DE69508787T2 - VAPOR COMPRESSION SYSTEM AND METHOD FOR OPERATING THE SAME - Google Patents

Abstract

A vapour compression system in which a quantity of a refrigerant circulates between at least two pressure levels in a condenser and an evaporator respectively, comprises a compressor (1) for increasing the pressure of refrigerant vapour; a condenser (5) for high pressure refrigerant vapour received from the compressor; an expansion device (13) such as a valve across which the pressure differential between the condenser and the evaporator is maintained, to control the withdrawal of liquid refrigerant from the condenser according to the volume of liquid refrigerant that is within or behind it; an evaporator (15) for liquid refrigerant received from the condenser; a receiver (21) into which refrigerant is discharged from the evaporator, with a vapour withdrawal conduit (25) through which vapour is withdrawn from the receiver for supply to the compressor, the receiver including a reservoir (23) into which liquid refrigerant discharged from the evaporator collects, to control supply of liquid refrigerant to the compressor; a liquid withdrawal conduit (27) through which liquid refrigerant is supplied from the reservoir into the vapour withdrawal conduit; and means (35) for controlling the rate of removal of liquid refrigerant from the reservoir in proportion to the amount of refrigerant that is removed from the receiver as vapour. The system can ensure that the wetness of the refrigerant discharged into the receiver from the evaporator is controlled to ensure that it is wet under normal operating conditions of the system, to optimise use of heat exchange surfaces of the condenser and the evaporator.

Description

The present invention relates to vapor compression systems such as those used in air conditioning, refrigeration and heat pumps, and to components of vapor compression systems such as condensers, evaporators and expanders. The invention is concerned with issues of control of such systems and components. The systems of the invention are suitable for use with mixtures of mutually soluble refrigerant substances having different boiling points (such that the mixture boils or condenses over a range of temperatures) and may enable energy savings attributed to the use of such mixtures to be achieved.

Conventional vapor compression systems include an evaporator, a condenser, and a compressor to increase the pressure of the refrigerant vapor from that prevailing in the evaporator (where the refrigerant absorbs heat) to that prevailing in the condenser (where the refrigerant loses heat). Condensed liquid refrigerant is fed from the condenser to the evaporator through an expansion device that maintains the pressure differential between the condenser and the evaporator and regulates the flow of refrigerant through the system. In many applications, the components of such systems are assembled into integrated sealed units.

The patent US-A-1884186 (Peltier) describes such a system in which a quantity of a coolant circulates between at least two pressure levels in a condenser or an evaporator, with

a compressor for increasing the pressure of the refrigerant vapor;

a condenser for high-pressure refrigerant vapor received from the compressor;

an expansion device for maintaining the pressure difference between the condenser and the evaporator to control the withdrawal of liquid refrigerant from the condenser according to the volume of liquid refrigerant in or behind it;

an evaporator for liquid refrigerant received from the condenser;

a receiver into which refrigerant flows from the evaporator, the receiver including a reservoir in which liquid refrigerant flowing from the evaporator is collected;

a vapor extraction line through which vapor is extracted from the receiver for supply to the compressor; and

a liquid discharge line through which liquid coolant is withdrawn from the reservoir and fed into the vapor discharge line in the case of a venturi device.

In particular, when a vapor compression system is required to cool one fluid over a range of temperatures while rejecting heat to another fluid which is heating over a range of temperatures, the efficiency of the system can be increased by using a refrigerant consisting of two or more mutually soluble substances which do not form an azeotropic mixture and therefore can condense or boil over a range of temperatures. The normal boiling points of the two substances are separated by about 15 to 60°C. By appropriate selection of substances for the mixed refrigerant, the changing boiling point of the mixed refrigerant as it condenses can be arranged to closely follow the temperature of the fluid being heated in the condenser over the length of the condenser, with the refrigerant and a heat transfer fluid flowing in countercurrent relationship to each other. Similar considerations apply to the evaporator. Consequently, less energy is required to drive the compressor because the pressure ratio required by the compressor is reduced.

For effective operation of a mixed refrigerant system, it is desirable that the relative proportions of the various components of the mixture remain substantially constant throughout the system. It is also preferred that the two phases of the refrigerant flow through at least the evaporator and the condenser in cocurrent so that the separate phases are well mixed and an effective mixture is produced between the phases. This condition may be referred to as equilibrium evaporation or liquefaction. It may arise, for example, when liquid and vapor are flowed in cocurrent with vapor flowing down the bore of the channel and liquid flowing along the walls of the evaporator or condenser effectively as a film of variable thickness around the flowing vapor. The equilibrium conditions of evaporation or condensation are preferably maintained over substantially the entire length (as the case may be) of the evaporator or condenser. This can be difficult to achieve because the phase change is accompanied by a large volume change which affects the flow condition of the two phases.

In WO-A-92/06339 a vapour compression system is disclosed which includes a two section evaporator which discharges refrigerant into a receiver at low pressure. The subject matter disclosed in that document is incorporated by this reference into the description of the present application. The first (or main) section of the evaporator receives liquid from the condenser through an expansion device and discharges refrigerant vapour together with a small amount of liquid into the receiver at low pressure from which vapour is supplied to the compressor. Liquid from the receiver is supplied to the second section of the evaporator and ensures that under steady state operating conditions of the system the discharge from the first section of the evaporator remains wet. The system includes as an expansion device a modulating float valve which opens when the amount of liquid in or behind it exceeds a predetermined level, the force required to open the valve being substantially independent of the pressure drop across it. The valve ensures that liquid accumulates in the condenser and is continuously supplied to the evaporator.

