Carbon Dioxide Part-II

8/9/2019 Carbon Dioxide Part-II

1/12

Heat Exchangers in Carbon Dioxide Cascade Systems

Part II. Practical Aspects of Carbon Dioxide Installations


2/12

3. Some design considerations for carbon dioxide installations.

3.1. Corrosion by carbon dioxide.

Carbon dioxide itself is inert to practically all metals andelastomers but some care should be taken:

Metals. When CO2 is mixed with water, the carbonic acid,H2CO3, is formed. This is corrosive, especially if oxygenis present as well. Stainless steels are not affected butcarbon steel, brass, copper and copper alloys are. Thecorrosive behaviour is impaired by the addition of corro-sive breakdown products of the oil. Stainless steel PHEs(PHEs) have not had any problem with corrosion due toCO2, but there are cases of compressor breakdowns dueto too high water content. Thus, some precaution shouldbe taken, 3.9.

Oil. The oil is not chemically affected by CO2 but CO2dissolves in some oils and at a pressure decrease therewill be foaming. Oil can deteriorate by wear and tearand high temperature and form corrosive products. Wa-ter and oxygen form corrosive organic acids with oil de-

composition products. Elastomers. CO2 will not corrode or affect these chemi-

cally but if high pressure CO2 diffuses into an elastomerit can sometimes break this when the pressure is re-leased and the elastomer removed.

3.2. Leaks.

A leak in a plant can be of either two types, figure 14:

A leak at a vessel or its adjoining pipes above theliquid level, figure 14.1.

There is an initial flashing down to the ambient pressurefollowed by an isobaric evaporation of the refrigerant. Ina well insulated vessel the evaporation is slow. It is pro-portional to the temperature difference and inverse pro-portional to latent heat. Ammonia has very little initialevaporation and the evaporation rate is slow, i.e. ittakes along time to empty a vessel. R508 leaves thevessel quickly. This is due to large initial evaporation,large t to the outside and low latent heat.

A leak below the liquid level

In a leak (figure 14.3a) is between the lowest point andthe liquid level, the vessel drains to the leak, followedby evaporation. If the leak is at the lowest point (figure14.3b), the vessel drains completely. Carbon dioxide isspecial. It solidifies, when it reaches the triple point andremains solid, i.e. it will not drain from a vessel. The ini-tial evaporation remains though. The behaviour is

somewhat devious though. Ice can get stuck, tightenthe leak and then release when the pressure increases.The positive aspect of it is that a large part of the fillingmight remain even for a leak at the bottom of a vessel.

Carbamate formation. A special case is the formationof ammonium carbamate when CO2 leaks into the am-monia side in a cascade condenser/evaporator. As theCO2 pressure is practically always higher than the am-monia pressure, the leak is into the ammonia side. Am-monium carbamate is corrosive and abrasive, i.e. it candestroy a compressor. It can be detected as white pow-der in the vapour after a direct expansion evaporator,e.g. by breaking a light circuit. The detection can berapid and if action is taken quickly compressor break-down can be avoided. Carbamate dissolves readily inwater and it can be decomposed to ammonia and CO2by heating to above 60 C. The temperature has to bekept until the gases have left the system otherwise theywill recombine.

Welded or semi welded PHEs are safer than HEs wherethe CO2 channel is entirely surrounded by ammoniachannels. The reason is that leakage through a weld ofa PHE is to the exterior.

Flooded flow evaporator are probably safer as the car-bamate is dissolved in the liquid ammonia and renderedrelative harmless what regards the compressors. It willaccumulate in the separator-evaporator loop. The de-tection will be more complicated though.

3.3. Carbon dioxide close to the triple point.

If a vessel with liquid CO2 is operating close to the triplepoint (-55.6 C), a sudden pressure decrease could causethe temperature to drop below this. Dry ice then forms and

as this it heavier than the liquid it sinks to the bottom of thevessel. A lump of dry ice can then move along a pipe toe.g. a circulation pump or a valve and destroy or block this.

If the exit from a vessel feeding a circulation pump ishydraulically correctly executed in order to prevent vor-tices, this danger is substantially decreased, see figure14.2. A vortex free exit should be horizontal. In a verticalexit, at the bottom of a vessel, vortices can easily form es-pecially in CO2 with its low viscosity. There are vortexbreakers inserted at the exit but the author is not fond ofinserts in vessels, which cannot be opened for inspection.

Note also, dry ice is heavier than liquid CO2 thus freezedamages as in water circuits will not occur. See also figure11 for pressure increase in enclosed spaces.

