WO2007088340A1

WO2007088340A1 - Reciprocating thermodynamic machine

Info

Publication number: WO2007088340A1
Application number: PCT/GB2007/000305
Authority: WO
Inventors: Michael William Dadd
Original assignee: Isis Innovation Ltd
Priority date: 2006-02-01
Filing date: 2007-01-30
Publication date: 2007-08-09
Also published as: GB0602035D0

Abstract

A first thermodynamic space (7) and a second thermodynamic space (10) for working fluid in a reciprocating thermodynamic machine such as a Stirling cycle cooler or engine are brought into fluid communication by a connection tube (9) which passes through off-axis apertures in suspension springs suspending the reciprocating parts of the machine.

Description

RECIPROCATING THERMODYNAMIC MACHINE

The present invention relates to a reciprocating thermodynamic machine, such as typically used for compressors, engines, heat pumps or coolers, and in particular to improvements in providing for transport of the working fluid in such a machine. Reciprocating thermodynamic machines are widely used as heat pumps, coolers, compressors and engines. Typically they are based on the Stirling or Gifford McMahon cycle, though other thermodynamic cycles are possible. In such a machine the working fluid, a gas, is transferred between expansion spaces and compression spaces. Conventionally a compression space is a component in which net work is done on the gas and the energy of the gas is increased. Likewise an expansion space is a component in which net work is done by the gas and the energy of the gas is decreased. Compression and expansion is often the result of obvious volume changes.

However there are also machines in which compression or expansion is effected by utilising acoustic properties of the gas.

Where the machine is being used as a heat pump or cooler, the expansion of the working fluid in the expansion space absorbs heat from outside the machine, the working fluid is then transferred to the compression space where it is compressed causing it to reject heat which is in turn rejected from the machine, before the working fluid is transferred back to the expansion space.

Heat exchangers are used for the absorption and rejection of heat in and out of the machine. In some machines conduction and convection via the walls of the compression and expansion spaces may suffice. Optionally the fluid can be arranged to pass through dedicated heat exchangers.

Often there is a significant temperature difference between the compression space and the expansion space and the fluid needs to be heated and cooled as it flows between the two spaces. Optionally the working fluid can pass through a thermal regenerator which improves efficiency by recycling the heat. Heat is taken from the working fluid on its way from the hot end to the cold end, and returned to the working fluid on its way back from the cold end to the hot end.

In a Stirling cycle cooler the desired phase between pressure variation and movement of working fluid between compression and expansion spaces can be achieved by using a displacer. In alternative coolers, known as "pulse tube" coolers, the phase is controlled by means of different types of flow control networks which do not involve moving components. These consist of acoustic equivalents of resistors, inductors and capacitors and determine the phase in a way analogous to an electrical filter. Where the machine is being used as an engine, on the other hand, working fluid is moved between a hot expansion space where it is in contact with a source of heat, and a cold compression space in contact with a thermal sink. The pressure variation caused by the expansion and contraction is used to drive a piston.

It will be clear, therefore, that such machines incorporate flow paths to connect compression and expansion spaces together, thus allowing the working fluid to be moved back and forth. It can be desirable for machines of this type to be arranged "in line", i.e. with expansion and compression spaces spaced along a linear axis along which the reciprocating parts of the machine reciprocate. This allows the machines to be balanced (thus reducing vibration in use). However, the arrangement of the machine affects how the compression and expansion spaces can be connected. In particular, the suspension of the reciprocating parts of the machine is often positioned between two or more of the compression and expansion spaces.

US-A-5,647,217 describes arrangements of such machines in which the fluid connection between the expansion and compression spaces is taken around the perimeter of the suspension of the reciprocating assembly, or alternatively axially through the reciprocating shaft itself. However, arranging for the connection to be outside the suspension complicates the outside housing of the machine and tends to form a tortuous route which increases flow resistance undesirably. Providing for the fluid connection via the central shaft of the reciprocating assembly avoids affecting the housing and provides a straighter route for working fluid, but complicates the design of the reciprocating assembly itself.

