Drive Arrangement For Rotary Valve In A Cryogenic Refrigerator

Abstract

A cryogenic refrigerator comprising a rotary valve which controls the flow of high pressure gas into the refrigerator and the return of the gas from the refrigerator, the cryogenic refrigerator further comprising rotary fluid motor arranged to drive the rotary valve.

Description

Cryogenic refrigerators have a number of applications such as the liquefaction of cryogenic gases, conduction cooling of cryostat components, direct and indirect cooling of superconductors and as a cold source for maintaining vacuum integrity. Such refrigerators typically utilise a rotary valve to manage the distribution of compressed gases such as helium, neon, nitrogen, argon or methane to and from the refrigerator's expansion chamber.

In an example of the prior art, a mechanically driven two stage Gifford McMahon refrigerator employs an electric motor, for example a stepper motor, to drive a gas displacer via a “scotch-yoke”. In addition, the electric motor drives a rotary valve which controls the flow of high pressure gas from a gas compressor into the refrigerator and the return of the gas to the gas compressor.

Cryogenic refrigerators may also employ gas-driven gas displacers. Alternative cryogenic refrigerators are of the pulse tube variety, in which the flow of compressed gas is controlled but there is no moving gas displacer. Such refrigerators still employ an electric motor to operate the rotary valve.

In certain circumstances, a rotary drive mechanism based upon electrical excitation is either impractical or undesirable (e.g. where high magnetic fields are present, in systems which are sensitive to electromagnetic interference generated by electric motors or where the environment contains flammable materials or risk of explosion). One possible solution is to distance the drive motor from the rotary valve, and provide mechanical drive connection to the rotary valve, for example by drive shafts or drive cables. In some applications, it is not desirable to have such mechanical arrangements. For this reason, it is desirable to provide a drive arrangement for the rotary valve wherein electrical excitation is not necessary.

The present invention accordingly provides a cryogenic refrigerator comprising a rotary valve which controls the flow of high pressure gas into the refrigerator and the return of the gas from the refrigerator, the cryogenic refrigerator further comprising a rotary fluid motor arranged to drive the rotary valve, wherein the rotary fluid motor is constructed of non-magnetic material for use in a high-strength, or sensitive, magnetic field.

According to the present invention, the electric motor conventionally employed to drive the rotary valve in a cryogenic refrigerator is replaced by a rotary fluid motor, for example a multi-vane type rotary fluid motor. The rotary fluid motor is constructed of non-magnetic material for use in a high-strength, or sensitive, magnetic field.

The rotary fluid motor may be arranged to drive the rotary valve directly, or may be arranged to drive the rotary valve via a gearing unit. It is constructed of non-magnetic material appropriate to its application and environment, i.e. for use in a high-strength magnetic field. The rotary fluid motor may be driven by any suitable fluid, such as air, gas, water or oil. As is conventional, a pump, typically operated by an electric motor and situated remotely from the rotary fluid motor, is provided to circulate fluid to the rotary fluid motor. This is advantageous in that the electric motor may be situated away from such hazards as flammable materials, risks of explosion or sensitive magnetic fields. The rotary fluid motor may also be manufactured to be tolerant of extreme temperatures, which would prevent or hinder the use of an electric motor to drive the rotary valve. The rotary fluid motor is manufactured to be non-magnetic, enabling the rotary fluid motor to be employed in regions of strong, or sensitive, magnetic fields, which would prevent or hinder the use of an electric motor to drive the rotary valve.

In the case of gaseous propulsion of the rotary fluid motor, the gas used to propel the rotary fluid motor may be derived from the compressed gas supply being switched in the driven rotary valve.

In a particularly advantageous embodiment, the present invention is applied to drive a rotary valve of a cryogenic refrigerator of a cryostat used to house magnet coils of a nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) system. Such refrigerators operate in a high strength magnetic field, which must be kept free from interference from outside magnetic fields. Such an environment is unsuitable for deployment of an electric motor since operation of the electric motor may be degraded by the high strength magnetic field, and an electric motor may generate electromagnetic interference sufficient to degrade the homogeneity of the magnetic field used for imaging. The rotary valve of the cryogenic refrigerator is used to switch fluids, typically at temperatures close to room temperature. A rotary fluid motor of nonmagnetic material may be provided at the rotary valve. Such rotary fluid motor will not be affected by the high strength magnetic field, and will not generate any electromagnetic interference. By careful selection of the material of the rotary fluid motor and the fluid used to drive the rotary fluid motor, the rotary fluid motor may be made tolerant of the temperatures encountered. In a particularly advantageous embodiment, the fluid used to drive the rotary fluid motor may be derived from a high pressure gas supply being switched by the rotary valve.

Claims

1. A cryogenic refrigerator comprising a rotary valve which controls the flow of high pressure gas into the refrigerator and the return of the gas from the refrigerator, the cryogenic refrigerator further comprising a rotary fluid motor arranged to drive the rotary valve, wherein the rotary fluid motor is constructed of non-magnetic material for use in a high-strength, or sensitive, magnetic field.

2. A cryogenic refrigerator according to claim 1 wherein the rotary fluid motor is a multi-vane type rotary fluid motor.

3. A cryogenic refrigerator according to claim 1, wherein the rotary fluid motor is arranged to drive the rotary valve via a gearing unit.

4. A cryogenic refrigerator according to claim 1, in association with a pump situated remotely from the rotary fluid motor, said pump being arranged to circulate fluid to the rotary fluid motor.

5. A cryogenic refrigerator according to claim 1, wherein the rotary fluid motor is propelled by a flow of gas derived from a compressed gas supply being switched in the rotary valve.

6. A cryogenic refrigerator according to claim 1, in association with a cryostat used to house magnet coils of a nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) system.

7. (canceled)