0

10

20

30

40

50

60

70

80

90

100

CO2

NH3

R22

R23

R32

R41

R116

R125

R143a

R404A

R407C

R410A

R507A

R508A

R290

R1270

R218

Initial evaporation, % Evaporation rate, % of R508A%

Figure 14. Leakage of refrigerants andthe formation of dry ice in CO2.The diagram is valid for a leakage above the liquid level.The absolute evaporation rate depends on insulation, re-frigerant mass, shape of the vessel, etc. See also text.

1. A leakage above the liquid surface.

2a. If the CO2 tem-

perature drops to be-

low the triple point,

dry ice can form and

sink to the bottom.

2b. Dry ice can then

destroy pumps and

block valves.

Initial liquid level

2c. In a tank with a

horizontal exit, both

vortex formation and the risk

for dry ice entering the pipe are

decreased.

3a. A leak below the liquid sur-

face.

3b. If the leakage is here, the

complete tank is drained, ex-

cept for CO2, see text.


3/12

3.4. Plant shut-down.

In case of a compressor shut down, the pressure in theCO2 circuit starts to increase. There are some different

responses to this, see figure 15 as well:

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

2. A managed release of CO2, similar to 1 but all pres-sures and temperatures are carefully monitored. Thiscan mean a saving of the lost CO2.

3. A special emergency cooling unit starts and condensesthe vaporized CO2

4. Pump the liquid CO2 to a vessel, which can stand thehighest possible pressure..

5. The HP system is built with a redundancy e.g. at leasttwo each of the critical components.

4 & 5 can obviously only be used in case of planned shut

down but not in case of a power outage.

3.5. Condensate subcooler/vapour superheater.

Sometimes subcooling of the condensate is done by su-perheating the suction vapour; mainly for three reasons:

Capacity increase. When the condensate is subcooledless refrigerant evaporates after the expansion valve,i.e. a higher liquid fraction remains. As it is the liquid,which gives the capacity, the capacity increases.

On the other hand, the vapour density at the suctioninlet decreases. As the volume flow is constant, themass flow decreases, which decreases the capacity.

Less refrigerant circulates but with a higher liquid frac-tion. The capacity thus depends on the balance be-tween increase of the liquid content and decrease ofthe total flow.

At the AC temperature program, 2/40 C, some refrig-erants e.g. R404A and R507A gain, others espe-

cially NH3 and R22, loose and the capacity decreases.In the temperature program studied here, -40/-10 C, allexcept the unimportant R116 and R508A, loose, but thedecrease is for most refrigerants unimportant.

Drying of the oil. Some compressor manufacturers re-quire that the suction inlet superheat is in the order of18 to 25 K in order to evaporate as much refrigerant aspossible from the oil droplets. Especially R1270 andR1290 but also carbon dioxide are very soluble in theoil, which otherwise might enter the compressor diluted.

The superheat is controlled after the condensate sub-cooler/vapour superheater, i.e. the evaporator can runwith little superheat or even wet. This increases theevaporator performance.

Two methods may be used, see figure 16:

1. A dedicated condensate subcooler/vapour superheateris used for larger superheats. PHEs are suitable but incase of some low pressure refrigerants, the low vapourdensity causes to many channels as the number ofchannels are controlled by the pressure drop. Carbondioxide with its large vapour density does not have thisproblem.

2. If the site permits, the vapour and condensate linescould run together inside the insulation. This is a cheapway of getting a sufficient superheat for oil drying but itshould not be used to control the superheat at the suc-tion inlet as the response time will be far too long.

Figure 15. Plant shut down.

4. A liquid receiver design for thevapour pressure at maximum am-bient temperature.

5. At least two of critical com-ponents.

1. The emergency valve opens, noother actions.

2. Managed release of CO2.

3. Emergencycooler unit.

Figure 16. Condensate subcooler/Vapour superheater.

Condenser

1. Dedicated condensate sub-cooler/vapour superheater

2. The condensate and vapour pipesare running together inside the insula-tion.


4/12

3.6. Vents, drains, compressor connections andequalization lines at condensers.

Figure 17 shows some dos and donts when arranging thecondenser piping.

A. Connection of the compressor discharge to the con-denser inlet (1a) versus to the liquid receiver (2a, b).

If the hot gas from the compressor passes the liquid re-ceiver (3) it heats up the liquid, but its temperature islowered, which reduces the stress on the condenser.The drawback is that a refrigerant close to the bubblepoint can cause cavitation in the pumps and in generala loss of capacity. If the vapour connection is at (2b)there is no larger heating of the condensate but a cer-tain dampening of pressure variations occurs

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

C. A drain from this vessel (4) to the liquid receiver shouldbe closed (5) during normal operation. See 3.6 K.

D. Vents (or drains) should never be placed directly on apipe (6-10) or a vessel, particularly not at low tempera-ture operation. Moisture can enter from the outside,freeze and block the valve.