According to the present invention there is provided a reciprocating thermodynamic machine, comprising a first thermodynamic space and a second thermodynamic space in fluid communication with each other, a reciprocating assembly for expanding, compressing or displacing a working fluid, the reciprocating assembly being suspended for reciprocation in the machine by at least one suspension member, and wherein the first thermodynamic space is positioned on the opposite side of said at least one suspension member from said second thermodynamic space, and wherein the fluid communication between the first thermodynamic space and the second thermodynamic space is provided by a connection tube, separate from said reciprocating assembly and extending through the envelope of said at least one suspension member.

Thus in general terms a reciprocating thermodynamic machine of this type can be regarded as comprising two assemblies A and B, which are either side of the moving components (the reciprocating assembly) and which are connected by the connecting tube. Assembly A is concerned with absorbing heat from a heat source at one temperature and transferring the energy to a heat sink at a different temperature. The operation of Assembly A requires a connection via the connection tube to the assembly B that is able to provide pressure oscillations from a mechanical power input (in a cooler or heat pump) or receive them for power output (in an engine).

The principal components of assembly A can be: an expansion space to absorb the heat from the heat source; and/or a compression space to reject heat to the heat sink. The expansion temperature can be higher than the compression temperature or vice versa. As these spaces are at differing temperatures a certain degree of spatial separation is required to minimise unwanted heat flows between them. The expansion and compression spaces are arranged to be at opposite ends of a single piston that is generally termed a displacer. Providing that the pressures at both ends of the displacer are not very different and that the displacer drive shaft is not too big, the work done on the displacer at one end equals the work done by the displacer at the other end. The movement of the displacer only requires a small power to overcome losses and this can be supplied a small motor or by the varying gas pressure. In a machine using a pulse tube the principle is actually very similar in that the compression and expansion spaces exist but they are not defined by the external boundaries. Assembly B has either a compression space for a mechanical power input or an expansion space (expander) for a mechanical power output. Generally for practical reasons Assembly B is operated close to ambient temperature (one does not want the compressor or engine components to be at the hot or cold end of the machine). For this reason Assembly B can be connected to either the expansion or the compression side of Assembly A - whichever is at ambient temperature.

Thus in principle the there are four possible combinations of temperature and point of connection for assemblies A and B that correspond to particular types of machine: Cooler:

Assembly A: Expansion temperature lower than compression temperature Assembly B: Compressor connected to compression side of Assembly A. Net heat rejected at ambient temperature. Net heat absorbed at low temperature. Net power into machine

Heat Pump: Assembly A: Expansion temperature lower than compression temperature

Assembly B: Compressor connected to expansion side of Assembly A

Net heat absorbed at ambient temperature.

Net heat rejected at high temperature.

Net power into machine

Engine (normal):

Assembly A: Expansion temperature higher than compression temperature

Assembly B: Expander connected to compression side of Assembly A

Net heat rejected at ambient temperature. Net heat absorbed at high temperature

Net power out of machine

Engine (cryogenic):

Assembly A: Expansion temperature higher than compression temperature Assembly B: Expander connected to expansion side of Assembly A Net heat absorbed at ambient temperature. Net heat rejected at low temperature Net power out of machine

It will be seen that in two instances (engine and heat pump) a compression space in one assembly is connected directly to an expansion space in the other assembly - i.e. one space needs to absorb heat and the other needs to reject it at the same temperature. Clearly it is not necessary to have two heat exchangers - although this is possible - it is simpler to have a single heat exchanger dealing with the combined heat flow. (This is why in the Stirling engine there is an expander/generator whilst still rejecting heat at ambient temperature.)

Thus with the invention the first thermodynamic space may be a compression or an expansion space, and the second thermodynamic space may be a compression or an expansion space.

The envelope of the at least one suspension member, namely the space occupied by it, may be defined by the radially innermost and outermost extent (relative to the axis of reciprocation) of the suspension member. The outer boundary of the envelope may be defined by arcs or straight lines joining the outermost parts of the suspension member.

The connecting tube may be entirely within, or only partly within, the envelope of the suspension member. It may extend through an off-axis aperture in the suspension member.

