Refrigeration – Cryogenic treatment of gas or gas mixture – Liquefaction
Patent
1996-04-17
1997-08-19
Capossela, Ronald C.
Refrigeration
Cryogenic treatment of gas or gas mixture
Liquefaction
62610, F25B 1902
Patent
active
056576358
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and a device for obtaining very low temperatures, less than approximately 1 K and, in particular, than 0.1 K.
Document EP-A-0,327,457, which corresponds to U.S. Pat. No. 4,991,401 and which cites as inventor one of the authors of the present invention, describes a cryostat which comprises a mixing point in which there is maintained a two-phase system comprising a solution phase of 3He in liquid 4He and a liquid phase formed by pure 3He. Liquid 3He and 4He are separately introduced continuously into a mixing point and the solution is extracted from the mixing point at such a rate that the 3He cannot flow back to increase the 3He content of the 4He and consequently render the latter less capable of dissolving the liquid 3He introduced. The mixing point is placed in an enclosure cooled at least to 2 K.
More precisely, in the mixing point, the two fluids create, by being mixed, a two-phase system comprising a phase rich in 3He and a dilute phase, the energy of dilution or of solution being used for the cooling, the progression of the two phases in the mixture outlet tube preventing the dissolved 3He from diffusing in countercurrent into the cold part of the system, whereas, at a higher temperature (above 0.5 K), the solubility of 3He in 4He increases, the mixture then includes only a single phase and the rate must be sufficient for the 3He not to be able to diffuse in countercurrent.
This cryostat has the advantage that it can operate in the absence of gravity because it does not comprise a distiller, which makes it particularly advantageous for use in space. When used in this context, the cryostat can operate by discharging into space the small quantities of 4He and 3He mixture which it produces. In the case when the vehicle is to return to earth, this mixture may also be stored in a reservoir, with a view to distilling it on the ground. If the cryostat is used on the earth, it can of course be coupled with a distillation apparatus, with the assembly then operating in a closed circuit.
One difficulty encountered in the use of this cryostat results from the necessity of having a superfluid helium reservoir for keeping the enclosure at less than 2 K, which constitutes a complication. It is known that such storage imposes particular constraints which are difficult to fulfil, in particular on board a spacecraft.
The object of the present invention is to provide a cryostat which operates according to the method described in EP-A-0,327,457 and which has a simple structure, is compact and consumes little energy, and more especially is free of the necessity of producing and/or storing superfluid helium for cooling the enclosure to 2 K or less.
In order to obtain this result, the invention provides a method for obtaining very low temperatures, according to which 4He and 3He, which are cooled with the aid of heat exchangers to a temperature of the order of 0.2 K or below, are continuously introduced into the point where they are mixed to absorb heat by dilution of the 3He in the 4He, thus producing cooling of the closed two-phase mixture, which mixture is extracted through a conduit designed so that the 3He cannot diffuse in countercurrent and reduce the dissolution of 3He, in which method a heat exchanger adjacent the mixing point is used to cool, by the extracted mixture circulating in the opposite direction, the fluids flowing toward the coldest point, the main feature of this method being that the 4He and the 3He which are intended to be mixed are cooled from their supply temperature to a temperature of less than 2.5 K by exchange with the extracted mixture, the power being absorbed by using a Joule-Thomson expansion of this mixture, thus permitting the system to operate with a supply temperature well in excess of 4 K.
The cooling power during the Joule-Thomson expansion depends only on the input and output pressures of the mixture. The best performance is obtained with pressures of the order of 2 to 15 bar on input and 1 to 15 millibar on output.
The invention results from
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Cryogenics, vol. 30, Sep. 1990, pp. 447-451.
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Advances in Cryogenic Engineering, vol. 35, Part B, pp. 1079-1086.
Benoit Alain Daniel
Pujol Serge
Capossela Ronald C.
Centre National d'Etudes Spatiales
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