US2943784A

US2943784A - Heater for a diffusion pump

Info

Publication number: US2943784A
Application number: US806924A
Authority: US
Inventors: Donald L Stevenson; John L Gallagher
Original assignee: Consolidated Vacuum Corp
Current assignee: Consolidated Vacuum Corp
Priority date: 1959-04-16
Filing date: 1959-04-16
Publication date: 1960-07-05
Anticipated expiration: 1977-07-05
Also published as: DE1841023U

Description

y 1960 D. L. STEVENSON ETAL 2,943,784

HEATER FOR A DIFFUSION PUMP Filed April 16. 1959 4 Sheets-Sheet 1 l6 INVENTORS.

00mm Z. .Szrznm wan BY JD/M/ Z. 6.41am?- July 5, 1960 D. L. STEVENSON ETAL 2,943,784

HEATER FUR A DIFFUSIQN PUMP 4 Sheets-Sheet 2 April 16, 1959 INVENTORS. 00mm Z. .YIZVIMMA/ Joby 1 6411.440! w, /a& (M

July 1960 D. L. STEVENSON ETAL 2,943,784

HEATER FOR A DIFFUSION PUMP Filed April 16. 1959 4 Sheets-Sheet 3 INVENTORJ'. 00mm A Jim mm o/m Z. 644mm y 1960 D. L. STEVENSON EI'AL 2,943,784

HEATER FOR A DIFFUSION PUMP Filed April 16, 1959 4 Sheets-Sheet 4 INVENTORS. 00mm Z. furl/way BY Joy/V 1.611046%? rm n [r5 "Consolidated Vacuum Corporation, Rochester, N.Y., a corporation of New York Filed Apr. 16, 1959, s... No. 806,924 8 Claims. (21. 230-101 This invention relates to diffusion type vacuum pumps, and more particularly is concerned with improved heater apparatus for such pumps.

Difiusion pumps for operatio'n at high vacuums are well known. Such pumps operate on the principle that a liquid having relatively heavy molecules is vaporized in the pump by raising its temperature. The vapor with its heavy molecules is directed by suitable nozzles in a direction away from the region to be evacuated, towards a'mechanical forepump. The accelerated molecules of vapor co'mpress against molecules ahead of the nozzle, forcing them toward the mechanical forepump and thereby reducing the pressure within the evacuated region. The vapors are recondensed on a cool wall of the pump where the liquid is permitted to return to the bottom the pump to be reheated and vaporized.

It has been the general practice heretofore to provide a'reservoir of oil, for example, at the bottom of the diffusion pump andto apply heat to the bottom of the diffusion pump by means of a flat electric 'heater attached to thebottom of the pump. The heat is transthe bottom and are provided with radially extending fins ferred to the oil'by conduction through the bottom wall of the pump. 'Thus the oil is caused to boil, producing vapors which rise inthe pump wherein they are directed through orifices which produce the proper jets of heavy molecule" vapors fof 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 'ho't spots exist which produce greater boiling activity in certain portions of the liquid. The liquid does not release the vapors in a steady fashion. The result is erratic and inefficient operation of the pump. V

In copending application Serial No. 649,890, filed April 1, 1957, under the names of George H. Bancroft, .Gordon Gerow, and Donald Stevenson, and assigned to the same assignee as the present invention, there is described 'an supplies heat more uniformly through thevolume of the liquid. The heater therein described includes acen; trally positioned cartridge type heater which projects up through the bottom of thepump, with fins attached thereto to distribute theheat through the liquid; The vapors are heated additionally after they leave the surface of the liquid by the heating fins which project upwardly above the liquid surface. j J

The present invention provides an. improved heater for a diffusion pump which increases the turbulence of the liquid and greatly increases the, energy with which the vapor is discharged from the liquid surface and sent up the central stack of the nozzle assembly. The present inventio'n is particularly applicable to large-capacity' pump; in which at least ,one,butpreferably a plurality of, cartridge-type heating elements project upwardly from projecting outwardly from the heating elements both below and above the surface of the liquid charge in the bottom of the pump. Each of the finned heating elements is surrounded by a hollow cylindrical memberwhich projects upwardly from the bottom of the pump above the surface of the liquid. The cylindrical mem-' bers localize the heating of the liquid, causing strong convection currents, both in the liquid and in the vapor rising above the surface thereof. The result is greater agitation of the surface of the liquid, giving rise to a more uniform and continuous discharge of vapor, and

greater velo'city of the vapor molecules in the nozzle stack, giving rise to increased pumping speed and efficiency.

