Refrigeration and air conditioning consumes a significant proportion of the electricity generated worldwide. It is therefore desirable to reduce the power consumptions of these refrigeration systems where possible.

Certain refrigeration systems cool to temperatures which could, at certain times of the day or year, be achieved by natural cooling, provided that the ambient temperatures are sufficiently low. However, it is difficult to achieve natural cooling without expending significant amounts of energy in filtration or in the pumping of recirculating coolants such as water.

Previous patent application EP0641978 Star Refrigeration Ltd. , discloses a refrigeration system employing a volatile primary refrigerant in natural thermosyphon circulation, which cools water in a secondary circuit for distribution to the areas requiring to be refrigerated. When the ambient temperature is sufficiently low, the primary refrigerant circulates by thermosyphon; and when the ambient temperature is not sufficiently cool to allow natural circulation, the refrigeration system reverts to mechanical refrigeration.

US Patent 5,400, 615 Star Refrigeration Ltd. , discloses a refrigeration apparatus in which the primary refrigeration circuit is a mechanically pumped vapour compression circuit.

This cools a pumped secondary circuit containing carbon dioxide as a volatile secondary heat transfer substance.

The volatile secondary refrigerant delivers cooling to desired locations.

It is an object of the present invention to improve upon such refrigeration systems.

Generally speaking, the present invention provides a primary refrigeration circuit which is able to operate in mechanical vapour compression mode or in thermosyphon mode depending upon the ambient temperature conditions, in thermal contact with a mechanically pumped secondary circuit employing a volatile secondary refrigerant.

Specifically, the present invention provides a refrigeration apparatus which comprises a primary refrigeration circuit arranged to cool a secondary refrigeration circuit having a thermal load; (i) the primary vapour-compression refrigeration circuit comprising a compressor for compressing a volatile primary refrigerant, a condenser for rejecting heat from the compressed refrigerant, and an expansion device for expanding the refrigerant into an evaporator for providing a cooling effect; and (ii) the secondary refrigeration circuit comprising a condenser for rejecting heat from a volatile secondary refrigerant, the condenser being in thermal contact with the primary evaporator and cooled thereby; and means for cooling the thermal load; the primary refrigeration circuit further comprising bypass means selectably operative to bypass the primary compressor and primary expansion device, so as to allow alternative refrigerant circulation through the primary refrigeration circuit by thermosyphon.

The invention also extends to a corresponding method of operating a refrigeration system.

Thus, the present invention consists of an apparatus using both a volatile primary refrigerant (for example, ammonia) and a volatile secondary refrigerant (for example, carbon dioxide). The primary refrigeration circuit has an evaporator, a compressor, a condenser and an expansion device as in a conventional vapour compression cycle. When refrigeration duty is satisfied, the compressor stops and the primary circuit defaults to a natural circulation, or thermosyphon mode, by providing that the expansion device and the compressor are bypassed by the bypass means.

Generally, the compressor is stopped responsive to the temperature of the thermal load, as controlled by a thermostat thereon. The condenser is located above the evaporator, so that when the temperature of the condenser falls below that of the evaporator, natural thermosyphon circulation is established.

Typically, with ammonia as primary refrigerant the condensing temperature in mechanical refrigeration mode is 35e to 45gC and in thermosyphon mode is 60 to 12°C (and generally 1 to 5gC below the evaporating temperature).

The secondary volatile refrigerant is condensed by being cooled in the evaporator of the primary refrigeration circuit. The condensed secondary liquid refrigerant is collected and pumped to the area requiring to be cooled (i. e. the thermal load). At this position, the secondary refrigerant absorbs heat and evaporates to provide a cooling effect. It thereafter returns to the condenser of the secondary system where the secondary refrigerant is re- liquified. A circulating pump may be provided in the secondary circuit.

The temperature of the secondary refrigerant, particularly carbon dioxide, may be controlled by varying the pressure of the volatile secondary refrigerant (for example, by throttling the return flow of secondary refrigerant vapour). This also enables the temperature of the thermal load to be controlled.

In a preferred embodiment of the invention, at least one primary refrigeration circuit is provided which cools a corresponding secondary condenser in the secondary refrigeration circuit. When mechanical refrigeration is required in the primary refrigeration circuit, one of the primary refrigeration circuits (the"lead unit") switches from thermosyphon to mechanical operation. This effectively switches off the thermosyphon modes in the other primary refrigeration circuits, since the temperature of refrigerant in the primary evaporator (and in the secondary condenser associated with the lead unit), will now be lower than the ambient temperature.

Preferably, the primary condenser (s) is provided with a cooling means, such as a fan, preferably provided with speed control means to prevent over-cooling in the event of ambient temperatures falling too low.

