WO2008000823A1

WO2008000823A1 - Method and system for distribution of an expanding liquid

Info

Publication number: WO2008000823A1
Application number: PCT/EP2007/056588
Authority: WO
Inventors: Claes Stenhede; Peter Kaden
Original assignee: Alfa Laval Corporate AB
Current assignee: Alfa Laval Corporate AB
Priority date: 2006-06-30
Filing date: 2007-06-29
Publication date: 2008-01-03
Anticipated expiration: 2008-12-30
Also published as: SE0601442L; SE531665C2

Abstract

A heat exchanger system has a circuit with a condenser (1), pressure altering means (2; 14), an evaporator (3) containing a distribution header (5) fluidly connected via restrictors (9-12) to several fluid channels (8) coupled in parallel and a compressor (4). The outlet of the condenser (1) is connected to the inlet of the pressure altering means (2; 14), the outlet of the pressure altering means (2; 14) is connected to the inlet of the evaporator (Z), the outlet of the evaporator (3) is connected to the inlet of the compressor (4), and the outlet of the compressor (4) is connected to the inlet of the condenser (1). The circuit contains a refrigerant and the system includes: (a) means for lowering the temperature of the refrigerant downstream of the condenser and upstream of the pressure altering means; and/or (b) means for changing the pressure including means capable of increasing the pressure of the refrigerant downstream of the condenser, so as to keep the refrigerant from flashing until it passes the restrictors (9-12), thereby obtaining a one-phase liquid refrigerant flow in the distribution header (5).

Description

METHOD AND SYSTEM FOR DISTRIBUTION OF AN EXPANDING LIQUID

Field of the Invention

The present invention relates to a method and a system for improved distribution of an expanding liquid in heat-exchanger systems. As to prior art, a system of this kind is disclosed in US-A-5.749,237.

Background in a plate heat exchanger adapted for direct expansion, a mixture of liquid and gas, i.e. a two-phase flow, enters through a port into a header and is subsequently distributed to channels, and as the mixture passes the chan- nets of the plate heat exchanger, heat from a medium surrounding the channels is absorbed by the mixture through evaporation of the mixture, in the case of a two-phase flow the liquid refrigerant enters an expansion valve upstream of the port at a high pressure, normally a pressure close to the condensing pressure. In the valve, the liquid expands or "flashes" to just above the evaporation pressure and a part of the liquid vaporizes. In order to achieve a precise distribution of capacity in the plate heat exchanger, each channel should ideally be charged with a precise amount of liquid and gaseous components. it is important to know that the expansion valve does not control the evaporation pressure, and the pressure drop over the valve is constant over short periods and is given by the refrigeration duty and the ambient conditions but not by the valve. The valve controls the flow (capacity) by changing the cross section, that is. the internal resistance.

A precise distribution can be either an equai distribution between the various channels, or any other well defined distribution corresponding to a certain case, e.g. when the evaporator has two sections, which are used to cool two different fluids, having different properties. In this case, the channels in the two sections shall have different, yet precise, flow rates.

A precise distribution is difficult to obtain since, at low velocities, the two-phase flow tends to separate, resulting in the liquid portion setting at the entrance of the header while the gaseous portion is distributed in the remainder of the space. As a consequence, the liquid preferably enters into the first channels. On the other hand, at high velocities, the inertia makes it difficult for the liquid to change direction and enter the channels. As a consequence thereof, most of the liquid refrigerant is collected in the furthermost part of the header, and thus enters the channels in said furthermost part.

The problem resulting from this behaviour is that the cooling properties will be negatively affected, both in terms of capacity and homogeneity, since the cooling properties will vary between individual channels.

The distribution can be improved if the pressure drops over the channels are great compared to the pressure drop in the header. The higher this ratio is, the less the pressure drop difference between the channels and the belter the distribution will be.