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

Fig. 1 is a side elevational view, partially in section, of a diffusion pump incorporating the improved heater of the present invention;

Fig. 2 is a sectional view taken substantially on the line 22 of Fig. l; v

Fig. 3 is a fragmentary sectional view of a modified form of the heater; Fig. 4 is a sectional view taken substantially on the line 44 of Fig. 3;

Fig. 5 is a fragmentary sectional view of a further modification of the heater; and e Fig. 6 is a sectional view taken substantially on the line 6-6 of Fig. 5. i

Referring to the form of the invention as shown in Figs. 1 and 2, the numeral 10 indicates generally the housing of the difiusion pump which includes a hollow cylindrical portion with a press-formed botom wall l4'. The bottom wall 14 is brazed, welded, or otherwise'secured to the bottom edge of the hollow cylindrical portion 12 to provide an hermetic seal therebetween. The

center portion of the bottom wall is raised to form an annular recess 15 with the cylindrical Wall portion 12 around the inside of the joint between the bottom wall and the side wall.

The upper end of the pump is provided with a suitable flange 18 to which a cooperating flange fo'rming a part of the system being evacuated may be secured. .The inlet to the pump is provided by a centrally'located opening through the flange 18 communicating width the interior of the housing 10.

The interior of the pump includes a suitable nozzle assembly, indicated generally at 20. The nozzle assembly may be of conventional design comprising a hollow central sectional nozzle stack having overlapping sections forming annular downwardly-directed orifices, such as indicated at 22, 24, and 26." Vapors rising in the center stack of the nozzle assembly 20 are directed out of the respective orifices in downwardly directed nozzles. The lower-cylindrical end 28 of'the nozzle assembly 20 is secured to and supported by a vapor-confining member 30 which extends outwardly substantially to the outer cylindrical wall12 and then downwardly to the bottom wall 14, the, vapor-confining member at its lower edge rests in the annular recess 15. Openings 32 ,are provided around the lower'edge of the vapor-confining member to permit the flow of condensed oil vapors to return to the pool of liquid in the bottom o'fthe pump. The'level of oil in the bottom of the pump is indicated by the dotted line 34.

An internal condensing wall, such .as formed-bya tapered metal sleeve 36, is secured at its'upper end to the flange 18. .The vapors directed downwardly out ofqthe nozzle orifices hit the condensing wall and condense into "oil droplets which run down to the vapor-confining Patented July "5, 1 960 member 30 and then ,nutwardly and downwardly to return to the oil charge. The condensing wall 35 may be cooled if desired by suitable cooling means such as watercooling coils (not shown) wrapped around the condensaswal it The h ater as embly to wh h Pre nt n s tien. is partieulanly directed; is located in the lower end of the honsing 1 for heating the liquid charge therein and generating vapors for operation of the pump nozzles. In the form of the invention shown in Figs. 1 and 2, the bottom wall 14 is provided with a pair of cup-shaped cylindrical recesses 38 and 40. Hollow casings, such as those indicated at 42 and '44, project upwardly in the recesses .58 and 40, the hollow casings being closed off at their upper ends and being hermetically sealed to the bottom wall 1.4 where they project through the bottom of the recessed portions 38 and 40 thereof; Each of the

Casings

42 and 44* have secured thereto a plurality of radial fins "46 which project at their upper ends above the urf c 'the qu d- Cartridge heaters 48 and 50 are positioned inside the hollow casings '42 and 44 respectively. Such type heaters are'well known and may be obtained in a great range of sizes. The cartridge heaters include outer metallic walls which fit snugly inside the hollow casings to provide heat conduction between the cartridge heaters and the casings and associated fins. Leads for connecting the cartridge heaters to a suitable source of electrical power (not shown) are brought out of the bottom of the casings.

Each of the heaters is surrounded by a hollow cylindrical pipe, such as indicated at 52 and 54, which projects upwardly from the bottom of the recessed regions 38 and in the bottom wall 14. The lower end of the pipes 52 and 54 are provided with openings 56 which permit free flow of oil into the interior regions of the pipes.

The upper end of thepipes 52 are tapered to a smaller diameter and extend above the heater casing and associated fins. Additional centrally-positioned cylindrical membersSS and 6ft are positioned above the heaters and are supported from the pipes 52 and 54 respectively, as by radial supports 62.

