US3497134A - Vapor vacuum pump - Google Patents

Description

/l 1 lll/lll flllllllull G. W. HORN VAPOR VACUUM PUMP Filed. Feb; 2, 1968 Feb. 24, 1970 INVENTOR. GEORGE w. HQRN United States Patent O 3,497,134 VAPOR VACUUM PUMP George W. Horn, Brookline, Mass., assignor, by mesne assignments, to Middlesex General Industries, Inc., Somerville, Mass., a corporation of Massachusetts Filed Feb. 2, 1968, Ser. No. 702,590 Int. Cl. F04f 9/00; F04b 37/02 U.S. Cl. 230-101 5 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for reducing backstreaming in a vapor vacuum pump. The method includes the steps of providing an ionized vapor as the vapor emerges from the nozzle of the pump and applying a field to the ionized vapor to deflect it selectively out of the space to be evacuated without interfering with the flow of non-ionized molecules out of the space to be evacuated. The apparatus for performing the method includes a vapor vacuum pump having a nozzle. A voltage source may be connected across the nozzle to ionize the vapor as it leaves the nozzle if the vapor is not sufficiently ionized. A field generator is located between the nozzle and the space to be evacuated to prevent the ionized molecules of vapor from backstreaming into the space to be evacuated.

My invention relates to vapor vacuum pumps, and more particularly to a novel method and apparatus for limiting backstreaming in such pumps.

Numerous methods, and .many forms of apparatus, have been devised for evacuating chambers to pressures below about -1 torr. The basic mechanism upon which many of the most efiicient methods and apparatus rely is the creation of a stream of vapor of molecules of a substance having a relatively low vapor pressure, into which stream the molecules in the space to be evacuated are diffused, entrained or otherwise induced. The stream is then condensed.

The lowest pressure that can be reached with a vapor vacuum pump is related to the leak rate in the evacuated chamber and the pumping speed. Typical substances which are suitable for use in vapor vacuum pumps are metals, commonly mercury, and various oils that have been developed.

Backstreaming of the vapor into the space to be evacuated, which occurs through various mechanisms familiar to those skilled in the art contaminates the space to be evacuated and results in a minor reduction in pumping speed. A number of such backstreaming mechanisms are discussed in U.S. Patent No. 2,919,061, dated Dec. 29, 1959 to Power for Vapour Vacuum Pumps.

Backstreaming not only increases the minimum pressure that can be obtained in the space to be evacuated, but more importantly in many instances precludes the use of vapor techniques because the chemicalcomposition of the residual gases in the space to be evacuated is of concern to the user. Accordingly, much effort has been directed to devising techniques for" reducing backstreaming. A number of techniques `have been used prior to my inventionv to reduce"backstreaming in vapor vacuum pumps. These include the placing of mechanical baliies in the path of the backstreaming molecules and the use of cool surfaces to condense the backstreaming molecules.

When these techniques are used, theresult'is generally a reduction in backstreaming at the expense of considerable reduction in pumping speed. The reason is that such devices as baiiies or cool surfaces in the path of vapor molecules going in the wrong direction also serve as barriers to molecules from the space to be evacuated that are going in the desired direction. Another prior method 3,497,134 Patented Feb. 24, 1970 ICC of reducing backstreaming, particularly with oil diffusion pumps, was to heat the top cap to prevent premature oil condensation thereon. However, this is merely a remedy for a poorly designed pump and backstreaming because of condensation can be avoided by proper design.

The principal object of my invention is to greatly reduce backstreaming while evacuating a space with a stream of vapor without substantially impeding the flow of molecules from the space to be evacuated.

Briefly, the above and other objects of my invention are attained by providing an ionized vapor stream at the nozzle, and introducing a field in the region between the nozzle and the space to be evacuated. The field may be either electrostatic or electromagnetic, but in any event is so directed as to accelerate ionized molecules away from the chamber to be evacuated, without in any way impeding the progress of non-ionized molecules from the space to be evacuated. By this method, it will be apparent that backstreaming can be reduced to a degree that is dependent primarily on the strength of the deflecting field relative to the temperature in the vapor space, and is relatively independent of the backstreaming cross-section.

The details of the manner in which the apparatus of my invention is constructed, and the preferred mode of carrying out the method of my invention, will best be understood in the light of the accompanying drawing, together with the following detailed description, of a preferred embodiment of my invention.

In the drawing, the sole figure comprises a schematic elevational view, with parts shown in cross-section and parts broken away, of a vapor vacuum pump in accordance with my invention.

In the drawing, I have shown a vapor vacuum pump incorporating my invention associated with a chamber 1 to be evacuated and communicating therewith by means of a conduit 3. The conduit 3 may be provided with conventional mechanical baiiles as indicated at 5 and 7. As will appear, the number and area of such conventional bafiies may be greatly reduced by the method and apparatus of my invention without decreasing the efficiency of the apparatus, and those mechanical bafiles that are provided will be rendered considerably more efficient.

The apparatus of my invention includes a conventional vapor vacuum pump, schematically indicated generally at 9, having its inlet connected to the conduit 3 and an outlet conduit, indicated at 11, that is adapted to be connected to a conventional forepump. While a pump comprising a single stage is shown, it will be apparent from the following description that the invention is equally applicable to multiple stage pumps.

The pump 9 includes a conventional boiler 13 containing a suitable substance, such as liquid oil or the like, as indicated at 15. A heater schematically indicated at 17 is provided to cause the substance 15 to vaporize and to pass upwardly through a chimney 19, and thence.out wardly and downwardly through a nozzle defined by the upper end of the chimney 19 and an umbrella cap 21.

