EP0223868A1 - Process for the reliquefaction of helium at or in a closed loop-operated bath cryo pump - Google Patents

Abstract

It is envisaged that the helium gas running in the return run of the uninterrupted circuit to the compressor is isobarically compressed in a compressor (2) and is then, via a heat exchanger (IIIa) which is arranged in a cooling housing (III) and ensures the forced guidance of the gas, cooled isobarically to the outlet connection (3) starting from the inlet connection (2). The cooling process is supported by nitrogen which is conducted through the cooling housing (III) in a second heat exchanger (VI) and is then relieved via a Joule-Thomson throttle valve (IV) and further cooled to below 4.3 DEG K, an additional LN2 heat exchanger (VIa) also supporting this process. The subsequently expanding gas is again conveyed to the housing (III) and, starting from the latter, the process is continued via the connection (1). <IMAGE>

Description

The invention relates to a method for reliquefying helium in or in a bath cryopump operated in a closed circuit.

These can be used in the context of ion etching systems, implementation systems, coating systems and also large coating systems for physical vapor deposition.
In research, this process can be used for the operation of fusion machines or for carrying out neutral radiation injections.

This list is not intended to outline a "restricted" program, but merely to give an indication of the breadth of the possible uses of the method.

The problem of the re-liquefaction of gases as such is known, the refrigerator cryopumps in their various designs probably having to be referred to as the closest prior art.

In general, these pumps consist of a two-stage cold head, which is connected to a compressor unit and only reaches a temperature of 8 ° -12 ° K on the cold surface. In order to be able to pump He, this cold surface is laminated with activated carbon, which enables adsorption.

In the case of bath cryopumps, their reservoir must be supplied from a dewar.
The LHe container is generally secured by an optically dense chevron baffle made of blackened Cu or A1, while the LN₂ container, ie the nitrogen container, is also largely protected against radiation by a corresponding outer jacket.
The relatively complicated structure, as well as the helium feed, which is carried out in pulse mode, requires regular maintenance of the pumps in relatively short time intervals, so that considerable operating costs are incurred in connection with the time required for refilling the liquid helium.

The additional nitrogen supply required is not to be regarded as problematic, since the operators of vapor deposition systems and vacuum systems generally have liquid LN₂ large stocks, so that an external supply of LN₂ to the cryopump, the radiation protection shield and for pre-cooling the helium re-liquefaction via appropriate lines is possible.

Taking this situation into account, it is therefore the object of the invention to name a method of the type described at the outset which is self-sufficient, ie does not require any refilling processes with liquid helium and liquid nitrogen, and which is furthermore due to the unproblematic, simple, uninterrupted cycles,
has an unlimited service life compared to the known refrigerator cryopumps and also copes with long service lives in the event of an O gas attack.

The solution to this problem according to the invention provides
that the helium gas leading in the return of the circuit to the compressor isothermally compressed from a connection to the outlet and then isobarically cooled from the compressor outlet to the outlet passage via a heat exchanger arranged in a cooling housing, starting from the compressor outlet via a heat exchanger which ensures positive guidance of the gas Countercurrent to the helium flow through the nitrogen LN₂ cooling housing is guided via a likewise positively driven, second heat exchanger to support helium cooling, and the cooled helium between connections is relaxed via a Joule-Thomson throttle valve, and the like. supported by a supplementary LN₂ heat exchanger in which the cryopump is cooled to <4.3 ° K, and
that the gas which then relaxes at> 4.3 ° K is fed back into the housing of the input heat exchanger and, starting from this housing, the process is continued via the two connections (compressor inlet and outlet) and is repeated continuously,
wovei the cycles for the LHe and LN₂ take place in connected phases.

The interaction of the helium and nitrogen circuits ensures the necessary pre-cooling with optimum efficiency in order to achieve a sufficient final cooling capacity at a temperature of below 4.3 ° K for the helium.
Here, the first stage formed by the compressor is the prerequisite for the isobaric cooling of this gas which takes place in the forging stage, which takes place in connection with the LN.sub.2 flowing through this stage in countercurrent, so that when the helium exits Gas from this stage has a temperature of approx. 7 ° K.

The subsequent use of the Joule-Thomson effect, in which the He is passed through a porous throttle point, creates - in contrast to the conditions when using ideal gases - temperature changes in real gases in front of and behind the throttle point, the temperature change Δ T is proportional to the pressure change Δ P.

To further explain this process, it should be noted that the Joule-Thomson effect is composed of two essential components, which can be seen from the correction terms in van der Waal's equation of state.

) (v-b) = RT
Die Korrektionsgröße für den Druck a/v² bedeutet den Kohäsionsdruck, der proportional mit v^-2 gesetzt wird. Das Kovolumen b berück­sichtigt das Eigenvolumen der Moleküle; bei engster Packung würden diese, den klassischen Vorstellungen entsprechend, etwa den Raum b/4 einnehmen. Die Konstanten a und b sind Stoff­konstanten, daher für jedes Gas von anderem Wert."For the sake of simplicity, the explanations according to Hermann Franke, "Lexikon der Physik", Volume 3, 1969, Frankh'sche Verlagbuchhandlung Stuttgart are cited as follows:
"Above all, the thermal equation of state has been the subject of numerous considerations. Attempts have been made to record the deviations from the ideal state by means of additional terms (taking into account the finite extent of the molecules and the force effect on one another). Based on gas kinetic ideas, BERNOULLI (1738) takes into account the volume of the Molecules as an additional link at v; a century later (1846), from compressibility measurements, RITTER found that a correction had to be made to the pressure, which was given as a / v², Van der Waals made both corrections at the same time and thus established the equation of state named after him , which largely captures the behavior of real gases.
This form of the equation of state, although not fully satisfactory in quantitative terms and not theoretically justifiable, has gained great importance due to its simplicity and clarity.
it is
(p +

