GB2273975A - Refrigerator for cryogenic temperatures - Google Patents

Abstract

It is known that there are many benefits gained by operating electromagnetic sensors and electronic circuits at cryogenic temperatures. The present refrigerator offers a long working life with minimum maintenance. The design consists essentially of a warm chamber (1) approximately ambient temperature connected to a cold chamber (2). The refrigerant fluid is a gas which undergoes a sequence of approximately isothermal changes at different temperatures in order to pump thermal energy from the cold chamber to the warm chamber. An oscillating piston or diaphragm (3) inside the warm chamber partitions it into two variable volume parts. The two parts of the dual chamber process the gas in antiphase with the space ahead of the advancing piston being used for gas compression while the increasing space behind the piston allows gas expansion. Each end of the warm chamber is fitted with a simple inlet valve and a spring loaded outlet valve (7 - 10). These regulate the phase angles during the piston/diaphragm cycle at which gas enters or leaves the warm chamber. As a result net negative work is done in the cold chamber. This produces a refrigeration effect in the cold chamber. piston/diaphragm movements and associated electric motors are restricted to the warm chamber, there are no moving mechanical parts in the cold chamber. This means that there are no problems associated with mechanical or electromagnetic vibrations in the region of the electronic circuits being served. The cold chamber may be honeycombed to enhance heat transfer and can be shaped to fit in with preferred working circuit (12) and Dewar insulation (11). The motive power can be related to an armature embedded into a piston or diaphragm which separates the two part volumes of the warm dual chamber. This design eliminates the necessity for a piston shaft and flywheel and allows the refrigerator to be permanently welded closed at the assembly stage. The symmetry of the dual warm chamber arrangement permits the addition of helical springs to the piston in order to store potential energy and smooth out the power supply current flow A regenerator (13) provides heat exchange between the relatively warm gas entering the cold chamber (2) and the cold gas leaving the chamber. If the gas transfer line connected to the cold chamber is flexible it offers a low inertia unit suitable for rapid movement scanner systems. Some embodiments include additional, beneficial Joule-Thomson cooling. <IMAGE>

Description

Refrigerator for Cryogenic Temperatures This invention relates to refrigerators which are capable of cooling sensors or electronic circuits to cryogenic temperatures.

It is known that there are many benefits gained by operating electromagnetic sensors and electronic circuits at cryogenic temperatures. Refrigerators for reaching and maintaining these temperatures are a well developed area of technology. Existing refrigerators are however a very strict compromise between many conflicting design requirements.

The present invention is a novel refrigerator which offers a long working life with minimum maintenance. The design consists essentially of a warm chamber at approximately ambient temperature connected to a cold chamber. The refrigerant fluid is a gas which undergoes a sequence of approximately isothermal changes at different temperatures in order to pump thermal energy from the cold chamber to the warm chamber. An oscillating piston or diaphragm inside the warm chamber partitions it into two variable volume parts. The two parts of this dual chamber process the gas in antiphase with the space ahead of the advancing piston being used for gas compression while the increasing space behind the piston allows gas expansion. Each end of the warm chamber is fitted with a simple inlet valve and a spring loaded outlet valve.These regulate the phase angles during the piston/diaphragm cycle at which gas enters or leaves the warm chamber. As a result net negative work is done in the cold chamber. This produces a refrigeration effect in the cold chamber.

Piston/diaphragm movements and associated electric motors are restricted to the warm chamber, there are no moving mechanical parts in the cold chamber. This means that there are no problems associated with mechanical or electromagnetic vibrations in the region of the electronic circuits being served. The cold chamber may be honeycombed to enhance heat transfer and can be shaped to fit in with preferred working circuit and Dewar insulation shape.

The motive power can be related to an armature embedded into a piston or diaphragm which separates the two part volumes of the warm dual chamber. This design eliminates the necessity for a piston shaft and flywheel and allows the refrigerator to be permanently welded closed at the assembly stage. The symmetry of the dual warm chamber arrangement permits the addition of helical springs to the piston in order to store potential energy and smooth out the power supply current flow.

Substantial cooling occurs during a large fraction of the operating cycle. This offers a good power to size ratio compared with Stirling cryocoolers.