Prior art solutions include distributors that are arranged close to the entrance of each (or every second/third) channel The distributors generally comprise a fixed restriction of the channel cross section, which results in a pressure drop prior to the channel but after the mixture has been distributed along the length of the header, as opposed to the previously described situation in which the main pressure drop occurs in the valve, before the mixture is distributed. The channel pressure drop is now increased as compared to the header pressure drop This type of solution solves part of the problem, yet only in a static arrangement. The flow in the header is still a two-phase mixture, with the behaviour described above

A device for uniform expansion of a liquid/gas two-phase refrigerant mass flow in a plate evaporator is described in US-A-5 806 586. The evaporator has a distribution duct, which is capable of being loaded on the inlet side with the refrigerant mass flow coming from an expansion valve. The evaporator further has a plurality of exchanger sections branched off essentially perpendicularly to the distribution duct along the latter at a distance from one another. In order to achieve a uniform distribution of the mass flow to the exchanger sections a porous body is arranged in the distribution duct. By this expansion valve the distribution is improved but is still far from ideal

of the Invention

The object of the present invention is to eliminate or at least alleviate the above referenced drawbacks and to provide an improved method and a system for precise distribution of a refrigerant into each channel of a heat exchanger, particularly during part load conditions

This object, and other objects which will appear in the following description, are achieved by a method and a heat-exchanger system according to the invention having the features set forth in the appended independent claims.

Preferred embodiments are defined in the related sub-claims.

By the provision of a number of restrictions before each channel the expansion of the liquid is made immediately before each channel. Consequently, the pressure drop in the distribution header will be comparatively low. while the pressure drop in the channels is high, i.e. close to the differential pressure between the condensing and evaporation pressures. According to the invention, each restrictor will be fed with a precise amount of liquid so that each channel will be fed with a precise amount of liquid and vapour.

Either of the steps above, or a combination of both, will result in the refrigerant being prevented from flashing as it passes the pressure altering means. This results in a one-phase flow in the distribution header, which implies a low pressure drop and a precise distribution. It should be noted that parameters like condensation pressure/temperature and evaporation pressure/temperature, and the like, are species specific and will vary between different refrigerants.

In one embodiment, the pressure altering means comprise a pump, and the step of changing the pressure is performed by said pump. A pump arranged upstream of the evaporator is a convenient way of achieving an increased pressure. The increased pressure before the evaporator makes it possible to vary the pressure upstream of the restrictors within a larger interval without risking that the refrigerant flashes. Since the pressure drop over the restrictors will govern the flow through the channels the larger pressure interval will result in a larger flow interval and thereby a larger capacity interval. in another embodiment, both a pump and a valve are used. If the pressure generated by the pump is difficult to control, the valve is arranged downstream of the pump. The valve makes it possible to lower the pressure up- streams of the restrictors and the variable pressure drop over the restrictors results in a changed capacity (flow).

In one embodiment the pressure generated by the pump is regulated by a motor of variable speed. This is a straightforward way of achieving a variable pressure prior to the evaporator. In such an embodiment the valve is obsolete.

In an alternative embodiment, the condenser is disposed in an elevated position to create a static pressure difference through a liquid column. A liquid column can atso be obtained by a tank in an elevated position containing refrigerant.

The step of changing the temperature is preferably performed by a subcooler arranged upstream of the evaporator. The subcooler is capable of lowering the temperature of the refrigerant in such a way that the refrigerant will remain in a liquid state even after the decrease in pressure experienced in the pressure altering means. The pressure altering means preferably comprise a valve but couid also consist of the previously mentioned pump.

In one embodiment, the subcooler constitutes a separate section of the evaporator, such that the refrigerant flows from the condenser into said separate section of the evaporator, where it is cooled down, and then to the valve. The subcooling can also be effected with refrigerant that is bypassed from the heat-exchanger circuit, in a system called an economizer in the field of refrigeration. In yet another embodiment, the subcooler comprises an individual cooling circuit in which a sufficiently cold cooling media flows. An individual circuit is a convenient way of making the subcooler independent of the rest of the heat-exchanger system.

An inventive heat-exchanger system comprises means for executing the above steps.

Other objects, features, advantages and preferred embodiments of the present invention will become more apparent from the following detailed description when taken in conjunction with the drawings and the appended claims.

Brief Description of the Drawings

Preferred embodiments of the invention will now be described in more detail below, reference being made to the accompanying drawings, in which

Fig. 1 is a schematic of a basic refrigeration cycle; Fig. 2 is a partial schematic of a plate heat exchanger according to prior art;

Fig. 3 is a schematic of a plate heat exchanger in accordance with Fig. 2. provided with a main expansion valve;

Fig. 4 is a schematic of a plate heat exchanger in accordance with Fig. 2. provided with individual expansion valves for each channel; Figs. 5-7 are schematics of a plate heat exchanger in accordance with Fig. 2. provided with individual fixed restrictors for each channel and a main expansion valve;

Fig. 8 is a schematic of a plate heat exchanger according to a first em- bodiment of the present invention; and

Fig. 9 is a schematic of a plate heat exchanger according to a third embodiment of the present invention.

