BACKGROUND In a typical supermarket cooling plant, cooled liquid refrigerant is distributed to various cooling devices at various locations throughout the store such as cold display cases, sub-zero freezers for storage and display of frozen foodstuffs, and walk-in cold rooms. The spent cold refrigerant vapour is returned to a bank of compressors supplying a condenser which is normally fan cooled to dissipate heat from the compressor exhaust gas to the external environment.

The costs associated with running such plant are high. Various systems have been proposed for increasing plant efficiency. In US 5 383 339 an ice storage unit is coupled to the cooling circuit via an auxiliary condenser to reduce power consumption during peak operating periods or extend the operating capacity of a refrigeration unit during periods of hot weather. The ice storage unit includes an ice tank filled with freezable liquid medium such as water or a mixture of water and ethylene glycol which is capable of forming a pumpable ice/water slurry. The ice tank is coupled to a secondary cooling circuit which includes a compressor, a condenser, an expansion valve and an evaporator, so that the plant may operate as a two-stage refrigeration system when desirable. The secondary circuit is typically run to generate ice slurry at off-peak times. The ice slurry can be pumped through an auxiliary heat exchanger which is connected in parallel with the normal air-cooled condenser to remove heat from the refrigerant of the primary circuit. A'drop-leg'or liquid trap is described which automatically switches from air-cooled condensing (and single stage operation) to ice- cooled condensing in the auxiliary condenser (and two-stage operation) when ice slurry is pumped to the auxiliary condenser.

Whilst such a system may seem to offer energy savings due to the two- stage operation, under certain circumstances the operating efficiency of the plant could be significantly impaired. For example, long liquid refrigerant distribution lines can lead to the formation of vapour bubbles in these distribution lines, which can cause a complete failure of the primary refrigeration system. Furthermore the use of ice store cooling alone may sometimes result in reduced operating efficiency.

Recently, a new generation of refrigerants has become available which have the advantage of greatly improved cycle efficiency. For example, R41 OA is a new high efficiency refrigerant which offers an improvement in cycle efficiency of between 10 and 15% compared with existing refrigerants such as R502 and R404A. However, its use in supermarkets and similar installations is generally rejected as the liquid line pressure can become very high.

The present invention seeks to provide a new and inventive form of cooling plant which is cost effective to install and maintain, which is suitable for use with high efficiency refrigerants, which avoids the occurrence of high pressures in the refrigerant distribution network, and which exhibits generally improved operating efficiency and reliability compared with known systems, resulting in significantly reduced running costs.

SUMMARY OF THE INVENTION The present invention proposes cooling plant having : - a primary cooling circuit including at least one compressor and an externally cooled heat exchanger arranged to supply refrigerant to cooling means which comprises at least one cooling device; and - a secondary cooling system of the vapour compression type which is thermally coupled to the primary cooling circuit via heat exchange means to cool the refrigerant of the primary cooling circuit; characterised in that the primary cooling circuit includes a circulation pump arranged to supply cooled refrigerant to said cooling means, and a bypass line is provided to return a proportion of the cooled refrigerant from the output of the circulation pump to the heat exchange means before it supplies said cooling means.

Such recirculation substantially reduces the risk of vapour bubbles forming in the supply lines to the cooling means, especially when there is a reduced cooling demand, and permits lowered operating pressure in the refrigerant distribution network.

Further improvement may be achieved if the distribution pipes and the return pipes serving the cooling devices are disposed in a thermally controlled environment defined by a common thermal insulation jacket.

The invention further provides cooling plant having: - a primary cooling circuit including at least one compressor and an externally cooled heat exchanger arranged to supply refrigerant to cooling means which comprises at least one cooling device; and - a secondary cooling system of the vapour compression type which is thermally coupled to the primary cooling circuit via heat exchange means to cool the refrigerant of the primary cooling circuit; characterised in that the heat exchange means includes a first direct or indirect heat exchanger arranged in series with the externally cooled heat exchanger to receive cooled refrigerant therefrom.

