US3579997A

US3579997A - Cryopumping installations with high flow-rates

Info

Publication number: US3579997A
Application number: US840734A
Authority: US
Inventors: Michel Rapinat
Original assignee: Air Liquide SA
Current assignee: Air Liquide SA; LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority date: 1968-07-30
Filing date: 1969-07-10
Publication date: 1971-05-25
Anticipated expiration: 1988-05-25
Also published as: CH512675A; LU59124A1; DE1937821A1; SE353944B; BE736646A; NL161573C; FR1584067A; NL161573B; GB1216448A; NL6911437A

Abstract

A cryopump is constituted by a fluidtight casing inside which is arranged a plurality of condensation plates carried by a cooling coil supplied with very cold fluid, the whole disposed externally of and around a baffle formed by a plurality of frustoconical rings arranged in the extension of a stop valve. The baffle is preferably cooled by a moderately cold fluid (80* K) though conduits, while a coil embedded in the wall of the casing and traversed by a moderately cold fluid cools an internal wall of the casing. This cryopumping device is adapted to extract high flow rates at a relatively moderate pressure of the order of 10 2 torr.

Description

United States Patent [72] Inventor Michel Rapinat Fontaine, France [2l] Appl. No. 840,734 [22] Filed July 10, 1969 [4S Patented May 25, 1971 [73] Assignee L`Air Liquide, Societe Anonyme pour Letude et Lexploitation des procedes Georges Claude [32] Priority July 30, 1968 [33] France [3 l 16 1,16 l

[54] CRYOPUMPING INSTALLATIONS WITH HIGH FLOW-RATES 12 Claims, 2 Drawing Figs.

[52] U.S. Cl 62/55.5 [5I] Int. Cl..... .t B01d S/00 [50] Field of Search 62/55.5

[56] References Cited UNITED STATES PATENTS 2,703,673 3/1955 Winkler 62/55.5

Primary ExamineF-William J. Wye Attorney- Young and Thompson ABSTRACT: A cryopump is constituted by a fluidtight casing inside which is arranged a plurality of condensation plates carried by a cooling coil supplied with very cold fluid, the whole disposed externally of and around a baffle formed by a plurality of frustoconical rings arranged in the extension of a stop valve. The baffle is preferably cooled by a moderately cold fluid (80K) though conduits, while a coil embedded in the wall of the casing and traversed by a moderately cold fluid cools an internal wall of the casing. This cryopumping device is adapted to extract high flow rates at a relatively moderate pressure of theorder of l0l2 torr.

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CRYOPUMHNG INSTALLA'HONS Wl'llll HlGlli FLOW- RATES The present invention relates to improvements in cryopumping installations with high flow rates, and in particular to such installations which can operate continuously up to moderately high vacua, of the order of lOz torr, for example.

ln cryopumps, by means of which a vacuum can be obtained and then maintained in a chamber, the gas coming from the chamber to be exhausted is deposited in the solid state on collecting means, constituted for example by a condensation or sublimation surface cooled by the vapors of a liquefied gas or by a refrigerator working in a closed circuitA Periodically, after having stopped and isolated the pump from the chamber, it is necessary to regenerate the cryopump by heating the condensation surface so as to liquefy or sublimate the solid deposit and to ensure the elimination of this latter by simple evacuation or drawing off. lf the pumping of a chamber is to be effected continuously, two cryopumps are then arranged in parallel, one being in course of regeneration while the other is in operation.

Up to the present time, cryopumps have been especially utilized for ultravacua, that is to say for high vacua which may attain 1019 tori'A ln this case, the mass flow is very low and the thickness of the condensate always remains small, even if the condensation surface is relatively modest, and although this type of cryopump may be in operation for long periods in order to maintain a very high vacuum, such continuous working does not present any problems since it is not necessary to carry out regeneration operations. The ultravacuum cryopumps constructed at the present time are thus characterized by thc existence of a single pumping element or unit, and by condensation walls having a relatively modest surface area.

On the other hand, when the depression level is much less considerable, for example of the order of lO12 torr, the mass flow then becomes large and the volume of condensate deposited in a given time is relatively high. When the thickness of the condensate increases, the thermal exchanges between the gas to be solidified and the condensation surface deteriorate and this results in a reduction of the speed of pumping together with a rise in the limiting pressure. In order to reduce the effect of this additional thickness of condensate, it is then necessary to increase the surface area of the heat-exchange walls and to regenerate the cryopump from time to time. It thus becomes necessary to arrangeA two cryopumps which operate cyclically.

