US3769806A

US3769806A - Method of and apparatus for cryopumping gas

Info

Publication number: US3769806A
Application number: US00198178A
Authority: US
Inventors: J Boissin; J Thibault
Original assignee: PROCEDES GEORGES CLAUDE SA
Current assignee: PROCEDES GEORGES CLAUDE SA; S A Procedes Georges Claude fr
Priority date: 1970-11-13
Filing date: 1971-11-12
Publication date: 1973-11-06
Anticipated expiration: 1990-11-06
Also published as: NL172590C; CA947990A; IT941125B; NL172590B; JPS56637B1; SE377830B; GB1375734A; FR2114039A5; BE775243A; LU64250A1; DE2155994A1; CH550941A; NL7115666A

Abstract

This invention relates to a method of and apparatus for pumping gas and particularly to a system in which a primary vacuum of the order of 10 1 to 10 4 Torr approximately is produced, followed by the production of a secondary vacuum. According to the invention, the main trapping surface effected in a main cryopumping operation is arranged in two areas, one being of easy access to the gas to be pumped and the other more difficult of access. A cryodeposit of the adsorbing gas is positioned practically exclusively in the area of most difficult access, in order to adsorb the residual gases. The most important residual gas referred to is helium.

Description

United States Patent [191 Boissin et al.

[ METHOD OF AND APPARATUS FOR CRYOPUMPING GAS [75] Inventors: Jean-Claude Boissin,

Montbonnot-Saint-Martin; Jean-Jacques Thibault, Saint-Martin-DUriage, both of France [73] Assignee: LAir Liquide, Societe Anonyme Pour LEtude et LExploitation des Procedes Georges Claude, Paris, France [22] Filed: Nov. 12, 1971 [2]] Appl. No.: 198,178

[30] Foreign Application Priority Data Nov. l3, I970 France 7040599 [51] Int. Cl B0ld 5/00 [58] Field of Search 62/555, 56

[56] References Cited UNITED STATES PATENTS 3,122,896 3/1964 Hickey 62/55.5

[451 Nov. 6, 1973 3,252,652 511966 Trendelenburg 62/555 3,364,654 l/1968 Westbrockm, 62/555 3,668,881 6/1972 Thibault et al. 62/555 Primary Examiner-Meyer Perlin Atlorneylrvin S. Thompson et al.

57 ABSTRACT This invention relates to a method of and apparatus for pumping gas and particularly to a system in which a primary vacuum of the order of 10" to 10 Torr approxi mately is produced, followed by the production of a secondary vacuum. According to the invention, the main trapping surface effected in a main cryopumping operation is arranged in two areas, one being of easy access to the gas to be pumped and the other more difficult of access. A cryodeposit of the adsorbing gas is positioned practically exclusively in the area of most difficult access, in order to adsorb the residual gases. The most important residual gas referred to is helium.

12 Claims, 10 Drawing Figures "00 my ,9? 7 7 Meg 5:- 412 ==f///7/\\\\\ -4-2 I06 1084M Y 110 m mew 5 I081 [L 105 10811 5708 g ma 5M no Patented Nov. 6, 1973 4 Sheets-Sheet l Mm t om AWE Patented Nov. 6, 1973 4 Sheets-Sheet 2 Patented Nov. 6, 1913 3,769,806

4 Sheets-Shoot 204 204 #203 J 203 J I r r w it: fit? METHOD OF AND APPARATUS FOR CRYOPUMPING GAS The present invention relates to a method of and apparatus for pumping gas and more particularly with a view to obtaining a very high vacuum.