The design requires fewer precision engineered components than equivalent Stirling systems which indicates lower manufacturing costs.

If the gas transfer line connected to the cold chamber is flexible it offers a low inertia unit suitable for rapid movement scanner systems.

For some versions of the invention additional, beneficial Joule-Thomson cooling also occurs.

This patent application also explains how the novel heat pumping arrangement may be exploited using conventional designs of piston or diaphragm pumps.

Illustrative embodiments of the invention will now be described.

The basic construction techniques and materials required have much in common with those used for the design of Stirling and Joule-Thomson cryocoolers. Those with a knowledge of these cryocoolers will be able to construct the cooler to be described.

Figure 1 is a schematic diagram ot the refrigerator and is not intended to indicate the working size, shape or exact positions of the component parts.

The movable part of the motor which motivates the piston is a runner or armature which is embedded in the crown of the piston. The stator passes through a hole in the centre of the crown. According to the current invention the motor may take the form of any tubular linear motor currently known to electrical engineers. For simplicity the first design will be described as having a simple stator consisting of a ferromagnetic rod and an armature which is a current carrying coil.

In figure 1,1 is the warm chamber at approximately ambient temperature and 2 is the cold chamber.

A sliding fit piston, 3 includes a centrally mounted armature, 4 which in this version is an annulus or ring shaped permanent magnet. 5 is an electromagnet which includes an alternating power supply.

The electromagnet forms part of a magnetic circuit including a soft ferromagnetic shaft, 6 which passes through the armature. The piston oscillåt8 in rsponse to changes in magnetic flux through the shaft.

A small amount of the working gas can flow through the narrow gap between the armature and the shaft and between the piston skirt and the chamber sidewalls in order to provide lubrication.

The addition of an armature adds to the inertial mass of the piston but this is offset by the absence of a piston crankshaft. The length of the piston stroke can be kept short compared with the piston diameter in order to minimise acceleration and inertia forces.

The two halves of the warm chamber exchange the working gas with the cold chamber via inlet and outlet valves mounted on the warm chamber walls. These valves are placed in an optimum position to minimise asymmetrical forces on the piston. 7 and 8 are the inlet valves The inlet valves are generous sized flap, ball, disk valves or the like. 9 and 10 are outlet valves which include springs or similar mechanisms which will only allow the working gas to be released when a pre-set excess pressure has developed.

The movement of the outlet valves can be considerably restricted so that even when fully open they act as Joule-Thomson expansion nozzles. This means that provided a suitable working gas is chosen for the temperatures and pressures of operation then Joule-Thomson cooling will occur when gas is released from the warm chamber.

A facility to alter the excess release pressure may be included. This facility can be a simple mechanical device or an electronic system which is manually adjusted or responds to a feedback signal.

An alternative to the spring loaded outlet valves described is to replace them with Joule-Thomson nozzles as known to designers of these devices.

The cold chamber is typically surrounded by Dewar insulation, 11 and is in good thermal contact with the circuit or detector being serviced, 12. When the system is operating the gas flowing through the pipe work connecting the chambers is at a temperature below ambient temperature. Any pipe work not insulated by the Dewar is insulated by lagging.

At the instant shown in figure 1 the piston is moving upwards, compressing the gas in the upper half of the warm chamber and drawing gas into the lower half. (For the sake of simplicity the warm chamber is referred to as having an upper and lower half, but it is to be understood that the cooler is designed to work in any orientation.) The compression process means that positive work is being done on the gas in the upper half chamber. Cooling fins or an other form of heat sink (not shown on the diagram) allow the consequent heat to be dissipated so that to a first approximation the compression takes place isothermally. Until the release pressure is achieved no gas leaves the upper part of the warm chamber.

At the stage in the cycle prior to this release the cold chamber is freely connected to the lower half of the warm chamber so that gas expansion takes place into the lower half of the warm chamber and the gas pressure drops in the cold chamber. This means that in both of these parts of the system negative work is being done. The negative work done in the cold chamber leads to the extraction of thermal energy from the cold chamber.

When the pressure release valve opens the gas driven out of the upper half of the warm chamber is partially transferred into the lower half of the warm chamber with the balance of the gas entering the cold chamber. The compressed gas moving out of the warm chamber has to do work against the spring loaded valve. The work done is equal to the drop in pressure on leaving the chamber multiplied by the volume swept out by the piston while the valve is open.

