Refrigerator

Abstract

 A refrigerator, more especially for cooling miniature devices to low temperatures, makes use of a closed cycle system based on the Sterling Cycle, with the moving elements of the system being actuated independently in the appropriate sequence.

Description

[0001] This invention relates to a refrigerator capable of cooling miniature devices to low temperatures. Such a refrigerator could have application, for example, for the spot cooling or freezing in biological or medical procedures, in micro-chemical analysis, for the spot cooling or temperature stabilization of certain instruments or equipment, and for the cooling of miniature super-conducting devices.

[0002] At the present time refrigeration to temperatures of the order of 80K is usually achieved using liquid nitrogen or other liquefied gases, and an object of the present invention is to provide a form of refrigerator capable of achieving these and even lower temperatures.

[0003] According to the invention a refrigerator comprises a closed cycle system employing a compressor communicating through a regenerator with an expander, and a pair of displacers communicating respectively with the passages between the compressor and regenerator and between the expander and regenerator, these elements being actuated independently in a Stirling cycle sequence to remove heat from the region around the expander.

[0004] The elements may be in the form of pistons operating in cylinders or movable diaphragms, actuated by electric or electronic means in the appropriate sequence, for example utilising thermal expansion, or electro-static or electro-magnetic forces. By arranging for the elements to be operated independently it is possible to achieve efficiencies much closer to that of a theoretical Stirling cycle than is possible with refrigerators of known kind, in which displacement, compression and expander piston motions are linked, for example by rotation of a crank, so that their movement is inherently less than a close approximation to that of an ideal Stirling cycle.

[0005] The present inventions is particularly applicable to micro-refrigerators, and by appropriate construction of heat exchange surfaces, high efficiencies and low temperatures can readily be achieved.

[0006] Preferably for such an application the compressor, expander and displacers each has a displacement volume of not more than 2.0 x 10⁶ µm.

[0007] The heat exchange surfaces can be formed by micro-machining or etching micro-sized grooves in the surfaces, i.e. grooves having a width and depth of the order of 10 microns or less.

[0008] The grooved surfaces may be formed of, or plated by a metal having high thermally conducting properties, such as silver or copper. In the case of the regenerator the platings may be broken a plurality of times in the flow direction to provide high axial thermal resistance to improve its performance.

[0009] By utilising nitrogen as the working substance it is possible to achieve temperatures as low as 100K, and for lower temperatures helium can be used.

[0010] The invention will be further explained by describing, with reference to Figures 1 to 6 of the accompanying schematic drawings one micro-cryorefrigerator in accordance with the invention and its manner of operation.

Figure 1 illustrates in diagrammatic form only, the elements of the refrigerator;

Figure 2 represents a Temperature-Entropy diagram;

Figure 3 illustrates volume variations of the working gas of the refrigerator over a complete cycle of operations in an ideal situation;

Figure 4 shows how the ideal motions may be replaced by simple harmonic motions;

Figures 5A and 5B represent sections of a suitable regenerator, and

Figures 6A, 6B and 6C illustrate suitable sections of ducts adjoining the compressor and expander.

[0011] Referring first to Figure 1, the refrigerator comprises a compressor C and an expander E connected by a duct 10 in which there is located a regenerator 11. Side ducts 12 and 13, located respectively between the compressor and regenerator and between the regenerator and expander, communicate with displacers A and B. For ease of explanation the compressor, expander and displacers are shown as piston and cylinder units, although in practice these may be replaced by devices incorporating movable diaphragms; the reciprocating movement of the pistons/diaphragms, which will hereinafter be referred to as pistons for simplicity, are independently controlled, for example by electro-static or electro-magnetic forces. The pistons of the displacers A and B are arranged to operate in anti-phase as indicated by the broken line 14, although they are not necessarily mechanically linked.

[0012] The system contains a working substance in the form of a gas, which may, for example be nitrogen, helium or hydrogen.

[0013] At the commencement of a cycle of operations (Position 1 of Figures 2 and 3) the piston 15 of the compressor C is in its withdrawn state as shown, corresponding to the maximum volume condition, and is moved to compress the gas in an adiabatic, isentropic manner, the volume V_C being thereby reduced (Position 2). Between positions 2 and 3 of the cycle the piston 15 remains stationary and heat produced during compression is rejected to atmosphere at constant pressure.

