AIR CYCLE COOLING SYSTEMS

Abstract: An open loop air cycle system (10) for delivering a flow of cooling air to an enclosure (11) is described. The system (10) comprises means for delivering a variable flow of cooled air for mixing with recirculated enclosure air in an enclosure air recirculation loop. The system (10) further comprises means for supplying a variable flow of air at ambient temperature and pressure to the enclosure air recirculation loop and means (40) for controlling the flow of cooled air for mixing with the recirculated enclosure air and for controlling the flow of ambient air to the enclosure air recirculation loop such that in operation, substantially constant volume flow of cooling air is delivered to the enclosure (11).

Description: Title: AIR CYCLE COOLING SYSTEMS Description of Invention This invention relates to air cycle cooling systems and is particularly concerned with an open loop air cycle cooling system for delivering a flow of cooling air to condition a heat load in an enclosure. To reduce the size and cost of an open loop air cycle cooling system for conditioning a heat load in an enclosure, such as a passenger compartment of a ground transportation vehicle, it is usual to recirculate a predetermined volume flow of warm air from the enclosure and mix it with a volume flow of cold air delivered by air cycle machinery provided as part of the system. Since in a ground transportation vehicle a source of high pressure air is not usually available, the air cycle machinery includes at least one compressor which is power driven to increase the pressure of ambient air. This increased pressure air is then expanded across a turbine wheel to obtain the volume flow of cold air which is mixed with the volume flow of warm recirculated enclosure air and the resultant mixture is delivered to the enclosure for cooling purposes. For the comfort of occupants of the enclosure it is desirable for a constant volume of air to be maintained within the enclosure, so that, ideally, at all times the volume flow of air supplied to the enclosure should be equivalent to the volume flow of air leaving the enclosure. This requirement can be met by arranging for a predetermined volume flow of air to be vented from the enclosure to ambient. However, when there is a lower heat load in the enclosure, such as may be the case when temperature ambient of the enclosure falls due to a change in the weather, the operating speed of the air cycle machinery is reduced to increase the temperature of air delivered by the machinery with a consequential reduction in the volume flow of air for mixing with recirculated air so that the volume flow of air supplied to the enclosure is less than the total volume flow vented from the enclosure. An objective of the present invention is the provision of an open loop air cycle cooling system which will deliver a substantially constant volume flow of cooling air at required temperature for conditioning a heat load in an enclosure. Accordingly, in its broadest aspect the present invention provides an open loop air cycle cooling system for delivering a flow of cooling air to an enclosure, comprising means for delivering a variable flow of cooled air for mixing with recirculated enclosure air in an enclosure air recirculation loop, means are provided for supplying a variable flow of air at ambient temperature and pressure to the recirculation loop and means are provided for controlling the flow of cooled air for mixing with the recirculated enclosure air and for controlling the flow of ambient air to the enclosure air recirculation loop such thatin operation, a substantially constant volume flow of cooling air is delivered to the enclosure. The means for delivering a variable flow of cooled air may comprise a variable speed air compressor wheel which is driven by a variable speed motor, to deliver air at increased pressure to an air expansion turbine wheel. The variable speed motor may be a variable speed switched reluctance motor. In one embodiment of the invention air flows from the motor driven compressor wheel to the turbine wheel by way of a second compressor wheel mounted for rotation on a common shaft with the turbine wheel. Preferably, air flows from the second compressor wheel to the turbine wheel by way of a primary heat exchanger and a reheater-condenser heat exchanger. The enclosure air recirculation loop may comprise a cooling air supply duct connecting between an outlet from the turbine wheel and an inlet to the enclosure, and a recirculation air duct connecting between an outlet from the enclosure and the cooling air supply duct. The connection between the recirculation air duct and the cooling air supply duct may be by way of a mixing box which is further connected for receiving cooled air from the turbine wheel. The enclosure air recirculation loop preferably includes a motor driven fan. The motor driven fan may comprise a constant speed motor driven fan which is preferably located in the cooling air supply duct. The means for supplying a variable flow of ambient air to the recirculation loop may comprise an ambient air supply duct which connects at one end with the recirculation air duct and is open to ambient air at its other end. In an embodiment of the invention the other end of the ambient air supply duct is connected with an ambient air inlet to the motor driven compressor wheel. In the ambient air supply duct there may be an ambient air flow control valve means which may comprise a valve member movable by a motor under the control of the control means, between a position in which it closes the duct and a position in which the duct is open to permit ambient air to flow to the recirculation air duct. The means for controlling the flow of cooled air and the flow of ambient