BACKGROUND OF THE INVENTION

The present invention relates to a cooled air cycle system as specified in the preamble of claim 1 and to a method for operation such a system.

Refrigeration systems usually employ a two-phase cycle with a refrigerant that alternates between gaseous and liquid states. Such systems have a large cooling capacity due to the amount of heat that can be taken up when the refrigerant evaporates and are therefore superior to any alternative in most applications, normally also for air-conditioning. However, the equipment in such a system is circumstantial and expensive and requires a closed piping circuit to isolate the usually pollutional refrigerant from the environment. And since refrigeration systems at the end transfer the cold through air or another gas, these conventional systems incorporates both a primary cooling medium, i.e. the refrigerant in the closed circuit and a secondary cooling medium, i.e. the air that transfers the cold to the object to be cooled.

Since long it has been known that refrigeration also can be attained by a cooled air cycle system, also called open reversed Brayton cycle. In such a system the air itself is the working medium in the refrigeration cycle. There is thus no need for a special refrigerant that is cyclically condensed and evaporated and there is no need to transfer the cold from a refrigerant to the air, since they are one and the same. Such systems therefore also eliminate the risk for pollution. In a cooling system according to this principle the gas, normally air is first compressed, whereby the pressure and the temperature rise. The warm compressed gas is then cooled in a heat exchanger and thereafter expanded with substantially the same pressure ratio as that of the compressor. The air leaving the expander will be cooler than the air entering the compressor and can perform its cooling purpose, e.g. air-conditioning, which normally takes place at atmospheric pressure, i.e. the same pressure as the compressor inlet pressure if located at ground level.

Examples of system operating according to the cooled air cycle are disclosed in US 3 045 4 US 3 686 893, US 3 965 697 and DE 42 18 299.

Cooled air cycle systems have small cooling capacity in comparison with conventional cooin systems, which is the main reason why they rarely have come into use. For some application however, where conventional cooling systems due to environmental and other reasons are n suitable a cooled air cycle system might be an advantageous alternative. The low efficiency such a system, however, calls for improvements in this respect in order to make the cooled cycle system more competitive, both for cooling applications e.g. air conditioning and for he pump applications.

The object of the present invention thus is to improve the efficiency of a cooled air cycle system.

SUMMARY OF THE INVENTION

This has been attained in that a system of the kind specified in the preamble of claim 1 inclu the features specified in the characterizing portion of that claim, and in that a method for operating such a system includes the features specified in the characterizing portion of claim

In a system of this kind used for air-conditioning a large part of the cooling effect for coolin the compressed air before reaching the expander is consumed for the condensation of the water vapour contained in the incoming air from ambient. In a typical example when the incoming air has a temperature of 40 °C and a humidity of 70 % and the cooling air leaving the expander has a temperature of 3°C and being saturated, the energy for cooling the compressed air will be about 35 kJ/kg air and the energy for condensing the water vapour i the air will amount to about 80 kJ/kg air. Most of the cooling effect thus is consumed for condensing the water in the air.

By providing the regenerative dryer in the conduits between the compressor and the expan a large amount of the effect used for the condensation is saved, and the efficiency of the system will be considerably increased. This kind of dryers is well-known for drying atmospheric or compressed air and forms per se no part of the invention.

In a preferred embodiment of the invention both the compression and expansion take place in rotary screw machines, whereby a particularly efficient system is attained due to the high volumetric capacity of such machines.

It is advantageous to arrange the dryer so that water is adsorbed in two steps and provide heat exchangers for cooling the air before and after each step, and a suitable dryer is of the rotating type.

These and other advantageous embodiments of the invention are specified in the dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained through the following detailed description of a preferred embodiment thereof and with reference to the accompanying drawings, of which

Fig. 1 is a functional block diagram representation of a system according to the invention and fig. 2 is a schematic section along line II-II of fig. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 1, schematically illustrates the invented system in an application producing cold dry air for air-conditioning. A rotary screw compressor 1 receives atmospheric air through an inlet conduit 8 and delivers the compressed air to an outlet conduit 9 at a raised pressure, typically about 3 bars and at a raised temperature due to the compression. The air then flows through conduits 9 to 15 to a rotary screw expander 2 in which the air is expanded to a pressure slightly above the compressor inlet pressure and leaves the expander 2 to a delivery conduit 16.