The system disclosed in WO-A-92/06339 operates satisfactorily, allowing the heat exchange surfaces in both the evaporator and the condenser to be used optimally, regardless of the performance required by the system. This allows the energy consumption of the compressor to be reduced. It has been shown that the energy saving benefits of mixed refrigerants (long considered possible) can be realised.

A two section evaporator, such as that contained in the system disclosed in WO-A-92/06339, can be complicated, particularly when liquid refrigerant must be distributed to an array of separate tubes fed from the receiver in order to maintain an appropriate flow rate of refrigerant through the second section of the evaporator. The distribution should be such that every tube in the array remains active even when the load on the system is low.

According to the present invention, it has been found that in a closed system, the humidity of a refrigerant flowing out of an evaporator into a receiver can be ensured by the controlled, continuous withdrawal of a small amount of liquid refrigerant from the receiver in proportion to the amount of refrigerant withdrawn from the receiver as vapor in order to control the humidity of the refrigerant flowing out of the evaporator into the receiver to ensure that it is humid under normal operating conditions of the system.

The invention accordingly provides in one aspect a method of operating a vapor compression system in which a quantity of a refrigerant is circulated between at least two pressure levels in a condenser and an evaporator, respectively, wherein the refrigerant consists of two or more mutually soluble refrigerant substances which do not form an azeotropic mixture, comprising:

(a) a compressor for increasing the pressure of a refrigerant vapor;

(b) the condenser for high pressure refrigerant vapour received from the compressor;

(c) an expansion device for supplying liquid refrigerant from the condenser to the evaporator at the rate at which it is produced in the condenser, the device maintaining the pressure difference between the condenser and the evaporator to control the withdrawal of liquid refrigerant from the condenser according to the volume of liquid refrigerant that is in or behind the expansion device;

(d) the evaporator for liquid refrigerant received from the condenser; and

(e) a receiver into which refrigerant flows from the evaporator, the receiver containing:

a reservoir for collecting liquid refrigerant discharged from the evaporator to control the supply of liquid refrigerant to the compressor;

a vapor extraction line through which refrigerant vapor is extracted from the receiver for supply to the compressor; and

a liquid discharge line through which liquid coolant is withdrawn from the reservoir and fed into the vapor discharge line for supply to the compressor,

the method comprising: supplying a wet refrigerant to the compressor and controlling the rate of withdrawal of liquid refrigerant from the receiver in proportion to the amount of refrigerant withdrawn from the receiver as vapor to control the humidity of the refrigerant exiting the evaporator into the receiver to ensure that substantially all of the heat exchange surface of the evaporator remains wet under normal operating conditions of the system.

It has been demonstrated by the present invention that the removal of a controlled small amount of liquid coolant from the receiver at a constant rate can result in a controlled low level of moisture in the effluent from the evaporator, which in turn can result in substantially the entire heat exchange surface of the evaporator remaining moist. Accordingly, a high heat transfer coefficient can be maintained as a result of heat transfer along the entire length of the evaporator, substantially independent of the load applied to the system and the composition of the coolant. The rate of liquid removal from the receiver is preferably such that the moisture of the coolant effluent from the evaporator is no more than about 5% by weight, more preferably no more than about 3.5% by weight, especially no more than about 2.5% by weight, e.g. between about 1 and 2% by weight.

The invention can also ensure that the liquid content in the coolant supplied to the compressor is controlled so that the amount of liquid can be increased without noticeable fluctuations. The humidity of the refrigerant supplied to the compressor may be similar to the humidity of the refrigerant flowing from the evaporator to the receiver. However, in many circumstances the two humidities are different. The differences between the two humidities may arise from, for example, evaporation of some of the liquid refrigerant removed from the receiver. The differences may also be compensated by the receiver.

In another aspect, the invention provides a vapor compression system in which a quantity of a refrigerant is circulated between at least two pressure levels in a condenser and an evaporator, respectively, comprising:

(a) a compressor for increasing the pressure of the refrigerant vapor;

(b) the condenser for high pressure refrigerant vapour received from the compressor;

(c) an expansion device for supplying liquid refrigerant from the condenser to the evaporator at the rate at which it is produced in the condenser, the device maintaining the pressure difference between the condenser and the evaporator to control the withdrawal of liquid refrigerant from the condenser according to the volume of liquid refrigerant that is in or behind the expansion device;

(d) the evaporator for liquid refrigerant received from the condenser;

(e) a receiver into which refrigerant flows from the evaporator, the receiver including a reservoir in which liquid refrigerant flowing from the evaporator is collected to control the supply of liquid refrigerant to the compressor;

(f) a vapour extraction line through which vapour is extracted from the receiver for supply to the compressor;

(g) a liquid withdrawal line through which liquid refrigerant is withdrawn from the reservoir into the vapor withdrawal line for supply to the compressor; and

(h) means for controlling the rate of withdrawal of liquid coolant from the reservoir in proportion to the amount of coolant withdrawn from the receiver as vapour so that the refrigerant supplied by the control device to the compressor has a composition such that the refrigerant subsequently discharged from the evaporator into the receiver is moist under normal operating conditions of the system.