The obtaining of a very high vacuum by cryopumping meets with great difficulties. Generally, a succession of means is used to obtain such a vacuum, namely mechanical pumps, for obtaining a'primary vacuum then a cryopump for obtaining the final or secondary vacuum. In fact, on the one hand, a cryopump which traps the gases by condensation on walls at a very low tem perature (2 to 25 Kelvin) obviously cannot trap certain gases which are extremely difficult to condense such as helium, hydrogen or neon and, on the other hand, a mere trace of these gases is sufficient to prevent the desired vacuum from being obtained. In order to overcome this drawback, ithas already been proposed to inject over the coldest trapping surface sorbing gases such as argon, carbon dioxide gas, which form cryodeposits and it has been proven that such cryodeposits of sorbing gases'havethe tendency to adsorb these gases which are difficultto condense. However, experience hasshown that thisaction of cryosorp tion is relatively weak due to the fact that the simultaneous input of the cryosorbing gases and the gases to be condensed has the effect of drowning'the cryosorbing gases in a matrix of easily condensable gases such that the efficiency of the cryosorbing condensates is considerably less with regard to the gases that are difficult to be condensed. Moreoventhe use of pumps for the primary vacuum of the type referred to above, offer this drawback of entraining by retrodiffusion certain vapours, for example oil vapours, and these vapours pollute the cryopump and the enclosure to be evacuated limiting the efficiency of the cryopump and, consequently, the quality of the vacuum which it can produce.

The present invention has for an object a method which enables an extremely high vacuum to be obtained rapidly by means of a cryopump, the main pumping surface of which at very low temperature (2 to 25 K) is surrounded at a distance by heat screens at a temperatureof the medium cold screens (77 to 120 K), in the case where a condensable cryosorbing gas is being used.

The invention relates to a method of pumping gases of the type wherein a primary vacuum is produced of the order of to 10 Torr approximately followed by the production of a secondary vacuum firstly by main cryopumping by means of a trapping area completed by sorption on a cryodeposit provided by injections, on the trapping surface of the cryosorbing gases.

This method is characterised in that the main trapping surface is arranged in two areas, one easily accessible to the gases to bepumped, theother'more difficult of access by the said gases and in that a cryodeposit of the said sorbinggassuch as argon or carbondioxide gas is positioned practically exclusively in the most inaccessible area, to sorb the residual gasessuch ashelium, hydrogen or neon.

Due to this method, the sorbent gas enables rapid pumping of the residual gases that have escaped main cryopumping, such as hydrogen, helium and neon; since these gases are thus adsorbed immediately on the sorbent gases at a very low temperature, without being impededby the main cryopumping-condensates themselves'trapped on the easy access part.

The invention relates also to certain arrangements .combinedwith a method of pumping of the type mentioned above, being characterised in that the primary vacuum is effected by a sorption pump enabling a residual gas of primary vacuum to exist having no more constituents than the gas before operation of the said pumping by sorption.

The present invention also has for an object theprovision of apparatus comprisinga sorption pump and a cryopump, the latter having a pumping chamber, a pumping body, at a distance from the chamber and intended to form a main cryopumping area; means for cooling the said body to a very low temperature and, if desired, means forming a heat screen at an average cold temperature about and at a distance from the main cryopumping body, such that the inner wall of the heat insulation chamber and/or reflector baffle is permeable to gases, as well as means for introducing a gas intended to form the sorbent gas for additional pumping.

One particular advantage of the invention is that the pumping body has, with respect to the pumping orifice, a trapping surface part of closer access than the remaining part, the latter being associated with means for injecting the cryosorbing gas. 1

In one modification, the pumping body has a wall partially closed on itself, having a wide access passage inwardly, whilst the neighborhood of the external face of the said wall is free from any substantial obstacle to channelling of the gases to be pumped and that the injection means are directed towards the interior of the said wall.

In another modification, the wall comprises a collar for trapping the condensable gases and a collar base, support of the cryosorbing gas, the injection means beingdirected towards the external face of the bottom.

The invention also relates to a device intendedto be attached as an accessory to this cryopump for applying this method.

According to this method, intended more particularly for pumping helium and, if desired, hydrogen and neon, first of all a preliminary vacuum of 10' to 10* torr is produced by means of a sorption pump, then the cryopump is cooled which stabilizes the pressure be tween 10' to 10* torr, then a first amount of the gas is introduced before forming the cryodeposit, by con necting the cryopump through a coil permitting a drop in pressure with a chamber containing the said gas for generating the cryodeposit, and, if necessary, successively one or more other quantities of this gas until the desired operating pressure is obtained.

