Pulse tube refrigerator

A pulse tube refrigerator comprising a cold head, wherein the cold head comprises at least one pulse tube (6) and at least one regenerator (7); the cold head having a cold end (8) and a warm end (5), each end being provided with respective heat exchangers (12, 13); wherein refrigerant is supplied (3) to the cold head; and wherein the warm end heat exchanger (13) is provided with a secondary cooling mechanism (16, 17) to improve the efficiency of the PTR.

Pulse Tube Refrigerators (PTR's) are an effective method of producing cooling at cryogenic temperatures and can be applied to a diverse range of applications where cryogenic cooling is required. e.g. GB2310486. Single or multiple stage cooling devices can be used to assist conservation of liquid cryogens. In applications like MRI, NMR and other large scale uses of superconducting magnets it is desirable to reduce the consumption of the cryogenic liquid, usually liquid Helium cooling the magnet. Other cryogenic liquids are used for high temperature superconducting magnet systems. During operation of the PTR heat is extracted from the magnet system at the low temperature heat stations (cold end) and rejected at a higher temperature heat station (warm end) through heat exchangers. The principle of operation of PTR systems is comprehensively reported in technical literature e.g. GB 2318176. The heat exchanger located at high temperature, usually operates close to room temperature. In applications such as MRI where this heat rejection is considerable and can be typically 100W in the steady state under load or increased to 1000W during cooldown, significant temperature increase is evident at this heat exchanger. The mechanism is concerned with removing the considerable heat generated in MRI applications the size of the additional cooling area is provided as observed in experiments.

In accordance with the present invention, a pulse tube refrigerator comprises a cold head, wherein the cold head comprises at least one pulse tube and at least one regenerator; the cold head having a cold end and a warm end, each end being provided with respective heat exchangers; wherein refrigerant is supplied to the cold head; and wherein the warm end heat exchanger is provided with a secondary cooling mechanism to improve the efficiency of the PTR.

In general, a substantial secondary cooling mechanism is required and this provides additional efficiency and temperature control of the warm end. Reducing the temperature of the warm end without significantly affecting the cold end temperatures directly affects the Carnot efficiency of the PTR cycle, thereby making the system more efficient.

Preferably, the secondary cooling mechanism comprises fins and an air supply, such that the cooling is provided by airflow over the fins.

Alternatively, the secondary cooling mechanism comprises an additional heat exchanger.

In order to cool the additional heat exchanger, preferably the refrigerant is fed to the additional heat exchanger before being supplied to the cold head.

This uses the high pressure (HP) refrigerant from the compressor making the cooling circuit self contained. Using only the refrigerant flow from the compressor to effect the additional cooling, enables the system to be self contained.

Alternatively, a supplementary coolant is provided for the additional heat exchanger.

Preferably, the supplementary coolant is provided for the additional heat exchanger by bleeding a small flow of gas from the compressor high pressure side through the heat exchanger and back to the low pressure side of the compressor.

The present invention enables the high temperature heat station temperature to be controlled by providing additional cooling to the warm end heat exchangers, which thereby increases the efficiency of the PTR system.

FIG. 1shows a conventional system configuration. A compressor1compresses refrigerant fluid, such as Helium or other suitable gas. The compressed Helium is fed from a high pressure (HP) supply outlet2to the HP supply gas line3. At this point the refrigerant is at a temperature dependent upon the cooling scheme employed by the compressor. Refrigerant is fed to a valve system4. This distributes the gas into a cold head comprising a high (room) temperature end5, a first pulse tube6and first regenerator7connected to a first stage8, the cold end; and a second pulse tube9connected between the high temperature end5and a second stage10, also the cold end and a second regenerator11. Gas flow in the cold head is ac flow in that it flows in and out through the same flow passages. Operation of the PTR produces cooling of the stages, in this case the first8and second10for a two stage refrigerator. The heat flow from the first and second stages8,10is extracted through cold end heat exchangers12at the cold end of the first and second pulse tubes6,9. The corresponding heat rejection created by the PTR refrigeration cycle is rejected through warm end heat exchangers13at the high temperature end. The gas supply returns to the compressor from the valve system4via a low pressure (LP) return gas line14to the LP return input15. Commercially available PTR systems tend to use only the refrigerant gas flowing from the system compressor to cool the valve part of the system. This gas is usually cooled at the compressor by water or air and distributed to the PTR cold head by means of the valve system. The valve may be attached to the cold head or remote from it. In either case the gas transport from the compressor is used in a ‘passive’ way to provide cooling to the high temperature heat exchangers.

In a first example of the present invention in which the temperature of the warm end heat exchangers is controlled by means of additional active cooling, a secondary cooling mechanism is provided in which a surface cools the high temperature end using forced air or natural air convection around the PTR. As shown inFIG. 2, natural convection air cooling fins16are provided at the high temperature end5to control the warm end heat exchanger13temperature. These fins16are substantial additions. In an example of a two stage cooler for MRI applications, a typical surface area of 5000 mm2per Watt is needed to attain the control required. Reducing the temperature of the high temperature end5, without significantly affecting the cold end temperatures, directly affects the Carnot efficiency of the PTR cycle making the system more efficient. Additional efficiency and temperature control of the high temperature end5is possible by passing a forced airflow over the fins16.

In a second example of the present invention, as shown inFIG. 3, forced cooling using a heat transfer fluid to a secondary heat exchanger is proposed. The main refrigerant Helium gas can be used, or any other suitable liquid or gas. If the main refrigerant fluid is not used, then a separate flow circuit is required. To control the warm end heat exchanger5temperatures using main refrigerant flow an additional heat exchanger17is added to the high temperature end5. Gas flow from the compressor1is fed to the heat exchanger in supply gas line18before passing to the valve system4via supply line19. The heat exchanger17is a substantial addition. In an example of a two stage cooler for MRI applications a typical surface area of 200 mm2per Watt is required to attain the control required. Reducing the temperature of the high temperature end5, without significantly affecting the cold end temperatures directly affects the Carnot efficiency of the PTR cycle making the system more efficient.

Alternatively, if the main refrigerant from the compressor is not passed through the heat exchanger, but a supplementary coolant, water for example is used instead, then a separate flow circuit (not shown) is used to pass the fluid around-the heat exchanger17. Any suitable fluid can be used. One such arrangement is to bleed a small amount of high pressure gas from the compressor through the heat exchanger and direct back to the compressor low pressure side entering the PTR.

The methods outlined here describe how a suitable supplementary heat exchanger is fixed as an integral or additive feature to the high temperature heat exchangers on a two stage PTR. The methods are generally applicable to a PTR with any number of stages.