The collecting vessel is preferably arranged so that liquid coolant contained in the reservoir is retained in the reservoir when the system is switched off.

Control of the flow of liquid refrigerant from the receiver can be achieved using a liquid withdrawal line through which liquid is supplied to the vapor supply line from the receiver to the compressor due to a pressure drop along the vapor withdrawal line downstream of the reservoir. Control can be achieved using a receiver into which refrigerant flows from the evaporator and which contains:

- a reservoir for liquid coolant,

- a vapor extraction line through which refrigerant vapor is delivered from the receiver to the compressor, and

- a liquid discharge line through which liquid coolant is supplied from the reservoir to the vapor discharge line,

the vapor withdrawal line being arranged so that the pressure of vapor flowing therein at a point downstream of the reservoir is reduced with respect to the pressure in the reservoir so that liquid refrigerant in the reservoir is drawn through the liquid withdrawal line into the vapor withdrawal line. A system incorporating such a receiver with means for withdrawing a controlled amount of liquid refrigerant has the advantage that appropriate control of the humidity of the refrigerant supply to the receiver can be achieved without having to include a two section evaporator. This enables energy savings to be achieved from the use of mixed refrigerants, whilst also minimising plant costs by avoiding the use of certain complicated multi-tube heat exchanger designs. By appropriately designing the flow resistance of the vapor and liquid withdrawal lines, including their arrangement with respect to the reservoir, it is possible to ensure ensure that, during steady-state operation of the system, the liquid supplied from the receiver to the vapor exhaust line (for supply to the compressor) is such that the refrigerant discharged from the evaporator to the receiver has a suitably low moisture content. Such operation of the system involves optimum use of the heat exchange surfaces of the evaporator and can enable the benefits of reduced energy consumption from the use of mixed refrigerants to be realized.

A further advantage arising from the use of the receiver, as referred to above, is that the optimized use of the heat exchange surfaces of the evaporator is achieved without deterioration of the control due to accumulation of compressor oil. This is in contrast to the system disclosed in WO-A-92/06339, in which compressor oil can easily accumulate excessively in the second evaporator section, particularly if the velocity of the refrigerant in the second section drops too low, which can happen if the tubes in the second section are not appropriately branched. Such accumulation of oil can cause operating instability, particularly when the power required of the system is reduced or when the system is restarted after a temporary shutdown.

The amount of liquid coolant withdrawn from the reservoir is controlled so that it is withdrawn at a substantially constant rate. The rate at which liquid coolant is withdrawn from the reservoir is preferably determined in relation to the amount of coolant withdrawn as vapour; this can be achieved by so-called metering devices. Details of such devices are set out below.

Provided that the flow rate of liquid refrigerant is substantially constant, it has been found that the amount of liquid that must be withdrawn from the reservoir and supplied to the compressor to support adequate wet discharge from the evaporator need not give rise to mechanical difficulties in the operation of the system or adversely affect the efficiency of the compressor.

The receiver is preferably arranged so that no more than about 4% by weight of the compressor flow rate of coolant passes through the liquid discharge line, with the remainder flowing through the vapor discharge line. More preferably, the liquid discharge line carries no more than about 3% by weight of the compressor flow rate. The liquid discharge line preferably carries at least about 0.5%, more preferably at least about 1% of the compressor flow rate. For example, the receiver may be arranged so that about 2% by weight of the compressor flow rate of coolant passes through the liquid discharge line.

The junction between the vapor and liquid withdrawal lines is preferably at a level that is at or slightly above the level of liquid coolant in the reservoir when the system is in steady state operation. This has the advantage of reducing the tendency of liquid coolant to leak into the vapor withdrawal line during a temporary shutdown of the system. The junction level should preferably be only slightly above the steady state liquid level so that the system delivers approximately the same ratio of liquid injected into the liquid withdrawal line over a range of performance.

The opening for vapor entry into the vapor exhaust line for supply to the compressor is preferably above the level of liquid refrigerant in the reservoir and preferably at or toward the top of the reservoir. This arrangement has the advantage of reducing the tendency for liquid refrigerant to be drawn with refrigerant vapor from the reservoir or the effluent from the evaporator or both and transferred to the compressor suction pipe. In this arrangement, the vapor exhaust line generally includes a section which descends to a level below the liquid level in the reservoir when the system is operating.

The opening for liquid to enter the liquid discharge line to flow to the steam feed line is preferably located near the lower part of the reservoir, more preferably in the bottom of the reservoir, so that from the reservoir voir continuously draws fluid even when the fluid level in the reservoir is low.

The opening from the liquid discharge line into the vapor discharge line preferably discharges liquid coolant into the vapor discharge line at a point that is at least about one-quarter of the distance across the vapor discharge line, more preferably at least about one-third of that distance. This has the advantage of assisting in the distribution of the liquid coolant into the vapor in droplet form.

The cross-sectional area of the vapor discharge line may be larger at a point downstream of the junction with the liquid discharge line than at a point upstream of that junction so that the total pressure drop in the compressor suction pipe is minimized.