An embodiment of the present invention will now be described by way of example only with reference to Figure 1 of the attached drawings.

Figure 1 is a schematic diagram of a refrigeration apparatus according to the present invention comprising a primary refrigeration circuit and a secondary refrigeration circuit.

The refrigeration apparatus shown in Figure 1 comprises generally a primary refrigeration circuit 100 employing ammonia as the primary refrigerant and a secondary refrigeration circuit 200 employing carbon dioxide as the secondary refrigerant.

The primary vapour-compression refrigeration circuit 100 comprises primary condenser 1. Ammonia gas is compressed in primary compressor 2 and is fed through oil separator 3 to the primary condenser. Ammonia gas is delivered from the primary evaporator 4, which also serves as the condenser of the secondary refrigeration circuit.

Primary refrigerant cools by passage through an expansion device 5 (such as an expansion valve, capillary etc. ). The expansion device 5 receives cooled liquid ammonia from the primary condenser 1.

In the mechanical vapour-compression mode, ammonia primary refrigerant is condensed to liquid in primary condenser 1. The ammonia liquid expands through expansion device 5 and enters the primary evaporator 4, which becomes cooled (thereby cooling the secondary refrigerant in thermal contact therewith). The gaseous ammonia is compressed by primary compressor 2 where it heats up. Heat is rejected therefrom in the primary condenser 1. When the ambient temperature surrounding primary condenser 1 falls to a particularly low level (i. e. below the temperature of evaporator 4), thermosyphon operation is established when the compressor is stopped by operation of a thermostat 21 sensing the temperature of the thermal load and thus operating bypass means to bypass the primary compressor 2 and the primary expansion device 5. The primary compresser 2 and oil separator 3 are bypassed by operation of the three-way valve 7; which operates in conjunction with the two-way valve 6 which bypasses the expansion device. In this way, thermosyphon circulation is established in the primary refrigeration circuit under the effect of gravity, since the primary condenser is located at a higher level than the primary evaporator.

The secondary refrigeration circuit 200 employs carbon dioxide as the volatile secondary refrigerant. Gaseous carbon dioxide returns along the manifold 9 and passes into secondary condensers 14,14a, 14b and 14c, which are in thermal contact with corresponding primary refrigeration circuits analogous to primary circuit 100 (for clarity only a single primary refrigeration circuit is shown). These additional primary refrigeration circuits allow some provision for diversity and standby in use. Heat is abstracted from the carbon dioxide gas in the secondary condensers and the carbon dioxide becomes liquefied and passes into a liquid carbon dioxide receiver 10. Liquid carbon dioxide therefrom is then pumped by secondary pumps 11 into a liquid carbon dioxide supply line 12 which provides liquid carbon dioxide to a cooling unit 20 constituting the thermal load requiring to be cooled, where the liquid carbon dioxide is allowed to evaporate. The heat extracted from the thermal load can be controlled by throttling the return flow of carbon dioxide gas to manifold 9, by throttling means 8, thus raising the evaporating temperature of the carbon dioxide.

Typical condenser evaporating temperatures and pressures for pumped and thermosyphon mode are as follows: Design load temperature: 15°C Ambient temperature : 30°C Mechanical refrigeration (pumped) mode: Ammonia condensing temperature: 40°C (pressure: 15.6 Bar A) Evaporating temperature: 10°C (pressure: 6.15 Bar A) C02 condensing temperature: 12°C (pressure: 47.3 Bar A) C02 evaporating temperature: 13gC (pressure: 48.5 Bar A) Ambient Temperature 5°C Thermosyphon Mode: Ammonia condensing temperature: 99C (pressure: 5.9 Bar A) Evaporating temperature 10°C (pressure 6.15 Bar A) C02 condensing temperature: 12°C (pressure 47.3 Bar A) C02 evaporating temperature: 13gC (pressure 48.5 Bar A) The advantages of the refrigeration apparatus of the present invention over a conventional thermosyphon system using a volatile refrigerant to cool a recirculating liquid, are that the pumping power required for the secondary refrigerant is much reduced compared to that which would be required for a secondary refrigerant liquid providing cooling by sensible heat only. A further advantage is that there is no substantial temperature change during the condensation of the ammonia and the evaporation of the ammonia in the primary refrigeration circuit; and the condensation of the carbon dioxide and the evaporation of the carbon dioxide in the secondary refrigeration circuit.

This means that the temperature difference between the ambient (around primary condenser 1) and the refrigerating load (served by the secondary refrigeration circuit) can be much reduced compared to prior art systems where non- volatile liquids are employed in a secondary refrigeration circuit. A third significant advantage is that the size of the pipework required for the volatile secondary system, particularly employing carbon dioxide as secondary refrigerant, is much smaller than would be required for a non-volatile secondary refrigerant liquid.