Such an arrangement may exhibit improved efficiency. Underwarm ambient conditions the heat exchange means acts as the principal condenser for the primary refrigerant, but when the ambient air temperature falls the externally cooled heat exchanger progressively removes the load on the secondary cooling system, until condensation occurs in the externally cooled heat exchanger.

In one preferred embodiment the plant includes means for thermally uncoupling the secondary cooling system from a first indirect heat exchanger such that cooling of the liquid refrigerant in the first indirect heat exchanger stops and the externally cooled heat exchanger operates as a condenser.

The heat exchange means preferably includes a second indirect heat exchanger which acts as a sub-cooler. The use of two such indirect heat exchangers ensures that the liquid refrigerant output is sub-cooled under all operating conditions. Preferably a refrigerant receiver is coupled between the first and second indirect heat exchangers to contain liquid refrigerant and refrigerant vapour, and the second indirect heat exchanger is supplied with liquid refrigerant from the receiver.

In another preferred embodiment a refrigerant receiver/flash chamber is connected to the output from the externally cooled heat exchanger of the primary cooling circuit to contain liquid refrigerant and refrigerant vapour, and the heat exchange means comprises a secondary vapour compression system in which one or more compressors draw vapour from the liquid receiver/flash chamber, compress the vapour, which passes to an externally cooled condenser from which the liquid is fed through an optional indirect heat exchanger where it is cooled by vapour leaving the receiver/flash chamber, to boost the cycle efficiency, and then to an expansion device, the cold vapour from which cools the exhaust stream from the compressor of the primary circuit by direct heat exchange (mixing) in the receiver.

BRIEF DESCRIPTION OF THE DRAWINGS The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figures 1 to 3 are schematic diagrams of three forms of supermarket cooling plant in accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS The supermarket cooling plant shown in Fig. 1 has a primary cooling system which is preferably charged with a high pressure, high efficiency refrigerant such as R410A. The cooling system supplies a range of cooling devices commonly found in large stores such as a walk-in cold room 18 and banks of display cabinets 15,16 and 17. It will be understood that each of the devices is supplied with refrigerant which flows through a solenoid valve, an expansion device then the evaporator coils. Spent low pressure, low temperature refrigerant vapour passes through return pipes 20 to a bank of compressors 1 which deliver high pressure, high temperature refrigerant vapour to an externally-cooled heat exchanger 4 which in this example is cooled by ambient air using electric motor-driven fans 3. The external cooling could be provided by other means such as water or any other coolant fluid. The cooled refrigerant then passes through a first indirect heat exchanger 6 following which the liquid condensate is collected in a liquid receiver 8 which holds the refrigerant in liquid and vapour phase to determine the system pressure. Liquid refrigerant is removed from the receiver and passes through a second indirect heat exchanger 10 following which a circulation pump 12 assists the distribution of refrigerant through a network of distribution pipes 13 to the cooling devices 15-18.

A bypass line 19 is connected to the input of the final cooling device in each branch of the distribution pipe network to return a proportion of the cold liquid refrigerant to the input of the second indirect heat exchanger 10.

Thus, the pump 12 maintains a continuous circulation of refrigerant around the loop 13,19, 10 with most of the liquid normally passing through the distributed solenoid valves of the devices 15-18. The precise pointto which the recirculated liquid refrigerant is returned is relatively unimportant. It could, for example, be returned to the receiver 8, or any other point between the compressors 1 and the second indirect heat exchanger 10.

It is also important to note that an advantage may be gained if the distribution pipes 13, the return pipes 20 and the bypass lines 19 are run together in a common sheath of heat-insulation material 14 wherein there is provided a common temperature environment.

The plant includes a thermal storage unit 23, which in this example is an ice tank filled with a freezable mixture of water and ethylene glycol capable of forming a pumpable ice slurry. Other pumpable thermal storage media could be used. When cost or load considerations dictate a pumpable ice slurry is generated by one or more secondary cooling circuits 21 of the vapour compression type. Since the secondary cooling circuits are of known form they are not shown in detail. The or each secondary cooling circuit includes a compressor which supplies compressed hot refrigerant vapour to a condenser in which the refrigerant is cooled and condensed to a liquid.