The progressive introduction of a quantity of argon, if this is the gas in' question, causes a decrease in the partial pressure of helium. If the first introduction is not sufficient others thereof are effected.

For applying this'method, the invention proposes a device intended to be attached as an accessorytothe cryopumpsused, the said devicebeing characterised in that it comprisesa measuring chamber containing the gas intended to form the cryodeposit, produced by meansof valve means and a coil permitting a drop in pressure having a distributor arranged in the cryopump.

According to another feature of the invention, in the case where liquid helium is used as thevery cold fluid, this distributor comprises orifices directed to the surfaces of the cryopump which are the last in contact with the liquid helium when the cryopump empties.

In order that the invention may be more clearly un derstood, reference will now be made to the accompanying drawings which show some embodiments thereof by way of example, and in which:

FIG. 1 shows a diagrammatic view of a pumping installation equipped with a device according to the in vention,

FIG. 2 shows how such a device may be applied to a cryogenic pump of a type available commercially,

FIG. 3 to 5 show diagrammatically embodiments of distributors with which the preceding devices may be fitted,

FIG. 6 shows a complete diagrammatic view of an installation according to the invention,

FIG. 7 shows a view in detail of FIG. 6,

FIG. 8 shows means for reheating the cryodeposit, and

FIGS. 9 and 10 show modifications of the device for introducing gases for the cryodeposits.

Referring now to the drawings, FIG. 1 shows an installation which comprises a cryopump 10 having a reservoir 11a of liquid helium having a base 11b and associated with this cryogenic pump, a device according to the invention consisting of a metering chamber 12 of small volume connected on one side by a valve 13 to a coil 14 permitting a drop in pressure being extended by a distributor 17 arranged below the reservoir 11, and supplied on the other side by a source ofgas l8 (argon, carbon dioxide or the like) by means ofa pressure reducer l9 and a valve 20.

For placing an enclosure under vacuum, the following procedure is adopted:

By means of a sorption pump, the coupling of which will be described hereinafter with reference to FIG. 6, a preliminary evacuation of the installation is effected. Thus, a vacuum of the order of 10' to ID torr is obtained, the residual pressure being due primarily to the helium and neon contained in the atmospheric air.

To this end, the use of a sorption pump cooled by liquid nitrogen is advisable, over and above the advantage which it has of not introducing organic oil vapours into the atmosphere to be pumped, for example, which would be prejudicial to obtaining a clean vacuum. IN fact, the speed of pumping is greatly increased which enables a substantial vacuum to be achieved rapidly.

The liquid helium cryogenic pump 10 is cooled; to this end, it is necessary to achieve a consumption of liquid helium which is as small as possible during filling the cryogenic pump: which leads to reducing the heat flow to the cryogenic pump. Helium vapourises extremely easily and, if the pressure existing in the chamber evacuated by the cryogenic pump at the instant of transfer of liquid helium in the cryogenic pump, is due substantially to the helium, then the heat exchanges between the hottest part of the chamber (walls at ambient temperature, screens cooled to 77K) and the cryosurfaces to be cooled to 4.2K are very large; for even if part of the cryosurfaces are already at 4.2I(, the helium is never condensed and it continues to transport calories to the cryopump, (moverover, the heat conductivity of helium is slightly greater than that of the other gases).

Firstly, the pressure in the chamber has fluctuations of great amplitude, then these amplitudes decrease in intensity. When the vapourisation phase of the helium in the cryopump is terminated, the pressure is stabilised at a low value between 10 and 10'' torr. It can be established that there is a certain amount of liquid helium in the cryopump 10 (a gauge level may be used during this operation).

Following that, one or more injections or argon are effected through the nozzle 17; the pressure decreases very rapidly from 10 to 10 torr. Then the maintenance transfer of helium is ensured in a satisfactory manner.

