GB2307734A

GB2307734A - Cooling habitable enclosures

Info

Publication number: GB2307734A
Application number: GB9524596A
Authority: GB
Inventors: Peter William Fitt
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-12-01
Filing date: 1995-12-01
Publication date: 1997-06-04
Anticipated expiration: 2015-12-01
Also published as: GB9524596D0; GB2307734B

Abstract

A system for using air as a refrigerant to provide cooling for a habitable enclosure incorporates an assembly of air compressor, (2) air expander, (4) and variable speed driving motor, 11, all mounted on a common shaft, 5. Air from the atmosphere, 1, enters the compressor, 2, where its pressure and temperature are raised before being cooled in a first heat exchanger, 3, after which it enters the expander, 4. The air leaving the expander enters a second heat exchanger, 6, where it receives heat from the habitable enclosure, 7, before passing through the first heat exchanger, 3, to cool the air leaving the compressor, 2, before being discharged to the atmosphere, 1. The refrigerant duty of the system is controlled by varying the speed of the motor, 11. The habitable enclosure, which may be the passenger compartment of a vehicle, is cooled via a heat exchanger (9) and a fan (10); anti-freeze being pumped around a closed system. The closed system may include a heating circuit linked to the engine cooling system.

Description

IXPROVEMENTS IN OR RELATING TO THE AIR CODITIOKING OF HABITABLE ENCLOSURES The present invention relates to improvements in systems which use air as the primary refrigerant for the air conditioning of habitable enclosures and, in particular, the passenger compartment of motor vehicles.

Refrigeration systems using air as the working fluid conventionally incorporate expansion means, compression means, and one or more heat exchangers. In these systems, refrigeration is achieved by extracting energy from the air, in the form of work, as the air passes through the expander from one pressure to a lower pressure. In the most common, 'open', system the pressure difference across the expander is established by a compressor, or compressors, upstream of the expander. Typically, air from the atmosphere is raised in pressure by the compressor, and passes through a 'hot' heat exchanger to reject heat to the atmosphere, before entering the expander. The cold air leaving the expander is delivered directly into the enclosure to be refrigerated.

This type of system can exhibit a number of unsatisfactory features.

These include a tendency for the air leaving the expander to contain fog or ice crystals, gaseous contaminents from the atmosphere, oil mist from the rotating machinery, if it is lubricated, and noise from the expander.

In some systems, these features are alleviated or ameliorated by isolating the refrigerant air from the enclosure to be refrigerated by introducing a second, 'cold', heat exchanger into a duct which connects the outlet of the expander to the inlet of the compressor. Heat from the enclosure to be refrigerated is then transferred to this second heat exchanger, and thus to the refrigeration circuit, by circulating air from the enclosure through this heat exchanger. Such a 'closed' system resembles closely a conventional vapour compression system and the 'cold' heat exchanger can exhibit similar problems to those experienced with the evaporators of vapour compression systems, the most serious of which is the tendency for ice or snow to be deposited within the passageways of the heat exchanger by the air from the enclosure.This form of system still requires the provision of a 'hot' heat exchanger downstream of the compressor to reject heat to the atmosphere.

According to the present invention, there is provided a system for using air as a refrigerant to provide cooling for a habitable enclosure comprising an assembly of an air compressor and an air expander and a driving motor all mounted on a common shaft and with the outlet of the compressor connected by duct means to the inlet of the expander, the duct means incorporating a first heat exchanger or recuperator, a second heat exchanger downstream of the expander by means of which heat from the habitable enclosure is transferred to the cold air leaving the expander, a further duct means to return the air leaving the second heat exchanger in counter flow through the first heat exchanger or recuperator to cool the air leaving the compressor, means to admit ambient air to the compressor and a means to permit air leaving the first heat exchanger to return to the atmosphere, and a controllable source of power for the motor which permits the motor speed to be varied.

The heat from the enclosure is conveyed by a secondary refrigerant fluid which is circulated to pass through both the second, or'cold', heat exchanger and a fan-assisted heat exchanger in the enclosure. To prevent inadvertent freezing of this secondary refrigerant it should be of an anti-freeze nature. The deposit of snow or frost within the air side of the 'cold' heat exchanger, by the very cold air leaving the expander, is avoided by suitable choice of fin construction, the relative movement of cold air and secondary refrigerant through this heat exchanger, and a control system which responds to the formation of ice or snow within this heat exchanger.In situations where the enclosure needs to receive heat, the same secondary fluid and fan-assisted heat exchanger can be used, the secondary fluid bypassing the 'cold' heat exchanger and being heated by a suitable external heat source.