During the next half of the cycle the piston moves down and the roles of the upper and lower halves of the warm chamber are reversed but the role of the cold chamber is the same.

After the first few cycles of operation the working gas travels towards the entrance of the cold chamber at a higher temperature than the mean temperature at which the gas previously left the cold chamber. A regenerator, 13 provides recuperative heat exchange between the relatively warm gas approaching the cold chamber and the cold gas which previously left the chamber. This operates in a similar manner to regenerators currently used for Stirling cryocoolers.

The desired function of this invention is to produce a refrigeration effect in the cold chamber.

At two stages in the cycle transient cooling of the warm chamber walls also occurs, even though the net effect is one of warming of the warm chamber walls.

1) The walls of the warm chamber are cooled when they come in contact with the expanding fraction of the gas prior to the opening of the spring loaded valve.

2) The gas which enters the warm chamber after the opening of the spring loaded valve has also been cooled because of the work done by the gas against the spring valve as it left the complimentary section of the warm chamber.

A desired effect in the warm chamber is one of isothermal compression ahead of the piston. This cannot be wholly achieved in practice but the prior cooling of the enclosing walls during the expansion phase assists in the tendency towards this condition. The wall cooling feature also reduces the maximum temperature reached during the operating cycle. The well known Carnot efficiency equation predicts that this reduction in maximum operating temperature leads to an increase in operating efficiency.

The force on the piston is along the direction of the shaft, eliminating the problems of side slap associated with driving systems which involve flywheels. The inlet and outlet valves can be placed at the ends of the chamber, concentrically around the shaft in order to provide even loading on the piston.

The elimination of side slap combined with the lubrication provided by the slight internal leakage of the working gas mean that no liquid lubrication is required. Problems associated with liquid lubrication are eliminated and the system can be welded closed to prevent leakage of the working gas.

The cold chamber has several features which make it superior to the cold chamber of existing Stirling cryocoolers: 1) It contains no solid moving parts.

2) The absence ot piston movement abrasion in the cold chamber broadens the choice of construction materials. The cold chamber can be constructed from plastic, epoxy-fibre glass, ceramic or non-ferromagnetic stainless steel. This reduces electromagnetic noise problems.

3) It can be internally finned or honeycombed to facilitate heat exchange between the cold gas, the chamber walls and the circuit being served.

4) The external shape of the cold chamber can be configured to a preferred shape to encompass any electronic circuits being served and optimize Dewar insulation performance.

The ratio of the volume of the warm chamber (Vw) to that of the cold chamber (Vc) is a design variable, depending on the ratio of working to ambient temperatures, acceptable total system mass, speed of cooling required, level of excess cooling capacity preferred etc. The cooling which takes place in the cold chamber causes the pressure of the working gas in the system, as measured at the end of a stroke, to drop compared with the initial pressure when the whole of the system is at ambient temperature. As Vw/Vc increases the drop in system pressure which occurs for a given final temperature in the cold chamber decreases. The total mass of gas in the system remains constant even though the pressure drops. This is accounted for by the mass of gas in the cold chamber being greater at the working temperature than at the ambient temperature.

The cooling cycle described offers substantial cooling during a large fraction of the piston cycle. This means that for a given cooling operation the refrigerator can have a significantly smaller mass and volume than the equivalent Stirling system.

The gas transfer lines which link the chambers can be flexible fine-bore tubes. The combination of low cold chamber inertial mass with flexibility of movement will make the present invention a preferred choice for high acceleration applications, for example infra-red scanning.

The cooler is fitted with a fill port (not illustrated) for use in the initial gas purging and for filling with fresh gas if leakage or contamination occurs.

At least one demand on the electric power supply calls for an alternating current. This can be supplied from a direct current source provided that the operating circuit includes an appropriate current chopping electronic circuit.

The outer body of the warm chamber is made from metal or at least has a continuous, electrically conducting external surface, enabling the body to act as a Faraday cage. This allows the electromagnetic emissions from the circuit components inside the body to comply with current electromagnetic emission regulations.

TOmpOraturs control may ipu maintain HSing ii Xh,ermostat or demand control system. Those with a good knowledge of electronics will be able to construct the necessary circuits.

