US3168819A

US3168819A - Vacuum system

Info

Publication number: US3168819A
Application number: US93770A
Authority: US
Inventors: Donald J Santeler
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1961-03-06
Filing date: 1961-03-06
Publication date: 1965-02-09
Anticipated expiration: 1982-02-09

Description

The present invention relates to vacuum systems and, more particularly, to vacuum systems employing diffusion pumps and refrigerated traps.

In recent years, vacuum technology has received greatimpetus as a result of increased interest in associated fields such as space simulators and vacuum metallurgy. Vacu- 'um environments are usually achieved and maintained through the use of diffusion pumps combined with me chanical pumping means and in some instances, further combined with cryogenic pumping means. Cryogenic pumping is usually performed by condensing the gas molecules in an evacuated chamber on a refrigerated surface. Diffusion pumps utilize the action of a large number of individual collisions between heavier molecules of a pumping fluid with the molecules of the fluid being pumped. Usually, the pumping fluid is a hydrocarbon oil which is discharged through orifices in the form of jets having velocities approximating the speed of sound. These jets give the fluid stream a strong thrust toward a condensing surface. Molecules of gas being pumped and traveling in a random fashion enter the area of these jets and suffer collisions with the heavier pumping fluid molecules which propel the pumped fluid toward the condensing surfaces Where the fluid is compressed and ultimately discharged.

As lower pressures are sought, it has been found that evacuated chambers may be contaminated by the working fluid of the diffusion pump and also that the surfaces within the evacuated chamber are subject to a degassing action wherein gas is evolved over varying periods of time from the chamber surfaces and bulk materials in the evacuated environment. Contamination by Working fluid from the diffusion pump may occur by the working fluid passing by volume migration through openings which place the evacuated chamber in communication with the pump and also by surface migration wherein the contaminating molecules migrate along the surfaces which connect the evacuated chamber with the pump.

Liquid nitrogen traps and similar refrigerated traps are used in vacuum systems for a twofold purpose. The first purpose is to isolate the pumping fluid in the pumping system from the evacuated chamber. The second purpose is to cryogenically pump the condensible gases located within the evacuated chamber, for example, gases such as water vapor which may constitute a large portion of the gas being evacuated.

Despite the provision of nitrogen or otherwise cooled traps in vacuum systems, diffusion pumps have been found to be a source of contamination in that the hydro carbon oil used as a pumping fluid may migrate through the trap. In order to be effective, a trap preferably should have anti-migration features, for example, some means for eliminating continuous warm walls extending from the outlet to the inlet of the trap. In addition thereto, a trap desirably should have features such as optical sealing means so that no oil molecules can pass from the inlet to the outlet of the trap without making one or more collisions with condensing surfaces. In addition, the trap should also have a high conductance for gas flow so as not to restrict the pumping speed of the diffusion pump. A trap should also possess extensive condensing surfaces facing the evacuated chamber to provide large cryogenic pumping capacity for condensible gases. Another desirable feature in a trap is a thermal United States Patent 3,168,819 Patented Feb. 9, 1965 gradient barrier of a type which reflects gas molecules toward a condensing surface to avoid undesirable degassing of concealed portions of the system during use.

The chief object of the present invention is to provide an improved vacuum system.

Another object of the invention is to provide an improved refrigerated trap for use in vacuum systems.

A still further object of the invention is to provide a refrigerated trap having a thermal gradient barrier therein situated to avoid extended degassing periods during use.

A further object is to provide a vacuum system with an improved refrigerated trap having a high conductance for gas flow with provision for isolating pumping fluids and having improved means for cryogenically pumping eondensible gases located in the chamber to be evacuated.

These and other objects of my invention will be more readily perceived from the following description.

One of the features of my invention is a vacuum system including an evacuated chamber having associated there with a refrigerated trap and a diffusion pump, the trap being located on one end of the chamber and including a heat exchange member located adjacent a continuous wall, the heat exchange member having an opening there in with associated baflles for optically sealing the opening. A continuous thermal gradient barrier may be located between the heat exchange member and the continuous wall to condense on the heat exchange member molecules reflected from the surface of the thermal gradient barrier.

