Refrigeration – Low pressure cold trap process and apparatus
Patent
1995-11-20
1997-03-18
Capossela, Ronald C.
Refrigeration
Low pressure cold trap process and apparatus
165 4, 417901, B01D 800
Patent
active
056112084
DESCRIPTION:
BRIEF SUMMARY
The invention relates to cryopumps and in particular to a pumping device of the cryogenic diffusion type in which the pumping function is boosted by the addition of a thermal gradient barrier diffusion pump.
A cryogenic diffusion pump is a device which can pump and to a certain extent, separate mixtures of gases at cryogenic temperatures.
Known pumps such as those described in J. Vac. Sci. Technol. A8(1) Jan/Feb (1990) 141-144 and Fusion Technology 21 March (1992) 276-281 usually consist of a vertical "dipstick" or "finger" cold trap in which the inner wall has an axial thermal gradient between the top (hot) and the bottom (cold) of the finger. Other shapes and arrangements for the pump are however possible.
The principle of the pump is that as a mixture of gases enters the cold trap the mixture thermally accommodates to the pump wall. As this happens the gas density increases and the mean free path of the individual gas molecules decreases accordingly so that the constituents are eventually pumped by cryocondensation, i.e. forming a frost layer on the cold wall depending on temperature and the respective vapour pressure characteristics of the gases.
It is possible however, using the pump, to effect a separation of one or more gases in the mixture from the others by setting the temperature of the walls such that one or more components of the mixture do not freeze out on the pump walls but remain in the gas phase. These can then leave the pump by a return tube or other outlet at a pressure close to (up to 80%) the total pressure of the mixture at the pumps inlet.
A particular advantage of cryogenic diffusion pumps is that they can pump gases at very low pressures.
One application for which the cryogenic diffusion pump has proved useful is pumping the exhaust gases from a thermonuclear fusion reactor. The exhaust contains mixtures of hydrogen isotopes H.sub.2, D.sub.2 and T.sub.2 including heteromolecular species such as HT and DT and helium isotopes .sup.4 He and .sup.3 He. .sup.4 He is a reaction product of the fusion reaction and must be removed in order to keep the .alpha.-particle concentration in the burning plasma below 5 to 10%. This results in a partial pressure of helium in the effluent gas of about 2 to 4%.
If a cryogenic diffusion pump is used to pump the exhaust gases from a fusion reactor the cold end of the gradient should be about 4.degree. K. and the warmer end about 77.degree. K. At these temperatures the hydrogen isotopes and other impurities condense onto the pump wall but helium remains in the gaseous phase and will be driven to the pump exit.
As mentioned before the pressure of gas at the pump outlet is dependant on the inlet pressure and there is usually a pressure drop of about 20% between the inlet and the outlet. A particular problem encountered in pumping exhaust gases from a fusion reactor is the relatively low pressure of said gases and the simultaneously high flow rate (e.g. typically a pressure of 0.1 Pa and a volume flow rate of 1000 m.sup.3 s.sup.-1) such that the gases leaving the outlet of the cryogenic diffusion pump cannot be further pumped or compressed by presently commercially available conventional pumps (such as turbomolecular and/or positive displacement pumps). Such pumps of large size to deal with the low pressures and high mass flow rates would have to be specially constructed.
Thus the present inventors have developed a modified cryogenic diffusion pump which contains an additional pumping means which acts to boost the outlet pressure. It has been discovered that the outlet pressure of the cryogenic diffusion pump can be satisfactorily boosted if it is combined with a thermal gradient barrier diffusion pump. This raises the gas pressure to a level where commercially available pump sizes are sufficient for further compression of the gas to atmospheric pressure.
Thermal gradient barrier diffusion pumps are known from J. Vac. Sci. Technol A6(1) Jan/Feb 1988 pages 148-149. The working principle of such a pump is based on thermal transpiration or the "Knudsen" effect
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Patent Abstracts of Japan, vol. 10, No. 378 (M-546) (2435), Dec. 17, 1986.
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Hemmerich Johann L.
Milverton Paul
Capossela Ronald C.
European Atomic Energy Community
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