WO1990012961A1

WO1990012961A1 - Stirling cycle machine and compressor for use therein

Info

Publication number: WO1990012961A1
Application number: PCT/GB1990/000593
Authority: WO
Inventors: Gordon Davey
Original assignee: Isis Innovation Limited
Priority date: 1989-04-21
Filing date: 1990-04-19
Publication date: 1990-11-01
Also published as: GB8909045D0; GB9028045D0; WO1990013170A1; GB2239494A

Abstract

A compressor (32) has a housing (34) to which is fixedly secured a piston (36) disposed axially of the housing. A cylinder (40) is arranged for axial movement relative to and coaxially with the piston to define with the piston a variable volume compression chamber (58). The cylinder carrying an electromagnetic drive coil (62) and the housing contains a magnetic circuit cooperable with the drive coil to cause said axial movement. By arranging for movement of the cylinder relative to a fixed piston, increased reliability and compactness can be achieved.

Description

Title of the Invention

STIRLING CYCLE MACHINE AND COMPRESSOR FOR USE THEREIN

Field of the Invention

This invention relates to a Stirling cycle machine and to a compressor suitable for use therein.

Background to the Invention

The manufacture of refrigeration units has become increasingly specialised, depending on the application of the unit. For example, in military and aircraft applications size and weight are vital whereas in space applications accuracy and reliability of operation are all important. It follows from this that the design of a compressor for use with a refrigeration unit for any particular use must be optimised to that particular use. However, some problems are universal and new concepts are needed to overcome such problems.

This invention is particularly concerned with compressors which are activated by an electromagnetic drive system in which a drive coil is arranged in the air gap of a magnetic circuit such that it intersects the field lines so that an E.M.F. supplied to the drive coil causes movement of a movable component secured to the drive coil. In known compressors the drive coil is attached to the piston of a piston and cylinder arrangement.

One application of such compressors is in closed cycle miniature refrigerators used to cool electronic devices in remote environments. This application requires high reliability, high efficiency and long maintenance-free life. In some environments, the refrigerator must have a small size, low weight, low vibration and the ability to operate in high ambient temperatures and large acceleration fields. A closed cycle refrigerator designed to meet many of the above requirements is described in an article entitled "Miniature Stirling cycle cooler" by G. Davey and A.H. Orlowska published in Cryogenics Vol. 27 March 1987 pps. 148-151.

This refrigerator is a split cycle Stirling cycle machine consisting of a compressor and a cold head connected by a pipe. The compressor has a piston rigidly attached to a shaft and mounted for reciprocal movement within a cylinder. The shaft is suspended from two sets of flat springs, one attached near the piston and supported at its outer edge from the refrigerator case, the other at the opposite end of the shaft and supported by a magnetically soft iron component providing a magnetic circuit. The suspension springs hold the piston clear of the cylinder and constrain it to move along the axis of the cylinder. Beryllium copper springs have been tested and found to have an effectively infinite fatigue life under the operating conditions encountered in the refrigerator. Other materials have also been found to be suitable. The compressor piston is driven by applying an alternating E.M.F. to a coil arranged in an air gap of the magnetic circuit.

The cold head contains a displacer and regenerator which consists of a stack of gauze discs. The displacer reciprocates to cause gas to flow to and fro through the regenerator. The refrigerator is filled to the operating pressure with helium gas which has been passed through a nitrogen trap to remove condensable impurities. If the refrigerator is operated correctly, heat is extracted from the cold end of the cold head. Reference is also made to U.S. Patent No. 4,397,155 and British Patent No. 2078863B which describes a Stirling cycle machine in which the compressor is connected to an electromagnetic device and in which the relationship between the movements of the displacer and the compressor is controlled by a second electromagnetic device.

In the arrangements described above, both the displacer plunger and the compressor piston are driven, and the electromagnetic drives are connected by systems whereby the phase difference can be controlled. However, some forms of Stirling cycle machines are known in which the displacer plunger acts as a "free piston" and in which, by appropriate choice of the design parameters the natural frequency of the the displacer can be selected so that the displacer responds to the compressor output with movements that show the correct difference in phase from those of the compressor itself.

A particular problem arises with known compressors in constraining a long spring-mounted piston shaft to travel axially. Some compressors rely on clearance gas seals for their operation. Any bend or sideways motion of the piston destroys this clearance and disrupts operation of the compressor.