E. The same is valid for safety valves.

F. In general, place safety valves, drains, vents, equalizationlines valves, etc. well away from vessels and main pipes.

G. The discharge vapour in (11) proceeds in a straight flowfrom the discharge exit, possibly via a muffler (4), into con-denser and finally into the through liquid receiver (TLR).There might be pressure drops along the flow path butthese will not disturb the flow.

As the inlet to the TLR is flush with the shell, thus freeaccess to the vapour space, the condensate drips intothe TLR together with possible inerts. The inerts can bevented from the TLR (12) but also at the condenser exit,as shown for the SLR (13).

H. The other liquid receiver is of the surge type (though itis not exactly a true surge SLR). The condensate pipeends at the very bottom of the SLR, well below the liquidsurface.

Inerts cannot pass this lock and are vented at the con-densate exit (13). Vent (7) has a double error; inerts can-not be vented from here and it is too close to the shell.

I. The refrigerant leaves through the pump (14b) to theflooded flow evaporators and returns partly vaporized at(15). The two-phase mixture separates and the liquid re-turns to the pump (14b)

The discharge vapour from the compressor (2a or 2b)of the DX system enters the liquid receiver as well.

The vapour flows, via the equalization line (16), back tothe condenser inlet (1b) for recondensation.

However, the pressure in the SLR is lower than at thecondenser inlet, there is the condenser pressure drop,

Pcond, and possibly others.At start up, the pressure is equal in all points 1b, 3 &16

18. The liquid levels in the pipe (18) and the vessel(3) are equal. Once the cold water enters at the lowerport, the vapour starts to condense and the pressuredecreases to reflect the liquid temperature in 17. Thelower pressure sucks vapour from the inlet port and thepressure drop but it also cause the liquid to mount inthe pipe (18) until there is a balance between the result-ing liquid column and the pressure drop.

The process is the same as drinking through a strawfrom a glass of water. The pressure is lower at the con-denser exit (the mouth) than in the liquid receiver (thewater surface in the glass) and refrigerant (water)

mounts in the condensate pipe (the straw).J. Theoretically, the two-phase flow from the flooded

evaporators could enter directly to the condenser but itis very difficult to design a condenser, where all the liq-uid and vapour a distributed equally from channel tochannel. More important, the additional liquid gives anextra resistance to the heat transfer

K. Note! A faulty placed or open equalization line is afrequent cause of underperforming condensers.

L. The error can be insidious. The equalization line wasmaybe not installed deliberately as such, it is simple aconnection, through various pipes and vessels from thecondenser inlet to the liquid receiver.

Figure 17. Vents, drains, compressor connections and equalization lines.

1a4

2a

Through liquid receiver (TLR). Surge liquid receiver (SLR).

58

13

6

12

7 15

10. Drain.To DX evaporators.

11

Cascade condenser with two separate condenser circuits.

Place all inlets at one end of theLR and all exits at the other.

14b. To flooded evaporators.

16

18

1b

2b

9. Drain.

3

17

PLR

PCond

14a. To DX evaporators.


5/12

3.7. Detection of inerts and venting,figure 18.1. Ammonia is the easiest refrigerant what regards detec-

tion and venting. Connect a hose to the vent, dip theother end as deep as possible into a bucket of cold wa-ter. The result is clear

Bubbles emerge => Inerts are presentNo bubbles emerge => Inerts are not present

Bubbles or no bubbles, practically no ammonia smell isnoticeable. Other refrigerant are harder to detect. Thereare mainly three methods to indicate inerts:

2. A temperature difference, between condensate out andcooling medium in, of less than a couple of degrees.

3. There is a large temperature drop from the saturation

temperature in to the condensate temperature out.Note! It is practically impossible to distinguish between acondensate flooding and inerts by using methods in 2 or3. See also 6 below.

4. A vibrating needle in an undampened pressure gauge,type Bourdon, indicates the presence of inert gases.

If inert gases are suspected, venting has to be done.

5. In case of H(C)FCs, venting should only be done into arecovery unit.

6. Carbon dioxide can be vented directly to the atmos-phere but to an outside location. Note, valve well awayfrom the vessel and no pipes after the valve.

Carbon dioxide does not actually need any detection, it

can be vented and the result checked. However, if thereis a lot of inerts, the venting can take quite some time.The author once vented an ammonia system for fourhours. Thus some detection is useful.