The warm chamber can also be designed so that gas is released at a pre-set compression volume, rather than at a set compression pressure. One way of achieving this is to include an extension to the piston skirt which includes an aperture. This aperture exposes a complimentary aperture in the walls of the chamber at a certain volume, only allowing gas release at this volume. An alternative design is to add protrusions to the piston which open the outlet valves as the piston approaches them. Those with a knowledge of valve systems will be able to construct other mechanisms which offer similar benefits.

Figure 2 shows variations on the design. The piston has been replaced by a diaphragm, 1 and the armature, 2 is an electromagnet. The use of a diaphragm or piston mounted electromagnet overcomes the coercivity problems associated with permanent magnets. The power supply lines for the electromagnet can be embedded into the diaphragm or into springs 3 and 4 which cushion the piston at the end of each stroke. The springs also store potential energy and even out the current loading, reducing the 12R losses of the electromagnet. The springs have a natural length such that they are unstressed when the diaphragm is at rest in the centre of the chamber. The springs offer a similar energy smoothing benefit to the flywheel in existing Stirling cryocooler designs but have a lower inertial mass.All of the versions of the cooler described in this application should be assumed to be fitted with energy storing springs. For the sake of clarity these have been left out of some of the diagrams.

In this design the only route for gas leakage inside the chamber is via the gap between the diaphragm electromagnet and the central shaft. By using a long solenoid electromagnet this flow is reduced.

A number of variations on the wiring of the electromagnets are possible. For example: a) The current flow may be such that the central shaft carries a steady magnetic flux and the diaphragm electromagnet current is alternating. This reduces the magnetic hysteresis losses within the system.

b) The diaphragm (or piston) electromagnet may be wired as two a.c. current carrying back-to-back electromagnets such that the two ends of the solenoid are of the same polarity. This has the advantage that the central soft magnetic shaft may be extended as as a continuous yoke, joining the two external ends of the shaft in order to produce a continuous high magnetic permeability circuit. This design is advantageous in situations where electromagnetic interference caused by the electric circuits may be a problem. The back-to-back electromagnets act as dipoles generating opposing and neutralising magnetic flux at a distance, in a plane at right angles to the direction of the central shaft. The dipoles are also enclosed in the Faraday cage created by the electrically conducting casing of the warm chamber. Any external electromagnets associated with the yoke carry d.c. currents.

Figure 3 is a diaphragm system which eliminates the central shaft and uses a permanent magnet or electromagnet as the armature.

This variation completely eliminates internal leakage of gas inside the warm chamber but is only suitable for small pumps because of the low magnetic permeability of the gap between the armature magnet and the electromagnets embedded in the end walls of the chamber. Springs to prevent overshoot of the diaphragm may be added as in figure 2.

In the design of the driving mechanism a wide range of magnetic components can be used including a combination of permanent and electromagnets similar to those used for moving coil loud speaker systems. Tubular linear motors as used for short stroke thrusters are also a suitable source of motive force.

This patent application is an extension of the intellectual property described in patent application number GB 9304961.7. The magnet configurations described therein can be applied to the present refrigeration pump.

The cooler can be offered with more complex gas transfer lines to take advantage of the cooling effect which takes place during the expansive phase in the warm chamber. A wide bore transfer line can carry the gas from the half of the chamber currently undergoing compression through the piston or along the chamber side wall so that it is cooled by the adjacent half of the chamber.

This mechanism may be used to cool the gas below its maximum inversion temperature before it is subjected to Joule-Thomson expansion.

An alternative design which includes a mechanism for cooling the the working gas below its maximum inversion temperature is shown in figure 4.

In this diagram the outlet valves have been replaced by four small diameter nozzles, 1, 2, 3 and 4.

These are sufficiently small in diameter to produce a pressure drop comparable to the spring loaded outlet valves. For this design the gases enter and leave the warm chamber by different tubes, 5, 6, 7 and 8. The fraction of the gas which does not enter the cold chamber passes through nozzles 1 or 2.

During operation the gas which enters the cold chamber passes through the regenerator, 9 and is cooled before emerging through nozzle 3 or 4.

Figure 5 shows a cooler which exploits the cooling cycle described for the current invention but uses an external motor to provide motive power.