The attached drawing illustrates a vacuum system em ploying the present invention.

The vacuum system basically may comprise a chamber adapted to be evacuated having associated therewith a refrigerated trap which is connected to a diffusion pump and mechanical pumping means. In the drawing, housing 2 comprises a cylindrical shell 3 having attached thereto an end member 4 with a discharge opening .5 connected to a suitable diffusion pump 7. The opposite end of shell 3 has mounted thereon an end member 6 which sealingly engages flange 8 attached to shell 3. Sealing means such as O-

rings

9 and 10 may be located between end member 6 and flange 3 which may be bolted together. In order to maintain the effectiveness of the seal provided by these O-rings, rough pumping means may be associated with the annular space between the O-rings by connecting vacuum line 11 thereto. Similarly, flange 12 of diffusion pump 7 may be attached to the end member 4 by bolt means and O-rings 13 and 14 may be utilized to seal the connection. To provide an eflective seal, the rough pumping means utilized with respect to O-rings 9 and it) may also be connected through line 15 to the annular space between O-rings 13 and 1 In the present embodiment, cylindrical'shell 3 with end members 4 and 6 define a housing for evacuated chamber 16. This housing may also provide space therein for a refrigerated trap, more specifically, nitrogen-cooled trap 17. As previously noted, the refrigerated trap used in vacuum systems may be provided for two different purposes. The first purpose is to isolate the pumping fluid in the pumping system, that is, the fluid in the diffusion pump from the main vacuum chamber which is chamber 16 in the present embodiment. The second purpose of the refrigerated trap is to provide large pumping capacity for condensible gases, such as water vapor, which originate within evacuated chamber 16, this pumping preferably being provided by cryogenic pumping means.

While the present invention describes an apparatus wherein shell 3 substantially defines a housing for chamber 16 and also trap 17, it will be appreciated that this feature is attractive but not essential to the practice of the present invention. In the described embodiment, the portion of shell 3 adjacent the trap and chamber 16 may be considered the inlet opening of the trap. If desired, separate housing members may be provided for chamber 15 and trap 17 with suitable connecting means therebetween. However, it has been found that utilizing a single shell obviates the need for extensive sealing means between the connecting portions of separate housings.

Trap 17 includes a torus shape heat exchange member 29 having an annular hollow portion defining a chamber adapted to receive liquid refrigerant such as liquid nitrogen or other low boiling point refrigerant. In the present embodiment, the torus shape member which comprises eat exchange member Ztl'is concentrically located closely adjacent shell 3. Since it is undesirable to have the re frigerant which may be liquid nitrogen at 77 K. in direct heat exchange relation with shell 3 at ambient temperature, there is a discrete space between shell 3 and hea exchange member 20. In the cross-sectional view illustrated in the drawing, torus shape member 29 has a substantially rectangular cross section and defines a concentric opening 21 which is adapted to place chamber 16 in communication with diflusion pump 7. Liquid refrigerant such as liquid nitrogen may be supplied through a suitable filler tube 22 which extends through cylindrical shell 3 to the hollow portion or defined chamber of heat exchange member 20. In order to insulate filler tube 22 from t. e shell and to maintain the sealed nature of chamber 16, a sheath 23 may be provided fabricated of a low heat conductive material, for example, stainless steel.