A vent placed on the upper side of the condensate exitcan be used to detect if too low capacity is due to flood-ing or inert gas presence. When the valve is opened:

If liquid droplets leave, flooding is likely.

If no liquid droplets leave, inerts are likely.

It can be difficult to detect liquid droplets, though

7. Question? Can CO2 be absorbed if vented intoa bucket with ammonia water and thus detected?

3.8. Temperature difference in a cascade unit.Figure 19 shows the temperatures in a cascade conden-ser evaporator. The vapour enters superheated at 50 C,condenses at -10 C and leaves slightly subcooled at may-be -11 C. The cooling refrigerant evaporates at -14 Cand superheats with 5 K to -9 C.

If the vapour temperature is decreased, e.g. by a desuper-heater or mixing with vapour from a flooded evaporator,the temperature difference to the evaporating refrigerantdecreases and it might even be impossible to keep thecondensing or evaporating temperatures. The pinchpoint is then approached and the evaporator size ap-proaches infinity.

Compare also with an evaporator, 2.8, figure 13.

3.9. Carbon dioxide quality.There are various carbon dioxide qualities, differing mainlyin the water content. Check with the compressor maker ofthe CO2 quality and oil type to be used.

A compressor in the CO2 cycle. R744, Refrigerantquality 4.0 (Ref. 3) with < 10 ppm O2 and < 10 ppm H2Oshould be used. This is expensive, though.

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

Pump circulation (without compressor). Practicallyany CO2 can be used. PHEs has been used for manydecades in treating all type of CO2 qualities, includingwith a high water content without any problem. Other

components, e.g. valve & pumps, could be more sensi-tive. As for compressors: check with makers for a suit-able CO2 quality.

3.10. Fouling in carbon dioxide circuits.

Fouling usually ends up in the evaporators, especiallyflooded evaporators, which then should be inspected regu-larly and cleaned if necessary.

A source of fouling in the refrigerant circuit is oil and itsdecomposition products. As no oil is completely insolublein carbon dioxide, it is flushed away be the liquid CO2 .

Fouling is usually a sign of excessive wear and tear or cor-rosion somewhere else and excessive fouling should thusentail an investigation to the causes.

Fig. 19. The temperature program in acascade unit.

50 C

-14 C

-10 C

The inlet vapour temperature is substantially de-creased, from 50 C to 0 C in a desuperheater.Note, the superheat is decreased as well.

5 K

0 C

The pinch point isapproaching.

-10 C

-14 C

5 K

Figure 18. Detection of inerts andventing.

1. Detectionand venting ofammonia

5. Venting and re-covery of HFCs.

6. Venting of CO24. Detection of inert gases, avibrating needle.

2. t to inlet

3. t to cold

side.

7. Can CO2be vented intoammonia wa-ter?

To waste water treatment


6/12

3.11. Carbon dioxide filling.

Follow the gas manufacturers instruction. In general, thefilling should start from the gas phase until a pressure wellover the triple point (5.2 bar) is reached otherwise dry icecan form and block valves and pipes. When the pressureis reached continue the filling from the liquid phase.

3.12. One or two liquid receivers.

Figure 20 gives an overview of one or two liquid receivers.The advantages and disadvantages can be summed up as:

The CO2 in the isothermal system is not very sensitiveto the water content and more important, it is oil free, animportant point for the operation of heat exchangers

The compression cycle needs CO2 of a higher quality,which is more expensive.

As a compression cycle usually contains oil, an oilmanagement system is necessary. Insoluble oil is alsodetrimental for the operation of heat exchangers as itcan cover the nucleation sites, which are responsiblefor a major part of the boiling heat transfer coefficients.

It is questionable to use two qualities of a refrigerant inthe same plant. Sooner or later, the qualities will bemixed up, with compressor break down as a result.

The benefit of two separate circuits is thus nullified, e.g.two emergency cooling circuits are necessary,

As the circuits are separate, they can operate at differ-

ent temperature levels, here one is condensing at-10 C and the other at -15 C, albeit the temperaturesshould not be too different.

Another better - option is two separate cascade units.

3.13. Dry expansion evaporators.

The installation and selection of expansion valves andother components do no differ from other high pressurerefrigerants, there are sufficient components available.The main points to consider for carbon dioxide are:

Flash gas before the expansion valve. Liquid carbondioxide is sensitive to overheating of the condensate,less so for a moderate pressure decrease in a sub-cooled condensate.

Be aware of dry ice formation if evaporation just abovethe triple point (-56.6 C).

There is no problem with distribution of liquid carbondioxide to parallel connected evaporators as is the casewith pump circulation, see 3.15. The high pressuredrop in the parallel legs compared to the header pipes,ensures an almost perfect distribution.