air may comprise an electronic control unit (ECU) which receives signals from temperature sensor means sensing air temperature internal of the enclosure, the temperature of ambient air, and preferably, the ECU is also connected to temperature sensor means sensing air temperature in the cooling air supply duct. The ECU may hold a look-up table of desirable enclosure air and cooling air temperatures for a range of ambient air temperatures. In operation, on receipt of enclosure air, ambient air and cooling air temperatures the ECU looks up desired values of enclosure air and cooling air temperatures for the prevailing ambient air temperature, computes the difference between the actual temperatures and the desired temperatures and processes this data to produce signals for varying the flow of cooled air and of ambient air into the air recirculation loop. When a heat load in the enclosure is below the maximum conditioning capacity of the system, the ECU signals the cooled air flow control means, e.g. the variable speed motor to reduce the rotational speed of the compressor wheel thereby reducing the pressure drop across the turbine wheel, resulting in the turbine wheel delivering higher temperature air at lower volume flow. To compensate for the lower volume flow from the turbine wheel the ECU signals the ambient air flow control means to allow ambient air, or more ambient air, to enter the recirculation duct so as to maintain a constant volume flow to the enclosure. Alternatively, the ECU may comprise a systems computer including a computer software program which manages the motor driven compressor wheel and the ambient air flow control valve so as to balance the enclosure air temperature to a desired value with respect to sensed temperature ambient of the enclosure. The invention will now be further described by way of example and with reference to the accompanying drawings in which: Figure 1 is a diagrammatic illustration of an air cycle cooling system in accordance with one embodiment of the invention; and Figure 2 is a diagrammatic illustration of an air cycle cooling system in accordance with another embodiment of the invention. An air cycle cooling system 10 in accordance with one embodiment of the invention, as shown in Figure 1, supplies a flow of cooling air to an enclosure such as a passenger compartment 11 of a ground transportation vehicle such as a railway train. The system 10 comprises a first compressor wheel 12 connected for being driven by a variable speed electric motor 13 which may be a switched reluctance motor. The compressor wheel 12 is connected on its inlet side for receiving air from an ambient air inlet 14. On its outlet side the compressor wheel 12 is connected by a duct 15 with an inlet side of a second compressor wheel 16. The second compressor wheel 16 is mounted for rotation on a common shaft 17 with a turbine wheel 18. On its outlet side the second compressor wheel 16 is connected by a duct 19 with an inlet side of the turbine wheel 18. Incorporated in the duct 19 are a primary heat exchanger 20 and a reheater- condenser heat exchanger 21. On its outlet side the turbine wheel 18 is connected with an air mixing box 23. A cooling air supply duct 24 which incorporates a constant speed motor driven fan 25 connects the mixing box 23 with the compartment 11. A first set of vents 26 located internally of the compartment 11 are connected to a vent duct 27 which incorporates a constant speed motor driven fan 28 whereby a desired volume flow of air from the compartment is vented to ambient. A second set of vents 29 located internally of the compartment connect with one end of a recirculation duct 30 which at its opposite end connects with the mixer box 23. The ducts 24 and 30 together with the mixer box 23 combine to form a compartment air recirculation loop which for the comfort of passengers should maintain a substantially constant volume flow of cooling air to the compartment in replenishment of the total volume flow of air extracted therefrom by way of vents 26 and 29. An ambient air duct 31 connects at one end with ambient air inlet 14 and at its opposite end with recirculation duct 30. The duct 31 incorporates an ambient air flow control valve 32 connected for movement between open and closed positions by an electric motor 33, whereby flow of ambient air to the recirculation duct may be controlled. A duct 34 connects, by way of the coolant side of the reheater- condenser heat exchanger 21, between the cooling air supply duct 24 and the recirculation duct 30. The duct 34 connects with duct 24 at a location between the motor driven fan 25 and the compartment 11 and, in operation, the fan induces a flow of cooling air through the duct 34. Water condensed in heat exchanger 21 is collected and fed by a conduit 35 to a nozzle 36 by which it is sprayed into coolant air passages of the heat exchanger 20, a flow of ambient air for cooling purposes being induced through the coolant air passages by a constant speed motor driven fan 37. A temperature sensor 38 located internally of the compartment 11 is connected by a line 39 for inputting compartment air temperature signals to an electronic control unit (ECU) 40. A temperature sensor 41 located externally of the compartment is connected by a line 42 for inputting ambient air temperature signals to the ECU 40. A temperature sensor 43 located in the cooling air supply duct 24 is connected by a line 44 for inputting to the ECU 40 signals representative of the temperature of cooling air flowing to the compartment 11 through duct 24. The ECU 40 is further connected by line 45 to the variable speed motor 13 driving compressor wheel 12 and by line 46 to the motor 33 of the ambient air flow control valve 32. In operation of the system the compressor wheel 12 