US2905373A

US2905373A - Diffusion pump

Info

Publication number: US2905373A
Application number: US703140A
Authority: US
Inventors: Thomas L Scatchard
Original assignee: New York Air Brake LLC
Current assignee: New York Air Brake LLC
Priority date: 1957-12-16
Filing date: 1957-12-16
Publication date: 1959-09-22
Anticipated expiration: 1976-09-22

Description

T. L. SCATCHARD DIFFUSION PUMP Filed Dc.

2 Sheets-Sheet 1 QM @H 0N 0H iiil Hm JNH AN ANN. mam 0 0 0 0 0 0 0 m NN @N N BN W 0 0 0 mm mm 2 NH ma WW INVENTOR.

Ai tern ey s ThomasLScacchard dj M A 2 q T 9 mm fi g PAN Q Q Q Q Q Q NN NM wm Q Q Q u n i m mm m wm. MN HM h 5 fl ww Lie Sept. 22, 1959 Filed Dec. 16} 1957 1". L. SCATCHARD DIFFUSION PUMP 2 Sheets-Sheet 2 INVENTOR. ThomasLfica-tchard,

BYplnlg-nv DIFFUSION PUMP Thomas L. Scatchard, Philadelphia, Pa., assignor to The JNew York Air Brake Company, a corporation 'of New ersey Application December 16,1957, Serial No. 703,140

4 Claims. (Cl. 230-101) This invention relates to vacuum pumps, and more particularly to vapor jet difiusion pumps.

Pumps of this type usually include a cylindrical jet chimney containing a plurality of jet openings, and an annular jet skirt which extends from the chimney toward a surrounding cylindrical casing. The annular area between the outer margin of the skirt and the casing constitutes the entraining area of the jet. It is well known that as the pressure downstream of the jet (fore pressure) increases, the pumping action of the jet gradually breaks down. Furthermore, it is known that the maximum tolerable fore pressure varies in inverse relation to the width of the entraining area, i.e., the radial distance between the outer edge of the skirt and the casing. On the other hand, it is recognized that the speed of the pump, i.e., the rate of entrainment of gas molecules by the vapor jet, is directly proportional to the size of the entraining area. These two design considerations, viz., fore pressure tolerance and pump speed, present conflicting requirements and consequently the design of the diifusion pump represents a compromise.

Because of this compromise, most difusion pumps in use today employ a plurality of jet stages spaced along the longitudinal axis of the pump. The width of the entraining areas decreases progressively from the high vacuum to the fore vacuum side of the pump so that those stages near the inlet yield high pumping speed while those near the outlet aiford good fore pressure tolerance. In many cases this arrangement results in an extremely long pump which is difiicult to handle and which occupies a large amount of space.

The object of this invention is to provide a diffusion pump which, for a given amount of space occupied, will afford higher fore pressure tolerances and pumping speeds than the concentric cylinder pumps of the prior art. In its preferred form, the pump comprises a concave-convex circular housing in which is mounted a coaxial concavoconvex toroidal jet chimney having one or more series of jet openings in both its upper and lower surfaces.

Associated with each series is an annular jet skirt which extends from the chimney toward the adjacent housing wall to define an annular entraining area. Gas molecules enter the housing through a coaxial opening in the upper wall of the housing and vapor molecules are supplied to the chimney by a coaxial conduit extending through the lower wall of the housing and a plurality of branched radial passages connecting this conduit with openings in the inner peripheral wall of the chimney. Because of this geometrical arrangement, two annular entraining areas are afforded at each pumping stage; one area on top of the toroidal chimney and one on the bottom. As a result of this increase in entraining area, the speed of the pump, for a given amount of space occupied, is increased. Furthermore, since the available entraining area has been increased greatly, the width of this area can be decreased to improve fore pressure tolerance, without reducing the speed below that of a conventional concentric cylinder pump of comparable 2,905,373 Patented Sept. 22, 1959 size. Thus the present invention provides a pump which, while occupying no more space than prior pumps, nevertheless aifords higher fore pressure tolerance or speed or both.

The preferred embodiment of the invention will now be described in relation to the accompanying drawings, in which:

Fig. 1 is a sectional view of the pump taken on line 1-1 of Fig. 2.

Fig. 2 is a plan view of the pump in which a portion of the upper wall of the housing has been removed to show internal structure.

As shown in the drawings, the pump comprises a housing 11 having upper and lower circular walls 12 and 13, respectively, joined at their outer peripheries by a two-part collecting ring 14. Both walls are curved upward to aiford a concave-convex shape for the housing. The housing is provided with a coaxial flanged inlet pipe 15 extending through inlet port 15 in the upper wall 12, and a plurality of outlet passages or ports 16 which extend through the outer periphery and are joined by collecting ring 14. A cooling coil 17 surrounds the housing.