3.14. The flooded evaporator.

The condenser-liquid receiver has been treated in chapter3.6 and 3.7. Here we will give some points on the designand installation of flooded evaporators, either the cascadeevaporator in flooded flow mainly ammonia or a proc-

ess cooler with CO2 as refrigerant, see figure 21.The main aspects are on PHEs, but most information arevalid for other evaporator types as well.

In flooded flow the refrigerant leaves the evaporator wet,sometimes as little as ten weight percent is vapour.

The driving force is a liquid column L, which has toovercome the pressure drops in the drop leg Pd,evaporator Pe and return leg Pr. It can do this as the

Fig. 20. Single or double carbon dioxide circuits.A. Single circuit.

Advantages: Less costly, only one liquid receiver.

The same CO2 quality but expensive - isused in both the DX and the flooded,pumped circuit.

Disadvan- Better quality more expensive CO2 istages: necessary.

Oil in the flooded circuit will foul the evapo-rators and decrease the performance.

B. Double circuit.

Advantages: CO2 of a lesser quality but cheap can beused in the flooded circuit.

The flooded section is oil free, important forthe performance of the evaporators.

Disadvan- It is easy to mix up the CO2 qualities for thetages: DX and the flooded circuits.

Expensive, two liquid receivers and two con-denser sections are necessary.

DX evaporators

Liquid receiver

Flooded evaporators

with pump circulation

Single section cascade condenser/evaporator Double section cascade condenser/evaporator

Liquid receiver &DX evaporators

LR & floodedevaporators.

-10 C

CO2 -10 C

NH3 -15 C

CO2-40 C

CO2 -40 C

CO2 -10 C

Fig. 21. The separator-evaporator.

Pr(eturn) leg

Pe(vap)Pd(ropLeg) H

Pump head

LLiquid head

Separator

Evaporator


7/12

density in the drop leg is much higher than the two-phase density in the evaporator and the return leg.

It is also possible to add a pump to give the natural cir-culation a boost.

Natural circulation is normally used if the evaporatorand separator are in the immediate vicinity. Pumpedflow is used for far away evaporators.

The optimal circulation rate - inverse of the exit vapourfraction ranges from less than 1.1 to 10 for CO2 and1.15 to 2 for ammonia. It varies with the thermal duty,evaporator type and pipe length. As the evaporator isusually the most critical component the manufacturer

should be consulted to get a proper circulation rate. The design of the return leg is especially critical in case

of natural flow, also called thermosiphon. A too smallpipe diameter, too many bends or too long pipes couldlead to a far too small circulation with an impaired heattransfer. A too large pipe diameter could mean that thevapour cannot lift the liquid in the vertical sections.

If the pressure drop in the return leg is too large there isa danger of oscillation; the vapour cannot leave the re-turn leg as fast as it is produced and the liquid ispushed out backwards into the drop leg.

When no more liquid in the evaporator, the vapour flowdecreases, the vapour leaves the return leg and the li-

quid enters the evaporator again, too much vapour isproduced and the process repeats.

A good design rule is to maximize the return leg pres-sure drop to 25 % of the total.

Pumped flow systems are less critical. The pump canbe rated for a fairly large circulation. If the pressuredrops in the system turn out to be larger than expected,the circulation decreases but this is already taken intoconsideration and most evaporators are fairly flexible.

Figures 22 and 23 shows some dos and donts for sepa-rators-evaporator systems.

If design conditions permit, a horizontal exit as in (22C)

allows very large load variations. An inclined return legas in (22D) should be avoided, especially for low pres-sure drop or expected very low load as instable flowcould result. Top inlet (22G) means an extra lift andshould be avoided. If the liquid level is used for control,injection of the flash gas in the liquid body (22H) is un-suitable as the level will be unstable.

Ejector designs (22J) have in general proved to be un-satisfactorily as the two-phase mixture tends to distrib-ute unevenly over the plate pack. Especially unsuitableis a simple pipe (22K) in the inlet; back flow is assured.

A long and slender separator is cheaper and performsbetter than a short and wide (22 L & O).

Figure 22. Separator placement.

Side placed (A)with three bends.

E. The pump is placed in a pitto reduce cavitation danger.

A B

C

D

EF

F. A downwards loop makesback flow more difficult.

Avoid inlet from the top (G)- P with outbenefits - as well as letting the flashvapour enter the liquid (H).

G

H

An ejector inlet (J) is questionable asthere will be maldistribution in theevaporator. A simple tube (K) is useless.