) (vb) = RT
The correction value for the pressure a / v² means the cohesion pressure, which is set proportionally with v ^-2 . The covolume b takes into account the intrinsic volume of the molecules; with the tightest packing, these would occupy space b / 4, according to the classic ideas. The constants a and b are substance constants, therefore of different value for each gas. "

Without wanting to discuss the scientific background in this context, it can be stated that the gas cools down as a result of the link a / v². It does the external work p₂ v₂ - p₁ v₁, which is not zero in real gases.
The second term b causes the throttling to heat up. Most often, however, the first size predominates.

The Joule-Thomson effect causes e.g. with air of 20 atm and minus 150 ° C 1.2 ° C / atm, while at room temperature only about 1/4 ° C / atm is reached.

Exact data are not available, in particular for helium, although the surprising knowledge gained from experiments in-house allows cooling of the helium to below 4.3 ° K and thus reliable liquefaction.

The directly associated with the bad cryopump LN₂ cooling reduces the radiation to a considerable extent, so that the temperature remains relatively constant over a long distance and the isobaric heating before the isothermal compression given in the compressor only begins during the return to the compressor.

In the next stage, as mentioned, isobaric cooling is repeated in a cycle.

Irrespective of this, it is expedient to combine the assembled parts, which make the process sequence possible, in one insulating housing.
The described invention thus takes full account of the requirements of the task.

In summary, it should be noted once again that the He gas coming from the pump has a temperature of approximately 5 ° to 6 ° K. This gas runs through a heat exchanger and reaches the compressor at 300 ° K. The 300 ° K gas stream coming from the compressor is pre-cooled with LN₂ and then cooled to 6 ° K from the aforementioned cold gas in the heat exchanger.

The method according to the invention is explained in more detail in an exemplary sequence by the attached schematic drawing.

Here, the Roman numerals I to VIa indicate the required basic apperative effort, while the positions provided with Arabic numerals 1 to 5 indicate the connecting cable runs or their end points.

The motor I is the drive source for the compressor II, which compresses isothermally in the course of the process. The heat exchanger housing III contains the heat exchanger IIIa for the helium to be cooled, the return of the helium gas stream, which is isobarically heated from the bath cryopump V, to the compressor II, the partially heated gas stream forming a further part releases its residual cold to the exchange surfaces of the He heat exchanger IIIa and cools it further. The cooling of the He in the heat exchanger IIIa is additionally supported by the LN₂ heat exchanger VI, which is also countercurrently guided, so that a temperature of about 7 ° K is reached when exiting the heat exchanger IIIa.

Between IIIa and V a Joule-Thomson throttle valve IV is arranged, which in the exit to the cryopump V due to the relaxation of the He given here, its cooling to below 4.3 ° K, and thus its liquefaction.

In addition, heat is extracted from the area of the cryopump V by the LN₂ heat exchanger VIa.

Regarding the connections indicated by the Arabic numerals, it should be noted that, starting from the outlet connection 5 of the cryopump V, the helium is transferred via III to 1, i. the compressor connection, isobarically heated and compressed in the compressor isothermally up to the outlet connection II, and isobarically cooled within the housing III, the residual cold of the He gas flow conducted by the cryopump V via the connection 5 in counterflow through the housing III on the way to Suction port 1 of the compressor is released, and further the isobaric cooling in the heat exchanger IIIa is supported by the LN₂ heat exchanger VIa, which is also guided in countercurrent through the housing III.

Between the Arabic positions 3, ie the exit from the heat exchanger IIIa and the position 4, the entry into the cryopump, the Joule-Thomson arrangement IV is provided, as mentioned, through its relaxation effect up to the connection port 4 of the cryopump V the He - gas is liquefied,
and in this state up to the outlet connection 5, which, as described, leads again to the suction connection 1 in counterflow through the housing III, is fed back to the compressor.

Claims (1)

Process for the reliquefaction of helium in or in a bath cryopump operated in a closed circuit, characterized in that
that the helium gas leading in the return of the circuit to the compressor (II) isothermally compressed from the connection (1) to the outlet (2) and then via a heat exchanger (IIIa) arranged in a cooling housing (III) from the compressor outlet (2) via a one Forced operation of the gas-securing heat exchanger (IIIa) isobarically cooled from the compressor outlet (2) to the outlet passage (3), with nitrogen LN₂ in countercurrent to the helium flow through the cooling housing (III) via a likewise forced second heat exchanger (VI) to support the Helium cooling is performed, and the cooled helium between the connections (3 and 4) via a Joule-Thomson throttle valve (IV) expanded, and supported by an additional LN₂ heat exchanger (VIa) in the cryopump (V) to <4, 3 ° K is cooled further, and
that the gas that then relaxes at> 4.3 ° K again the housing (III) of the input heat exchanger (IIIa) and starting from this housing, the process continues via the two connections (1 and 2) and is repeated continuously, the loops for the LHe and LN₂ taking place in coherent phases.

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