The change in the cross-sectional area of the vapor extraction line may be associated with a restriction in the line. The restriction may be such that a venturi is provided in the vapor extraction line. The venturi is preferably mounted horizontally with its centerline at about the normal liquid level in the reservoir when the system is operating in a steady state. It has been found that injection of a small amount of liquid refrigerant into the stream of refrigerant vapor at the throat of a venturi does not appreciably adversely affect pressure recovery. As a result, the pressure drop in the vapor line between the receiver and the compressor suction pipe remains low, providing energy efficient operation. An arrangement utilizing a venturi may result in liquid refrigerant being withdrawn from the reservoir in proportion to the amount of refrigerant withdrawn as vapor.

The liquid withdrawal line may comprise an n-shaped part with two legs and a connecting part extending between them, in which liquid is drawn from the reservoir and flows from the reservoir initially upwards along a first leg and downwards along the other leg to the junction with the vapor flow line, thus acting as a siphon. The height of the n-shaped part of the liquid withdrawal line above the normal The level of liquid refrigerant in the reservoir is selected so that the n-shaped portion is at least as high as the highest expected level of liquid that will be contained in the reservoir at any time during operation of the system to ensure that liquid refrigerant does not leak from the reservoir to the compressor, particularly at shutdown. The second leg for downward flow preferably includes the resistive line for capillary flow. An arrangement in which the liquid withdrawal line includes an n-shaped portion can result in liquid refrigerant being withdrawn from the reservoir in proportion to the amount of refrigerant withdrawn as vapor.

The reservoir into which coolant flows from the first evaporator section is generally arranged so that coolant collected therein has a large surface area. The surface area of the liquid coolant may, for example, be at least about twice the square of the height of the reservoir, preferably at least about three times the square of that height. This has the advantage that a change or fluctuation in the amount of liquid coolant contained in the reservoir does not appreciably affect the depth of the liquid and foaming of the coolant in the reservoir is less likely to result in liquid coolant being supplied to the compressor. This enables a appreciable gap to be maintained between the upper surface of the collected liquid coolant and the outlet through which vapor is supplied to the compressor, thus minimizing and preferably avoiding the possibility of liquid coolant being supplied to the compressor in large quantities under all possible operating conditions.

It is particularly preferred to use a vapor extraction line having a venturi device provided therein by a constriction together with a liquid extraction line containing an n-shaped part as described above. The venturi device may cause a significant pressure difference between the reservoir and the outlet from the vapor extraction line at the junction with the liquid extraction line. The pressure difference may be arranged so that liquid coolant passes the first angle of the n-shaped portion of the liquid discharge line at a rate suitable to keep the effluent from the evaporator moist as discussed above. This arrangement has the advantage of reduced energy loss due to frictional effects in the vapor discharge line because the pressure loss associated with accelerating the vapor through the throat of the venturi can be substantially recovered in the diverging diffuser section.

The liquid withdrawal line is designed to have a total pressure drop when liquid coolant is supplied at the desired flow rate equal to the total pressure drop in the outlet vapor line between the receiver and the point of liquid injection into the liquid line. This can be achieved by selecting the arrangements of the vapor and liquid withdrawal lines so that the pressure drop in the vapor withdrawal line between the reservoir and the junction with the liquid withdrawal line provides a controlled liquid flow along the liquid withdrawal line from the reservoir to the junction in proportion to the suction flow rate of the vapor to the compressor. The selection includes parameters such as cross-sectional areas and lengths of the lines between the junction between them and the reservoir. Accordingly, the line can be in the form of a capillary tube with a small cross-section along at least part of its length. Alternatively or additionally, the cross-sectional arrangement of the vapor discharge line may differ between the upstream and downstream parts of the junction with the liquid discharge line.

The vapor extraction line may comprise a U-shaped portion having two legs and a connecting portion extending between them. The upstream leg of the U-shaped portion may then provide a downwardly extending portion of the vapor extraction line from the opening for vapor entry into the vapor feed line for supply to the compressor above the level of the refrigerant liquid. In this arrangement, it is generally preferred that the junction between the vapor extraction line and the liquid liquid discharge line is arranged in the downstream leg of the U-shaped part of the steam discharge line.

The pressure drop in the vapor discharge line between the reservoir and the junction with the liquid discharge line preferably corresponds to a column of liquid coolant of between 45 and 200 mm, more preferably between 65 and 160 mm, in particular between 80 and 130 mm.

The liquid withdrawal line may be arranged so that between an exit into the line from the reservoir and an exit from the line into the vapor withdrawal line, liquid contained therein is arranged in heat exchange relationship with liquid refrigerant flowing out from the condenser so as to be heated by the liquid refrigerant. In this construction, the liquid withdrawal line includes a restriction therein by which the flow of refrigerant is controlled along the line. It is preferred that this flow of refrigerant flows generally upwards while being in heat exchange relationship with the condensate.