At least the chimney 19 and the cap 21 are preferably made of metal, and the cap 21 is electrically insulated and separated from the chimney 19. The cap may be supported in the apparatus by any conventional insulating means, here schematically shown as an insulating stud 23 connected between the side wall forming the conduit 3 and the cap 21.

The vapor stream emitted from the nozzle formed by the chimney 19 and the cap 21 emerges at high velocity, and mingles with molecules from the chamber 1 by the mechanisms of diffusion and intermolecular collision. The vapor ultimately condenses on the walls of the apparatus, which may be cooled by conventional means here shown schematically as a water jacket 25, and is ultimately returned to the boiler 13 by conventional means,

here shown as involving an intermediate sump of condensate 27.

A conducting grid 29 of open wire work or the like is mounted in the space above the cap 21. The grid may be mechanically and electrically connected to the cap 21 by means here shown as a metal riser 31.

The positive terminal of a suitable source of relatively high voltage is connected to the conducting riser 31. The negative terminal of the voltage source is returned to the chimney 19, as indicated by the ground connection at 33.

To illustrate the voltage requirements, where the nearest part of the chimney 19 is 11/2 to 2 millimeters from the cap 21 at the nozzle, 500 volts is satisfactory for Dow- C-orning 704 silicon oil at 200 C. The field intensity is then sufficient to ionize enough of the molecules of vapor emitted from the nozzle to prevent backstreaming. In general, the required voltage for ionization depends upon the pumping material and its temperature as well as the spacing of the ionizing electrodes.

It will be apparent that the field produced by the grid 29 relative to the other metallic elements of the apparatus will accelerate any ions of vapor, coming into the region above the cap 21, away from the grid and downward towards the reservoir. The grid itself need only occupy a very small proportion of the cross-sectional area of the conduit 3, so it will offer substantially no barrier to non-ionized molecules streaming `from the chamber 1.

While I have shown as the field source a grid connected to the source of voltage that also ionizes the vapor stream,

it will be apparent to those skilled in the art that different sources could be used, and that substantially the same effect could be produced by a grid that was electrostatically charged, or by a permanent magnet or electromagnet surrounding the apparatus and producing an electromagnetic field in the region above the cap 21 to deflect the ionized molecules of vapor.

The method and apparatus of my invention have the additional advantage that the eiciency of mechanical baies such as 5 and 7 is greatly improved because such ionized molecules as may pass the grid 29 will have a higher probability of sticking to the surfaces of the walls and baliies than will non-ionized molecules from the chamber 1. Also, by reason of the preferred sticking probability of the ionized vapor molecules, the requirements on the coolant in the jacket 2S are reduced, and somewhat higher temperatures of the cooling fluid can be employed for the same efficiency as compared with a conventional vapor pump.

In the foregoing description, I provide a positive voltage on the cap and on the grid 29. It will be vapparent that if the grid is insulated from the cap 21, the voltage applied to the cap may be negative and still provide an ionized stream. The potential on the grid should always be positive with respect to ground, however.

Further, in the embodiment described above, ionization of the vapor from the nozzle is achieved by an electrostatic field. However, my invention is not limited to this method of providing an ionized stream of molecules.

charged'through the nozzle. For some materials there may be inherently a suicient number of ionized molecules in the vapor stream so that backstreaming will be substantially reduced.

While I have described my invention with reference to various illustrative and preferred details, many changes and variations will be apparent to those skilled in the art upon reading my description, and such can obviously be made without departing from the scope of my invention.

Having thus described my invention, what I claim is:

1. A vapor vacuum pump comprising, in combination, means forming a pump housing, said housing including means forming an entrance passage and an outlet passage, meas located within said housing including a nozzle to provide a stream of pumping vapor to entrain molecules of gas entering said pump housing through said entrance passage, said means for providing a stream of pumping vapor including a boiler for vaporizing' said pumping fluid when in liquid form, a chimney having an entrance and a exit end, the entrance end of said chimney receiving vapors from said boiler, means forming a cap about and spaced from the exit end of said chimney, said cap and said chimney forming said nozzle, means for applying a Voltage across said nozzle to ionize said pumping vapor passing therethrough, and means for generating a field within said entrance passage to preclude particles of said vapor from passing through said entrance passage.

2. The apparatus of claim 1 including a grid of spaced conducting members located in said entrance passage, and means for connecting said grid to a source of potential.

3. The combination defined in claim 2 in which said grid and said chimney cap are electrically connected.

4. In the method of evacuating gas from a chamber which includes the step of forming a stream of vapor directed away from said chamber to entrain molecules of gas emerging from the chamber to be evacuated, the process of reducing vapor backstreaming comprising in combination the steps of ionizing the vapor stream before it entrains said molecules of gas and producing an ion deflecting eld comprising a magnetic field in the region between the vapor stream and the chamber thereby to preclude said ionized vapor particles from backstreaming into said chamber.

5. The combination as defined in claim 1 in which said means for applying a voltage across said noule comprises:

applying a potential across said cap and chimney.

References Cited UNITED STATES PATENTS 2,246,327 6/1941 Slepian 230-101 3,053,436 9/1962 Sedlacsik 230--101 3,365,119 1/1968 Kainer 230-69 WILLIAM L. FREEH, Primary Examiner WARREN J. KRAUSS, Assistant Examiner U.S. Cl. X.R. 230-69

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