US2063665A - Diffusion vacuum pump - Google Patents

Description

Dec.8, 1936. H. w.- EDWARDS 2,063,665,

- I DIFFUSION VACUUM PUMP Filed March 21, 1954 I 2 K Ill I T 41 31 I ci l 50 ai 15 F gl Jr |'.l 21 .36 l H E 15 2.. (L r '17 l 1%)" fiiidirards, by:

' .fliiorneg/ Patented Dec. 8, 1936 UNITED STATES PATENT OFFlCE DIFFUSION VACUUM PUMP and L. H. McGowan Application March 21, 1934, Serial No. 716,639

8 Claims.

My invention relates to diffusion pumps of vapor type, which are adapted for obtaining high vacuum in closed receptacles, such as vacuum chambers used for condensing metal in vapor phase on mirror bodies and for producing high vacua in any other closed receptacles desired. Among the objects of my invention is the production of a high speed vacuum pump which shall be as fast and efficient in operation as possible and which shall be extremely simple in construction, and free from moving parts. One of the characteristics of pumps of this type heretofore in use is high resistance in the passage or conduit leading from the source of vapor to the condenser, which has impaired the operation of the pump, increased the amount of heat required and slowed down the effective results obtained. One object of this invention is to reduce this resistance to a 20 minimum and permit the vapor to be given a correspondingly high velocity between its source and the condenser. Another object of the invention is to cause practically all the condensed liquid to return freely to the source of vapor. Another object is the introduction of a nonmetallic vapor into the inethod of operating my improved pump, whereby the presence or even minute traces of vapor of metallic origin from the pump, in the vacuum chamber is prevented. 30 This is an advantage when metallic alloys of fine proportions in vapor phase are being condensed in the vacuum chamber. A still further object or feature of my invention is the use of oil of low heat of vaporization, in my improved pump, 35 whereby quicker and more eilective results are attained.

In the accompanying drawing forming part of this specification, Fig. 1 is a diagrammatic view, showing the application of my improved 40 diffusion pump in use; Fig. 2 is a vertical, central section, of my improved difiusion pump, taken on line 2-2 of Fig. l, and Fig. 3 is a perspective view of one of the baiiies.

The difiusion pump forming the subject of 45 this invention is designed particularly, although not exclusively for use with a mineral oil which has a low vapor pressure and may therefore be used without a liquid air or solid carbon dioxide trap, which is necessary in most cases if low 50 pressures are to be obtained. The particular pump herein described will produce about one millionth of a millimeter of mercury pressure and has a diameter of three inches. This diameter may be varied and wherever specific dimensions are mentioned throughout this speci- 5 fication, I do not wish to .be limited thereby, within the spirit of my invention.

My improved pump consists of a tubular casing A, made of metal or other suitable material, whichis intended to be used in substantially upright position. The lower end of this casing is sealed with a circular plate forming a floor. H), of the same material as the tube and the upper end is intended to be connected in any suitable manner toa vacuum chamber B or any other device from which air is to be exhausted. The lower closed end of the casing serves as a vaporizer and reservoir for a supply of oil H or other suitable vaporizing material. I have found that by using oil of low heat vaporization, quicker and better results are obtained in the operation of my improved pump. A condenser, resembling a water jacket C and using water or other cooling medium circulated through ingress and egress ducts l2 and I3 is located approximately in the median region of the casing and adapted to condense the oil or other vapor. A-suitable heater of any kind desired is placed below and in contact with the lower end it of the pump in order to heat the oil suificiently to supply a stream of oil vapor upwardly. Said heater is indicated diagrammatically by the electric resistance coil M in the receptacle I5, the latter being preferably a poor heat conductor made of asbestos or other suitable heat resistance material, within which the lower end of the tube A is enclosed, thus surrounding the region containing the vaporizing oil.

The internal parts of the pump are all removable from its upper end to facilitate cleaning when desired. These internal parts are a chimney D, a deflector E, and the baflie system F. The chimney is preferably made out of any good heat conducting material such as copper. Its lower end for a certain distance upwardly, about the height of the receptacle l5, conforms closely with the inner diameter of tube A and is removable upwardly therefrom. Above its lower endthe chimney contracts into a long cylindrical,

upwardly extending body bearing a definite ratio to the inside diameter of the pump which has been found to be suitable for a maximum speed at a definite pressure. Approximately this ratio is as follows: when tube A is fifteen inches long and three inches inside diameter, the cylindrical body of the chimney is four and one half inches long and one and one half inches outside diameter. These dimensions may be as above indicated varied within the spirit of my invention. Two insulating Jackets l8 and ll of tubing, made out of glass or any other heat resisting material are placed endwise closely over the body of the chimney, the lower jacket being supported by a collar l8 around the lower end of said body. The function of these jackets is to partially insulate, thermally, the copper body of the chimney from the gases which may bevin the region between the insulating Jackets and the pump casing. Near the lower end of the constricted body of the chimney, a horizontal circular disk 20 (see Fig. 3) of thin metal is removably held between the two jackets. This disk is of such diameter that its outside edge is everywhere within a half, of a millimeter (more or less) from the pump casing wall. Thus the disk performs the function of a baille immediately above the vaporizer and is used to prevent any oil vapor which may be found in the space below it from rising and interfering with the free movement of gases which are driven down from above and which are drawn out of the side outlet 2| by the pump F to be hereinafter described. Condensed oil from above seals the outer edge of the disk 20 to the casing wall A and at the same time flows downwardly past the disk. A small drain tube 22 made out of copper or other suitable material is supplied in the lower part of the chimney. Its lower end dips into and is sealed in the oil in the reservoir and its upper end connects through the wall of the chimney with the space in the casing A immediately above the lower end portion of the chimney and the baiile disk 20.