Examples of materials suitable for use as refrigerants in a single refrigerant system include those designated by the trademarks R22 and R134a. A particular advantage of the system of the invention is that it is well suited to the use of a wide boiling, non-azeotropic mixed refrigerant, wherein it is particularly desirable that at all locations within the condenser and evaporator liquid and vapor refrigerants flow together in cocurrent fashion and be in equilibrium, while the mixed refrigerant flows in substantially countercurrent relationship with the fluid with which it exchanges heat. This object can be achieved by the system of the invention, particularly when it also includes an expansion valve where the force required to open it is substantially independent of the pressure drop across it. The vapor compression system of the invention therefore enables the energy saving that can be obtained from the use of a wide boiling, mixed refrigerant. In addition, further energy saving can be achieved because of the ability of the system of the invention to to adapt to changing power, starting conditions, ambient conditions, etc. while operating at optimum efficiency.

The refrigerant thus preferably consists of two or more mutually soluble refrigerant substances which do not form an azeotropic mixture. Examples of suitable mixed refrigerants include those designated by the trademarks R23/R134a and R32/R227. Suitable mixtures of refrigerant substances have boiling points separated by at least about 10°C, e.g. at least about 20°C. The difference in boiling points is often less than about 70°C, preferably less than about 60°C, e.g. less than about 50°C.

It is understood that the term "refrigerant" used in this document to refer to the fluid circulating in the vapor compression system is applicable to the fluid circulating in systems operating as air conditioning or heat pumps.

The capacity delivered by the vapor compression system is determined by suitably adjusting the flow rate of the refrigerant vapor through the system. This can be achieved in several ways: for example, the throughput of the compressor can be adjusted by adjusting its speed, for example, or by unloading one or more cylinders, or more than one compressor can be provided, some or all of which are used according to the amount of refrigerant to be circulated.

Alternatively, the desired performance can be obtained as needed by selectively switching the compressor on and off.

Control of compressor flow can be in response to a detected temperature change in the medium to be heated or cooled by the system. For example, in a refrigeration system, a temperature sensor can be used to cause the flow of a compressor to increase when a temperature increase is detected in a cold chamber.

If air is used as the heat transfer medium in the condenser or evaporator, or in cases where the performance of the unit varies greatly, fans with variable output power to modulate air flow and conserve energy.

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic illustration of a vapor compression system according to the present invention;

Fig. 2 is a schematic illustration of a collection vessel suitable for use in the vapor compression system shown in Fig. 1;

Fig. 3 is a schematic illustration of another embodiment of a collection vessel;

Fig. 4 is a schematic illustration of another vapor compression system according to the present invention; and

Fig. 5 is a schematic illustration of a vapor compression system in which liquefied refrigerant is divided into two streams which are divided between two evaporators.

Referring to the drawings, Fig. 1 shows a vapor compression system comprising a compressor 1 for increasing the pressure of a refrigerant vapor and forcing the vapor through a first conduit 3 to a condenser 5. The condenser 5 comprises an array of both series and parallel connected condenser tubes 7 attached to a plurality of fins which facilitate heat transfer between a cooling medium flowing over the fins and the refrigerant contained in the condenser tube. The medium could be air, for example, if the system forms part of an air conditioning unit or a refrigerator. The flow directions of the two fluids are substantially opposite to each other so that this arrangement is suitable for mixed refrigerants as well as pure refrigerants.

Refrigerant flows out of the condenser 5 through a valve 13 into a second line 11. A vapor return pipe 14 is provided to ensure that the inlet to the valve 13 is not blocked by vapor. The valve 13 is arranged to open when the amount of liquefied liquid refrigerant behind or within it exceeds a predetermined level. The design of a suitable Valve is disclosed in WO-A-92/06339. A suitable valve is one in which the force required to open it is substantially independent of the pressure drop across it.

The refrigerant from the condenser flows through the valve 13 and the second line 11 to an evaporator 15. The evaporator 15 comprises an array of tubes connected in series and in parallel. It further comprises evaporator fins over which a fluid flows to transfer heat and evaporate the refrigerant. As a result, the fluid is cooled. The fluid could be air, for example, if the cooling system forms part of an air conditioning unit or a refrigerator.

From the evaporator 15, the refrigerant flows into a receiver 21. The liquid refrigerant flowing out of the evaporator collects in the receiver's reservoir 23, which can provide an intermediate storage of the liquid refrigerant. A vapor extraction line 25 extends from the upper end of the reservoir 23 to convey the majority of the refrigerant as vapor (i.e., substantially liquid-free vapor) from the reservoir to the compressor. It will therefore be understood that the refrigerant in the receiver can be separated into liquid and vapor phases.

The receiver contains a liquid withdrawal line 27 through which liquid coolant is supplied from the reservoir into the vapor withdrawal line 25. By selecting its diameter and length, taking into account flow restrictions such as those imposed by bends, the vapor withdrawal line is arranged so that the pressure of the incoming vapor at a point downstream of the reservoir is reduced relative to the pressure in the reservoir, so that liquid coolant in the reservoir is drawn through the liquid withdrawal line 27 into the vapor withdrawal line 25 at a rate proportional to the vapor flow. The pressure drop corresponds approximately to a column of liquid coolant of about 100 mm.