The suspension member may comprise a spring, for example a flat spiral spring having a plurality of spiral arms, and which may be clamped at its inner and outer periphery to suspend the reciprocating assembly. The arms may be connected together in an integral suspension spring, or the suspension member may comprise an assembly of separate spring arms, which may be clamped at their inner and outer ends to suspend the reciprocating assembly. Thus the suspension member may have clamped and undamped (free) portions. The connection tube can pass between the spiral arms, or partly or completely through the clamped portions of the suspension member, or through an aperture in an arm.

A plurality of the connection tubes may be provided, passing through the same or different spaces between the arms. The reciprocating assembly may comprise a piston, which may be connected to a linear motor and the connection tube may extend through the linear motor.

The reciprocating assembly may comprise a displacer and the compression space may be connected to a compressor. The displacer may be driven by a motor or may be "free", i.e. be driven by the working fluid itself. The reciprocating thermodynamic machine may comprise a cold head assembly coaxially mounted with a compressor and the connection tube may extend off-axis within the cold head assembly and compressor. The cold head assembly comprises a pulse tube or displacer and the compressor may comprise a pair of opposed reciprocating pistons between which said compression space is defined. The connection tube, or a plurality thereof, may provide the only flow path for working fluid between the expansion space and compression space, thus avoiding the need for a flow path through the elements of the reciprocating assembly, e.g. the shaft and pistons, or a flow path around the outside or through the outer housing of the machine.

The machine may be a cooler, heat pump, engine or compressor operating according to one of the known thermodynamic cycles such as the Stirling cycle or the Gifford McMahon cycle, and the invention may be applied in arrangements which are balanced, either inherently or by controlling the amplitude and/or phase of the reciprocating parts, or which use a balancer. It may also be applied in single or multistage machines, and double-ended arrangements.

The invention will be further described by way of example with reference to the accompanying drawings in which:- Figure 1 schematically illustrates an in line balanced cooler of the free piston type according to a first embodiment of the invention;

Figure 2 schematically illustrates the suspension spring from such a cooler and the space used to accommodate the connection tube;

Figure 3 schematically illustrates an in line pulse tube balanced compressor according to a second embodiment of the invention;

Figure 4 schematically illustrates an in line displacer/balanced compressor combination according to a third embodiment of the invention;

Figures 5a, b and c schematically illustrate in cut-away, side and plan views respectively an in line balanced cooler with dual displacer and pulse tube cold heads according to a fourth embodiment of the invention;

Figure 6 schematically illustrates an in line balanced engine with a displacer expander according to a fifth embodiment of the invention;

Figure 7 schematically illustrates an in line balanced engine with a displacer/pulse tube hybrid expander according to a sixth embodiment of the invention;

Figures 8(A) to (E) schematically illustrate a variety of connection tube and spring arrangements; and