J

K

L. Short but wide separator. The design isexpensive and the separation efficiency isquestionable, see also (O).

L

NM

O

M. Two symmetrical return legs. Atleast the return leg at the movableframe plate should have a flange.

N. Two symmetrical exits, joined beforeentrance to the separator. It is probablymore expensive than (M).

O. Avoid asymmetric return legs. A long andslender separator is cheaper than (L) and theflow is more stream lined with better efficiency.

Top placed (B) or horizontalexit (C), both with two bends.Inclined (D) is questionable.


8/12

Figure 23 shows some design elements for a thermosi-phon separator-evaporator loop.

Place all inlets at one end and all exits at the other.

The separator can be divided into a separator (7) and areceiver part (1, 1c). The liquid filling is minimized and ifthe liquid level is used to control an expansion valve,the operation will be more stable.

Use a horizontal exit (1a, 1b & 1c) to the drop leg ifpossible. Vortex formation is suppressed and in case ofCO2, dry ice is less likely to enter the pipes.

A valve (2) in the drop leg can be used to suppress un-stable evaporation.

Never put a control valve in the return leg. If a stop valveis necessary use a ball or gate valve, globe valves arequestionable as the pressure drop is too high.

In case of insoluble oil, heavier than the refrigerant,drain the oil at the lowest point (3). Oil separation is im-proved if the pipe inclines slightly upwards.

Especially for a low pressure vapour, the return leg (4)should be as smooth as possible. The pipe exit shouldbe slightly pointing downwards, here by a 45 cut of apipe (5). Elbow bends (6) are very suitable.

Figure 23 shows a two pass design on the liquid side. Itis used for cooling of water/brine to a temperature close

to the evaporation temperature. The liquid inlet and ex-its are at the top. In this way the evaporating refrigerantmeets roughly the same liquid temperature when enter-ing the channels at the bottom

A plate heat exchanger is one of the few evaporatortypes, which can cool water close to the freezing pointwithout evaporator damage should freezing occur.

Design of a two-phase circuit is difficult but an ex-perienced evaporator manufacturer should be ableto assist in this.

3.15. Pumped flow evaporators.

The previous chapter dealt mainly with a separator in theimmediate vicinity of flooded flow evaporators pumpedor natural circulation - typically plate or tube evaporatorsfor cooling of brine. It is also usually a compressor system.The refrigerant, ammonia, carbon dioxide or other isevaporated, compressed and condensed in a normalcompressor cycle.

This chapter deals with pumped flow evaporators whereevaporating carbon dioxide is used instead of brine. Thegeneral properties of carbon dioxide in this application wasdescribed in 2.4. The evaporators unit coolers, platefreezers, tubes in an ice rink, freeze driers etc. are furtheraway and the carbon dioxide has to be pumped apprecia-ble distances.

It is usually but must not be - an isothermal circuit, seefigure 20. The carbon dioxide evaporates and condenses

at basically the same pressure except for the small pres-sure differences necessary for the circulation.

Figure 24 shows some basic layouts of the separator-pump-evaporator circuit. Distribution of a fluid betweenparallel connected vessels of different types is a difficultproblem; for a single phase fluid and still more so for atwo-phase fluid. Only some points can be treated here.

As figure 24 implies, each group of heat exchangersshould be fed by its own pump, at least if the groupsare far apart, at different heights or of different types.

In figure 24A a number of unit coolers are placed at dif-ferent heights. The exits from the UC join a commonheader below the UC. If the pressure drop for all

the UCs and the attached pipe work are equal, then the

distribution could be as required. The danger is that ifthe capacity of lowest placed UC decreases suddenly,the pressure drop decreases as well. It could the stealCO2 from especially the highest placed UC.

Better is to join the exits above the UC as in figure 24B.If the pressure drops in the UC is lower than the corre-sponding head H, the flow is better distributed over theUC and capacity changes are easier accommodated.

If the capacity in one of the UC - # 3 - decreases to zero,i.e. no evaporation at all, the pressure drop changes.

It increases if the friction pressure drop is small and thestatic two-phase pressure drop is large as this part isreplaced by liquid with higher density.

It decreases if the friction part is large and the statictwo-phase part is small. The friction part goes to zeroand replacement of the two-phase mixture with liquidhas no importance if this pressure drop is small.

There will be no flow in the non evaporating UC and inthe pipe a static liquid column LC - forms, which cor-responds to the pressure drops in the other UCs.

The other UCs get a little more refrigerant, which usu-ally has no adverse effect.

If very large height difference between the UC, theremight be a temperature penalty in shape of a boilingpoint increase. In such a case, the UC should be di-vided into groups, each group fed by its own pump.

One method to even out the refrigerant distribution is to

arrange the UC with symmetrical exits as in figure 24C.