The refrigerant then passes through an expansion device followed by evaporator coils from which spent refrigerant vapour returns to the compressor. The thermal storage fluid is removed from the ice store 23 via pipes 32 and cooled by the evaporator coils in unit 21 before being returned to the ice store via pipes 22. Ice slurry stored in the ice store 23 can be circulated through the indirect heat exchanger 6 by means of pump 25, thereby cooling the refrigerant flowing through the indirect heat exchanger in the primary circuit. The ice slurry is returned to the ice store via return pipe 27.

Similarly ice slurry could also be pumped through indirect heat exchanger 10 to further cool the liquid refrigerant of the primary circuit leaving the receiver 8, using pipework and a pump, not shown. However the preferred method of further cooling the liquid refrigerant of the primary circuit using indirect heat exchanger 10 is to bleed off liquid refrigerant from the pipe 9 using pipe 60, passing this bleed flow through expansion device 61 to create cold vapour which flows to indirect heat exchanger 10 through pipe 62, and flows from indirect heat exchanger 10 through pipe 63 to enter pipe 20 which returns refrigerant vapour to the suction port of compressors 1. The rate of flow of the bleed flow around loop 60,61, 62,10, 63 is controlled thermostatically using temperature sensor 11 in the liquid discharge line 13 from indirect heat exchanger 10 and sensor 64 in the vapour discharge line 63. Pipes 60 and 63 could be connected to any other vapour compression system to provide cooling in indirect heat exchanger 10.

The heat transfer processes occurring in the air-cooled heat exchanger 4 and the first indirect heat exchanger 6 depend, amongst other things, on the relative temperatures of the compressor exhaust vapour and the external air.

For a given compressor exhaust vapour temperature and flow rate, when the external air temperature is low enough complete condensation of the exhaust vapour may occur in the air-cooled heat exchanger 4. At higher ambient air temperatures the air-cooled heat exchanger may serve as a de- superheater, removing some of the sensible heat from the superheated compressor exhaust vapour, with condensation occurring in the first indirect heat exchanger 6, where cooling is supplied by ice slurry circulating from the ice store 23 using pump 25. Thus heat exchanger 4 may serve as either a de-superheater or a condenser or both.

The liquid condensate leaving the receiver 8 is then sub-cooled by the second indirect heat exchanger 10.

If the external air temperature falls to the point at which condensation is accomplished in the air-cooled heat exchanger4 and the temperature of the condensate leaving heat exchanger 4 is close to the temperature of the ice slurry circulating through indirect heat exchanger 6, then most of the cooling load on the thermal storage system is removed. Therefore, to a large extent the system is self-regulating. However, in such circumstances it is also advantageous to uncouple the ice store from the first indirect heat exchanger 6, conveniently achieved by stopping the pump 25, which effectively converts the system to single stage operation. When pump 25 is turned off refrigerant fluid still flows through the indirect heat exchanger 6, which now acts as part of the pipework rather than a heat exchanger. When the first indirect heat exchanger 6 is not operational the primary circuit acts as a conventional single stage system with sub-cooling. Hot refrigerant vapour is supplied to the air-cooled heat exchanger 4, which functions as a condenser to supply liquid refrigerant to receiver 8, sub-cooler 10, circulating pump 12, cold room 18 and the display cabinets 15-17. The point at which it becomes advantageous to turn off the pump 25 depends upon the aims of the system operator. If the principle aim is to minimise energy consumption the pump 25 is shut down when the energy consumption of the ice generator 21 and pump 25, which is required to meet the cooling demand of the indirect heat exchanger 6, becomes greater than the additional energy requirement of the compressor bank 1 when operating without the cooling effect of indirect heat exchanger 6. If the principle aim is to minimise running costs then the point at which pump 25 is shut down depends upon the relative costs of electrical power at peak and off-peak rates. If ice is generated by the secondary cooling system 21 during off- peak periods then the relative cost of using this ice to reduce the power consumption of compressors 1 running at peak tariff times will determine when pump 25 should be switched off. If the principle aim is to minimise the production of carbon dioxide as a by-product of power generation then pump 25 is switched off when carbon dioxide production associated with the use of the ice generator to meet the cooling load of the indirect heat exchanger 6 is higher than the carbon dioxide production associated with the use of additional compressor power required to operate the system when indirect heat exchanger 6 is inoperative.