The deflector E is mounted rigidly upon the upper end of a shaft 25 by bolt connection 26 and said shaft extends downwardly and rests upon the bottom plate I!) of the chimney and holds the deflector spaced a short distance above the chimney in such a position that proper clearance is obtained. Suitable spacing pins 21 radiating outwardly serve to hold the shaft coaxial within the chimney. The lower surface 28 of the deflector E commencing at the junction of shaft 25 outwardly to its margin is concave so as to evenly and continuously deflect vapor passing upwardly from the vaporizer through the chimney outwardly and downwardly into the space between the outer wall of the jacket and the inner wall of the casing. Both the shaft 25 and deflector are made out of a good heat conductor such as copper or other suitable material, and thereby serve to conduct heat readily from the floor of the vaporizer to the deflector and prevent the oil vapor from condensing on the inside of the deflector.

The baflle system F as shown comprises three parts but may be extended to include as many parts as desired, their function being to prevent oil vapor from ascending into the vacuum chamber B. Each of the three parts (as shown) consists of a portion of a conical surface. The central part 30 is made out of one solid piece with.

the outside surface cylindrical in shape to fit snugly into the pump casing and at the same time permit easy withdrawal and with its inside surface 8l frustro conical and pointing downwardly.

The lower and upper members are thin conical shells 32 and, 33, pointing upwardly and held in series spaced relation on the median member 30' by sets of pins 34 and 35. The baflie assembly is supported in the, proper vertical position above the deflector E by means of the vertical pin 36, secured rigidly to the lower member'32 and extending downwardly with its lower end resting on theupper end of the shaft 25. The angle of the conical surface of each baflie member of the baflle assembly should be comparatively small for the most eflicient results.

When desired the bafile disk 20 may be provided with a pair of deflecting surfaces 31 (see Fig. 3) slanting downwardly from a point uppermost diametrically opposite the forepump outlet 2|, circumferentially around the insulating jacket I1 and gradually merging into the plane of the disk for more effectively streamlining and directing down draft into said outlet. According to the results desired the number of baflie elements may be increased or decreased or removed entirely.

My improved diflusion pump is shown in Fig. 1 connected with a vacuum chamber and forepump as when in operation. In the drawing the upper end of the diffusion pump casing A is connected by sealing to the wall of the vacuum chamber B. The outlet duct 2! from the difiusion pump is connected through a three-way valve 40 with the fore pump F. A branch passage 4| from said valve to the vacuum chamber B is arranged so that by turning the valve into one position the forepump acts to exhaust air directly from said vacuum chamber for preliminary evacuation. By turning the valve into another position the forepump acts through my improved diffusion pump to quickly produce a higher vacuum, and by turning the valve into a third position both connections from the forepump with the vaccum chamber B and diffusion pump are closed.

Operation Assuming that a high vacuum is desired in chamber B, the valve 40 is turned into the position shown in Fig. l and the forepump operated. Air or gases are thus exhausted from the chamber B down through the diffusion pump and exhaust outlet 2|. During this operation oil in the vaporizer is heated by the heater I4 to a vaporizing temperature. As the oil boils its vapor ascends to the upper regions through the chimney. Since the oil has a comparatively low heat of vaporization the chimney D, shaft 25 and deflector E, which are good heat conductors, prevent the oil vapor from condensing on the lower surface of the deflector E. This vapor is deflected by the gradually curved lower surface 28- of the deflector E downwardly into the bombarding region between the outside of the chimney and-inside of the condenser C. The introduction of this gradually and continuously curved deflector surface increases the pumping speed of the diffusion pump materially. During this operation gases from the vacuum chamber B are drawn down into the region between the chimney and the condenser in the diffusion pump casing and are exhausted through the outlet 2| by the forepump. During this operation the oil vapor being directed downwardly propels the gases in said bombarding region which it bombards downwardly towards the lower baffle 20. From the lower portion of said bombarding region immediately above this baflle the gas is drawn off by the forepump through the outlet 2|. The oil apor condenses for the most part, on the inner wall of the condenser and in liquid form returns to the oil vaporizer or reservoir through drain tube 22. The action of the bombarding gases which are exhausted from the vacuum chamber by vapor to produce a pumping effect is well known and hence a detailed description of the process occurring in said bombarding region is omitted. Suffice it to say that directing the current of oil vapor into and along with the current of gases received from the vacuum chamber reduces resistance to the onward movement of the gases and vapor to a minimum and thus reduces pressure of gases in the 'vacuum chamber effectively in'a shorter space of time than heretofore. vapor is prevented from escaping to the vacuum chamber by the bailieasystem F substantially without impeding the progress of the exhaust flow from the vacuum chamber. For many purposes the presence of oil or other vapor or even minute traces thereof from the diffusion pump in the vacuum chamber is highly undesirable. The bafiie system of conical surfaces of long altitude characteristic serves to retard upward passage ofvapor while offering a minimum obstruction to the downward movement of the gases which are removed from the vacuum chamber. This characteristic in effect is to assist the onward movement of the gases and that characteristic is also present throughout the entire apparatus and produces more effective and time saving results. In tests which have been made it has been determined that at a pressure of .0001 of a millimeter of mercury, the mean free path of the gas molecules in the diffusion pump is approximately 75 C. M. long. Because of this fact it is possible, in the baffle system disclosed to disregard the collisions which occur between individual molecules of the gas and replace stream line" surfaces with those surfaces from which molecules bounce in a forward direction, in the larger number of cases at least. Long na'rrow tapered conical surfaces will supply this improvement as has been clearly indicated in practice.

