Gas-cooled bushing in cryotanks for superconducting applications

Refrigeration – Storage of solidified or liquified gas – Including cryostat

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622592, 1741251, 335216, F25B 1900

Patent

active

057201730

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

In the application of the superconducting effect, the object which is to be cooled to such a temperature that the electrical conductors associated with the object become superconducting is normally disposed in a so-called cryotank. In the lower part of the cryotank the refrigerant is arranged in the form of a cryogenic liquid surrounding the object. The space is the cryotank above the liquid level is occupied by the refrigerant in gaseous state. The electric current connection to the object is brought about by current leads in bushings which via fixing flanges are attached to the lid of the cryotank. Inside the cryotank the bushings are open at the bottom, whereby the refrigerant in gaseous state surrounds the current leads and cools these. The invention relates to a special design of the bushings and the current leads to attain an optimum cooling effect of the current leads.


BACKGROUND ART, THE PROBLEMS

The walls and bottom of a cryotank are very well thermally insulated against the surroundings with the aid of outer layers with radiation shields where air has been evacuated, that is, where a vacuum is created. Thus, there is no mentionable influx of heat to the interior of the cryotank from the surroundings. However, as will be clear from the following, the temperature of the gas at the inside of the lid attain a temperature which practically corresponds to the temperature of the surroundings. The temperature of the gas at the boundary layer between gas and liquid obtains the boiling-point temperature of the liquid at the pressure considered. The temperature difference which prevails between the lower and upper parts of the gas is distributed over the height of the gas column and leads to a certain heat influx to the liquid in the cryotank.
A problem which arises in connection with superconducting applications is the heat influx to the cryogenic liquid which takes place because of the temperature difference between the surroundings and the object. This is due to the fact that good electric conductors are also good thermal conductors. In addition, at least at high currents, heat is developed in those parts of the current leads which are located in the gas-filled part of the cryotank due to the current which traverses the conductors. The electric heat generation takes place as a result of the ohmic resistance in the current leads. In case of alternating current, there is also the generation of heat because of the occurrence of eddy currents. The increased resistance arising because of the skin effect must also be taken into account.
Without quantifying the heat influx to the cryotanks and the internally developed heat in greater detail, it can be determined that to minimize the heat influx to the liquid, this heat has to be discharged in some way. Automatic means therefor is built into the relationship between the liquid and vapour phase of the refrigerant. If there is a tendency to increasing gas temperature, an increasing part of the liquid will change into gaseous state. The gas thus developed is allowed, via an opening on that part of the bushing which is located outside the cryotank, to flow freely out into the surrounding air.
The above means that a gas flow, which at the interface between liquid and gas largely maintains the temperature of the liquid, on its way up to the lid and the discharge into the surrounding air where it obtains the temperature of the surrounding air, flows around the current leads and hence can be used for cooling thereof. Since the direction of the gas flow is opposite to the heat influx, this gas cooling is often called counter-flow cooling. To make this cooling as efficient as possible, the current leads are designed as heat exchangers. As such, the current leads in the gas-filled part of the cryotank may have various designs. In
Superconducting Magnets, Clarendon Press, Oxford 1983, page 272, the current leads are described as electrically parallel-connected foils which are mounted at a certain distance from each other for passage of a refrigerant alon

REFERENCES:
patent: 4340405 (1982-07-01), Steyert, Jr. et al.
patent: 4739633 (1988-04-01), Faris
patent: 4813244 (1989-03-01), Brzozowski
patent: 5065582 (1991-11-01), Seifert
patent: 5166776 (1992-11-01), Dederer et al.
patent: 5299425 (1994-04-01), Hingst
patent: 5317879 (1994-06-01), Goldberg et al.

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