US3182896A

US3182896A - Diffusion vacuum pump apparatus

Info

Publication number: US3182896A
Application number: US240902A
Authority: US
Inventors: Werner G Bachler
Original assignee: Leybold Holding AG
Current assignee: Leybold Holding AG
Priority date: 1962-11-29
Filing date: 1962-11-29
Publication date: 1965-05-11
Anticipated expiration: 1982-05-11

Description

May 11, 1965 w. G. BAcHLER 3,132,396

DIFFUSION VACUUM PUMP APPARATUS Filed Nov. 29, 1962 2 Sheets-Sheet 1 ,J-is 26 a 2 3 i it I6- X 1? INVENTOR. w. alien LER ATTORNEY May 11, 1965 W. G. BACHLER DIFFUSION VACUUM PUMP APPARATUS Filed Nov. 29, 1962 2 Sheets-Sheet 2 FIG 4 244c. 244c.

' TIME (om a Mom 0 DNM MA). MAx. 250c. 260 c. 260%.-

' -=(ouT)b ouT) b noun b on. TEMPERATURE NEAR BOT 0 o a A TOM WITH STIR- 244 c MINUTE 244 c- RER'IN MOTION AND ARRESTED 4oN u ON)u (ON)u MAX, MAX 250 c. 260 c. 260 c.

OUT)b (OUT)b Wounb 2T c. .L Z44PCTEMPLERATURE 25o WATTS 350WATTS 45OWATTS F |G.5 60 (WATTSf 1000 FLUCTUATIONS OF TOTAL MINUTE PRESSURE FOR VARIOUS QUANTITIES OF PUMP FLUID AND HEATER INPUTS, NORMAL DIFFUSION PUMP I200 I00 cm? on. zoom. on. A 3OOcm. O|L -H-IO' TORR (wATTs)- F I G so'o FLUCTUATIONS OF TOTAL PRESSURE FOR VARIOUS QUANTITIES OF PUMP moo FLUID AND HEATER INPUTS,

MINUTE DIFFUSION PuMP WITH BUILT-IN STIRRER I200 INVENTOR, r W. BACHLER' BY N I400 9 -67 6 (No-8 TORR! lOOcm5 on. 2oom5o|L aoomi' olL ATTORNEY United States Patent Q 3,182,896 DIFFUSION VACUUM PUMP APPARATUS Werner G. .lliichler, Cologne, Germany, assignor to Leyhold Holding A.G., Zug, Switzerland Filed Nov. 29, 1962, Ser. No. 240,902 Claims. (Cl. 230-401) This invention relates to diflusion type vacuum pumps and more particularly relates to an improved boiler apparatus for suchpumps. a

The operation of high vacuum diffusion pumps is generally well known. A pumping fluid is evaporated in a heated boiler of the pump and the resulting vapor directed at supersonic velocity through a nozzle system to be finally condensed on a cold surface. As the high speed vapor stream passes between the nozzle system and the condensing surface, it accepts by diffusion gas molecules from the system being evacuated and compresses these molecules into a higher pressure area which normally communicates with a mechanical backing pump. The liquid produced on the condensing surface returns to the pump boiler to be reheated and re-evaporated.

The ultimate pressuresobtainable with such pumps is dependent upon a-number of factors. Gas desorption from the walls of the vacuum system, degassing of demountable seals :and gasket material, virtual leaks and actual leaks, can all seriously limit the ultimate pressure obtainable, In addition, erratic and ineflicient operation of, the diffusion pumps themselves can seriously limit the base pressure obtainable within the vacuum system.

Tests have shown that much of the inconsistent and erratic performance of diffusion pumps is a result of localized overheating in the pump boiler which causes, during an accidental cavitation, the spontaneous development of vapor bubbles. These vapor bubbles then burst through the surface of the pumping fluid producing eruptive boiling. The eruptive boiling problem is less extensive when the pumping fluid contains large quantities of gas and dirt particles, since the individual moving gas molecules and. dirt particles within the fluid tend to re lieve the pressure at local hot spots, thereby preventing the formation of larger eruptive gas or vapor bubbles. However, the eruptive boiling problem can become quite substantial when the boiling fluid is relatively clean, i.e., freev of gas. This is precisely thecase after prolonged operation of a diffusion pump inthe ultra high vacuum range, which operation tends to purify the pumping fluid by purging it of suspended gas molecules.