As previously noted, the liquid nitrogen trap in the present invention is provided to supply large pumping capacity for condensible gases. For this reason, it is desirable to having openin: 21 of heat exchange member 2% connecting chamber 16 and diffusion pump 7 through a seal that is optically tight to insure that molecules passing from the chamber or from the diffusion pump encounter at least one refrigerated condensing surface. A seal of this type may include a plurality of

bafilcs

25, 27, and 29. First bafile 25, which is located adjacent chamber 16, has a concentric opening 26 therein. Below baflie 25 and spaced therefrom is mounted bafile 27 having a plurality of annularly disposed openings 28 located adjacent heat exchange member 20. Baffle 29 which is similar in' construction to baffle 25 may be mounted below baflle 27 and may have a center opening 30. From the described construction, it can be seen that molecules passing through opening 21 either strike the surface of the heat exchange member or one of the

baflie members

25, 27, and 29 which are in heat exchange relation with refrigerant inside heat exchange member 20. It is also noted that since the heat exchange member 26 is only slightly smaller in outside diameter than the inside diameter of shell3, heat exchange member 20 and the surfaces of the baffles provide condensing heat exchange surfaces having substantially an area as great as the cross-sectional area of the evacuated chamber taken through a plane normal to the axis of cylindrical shell 3. In other terms, the presented heat exchange surface for condensing purposes is substantially as large as the trap inlet opening.

A continuous thermal gradient barrier is required to prevent the passage of molecules through the annular space between the inner surface of shell 3 and the outer surface of heat exchange member 20. For this purpose a frustro-conical ring member 32 fabricated of a comparatively poor heat conducting material extends laterally from shell 3 at point 34 directly to heat exchange member 20 at point 35 which is adjacent the defined chamber. Member 32 is illustrated in FIG. 1 as a single metal sheet or plate of relatively short height as compared to the height of shell 3. In one preferred embodiment of this invention as illustrated, member 32 is angularly directed between heat exchanger 20 and shell 3 and connected with its apex end at the bottom peripheral portion of heat exchanger 20 while its base end is connected to the shell 3 at a point generally laterally spaced from the top peripheral portion of the heat exchanger 20. Usually shell 3 is at ambient temperature and heat exchange member 255 may be at temperatures of approximately 77 K. and lower. it has been found that at the intermediate temperatures on the thermal gradient barrier, condensed molecules may rte-evaporate and degassing may occur over an extended period thereby greatly hampering the'pumping down of chamber 16 to desired vacuum levels. In the present embodiment, the inner surface of frusto-conical member 32 which is on the side of the trap in communication with chamber is also substantially facing the outer periphery of heat exchange member Ztl, the surface of which may be refrigerated by liquid nitrogen. Accordingly, re-evaporated molecules from barrier 32 are condensed on heat exchange member 2% and do not provide an outgassing problem.

If desired, there may be provided below heat exchange 7 member 20 a suitable radiation shield 37. Plate as may be mounted adjacent the trap and spaced from opening 5 in member 4. Plate 4% is a condensing surface adapted to be in heat exchange relation with cooling medium such as water passed through a tube 41 supplied from tube The water may be discharged through line 43 which is further connected to coil 44 in heat exchange relation with the casing of diffusion pump 7. By placing tube 41 in heat exchange relation with plate iii, a bafiie is provided having the temperature of the cooling water for the purpose of condensing pumping fluid which may pass from the diffusion pump 7.

Diffusion pump 7 may take many forms which are presently available and which are well known in the art. In the present embodiment, diffusion pump 7 comprises a lower boiler portion 56 wherein the pumping fluid which may be a hydrocarbon oil is heated and passed upwardly through three

cylindrical tubes

51, 53, and 55. These cylindrical tubes are of diminishing cross-sectional area and tube 51 terminates close to boiler 5b. Tube 51 terminates in an annular discharge orifice 52. Tube 53 terminates in annular orifice 54 above orifice 52. Tube 55 terminates at anular orifice 56 which is the orifice furthest from boiler 50. Located below the orifices between the pump casing and tube 51 may be yieldable shield 58 which acts as a check valve to permit the pump ing fluid discharged from the orifices to return to boiler 50. Foreline 55 of diffusion pump '7 may discharge compressed gases from the casing at a point above yieldable shield 58, foreline 59 may be connected to a mechanical pump 60 in a manner well known in the art. In the present embodiment, lines 11 and '15 which are associated with the O-rings comprising the sealing means for flanged connections in the apparatus, may be connected to line 59 and in this manner mechanical pump 60 provides the rough pumping for maintaining these seals. If desired, pressure-measuring

instruments

61 and 62 may be utilized to measure pressures in chamber 16 and foreline 59. These instruments may be ion gages, thermal conductivity gages, or similar gages depending on the vacuum being measured.