Located within housing 11 is a coaxial concave-convex toroidal jet chimney 18 which is supported by a plurality of brackets 19. The chimney contains two circular series of

jet openings

21 and 22 in its upper surface 23 and two similar series 24 and 25 in its lower surface 26.

Associated with these series are annular jet skirts 27,

' nular entraining

areas

32, 33, 34 and 35. According to the well known practice in staged diirusion pumps, the width of entraining

areas

32 and 34, adjacent the fore vacuum side of the pump, is less than the width of

areas

33 and 35 near the inlet or high vacuum side. This arrangement affords a workable compromise between fore pressure tolerance and pump speed.

Pumping vapors, either oil or mercury, are generated in an electrically heated boiler 36 and transmitted to jet chimney 18 by axial conduit 3'7, and branched radial passages or tubes 38. Because of the dished shape of the housing 11, Vapor condensate which forms on the cooled walls 12 and 13 will accumulate in the outer portion 39 of the housing. This condensate is returned to the boiler by drain pipes 41.

During operation, the inlet pipe 15 would be connected with the system to be evacuated and the collecting ring 14 with a fore pump (not shown) by outlet pipes 42. Vapors generated in boiler 36 and flowing to chimney 13 via conduit 37 and passages 38, will issue as jets from

openings

21, 22, 24 and 25 and traverse entraining

areas

32, 33, 34 and 35. Some of the gas molecules entering the pump through inlet pipe 15 will pass between the

upper walls

12 and 23 of the housing and chimney, respectively, and the balance will move down through the openings between radial passages 38 and pass between the lower housing wall 13 and the lower chimney wall 26. In a well known manner, these gas molecules will be entrapped by the vapor jets and directed toward the outlet passages 16. The pumping vapors will condense on the cooled walls of housing 11 and be returned to boiler 36 by drain pipes 41. The gas molecules, on the other hand, will be withdrawn by the fore pump through outlet pipes 42.

As stated previously, the drawings and description relate only to a preferred embodiment of the invention. Since many changes can be made in the structure of this embodiment without departing from the inventive concept, the following claims should provide the sole measure of the scope of the invention.

What is claimed is:

1. A vapor pump comprising a housing having upper and lower circular walls joined at their outer peripheries; a toroidal ljet chimney located within and coaxial with the housing, the upper and lower surfaces of the chimney being spaced from the corresponding walls of the housing; at least one circular series of openings in both the upper and lower surfaces of the chimney; an annular jet skirt for each series of openings, the skirts extending from the chimney toward the upper and lowerjwalls of the housing to define annular entraining areas; an'inlet port formed in the center of the upper wall; :a plurality of outlet ports formed in and spaced around the outer periphery of the housing; a vapor boiler; -a vapor conduit connecting the boiler with the interior of the jet chimney; means :for cooling the Walls of the housing to thereby {provide surfaces on which the vapors issuing from the openings in the chimney may condense; and

openings in both the upper and lower Walls of the chirnpay; an annular jet skirt for each series of openings, the skirts extending from the chimney toward the housing walls but terminating short thereof to define annular en- ?training areas; an inlet port formed in the center of the upper wall; a plurality of outlet ports formed in and spaced around the outer periphery of the housing; a vapor boiler; a vapor conduit connecting the boiler with the interior of the jet chimney, said conduit including one portion coaxial with the housing, and another portion comprising a plurality of radial tubes extending from the first portion through the inner peripheral wall of the chimney; means for cooling the walls of the housing to thereby provide surfaces on which the vapors issuing from the openings in the chimney may condense; and vapor condensate dnains located adjacent the outer periphery of the lower housing wall.

3. vapor pump comprising a concavo convex hollow circular housing; a concave-convex toroidal jet chimney located within and coaxial with the housing; at least one circular series of openings in both the concave and convex surfaces of the chimney; an annular jet skirt for each series of openings, said skirts extending from the chimney toward the housing to define annular entraining areas; an inlet port formed in and located on the axis of the housing; a plurality of outlet ports formed in and spaced around the outer periphery of the housing; a vapor boiler; a vapor conduit connecting the boiler and the chimney; means for cooling the Walls of the housing to thereby ,provide surfaces on which the vapors issuing from the openings in the chimney may condense;

and vapor condensate drains located in the bottom of the housing.

'4. The vapor pump defined in claim 1 in which the vapor conduit includes one portion coaxial with the housing and the chimney, and another portion comprising a plurality of radial tubes extending between the first portion and the ,jet chimney.

References Cited in the file of this patent UNITED STATES PATENTS 2,289,845 Litton July -14, 1942