It will be pointed out that there are two rather critical spacing dimensions which efiect the pumping speed of the pump at any particular pressure. One of these is the distance between the outer edge of the deflector disc 20 and the inner wall of the pump casing. For a pressure of about 104 m. m. this distance should be about m. m. The other spacing distance is the vertical clearance between the upper edge of the chimney ,D and the lower surface of the deflector E. For this pressure I have found the optimum distance to be about '7 m. m. All surfaces with which the gases and oil vapor come into contact should be well polished and smooth for the most effective results.

In accordance with the patent statutes, I have described the principles of operation of my invention, together with the apparatus which I now consider to. represent the best embodiment thereof but I desire to have it understood that the construction shown is only illustrative and that the invention can be carried out by other means and applied to uses other than those above set forth within the scope of the following claims.

Having described my invention, what I claim as new and desire to secure by Letters Patent is:

1. In apparatus for exhausting a closed receptacle, a main duct connected with said receptacle, a vaporizer supported in the lower end ceptacle, a duct communicating with said receptacle, a vaporizer supportedin the lower portion of said duct, a double-walled chimney of reduced diameter communicating with said vaporizer and providing between it and the duct wall a condensing chamber, the outer wall of said chimney being formed of insulating material, and the inner wall of said chimney being formed of heat conducting material, said chimney walls being joined to prevent the passage of gases between them.

3. In apparatus for exhausting a closed receptacle, a duct communicating with said receptacle, a vaporizer supported in the lower portion of said duct, a chimney of reduced diameter communicating with said vaporizer, and an insulating wall surrounding said chimney wall and providing between it and the wall of said duct 2.

condensing chamber, said walls being closed to the passage of gases between them.

4. A diffusion pump for exhausting a closed receptacle, comprising: a duct communicating with said receptacle, a vaporizer having a reservoir in the lower portion of said duct, a vaporconducting chimney extending from said vaporizer upwardly in said duct, cooling means extending about a portion of said duct adjacent said chimney, and insulating means adjacent said chimney and between said chimney and said cooling means.

5. In a diffusion pump for exhausting a closed receptacle, a duct communicating with said receptacle, a vaporizer supported in the lower portion of said duct, a chimney communicating with said vaporizer and extending upwardly therefrom, and a glass tube within said duct which encircles said chimney, said tube and chimney being closed to the passage of gases between them.

6. A diifusion pump for exhausting a closed receptacle comprising: a duct communicating with said receptacle; a vaporizer having a reservoir in the lower portion of said duct; a vaporconducting chimney extending from said vaporizer upwardly in said duct; cooling means extending about a\ portion of said duct adjacent said chimney; an insulating wall surrounding said chimney; and an annular disk extending radially from said insulating wall and having an external diameter less than that of the inside of said duct; said disk and said insulating wall providing with the wall of said duct a condensing chamber.

7. A diffusion pump for exhausting a closed receptacle comprising: a duct communicating with said receptacle; a vaporizer having a reservoir in the lower portion of said duct; a vaporconducting chimney extending from said vaporizer upwardly in said duct; cooling means extending about a portion of said duct adjacent said chimney; an insulating wall surrounding said the wall of said duct an annular passage for the withdrawal of condensate from said chamber.

8. 4i difl'usion pump for exhausting a closed receptacle comprising: a duct communicating with said receptacle; a vaporizer having a reservoir in the lower portion of said duct; a vaporconducting chimney extending from said vaporizer upwardly in said duct; cooling means extending about a portion of said duct adjacent said chimney; an insulating wall surrounding said chimney; an annular disk extending radially from said insulating wall and having an external diameter less than that of the inside of said duct, said disk and said insulating wall providing with the wall of said duct 9. condensing chamber; and a deflector supported on said disk for guiding the draft within said cooling chamber.

HIRAM W. EDWARDS.

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