driven by the motor 13 draws in air through ambient air inlet 14 and delivers this air at increased pressure to the second compressor wheel 16. The second compressor wheel further increases the pressure of the air before it is delivered by way of duct 19 to the turbine wheel 18. In flowing from compressor wheel 16 to turbine wheel 18 the air passes through the ambient air heat exchanger 20 where its temperature is reduced and then through heat exchanger 21 where a substantial amount of any water vapour in the air is condensed and removed before the air is supplied to the turbine wheel 18. The air is expanded across the turbine wheel 18 and delivered to the mixing box 23. In expanding across the turbine wheel 18 the air drives the turbine wheel 18 and with it the compressor wheel 16. In the mixing box 23 the cold air delivered by the turbine wheel 18 mixes with warm air which flows into the box 23 from recirculation duct 30. Cooling air at a temperature between that of the turbine wheel cold air and the recirculation warm air is delivered from the mixing box 23 to the compartment 11 by way of duct 24. On start-up of the system on a hot day, there will be a high heat load in the compartment to be reduced before it is occupied. Temperature sensors 38, 41, 43 sense the temperatures internal of the compartment 11, external of the compartment 11 and in duct 24, respectively, and signals representative of these temperatures are input to the ECU 40. In this embodiment the ECU 40 holds a look-up table of desirable compartment air temperatures and cooling air temperatures for a range of ambient temperatures external of the compartment. The ECU 40 enters the look-up table to look up desirable values for compartment air and cooling air temperatures for the prevailing ambient air temperature. The ECU 40 then computes the difference between the actual and desirable temperatures and processes this data to output a signal for control of the variable speed motor 13 which is driven at or near its maximum speed so as to maximise the pressure drop across the turbine wheel 18 whereby air at a sub¬ zero temperature which is appropriate to the sizing of system components is delivered to the mixing box 23. At this stage the ambient air flow control valve 32 is biased closed. The motor driven fans 25, 28 and 37 are also switched on at system start-up. The sizing and speed of motor driven fan 28 is such as to generate a predetermined flow of air from the compartment 11 to ambient through the vents 26 and vent duct 28. Similarly, the motor driven fan 25 is sized to generate a predetermined volume flow of air from the compartment 11 through vents 29 and recirculation duct 30. This recirculation air mixes with cold air in the mixing box 23 and is returned to the compartment 11 by way of the cooling air supply duct 24. When the heat load in the compartment 11 reduces so that the compartment air and cooling air temperatures are sensed to be at the required values, the ECU 40 signals the variable speed motor 13 to reduce the speed of the compressor wheel 12 so that the required compartment air temperature is maintained and there is a substantially constant volume flow of air to the compartment 11 through the cooling air supply duct 24 to replenish the total volume flow of air extracted through vents 26 and 29. Should the ambient temperature fall so that for the comfort of passengers the compartment air temperature requires to be increased, the ECU 40 signals the variable speed motor 13 to decrease the rotational speed of the compressor wheel 12 thereby reducing the pressure drop across the turbine wheel 18. This results in air at increased temperature than previously being delivered by the turbine wheel 18 with a consequential increase in the temperature of cooling air delivered to the compartment 11 by the cooling air supply duct 24. However, when the rotational speed of the compressor wheel 12 is reduced there is a resultant reduction in the volume flow of air delivered by the turbine wheel 18. The present invention compensates for this reduced volume flow from the turbine wheel 18 by providing for the ECU 40 to signal the motor 33 to move the ambient air flow control valve 32 towards opening so that ambient air is induced by motor driven fan 25 to enter the duct 31 and flow to the recirculation duct 30 to maintain the volume flow in the recirculation loop comprised by ducts 30 and 24 substantially constant. In another embodiment of the invention shown in Figure 2 and for which the same numerals are used to reference common elements of the systems of Figures 1 and 2, the reheater-condenser heat exchanger 21 is located in the cooling air supply duct 24 so that all of the volume flow of cooling air to the compartment 11 is by way of this heat exchanger. It is to be appreciated that these embodiments are by way of example only and other modifications are possible. For example, in the Figure 2 embodiment, the constant speed motor driven fan 25 could be located upstream of the heat exchanger 21 in the cooling air supply duct 24. The ECU 40 could be an on-board systems computer including a computer software program which manages the motor driven compressor wheel 12/13 and ambient air flow control valve 32 so as to balance the compartment air temperature to a desired value with respect to sensed temperature ambient of the enclosure. In either embodiment, instead of the speed of motor 13 being controlled to vary the flow of cooled air introduced into the air recirculation duct 24, any other means for achieving variable flow may be provided, such as a variable geometry compressor fan which is driven at a constant speed. Any means for varying the flow of ambient air into the recirculation loop may be provided as an alternative to the valve 32 as described.

