US2943783A

US2943783A - Heater for a diffusion pump

Info

Publication number: US2943783A
Application number: US649890A
Authority: US
Inventors: George H Bancroft; Gordon P Gerow; Donald L Stevenson
Original assignee: Consolidated Vacuum Corp
Current assignee: Consolidated Vacuum Corp
Priority date: 1957-04-01
Filing date: 1957-04-01
Publication date: 1960-07-05
Anticipated expiration: 1977-07-05

Description

July 5, 1960 e. H. BANCROFT ETAL 2,943,783

HEATER FOR A DIFFUSION PUMP Filed April 1. 1957 I I J i I g 60 caoleaa lly l orr ORDON F. GEROW BY ONALD L. SEVENSON ATTORNEYS July 5, 1960 e. H. BANCROFT A 2,943,783

HEATER'FOR A DIFFUSION PUMP Filed April 1, 1957 2 Sheets-Sheet 2 INVENTOR.

NSO/V AT TOR/VEYS GEORGE H. BANCROFT coma/v P smow BY DONALD L. STEVE 0 2 w 2 |I M :..l\ n w I i w IHWH m .5 i I! I,

FIG. 3.

HEATER, FOR A, DIFFUSION PUMP George H. Bancroft-and Gordon P. Gerow, Rochester, and'Donald L. Stevenson, Albion, N.Y., assignors, by mesne assignments, to. Consolidated Vacuum Corpora tion, Rochester, N. Y., a corporation of New York.

' Filed Apr, 1,1957, Ser.No. 649 ,89.0 2 Claims. 121.230.5401

This invention relatesto diffusion typevacuum pumps, and more particularly, is concernedwith improved heater apparatus for such pumps. T

Diffusion pumps for operation at highvacuums are well known. Such pumpsoperate on the principle that a liquid having relatively heavy molecules is vaporized by raising its temperature. The vaporwith its-heavy molecules is directed by suitable jets indirection away from the region to be evacuated towards a mechanical forepump. The accelerated molecules of vapor compressthe gas molecules ahead of the jet, forcing them toward the mechanical forepump and thereby reducing the pressure within the evacuated region. The vapors are recondensed on the cooled walls of the pumpwhere the liquid is permitted toreturn to the bottom of the pump to be reheated and vaporized.

It has been the general practice heretofore toprovide a reservoir of oil, for example, at: the bottom of the diffusion pump and apply heat to the bottom of the diffusion pump by means of a flat electric heater positioned under the bottom of the pump, the heat. being transferred. to the oil by conductionthrough the walls of the pump. The oil is thus caused to boil, producing the vapors which rise in the pump and are directed through orifices which provide the proper jets of the heavy molecule vapors of the oil. In addition to being an ineflicient means of transferring heat to the pump liquid/such known heating methods do not cause the liquid to boil uniformly. Local hot spots exist which produce greater boiling activity in certain portions of the liquid; The liquid: does not release the vapors in a: steady fashion. Greater agitation of the liquid in spots causes liquid droplets to be carried along with the vapor. The result is erratic and inefficient operation of the pump.

The present invention. provides improved heating means for a diffusion pump which applies heat more uniformly through the volume of the liquid. Moreover, the heater of the present invention provides a means for heat ing the vapors after they leave the surface of the liquid. In this manner the vapors are super-heated to increase the energy of the vapor molecules, which in turn increases the velocity of the molecules in the jets of the pump to increase the pumping efiiciency.

To this end, the present invention provides novel heat ing means in a diifusion pump having a cylindrical outer wall and a bottom wall, and a jet element having a lower cylindrical wall portion positioned concentrically within the outer wall of the pump against the lower end thereof. The heater means for boiling the diffusing liquid of the pump comprises a lower cylindrical casing which projects upwardly from the bottom wall of the pump within the hollow jet element. The casing is joined to the bottom wall in a hermetically sealed joint, the projecting upper end of the casing being closed off and hermetically sealed. A cartridge-type electric heating element is positioned in the casing, and a plurality of heat-conductive fins joined to the casing, the fins projecting radially outward from the casing. Part of the casing and projecting fins is below Patented Jul-y. 5,; 1.9 9:

the. surface of the liquid and part is above the surface; of the liquid, whereby the vapors from the boiling liquid; are superheated by the casing and fins, as they rise, in,. th e, jet element.

For a better understanding of the invention, reference should be had to the accompanying drawings wherein: i

Fig. 1 is an elevational view. of .a' diffusion pumputiliz: ingthe improved heater of the present invention;

Fig. 2 is a sectional view of the'dilfusion pump. showing the interior. construction thereof;

Fig. 3Jis a sectional view takensubstantially; onthe line..3-3 of Fig. 2; and

Fig. 4..is a sectional view similarlto Fig. 3. but

a. modified construction. of.:the heating means.