The result of localized eruptive boiling is to produce brief periods during which the thermal energy accumulated at the pumping fluid hot spots is released in addition to the energy continuously supplied by the heater element. This combined effect increases the pumping fluid evaporation rate causing the density of the vapor stream projecting from the nozzle to become abnormally high. The increased vapor density causes a corresponding increase in the backstreaming rate of pumping fluid molecules into the high vacuum side of the vacuum system. This is especially true of the light weight hydrocarbon molecules which are produced at an increased rate during the brief. period of overheating and eruptive boiling within the pump boiler.

Conversely, during non-eruptive boiling periods within the pumpboiler, the pumping fluid evaporation rate and density of the vapor stream leaving the nozzles is reduced. The lower vapor density allows gases on the fore vacuum side with a high diffusion coefficient (for example, hydrogen and helium) to penetrate the vapor stream in the reverse direction and enter the high vacuum side of the 3,182,896 Pafented May 11, 1955 ice vide a uniform vapor stream density which is high enough to prevent extensive back diffusion of high diffusion coeflicient gas molecules into the high vacuum' side of the system and small enough to eliminate'excessive backstreaming of pumping fluid molecules.

. Many. heater designs have. been. utilized in an attempt to alleviate eruptive boilingproblems in diffusion pumps,.such as, for example, immersion of the heating element Within the pumping fluid, 1machining of the pump boilers internal surface to various configurations disadvantages, such as, for example, requirement of difficult vacuum seals, providing internal surfaces and compartments which are extremely diflicult to clean, increasing the necessary dimensions of the pump, etc. i

An even more serious problem associated with the use of such mechanicaldesigns is the fact that the magnitude of eruptive boiling within the pump boiler appears to depend upon a number of factors such as geometry of the boiler, amount of power supplied by the pump heater, the specific load of the pump, the amount of pumping fluid, the degree of the pumping fluid purity, etc. Because of the numerous variable factors involved, it becomes extremely difficult to design a diffusion pump which will exhibit an absence of eruptive boiling over wide operational ranges andfor extended periods of operational time. V :1

It is. therefore, the object of the present invention to provide 'a diffusion vacuum pump which exhibits an extremely uniform rate of pumping fluid evaporation and a correspondingly uniform vapor stream density at the pump nozzle over a wide range of operating conditions.

One feature of the present inventionis the provision in a diffusion vacuum pump of a movable member for agitating the pumping fluid so as to establish an even temperature distribution throughout the pumping fluid.

Another feature of this invention is the provision of a diffusion vacuum pump of the above featured type wherein the movable member is powered by a driving apparatus requiring no lead throughs in the vacuum walls of the pump. 7 e V Another feature of this invention is the provision of a diffusion vacuum pump of the above featured type wherein the driving apparatus is aturbine device powered by the evaporated pumping fluid vapor.

Still another feature of this invention is the provision of a diffusion vacuum pump of the above featured type wherein the movable member is a rota-ting vane positioned within the pumping fluid of the diffusion pump boiler. Another feature of this invention is the provision in a diffusion vacuum pump of the above featured type of a support assembly for the pumping fluid agitator which is uniquely suited for use in a diffusion vacuum pump.

Another feature of this invention is the provision of a diffusion pump of the above featured type wherein the turbine device includes a specially designed statorand rotor assembly particularly suited for use in a diffusion pump wherein the pressure differential between the two sides of the turbine device is relatively small.

Another feature of this invention is the provision in a diffusion pump of the above featured type of a braking device positioned within the pumping fluid and adapted to prevent the pumping fluid surface from assuming a convex shape as a result of the centrifugal forces involved. V

These and other features and advantages of this invention will be more clearly understood upon a perusal of the following specifications taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a sectional elevation of a diffusion pump according to this invention;

FIG. 2 is a partial sectional plan view showing the rotating vane and braking device positioned within the pumping fluid;

FIG. 3 is a partial sectional view showing in detail the driving turbine device;

FIG. 4 is a reproduction of a recording trace of the oil temperature near the bottom of the pump boiler with the agitator both in motion and at rest and for three different power inputs;

FIG. 5 is a reproduction of the recording traces representing fluctuations of total pressure at the high vacuum side of the pump operating without a pumping fluid agitator for three different quantities of pump fluid and a variety of power inputs; and

FIG. 6 is a reproduction of the recording traces representing fluctuations of total pressure at the high vacuum side of the same diffusion pump operating under similar working conditions but utilizing a pumping fluid agitator according to the present invention.

