Cooling systems, or refrigerating systems, are based normally on a two-phase cycle in which a primary refrigerant, for instance freon, is caused to alternate between a liquid phase and a gas phase. A system of this nature will generally have a high cooling capacity and will take up large quantities of heat as the primary refrigerant passes from its liquid phase to its gas phase. It is therefore the system that is most used and most pre- ferred in the majority of applications, including air-conditioning. However, the equip- ment required in such a system is both extensive and expensive.

These known systems require a closed circuit for a typical environmentally haz- ardous refrigerant, which must be isolated from the surroundings. Actual cooling of the objects is achieved with a secondary medium, for instance air, which is cooled by the primary medium in the closed circuit, through the medium of a heat exchanger.

It has long been known that cooling can also be achieved with a cooling system that operates in accordance with the cold air cycle, designated the open reversed Joule-Brayton cycle. Air (or some other gaseous substance) is the only working medium used in the cooling cycle of this system.

In a cooling system according to the cold air cycle, gas, normally air, is first compressed from ambient pressure, therewith significantly increasing the temperature of the gas. The hot compressed gas is then cooled in a heat exchanger with small pressure losses and thereafter caused to expand to essentially ambient pressure, or preferably a somewhat higher pressure. The heat exchange effected in the heat exchanger is normally with ambient air. This expanded gas is colder than the gas introduced into the compres- sor and can therefore be used for cooling purposes, for instance air-conditioning, which normally takes place at ambient pressure, in other words at the same pressure as the compressor inlet pressure.

In a system that operates in accordance with the cold air cycle, there is therefore no need to circulate a primary medium that is cyclically condensed and vaporised and from which cold is transferred to a secondary medium, such as air.

The system operating in accordance with the cold air principle has a lower cool- ing capacity than a conventional cooling system that has a closed circuit which includes a primary refrigerant such a freon. Consequently, the components of a system operating in accordance with the cold air principle will be relatively large and also expensive in relation to cooling capacity.

If a cooling cycle according to the cold air principle is used in a system where the nature of the objects that need to be cooled is such that a relatively large pressure drop will be obtained, the requirements placed on the cooling equipment components will be higher, since the pressure from the cooling equipment must be significantly higher than the ambient pressure. As a result of the requisite overpressure, the temperature at the ex- pander outlet will be higher than if the expander outlet pressure had been closer to the inlet pressure of the cooling equipment. It is necessary to compensate for this tempera- ture difference, by means of correspondingly higher compression in the compressor, so as to obtain a satisfactorily low expander outlet temperature. One situation in which this problem arises is when the cold air is used to cool electronic equipment, for instance electronic equipment in aircraft. Since the air has to pass through a large number of very confined ducts in this latter application, the air that leaves the expander will have a sig- nificant overpressure due to the flow resistance in the ducts.

Cooling systems that operate in accordance with the cold air principle for gener- ating cold air for the purpose of cooling objects that need to be cooled, for instance elec- tronic components in aircraft and also aircraft cabins are known to the art. For instance, such a cooling system is described in Swedish Patent Specification 9403966-6 (publica- tion number 504967). The system described in the Swedish patent specification includes two parallel compressors having inlet and outlet means, a heat exchanger, an expander having inlet and outlet means, compressor inlet conduits, connecting conduits that ex- tend from the compressors to a common heat exchanger inlet, and a conduit that extends from the expander outlet means to the object in need of cooling, for instance electronic components.

This described known system is still expensive, because it requires two compres- sors. The use of two compressors enables the gas supply to the expander to be adjusted, since at least one of the compressors is driven by a separate drive means and since its compressed gas volume can be varied with time. Another described method of adjusting

the flow to an object in need of being cooled is to release expanded cold air to the sur- roundings via a valve in a branch conduit, optionally in heat exchanging contact with the heat exchanger of the system.

Accordingly, an object of the present invention is to provide a controllable sys- tem for generating a cold air flow whose temperature can be regulated in accordance with cooling requirements.

Another object is to provide a controllable system that includes only one com- pressor and a helical screw expander, and also a method for controlling such a system.

These objects have been achieved in accordance with the invention with a device for generating cold air for cooling objects that require cooling, said arrangement includ- ing a compressor having inlet and outlet means, a heat exchanger, a helical screw ex- pander having inlet and outlet means, and a first conduit that connects the compressor outlet means to the heat exchanger, a second conduit that connects the heat exchanger to the expander inlet means, and a third conduit that extends from the expander outlet means to the object that needs to be cooled. The compressor and the helical screw ex- pander operate with a generally constant volumetric flow. The compressor may conven- iently be a helical screw compressor. The present invention is characterised in that a temperature sensor is placed in the expander outlet means or in the third conduit, and in that the temperature sensor functions to actuate means for regulating the flow to the ex- pander inlet means in accordance with the sensed temperature.