Claims: CLAIMS 1. An open loop air cycle cooling system (10) for delivering a flow of cooling air to an enclosure (11), comprising means for delivering a variable flow of cooled air for mixing with recirculated enclosure air in an enclosure air recirculation loop, characterised in that means are provided for supplying a variable flow of air at ambient temperature and pressure to the enclosure air recirculation loop and means (40) are provided for controlling the flow of cooled air for mixing with the recirculated enclosure air and for controlling the flow of ambient air to the enclosure air recirculation loop such that in operation, a substantially constant volume flow of cooling air is delivered to the enclosure (11). 2. An open loop air cycle cooling system (10) according to Claim 1 characterised in that the means for delivering a variable flow of cooled air comprises variable speed air cycle machinery having a variable speed motor (13) which drives a first compressor (12) such that in operation air at increased pressure is delivered to an air expansion turbine (18). 3. An open loop air cycle cooling system (10) according to Claim 2 characterised in that air flows from the first compressor (12) to the turbine (18) by way of a second compressor (16) mounted for rotation on a common shaft (17) with the turbine (18) and in that air flows from the second compressor (16) to the turbine (18) by way of a primary heat exchanger (20) and a reheater-condenser heat exchanger (21). 4. An open loop air cycle cooling system (10) according to Claim 2 or 3 characterised in that the enclosure air recirculation loop comprises a motor driven fan (25), a cooling air supply duct (24) connecting between an outlet from the turbine (18) and an inlet to the enclosure ( 11), and a recirculation air duct (30) connecting between an outlet from the enclosure (11) and the cooling air supply duct (24), and in that the connection between the recirculation air duct (30) and the cooling air supply duct (24) is by way of a mixing element (23) which is further connected for receiving cooled air from the turbine (18). 5. An open loop air cycle cooling system (10) according to Claim 4 characterised in that the motor driven fan (25) comprises a constant speed motor driven fan which is located in the cooling air supply duct (24). 6. An open loop air cycle cooling system (10) according to any preceding claim characterised in that the means for supplying a variable flow of ambient air to the enclosure air recirculation loop comprises an ambient air supply duct (31) which connects at one end with the recirculation air duct (30) and at its other end is open to ambient air, there being ambient air flow control means (32) in the ambient air duct (31) to vary the flow of ambient air along the duct (31). 7. An open loop air cycle cooling system (10) according to Claim 6 characterised in that the ambient air flow control means (32) comprises a valve member located in the ambient air supply duct (31) movable under the control of the control means (40) by a motor (33) between a position in which the valve member closes the duct (31) and a position in which the duct (31) is open to permit ambient air to flow to the recirculation air duct (30). 8. An open loop air cycle cooling system (10) according to any preceding claim characterised in that the means (40) for controlling the flow of cooled air and the flow of ambient air comprises an electronic control unit (40) which receives signals from temperature sensor means (38,41,43) sensing air temperature internally of the enclosure (11), temperature of the ambient air and the temperature of the air in the cooling air supply duct (24). 9. An open loop air cycle cooling system (10) according to Claim 8 characterised in that the electronic control unit (40) comprises a look-up table of desirable enclosure air and cooling air temperatures for a range of ambient air temperatures, and in operation, on receipt of signals indicating enclosure air, ambient air and cooling air temperatures the electronic control unit (40) obtains from the look-up table desired values of enclosure air and cooling air temperatures for the prevailing ambient air temperature, computes the difference between the actual temperatures and the desired temperatures and generates signals for varying the flow of cooled air and ambient air into air recirculation loop.

Inventors: GILES GEORGE RICHARD, RICHARDS DONALD JAMES, ROOTS CHRISTOPHER FRANCIS, GILES, GEORGE, RICHARD, RICHARDS, DONALD, JAMES, ROOTS, CHRISTOPHER, FRANCIS
Applicants: GILES GEORGE RICHARD, NORMALAIR GARRETT LTD, RICHARDS DONALD JAMES, ROOTS CHRISTOPHER FRANCIS, GILES, GEORGE, RICHARD, NORMALAIR-GARRETT (HOLDINGS) LIMITED, RICHARDS, DONALD, JAMES, ROOTS, CHRISTOPHER, FRANCIS
Application year: 1995
Main classification: F24F11
IPC codes: B60M1, F24F5, F24F11, F25B9