The evaporator, receiver and flow metering device together ensure that the entire evaporator surface is used for heat transfer regardless of the power required by the system. When the system is running, the arrangement of the vapor and liquid withdrawal lines in the receiver ensures that liquid is drawn from the reservoir at a controlled rate. Consequently, the liquid level in the reservoir is more likely to fall. The use of an expansion device which opens when the amount of liquefied liquid refrigerant in or behind it reaches a predetermined level ensures that liquid cannot collect anywhere in the system other than the reservoir because the expansion device ensures that liquid does not collect in the condenser. Liquid is supplied from the condenser to the evaporator at a rate at which it is produced in the condenser. The system will therefore tend to a steady state in which the liquid refrigerant withdrawn from the reservoir through the liquid withdrawal line is exactly compensated by the liquid component of the two-phase refrigerant flowing from the evaporator into the reservoir. The pressure in the evaporator will automatically adjust itself to achieve this equilibrium. This means that the entire evaporator surface must be wet during such steady state operation.

Figures 2 and 3 show more detailed designs of receivers. Referring first to Figure 2, the disclosed design includes a reservoir 31 into which coolant flows from the evaporator through a discharge line 33. The outlet from the discharge line is toward the top of the reservoir 31. The vapor exhaust line 25, from which the coolant vapor is delivered from the reservoir 31 to the compressor, is toward the top of the reservoir.

The vapor extraction line 25 has a U-shaped portion 35 immediately downstream of the reservoir 31. The U-shaped portion has first and second legs 37, 39 and a connecting base portion. The base portion is at a level well below the normal level 41 of the liquid coolant contained in the reservoir 31 when the system is running in a steady state.

The second leg 39 of the U-shaped part of the steam exhaust line is conically widened.

A liquid discharge line 43 runs from the bottom of the reservoir 31 (well below the normal liquid level 41) and is connected to the second leg 39 of the U-shaped The junction between the liquid and vapor exhaust lines 25, 43 is at a point just upstream of the flare in the vapor exhaust line. The opening from the liquid exhaust line into the vapor exhaust line discharges liquid coolant into the vapor exhaust line at a point about one-third of the distance up the vapor exhaust line so that the liquid coolant discharged into the vapor exhaust line atomizes slightly as it flows out.

The liquid withdrawal line 43' is formed by a capillary tube. The diameter and length of the capillary tube are selected so that for a rate of liquid injection into the vapor withdrawal line 25 at the desired ratio (e.g., about 2% by weight of the flow rate of refrigerant through the compressor), the pressure drop across the liquid withdrawal line is equal to the pressure drop in the vapor withdrawal line 25. The junction between the vapor and liquid withdrawal lines is at a level that is at or slightly above the level of liquid refrigerant in the reservoir when the system is in a steady state operating condition.

Fig. 3 shows an alternative construction of the collecting vessel 51. It has a reservoir 53 into which coolant flows from the evaporator through a line 55.

A vapor exhaust line 57 has an opening toward the top of the reservoir for entry of vapor for delivery to the compressor. The vapor exhaust line includes a downward portion and a portion that runs approximately parallel to the surface of the liquid refrigerant contained in the reservoir at about the liquid level when the system is running at a steady state.

A restriction in the steam exhaust line provides a venturi device 59, by which the pressure of the steam in the steam exhaust line is reduced and then increased.

A liquid discharge line 61 is provided in the form of an n-shaped tube, with its opening 63 arranged for an entry of liquid towards the bottom of the reservoir 53.

The opening 65 for the liquid coolant to exit into the vapor exhaust line 57 is arranged in relation to the valve turi device 59 is arranged so that liquid coolant is drawn from the reservoir 53 through the liquid discharge line into the vapor discharge line as a result of the pressure changes imposed on the vapor in the vapor discharge line by the venturi device.

The amount of liquid drawn into the vapor exhaust line is controlled, at least in part, by the dimensions of the venturi device.

Fig. 4 shows a vapor compression system having a compressor 81 for increasing the pressure of a refrigerant vapor and forcing the vapor through a first line 83 to a condenser 85. From the condenser, refrigerant flows through a valve 93 into a second line 91. A vapor return tube may be provided to ensure that the inlet to the valve is not blocked by vapor. The valve is arranged to open when the amount of liquefied liquid refrigerant behind or within it exceeds a predetermined level.

The refrigerant from the condenser flows through the valve 93 and the second line 91 through an evaporator 95. The refrigerant flows out of the evaporator 95 into a receiver 101. The liquid refrigerant flowing out of the evaporator collects in the receiver reservoir 103. A vapor extraction line 105 extends from the top of the reservoir to carry the majority of the refrigerant as a vapor from the reservoir to the compressor.

The receiver contains a liquid withdrawal line 107 through which liquid refrigerant is supplied from the reservoir into the vapor withdrawal line. The liquid line contains a restriction 108 which provides a flow resistance. Liquid refrigerant in line 107 is subjected to heat transferred by liquid refrigerant flowing out of the condenser as it flows generally upwardly through a heat exchanger 109 (so that the entry point into the heat exchanger is lower than the exit point) so that the liquid refrigerant is at least partially vaporized. The refrigerant from the heat exchanger is then injected into the vapor withdrawal line 105 for supply to the compressor.

The valve 93 includes a float 111 exposed to saturated liquid coolant from the condenser and valve orifices 113 exposed to liquid coolant that has been subcooled by passage through the heat exchanger 109. The float 111 and the needles that close the valve orifices are connected by an elongated rod 115 in a tight fitting tube such that the flow of liquid allowed between the float chamber and the valve orifices through the resulting annular passage is negligible. Accordingly, saturated coolant condensate is caused to flow from the bottom of the float chamber through the heat exchanger into the valve body where it expands through the orifices of the valve.