Figure 9 schematically illustrates a further example suspension member and connection tube arrangement. Figure 1 illustrates an in line balanced cooler according to a first embodiment of the invention. It comprises a cold head assembly 1, a compressor 3 and a balancer 5 arranged in that order along an axis of reciprocation 2. The compressor 3 and balancer 5 are conventional and will not be described in detail. The compressor 3 includes a first compression space 7 where gas, as the working fluid, is compressed by piston 4 and heat is rejected via a heat exchanger 8. The piston 4 is driven by motor 6. The compressed gas from the first compression space 7 is transferred by connection tube 9 to the cold head assembly 1. A second compression space 12 exists under a displacer 13. A second heat exchanger could be used to remove the heat of compression but in this embodiment the mixing of the compressed gas allows such heat to find its way to heat exchanger 8 where it is rejected. The combined compressed gas from compression spaces 7 and 12 is transferred via a regenerator 21 to expansion space 10 in a cold finger 11 which is a coaxial, cylindrical, diameter- reduced part of the housing of the cold head assembly 1 distal from the compressor 3. In this embodiment the regenerator 21 is housed within the displacer 13. Also the heat transfer at the cold head 22 occurs between the gas and the walls of the expansion space 10 - in this case a separate heat exchanger is not provided, hi this embodiment the movement of working fluid is caused by the displacer 13 which is driven by the working fluid itself. The displacer 13 is fixed to an axially extending shaft 15 by which it is connected to a damper 17 mounted coaxially in a diameter- increased part of the housing of the cold head assembly 1. The displacer 13, shaft 15 and damper piston constitute a reciprocating assembly which is suspended by a suspension member formed, in this embodiment, by flat spiral springs 19a and 19b coaxial with the shaft 15 and mounted at their perimeter to the inside of the diameter- increased part of the housing of the cold head assembly 1. As can be seen in Figure 1 the connection tube 9 is separate from the reciprocating assembly, and passes, off- axis, through apertures 18a and 18b in the springs 19a and 19b, these apertures being between the innermost and outermost radial extent of the springs. The connection tube 9 can be a simple push fit at one end into the wall joining the diameter-reduced and diameter-increased parts of the cold head assembly 1, and at its other end into the compressor 3. The cold head assembly 1 may be connected to the compressor 3 by an annular weld around the outer housing 23 of the cold head assembly 1. Figure 2 illustrates in plan view the springs 19a, 19b which form suspension members for the reciprocating assembly. In this embodiment they are formed by a circular outer ring 25 and a circular inner ring 27 which are connected by three spiral arms 29a, b, c. The connection tube 9 passes between any two of the spiral arms, two alternative locations being shown in Figure 2. It will be appreciated that more than one connection tube 9 may be provided, in which case more of the spaces between the arms may be used. Further, the connection tube 9 does not need to be circular. The flow area of the connection tube may be increased by using a tube shape which fills more of the space between the spiral arms, for example oval tubes. Other illustrative arrangements will be described later with reference to Figure 8 and 9.

The invention can be applied to other types of machine and other geometries. Two examples will be described with reference to Figures 3 and 4 which show balanced compressor arrangements in which additional apertures in the springs and motor components of one of the compressors allows the connection tube to be connected between the compression space and the expansion space.

Figure 3 illustrates schematically an example of an in line pulse tube balanced compressor combination as a second embodiment of the invention. In the illustrated embodiment the cold head assembly 1 includes a pulse tube assembly 34 having side- by-side a tube housing a regenerator 34a, and a pulse tube 34b, though other arrangements are also possible, for example a coaxial arrangement. The pulse tube 34b can be considered as an expander in which a "gas piston" 35 acts as an expansion piston, the movement of this piston being controlled by the acoustic impedance of the flow control network 39. An expansion space 10 can be identified as the swept part of the pulse tube adjacent to the cold head 33 as is shown in Figure 3. For efficient operation it is desirable that the gas in the pulse tube remains stratified. Flow straighteners 31a,b may be provided at the ends of the pulse tube to minimise mixing. In Figure 3 they are shown as separate components but they are often also used to provide heat transfer. A key feature of a pulse tube is that its operation does not require moving components and it therefore generates little vibration. In this embodiment the compression space 7 is in fluid communication with the expansion space 10 in the cold head assembly 1 by means of the connection tube 9 and regenerator 21. The connection tube 9 passes through apertures 37a and b in the pair of suspension springs 36a of the left-hand (as viewing the drawings) reciprocating assembly formed by the piston 30a and moving part of the linear motor 32a. The apertures 37a and 37b may be specially formed, or may be the spaces between spiral legs of spiral springs as illustrated in Figure 2.

In the illustrated embodiment heat rejected by the working fluid in the compressor 3 is rejected from the machine by means of a heat exchanger 38 positioned to absorb heat from the working fluid before it enters the connection tube 9. However it is alternatively possible to reject the heat from the cold head assembly 34.

The use of the balanced pair of pistons 30a and 30b with respective motors 32a and 32b, and the in line pulse tube 34 mean that the cooler illustrated in Figure 3 is well balanced. It should be noted that the illustration does not show all of the pulse tube components and connections as these are not relevant to the explanation of the presence and positioning of the connection tube 9. Such components and connections are, however, well-known to those skilled in the art.