The introduction of quantities of argon, of the order of 10cm is effected by simply opening the valve 13. The coil 14 introduces a drop in pressure and enables the flow of argon to be limited. This argon is directed through the distributor 17 to the surfaces which remain the last in contact with the liquid helium when the cryopump is emptied of liquid helium. This is to avoid possible desorption of helium due to reheating after the cryodeposition when the level of helium drops in the cryogenic pump. 7

The possibledividing up of the introduction enables benefits to be gained from the preferential storage of helium between the successive layers or argon. The total quantity of argon to be introduced depends upon the temperature and pressure which it is desired to obtain. In numerous applications, operation is at 4 .2K and a pressure of 1.10 torr is sufficiently low. It is thus necessary to count 1,000 molecules of argon per molecule of helium to be condensed. A numerical example will hereinafter be referred to.

It will be noted that, when the desired residual pressure has been reached, it is advantageous to introduce an additional quantity of argon so as to pump the helium which could enter the chamber as a result.

If it is intended to operate at 2.7K, it is advantageous to form the cryodeposit of argon after having obtained this temperature.

Naturally, this pumping programme can be applied to any gas other than argon to form the cryodeposit: CO H O, N 0, N SF FIG. 2 shows in a more accurate manner one embodiment of a device according to the invention associated with a liquid helium cryopump available in commerce and made by LAIR LIQUIDE Company of France. This cryopump is described in greater detail in French Pat. No. 1,519,847 and forms no part of the present invention as such and so need not be described in greater detail.

The members similar to those of FIG. 1 are given the same reference numerals.

The chamber 12 in this example consists of a helical tube made from stainless steel having an inner and outer diameter of 4 and 5 mm respectively and a length of 78 cm, and the coil 14 is made of stainless steel and has an inner and outer diameter of 0.6 and 0.8 mm respectively and a length of cm.

The volume of the chamber is 10cm and the drop in pressure selected enables introduction to be spread out over 2 minutes. 7

In this embodiment, the volume (chamber to be evacuated plus cryopump) is 250 liters.

The pressure at the end of preliminary evacuation is 5.10 torr.

The pressure at the end of cooling the helium is approximately 1.10 torr, constituted by helium.

The quantity of helium to be pumped 250 l X 1.10 l= 2.510 3 Lt. in which 1 indicates the volume in liters and t indicates. the time in minutes).

Number of introductions required over 2 minutes:

Thus, with an introduction or argon over 2 minutes, the pressure in the chamber can be dropped to below torr.

FIGS. 3 to 5 show various forms possible for the dis tributor 17.

In FIG. 3, this distributor is of annular shape, the discharge orifices being distributed around its periphery.

In FIG. 4, the distributor is spherical, whilst in the modification of FIG. 5 it consists of a series of nozzles 32 each terminated by an orifice 33.

Referring to FIGS. 6 and 7, a pumping installation according to the invention comprises, attached in sealed manner to a chamber to be emptied 101, on the one hand, a mechanical pump 102, referred to as a dry pump, in the sense that there is no longer any risk of oil contamination, and a sorption pump 103, on the other hand, a cryopump 104. The sorption pump 103 may be of any suitable type and advantageously of the type described in French Pat. No. 2,048,253 in the name of LAir Liquide. The cryopump 104 is formed from a heat insulated enclosure 105 having two walls 106 and 107 on the interior of which is placed by suspension means (not shown) a trapping wall 108, the construction of which will be described hereinafter, this cryopumping wall 108 is cooled by a tubel09 which is supplied with a very cold fluid, for example, liquid helium or gaseous helium issuing from a cryogenerator fed by an input duct 110 and the vapours from which are evacuated through an evacuation'duct 110', the ducts 110 and 110' passing into a feed through 111. This trapping wall 108 is protected from heat radiation issuing from the exterior by heat screens such as the inner wall 107 of the enclosure 105 and an optically sealed baffle 114, cooled by liquid nitrogen, circulating in the

coils

112 and 115 supplied from a reservoir 113.

In the embodiment shown in FIG. 7, the cryopumping wall 108 has the shape of a cylinder 108' with a base 108a, a side wall 10811 and an edge 108a; a plate 108d rests on the upper part of the cylindrical wall 108'.