At very high external temperatures it may be advantageous to bypass the air cooled heat exchanger 4 to prevent the high external temperatures from actually heating the refrigerant rather than cooling it.

As explained, the pump 12 maintains a circulation of sub-cooled refrigerant around the loop 13,19, 10 at all times, so that, combined with the common temperature environment provided around the recirculation pipes, the risk of boiling of liquid refrigerant in the distribution lines is greatly reduced or even eliminated. The recirculation of refrigerant through the second heat exchanger 10 ensures that a low temperature is maintained in the distribution pipes so maintaining a relatively constant flow rate of liquid in the distribution network.

Pumped recirculation of ice slurry from the ice store 23 may also supply the cooling requirements of an air conditioning system (not shown) and other cooling units, such as milk and vegetable storage cabinets. This can further reduce operating costs.

In regions where there is no off-peak energy tariff there may be no advantage in using a thermal storage medium. In such cases the installation costs and the power consumption can be reduced by eliminating the ice store 23. A preferred way of achieving this is shown in the modified refrigeration plant of Fig. 2 in which the items which have already been described above retain the same reference numbers. In this embodiment the primary cooling circuit is directly coupled to a secondary cooling circuit via common liquid receiver 8 which functions as a flash chamber. The secondary cooling circuit includes compressors 50 which exhaust hot refrigerant vapour to externally cooled condensers 51. Liquid refrigerant flows from condensers 51 through pipe 52 to an optional indirect heat exchanger 53 and then to expansion device 55 from which cold vapour passes to the receiver/flash chamber 8. Refrigerant vapour from chamber 8 is drawn through pipe 54, through optional indirect heat exchanger 53 to the suction port of compressors 50. When optional indirect heat exchanger 53 is used the vapour in suction pipe 54 may be used to cool the liquid supply to expansion device 55, thereby increasing the cycle efficiency.

Indirect heat exchanger 10 sub-cools the liquid from the receiver 8 to assist in preventing the liquid boiling in the distribution pipes. In cases where it is undesirable to mixthe refrigerants of the primary and secondary refrigeration circuits in the receiver 8 then heat exchange between the two circuits can be via the indirect heat exchanger 6 which is connected directly to the secondary refrigeration circuit. As described above, it may become advantageous to thermally decouple the primary and secondary cooling circuits and in the system shown in Fig. 2 this is achieved by switching off compressors 50.

In the examples described thus far the primary and secondary cooling circuits are single stage systems, but it will be appreciated that in any embodiment of the invention the ice generator or secondary cooling circuit could itself be a two stage system. Furthermore, the primary system could also be configured as a two stage system.

In some installations the distribution of normal refrigerants may be undesirable or even prohibited, in which case a secondary cooling loop can be used as shown in Fig. 3. Again, items which have already been described retain the same reference numbers. Instead of being distributed to the cooling devices 15-18 the refrigerant in the primary cooling circuit is circulated through an expansion valve 47 followed by an indirect heat exchanger 40. A pump 45 circulates a cooling fluid such as brine through the indirect heat exchanger 40 in which it is cooled by the primary cooling circuit. The fluid then travels via distribution pipes 42 to the cooling devices 15-18, from which the spent fluid is returned to a buffer store 44 via return pipes 43. The secondary cooling loop could include a second thermal storage unit (not shown), e. g. an ice tank filled with a freezable mixture of water and ethylene glycol or other pumpable thermal storage fluid. In order to prevent boiling of the refrigerant in the primary refrigeration circuit a bypass line 19 is connected to the output of pump12 to return a proportion of the cold liquid refrigerant to the input of the second indirect heat exchanger 10 so that the pump 12 maintains a continuous recirculation of refrigerant. Again, the recirculated refrigerant could be returned to the receiver 8, or any other point between the compressors 1 and the second indirect heat exchanger 10.