Detailed Description of Embodiments of the Invention The basic compressor refrigeration cycle according to prior art is shown in Fig. 1. The actual use of the cycle can obviously be in an air conditioning apparatus/plant, a heat pump as well as in a proper refrigeration apparatus/plant.

In the condenser 1 , high pressure gaseous refrigerant condenses. The condensed refrigerant fluid then flows to the expansion valve 2. In the expansion valve 2 the liquid passes a restricted cross section. This results in a high pressure drop, so that the pressure falls to close to the evaporation pressure. In the process, part of the liquid refrigerant evaporates and the mixture is cooled down to close to the evaporation temperature. At the exit of the valve 2, a cold two-phase mixture leaves the valve 2. The two-phase mixture enters the evaporator 3 in which the cold liquid refrigerant evaporates and cools the process fluid. There are several commonly used process fluids_: such as air, water, brine, a process liquid, etc. The cold low pressure gaseous refrigerant then enters the compressor 4. Here the refrigerant pressure is increased to a pressure level, which is sufficiently high for the intended refrigerant media to be able to condense in the condenser 1.

The control of the valve 2 in conjunction with the evaporator 3 contributes to a good functioning of the cycle. When the cooling requirements change, the valve 2 has to be adjusted accordingly. If too much refrigerant leaves the valve 2. liquid might not evaporate completely in the channels. This results in liquid leaving the evaporator 3, which in certain cases can damage the compressor 4. If too little refrigerant passes through the valve 2, the required capacity cannot be maintained. It is therefore essential that the expansion valve 2 is accurately controlled. Subcooling the refrigerant by cooling, increased pressure or a combination of both leads to a control range. This control range is relatively wide and used to control the expansion valve 2 and thereby the capacity of the evaporator 3. in a heat exchanger, see Fig 2, composed of a number of parallel channels 6, it can be difficult to obtain a precise distribution of a fluid from a distribution header 5 to the parallel channels 6 and then into a collection header 7. The distribution header 5, or header", is a distribution manifoid from which the channels 6 are branched off. In Fig. 2 oniy the flow pattern is shown, the heat exchanger is just indicated as separated surfaces 8. It can be composed of any type of channels connected in parallel.

Assume that the pressure drops A - B and C - D are large compared to A - D and B - C. As the pressure drop must be equal from inlet to exit regard- less of whether the path is A - D or A - B - C - D, it follows that the pressure drop A - D is higher than B - C. As the pressure drop is the driving force for the flow, it follows that the flow of refrigerant will be different for different channels.

A correct distribution is still more difficult for a two-phase flow, e.g. for an evaporator in a refrigeration apparatus/plant. Fig. 3 shows the vaive/evaporator assembly. Saturated or almost saturated liquid refrigerant enters the valve 2 at a high pressure, usually close to the condensing pressure. In the valve 2 it expands to just above the evaporation pressure, whereby a part of the liquid vaporizes. The resulting two-phase fluid has a large volume, which increases the pressure drop in the header 5, which compounds the problem. If the refrigerant velocity is low in the distribution header 5, the liquid part settles at the entrance part of the distribution header 5 and enters preferentially in the first channels, extending from that part of the distribution header 5. If the refrigerant velocity is very high, inertia will result in the liquid refrigerant having difficulty changing direction and entering the channels. In this case, liquid refrigerant will build up in the furthermost part of the distribution header 5 and subsequently enter the furthermost channels 6. Consequently, the refrigerant flow velocity in the header 5 is a parameter that affects the performance of the heat exchanger in an unwanted fashion. Both problems could be solved if the expansion of the liquid occurs just before each channel δ. In Fig. 4 a number of variable restrictors 2' are placed before each channel β. In the distribution header 5 there is now only liquid, thus no problem with phase separation, and the pressure drop A - B will be low. The pressure drop C-O is still comparatively low, while the pressure drop A-A'-D and B-B'-C is high, practically corresponding to the differential pressure between the condensing and evaporation pressures. The flow of refrigerant through different channels will be precise, since there is a precise amount of liquid refrigerant passing each variable restrictor 2'. There is consequently no distribution problem as there is only liquid refrigerant and no gaseous refrigerant in the distribution header 5. The solution of using a large number of small, adjustable restrictors is costly, and to actually integrate the valves into the evaporator 3 is difficult. Further the maintenance and cleaning of such valves or restrictors are difficult.