The method of controlling the inventive arrangement comprises sensing the tem- perature of air exiting from the expander in a position in or downstream of the expander outlet, and by adjusting the flow of air to the expander on the basis of the sensed tem- perature.

A change in the air flow to the expander will result in a change in the extent to which the pressure is lowered in the expander and therewith also to a change in the tem- perature of the outgoing air. A reduction in the air flow to the expander inlet will result in a lower expander inlet pressure and therewith in lower pressure reduction during the expansion phase. This lower pressure reduction results in a lower temperature drop, i. e. in a higher temperature of the outgoing air.

One advantage afforded by the present invention is that the system pressure, that is to say the pressure between the compressor outlet means and the inlet means of the

helical screw expander, can be reduced when cooling requirements decrease. Because the compression in a cold air system requires the highest mechanical effect, less power is required when the system pressure can be reduced.

Preferred embodiments of the present invention will be apparent from the de- pending Claims.

The invention will now be described in more detail with reference to a detailed description of preferred embodiments and also with reference to the accompanying drawings, in which Figure lisa schematic illustration of a first embodiment of an inventive cold air system; Figure 2 is a schematic illustration of a second embodiment of an inventive cold air system; Figure 3 is a schematic illustration of a third embodiment of an inventive cold air system; and Figure 4 is a schematic illustration of a fourth embodiment of an inventive cold air system.

Figure 1 illustrates schematically an embodiment of an inventive arrangement which includes in series a compressor 1 which operates with a substantially constant volume flow, a heat exchanger 2 which is in heat exchange contact with ambient atmos- phere, and a helical screw expander 3. The expander 3 also operates with a substantially constant volume flow. A first conduit 4 connects the compressor outlet means with the heat exchanger inlet means, a second conduit 5 is disposed between the heat exchanger outlet means and the inlet means of the helical screw expander 3. A third conduit 6 ex- tends from the expander outlet means to an object or objects (not shown) that needs/need to be cooled, for example electronic components of a stationary aircraft on the hard- standing. The object that needs to be cooled may be included as part of the conduit 6. An inlet conduit 15 may be connected to the inlet means of the compressor 1. The compres- sor 1 is driven by a drive means, not shown.

A branch conduit 10, which short-circuits the expander inlet and outlet means, is disposed between the second conduit 5 and the third conduit 6. A regulating valve 9 is mounted in the branch line 10. The regulating valve 9 can be adjusted between a closed position and a fully open position.

In the illustrated case, a temperature sensor 11 is placed in the third conduit 6, although it may alternatively be placed in the outlet means of the expander 3. The sensor 11 is connected electrically to a processor 7 which, in turn, is connected to a control de- vice 8. The control device 8 adjusts the throughflow area of the regulating valve 9 in re- lation to the temperature sensed by the temperature sensor 11.

When starting-up the arrangement, air is delivered to the inlet means of the com- pressor 1, via the inlet conduit 15. The supply air is compressed in the compressor 1 to a pressure of up to 3 bar, for instance. This compression means that the temperature of the air leaving the compressor outlet means will be much higher than the temperature of the air delivered to the compressor 1. The compressed air is delivered through the conduit 4 to the heat exchanger 2, in which an exchange of heat takes place with the ambient at- mosphere with only slight pressure losses, therewith cooling the air. The cooled, com- pressed air is delivered to the expander 3 through the conduit 5. The valve 9 is closed, so that no air can flow through the branch conduit 10. The pressure of the air is reduced in the helical screw expander 3, wherewith the temperature of the air falls significantly to a temperature that lies beneath the temperature of the compressor inlet air. The air leaving the expander 3 has a higher pressure than the ambient atmosphere, for instance a pres- sure of 0.11-0. 15 MPa.

When the temperature sensed by the temperature sensor 11 is lower than a pre- determined lowest temperature, the processor 7 sends a signal to the control device 8 which responds by opening the valve 9. Air can then flow through the branch-conduit 10 to the conduit 6. As air begins to flow through the valve 9, the pressure downstream of the compressor 1 drops. This means that the power required to drive the compressor 1 decreases. The pressure drop across the helical screw expander 3 also decreases, result- ing in a lower temperature drop.

