GB2234053A

GB2234053A - Gas-cycle refrigerator

Info

Publication number: GB2234053A
Application number: GB8915330A
Authority: GB
Inventors: Stephen Forbes Pearson
Original assignee: Star Refrigeration Ltd
Current assignee: Star Refrigeration Ltd
Priority date: 1989-07-04
Filing date: 1989-07-04
Publication date: 1991-01-23
Anticipated expiration: 2009-07-04
Also published as: GB8915330D0; GB2234053B

Abstract

A gas-cycle refrigeration apparatus comprises a centrifugal rotary compressor (1), a positive displacement screw compressor (8) driven directly by an electric motor (14), and an expansion turbine (16) which drives the centrifugal compressor at high speed. Interstage coolers (6, 14) are provided after each compressor. The apparatus is able to provide very cold air (e.g. - 50 DEG C) using an electric motor as prime power source. <IMAGE>

Description

GAS-CYCLE REFRIGERATOR The present invention relates to a refrigeration apparatus using a gas, such as air, as the working fluid.

Air cycle refrigeration was commonly used in the latter part of the 19th century. It fell out of use because the efficiency of the cycle was much less than the efficiency of the vapour compression cycle.

In the conventional air cycle refrigerator, an engine or motor drove an air compressor which induced air, usually from the space being refrigerated, and compressed it to a predetermined pressure. As a result of being compressed the air became warm. Heat was removed from the warm air by some form of heat exchanger. The high pressure air, now cooled to near ambient temperature, was then expanded in an engine. The effect of expanding the air in the engine while doing work was to cause the air to discharge from the engine at a low temperature. The work done in the expansion engine was usually utilised to provide part of the power input to the compressor. In the late 19th century the compressors and the expansion engines were invariably of the reciprocating type. A major advantage of the air cycle refrigerator is that the working fluid is cheap, readily available and non-toxic.

The major disadvantages were that the efficiency was low and that snow might be formed in the expansion engine, giving rise to mechanical damage.

In more recent times, air cycle refrigerators have largely fallen from favour, except for specialised applications in aircraft air conditioning systems where compressed air is readily available from the jet engines.

Modern air cycle refrigerators use expansion turbines rather than reciprocating expansion engines.

Concern about the environmental consequences of continuing to use chlorofluorocarbon (CFC) refrigerants gives rise to a renewed interest in the air cycle refrigerator. The availability of pure rotary high speed turbines and compressors allows the construction of compact and efficient machines. Modern continuous freezing processes use air temperatures much lower than the temperatures which were in use in the late 19th century. Under these circumstances the low exhaust air temperature which is a feature of the air cycle refrigerator is no longer a disadvantage. However, a major disadvantage of the rotary air cycle machine is that the compressor and turbine must run at very high speeds.

The cost of a motor to drive the compressor at such high speeds would be high and capacity/speed control would be difficult.

It is an object of the present invention to provide a gas-cycle refrigeration apparatus wherein the drive source can be run at conventional motor speeds and which has simple provision for varying the refrigeration capacity.

The present invention provides a gas-cycle The present invention provides a gas-cycle refrigeration apparatus which comprises: - first rotary compressor means for compressing the gas to a first pressure; - second rotary positive displacement compressor means driven by a drive source for further compressing the compressed gas to a second pressure; - expansion turbine means through which compressed gas at the second pressure is expanded thereby driving the turbine and producing refrigerated gas, the expansion turbine means being operatively connected for directly driving the first rotary compressor means; and - interstage air cooling means for removal of heat from the compressed gas at the first and second pressures.

The first rotary compressor means may be any suitable rotary compressor, such as a centrifugal compressor or turbine. The second rotary compressor means is a positive displacement compressor wherein a volume of gas is trapped and compressed.

If necessary, the apparatus may comprise more than one second rotary compressor means in order to provide further compression stages. The compressed gas from the final compressor is then expanded through the expansion turbine means.

The drive source for the second rotary compressor means is usually an electric motor driven at conventional electric motor speeds, for example 1500 to 3,000 revolutions per minute. This is possible since part of the compression work is undertaken by the first rotary compressor means which is directly driven by the expansion turbine means, thereby avoiding the need for any high speed motors.

The capacity of the refrigeration apparatus may be varied by loading or unloading the second rotary compressor means, for example using a slide valve, or by varying the speed of the drive source.

An embodiment of the present invention will now be described by way of example only with reference to the attached drawing wherein: Figure 1 is a schematic flow diagram of an air-cycle refrigeration apparatus.

The refrigeration apparatus comprises a first centrifugal compressor 1 rotatably mounted on a shaft 2 and having an air inlet 3 and air outlet 4. Compressed air from the centrifugal compressor at a first pressure, typically 2 Bar A, then passes to first interstage cooler 6, which is an air-to-air cooler where the hot compressed gas is cooled before being introduced into positive displacement rotary compressor 8 via inlet 10. Rotary compressor 8 is driven by a two-pole electric motor 14 operating at 3,000 rpm. The air is compressed still further before passing through air outlet 12 to second interstage air-to-air cooler 14 where it is cooled once again.

In order to provide a cooling effect, the compressed air is then allowed to expand through a turbine 16 where it expands and becomes cool, for example to minus 5O0C, before leaving the turbine at cold air outlet 18.

The expansion turbine 16 is mounted on the same shaft 2 as the centrifugal compressor 1. Expansion of the compressed air through the turbine 1 6 drives the turbine at high speeds, which in turn drives the centrifugal compressor 1 at high speed. The net result is to provide an air cycle refrigerator driven at 3,000 rpm by a two-pole electric motor, but which contains a high speed shaft operated by a high speed turbine driven by the action of the compressed air, which in turn drives a high speed intake compressor.

Claims

1. A gas-cycle refrigeration apparatus which comprises: - first rotary compressor means for compressing the gas to a first pressure; - second rotary positive displacement compressor means driven by a drive source for further compressing the compressed gas to a second pressure; - expansion turbine means through which compressed gas at the second pressure is expanded thereby driving the turbine and producing refrigerated gas, the expansion turbine means being operatively connected for directly driving the first rotary compressor means; and - interstage air cooling means for removal of heat from the compressed gas at the first and second pressures.

2. An apparatus according to claim 1 wherein the first rotary compressor means is a centrifugal compressor or turbine.

3. An apparatus according to any preceding claim wherein the second rotary positive displacement compressor means is a screw compressor.

4. An apparatus according to any preceding claim which comprises more than one second rotary positive displacement compressor means arranged in series in order to provide further compression stages.

5. An apparatus according to any preceding claim wherein the or each second rotary positive displacement compressor means is driven directly by an electric motor.

6. An apparatus according to claim 5 wherein the electric motor operates at 1500 to 3000 rpm.

7. An apparatus according to any preceding claim wherein the refrigeration capacity is varied by loading and unloading the second rotary positive displacement compressor means.

8. A gas-cycle refrigeration apparatus substantially as described in conjunction with the drawing.