A practical solution is then to keep the main expansion valve 2 and to introduce fixed restrictors 9, 10, 11 instead of the variable restrictors 2', as illustrated in Figs. 5-7. The fixed restrictors 9-11 can be in the form of a pipe with fixed restrictors 9 in its peripheral wall, said pipe being inserted in the distribution header 5, see Fig. 5. In the case of a plate-heat exchanger the pfates can be formed into a plate-like restrictor 10 at each channel infet. see Fig. 6, or disks with drilled restrictors 11 , inserted in a port hole of each channel, see Fig. 7. The fixed restrictors 9-11 , if allowed to take the full differential pressure, will do a good job distributing the refrigerant. However, in order to operate at part load the valve 2 has to be used to realise the necessary pressure drop in order to vary the flow through the channels, and the larger the pressure drop, the more of the previously mentioned problems with the two- phase flow will recur. Further, the optimal size of the restrictors varies with nominal capacity, pressure, type of refrigerant among others, i.e. each refrigeration system needs individually tailored restrictors. Thus, the use of fixed restrictors in the above context will be an inflexible solution.

A first embodiment of the present invention will now be described with reference to Fig. 8. The invention solves the practical problems related to the occurrence of a two-phase flow. Fig. 8 shows a partial schematic view of a plate heat-exchanger system according to the first embodiment of the invention, showing the evaporator 3 with its distribution header 5 and channels 6 connecting the distribution header 5 to the collection header 7. Fixed restrictors 12 are distributed along the length of the distribution header 5, said re- strictors 12 fluidly connecting the inside of the distribution header 5 and the channels 6. The restrictors 12 can be of a type described earlier with reference to Figs. 5-7. Upstream of the valve 2 a subcooler 13 is arranged for cooling of the refrigerant. The subcooler 13 can constitute a part of the evaporator 3 in such a way that the refrigerant flows from the condenser 1 into a separate section of the evaporator 3. where it is cooled and then to the valve 2. The subccoler 13 can also comprise a separate cooling circuit or an economizer. The subcooler 13 cools the refrigerant with a cooling fluid, and in gen- eral. the cooling fluid can be any sufficiently cold fluid, including a flow from an economizer. Now, the problem with varying the capacity using the expansion valve 2 was that flashing occurred as the refrigerant expanded in the valve 2. If the liquid refrigerant flashes, the problems of uneven distribution related to the two-phase flow appears. This first embodiment of the invention solves the distribution problem in the following way: Consider a situation where flashing occurs at the expansion valve 2. With the inventive system the refrigerant can be cooled down by the subcooler 13 before entering the expansion valve 2. As the refrigerant passes the expansion valve 2 it will ex- perience a pressure drop, but since the temperature is lowered the refrigerant will not flash. Thus, a one-phase flow is present in the distribution header 5, which eliminates said drawbacks.

The previous method of ensuring a precise distribution is based on the fact that when the refrigerant cools down from the saturated condensing tem- perature to a temperature somewhat above the evaporation temperature, the pressure can be lowered in the distribution header 5. from which the restric- tors 12 lead into the channels 6, without causing any flashing. It should be noted that the relevant pressures and temperatures vary between different types of refrigerant and that the inventive concept is not limited to any particu- iar refrigerant. Once a specific refrigerant is chosen, the temperatures and pressures are given. The variable differential pressure over the restrictors 12 is the parameter which controls the refrigerant flow and thus the capacity. In this first embodiment, the maximum capacity of the evaporator 3 occurs when the restrictors 12 essentially discharge the total differential pres- sure. The total differential pressure corresponds to the difference between the condensing and evaporation pressures. The differential pressure over the restrictors 12 is the driving force for the flow. In order to lower the capacity, the pressure drop over the valve 2 is increased, which reduces the pressure drop over the restrictors 12 and thus the flow through the channels 6. According to a second embodiment, see Fig. 9, a pump 14 is arranged upstream of the expansion valve 2. The pump 14 makes it possible, for example, to increase the pressure before the valve 2. In the second embodiment of the invention, the temperature in the flow between the condenser 1 and the evaporator 3 is constant while the pressure is increased by means of a pump 14. The pump 14 increases the pressure (a') after the condenser (not shown) to a higher pressure (a). The variable valve 2 decreases this pressure (a) to a lower pressure (b). Even though the pressure drops in the valve 2, the liquid refrigerant pressure is all the time above the saturation pressure and no flashing occurs until it has entered the restrictors 12. The controlling variable for the refrigerant flow is the variabie pressure difference over the restrictors 12. In the second embodiment, the restrictors 12 are laid out to give the required minimum capacity for the full differential pressure drop, that is. when the valve 2 is fully open and the pump 14 is inactive. When the pump 14 starts to increase the pressure before the restrictors 12. possibly modified by the valve 2. the driving differential pressure increases and consequently so does the capacity.