Figure 4 illustrates an in line displacer/balanced compressor combination according to a third embodiment of the invention. The compressor 3 is arranged similarly to that of Figure 3 and corresponding components are indicated by the same reference numerals. Thus there is a balanced pair of pistons 30a, 30b driven by respective linear motors 32a, 32b with respective suspension springs 36a, 36b all coaxially mounted. The compression space 7 is in fluid communication with the cold head assembly 1 by means of the connection tube 9 which passes through apertures 37a and 37b in the suspension springs 36a of the linear motor 32a and through an aperture in the linear motor 32a itself. A heat exchanger 38 is provided as before.

In the cold head assembly 1 the expansion space 10 is provided within a cold finger formed by a diameter-reduced part 41 of the cylindrical housing 40 of the cold head assembly 1. In this embodiment the cold finger houses an annular assembly containing an annular regenerator 43 and cold and warm end heat exchangers 45,49. Mounted within the annular regenerator is a displacer 42 carried by a displacer shaft 44 and whose motion is damped by a damper 48 all of which are coaxial with the compressor axis 2 and which form a reciprocating assembly. The displacer 42 is suspended by spiral springs 46 which are mounted at their perimeter to the inside wall of a diameter-increased part of the housing 40 of the cold head assembly 1, and the motion of the displacer is caused by the pressure variations in the working fluid itself as is normal in a free piston assembly. Although the movement of the displacer 42 gives rise to an out-of-balance force, this can be balanced, if desired, by modifying the input drive to one of the compressor halves. In this case the compressor halves (each formed by one of the linear motors and pistons) will have different amplitude and phasing, but because the out-of-balance force produced by the displacer is relatively small, the difference in amplitude and phasing required to balance the force is also small.

As can be seen in the drawing the connection tube 9 passes through off-axis apertures 47a, 47b in the displacer springs 46 before being connected to the diameter- reduced part 41 of the housing 40. Thus in this embodiment the connection tube 9 passes through the suspension members of two reciprocating assemblies, that of the left-hand compressor half formed by linear motor 32a and piston 30a, and that of the displacer.

An example will now be described with reference to Figure 5 which shows an arrangement where a compression space is connected so as to drive dual cold heads. Two further examples will be described with reference to Figures 6 and 7 which show Stirling cycle engines with both displacer and hybrid displacer/pulse tube expansion assemblies.

Figures 5a, 5b and 5c illustrate an in line cooler according to a fourth embodiment of the invention. In the illustrated embodiment there is a compressor 3, displacer cold head assembly 51 and balancer (not shown) arranged similarly to that of Figure 1. In addition there is also a pulse tube cold head assembly 53 mounted at the side of the displacer cold head assembly 51. Thus in this embodiment the compressor is connected to two cold heads. The compressed gas from compressor 3 is transferred by connecting tube 9 to the dual cold head assembly 50. At the dual cold head assembly 50 the connecting tube 9 splits into two connecting passages 55a and 55b. The first passage 55a connects compressed gas from the compressor via the pulse tube regenerator 21a to the expansion space (not shown) of the pulse tube cold head. Passage 55b connects compressed gas from the compressor via the displacer regenerator 21b to the expansion space 10 of the displacer cold head assembly 51. Figure 5a shows how the gas flow from connecting tube 9 is split to supply the two cold heads. Figure 5b shows a side view, and Figure 5c a plan view, that includes the disposition of the pulse tube not shown in Figure 5a. The pulse tube cold head does not have any moving components and the cooler is balanced as described for the embodiment shown in Figure 1. Figure 6 illustrates an in line balanced Stirling engine according to a fifth embodiment of the invention. This embodiment is similar to the cooler illustrated in figure 1 except that some processes are reversed. There is a hot head assembly 61 with an expansion space 10 above the displacer 13 that operates at high temperature e.g. 900 k. The compression space 7 of Figure 1 becomes an expansion space 62 and the likewise the compressor 3 becomes an expander 65 with (as an example of a load) the generator 67 producing an electrical power output. In the embodiment illustrated the regenerator 21 is housed in a separate tube 64 adjacent to the displacer tube 66. The hot end heat exchanger 63 is shown as a tubular assembly connecting the displacer and regenerator tubes as shown. There are two ambient heat exchangers 68a,b, one adjacent to the regenerator in the hot head assembly, the other adjacent to the expander. In this embodiment a separate motor 69 that is part of the displacer assembly controls the movement of the displacer. A potential advantage is that the engine power output can be controlled by varying the displacer drive. Also it would be possible to produce a three phase generator by using three individual engine/generator units appropriately synchronised by the displacer drives.