To the lower end of the cryopumping enclosure 101 is attached by means ofa flange l 19, a tube 120 for admitting a cryodeposit gas, connected to a gas source, not shown, such as argon or C0,. The tube 120 discharges substantially at the level of the orifice 108s in the trapping wall 108 by a nozzle formed as a diffuser 122 so that the gas which is introduced into the pumping enclosure is directed to the trapping wall 108. An electrical resistor 124, the use of which will be seen hereinafter, is arranged about the input tube 120.

Advantageously, there may be used a cryopumping surface as shown in FIGS. 9 and 10.

As shown in FIG. 9, a stainless steel receptacle 203 is provided for a liquid at a very low temperature, and has an inlet opening 204 and a bottom 202 which is surrounded by a copper collar 201 spaced from receptacle 203. Collar 201 is secured to bottom 202 by means of a conical wall 207. Wall 207 and collar 201 and receptacle 203 define an annular chamber opening toward the aperture 200 through which the gases to be pumped are admitted. A nozzle 205 is disposed below bottom 202 for the admission of the cryosorbing gas.

In FIG. 10, the nozzle is surrounded by a collar 206 welded to the bottom 202. The easily condensable gases are trapped on the bottom 202, the area adjacent this bottom being the most difficult of access to the gases and being surrounded by collar 206.

The operation of the cryopumping installation is as follows: in a first stage, a primary vacuum is produced in the enclosure to be pumped by means, firstly, of the mechanical pump 102, then of the sorption pump 103, and once this primary'vacuum is produced, cryopumping is used enabling the extremely high vacuum to be produced.

In a first phase, the installation operates by simple cryopumping, i.e., a substantial part of the residual gases condensed on the cold walls, those on the walls at an average cold temperature 107 and l 14 and, above all, on the wall 108d at a very low temperature and the outer edge of the cylinder 108' but cannot reach, on account of the cylindrical shape 108, the base 108a and the inner edges of the said cylinder 108'.

Once a pressure of the orderof 10 to 10 torr is obtained, the introduction of successive amounts of sorbent gas such as carbon dioxide gas or argon is effected through the tube 120. During this introduction and to avoid any inopportune condensation of the sorbent gas in the input tube, slight heating by the resistance 124 is used which ensures suitable transfer of the gas in the chamber of the cryopump. By reason of the particular shape of the wall of the cryopump 108 which is primarily cylindrical, and of the arrangement of the nozzle substantially at the opening level of the said cryopumping cylinder 108', on the one hand, and of the design of the input nozzle of the cryodeposit or sorbents gas, on the other hand, it is certain that a substantial part, if not the whole of this cryodeposit or sorbent gas, is immediately and directly trapped on the said trapping surface, without being diffused within the enclosure, i.e., without being able to reach the trapping surfaces. For easy access to the easily condensable gases, it is thus certain that the sorbent is immediately carried to the efficient temperature of cryosorption and that it will not be drowned by the easily condensable gases so that final pumping is ensured under the best speed conditions. According to the modification shown in FIG. 8, the means for reheating the cryodeposit or sorbent gases is formed in this case by a thermally conductive mass member positioned in the input tube 131 and having an extension 130a to a heat source which may be an electrical resistance 132.

The invention is useful in all applications where it is desired to use an extremely high vacuum of the order of 10 to 10 torr.

We claim:

1. A method of cryopumping gases, comprising producing in a gas a primary vacuum of the order of 10 to 10" Torr,and producing a secondary vacuum in said gas by thereafter cryopumping the gas on a refrigerated trapping wall means, said trapping wall means having portions that are so positioned as to be successively contacted by said gas with a first portion of said trapping wall means contacted by said gas prior to a second portion of said trapping wall means, and applying to only said second portion of said trapping wall means a series of layers of adsorbing gas in solid phase thereby to promote the deposition of gas to be pumped on each said layer.

2. A method as claimed in claim 1, and producing said primary vacuum by sorption pumping.

3. A method as claimed in claim 1, in which said adsorbing gas is a member selected from the group consisting of argon, CO H O, N 0, N and SP and said gas to be pumped is helium.