The valve 2 is only necessary if the pump 14 delivers a pressure (a) which is constant or difficult to control. If the pressure (a) provided by the pump 14 can be easily controlled by e.g. a motor with variable speed (not shown), the valve 2 can be removed. The pump 14 can be any suitable pres- sure increasing device, such as a mechanical or thermal pump.

As has been pointed out in the above, the inventive concept is universal in the sense that it can be used for many types of refrigerants. In the following tables, however, some exemplifying figures are listed for several different cases In the examples shown in the following tables, line pressure drops and other non-relevant pressure drops are omitted.

Tabie 1. Basic refrigeration system (refrigerant R507A).

+ refers to the fact that the pressure is slightly higher before the evaporator than the evaporation pressure, which ;s defined at the exit of the evaporator. An example of a system according to the first embodiment is exemplified in Table 2. below.

Tabfe 2. First embodiment.

♦ refers to the fact that the pressure is slightly higher before the evaporator than the evaporation pressure, which is defined at the exit of the evaporator

It should be noted that the term "expansion valve" is no longer strictly correct, since no expansion takes place. Also, in Table 2 the pressure drop is minimal over the expansion valve. The pressure before the fixed restrictions can. without the occurrence of flashing before the fixed restrictions, vary between 17.0 bar and 7.74 bar (the saturated pressure at 7°C), and consequently the capacity can be varied correspondingly, it should also be noted that the subcooling can be achieved with any of the devices described above. in the above example, the superheat can be used to control the expansion valve. In short, if the load decreases so does the superheat, which causes the expansion valve to close. This increases the pressure drop over the expansion valve, and thus decreases the pressure before the fixed restric- tions (still within the interval above), which lowers the capacity of the evaporator in order to meet the decreased load. The use of superheat as a control parameter can also be applied to the examples below. This parameter is then used to control the expansion valve or, where appropriate, the pump.

Table 3 exemplifies the second embodiment, in which a pump is arranged downstream the condenser. As mentioned earlier the expansion valve can be removed if the pump can supply a variable pressure. Also, a subcooler can be added.

Table 3.

Table 3. cont.

♦ refers to the fact that the pressure is slightly higher before the evaporator than the evaporation pressure which is defined at the exit of the evaporator.

The use of a subcooier 13 has certain advantages in conjunction with a pump. It can increase the thermodynamic efficiency of the system and moreover, it can improve the operation of the pump 14 as pumps sometimes have difficulties in pumping saturated liquids. Therefore, in a third embodiment, a subccoier 13 according to the first embodiment is arranged upstream of a pump 14 according to the second embodiment. This results in the stated advantages.

The restrictor can be either one restrictor or a number of restrictors each feeding one or more channels. In the later configuration the distribution will be improved, as previoυsiy described.

The invention thus relates to a method and a system for improved distribution of an expanding liquid in a heat-exchanger system by changing one or more intensive variables for a refrigerant flowing in a heat-exchanger system. It should be pointed out that the above described embodiments used as standalone solutions can be combined.

The invention makes it possible to control the refrigerant flow (capacity) by changing the distribution of the pressure drop between a valve and a restrictor and not by changing the flow by means of a variable cross section in a valve. It should also be noted that the inventive concept has bearing on several applications where a number of parallel channels are to be fed by a two- phase flow, such applications including unit coolers, etc.

Further, it should be understood that a small amount of vapour, smaller than what is used in prior art systems, after the pressure altering means will not compromise the inventive concept and is included in the scope of the claims.

