Refrigeration – Storage of solidified or liquified gas – Including cryostat
Patent
1989-09-25
1991-02-12
Capossela, Ronald C.
Refrigeration
Storage of solidified or liquified gas
Including cryostat
62467, F25B 1900
Patent
active
049914014
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process and an apparatus for obtaining very low temperatures.
Among the methods for obtaining very low temperatures, one of the most advantageous makes use of the dilution of the isotope 3He in ordinary helium 4He. Below approximately 2.17.degree. K., a 4He-3He mixture exhibits two phases, the limiting content of 3He in 4He being approximately 6%.
To describe the principle of known processes in a somewhat simplistic manner, let us assume that, in a "mixing box" placed in a vacuum enclosure cooled to a temperature of the order of 2.degree. K. or less, there are 3He and 4He in such quantities that there are two phases, a "solution" phase and a pure 3He phase, if we decrease the concentration of 3He in the solution phase, pure 3He will dissolve therein to reestablish the equilibrium concentration, and the energy needed for this dissolution will be taken from the mixing box, which will consequently be cooled.
The word "solution" is employed to simplify matters, it is known that the usual definitions do not apply well at very low temperatures.
In practice, the mixing box is used in combination with a column filled with solution at equilibrium, and at the top of which is placed an evaporator which allows 3He to be extracted from the solution. A concentration gradient is created in the column, and 3He migrates from the mixing box towards the evaporator, which results in the dissolution of pure 3He and the cooling of the mixing box.
For such a device, or cryostat, to operate continuously, it suffices to introduce liquid 3He into the mixing box, optionally mixed with a little 4He, to compensate for escapes.
It would be possible to, in a similar manner, operate a cryostat in which the mixing box contained a solution phase and a pure 4He phase.
With such cryostats, the presence of a distillation unit requires a pumping unit incorporating large-diameter pipes, and makes operation in all orientations or in the absence of gravity (for example use in space) difficult, or even impossible.
The purpose of the invention is to make it possible to get rid of these constraints by the construction of a cryostat comprising neither a distiller nor a low-pressure pumping line.
To obtain this result, the invention provides a process for obtaining very low temperatures, according to which there is created, in a mixing box placed in a vacuum enclosure cooled to a temperature of the order of 2.degree. K. or lower, a two-phase system comprising a phase of solution of 3He in liquid 4He, and a liquid phase consisting of the pure isotope 3He, this isotope is transferred into the solution phase, the energy of dissolution being taken from the mixing box in order to cool it, 3He is extracted from the mixing box in the state of solution and the 3He in the pure liquid state is introduced into the mixing box in a quantity equal to the quantity of this isotope which leaves the mixing box through the solution, a feature of this process being that the two-phase system is created by continuously feeding the mixing box with liquid 4He and 3He which are introduced separately, in that the solution phase is extracted in such velocity conditions that the 3He which it contains cannot diffuse countercurrentwise sufficiently in the solution to raise appreciably the content of 3He in the liquid 4He introduced and reduce the dissolution, in this 4He, of the liquid 3He introduced simultaneously.
The process of the invention introduces appreciable differences from the prior art. In both cases, an attempt is made to have a 4He with a low content of 3He, and in which the liquid 3He introduced will be capable of dissolving easily, with production of cold.
In the prior art, the content of 3He is lowered by making the latter diffuse through the solution in the direction of an evaporator. According to the invention, on the other hand, the solution is extracted at a velocity such that 3He cannot return backwards to raise the content of 3He in 4He and consequently to make it less capable of dissolving the liquid 3He. In the system of
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Capossela Ronald C.
Centre National d'Etudes Spatiales
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