Figure 7 illustrates an in line balanced Stirling engine according to a sixth embodiment of the invention and is similar to that illustrated in Figure 6. In this embodiment the displacer 13 no longer spans the hot and cold regions of the hot head assembly, instead the displacer 13 is limited to the low temperature region. The displacer cylinder is connected via an interface 77 and optional flow straightener 73b to a "pulse tube" 75 (or "thermal buffer tube" as it is often referred to in engine references). The expansion space 10 is situated at the hot end of the pulse tube 75 and defined by the action of a "gas piston" 76, the gas piston 76 transmits the power to the other end where it acts directly on the displacer 13. The net result of this combination is very similar to a conventional displacer alone. A potential advantage is that it greatly reduces the complexity and moving mass of the displacer component and allows a simpler design. The interface 77 connects the pulse tube to the displacer cylinder. Combined with a flow straightener 73b, it maintains a smooth gas flow and allows a pulse tube 75 with a different diameter to the displacer to be used. In this embodiment the displacer moving assembly is reduced to a minimum with one suspension member and no separate damper. This reduces the required length of the connecting tube 9 and allows the connecting tube 9 to be integrated with a heat exchanger 72 and the hothead assembly 71. The potential advantages are the reduction in the dead volume of the connecting passage ways and the effective use of a single heat exchanger.

The displacer expanders illustrated in Figures 1, 4, 5, 6 and 7 show a range of geometries for the inclusion of heat exchangers and a regenerator. These are equally applicable to the all types of thermodynamic machines both with single or multistage operation. Such components and connections are, however, well-known to those skilled in the art.

Although the arrangements illustrated here are single-stage, the invention may be applied to multi-stage machines. It may also be applied to double-ended arrangements, for example in which an additional cold head assembly, a mirror image of that shown on the left-hand side, is provided on the right-hand side of the machines of Figures 3 and 4 driven by the linear motor 32b. In this case a second connection tube 9 is provided extending in mirror image to that shown through the additional cold head assembly. The expander assemblies at each end can be any combination of displacer cold heads, pulse tube cold heads or engine expansion heads. If a cold head at one end is combined with an engine at the other end it can be arranged that the power required for the cold head comes principally from the engine side. A small electrical machine can be used to control the machine or to generate a modest electrical output. Figure 8 illustrates a variety of arrangements for the positioning of the connecting tube 9 in relation to the suspension springs 19a,b. Figure 8(A) is, for reference, the arrangement described above and shown in Figure 2. Figure 8(B) shows an arrangement in which the circular outer ring 25 is absent so that the outer parts of spring arms 29a,b,c are separate from each other. The envelope of the spring could be defined as its outer diameter - a circle through the radially outermost parts of the spring, or by arcs and straight lines (shown dashed in Figure 8(B)). Figure 8(C) shows a spring 19a,b with three additional spring arms 29d,e,f, making six in total. Further, areas 80a-f of the spring that are not stressed have been removed, the connecting tube extending through one of these areas 80c. Figure 8(C) and (E) also show that the connecting tube does not have to be entirely within the envelope of the springs (shown dashed in Figure 8(C)), but can be partly within and partly without. Figure 8(D) is similar to Figure 8(C) but uses four arms 29a,b,e,f rather than six. The outer envelope is shown as defined by a circle, but the dotted arcs could be replaced by straight lines, in which case the connecting tube 9 might be partly within and partly without the envelope.

It should be appreciated that in use the inner and outer parts of the suspension springs 19a,b form clamping areas 82 and 83 which are clamped to the machine and suspended assembly as shown in Figure 8(E). The connecting tube 9 may extend partly through the outer clamped area 83 of the springs 19a,b as shown.