The high value of the mass flow of gas to be cryopumped necessitates the production of a pumping unit which has at the same time a good inlet conductance for the gases to be collected and a large condensation surface area, and it is an object ofthe invention to provide a cryopump in which these two characteristics are arranged so as to obtain satisfactory operation of the pump with however a comparatively small overall size.

According to an essential characteristic feature of the invention, the collecting means comprise main collecting means brought to a cold temperature, arranged outside and around a baffle brought to a moderately cold temperature and constituted by an assembly of walls of revolution arranged at a distance from each other along a common axis forming an extension of the axis of the inlet valve, and having an internal diameter substantially in the vicinity of that of the said inlet valve. The baflle is a body composed of a plurality of deflecting walls, for example of frustoconical shape, arranged at a distance from each other along a common axis and permitting a radial distribution in steps towards the exterior of the gas which passes into the interior of the baffle, while forming a screen against direct radiation coming from the zone at ambient temperature, that is to say in the present case, essentially from the inlet valve.

The advantages of this arrangement are as follows:

The cross section of passage of the baffle, that is to say the sum of the individual sections between two frustoconical walls, is independent of the diameter of the valve, and it can easily be increased by providing a larger number of baffle walls, or in other words by extending the cryopump axially. lt is thus possible to obtain a very high inlet conductance of the gases in the direction of the main condensation surface.

The surface area of the condensation wall may be very large, since the latter surrounds the baffle. Furthermore, a small increase in external diameter is sufficient to produce a very large increase in the exchange surfaces.

ln the case of an installation which operates continuously with two pumping units, the regeneration of one cryopump unit is effected by introducing into the chamber a certain flow of hot gas which, by cooling and becoming liquefied, causes the sublimation or the liquefaction of the deposit. This method of regeneration is very simple, very rapid and permits good recovery of frigories.

The characteristic features and advantages of the invention will be further brought out in the description which follows below, given by way of example with reference to the accompanying drawings, in which:

FIG. l is a diagrammatic view of the whole of a cryopumping installation;

FIG. 2 is a view in cross section of a pumping unit according to the invention.

The following description gives an example of construction of a cryopump which can operate without interruption for several months, and which is capable of pumping a large mass flow at a pressure of l0l2 torr.

The installation according to the invention is composed of a cryogenerator l, two

pumping units

2 and 3 and accessories permitting continuous working to be effected.

The cryogenerator l produces a certain frigorific output at two temperature levels, which are respectively in the vicinity of 80 K and 25 K. The frigories produced by the cryogenerator are conveyed to the pumping units by means of a circulation of two fluxes of gaseous helium under pressure, respectively through the outward and return conduits 4 and S with stop valves 6 and 7 for the very cold fluid (25 K) and outward and return conduits 8 and 9 for the moderately cold fluid (80 l() towards the two

pumping units

2 and 3, which are connected by the

isolating valves

13 and 14 respectively to a chamber l2 in which a vacuum is to be maintained.

As shown in FIG. 2, each pumping unit 2 (3) is constituted by a fluidtight casing delimited towards the exterior by a wall with high heat insulation 21 composed of two parts 21a, 2lb

fitted into each other. Inside the casing is arranged an assembly of condensation surfaces constituted by plates 22 carried by a cooling coil 23 which is supplied with a very cold fluid by the circuit of the conduits 4 and S.

These plates 22, of which there is a very large number, are for example square with a 5 mm. side, a thickness of.l mm. and spaced apart by 3 cm. On each side of the plates 22 arev arranged, on one side a wall 25 fixed against the internal face of the wall of the casing 21a; on the other side a baffle 26, cornposed of a plurality of

frustoconical rings

26a, 26b, 26e, 26d, arranged coaxially with the vertical axis of the casing in the extension of the axis of the isolating valve (13 (14) and flared upwards. These rings 26a, 26h, 26e, 26d, are blackened so as to prevent reflection of the radiation produced by the surfaces at ambient temperature (especially the valves ll3 (14). The lower ring 26d is followed by a conical reflector 27, the apex of which is directed upwards.

These rings forming the baffle 26, the reflector 27, and also the walls 2S are supplied with fairly cold fluid by a cooling coil, constituted, level with the baffle 26, by a plurality of con duits in the form of

open rings

30a, 30b, 30C, 30d, the downstream extremity of one being connected to the upstream extremity of` the next following ring byva pillar element 28a, 28h, 23e, 28d, this latter supplying a ring 31 level with the reflector 27, while the return is effected by the conduit 28e and by a coil 32 embedded in the wall 2S, both supplied by the conduits 9 of fairly cold fluid.

lt will be noted that the wall 25 is extended downwards by a bottom collecting wall 29 with a funnel member 35 and having a portion in the form of a dome 34 with one or a number of laterial perforations 33.