In the operation of the'present invention, the material or device to be tested is placed within testing chamber 16. For example, a material may be placed in chamber 16 to determine its outgassing qualities. In another instance, the operation of a bearing and the behavior of its lubricant may be tested in a vacuum to determine its operativeness in a spatial environment. Having suspended or otherwise mounted the test object within chamber 16, cover 6 may be bolted down and roughing pump as may be started to maintain the seal between cover 6 and flange 8. In order to actuate the present apparatus, a suitable refrigerant, such as liquid nitrogen, may be supplied to tube 22, the refrigerant passing into torus shape heat exchange member 20 thereby bringing the temperature of the heat exchange member 20 and associated

baffle members

25, 27, and 29 to a temperature as low as 77 K. It will be appreciated that gases, such as water vapor in chamber 16 are cryogenically pumped by the refrigerated surfaces and in some instances, if desired, lower temperature refrigerants such as liquid helium and liquid hydrogen may be supplied to heat exchange member 20.

Diffusion pump 7 may be started by activating boiler 50. The hydrocarbon oil or other pumping fluid in the boiler is vaporized to permit passage through

tubes

51, 53, and 55. The pumping fluid vapor being discharged through

orifices

52, 54, and 56 is passed in streams toward the casing of the pump which is cooled by cooling Water passing through coil 44. The vapor condenses thereon and is returned to boiler 50 by passing around yieldable shield 58. The gases which are collected by the pumping fluid are passed through foreline 5? to mechanical pump 60 and discharged from the system.

The action of the pump depends upon collisions of oil vapor molecules with molecules of gases being pumped from the evacuated chamber 16. Jets of the pumping fluid, that is, oil vapor molecules are emitted from the

orifices

56, 54, and 52, which comprise three stages of compression in the pump. These jets are emitted at velocities approximating the speed of sound and are shaped to make the emitted pumping fluid vapor thrust strongly downward and outward. The molecules from the chamber 16, discharged from the material being tested, travel in a comparatively random fashion into the region occupied by these jets and suffer collisions with the much heavier pumping fluid molecules. The driving rain of pumping fluid molecules propels the gases being pumped generally downward thus compressing them. The pumping fluid is returned to the boiler as a film while the highly compressed collected gas is passed through foreline 59 to mechanical pump 60 where it is discharged from the system. A pressure of approximately l l() mm. of mercury and lower may be readily achieved by the use of such a diffusion pump alone.

During the operation of the diffusion pump, some of the pumping fluid molecules, for example oil vapor molecules, may condense on surfaces adjacent the orifices. Subsequently, these vapors may be re-evaporated in a manner to bypass the jets and enter into the area defined by opening 5. In order to prevent the migration of these oil molecules into the chamber being pumped, cooled batlle plate 40 is provided. The condensing surface provided by baffle plate 4t) provides a resting place for condensed oil molecules passing from the diffusion pump. This plate does not collect all the pumping fluid being lost by the diffusion pump thereby limiting the vacuum levels achieved by the system. As previously noted, this pumping fluid may contaminate the clean evacuated chamber 16 by volume migration wherein the pumping fluid passes in counter flow to the fluid being pumped from chamber 16. For this reason, opening 21 is bafiled to optically seal chamber 16 from the pump assuring that the pumping fluid molecules moving by volume migration will pass adjacent the various baffles 2?, 27, and 25 and surfaces of heat exchange member 20 in such a manner that these molecules may condense thereon.