The suspension members 19a,b may be an assembly of separate spring arms

290a,b,c as schematically illustrated in Figure 9. These are clamped to the machine and suspended assembly at inner and outer clamping areas 82, 83 as before, and the connection tube or tubes 9 may pass partly or completely through the inner or outer clamping areas or both, as shown.

Claims

1. A reciprocating thermodynamic machine, comprising a first thermodynamic space and a second thermodynamic space in fluid communication with each other, a reciprocating assembly for expanding, compressing or displacing a working fluid, the reciprocating assembly being suspended for reciprocation in the machine by at least one suspension member, and wherein the first thermodynamic space is positioned on the opposite side of said at least one suspension member from said second thermodynamic space, and wherein the fluid communication between the first thermodynamic space and the second thermodynamic space is provided by a connection tube, separate from said reciprocating assembly and extending through the envelope of said at least one suspension member.

2. A reciprocating thermodynamic machine according to claim 1 , wherein the first thermodynamic space is a compression or an expansion space, and the second thermodynamic space is a compression or an expansion space.

3. A reciprocating thermodynamic machine according to claim 1 or 2, wherein the reciprocating assembly reciprocates linearly along an axis and the envelope of said at least one suspension member is defined by the-radially innermost and outermost extents of the suspension member relative to the axis of reciprocation.

4. A reciprocating thermodynamic machine according to claim 1 , 2 or 3, wherein the connection tube is entirely within the envelope of said at least one suspension member.

5. A reciprocating thermodynamic machine according to claim 1, 2 or 3, wherein the connection tube is partly within the envelope of said at least one suspension member.

6. A reciprocating thermodynamic machine according to any one of claims 1 to 5, wherein the suspension member comprises a spring.

7. A reciprocating thermodynamic machine according to claim 6, wherein the spring is a flat spiral spring having a plurality of spiral arms, said connection tube passing between the spiral arms of said spring..

8. A reciprocating thermodynamic machine according to any one of the preceding claims, wherein the reciprocating assembly comprises a piston for compressing said working fluid.

9. A reciprocating thermodynamic machine according to any one of the preceding claims, wherein the reciprocating assembly comprises a displacer for moving the working fluid between the first thermodynamic space and the second thermodynamic space.

10. A reciprocating thermodynamic machine according to any one of the preceding claims, wherein the reciprocating thermodynamic machine is a Stirling cycle cooler.

11. A reciprocating thermodynamic machine according to any one of the preceding claims wherein the compression space is connected to a compressor.

12. A reciprocating thermodynamic machine according to any one of the preceding claims, wherein said reciprocating assembly is driven by a linear motor.

13. A reciprocating thermodynamic machine according to claim 12 wherein the connection tube extends through the linear motor.

14. A reciprocating thermodynamic machine according to any one of the preceding claims, wherein a plurality of said connection tubes are provided.

15. A reciprocating thermodynamic machine according to any one of the preceding claims, wherein at least one of a thermal regenerator, a heat exchanger, and further thermodynamic space is provided in the fluid path between the first thermodynamic space and the second thermodynamic space.

16. A reciprocating thermodynamic machine according to any one of the preceding claims comprising a cold head assembly coaxially mounted with a compressor and wherein the connection tube extends off-axis within the cold head assembly and compressor.

17. A reciprocating thermodynamic machine according to claim 16 wherein the cold head assembly comprises a pulse tube.

18. A reciprocating thermodynamic machine according to claim 16 wherein the cold head assembly comprises a displacer.

19. A reciprocating thermodynamic machine according to claim 16, 17 or 18 wherein the compressor comprises a pair of opposed reciprocating pistons between which said compression space is defined.

20. A reciprocating thermodynamic machine according to any one of the preceding claims wherein the connection tube, or a plurality thereof, provide the only flow path for working fluid between the expansion space and compression space.

21. A reciprocating thermodynamic machine according to any one of the preceding claims wherein the suspension member comprises portions which are clamped to at least one of the reciprocating assembly and machine, and also undamped portions.

22. A reciprocating thermodynamic machine according to any one of the preceding claims wherein the suspension member comprises a plurality of separate spring arms assembled together to suspend the reciprocating assembly.