The temperature of the air flowing through the branch conduit 10 is higher than the temperature of the air that has expanded in the expander 3. Thus, the temperature of the air in the conduit 6 increases after combining the air flows from the expander 3 and the branch conduit 10. This temperature change in the conduit 6 results in a reduction in the throughflow area of the valve 9.

Figure 2 is a schematic illustration of an embodiment of the arrangement illus- trated in Figure 1. Similar to the Figure 1 arrangement, the Figure 2 arrangement in-

cludes a compressor 1, a heat exchanger 2 which is in heat exchange contact with ambi- ent atmosphere, and a helical screw expander 3, these components being connected in series. The branch conduit 10 of the Figure 1 embodiment is replaced with a branch con- duit 12. The branch conduit 12 is also provided with a regulating valve 9 which is actu- ated by the control device 8. The branch conduit 12 extends from the conduit 5 and opens into a closed working chamber of the helical screw expander 3. In a helical screw expander, a working chamber has its smallest volume when closing against the expander inlet. During continued rotation of the expander, the volume of the closed working chamber will increase until the chamber comes into contact with the outlet of said ex- pander 3. The branch conduit 12 opens into a working chamber when its volume is greater than the smallest volume at the moment of closure.

Figure 3 illustrates schematically an arrangement which differs from the aforede- scribed arrangements by virtue of the fact that the compressor is a helical screw com- pressor 14 and that the branch conduit 10 of Figure 1 has been replaced with a branch conduit 16. Other components are identical to those of the Figure 1 embodiment. The branch conduit 16 of the Figure 3 embodiment also includes a regulating valve 9 which is actuated by the control device 8. The branch conduit 16 connects the supply conduit 15 to the outlet means of the compressor 14 or to the first conduit 4.

When starting-up the arrangement shown in Figure 3, air is delivered to the inlet means of the compressor 14 via the inlet conduit 15. The supply air is compressed in the compressor 1 to a pressure of, e. g., 3 bar. As a result of this compression, the tempera- ture of the air leaving the outlet means of the compressor 14 will be much higher than the temperature of the air supplied to said compressor. The valve 9 is closed, so that no air is able to flow through the branch conduit 16. The compressed air is delivered to the heat exchanger 2 through the conduit 4 and a heat exchange takes place in the heat ex- changer with the ambient atmosphere with only small pressure losses, therewith cooling the air. The cooled compressed air is delivered to the expander 3 through the conduit 5.

The pressure of the air is reduced in the expander 3, wherewith the temperature falls drastically to a temperature which lies beneath the temperature of the compressor supply air. The pressure of the air leaving the expander 3 is higher than the pressure of the am- bient atmosphere, e. g. a pressure of MPa.

When the temperature sensed by the temperature sensor 11 is lower than a pre- determined lowest temperature, the processor 7 sends a signal to the control device 8 which responds by opening the valve 9. Air is then able to flow back to the inlet conduit 15, through the branch conduit 16. The pressure downstream of the compressor 14 falls as air begins to flow through the valve 9. This means that less power is required to drive the compressor 14.

The temperature of the air flowing through the branch conduit 16 is much higher than the temperature of the ambient air supplied to the compressor 14. Consequently, the air entering the compressor 14 will have a higher temperature than the ambient atmos- phere and that the output temperature will also be higher. At the same time, the volume of air delivered to the heat exchanger and the helical screw expander 3 decreases, as does also the pressure upstream of the expander 3. The net result is that the air from the ex- pander 3 obtains a higher temperature, which results in adjustment to the throughflow area of the regulating valve.

The embodiment illustrated in Figure 4 is similar to the Figure 3 embodiment, but differs therefrom by virtue of the branch conduit 16 in Figure 3 having been replaced with a branch conduit 17. Instead of connecting the first conduit 4 with the inlet conduit 15, the branch conduit 17 in Figure 4 connects a closed working chamber in the com- pressor 14 with said inlet conduit 15.

In a helical screw compressor, a working chamber has its greatest volume when closing against the inlet means. As rotation continues, the volume of the closed working chamber decreases until the other end of said working chamber establishes fluid com- munication with the compressor outlet means. The branch conduit 17 is arranged in the compressor 14 so that a working chamber will come into contact with the orifice of the branch conduit prior to fluid connection with the outlet means being established and sub- sequent to establishing fluid connection with the inlet means.

The arrangement according to Figure 4 operates in a manner corresponding to the arrangement according to the Figure 3 embodiment.