The installation (see FlG. 1) further comprises accessories which enablethe saturated unit to be regenerated, and which ensure the continuous operation of the installation. These accessories mainly comprise: A pumping group 40 for the noncondensable gases, connected to a conduit 4l, 42, terminating at the bottom of each

pumping unit

2, 3, through the intermediary of a

valve

43 and 44 respectively. This group 40 is intended to eliminate the gases which are only condensable at a temperature colder than the temperature of the main condensation walls 22 (that is to say colder than 25 K). These gases are mainly hydrogen and helium. lt is particularly advisable to utilize a diffusion pump associated with a primary pump, in consequence of the relatively low flow rate of the pressure level (about lI3 torr) and the nature of the gases.

A prevacuum pump 45 connected by the conduits 46, 47 to valves 48, 49, in the interior of each

unit

2, 3. After each regeneration, the pressure rises in the unit to a fairly high value. Before being able to put this unit back into operation, it is necessary to reduce the pressure to a few torrs. A primary pump is particularly well suited to this use.

A receptacle 50 intended to collect the liquefied gases and communicating by a valve with atmospheric air, and into which discharges a pipe 5l connected by the

conduits

52, 53 to valves 54 (55), FIG. 2 on the funnel 35 at the bottom of each pumping .

unit

2, 3. This receptacle 50 permits the collection of the liquefied deposit during regeneration. It is possible to recover the frigories stored in this tank by immersing in it a coil 56 through which passes the helium coming from the head of fairly cold fluid 8, 9.

The regeneration of the pumping unit is effected in the following manner: A regenerated unit 2 (or 3) is put in parallel with the unit 3 (or 2) in course of operation, after which this unit 3 (or 2) respectively, is isolated by the valve 14 (or I3) respectively. The circulation of helium at very low temperature (25 K) is then interrupted to the unit 3 in course of regeneration, by closing the valve 7.

The valve 55 which puts the unit 3 into communication with the tank 50, is then opened. An inlet of dry atmosphere takes place which causes the liquefaction of the deposit. The liquid obtained is then collected by gravity in this same tank 50.

The unit 3 is then isolated from the tank 50 (closure of the valve 55) and is then put into communication with the prevacuum pump 45 by opening the valve 48. The pressure is then brought to a value of a few torrs. During the same time, the unit is cooled by opening the valve 7, by a small flow of helium at 25 K so as to prepare it for being put into operation,

When the temperature of the cryosurface is lower than 30 K, the pumping unit is regenerated and is immediately ready to work.

The installation described above, or any other installation derived therefrom, permits the production of a cryopump working continuously and capable of pumping a large mass flow at pressures of the order of l0lz torr. It is possible to contemplate the use of this installation in replacing pumping groups comprising Roots" or steam ejectors. The replacement of the Roots is particularly recommended when the gas to be pumped contains dust.

I claim:

1. A cryopump with a high flow rate, comprising a fluidtight casing thermally insulated from the exterior and having a wide inlet passage, an inlet valve associated with said passage, gas collection meanscomprising baffle means formed by an assembly of walls of annular form mounted at a distance from each other along a common axis in the extension of the axis of said inlet valve, said baffle means having an internal diameter substantially close to that of said inlet passage, main collecting means disposed externally around said baffle, means and means for circulating a moderately cold fluid for said baffle means and a very cold fluid for said main collecting means.

2. A cryopump with a high flow rate as claimed in claim l, in which said baffle means comprises a plurality of frustoconical stepped walls, spaced apart from each other.

3. A cryopump as claimed in claim 2, in which said frustoconical walls are mounted inside said casing so as to flare outwards towards the exterior in the direction of said inlet valve.

4. A cryopump with a high flow rate as claimed in claim l, in which said means for circulating a very cold fluid comprise a tubular coil mounted round and externally to said baffle, means said main collecting means comprising an assembly of metal plates carried by said coil.

5. A cryopump with a high flow rate as claimed in claim l, and further comprising a reflector located at the extremity of said baffle means opposite to said inlet valve, said means for circulating a moderately cold fluid incorporating a tube for cooling said reflector.

6. A cryopump with a high flow rate as claimed in claim 1, in which said means for circulating a very cold fluid comprise a fluid at a temperature substantially of the order of 25 K, while said means for circulating a moderately cold fluid comprise a fluid at a temperature substantially of the order of K.

7. A cryopump with a high flow rate as claimed in claim l, in which an internal wall of said casing is cooled by said means for circulating a moderately cold fluid.