The evaporator, receiver and flow metering device together ensure that regardless of the power required by the system, the entire evaporator surface is effectively used for heat transfer. When the system is running, the arrangement of the vapor and liquid withdrawal lines in the receiver ensures that liquid is drawn from the reservoir at a controlled rate. Consequently, the liquid level in the reservoir is more likely to fall. The use of an expansion device that opens when the amount of condensed liquid refrigerant in or behind it reaches a predetermined level ensures that liquid cannot accumulate anywhere in the system other than the reservoir because the expansion device ensures that liquid does not accumulate in the condenser. The system will therefore tend to a steady state condition in which the liquid refrigerant withdrawn from the reservoir through the liquid withdrawal line is exactly compensated by the liquid component of the two-phase refrigerant discharged from the evaporator into the reservoir. The pressure in the evaporator will automatically adjust itself to achieve this equilibrium. This means that the entire evaporator surface must be wet during such steady state operation.

Fig. 5 shows a system that can allow a temperature change in the evaporator that is significantly larger than that in the condenser, for example by a factor of up to two or more.

The temperature change across the condenser could, for example, be about 10ºC (e.g. between 19 and 29ºC), while the temperature change across the evaporator could be about 22ºC (in two steps of e.g. 27 to 16ºC and 16 to 5ºC).

The system includes a receiver 120 into which liquid flows from the condenser 122. Liquid from the reservoir is split into two streams, one of which supplies refrigerant to a first and a second evaporator 124, 126. The flow of refrigerant into the evaporators is controlled by valves 128, 130.

Refrigerant in liquid and vapor phases flows from the evaporators into respective reservoirs 132, 134, from which refrigerant vapor is withdrawn for supply to the compressor assembly 136. The reservoirs also supply liquid refrigerant in small controlled quantities to the compressor through liquid withdrawal lines 138, 140 which connect the vapor withdrawal lines in the manner described above with reference to Fig. 2 or Fig. 3.

The valves 128, 130 are controlled by level sensors for liquid in the reservoirs.

The compressor unit 136 has two separate compressors operating at high and low pressures respectively. Using two compressors in this manner facilitates operation of the system's two evaporators over different temperature profiles.

The components are designed so that the receiver can hold all of the free refrigerant in the system if the reservoirs are not holding any. The reservoirs are large enough to hold liquid refrigerant in the compressor without foaming.

Claims (27)