Claims

1. A method for distribution of an expanding liquid in a heat-exchanger system having a circuit comprising the components: a condenser (1), pressure altering means (2; 14). an evaporator (3) containing a distribution header (5) fluidly connected via restrictors (9-12) to several fluid channels (6) coupied in parallel, and a compressor (4), each component having an inlet and an outlet, wherein the outlet of the condenser (1) is connected to the inlet of the pressure altering means, the outlet of the pressure altering means is connected to the inlet of the evaporator (3), the outlet of the evaporator (3) is connected to the inlet of the compressor (4), and the outlet of the compressor (4) is connected to the inlet of the condenser (1). said circuit containing a refrigerant and said method comprising the steps of:

(a) lowering the temperature of the refrigerant downstream of the condenser and upstream the pressure altering means, and/or

(b) changing the pressure of the refrigerant using means capable of increasing the pressure of the refrigerant downstream of the condenser, so as to keep the refrigerant from flashing until it passes the restrictors

{9-12}, thereby obtaining a one-phase liquid refrigerant flow in the distribution header (5).

2. The method of claim 1 , wherein said pressure altering means comprise a pump (14) and said step of changing the pressure is performed by said pump (14).

3. The method of claim 1 , wherein said pressure altering means comprise a valve (2) and said step of changing the pressure is performed by said valve (2).

4. The method of claims 2 and 3. wherein said pump (14) is arranged upstream of an additional pressure altering means which comprises a valve

(2).

5. The method of claim 2 or 4. wherein the pressure generated by said pump (14) is regulated by a motor of variable speed.

6. The method of any preceding claims, wherein the step of changing the temperature of the refrigerant is performed by a subcooler (13).

7. The method of claim 6, wherein the subcooler (13) performs the cooling by causing part of the refrigerant to by-pass the heat-exchanger circuit.

8. The method of claim 7, wherein the subcooler (13) is a separate section of the evaporator (3).

9. The method of claim 6 or 7, wherein the subcooler (13) comprises an individual circuit, separate from the heat-exchanger circuit.

10. The method of any preceding claim, wherein said several fluid channels (6) in the heat-exchanger system form part of a plate heat ex- changer.

11. A heat exchanger system having a circuit comprising the components: a condenser (1), pressure altering means (2; 14), an evaporator (3) and a compressor (4), each component having an inlet and an outlet, wherein the outlet of the condenser (1) is connected to the inlet of the pressure alter- ing means (2; 14), the outlet of the pressure altering means (2; 14} is connected to the inlet of the evaporator (3), the outlet of the evaporator (3) is connected to the inlet of the compressor (4), and the outlet of the compressor (4) is connected to the inlet of the condenser (1). said circuit containing a refrigerant and said system comprising: means for lowering the temperature of the refrigerant downstream of the condenser (1) and upstream of the pressure altering means (2: 14), and/or means for changing the pressure comprising means capable of increasing the pressure of the refrigerant downstream of the condenser (1), c h a r a c t e r i s e d in that the evaporator (3) comprises a distribution header (5) fluidly connected via restrictors (9-12) to several fluid channels (6) coupled in parallel, and that said means for lowering the temperature of the refrigerant and/or said means for changing the pressure of the refrigerant keep the refrigerant from flashing until it passes the restrictors (9- 12), resulting in a one-phase liquid refrigerant flow in the distribution header (5).

12. The system of claim 11. wherein said pressure altering means comprise a pump (14) capable of changing the pressure.

13. The system of claim 11, wherein said pressure altering means comprise a valve (2) capable of changing the pressure.

14. The system of claim 12 or 13, wherein said pump (14) is arranged upstream of an additional pressure altering means which comprises a valve (2).

15. The system of claim 11 or 12, wherein the pressure generated by said pump (14) is regulated by a motor of variable speed.

16. The system of any one of claims 11-15, wherein the step of changing the temperature of the refrigerant is performed by a subcooler (13).

17. The system of claim 16, wherein the subcooler (13) performs the cooling by causing part of the refrigerant to by-pass the heat-exchanger cir- cuit.

18. The system of claim 17, wherein the subcooier (13) is a separate section of the evaporator.

19. The system of claim 16 or 17. wherein the subcooler (13) comprises an individuai circuit, separate from the heat-exchanger circuit.

20. The system of any one of claims 11-19, wherein said several fluid channels (6) in the heat-exchanger system form part of a plate heat exchanger.

21. The system of any one of claims 11-19, wherein said several fluid channels (6) in the heat-exchanger system form part of a unit cooler.