8. A cryopump with a high flow rate as claimed in claim 7, in which said casing is provided with a lower passage for the evacuation of liquid to the exterior, and in which said intemal wall of the casing has a portion of funnel shape engaged in said liquid-evacuation passage.

9. A cryopump with a high flow rate as claimed in claim 7, in which said casing is provided with a lower passage for the evacuation of gases towards the exterior, and in which said internal wall of the casing is adapted to form a dome perforated laterally above said gas-evacuation passage.

l0. A cryopumping installation with a high flow rate, cornprising at least two cryopumps each including a fluidtight casing thermally insulated from the exterior and having a wide inlet passage, an inlet valve associated with said passage, gas collection means comprising baffle means formed by an assembly of walls of annular form arranged at a distance from each other along a common axis in the extension of the axis of said inlet valve, said baffle means having an internal diameter substantially close to that of said inlet passage and main collecting means disposed externally of and around said baffle means, means for circulating a moderately cold fluid for said baffle means and a very cold fluid for said main collecting means, a single chamber to be exhausted coupled to each said pump by means of each of said inlet valves, a double flux cryogenerator supplying said means for circulating moderately cold and very cold fluids, stop valves on said circulating means, a pumping group, coupling means incorporating stop valves between said pumping group and each said cryopump, a prevacuum pump, coupling means including stop valves between said prevacuum group and each said cryopump, and means for recovery of liquids adapted to cooperate with said means for circulating the moderately cold fluid.

ll. A cryopump with a high flow rate as claimed in claim l, said annular baffle means being so shaped and disposed as to direct gas from within said annular baffle means radially outwardly toward said main collecting means.

12. A cryopump with a high flow rate as claimed in claim 1l, in which said main collecting means comprises a large number of metal plates.

Claims

2. A cryopump with a high flow rate as claimed in claim 1, in which said baffle means comprises a plurality of frustoconical stepped walls, spaced apart from each other.
3. A cryopump as claimed in claim 2, in which said frustoconical walls are mounted inside said casing so as to flare outwards towards the exterior in the direction of said inlet valve.
4. A cryopump with a high flow rate as claimed in claim 1, in which said means for circulating a very cold fluid comprise a tubular coil mounted round and externally to said baffle, means said main collecting means comprising an assembly of metal plates carried by said coil.
5. A cryopump with a high flow rate as claimed in claim 1, and further comprising a reflector located at the extremity of said baffle means opposite to said inlet valve, said means for circulating a moderately cold fluid incorporating a tube for cooling said reflector.
6. A cryopump with a high flow rate as claimed in claim 1, in which said means for circulating a very cold fluid comprise a fluid at a temperature substantially of the order of 25* K, while said means for circulating a moderately cold fluid comprise a fluid at a temperature substantially of the order of 80* K.
7. A cryopump with a high flow rate as claimed in claim 1, in which an internal wall of said casing is cooled by said means for circulating a moderately cold fluid.
8. A cryopump with a high flow rate as claimed in claim 7, in which said casing is provided with a lower passage for the evacuation of liquid to the exterior, and in which said internal wall of the casing has a portion of funnel shape engaged in said liquid-evacuation passage.
9. A cryopump with a high flow rate as claimed in claim 7, in which said casing is provided with a lower passage for the evacuation of gases towards the exterior, and in which said internal wall of the cAsing is adapted to form a dome perforated laterally above said gas-evacuation passage.
10. A cryopumping installation with a high flow rate, comprising at least two cryopumps each including a fluidtight casing thermally insulated from the exterior and having a wide inlet passage, an inlet valve associated with said passage, gas collection means comprising baffle means formed by an assembly of walls of annular form arranged at a distance from each other along a common axis in the extension of the axis of said inlet valve, said baffle means having an internal diameter substantially close to that of said inlet passage and main collecting means disposed externally of and around said baffle means, means for circulating a moderately cold fluid for said baffle means and a very cold fluid for said main collecting means, a single chamber to be exhausted coupled to each said pump by means of each of said inlet valves, a double flux cryogenerator supplying said means for circulating moderately cold and very cold fluids, stop valves on said circulating means, a pumping group, coupling means incorporating stop valves between said pumping group and each said cryopump, a prevacuum pump, coupling means including stop valves between said prevacuum group and each said cryopump, and means for recovery of liquids adapted to cooperate with said means for circulating the moderately cold fluid.
11. A cryopump with a high flow rate as claimed in claim 1, said annular baffle means being so shaped and disposed as to direct gas from within said annular baffle means radially outwardly toward said main collecting means.
12. A cryopump with a high flow rate as claimed in claim 11, in which said main collecting means comprises a large number of metal plates.