. A managed release of CO2, similar to 1 but all pres sures and temperatures are carefully monitored. This can minimize the loss of CO2.

B

Some practical aspects on the use of carbon dioxide. Response, C.

C. A special emergency cooling unit starts and condenses the vaporized CO2.

C

Some practical aspects on the use of carbon dioxide. Response, D.

D. Pump the liquid CO2 to a vessel, which can stand the highest possible pressure.

D

Some practical aspects on the use of carbon dioxide. Response, E.

E. The HP system is built with a redundancy, e.g. at least two each of the critical components.

E

E

EE

Some practical aspects on the use of carbon dioxide. Vents and drains, A.

Connection of the compressor discharge to the condenser inlet (1a) versus to the liquid receiver (2a, b).

1a

2b 2a

Some practical aspects on the use of carbon dioxide. Vents and drains, B.

If the hot gas from the compressor passes the liquid receiver (3) it heats up the liquid, but its temperature is lowered, which reduces the stress on the condenser. The drawback is that a refrigerant close to the bubble point can cause cavitation in the pumps and in general a loss of capacity. If the vapour connection is at (2b) there is no larger heating of the condensate but a certain dampening of pressure variations occurs.

1a

2b 2a

3“Through” liquid receiver “Surge” liquid receiver

Some practical aspects on the use of carbon dioxide. Vents and drains, C.

Another method to dampen excessive pressure and/or temperature variations is to connect a muffler, a vessel or the like (4), which can impart inertia to the flow.

4

“Through” liquid receiver “Surge” liquid receiver

Some practical aspects on the use of carbon dioxide. Vents and drains, D.

Vents , safety valves and drains should never be placed directly on a pipe (6-10) or a vessel, particularly not at low temperature operation. Moisture can enter from the outside, freeze and block the valve. The valve 7 has a double fault, to close and an unsuitable position for a vent, better is 7.


6 7

8

9 10

Some practical aspects on the use of carbon dioxide. Vents and drains, E.

Place vents, safety valves and drains on a pipe, well away from the vessel. Note that there should be no pipe connected to the exit of a vent or safety valve. Dry ice could form and block the exit.


Some practical aspects on the use of carbon dioxide. Vents and drains, F.

Be sure to make a sufficient liquid column available to equalize the pressures over the condenser at through liquid receiver. Note that a surge liquid receiver should have no equalization line or have it closed.


ΔP

h

Some practical aspects on the use of carbon dioxide. Arrangements of unit coolers.

Some practical aspects on the use of carbon dioxide. Defrosting, A.

A. Electrical

Some practical aspects on the use of carbon dioxide. Defrosting, B.

B. Glycol from the HT side.

Some practical aspects on the use of carbon dioxide. Defrosting, C.

C. A special HP compressor for producing hot gas.

Some practical aspects on the use of carbon dioxide. Defrosting, D.

D. Increase the pressure of the liquid, evaporate and superheat it and use the vapor for defrosting.

Some practical aspects on the use of carbon dioxide. Defrosting, E.

E. The unit to be defros-ted is connected to standby compressor and both are shut off from the rest of the system. The vapour is heated by the hot gas.

Some practical aspects on the use of carbon dioxide. Oil types.

Oil type.

Miscibility

Hydrolysis

Oil filtration

Oil return

Use together with ammonia

PAOPoly-Alpha-Olefin oil

Immiscible

Low

Active carbon, Multistage coalescing filters.

Difficult as the oil is lighter than CO2.

PAO can be used with both NH3 andCO2, i.e. only one oil in the plant

POEPolyOl-Ester oil

Miscible

Reacts with water.Stability?

As HCFC/HFC systems.

Oil evaporation as in HCFC/HFC systems.

Different oils for the NH3 and CO2 parts of the systems.

Some practical aspects on the use of carbon dioxide. Oil return, A.

An insoluble oil heavier than the refrigerant should be drained at the lowest point of the loop in a flooded system.

AA

Some practical aspects on the use of carbon dioxide. Oil return, B.

An insoluble oil lighter than the refrigerant should be drained from the surface. This implies a constant liquid level.

B

Some practical aspects on the use of carbon dioxide. Oil return, C.

An insoluble oil heavier than the refrigerant should be drained from the bottom. Normal practice in ammonia systems

NH3

Oil

NH3


An insoluble oil lighter than the refrigerant should be drained from the top. The principle could simply be a flipped NH3 type system

NH3

Oil

NH3

CO2 CO2


A soluble oil has to be separated by evaporating the liquid refrigerant. Here it is done by an oil evaporator. Heating medium for this usually the high pressure condensate but any suitable heat source can be used. Note that oil has to be pumped to the DX system to the left otherwise this will be starved on oil.

C

Some practical aspects on the use of carbon dioxide. Control.

CO2 systems have a relatively small content of refrigerant, a consequence of the small pipes.

Accordingly, they are sensitive to changes in the capacity of the compressors, especially compressors with large steps in the capacity control.

Varying speed drives are an advantage here.

Thank you

co2-ashrae

Documents

insoluble

surge liquid

initial liquid

insoluble

high water

carbon dioxide

temperature

hot gas