1. Vapor compression system in which a quantity of a coolant circulates between at least two pressure levels in a condenser (5) or an evaporator (15), with: (a) a compressor (1) for increasing the pressure of the refrigerant vapor; (b) the condenser for high pressure refrigerant vapour received from the compressor; (c) expansion means (13) for supplying liquid refrigerant from the condenser to the evaporator at the rate at which it is produced in the condenser, the means for maintaining the pressure differential between the condenser and the evaporator to control the withdrawal of liquid refrigerant from the condenser according to the volume of liquid refrigerant in or behind it; (d) the evaporator for liquid refrigerant received from the condenser; (e) a receiver (21) into which refrigerant flows from the evaporator, the receiver containing a reservoir into which liquid refrigerant flowed from the evaporator collects to control the supply of liquid refrigerant to the compressor; (f) a vapor extraction line (25) through which vapor is extracted from the collection vessel for supply to the compressor; (g) a liquid discharge line (27) through which liquid refrigerant is withdrawn from the reservoir into the vapor discharge line for supply to the compressor; and (h) means (35) for controlling the rate of withdrawal of liquid refrigerant from the reservoir in proportion to the amount of refrigerant withdrawn from the receiver as vapor, such that the refrigerant supplied to the compressor by the control means has a composition such that the refrigerant subsequently discharged from the evaporator into the receiver is wet under normal operating conditions of the system. 2. A vapor compression system according to claim 1, in which the collecting vessel is arranged so that the vapor contained in the reservoir liquid coolant is retained in the reservoir when the system is switched off. 3. A vapor compression system according to claim 1 or claim 2, in which the vapor extraction line is arranged so that the pressure of vapor flowing therein at a point downstream of the reservoir is reduced relative to the pressure in the reservoir, and in which the liquid extraction line is arranged so that liquid refrigerant in the reservoir is drawn through the liquid extraction line into the vapor extraction line. 4. A vapor compression system according to claim 3, wherein the arrangement of the vapor withdrawal line and the arrangement of the liquid withdrawal line are selected such that the pressure drop in the vapor withdrawal line between the reservoir and the point at which it is connected to the liquid withdrawal line provides a controlled flow of liquid along the liquid withdrawal line from the reservoir to the connection point. 5. A vapor compression system according to claim 4, in which the cross-sectional shape of the vapor extraction line differs between the parts upstream and downstream of the junction of the liquid extraction line. 6. A vapor compression system according to any one of claims 3 to 5, in which the junction between the vapor and liquid withdrawal lines is at a level which is approximately the level of the liquid coolant in the reservoir or slightly higher when the system is in steady state operation. 7. A vapor compression system according to claim 6, wherein the opening from the liquid discharge line into the vapor discharge line discharges liquid refrigerant into the vapor discharge line at a point at least about one-third of the distance down the vapor discharge line. 8. A vapor compression system according to claim 3, wherein the vapor exhaust line has a restriction therein. 9. A vapor compression system according to claim 8, wherein the restriction creates a venturi restriction. 10. A vapor compression system according to any one of claims 3 to 9, wherein the liquid discharge line comprises an n-shaped part with two legs and a connecting part extending between them, in which liquid from the reservoir and allowing it to flow from the reservoir along a first leg initially upwards and along the other leg downwards to the connection point with the steam flow line. 11. A vapor compression system according to any one of claims 3 to 10, wherein the liquid withdrawal line includes a portion that is a capillary. 12. A vapor compression system according to any one of claims 3 to 11, in which the opening for vapor to enter the vapor exhaust line is located at or towards the upper end of the reservoir, and in which the vapor exhaust line includes a portion which extends downward to a level below the liquid level in the reservoir when the system is in operation. 13. A vapor compression system according to claim 12, wherein the vapor exhaust line includes a U-shaped portion having two legs and a connecting portion extending therebetween, wherein the upstream leg of the U-shaped portion provides the downwardly extending portion. 14. A vapor compression system according to claim 13, in which the junction between the vapor extraction line and the liquid extraction line is in the downstream leg of the U-shaped portion of the vapor extraction line. 15. A vapor compression system according to claim 14, wherein the junction between the vapor extraction line and the liquid extraction line is at approximately the liquid level in the reservoir when the system is in operation. 16. A vapor compression system according to claim 1, wherein the liquid withdrawal line is configured such that liquid contained therein is disposed in heat exchange relationship with liquid refrigerant discharged from the condenser so as to be heated and at least partially vaporized by the liquid refrigerant between an exit into the line from the reservoir and an exit from the line into the vapor withdrawal line. 17. A vapor compression system according to claim 16, wherein the liquid withdrawal line includes a restriction therein for controlling a flow of coolant along the line. 18. A vapor compression system according to claim 16 or claim 17, in which the opening into the liquid withdrawal line for liquid from the reservoir is located in the bottom of the reservoir. 19. A vapor compression system according to any one of claims 16 to 18, in which refrigerant flows generally upwardly during a heat exchange relationship with the condensate. 20. A vapor compression system according to any one of claims 1 to 19, in which the means for controlling the rate of withdrawal of liquid refrigerant from the reservoir is arranged so that the humidity of the refrigerant flowing out of the evaporator into the reservoir is not more than 5%, preferably not more than about 3.5%. 21. A vapor compression system according to any preceding claim, which includes a refrigerant consisting of two or more mutually soluble refrigerant substances that do not form an azeotropic mixture. 22. A vapor compression system according to any preceding claim, including two evaporators arranged to sequentially cool a fluid over successive temperature ranges. 23. A vapor compression system according to claim 22, including respective reservoirs into which refrigerant is drained from the evaporators. 24. A vapor compression system according to claim 23, including valves for controlling fluid flow between the evaporators, the valves being controlled according to the level of liquid refrigerant in the reservoirs. 25. A method of operating a vapor compression system in which a quantity of coolant circulates between at least two pressure levels in a condenser (5) and an evaporator (15), respectively, wherein the coolant consists of two or more mutually soluble coolant substances which do not form an azeotropic mixture, the system comprising: (a) a compressor (1) for increasing the pressure of the refrigerant vapor; (b) the condenser for high pressure refrigerant vapour received from the compressor; (c) an expansion device (13) which supplies liquid refrigerant from the condenser to the evaporator at the rate with which it is produced in the condenser, the device maintaining the pressure difference between the condenser and the evaporator to control the withdrawal of liquid refrigerant from the condenser according to the volume of liquid refrigerant in or behind it; (d) the evaporator for liquid refrigerant received from the condenser; (e) a collecting vessel (21) into which the coolant from the evaporator is allowed to flow, the collecting vessel containing: - a reservoir for collecting liquid refrigerant discharged from the evaporator to control the supply of liquid refrigerant to the compressor; - a vapor extraction line (25) through which coolant vapor is extracted from the collecting vessel for supply to the compressor; and - a liquid discharge line (27) through which liquid coolant is discharged from the reservoir into the vapor discharge line for supply to the compressor, the method comprising: supplying a wet refrigerant to the compressor and controlling the rate of withdrawal of liquid refrigerant from the reservoir in proportion to the amount of refrigerant withdrawn from the receiver as vapor to control the humidity of the refrigerant discharged from the evaporator into the receiver to ensure that substantially all of the heat exchange surface of the evaporator remains wet under normal operating conditions of the system. 26. A method according to claim 25, in which the vapor extraction line is arranged so that the pressure of vapor flowing therein at a point downstream of the reservoir is reduced relative to the pressure in the reservoir so that liquid coolant in the reservoir is drawn through the liquid extraction line into the vapor extraction line. 27. A method according to claim 25, in which the liquid withdrawal line is arranged such that liquid contained therein is arranged in a heat exchange relationship with liquid coolant flowing out of the condenser so that it is heated and at least partially vaporized by the liquid coolant between an outlet from the re servoir into the line and an outlet from the line into the steam extraction line.