Further benefits of the cooling plant may be summarised as follows : 1. The primary cooling system typically operates with a suction temperature to match the application. By ensuring that the condensing temperature can be lower than ambient it is possible to use improved refrigerants with higher pressures and higher efficiencies such as R410A and the following benefits accrue: (a) Reduced and more constant refrigerant mass flow rate allows the use of smaller pipes and eliminates the need for motorised control valves and individual computers for each evaporator. The reduced flow rate, the low temperature of the liquid feed, and superior thermal properties of alternate refrigerants, mean that fewer/smaller compressors are required and these are under less strain. The reduced inventory of refrigerant means that leaks are less damaging and expensive.

(b) Refrigerants with better oil return characteristics improve the efficiency of condensers and evaporators, permitting the suction temperature to be raised, and lowering the condensing temperature.

(c) Using higher-pressure refrigerant creates the opportunity to use higher pressure on the suction side so that fewer compressors are needed and the increased vapour flow rate leads to a net improvement in oil return and cooling of the compressor motors.

(d) Lower liquid side pressures reduce the incidence of pipe failures.

2. The secondary higher temperature stage can be separate from the primary low temperature stage, thus opening the way to the use of other optimal refrigerants and can also be external to the supermarket.

3. The two stages can interact via a pumpable thermal storage medium which provides the means of relieving the load on the primary circuit during peak power periods as well as a stabilising effect in the operation and control of the overall system. It is also feasible to integrate the thermal storage system into an air-conditioning system as described, thereby conferring benefits of off-peak power to the air conditioning system. Milk and vegetable cooling can be supplied directly from the ice store thus transferring all of this cooling load to off-peak times with associated economic benefits.

4. As the outside temperature drops the efficiency advantage of two stages diminishes, but the new system can be adapted by switching off pump 25 which deactivates condenser 6 in the first preferred embodiment, or switching off compressor 50 in the second preferred embodiment.

5. The dual function air cooled de-superheater condenser 4 can cool the refrigerant vapour before it meets the ice bank condenser 6, improving efficiency.

6. Both liquid lines and suction lines can be run within the same insulation.

This will consequently raise the suction vapour temperature causing the discharge temperature of the compressors to increase, dissipating more energy in the externally cooled heat exchanger. In addition the further sub- cooling of the liquid without lowering the feed pressure will further assist the prevention of boiling in the liquid lines and will allow greater energy saving when the outside temperature is very low.

7. The pump 12 maintains circulation in the loop 13,19, 10, so that the risk of local boiling in the distribution lines is significantly reduced.

8. Serviceability and reliability are improved since the new design facilitates the rectification of faults during the day and there will be fewer vital compressors in the primary stage.

9. In situations where the use of thermal storage is not advantageous the thermal store can be dispensed with and the primary circuit refrigerant cooled by direct heat exchange with, the secondary circuit refrigerant, as indicated in Fig. 2. When direct contact between the fluids in the primary and secondary refrigeration stages is undesirable then heat exchange between the two circuits is accomplished via indirect heat exchanger 6 in Fig. 1.

10. In situations where it is desirable to use a secondary fluid to be distributed to the cooling devices via a secondary cooling loop then the primary circuit refrigerant fluid can be used to cool the secondary fluid in the secondary loop by means of an indirect heat exchanger 40, as indicated in Fig. 3. The primary circuit may operate with direct or indirect cooling from the secondary refrigeration circuit. The lower evaporator temperatures associated with secondary loop systems would indicate increased advantages of the new system.

In conclusion, compared with existing systems, the new system has lower running costs through a number of efficiency improvements (which cut CO2 emissions) and has the potential to transfer a substantial proportion of the remaining load to off-peak power (thus further cutting CO2 emissions by virtue of lowering the electrical power transmission losses).

It will be appreciated that the features disclosed herein may be present in any feasible combination. Whilst the above description lays emphasis on those areas which, in combination, are believed to be new, protection is claimed for any inventive combination of the features disclosed herein.