Between the compressor 1 and the expander 2 means a provided for cooling the air and for withdrawing water therefrom, so that the air reaching the expander 2 will have slightly higher temperature than the compressor inlet air. After the expansion the temperature of the air will be considerably lower and can be used for cooling purposes. The cooling and water

separating means consist of a first 3, a second 4 and a third 5 heat exchanger which cool th air, a water separator 6 connected to the first heat exchanger 3 and a regenerative dryer 7.

The hot air from the compressor 1 flows through conduit 9, a regenerating section A of th dryer 7 and conduit 10 to a first heat exchanger 3, in which the air is cooled.

A water separator 6 is connected to the first heat exchanger 3 for withdrawal of water contained in the air, which water has been condensed through the cooling in the heat exchanger 3. Thereafter the air flows through conduit 11 to a second section B of the drye in which the water adsorbing structure thereof adsorbs water vapour from the air, thereby decreasing its humidity. The air leaving the second section B of the dryer 7 then passes a second heat exchanger 4 further lowering the air temperature and is then conducted throu conduit 13 to a third section C of the dryer 7, where further vapour can be adsorbed due t cooling in heat exchanger 4. After leaving section C of the dryer 7 the air flows via a third heat exchanger 5 for further cooling to the expander 2. Due to the drying of the air in sect B and C of the dryer the humidity of the air reaching the expander is very low, representin dew temperature of about -20° C. The temperature fall of the air during the expansion therefore will not condense any water, and the air can be delivered for cooling without ris for condensation on the delivery side.

The regenerative dryer 7 is of the rotating type, driven by an electric motor 17 around axi at a speed of 5 to 10 rpm. A generally cylindrical rotor structure 18 capable of adsorbing water rotates by the motor 17 in a housing 19 sealingly co-operating with the rotor 18, w housing 19 is provided with openings connected to the air conduits. Dryers of this type ar available on the market. The rotor structure of such a dryer typically is built up by glass fi reinforced paper in a honeycomb pattern, which is impregnated with silicagel as the active substance.

Fig. 2 schematically illustrates the dryer 7 as seen along line II-II of fig. 1. Each of the thr sectorshaped sections A, B, C is delimited by a seal element (not shown) attached to the housing and contacting an end of the rotor structure along a loop, these loops being represented by the shaded areas in the figure. In the three sections the connections to the conduits 9, 12 and 13 are indicated by end views of arrow symbols illustrating in- and

outflow, respectively. It is to be noted that the connections of conduits 9, 12, 13 to the sections A, B, C lie above the plane of the drawing and end in correspondingly shaped spaces which face the rotor structure, so that the air is admitted to the entire of each section.

As the rotor structure 18 rotates, each part thereof passes the three sections A, B, C, delimited by the seal elements on each end of the rotor 18, and each section communicates with an inlet at one side and an outlet on the opposite side. In section B the rotor structure 18 adsorbs vapour from the air passing from conduit 11 to conduit 12 and in section C vapour from the air passing from conduit 13 to conduit 14, which vapour condensates as it adsorbs in the structure. When the structure carrying the condensed water reaches section A it comes into contact with the hot compressed air from the compressor, which has low humidity so that the adsorbed water will evaporate and flow with the air through conduit 10 to the cooling heat exchanger 3. The major part of this water vapour and that of the incoming air condenses due to the cooling and is withdrawn in the water separator 6. The structure thereby is regenerated when passing section A and will thus be capable to adsorb water when reaching sections B and C the next time.

By the use of the regenerative heat exchanger in the cooled air cycle system the cooling effect required for cooling the compressed air before the expansion has been reduced to less than a third at the cost of the almost negligible power for driving the dryer.