[0014] Further cooling of the gas takes place on passage through the regenerator 11 on movement of the pistons of the displacers A and B in opposition (Positions 3 to 4), the volume of gas remaining constant the volume V_A in the displacer A decreasing and the volume V_B in the displacer B increasing. The piston of the expander E is then withdrawn (Positions 4 to 5) to increase the volume V_E in the expander in an adiabatic isentropic manner, producing a further cooling of the gas. Heat is then transferred from the surroundings of the expander (Positions 5 to 6) and the gas is finally transferred back to the compressor C (Positions 6 to 1) with the pistons of the compressor C and expander E moving in opposite senses, and those of the displacers similarly returning to their initial positions, the total volume of gas being constant during this part of the cycle, and the gas cooling the regenerator 11 on its passage therethrough.

[0015] The ideal motions of the compressor C, expander E and the displacers A, B may be replaced by simple harmonic motions, as shown in Figure 4, in which the volumes of gas in the various elements are plotted against time, V_cl representing the minimum volumes, and V_cl + V_s the maximum volumes of the elements. The frequency of operation of the displacers A and B is half that of the compressor C and expander E, which are intermittently stopped over half the cycle. Thus compressor C is shown as stopped between π and 3π and the expander betwen 0 and 2π. Whilst the overall efficiency of the micro-refrigerator will be somewhat less than ideal, it will in general prove adequate for effective cooling to low temperatures depending upon the gas used as the working substance.

[0016] The regenerator 11 is conveniently constructed as illustrated in Figures 5A and 5B, which represent a longitudinal section, and a transverse section across the line A-A, respectively, the regenerator being in the form of a rectangular structure formed by two superposed plates 16, 17 approximately 10mm long and 4mm wide. The covered surface of the plate 16 is provided with a series of longitudinally extending micron-sized grooves 18, through which the gas is passed in use. In each of the grooves there are a number of deposits 19 of a good thermally conductive material, for example silver or copper, spaced apart longitudinally in order to provide high axial thermal resistance to improve the regenerator's performance.

[0017] The parts of the duct 10 adjacent the compressor C and expander E may be formed as shown in Figures 6A, 6B and 6C which represent a side section, a plan section and a transverse section respectively of the relevant parts of the duct. This comprises a main duct element 20 having a bore 21 which widens towards a central region 22. One side of the duct element 20 is formed with an opening over this central region 22 into which is fitted, so as to close the opening, a copper block 23 having its inner surface etched with longitudinal grooves 24, to increase the surface area contacting the gaseous working fluid on its passage through the duct, and thereby enhance the heat transfer qualities.

[0018] Suitable displacement volumes for a micro-refrigerator as above described are as follows :-
   V_A = 0.786 x 10⁶ (µm)³
   V_B = 0.550 x 10⁶ (µm)³
   V_C = 0.786 x 10⁶ (µm)³
   V_E = 1.02 x 10⁶ (µm)³.

[0019] It can be demonstrated that with nitrogen as the working gas it is possible to refrigerate down to temperatures of the order of 100K, with a temperature drop of from 300K to 208K in 3 seconds for a mass having a heat capacity of 4.2 micro J/K. With helium or hydrogen as the working gas temperature of down to 10K and 50K respectively may be achieved.

[0020] It will, however, be realized that the construction of the various elements of refrigerators in accordance with the invention may take many different forms, with dimension selected to suit the particular applications of the refrigerator.

Claims

1. A refrigerator comprising a closed cycle system employing a compressor communicating through a regenerator with an expander, and a pair of displacers communicating respectively with the passages between the compressor and regnerator and between the expander and regenerator, these elements being actuated independently in a Stirling cycle sequence to remove heat from the region around the expander.

2. A refrigerator according to Claim 1 wherein the compressor, expander and displacers are actuated in the appropriate sequence by electric or electronic means.

3. A refrigerator according to Claim 2 wherein the compressor, expander and displacers are actuated by thermal expansion or electro-static or electromagnetic forces.

4. A refrigerator according to any preceding claim wherein the compressor, expander and displacers each has a displacement volume of not more than 2.0 x 10⁶ µm³.

5. A refrigerator according to Claim 4 wherein the heat exchange surfaces are formed with grooves having a width and depth of not more than 10 microns.

6. A refrigerator according to Claim 5 wherein a grooved heat-exchange surface is formed of or coated by a metal having good thermally conductive properties

7. A refrigerator according to Claim 6 wherein the coating on the grooved heat-exchange surface of the regenerator is broken a plurality of times in the flow direction of the working gas to provide higher axial thermal resistance.

8. A refrigerator according to any preceding claim wherein the working gas is nitrogen, helium or hydrogen.

Drawing