A piston shaft, 1 passes through a sliding fit seal, 2. A rotating cam, 3 driven by a motor or other means of rotary power provides the motive force for the up-stroke of the piston. The mass distribution of the cam can be adjusted so that it acts as an energy demand smoothing flywheel. A return spring, 4 provides the motive force for the down stroke. 5 is a guide which eliminates unwanted transverse movement of the piston shaft.

Sliding fit seals can lead to working gas leakage. This can cause problems, especially if the working gas is helium, because of its ability to escape through very small gaps. The leakage problem can be overcome if the casing enclosing the cam and motor is filled with the working gas and then sealed by welding to the body of the cooler.

The rotation of the motor and cam stir up the gas inside the casing and the movement of the piston shaft in and out of the cooler causes pressure variations inside the case. Both of these cause parasitic energy losses not present in the other designs described above.

Figure 6 shows a variation on the above arrangement.

In this design a flexible diaphragm, 1 replaces the sliding fit seal. The diaphragm and area of contact with the piston shaft form a gas proof seal.

Those with a knowledge of the design of diaphragm pumps will be aware that the diaphragm and associated piston shaft may, alternatively be linked to a hydraulic driving system in order to even out the stress on the diaphragm membrane.

The cam and associated rotational drive motor indicated in figures 5 and 6 may be replaced by a linear drive motor which supplies a direct thrusting force to the piston.

The novel heat pumping principles described in this patent application may also be exploited by using a system which replaces the dual warm chamber with two separate single chambers. The volumes of the single chambers are varied in anti-phase by means of two pistons and the gas flow process is controlled using valves or Joule-Thomson nozzles as explained above. This arrangement allows tight fitting lubricated pistons to be used but is less thermodynamically efficient than the other designs.

Figure 7 shows a two piston design with the pistons connected to a rotating flywheel.

1 and 2 are the pistons which are linked to the flywheel 3 by connecting rods 4 and 5. The pistons sweep out the volumes of the warm chambers 6 and 7 in antcphase. The electric drive which powers the system is omitted from the diagram.

Figure 8 shows a two piston design with the pistons connected to a linear motor. 1. The oscillating armature 2 is directly connected to the pistons which operate in antl-phase. The energy absorbing springs 3 and 4 each have one free end attached to a piston and a hxed end attached to the casing.

Claims (11)

Claims

1 A refrigeration or cooler unit which includes a cold chamber with no solid moving parts which cools by gas expansion and is serviced successively by two variable volume warm chambers acting in anti-phase.

2 As four claim 1 but with the inclusion that the two warm chambers are separate parts of a single chamber separated by a movable piston or diaphragm.

3 As for either of the above claims for a piston or diaphragm which has an armature or other essential part of the driving mechanism inclusive in its structure such that an external driving connecting rod is dispensed with.

4 As for any of the above claims for a magnet or other linear motor arrangement inclusive in the piston or diaphragm which interacts with magnetic flux which is carried by a ferromagnetic shaft passing through an aperture perpendicular to the plane of the piston or diaphragm.

5 As for any of the above claims for a cooler which releases gas from the half of the warm chamber under compression when a pre-set pressure is reached.

6 As for claim 5 but including mechanical or electronic means for manually or automaticaliy adjusting the release pressure.

7 As for claims 1 to 4 for a cooler which releases gas from the half of the warm chamber under compression when a pre-set volume is reached.

8 As for the above claims for a cooler which includes a multiplicity of energy absorbing and storing springs which smooth out the power demands which the cooler makes on the driving motor during the course of the operational cycle.

9 As for the above claims for a cooler which includes internal moving parts in near contact which are lubricated by deliberate internal leakage of the working gas between the shearing surfaces.

10 As for any of the above claims for a cooler which also exploits the benefits of Joule-Thomson cooling.

11 As for any of the above claims for a cooler which includes a regenerator external to the cold chamber.

1 2 As for any of the above claims for a cooler which also cools the gas from the half of the chamber undergoing compression by bringing it into thermal contact with gas in the half of the chamber currently under expansion.

1 3 A system consisting of two or more of any of the above coolers in cascade.

1 4 A system consisting of any of the above coolers In cascade with a known cooling system.