Summary of the Invention

According to one aspect of the present invention there is provided a compressor having a housing to which is fixedly secured a piston member disposed axially of the housing and providing a piston head region, there being a cylinder arranged for axial movement relative to and coaxially with the piston head region to define with the piston a variable volume compression chamber, the cylinder carrying an electromagnetic drive coil and the housing containing a - A -

magnetic circuit cooperable with the drive coil to cause said axial movement.

As a cylinder can be manufactured to be far more rigid and torsion resistant than a thin shaft carrying a piston, the clearance seal can be maintained far more efficiently and with greater reliability by keeping the piston fixed and moving the cylinder. In addition, a cylinder has in its end annular surface a convenient mounting point for springs used to mount the cylinder in the housing. Furthermore, since a cylinder can be supported at both ends, in contrast to a piston, it is easier to maintain axial alignment, and to constrain the cylinder to travel axially.

A further problem arising in known compressors is that there is significant vibration when the compressor piston is reciprocating, resulting in noise and greater wear and tear.

A compressor which includes two pistons which move in antiphase in order to reduce vibration is known from an article by A.K. de Jonge and H.J. Helmonds, published in the Proceedings of the Twelfth International Cryogenic Engineering Conference, Southampton U.K. 12-15 July 1988.

In an attempt to reduce vibration of the compressor whilst also providing a compressor having a moveable cylinder, which therefore has the advantages discussed above, the compressor can be further provided with a second piston head region positioned in line with the first piston head region, and a second cylinder defining with the second piston head region a second variable volume compression chamber and arranged for axial movement relative to and coaxial with the second piston head region, the second cylinder carrying a second electromagnetic drive coil and the housing carrying a second magnetic circuit cooperable with the second drive coil to cause said axial movement, the compressor being controllable such that the two cylinders move in antiphase so that there is no or only negligible unbalanced momentum while the machine is in operation.

This balanced, moving cylinder configuration provides a compact, low vibration compressor.

The first and second piston head region may be provided by a common piston member.

The compressor may be combined with a cold head arranged to form a split-cycle machine, for example of the type described in the article by G. Davey and A.H. Orlowska referred to above, the compressor and cold head being, linked by means of a conduit which transmits pressure pulses therebetween.

Alternatively, the compressor may be combined with a cold head in an integral cycle machine which has a shared common compression chamber. The present invention provides in another aspect an integral cycle machine comprising a compressor having a housing to which is fixedly secured a piston member disposed axially of the housing and providing a piston head region, there being a cylinder arranged for axial movement relative to and coaxially with the piston head region to define with the piston a variable volume compression chamber, the cylinder carrying an electromagnetic drive coil and the housing containing a magnetic circuit cooperable with the drive coil to cause said axial movement; and a cold head, the cold head having a displacer plunger movable within a displacer cylinder, wherein the cold head is arranged so that a surface of the plunger remote from the closed end of the displacer cylinder faces into the compression chamber of the compressor, so that the compressor and displacer have a shared common compression chamber.

By providing the compressor and cold head in a single unit having a moving cylinder and stationary piston head_.region it is possible to produce a particularly light and compact machine of high efficiency, which also enjoys the advantages associated with a moveable cylinder.

Another problem which arises in known compressors concerns the magnetic circuit. In order to produce the required magnetic field, the magnetic material takes the form of a relatively flat disc, of large diameter and small axial length. This produces an axial magnetic field in the magnet and a radial magnetic field in the air gap, which linear motion devices have always relied upon for their operation. The use of an axial magnetic system, i.e. one in which there is an axial field in the magnet, can place undesirable constraints on the design of the compressor. In low size, low weight applications the size of the diameter of the magnet can be the limiting factor on the lowest possible size of the compressor. In large compressors, the cost of the magnet is a significant proportion of the overall cost, in some cases more than half.

The^' magnetic circuit can be arranged to provide a radial magnetic field.

The magnet may then be located in a part of the magnetic circuit where the flux is radial, but is not in the same part of the circuit as the air gap. The magnet can then be optimised independently of the conditions in the air gap. Further the flux density is only limited by saturation of the soft magnetic material, and it is possible to design a practical magnetic circuit, using pure iron, with an air gap flux density of 1.6 Tesla (and possibly higher) .

Particular forms of this magnetic circuit are described more fully in our copending International Patent Application No. (Page White & Farrer reference 66145/VRD)

The combination of a moving cylinder and a radially magnetised arrangement is particularly effective in reducing the size of the compressor to a minimum and in providing a compact and mechanically reliable arrangement.