4. Cryopumping apparatus comprising a sorption pump (103), a cryopump (104) communicating with said sorption pump, said cryopump comprising a pumping chamber (105), a pumping body disposed inside and spaced from the side walls of said pumping cham her and providing a main trapping wall (108), means (109) for cooling said pumping body to very low temperature, means defining a heat screen at a relatively low temperature above and at a distance from said trapping wall, the last-named means comprising an inner wall (107) of said chamber of said cyopump and reflector baffle means (114) permeable to gases, said chamber having a pumping opening (200) for the admission of gas to be pumped, said trapping wall having a portion (108b, 108d) relatively close to said pumping opening and a portion (108') relatively remote from said opening, and means to direct an absorbing gas against said relatively remote portion to form a solid cryodeposit to promote the adsorption of gas to be pumped on said cryodeposit.

5. Apparatus as claimed in claim 4, said trapping wall means comprising a receptacle (203) for a liquid at very low temperature and a collar (201) spaced from and surrounding said receptacle, said collar being secured to the bottom (202) of said receptacle by means of a wall (207), said wall and collar and receptacle defining an annular chamber that opens toward said opening (200), said directing means (205) being disposed on the side of said bottom (202) opposite said opening (200).

6. Apparatus as claimed in claim 5, and a collar (206) surrounding said directing means.

7. Apparatus as claimed in claim 4, said directing means (17) having a plurality of outlets to direct said absorbing gas in a plurality of streams.

8. Apparatus as claimed in claim 7, said heating means comprising a mass of thermally conductive element (130) disposed in said conduit (131) and having an extension (130a) extending outside said chamber to a heat source.

9. Apparatus as claimed in claim 4, said directing means comprising a conduit (120) extending a substantial distance into said chamber and reheating means (124) for said conduit in said chamber.

10. Apparatus as claimed in claim 9, said reheating means comprising electric resistance means disposed about said conduit.

11. Apparatus as claimed in claim 9, said conduit being connected at one end to a metering chamber (12) by means of valve means (13, 20) and a coil (14) producing a pressure drop, said conduit being connected at its other end to said directing means (17).

12. Apparatus as claimed in claim 11, said directing means having orifices (30) directed toward the surface of the cryopump which maintain the cryopump in contact with a very cold fluid when the cryopump is empty-

Claims

1. A method of cryopumping gases, comprising producing in a gas a primary vacuum of the order of 101 to 104 Torr, and producing a secondary vacuum in said gas by thereafter cryopumping the gas on a refrigerated trapping wall means, said trapping wall means having portions that are so positioned as to be successively contacted by said gas with a first portion of said trapping wall means contacted by said gas prior to a second portion of said trapping wall means, and applying to only said second portion of said trapping wall means a series of layers of adsorbing gas in solid phase thereby to promote the deposition of gas to be pumped on each said layer.

3. A method as claimed in claim 1, in which said adsorbing gas is a member selected from the group consisting of argon, CO2, H2O, N2O, N2 and SF6 and said gas to be pumped is helium.

4. Cryopumping apparatus comprising a sorption pump (103), a cryopump (104) communicating with said sorption pump, said cryopump comprising a pumping chamber (105), a pumping body disposed inside and spaced from the side walls of said pumping chamber and providing a main trapping wall (108), means (109) for cooling said pumping body to very low temperature, means defining a heat screen at a relatively low temperature above and at a distance from said trapping wall, the last-named means comprising an inner wall (107) of said chamber of said cyopump and reflector baffle means (114) permeable to gases, said chamber having a pumping opening (200) for the admission of gas to be pumped, said trapping wall having a portion (108b, 108d) relatively close to said pumping opening and a portion (108'') relatively remote from said opening, and means to direct an absorbing gas against said relatively remote portion to form a solid cryodeposit to promote the adsorption of gas to be pumped on said cryodeposit.

9. Apparatus as claimed in claim 4, said directing means comprising a conduit (120) extending a substantial distance into said chamber (105), and reheating means (124) for said conduit in said chamber.

12. Apparatus as claimed in claim 11, said directing means having orifices (30) directed toward the surface of the cryopump which maintain the cryopump in contact with a very cold fluid when the cryopump is emptying.