Referring now to FIG. 1 there is shown a diffusion pump 11 having a cylindrical wall 112 closed at its bottom end by a circular base plate 13 adapted to be heated by a. conventional diffusion pump heater. The open upper end of cylindrical side wall 12 includes a flange 14 adapted for connection to a chamber it is desired to evacuate. A cooling jacket 15 surrounds the upper portion of cylindrical side wall 12 while an aperture 16 in its lower portion communicates with a flanged exhaust tubulation 17 adapted to communicate with a rough pumping system.

A plurality of annular chambers 18 are formed by concentric hollow tubes 19 mounted on and hermetically sealed tothe heater base plate 13. Moving inwardly each successive concentric hollow tube 19 is longer than the preceding outer one and each is capped by a vapor nozzle assembly 21. The lower portion of the diffusion pump 11 is filled with a pumping fluid such as, for example, oil to a level indicated by the dotted line 22, thereby serving as a boiler divided into a plurality of fractionating compartments 23 by the hollow cylindrical tubes 18. The center hollow tube 24 does not extend all the way to the bottom of the diffusion pump 11 so that the innermost fractionating compartment 25 is in communication with innermost annular hollow chamber 18 as well as with the interior 26 of center hollow tube 24.

The operation of the diffusion pump 11 is generally well known wherein the pumping fluid 22 is heated to evaporation and the resulting vapor rises to be directed through nozzle assembly 21. The escaping vapor is then condensed on the cold surface of the diffusion pump wall 12 after which it returns to the fractionating compartments 23 to be again reheated and evaporated. The use of fractionating compartments wherein the more volatile constituents of the pumping fluid 22 are discharged only through the outer and higher pressure nozzles of the diffusion pump 11 is also well known.

Referring now to FIGS. 1-3, the pumping fluid agitator device 30 of this invention is shown within the innermost fractionating compartment 25. Rotatively mounted within the innermost fractionating compartment 25 is the shaft 31 which lies perpendicular to the circular base plate 13 and whose pointed lower end rests at the center thereof. The shaft 31 is supported within central apertures of the mounting bar 32 and the stator 33 which are in turn secured to the inner wall of innermost hollow tube 19. Mounted on the lower portion of and for rotation with the shaft 31 is the agitating vane 34 compris-' ing a plurality of radially extending arms 35. Mounted i on the upper portion of the shaft 31 directly above the stator 33 is the rotor 36.

Each of the stator 33 and the rotor 36 is a circular plate having cut pie shaped sections therein bent at an angle to the original plane of the plate. The pie shaped blades 37 of the stator 33 are bent in a direction opposite to that of the rotor blades 38. Therefore rising pumping fluid vapor within the innermost fractionating compartment 25 is directed by the stator blades 37 directly against the rotor blades 38 so as to cause rotation thereof. The rotation of rotor 36 produces rotation of the shaft 31 and the agitating vane 34 resulting in agitation of the pumping fluid 22.

The. smooth stirring of the pumping fluid by the agitating vane 34 prevents the development of hot spots and the resulting erratic performance discussed above. .Similar agitating mechanisms can also be used in the outer fractionating compartments 23. However, the embodiment shown is preferred because of its simplicity and also because the problems associated with eruptive boiling are of much lesser importance in the fractionating compartments which are not associated with the high vacuum nozzles. It is also preferable that the agitating vane 34 be located as near the base plate 13 as possible so as to agitate the extremely hot pumping fluid adjacent to the base plate surface. Also the blades of agitating vanes 34 should preferably be completely covered with the pumping fluid 22 so as to prevent splashing thereof.