Referring to the drawings in detail, the numeral: it) indicates generally. thehousing of the pump, which includes a hollow cylindrical portion 12 preferablyjm'ade of metal, and a lower end wall 14. The up'per endtwall of the housing. 10 includes a'metal' cap 16. which fits intothe endof the cylindricalwall 12. A seal is made by means ofan O-ring 18 which is squeezed outwardly against the inner Wall of the cylindrical portion 12 of the housing 10 by means of an annular plate 20 having a threaded stud 22- secured thereto. A nut24 engages the stud 22. When tightened up, the nut causes the O-ring to be squeezed out between the cap 16, and the annular plate 2th in sealing relationship with the cylindrical wall portion 12. V

The discharge side ofthe pump is provided by a pipe 26, oneend of'which opens into the lowerportion offth'e housing 10. A flanged coupling 28 provides, means for coupling the pumpto the system to be evacuated- TH: flange coupling 28 has an opening therethrough communieating with the upper end of the vacuum pump housing. A cooling coil in the form of a continuous metal tube 30 is Wound around the exterior of the pump housing and the discharge pipe 36. A suitable coolant can be-circulatedthrc'iughthetube 30 to provide means for cooling the walls of the pump. I

The interior of the pump includes a jet element indie cated generally at 32. The jet element is preferably constru-cted of three

cylindrical sections

34, 36, and 38 which are successively of diminishing diameter. The three cylindrical sections are located along a common axis with their ends being joined in. concentric overlapping relationship. Thus the cylindrical members 34 and 36 are joined by relatively thin spacers 40 and the cylindrical sections 36 and 38 are similarly joined by spacers 42.

The heater assembly to which the present invention is particularly directed is located in the lower end of the housing 10. The heater assembly includes a hollow cylindrical casing 44- which projects upwardly into the housing from the lower end wall 14. The lower end of the casing 44 is joined around its outer periphery to the bottom plate :14, a hole being provided in the bottom plate 14 to receive the casing 44. By suitable welding or brazing of the joint between the bottom plate 14 and the casing 44, an hermetically sealed joint is provided therebetween. The upper end of the hollow casing 44 is closed off by an end plate 46. Thus the hollow interior of the casing 44 is open to atmospheric pressure, while the outer wall of the casing 44 forms part of the wall of the evacuated pump interior.

A plurality of radially projecting conductive fins, such p 3 the liquid charge, preferably more than half the length of the fins and casing being above the liquid level. The liquid level is indicated by the dotted line 50.

Heat is supplied to the finned heating structure by means of a cartridge-type heater 52. Such type heaters are well known and may be obtained in a great range of sizes. The cartridge heater includesyan outer metallic wall 54 which has an outer diameter substantially equal to butjslightly less than the inner diameter of the casing 44, whereby the cartridge heater 52 can be inserted into the interior of the casing 44. A pair of leads 56 extend out of the end of the cartridge heater 52 for connecting toa suitable source of electric power. The cartridge heater 52 is held in position by means of a collar 58 secured tofthe 'bottom plate 14. The collar 58 extends around the opening through which the cartridge heater 52 is inserted int o the casing 44; A set screw 60 in the collar 58, when tightened, locks the cartridge heater into position.

- 'In the modification of Fig. 4, the fins 48 are provided at their lower ends with portions 49 extending substantially parallel to the bottom wall. This increases the heating area of the fins and also provides increased thermal agitation and movement of the liquid for more uniform of the pump with a substantial portion of the surface area generation of vapor over the whole surface of liquid. 7

' It will be noted that the finned heating structure is closely surrounded by the wall of the cylindrical section 34. This wall is of course cooler than the heater and tends to remove heat from the liquid charge. fining and cooling theliquid in the immediate vicinity of the heating structure, strong convection currents are produced in the liquid due to the combined heating of the heater and the cooling of the surrounding cylindrical wall.

This decreases the time anyof the liquid is exposed to the heating surface. .The result is more uniform heat distribution through the liquid charge, and the elimination of localized hot spots which cause isolated areas element positioned in the casing, a cylindrical wall encircling and arranged concentrically with said casing, a plurality of heat-conductive fins joined in heat conductive relationship to the casing, and extending lengthwise of the casing, the fins projecting radially outwardly from the casing within and extending substantially to the inner surface of said cylindrical wall to form a compartmented region for the liquid, the casing and fins projecting above the bottom wall of the pump and extending vertically partly below. and partly above the level of the liquid charge in the pump, whereby the vapors from the boiling liquid in said compartmented region are superheated by By conof turbulence otherwise present in conventional heating arrangements.

We claim: 1. A diffusion pump having a cylindrical housing and a bottom wall, and a jet element positioned vertically and concentrically within the housing wall of the pump comprising heater means for boiling the dilfusion liquid of the casing and fins as they rise within the jet element.

2. In a diffusion pump having an outer wall and a bottom wall, and a jet element having a lower wall portion positioned within the outer wallof the pump, means for boiling the diflfusion liquid of the pump which normally collects in the region of the bottom wall, comprising an electric heater projecting up from the bottom wall of the pump, the heater being secured to and supported by the bottom wall in a hermetically sealed joint, the heater being formed with a heating surface extending above and below the surface of the liquid in the bottom of the heater being above the liquid surface, a plurality of fins extending lengthwise of the heater and projecting radially therefrom, and means defining a confining wall closely surrounding the heater and associated fins, each of said fins including a lower portion extending at an angle to the remainder of the fin.

References Cited in the file of this patent UNITED STATES PATENTS 1,471,012 Snook Oct. 16, 1923 2,121,599 Kilgore et al. June 21, 1938 2,246,327 Slepian June 17, 1941 2,305,947 Armstrong et a1. Dec. 22, 1942 2,432,169 Morgan et -al. Dec. 9, 1947 2,437,897 Stoltenberg et al Mar. 16-, 1948 2,505,953 Flosdorf May 2, 1950 2,508,765 Morand May 23, 1950 r 2,630,266 Lawrance Mar. 3, 1953 2,721,025 Lawrance'et a1. Oct. 18, 1955 2,785,274 Andrus Mar. 12, 1957 FOREIGN PATENTS 100,429 Austria July 10, 1925 338,971 Germany July 9, 1921 686,659 Great Britain Jan. 28, 1953