Brief description of the drawings.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the acompanying dawings in which:

Figure 1 is a sectioned view of a displacer of a split-cycle Stirling cycle machine;

Figure 2 is a partially sectioned side-view of a balanced compressor arrangement of a split-cycle Stirling cycle machine;

Figure 3 is a sectioned view of a moving cylinder compressor arrangement with an improved magnet circuit;

Figures 4 and 5 are diagrammatic views of an axial and a radial magnet system respectively; and

Figure 6 is a sectioned view of an integral Stirling cycle machine.

By way of example only, the compressor arrangement of the invention is described as applied to Stirling cycle refrigeration units. Description of the preferred embodiments.

Figure 1 illustrates a cold head 2 which operates on a known principle. The cold head 2 comprises a displacer plunger 4 movable within a cylinder 6 having a regenerator 8 and separated from the regenerator by a small annular clearance 10. Movement of the plunger 4 to and fro within the cylinder 6 causes gas to be displaced through the regenerator 8 in alternate directions between the blind end 12 of the cylinder and the opposite end 14. Operation of the cycle causes t'he blind end 12 to become relatively cold and the end 14 relatively warm. The displacer plunger 4 is connected to one end of a rod 16 constrained to axial travel by two sets of flat spiral springs 18 which connect the rod 16 to the fixed structure of a housing 20. The cold head 2 is connected to a compressor 32 (shown in Figure 2) by a length of pipe indicated . diagrammatically by reference numeral 24 which serves to transmit pressure pulses from the compressor 32 to the cold head 2 these pulses being applied across a shaft clearance seal 24. The displacer plunger outer surface and the displacer shaft clearance seal 24 are lined with self-lubricating plastic material 26 so that accidental contact is not damaging.

The Stirling cycle machine described herein is a "free-piston" machine, that is the piston is free of any mechanical drives or linkages. The compressor can be arranged to run close to the resonant frequency formed by the moving mass/gas volume. In an embodiment of the invention, the resonant frequency is designed to be close to 50 or 60 Hz, which in fact is near the optimum for high efficiency of the transient heat transfer processes which occur in the regenerator 8. It is important that the displacer 4 moves in a proper phase relative to the compressor 32, and this may be achieved by suitable design of the resonant frequency of the displacer 4. The pressure pulses from the compressor 32 which are applied across the shaft clearance seal 24 in the cold head 2 perform the dual function of causing reciprocal movement of the displacer plunger 4 and providing the correct pressure variation for the Stirling cycle on which the machine operates.

Turning now to Figure 2, one example of a balanced, moving cylinder compressor arrangement will be described. The compressor 32 comprises a housing 34 in which an axially extending piston member 36 is fixedly mounted. The piston member provides two axially opposite piston head regions one of which 36a is shown in Figure 2. Two similar closed end cylinders, one of which 40 can be seen in Figure 2, are arranged for axial movement relative to the respective opposite piston head regions of the single piston member 36. The cylinder 40 is mounted by two sets of flat spiral spring suspensions 42, spaced apart lengthwise of the cylinder 40, to a fixed structure 44 of the housing 34. The annular end surface 40a of the cylinder 40- provides a convenient mounting point for one of the spring suspensions 42. The other of the spring suspensions can conveniently be mounted in a recessed annulus 40b on the base of the cylinder. The spring suspensions of the compressor piston and displacer plunger are similar, the springs being arranged in groups of six. Of course, any other number of springs could be used according to specific design requirements. The springs are manufactured from Beryllium copper sheets using photographic and etching processes. Other suitable materials could be used for the springs, in particular stainless steel. A polymer liner 48 is disposed adjacent the inner wall of the cylinder 40, spaced slightly from the inner wall to provide a clearance seal 50. Optimum radial clearances differ with the size of the compressor. There is an annular gap 52 between the piston 36 and the inner wall of the passageway 38 of the housing 34. The annular gap 52 communicates with the pipe 24 interconnecting the cold head 2 and the compressor 32 via a passageway 56. There is a compression chamber 58 defined by the piston head region 36a of the piston member 36, the closed end 41 of the cylinder 40 and walls 43 of the cylinder. A similar arrangement is mounted relative to the other end of the piston member 36 (not shown) the cylinders being arranged so that they face one another.