Figure 24D shows circuit, where the UC are arrangedwith asymmetric exits, i.e. the refrigerant in the further-most UC has a longer way to travel and has to passvarious bends & connections. This creates pressuredrops in both the feed and return leg. The driving headin the furthermost UC is then less than in the first.There is neither any appreciable vertical distance as infigure 24B to even out pressure drop variations.

A high pressure drop at each UC inlet is a commonmethod of distributing a fluid between parallel channels.This can be done by disks with tailored holes at the inletor better, as the figure shows, with valves.

Fig. 23. Separator elements.

7

1 1c

1a

1b

2

3

4

56

6


9/12

Both methods are questionable, especially the disks,which cannot easily changed as fine tuning of the pres-sure drops might be necessary.

The major drawback is that for the disks/valves to be ef-fective, the pressure drops have to be fairly large in or-der to be effective.

A high pressure drop means a danger of flashing afterthe disk/valve, especially if the liquid carbon dioxide hasbeen heated somewhat during the transport.

If the UC, or the freeze dryer, the plate freezer, the PHEis not designed for a vapour fraction at the inlet a se-vere maldistribution could occur in the unit.

Liquid systems use three way valves, which by-pass

part of the liquid but that is questionable in two-phasesystem as a by-pass simply means extra circulation.Note also that the pressure resistance of the valve hasto be comparable to that of the evaporator, otherwisethere will be no or only an erratic control function.

A train of evaporators as in figure 24, could be com-posed of various types of evaporators and with differentcapacities. The manufacturers are consulted and thensubmit the specifications. Most likely the pressure dropswill differ from the specifications.

Then, these are assembled to a circuit and the pump isstarted. However, the pressure drops between parallellegs in a well design circuit, e.g. figure 24C have to beequal but most likely they are not. The system solves

this by redistributing the liquid carbon dioxide until thepressure drops become equal. Thus one item can bestarved while others are overfed. This can be solved by:

o Instead of the exact, optimal circulation of liquid car-bon dioxide, the circulation rate is increased. This is aneasy and fairly safe method of assuring that each itemshould obtain at least its proper amount of refrigerant.

o Adjustment of the inlet valves as in figure 24D. Itprobably needs a lot of time consuming tinkering withthe valves.

o Request from the manufacturer of each item, howmuch refrigerant has to be fed to the item in order togive the requested nominal pressure drop.

The return leg should be downwards inclining 0.5 % -but this might be difficult to keep in practice. Note how-ever, that an ice rink has more than 100 m of two-phaseflow in perfectly horizontal pipes and with any problems.

The optimal circulation rate can vary considerably, tothe point that the term optimal has no meaning.

The special design of PHEs plates are removed oradded in parallel to increase the size means thatwhen the circulation rate increases, the K-value in-crease and the number of plates can be reduced butthe pressure drop increases.

Thus, natural circulation evaporators usually operatewith a very low circulation rate, in the order of 1.1 to 2.0.

Forced flow evaporators can operate for a little highercirculation rate, 1.1 to around 3.

However, if the circulation is too high, the pressure dropin all parts of the system becomes too large and theevaporation temperature increases, see 2.8. This isespecially serious for lower evaporation temperatures.This implies that the circulation rate should be lower,the lower the evaporation temperature is.

Unfortunately, there is a conflicting requirement, the liq-uid volume fraction should not be too low. For a givencirculation rate, it decreases with decreasing temperature.

This is an area, which has not been investigated verywell but as a reference we can take an ammonia ther-mosiphon evaporator operating around 0 C, which is

usually laid out for an exit vapour fraction around 0.8,circulation 1.25, but the actual circulation is probablylarger as the driving head usually is larger than the cal-culated pressure drop. Thus use a circulation 2.5.

The liquid volume fraction out is then 0.75 % and thisvalue is used to calculate the circulation rates for CO2.

At -10 C: Circulation: 1.12 (Ref. 1) 1.6At -40 C: 1.36 (Ref. 1) 2.4

(Ref. 1) recommends about the double circulation butan overfeed is advantageous, see the previous section.

In the end, the evaporator manufacturer should be con-sulted. Only he knows under which conditions an evapo-rator gives its best performance.

Figure 24. Arrangement of pumped flow systems.

A. Unit coolers with exits at different heights. B. Unit coolers with exits at the same height.

C. Symmetrical exits. D. Unsymmetrical exits.

H

LC

# 3

To cascade condenser.

From cascadecondenser.