Evacuated chamber 16 may also be contaminated with pumping fluid by surface migration, that is, pumping fluid molecules may pass along the inner surface of end member 4 along the inner surface of shell 3 until thermal gradient barrier 32 is encountered at the annular junction point 34. The temperature of the thermal gradient barrier changes from ambient temperature at point 34 until it reaches approximately the temperature of the refrigerant (77 K. for nitrogen) in heat exchange member 20 at annular junction point 35, where thermal gradient barrier is connected to heat exchange member 20. In the area immediately adjacent the heat exchange member, because of the lower temperature of the thermal gradient barrier, there is little difficulty with migrating oil molecules since the oil molecules condense and lose their mobility.

With respect to chamber 16, trap 17 provides large pumping capacity for condeusible gases and for this purpose, it is noted that the surfaces in heat exchange relation with the refrigerant in heat exchange member 20 provide a facing surface area substantially as great as the cross-sectional area of the chamber 16 taken through a plane normal to the axis of cylindrical shell 3.

The present invention provides a vacuum system having an improved refrigerated trap with an optically tight seal to prevent pumping fluid molecules, from an associated diffusion pump, from entering the evacuated chamber. This trap also supplies large condensing surfaces for cryogenically pumping condensible gases such as water vapor from the chamber. In addition thereto, the present invention provides a trap with a hidden thermal gradient barrier located adjacent a condensing surface which avoids commonly encountered outgassing problems in vacuum systems.

While I have described a preferred embodiment of my invention, it will be understood that my invention is not limited thereto since it may be otherwise embodied within the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a trap for use in a vacuum system, the combination of wall means defining a chamber, means defining inlet and discharge openings for the chamber, a hollow heat exchange member located adjacent the wall means and having an opening to place the inlet opening in communication with the discharge opening, said hollow heat exchanger defining a chamber adapted to contain a refrigerant therein baffle means in heat exchange relation with said heat exchange member and optically sealing the opening in said heat exchange member, said bafile means and said heat exchange member supplying heat exchange surface exposed to said inlet opening having an area substantially as great as said inlet opening, a tapered thermal gradient barrier concentrically located between and engaging said defined heat exchange chamber member and said wall means to act as a seal therebetween, a tapered surface of said thermal gradient barrier substantially facing a surface thermally associated with said heat exchange member to condense thereon molecules reflected from the side of the thermal gradient barrier in communication with the inlet opening.

2. In a trap for use in a vacuum system, the combination of means defining a chamber including a cylindrical Wall, said chamber having means defining an inlet opening and a discharge opening, a hollow circular heat exchange member located in the chamber concentric with and spaced from said cylindrical wall, said hollow circular heat exchange member defining a chamber adapted to contain a refrigerant therein said circular heat exchange member having an opening therein positioned in concentric relationship to said inlet opening, baflle means in heat exchange relation with said heat exchange member extending across said opening in said heat exchange member to optically seal said opening, said heat exchange member and said baflle means supplying extended heat exchange surface facing said inlet opening and having an area substantially as great as the cross-sectional area of the inlet opening, a continuous annular tapered thermal gradient barrier located between and concentric with said cylindrical wall and said circular heat exchange member, said barrier being directly connected laterally to said defined heat exchange chamber member and said cylindrical wall, the inner surface of said thermal gradient barrier angularly facing the heat exchange member to condense molecules thereon reflected from the thermal gradient barrier, said inner surface being on the side of the thermal gradient barrier in communication with the inlet opening.