The cylinder 40 has a radial flange with a depending periphery 60 carrying a drive coil 62 which is connected to a source 64 of alternating E.M.F. The coil 62 is mounted in the air gap 66 of a magnetic path formed by a soft iron component 46 which extends circumferentially of the cylinder 40 and has facing axially spaced annular, surfaces holding a permanent magnet 70. The magnet 70 in this embodiment is in the conventional form of a disc with a large diameter relative to its axial length.

In use, the coil 62 is activated by the source of E.M.F. 64, which, in cooperation with the magnetic path formed by the soft iron component 46 and the permanent magnet 70, causes the cylinders 40 to reciprocate relative to the piston member 36. This of course has a pulsating effect on the pressure in the compression chamber 58 which is transferred via the annular gap 52 and the passageway 56 to the displacer 2. It is an important feature of this embodiment of the invention that the housing 34 contains two "back-to-back" cylinders such as that designated at 40, arranged to operate in antiphase. This has the significant advantage that the compressor is balanced, that is there is no or only negligible out of balance momentum arising during compressor operation. A further advantage of using a pair of back-to-back compressors is that they both "see" a common gas spring and the same damping, so that the balancing is not affected by gas leakage. It is also pertinent to note that it is the cylinder, and not the piston, which reciprocates. This has the advantage that the overall compressor arrangement is more compact, the manufacture of long, straight shafts can be avoided and there is less wobble, enabling the clearance seal to be maintained without touching because the centre of gravity of the assembly is kept between the springs 42 rather than being outside them.

A position transducer, for example a strain gauge attached to one of the flat springs 42 can be used for measuring and controlling displacement of the cylinder 40. Balance could be maintained using a measure of cylinder displacement, and start-up of the machine could also be controlled. The control of the compressor is carried. out by electrical circuitry which affects the E.M.F. supplied to the coils 62, with feedback if necessary from the position transducer to maintain motion of the cylinders 40 in antiphase.

Although the compressor arrangement shown in Figure 2 has several significant advantages of the prior art, it has been found that the design can be improved still further by changing the magnetic circuit, as shown in Figure 3. In Figure 3 like numerals designate like parts as in Figure 2. Some aspects of the arrangement can however be seen more clearly in Figure 3. For example, the cylinder is seen to have a cylinder head 80 having a radial flange 85 secured to cylinder walls 82 by bolts 84. Bolts 86 hold the inner periphery of one of the disc spring stacks 42 in place relative to the cylinder walls. Reference numerals 86' designate securing bolts for the other spring stack 42. The cylinder wall 82 also has a radially extending flange to which is secured, by the bolts 86, a spider 88 which carrries the drive coil 62. Reference numeral 90 designates bolts used for securing the outer periphery of the disc springs 42 to a part 92 fixed to the housing. Apart from the magnetic system, the construction and operation of the Figure 3 embodiment is substantially the same as that described with reference to Figure 2, and so will not be described further.

Reference numeral 94 designates a magnet which provides a radial magnetic field. Reference is made to Figures 4 and 5 which show diagrammatically respectively a conventional, axial magnetic circuit and a radial magnetic circuit. It can be shown by calculation of the gap aspect ratio of the magnetic circuit that the diameter of a radial magnetic circuit can be made to be significantly less than that of the axial magnetic circuit, although its length is then slightly larger. This configuration is ideally suited to the moving cylinder compressor described in the present application since it enables the external diameter to be kept to a minimum. Further, it enables the bulk of magnetic material to be reduced quite significantly, which, for larger compressors, significantly reduces the overall cost. A more detailed description of the radially magnetised system is included in our copending International Patent Application No. (Page White & Farrer reference

66145/VRD) .

A further aspect of the invention is now described in relation to Figure 6. Although the embodiments previously described in relation to Figures 1 to 3 relate to so called "split-cycle" machines, the moving cylinder arrangement shown therein can equally be employed in many other machines.

Figure 6 shows an integral cycle machine in which the cold head and compressor are arranged in a single unit. A piston 100 is fixedly mounted on a housing 102. The piston has central passageway as described in more detail herein. A cylinder 104 is suspended on two sets of flat, spiral arm springs 106 which allow reciprocating movement of the cylinder 104 relative to the piston 100. A clearance seal 107 is provided between the piston 100 and cylinder 104.