An importantfeature of the agitating device 30 is the use of both a stator 33 for directing the rising vapors and a driven rotor 36 for providing motion. Tests have shown that the use of a rotor alone produces much less desirable results because the difference in gas pressure below and above the turbine are insufficient to provide satisfactory motive power. Also, the use of both a stator and a rotor provides a unitary turbine device which is optically dense in the vertical direction except during-the brief periods of operation when the openings between successive blades in each unit are aligned in the vertical direction. Since these periods comprise a small percentage of total operating time and since the openings themselves includea small percentage of the total surface area of each unit, the turbine device will function effectively as a barrier to drops of pumping fluid which are carried along with the rising vapor. That is; a very large percentage of such rising pumping fluid drops will be intercepted by the turbine device and prevented from reaching the vapor nozzle assembly 21.

The use of braking arms 39 is another extremely important feature of this invention. As shown in FIGS. 1-3 the plurality of flat rectangular braking blades 39 are secured to the inner wall of the innermost hollow table 19 immediately above the agitating vane 34. The braking blades are preferably positioned so as to project partly into the pumping fluid and partly above, as shown in FIG. 1. The braking blades 39 retard circular movement of the pumping fluid 22 thereby reducing the applied centrifugal forces and preventing the surface of the stirred pumping fluid 22 from taking an undesirable convex shape. The smooth pumping fluid surface maintained by the braking plates greatly facilitates the smooth performance of the diffusion pump 11.

The use of the shaft 31 with a pointed end rotating on the surface of base plate 34 and the simple apertured'supports 32 and 33 is also preferable over other more complicated designs such as,'for example; bearing supports. The support mechanism shown is extremely simple and is much less sensitive to failure or impaired operation as a result of clogging by dirt and other contamination within the pump boiler.

Referring now to FIG. 4, the reproduced recording traces show the effectiveness of the present invention in the distribution of heat throughout the pumping fluid (oil was used as the pumping fluid medium). The curves show the measured temperature at the bottom of heated base plate 13 for a variety of input powers. The agitating device was oh and on at one minute intervals with the points (a) indicating time agitator was started and points (b) representing times when motion of the agitator was discontinued. The curve for an input power of 250 watts shows a maximum temperature of approximately 250 C. with the agitating device not operating and a temperature of 240 C. with the agitating device operating. The curve for 350 watts input power shows a Widely fluctuating temperature with a maximum of about 260 C. for the non-operating agitator and a relatively constant temperature of 244 C. with the agitator operating. The curve for 450 watts input power also shows erratic temperature variations with a maximum neat 260 C. for an operating agitator and a relatively constant temperature of 244 C. with the agitator operating.

Thus, the temperature at the bottom of the diffusion pump boiler was maintained much more constant and up to 16 C. in temperature cooler with the agitating device of the present invention. There is also reason to assume that local temperature differences in the two cases were even larger.

The reproduced recording traces shown in FIGS. 5 and 6 indicate the effectiveness of the present invention in eliminating the erratic pressure fluctuations commonly found in diffusion pumps. In the curves, time is shown on the vertical scale and pressure at the high vacuum side of the pump on the horizontal scale with the vertical center of each curve representing 1 10 Torr. As shown in FIG. 5, a diffusion pump Without the agitating device of the present invention was operated with various amounts of pumping fluid oil and at input power ranging from 600 to 1400 watts. The curve for the pump operating with 100 cubic centimeters of pumping fluid shows extremely wide pressure fluctuations in all except the 600 Watt input power range. The same pump operating with 200 cubic centimeters of pumping fluid oil shows wide pressure fluctuations over the entire range of input power. The curve for the same diffusion pump operating with 300 cubic centimeters of pumping fluid oil shows relatively constant pressure performance in the 600, 1200 and 1400 input watt range but extremly wide pressure variations in the 800 and 1000 watt ranges.

FIG. 6 shows similar reproduced curves for the same diffusion pump utilizing the agitating device of the pres ent invention. As shown in the curves for each of the pumping fluid oil quantities, a relatively smooth pressure performance was obtained over the entire range of input power.

Thus, the present invention provides a vacuum diffusion pump having an extremely uniform rate of pumping fluid evaporation and a correspondingly uniform pressure performance. Furthermore, such a performance is obtained with apparatus which is relatively inexpensive to produce and is simple and trouble-free in use.