3. In a vacuum system, the combination of means defining a testing chamber having continuous wall means, means defining a trap located on one end of said chamber, a vacuum pump, means defining a discharge opening in said chamber for connecting said vacuum pump aiaa'eie annular heat exchangemeans located adjacent to and closely spaced to the Wall means and concentric therewith, said hollow annular heat exchange means defining a chamber adapted to contain a refrigerant therein said heat exchange means having an opening therein and concentric with said discharge opening, bafiie means optically sealing said openingin said heat exchange means and being in heat exchange relation with said heat exchange means, said bafiie means and said heat exchange means supplying extended heat exchange surface having an area substantially as great as the cross-sectional area of the chamber through said all means, a continuous annular tapering ring thermal gradient barrier located concentrically between said heat exchange means and said Wall means, said tapering ring being directly connected at its larger base end to said Wall means and at its smaller end to the said defined chamber of said heat exchanger to provide a seal therebetween, said thermal gradient barrier having an internal tapering surface angularly directed toward said heat exchanger means and facing a surface thermally associated with the heat exchange means to condense molecules thereon reflected from the side of the thermal gradient barrier in communication with the opposite end of the chamber,

, 4. In a vacuum system, means defining a testing chamber having a substantially cylindrical Wall, means defininga trap located in one end of said chamber, means defining a discharge opening, in said chamber, avacuum pump adapted to be connected to said discharge opening, said trap comprising an annular hollow heat exchange chamber member adapted to contain a low temperature fluid concentrically located closely adjacent the cylindrical Wall and having a concentric opening therethrough, bafile means optically sealing said heat exchange member opening and being in heat exchange relation With said heat, exchange member, said batlie means and said heat exchange member supplying heat exchange surface facing the opposite end of the chamber having an area substantiallytas great as the cross-sectional area of the chamber taken through the cylindrical Wail, a thin annular frusto-conical shaped thermal gra iient barrier being positioned concentrically with said hollow heat exchange member and said cylindrical Wall and angularly directed thereto, said barrier being thermally connected directly at its apex end directly to said heat exchanger chamber and at its base end to said Wall at a point substantially adjacent a lateral projection of a portion of said heat exchange chamber on said cylindrical Wall, said thermal gradient barrier having its inner tapered surface facing the heat exchange member-to condense molecules thereon reflected from the inner surface, said inner tapered surface of said thermal gradient barrier being in communication With the opposite end of the chamber.

References Cited in the file of this'patent UNITED STATES PATENTS A. OLEARY, Primary Examiner.

nERBEiiT L. MARTIN, CHARLES SUKALO,

' Examiners.

Claims

1. IN A TRAP FOR USE IN A VACUUM SYSTEM, THE COMBINATION OF WALL MEANS DEFINING A CHAMBER, MEANS DEFINING INLET AND DISCHARGE OPENINGS FOR THE CHAMBER, A HOLLOW HEAT EXCHANGE MEMBER LOCATED ADJACENT THE WALL MEANS AND HAVING AN OPENING TO PLACE THE INLET OPENING IN COMMUNICATION WITH THE DISCHARGE OPENING, SAID HOLLOW HEAT EXCHANGER DEFINING A CHAMBER ADAPTED TO CONTAIN A REFRIGERANT THEREIN BAFFLE MEANS IN HEAT EXCHANGE RELATION WITH SAID HEAT EXCHANGE MEMBER AND OPTICALLY SEALING THE OPENING IN SAID HEAT EXCHANGE MEMBER, SAID BAFFLE MEANS AND SAID HEAT EXCHANGE MEMBER SUPPLYING HEAT EXCHANGE SURFACE EXPOSED TO SAID INLET OPENING HAVING AN AREA SUBSTANTIALLY AS GREAT AS SAID INLET OPENING, A TAPERED THERMAL GRADIENT BARRIER CONCENTRICALLY LOCATED BETWEEN AND ENGAGING SAID DEFINED HEAT EXCHANGE CHAMBER MEMBER AND SAID WALL MEANS TO ACT AS A SEAL THEREBETWEEN, A TAPERED SURFACE OF SAID THERMAL GRADIENT BARRIER SUBSTANTIALLY FACING A SURFACE THERMALLY ASSOCIATED WITH SAID HEAT EXCHANGE MEMBER TO CONDENSER THEREON MOLECULES REFLECTED FROM THE SIDE OF THE THERMAL GRADIENT BARRIER IN COMMUNICATION WITH THE INLET OPENING.