The form of the cylinder arrangement is similar to that of the embodiment of Figure 2, that is, it includes a cylinder wall and a radial flange 108 with a depending periphery 110 which carries a drive coil 112 connected to a source of alternating E.M.F. (not shown in figure 6). The coil 112 is mounted in an air gap 114 of a magnetic path formed by a soft iron component 116 and pole piece 118 which extend circumferentially of the cylinder 104 and have facing axially spaced annular surfaces holding a permanent magnet 120. The magnet 120 in this embodiment is in the convential form of a disc with a large diameter relative to its axial length.

Alternatively, a radial type magnetic drive arrangement as was described in relation to Figure 3 could equally be employed here.

Turning now to the cold head, this includes a thin walled displacer cylinder 124, closed by a copper cap 126, in which the displacer plunger 128 is able to move. A lower part of the displacer extends within the passageway of the piston 100, a lower surface 130 of the displacer facing into a compression chamber 122 defined by the cylinder walls, a bush 138 which forms the cylinder head and a piston head region 131 of the piston 100, in addition to the lower surface 130. An annular gap 132 conducts gas from the compression chamber 122 to a regenerator 134, the heat of compression being removed as the gas passes through this gap 132. The regenerator 134 comprises a stack of metal mesh discs contained within the displacer plunger 128. The shaft 136 of the displacer passes through the bush 138 and is attached to spiral arm suspension springs 140. A clearance seal 142 is provided between the displacer shaft 136 and the bush 138.

In operation, the coil 112 is activated by the source of E.M.F. and causes the cylinder to reciprocate relative to the piston, the gas in chamber 122 being compressed and expanded. The changing gas pressure acting over the clearance seal 142 causes the required oscillation of the displacer.

As shown in Figure 6, the displacer shaft 136 is support by a single set of suspension springs 140. Alternatively, the shaft 136 can be lengthened and a second set of suspension springs attached some distance from this first set, in order to hold the displacer in more precise axial alignment, thus reducing the chances of contact between the displacer plunger 128 and the displacer cylinder 124, and displacer shaft 136 and bush 138.

By arranging the compressor and cold head in a single unit with the compression chamber located adjacent the lower surface of displacer plunger, it is possible to provide a particularly small overall size for the machine. Thus the machine is particularly useful in applications where low size and weight are important. Industrial Application

The compressor and Stirling cycle machine described above are particularly useful for refrigeration in hostile environments and where low weight and long term reliability are required.

Claims

CLAIMS ;

1. A compressor having a housing to which is fixedly secured a piston member disposed axially of the housing and providing a piston head region, there being a cylinder arranged for axial movement relative to and coaxially with the piston head region, to define with the piston a variable volume compression chamber, the cylinder carrying an electromagnetic drive coil and the housing containing a magnetic circuit cooperable with the drive coil to cause said axial movement.

2. A compressor as claimed in claim 1, further provided with a second piston head region positioned in line with the first piston head region, and a second cylinder defining with the second piston head region a second variable volume compression chamber and arranged for axial movement relative to and coaxial with the second piston head region, the second cylinder carrying a second electromagnetic drive coil and the housing carrying a second magnetic circuit cooperable with the second drive coil to cause said axial movement, the compressor being controllable such that the two cylinders move in antiphase so that there is no or only negligible unbalanced momentum while the machine is in operation.

3. A compressor according to claim 2 wherein the first and second piston head regions are provided by a common piston member.

4. A compressor according to any preceding claim in combination with a cold head arranged to form a split cycle machine, the compressor and cold head being linked by means of a conduit which transmits pressure pulses therebetween .

5. A compressor according to any preceding claim wherein the magnetic circuit is arranged to provide an axial magnetic field.

6. A compressor according to any one of claims 1 to 4 wherein the magnetic circuit is arranged to provide. a radial magnetic field.

7. A compressor substantially as hereinbefore described with reference to or as shown in Figures 2 or 3 of the accompanying drawings.

8. An integral cycle machine comprising a compressor having a housing to which is fixedly secured a piston member disposed axially of the housing and providing a piston head region, there being a cylinder arranged for axial movement relative to and coaxially with the piston head region to define with the piston a variable volume compression chamber, the cylinder carrying an electromagnetic drive coil and the housing containing a magnetic circuit cooperable with the drive coil to cause said axial movement; and a cold head, the cold head having a displacer plunger movable within a displacer cylinder, wherein the cold head is arranged so that a surface of the plunger remote from the closed end of the displacer cylinder faces into the compression chamber of the compressor, so that the compressor and displacer have a shared common compression chamber.

9. An integral cycle machine substantially as hereinbefore described with reference to or as shown in Figure 6 of the accompanying drawings.