Patent Application
20090301129
The invention pertains to the field of cryorefrigeration. The invention more particularly pertains to conserving liquid helium and liquid nitrogen in nuclear magnetic resonance and magnetic resonance imaging systems.
Liquid helium and liquid nitrogen are used to maintain low temperatures for superconductivity, magnets operated with superconducting elements, and instruments for imaging and spectroscopy referred to as Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance (NMR) spectroscopy. Many of these systems were designed and installed with the expectation that helium and nitrogen vapor would boil off and be exhausted as they were both plentiful and not environmentally harmful. Presently however it has been observed by the inventors that resupply of cryogens is both inconvenient and increasingly expensive as helium supply has not expanded at the rate of helium demand.
Cryocoolers comprising a compressor and a cold head are within the prior art. Both Gifford-McMahon and pulse tube cryocoolers are suitable in single stage and multiple stage form to perform helium and nitrogen reliquefication through recondensing. Recondensing nitrogen gas is performed in the degrees Kelvin operating range of 65K and 80K. Recondensing helium gas is performed in the degrees Kelvin operating range of 3.8 K and 4.8K. In a conventional cryocooler system, a cold head comprises manifolds, the pneumatic control electronics and valves, the expanders, the heat exchangers, and the regenerators of the first stage and the second stage. A conventional cryocooler system further comprises a compressor which intakes low pressure high purity working helium gas and compress helium gas to high pressure (200-350 psi) which absorbs heat load in a first stage heat exchanger and thermally couples it to the environment.
Prior art cryocoolers are further discussed in U.S. Pat. Nos. 6,167,707 and 6,330,800.
Conventional cryogen reliquefication systems have been observed to induce undesirable sidebands in NMR and artifacts in MRI images due to mechanical vibration of the high intensity magnets. To avoid this some magnetic resonance systems include a switch at the operator console to control the reliquefication system and others are designed to boil off cryogens ignoring cost of their resupply. Moreover, cryogen reliquefication systems, being mechanical, require periodic servicing and maintenance which if integrated into the cryostat interrupts the production schedule of the highly expensive NMR/MRI capital equipment.
Thus it can be appreciated that what is needed is a way to provide MRI and NMR cryostats designed without an integrated cryocooler with removeable and retrofittable apparatus to conserve liquid helium and liquid nitrogen.
Cryogen refrigeration systems cause undesired sidebands in spectroscopy and artifacts in images provided by nuclear magnetic resonance instruments yet cryogen boil-off is ever more expensive and inconvenient in materials, lost production time, and supply chain management.
A super insulated transfer line removably couples a served cryostat to a vacuum vessel containing at least one helium recondenser, the vacuum vessel is further attached to an isolation coupling which mechanically supports a cryogenic cold head.
The cost and inconvenience of resupplying liquid helium and liquid nitrogen is reduced. New shipments of superconducting magnets can be delivered in the chilled state without the cryogen refrigeration system installed which eliminates boil off of cryogen due to environmental heat in the unpowered refrigeration heat mass. Maintenance on the cryogen refrigeration equipment can be done by removing the transfer line and leaving the NMR/MRI system in production mode. Multiple client NMR/MRI cryostats may be served by a single high capacity refrigeration unit. Legacy NMR/MRI systems designed to vent helium to the atmosphere may be upgraded with a reliquefier without removal or time consuming modification.
The best mode embodiment of the present invention comprises a pulse tube cryogenic cold head coupled to a bellows coupled to a vacuum vessel. The vacuum vessel communicatively connects to a plurality of nitrogen transfer lines and a plurality of helium transfer lines which allow the cryogen reliquefying apparatus to be shared among several instruments. Each nitrogen transfer line passes through the vacuum vessel wall and connects to a condenser attached to a 65K to 80K heat shield can attached to the 1st flange of the cold head. Each helium transfer line passes through the vacuum vessel wall, and through the heat shield wall to connect to a recondenser assembly attached to the 2nd flange of the cold head.
The present invention is an apparatus for removably attaching cryogen reliquefication to one or more cryostats which may have been shipped and installed without an integrated cryogen reliquefier. The invention is an apparatus comprising a
The bellows comprises
The flexible helium transfer line comprises
The recondenser assembly comprises
The apparatus further comprises a cryogen refrigeration coldhead coupled to the bellows, said coldhead comprising a first flange coupled to said 77K can and a second flange coupled to said helium recondenser assembly. The apparatus further comprises a flexible nitrogen transfer line coupled to said vessel.
The apparatus further comprises a cryogen pump whereby the liquid may be reintroduced to the cryostat independent of gravity flow.
The apparatus further comprises a system purge loop and a refrigeration balancing heater whereby slightly positive pressure is maintained over ambient air pressure to prevent contamination.
The apparatus further comprises a plurality of helium transfer lines coupled to the helium recondenser assembly whereby a plurality of client cryostats may be served by one cryocooler. The coldhead is a pulse-tube coldhead.
A flexible transfer line for boil off gas and liquid cryogen return may be retrofitted to a cryostat which was not designed with cryogen refrigeration capabilities. A plurality of nitrogen transfer lines and helium transfer lines may be connected to serve multiple client cryostats.
It is the objective of the present invention to allow retrofitting of MRI and NMR systems with a cryogen conserving reliquefier.
The invention comprises a flexible well-insulated anti-vibration transfer line coupled to a cryocooler comprising a cold head and a bellows which may attach to a cryostat through the vents which previously exhausted helium vapor.
The invention is distinguished from conventional cryocoolers by having an anti-vibration transfer line to reduce vibrations transmitted to the magnet. The invention is further distinguished by supporting a plurality of cryostats. The invention is further distinguished by attaching to the nitrogen vent port and the helium vent port of many different designs of cryostats which allows retrofitting of installed cryostats without substantial modification. The invention is distinguished by having structural support for the cold head isolated from a plurality of cryostats, ie. the cold head is not itself coupled to the cryostat except by the anti-vibration transfer line.
Significantly, this invention can be embodied in other specific forms without departing from the spirit or essential attributes thereof, and accordingly, reference should be had to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.