10/12


11/12

3.15. Defrosting.

The condenser in a LT typically operates well below 0 C.Defrosting the LT unit cooler by condensing the hot gasfrom the compressor is thus not possible. For carbon diox-ide particularly there are some possibilities, see figure 26:

1. Electric defrosting, suitable for lower capacity systems.

2. Glycol from the HT system. A special defrosting circuit isnecessary in the unit coolers

3. A special high pressure compressor, which increasesthe pressure to condensation pressure of 10 C. (Ref. 2)has reported good experience with this arrangement.The disadvantage is that all components in the circuitmust be designed for an operating pressure of 45 bar.

4. Instead of increasing the pressure by compressing agas, it can be done by increasing the pressure of theliquid refrigerant (4a), followed by evaporation (4b),separation of vapour and liquid (4c) and superheating(4d). (Ref. 1) has reported good experience with this ar-rangement. As before, all components have to be de-signed for 45 bar.

In theory it might be possible defrost a unit cooler by usingthe hot gas only, i.e. with no condensation. Unfortunately,the defrosting time will be too long. The arrangementshown here could overcome this problem.

5. In the figure are shown three operating and one stand-by compressor, all equal. The standby compressor isused for defrosting. The unit cooler to be defrosted (5a)is connected to this compressor (5b) and both are dis-connected from the system. In the loop, just before thecompressor, is a superheater (5c) installed as well.

The compressor has two functions:

Act like a pump to circulate the vapour.

Lift the temperature after the vapour superheater.

The data are shown in 5d. With a power input of 3.5 kW,the defrosting capacity is 12.5 kW, which is delivered tothe unit cooler 78 C in and 54 C out. With the excel-lent heat transfer properties of CO2, this could very wellbe sufficient to defrost a unit cooler in sufficient shorttime.

o The advantages of this system are:

o The maximum design pressure is equal to the con-

denser design pressure.o No particular extra components, except the super-

heater are necessary (plus the standby compressor).

o A computer simulation where condensing R507A at10 C was compared with 26 bar CO2 cooled 78 to54 C gave a slightly lower heat transfer coefficient forCO2 than for R507A, but the temperature differencefor CO2 was almost three times as large, giving a verylarge advantage for CO2.

There are some restrictions:

The heat source temperature has to be sufficient highin order to heat the refrigerant vapour, to 70 C in thecase studied. An ammonia compressor in the HT cir-cuit can easily supply this temperature level, either

the oil or a special glycol circuit.

The discharge temperature can be lifted higher by in-creasing the compression ratio but this increases thecompressor power consumption as well.

The compressing ratio is outside the range specifiedby the manufacturers but this probably is due to thatsuch low ratios are usually not required rather thanany technical difficulties, but this has to be checked.

The higher the compressor efficiency is, the lower thedischarge temperature will be and the more heat hasto be supplied by the superheater. This is an advan-tage as this heat probably is cheaper than the elec-tricity for the compressor motor.

Figure 26. Defrosting of unit coolers.

1

2

3b 4d

4a

5c

4c

5b

5a

5d.Data for the defrosting circuit.

Compressors: 3 working, 84.4 kW each

Unit coolers: 3 * 4, 21.1 kW/eachUC. Type: AlfaCubic BL403CElectric defrost: 12.5 kW (If required)

Compressor when defrosting.

Power: 3.5 kWSuction: -12.5 C/24.7 barDischarge: -10 C/26.5 barDefrost cap.: 12.5 kWSuperheater: 9.0 kW

Ts = -10 C/3 K subcooling

Tsat = -10 C/Tsup = 49 C

Ts = -40 C/0 K subcooling

54 C

78 C

70 C

54 C 78 C

4b

Brazed PHEs are suitable for

all the positions 4b, 4d & 5c

3a


12/12

Acknowledgements.

For the content of this paper, I have had the invaluable helpof colleagues within Alfa Laval, in both Italy and abroad, butespecially Gran Hammarson, to whom all I express mythanks.

We also thank Friosol AG in Switzerland for the use of thecover photography .

References.1. Design Consideration when Using Carbon Dioxidein Industrial Refrigeration Systems.

Angus Gillies, B. Eng., C. Eng. &David Blackhurst BSc(Hons), C. Eng.Star Refrigeration Ltd.Glasgow UK.

2. Introducing a New Ammonia /CO2 Cascade Conceptfor Large Fishing Vessels.

Per Skrbk Nielsen and Thomas Lund.York Refrigeration, Marine and ControlsViby J, denmark

2003 Ammonia Refrigeration Conference & Exhibition.IIAR, Albuquerque, New Mexico, March 16 19, 2003

3. Safety leaflet Carbon Dioxide, Linde/AGA.

Documents

Carbon Dioxide Part-II