A further important feature of the invention is the integration of the compressor, expander, and a variable-speed motor into a single assembly having a common shaft. In this way the work generated in the expander is directly used to aid the compression process, the additional compression power required being provided by the integral motor, most conveniently being electrically-powered from any suitable generating source.

Specific embodiments of the invention will now be described, by way of example only, in which the habitable enclosure is the passengers' compartment of a motor car. In this description, reference will be made to the accompanying drawings in which: Figure 1 shows the circuit diagram for a system incorporating a compressor, expander and motor assembly, together with a recuperator, a heat exchanger, and a secondary refrigerant circuit to transfer heat from the passenger compartment.

Figure 2 shows the circuit for a system similar to that of Figure 1 in which the secondary refrigerant is the vehicle engine's anti-freeze coolant liquid and valves are incorporated to enable the enclosure to be heated by rejected heat from the engine or from the compressed air.

Figure 3 illustrates one possible configuration of expander, compressor and electric motor appropriate to either system.

In the first embodiment, illustrated by Figure 1, air from the atmosphere, 1, enters the compressor, 2, and upon leaving passes through a recuperator, 3, where its temperature is reduced. This air then enters the expander, 4, where the pressure and temperature both fall.

During this expansion process work is transferred by means of a shaft, 5, the compressor, 2. The cold air leaving the expander, 4, enters a heat exchanger, 6, to receive heat, conveyed by means of a secondary fluid, zrom the passenger compartment, 7, of the car. This secondary fluid is conveniently an anti-freeze liquid which is circulated by means of a pump, 8, through a second heat exchanger, 9, within the car and through which air within the car is circulated by a fan, 10.

After leaving the heat exchanger, 6, the refrigerating air passes into the recuperator, 3, where it absorbs much of the compression heat before being discharged to the atmosphere, 1. The work required to provide the compression process exceeds the work available from the expansion process and additional power is provided by the variable-speed electric motor, 11, through the common shaft, 5. To accommodate variations in the cooling demand of the enclosure, the speed of the motor is adjusted to alter the compression ratio and mass flow of the air.

To prevent the accumulation of ice or snow which may be deposited by the cold air within the heat exchanger, 6, it is preferrable for the antifreeze to enter this heat exchanger at the same end as the cold air, and to flow in the same general direction as the air before returning to the fan-assisted heat exchanger, 9. By means of a sensor, 12, the temperature of the air leaving the heat exchanger, 6, is limited to a minimum of OOC by controlling the speed of the motor, 11. .4ny condensate deposited in the air side of the heat exchanger, 6, is conveyed forward towards its outlet to pass into the low pressure side af the recuperator, 3.

During certain operating conditions, atmospheric moisture may condense in the high pressure side of the recuperator, 3. This condensate is directed from the high pressure side, by means of a duct, 13, into the low pressure side where it evaporates and assists the intercooling process.

When the car engine is operating, the power required by the electric motor, 11, is provided by a generator driven by the engine. At times when the engine is stationary, and the car is suitably parked, the electrical power can be obtained from any convenient external source.

In the second embodiment, illustrated by Figure 2, the vehicle compartment cooling system, as described in the first embodiment, and illustrated by Figure 1, has the addition of a heating circuit using hot antifreeze solution from the cooling system of the vehicle engine, 14.

When heating is required a three-way valve, 15, is operated to isolate the heat exchanger, 6, from heat exchanger, 9, and to permit engine coolant to circulate through said heat exchanger, 9, under the influence of the coolant circulating pump, 16. During this heating mode the motor, 11, and pump, 8, are each inactive. When cooling is required the threeway valve, 15, is operated to connect the heat exchanger, 6, to the fanassisted heat exchanger, 9, and to isolate the engine cooling system.

The motor, 11, and pump, 8, are both activated to provide cooling as described in the first embodiment. In situations when the use of engine coolant for heating is unsuitable, a by-pass valve incorporating a flow restrictor, 17, is opened to enable hot air to pass directly from the compressor, 2, to the heat exchanger, 6. By this means, the compartment can be indirectly heated by the work energy provided by the variable speed motor, 11.

Figure 3 illustrates one arrangement for the assembly of expander, compressor and variable speed motor. For compactness, which is an essential requirement where installational space is limited, as in a motor car, the expander and compressor are both of a rotary turbo type.