It will be undertsood that various changes in details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

What is claimed is:

1. A vacuum diffusion pump apparatus comprising a sealed outer wall having an opening adapted for connection to a chamber to be evacuated and an exhaust aperture, a boiler section enclosed by the lower portion of said sealed outer wall and adapted to contain a pool of pumping fluid, heater means for applying heat to said boiler section so as to heat and cause evaporation of the pumping fluid contained therein, a vapor direction means supported by said outer wall and extending into said boiler section, said vapor direction means including a nozzle assembly adapted to direct evaporating pumping 6 fluid against the inner surface of said outer wall, and movable means so positioned within said boiler section as to be substantially covered by the pumping fluid pool and to cause agitation thereof.

2. Apparatus according to claim 1 wherein said movable means comprises a plurality of rotatable arms adapted to be covered by the pumping fluid.

3. Apparatus according to claim 1 including mechanical braking means positioned within said boiler portion and adapted to retard circulatory movement of the pumping fluid.

4. Apparatus according to claim 1 including a driving means positioned within said boiler section for imparting motion to said movable means, and a turbine device included in said driving means adapted to be driven by the stream of vapor provided by the evaporating pumping fluid.

5. Apparatus according to claim 4 wherein said movable means comprises a plurality of rotatable arms adapted to be covered by the pumping fluid and to produce circular movement thereof.

6. Apparatus according to claim 5 including mechanical braking means positioned within said boiler portion and adapted to retard circular movement of the pumping fluid.

7. Apparatus according to claim 4 wherein said turbuine device is positioned between said nozzle assembly and the pumping fluid.

8. Apparatus according to claim 4 wherein said turbine device comprises a rotor and a stator, said stator adapted to direct the stream of vapor against said rotor, and said rotor adapted to provide motion to said movable member.

9. Apparatus according to claim 8 wherein said rotor and said stator are adjacent each other so as to form a double barrier between said nozzle assembly and the pumping fluid thereby serving as an effective screen to rising drops of pumping fluid.

10. Apparatus according to claim 9 including mechanical braking means positioned within said boiler portion and adapted to retard circulatory movement of the pumping fluid.

11. Apparatus according to claim 9 wherein said movoable means comprises a plurality of rotatable arms adapted to be covered by the pumping fluid.

12. Apparatus according to claim 9 including a rotatable shaft joining said rotor and said movable means, said rotatable shaft having a pointed end adapted to bear gainst the inner side of said sealed outer wall.

13. Apparatus according to claim 12 including mechanical braking means positioned within said boiler portion and adapted to retard circulatory movement of the pumping fluid.

14. Apparatus according to claim 12 wherein said movable means comprises a plurality of rotatable arms adapted to be covered by the pumping fluid and to'produce circular movement thereof.

15. Apparatus according to claim 14 including mechanical braking means positioned within said boiler portion and adapted to retard circular movement of the pumping fluid.

References Cited by the Examiner UNITED STATES PATENTS 1,683,949 9/28 Bergdoll 103-89 2,521,345 9/50 Cortright 230101 FOREIGN PATENTS 1,202,482 7/59 France.

781,159 8/57 Great Britain.

LAURENCE V. EFNER, Primary Examiner.

WARREN E. COLEMAN, Examiner.

Claims

1. A VACUUM DIFFUSION PUMP APPARATUS COMPRISING A SEALED OUTER WALL HAVING AN OPENING ADAPTED FOR CONNECTION TO A CHAMBER TO BE EVACUATED AND AN EXHAUST APERTURE, A BOILER SECTION ENCLOSED BY THE LOWER PORTION OF SAID SEALED OUTER WALL AND ADAPTED TO CONTAIN A POOL OF PUMPING FLUID, HEATER MEANS FOR APPLYING HEAT TO SAID BOILER SECTION SO AS TO HEAT AND CAUSE EVAPORATION OF THE PUMPING FLUID CONTAINED THEREIN, A VAPOR DIRECTION MEANS SUPPORTED BY SAID OUTER WALL AND EXTENDING INTO SAID BOILER SECTION, SAID VAPOR DIRECTION MEANS INCLUDING A NOZZLE ASSEMBLY ADAPTED TO DIRECT EVAPORATING PUMPING FLUID AGAINST THE INNER SURFACE OF SAID OUTER WALL, AND MOVABLE MEANS SO POSITIONED WITHIN SAID BOILER SECTION AS TO SUBSTANTIALLY COVERED BY THE PUMPING FLUID POOL AND TO CAUSE AGITATION THEREOF.