Machines of this type, and of the size which applies to motor cars, operate with a very high maximum rotational speed often in the range from 40,000 to 100,000 rpm. To avoid the use of gearing, an electric motor, 11, capable of operating at these high speeds must be used, a suitable type being known as a Switched Reluctance Motor which is also capable of variable speed control. As shown, the three components, compressor, 2, expander, 4, and motor, 11, are mounted on a common shaft, 5. In the arrangement shown, the motor is supported between bearings, 17, with the turbine wheel located at one end of the common shaft, and the compressor wheel located at the other end. It is to be understood that there is no fundamental reason why the motor, 11, should not be located at either of the ends of the shaft, with the compressor, 2, and expander, 4, closely associated with each other.

Claims

CLAIMS:

1. A system for using air as a refrigerant to provide cooling for a habitable enclosure comprising an assembly of an air compressor and an air expander and a driving motor all mounted on a common shaft and with the outlet of the compressor connected by duct means to the inlet of the expander, the duct means incorporating a first heat exchanger or recuperator, a second heat exchanger downstream of the expander by means of which heat from the habitable enclosure is transferred to the cold air leaving the expander, a further duct means to return the air leaving the second heat exchanger in counter flow through the first heat exchanger or recuperator, to cool the air leaving the compressor, means to admit ambient air to the compressor and a means to permit air leaving the first heat exchanger to return to the atmosphere, and a controllable source of power for the motor which permits the motor speed to be varied.

2. A system as claimed in Claim 1 in which heat is conveyed from the habitable enclosure to the cold air passing through the first heat exchanger by a secondary refrigerant which receives heat from the air within the enclosure by means of a fan-assisted heat exchanger, or heat exchangers, located within the enclosure.

3. A system as described in Claims 1 and 2 in which the secondary refrigerant is the antifreeze solution used in the engine cooling circuit of a motor vehicle, valve means being incorporated to isolate the engine cooling circuit from the first heat exchanger and from the heat exchanger within the enclosure, said enclosure being the passenger compartment of the motor vehicle.

4. A system as described in Claims 1, 2, and 3 in which the fan-assisted heat exchanger which receives heat from the air within the enclosure is also used to deliver heat to the air within the enclosure, valve means being incorporated to permit the engine cooling circuit to be coupled to the fan-assisted heat exchanger and to be isolated from the first heat exchanger.

5. A system as claimed in any one of the preceding claims, wherein the motor forming part of the motor/compressor/expander assembly is an electric motor.

6. A system as claimed in any one of the preceding claims in which power for the electrical motor described in Claim 5 is provided by a generator driven by the engine of the motor vehicle carrying the enclosure.

7. A system as claimed in Claim 6 in which the generator operates to provide an alternating current at a voltage in excess of 24volts which is processed electronically to deliver a range of frequencies and/or voltages to the electric motor driving the compressor/expander combination to accommodate variations in cooling duty by varying the motor speed.

8. A system as described in any of the previous claims in which the first heat exchanger is constructed so that the secondary fluid flows in essentially the same direction as the cold air, and any condensate deposited within the cold air passageways is carried towards the cold air outlet.

9. A system as described in Claim 8 in which the motor speed is controlled to ensure that the temperature of the cold air leaving the first heat exchanger does not fall below freezing point.

10. A system as described in Claim 1 in which the second heat exchanger, or recuperator, is constructed so that any condensate deposited in the high pressure passageways flows into a drain which transfers the condensate to the low pressure passageways of the said heat exchanger, or recuperator.

11. A system as claimed in Claim 7 in which the generator used to power the motor of the cooling system is also used to provide all other electrical needs within the motor vehicle, the alternating voltage being electronically processed to deliver the standard nominal battery voltage of 12volts DC or 24volts DC normally used on motor vehicles.

12. A system as claimed in any one of the preceding claims in which provision is made to enable an external source of electrical power to be used to drive the motor when the motor vehicle engine is not in operation.

13. A system as claimed in any one of the preceding claims in which the fan-assisted heat exchanger is used to deliver heat to the enclosure, duct means and valve means being provided to enable the outlet of the compressor to be connected through a restrictor to the inlet of the second heat exchanger.

14. A system substantially as herein described with reference to the accompanying drawings used to refrigerate a habitable enclosure by the use of an independent source of generated power.