Figs. :3 and 4 show a modified form .of the multiple heater arrangement described above in connection with Figs. 1 and 2. In this form of the invention the heaters are not recessed, but rather, the bottom wall 14 is made relatively flat. Casings 42' and "44- extend up through the flat bottom Wall and-are provided with radially extending fins, in the same manner as the heating arrangement of Figs. 1 and 2. The surrounding cylinders are provided by metallic cup-shaped

members

64 and 66,

with the bottoms of the cup-shaped members extending under the fins of the heaters. The sides in turn extend up and surround the fins of the respective heaters. The open tops of the cup-shaped members extend above the fluid level but do not extend above the tops of the heaters. Holes 67 permit fluid to flow into the

members

64 and 66. This arrangement is easier and less expensive to construct than that of Fig. 1.

The use of pipes or cylindrical enclosures surrounding each of the heaters has been found most effective in multiple heater units. In multiple heating units without the cylindrical skirts it has been found, for. example, that at high surface temperature of the heaters where they project above the fluid level, decomposition of the fluid results. However, with the addition of the skirts surrounding each of the heaters, the concentration of vapors within the, direct vicinity of the heater causes the heat to be dissipated at a rate which prevents burning, and decomposition of the fluid, and at the same time increases the pumping speed. The improved heaters with the skirts have. also been found 'to improve the operation 'of pumps using a single finnedcartridge heater arrangement, by increasing turbulencethrough the formation of strong convection currents in' the 'fluid Asingle heater unit of this type is shown in Figs. 5 and 6. The diffusion pump of Figs. 5

and 6 may be essentially identical to that described in the above-mentioned copending application in which the housing consists of an outer cylindrical wall 70 and an enclosing flat bottom wall 72. The heater casing 74 extends through the bottom wall 72 and is provided with a plurality of heat-conduetive radially extending fins 76. The er end o th i fig e ind ca d a 7 rests on the wall 72. The'cartridge heater- 78 extends up into the-casing 7'4 and is secured into position by a set screw 89 extending through a collar 82 welded to the bottom wall 72. A cylindrical skirt 84 surrounds the heater with its fins 76, the cylindrical skirt 84 extending above the normal fluid level indicated by the dotted line 86. The cylindrical skirt 84 is provided with openings 88 adjacent the lowerxedge thereof through which the oil enters into the interior region of the skirt 84 adjacent the heater. This design also results in greater effective heating by reducing the amount of heat transferred to the outer wall of the pump. and to the wall of the nozzle assembly.

The heater arrangement, whether in single or multiple units, permits greater watts input to be supplied to the pump without decomposition of the oil charge. The result is significantly faster pumping. The heater arrangement of the present invention has been found to produce violent agitation, providing better heat distribution through the liquid and preventing decomposition due to localized hot spots.

What is claimed is:

1'. A diffusion pump comprising an outer sealed housing having input and output conduit connections thereto, a hollow nozzle assembly positioned vertically in the housing and open at the lower end thereof; a liquid charge in the housing; and means for boiling the liquid to continuously generate vapor within the nozzle assembly including a plurality of hollow casings projecting upwardly through the liquid from the bottom of the housing, a plurality of heat conductive fins joined to each of the casings and projecting radially outwardly from the casings, the fins extending above the surface of the liquid,

and a cartridge-type heater within each of the hollow casings; and a plurality of hollow cylindrical members, one of-said members surrounding each of the casings and associated fins, the cylindrical members projecting above the liquid surface and being-formed with an opening in the wall thereof below the surface.

2. Apparatus as defined in claim 1 wherein the cylindrical members each include two. sections of different diameter, theuppermost section being of smaller diameter and concentrically supported from the larger diameter section.

3. A diffusion pump comprising an outer sealed housing, having input and output conduit connections thereto, a hollow nozzle assembly positioned vertically in the housing and open at the lower end thereof; a liquid charge in the housing; and means for boiling the liquid to continuously generate vapor within the nozzle assembly in-- cluding a plurality of hollow casings projecting upwardly through, the liquid from the bottom of the housing, a plurality of heat conductive fins joined to each of the casings and projecting radially outwardly from the casings, the fins extending above the surface of the liquid,

and heater means within each of the hollow casings; and a plurality of hollow cylindrical members, one of said members surrounding each of the. casings and associated fins, the cylindrical members projecting above the liquid surface and being formed with an opening in the wall thereof below the surface.

4. In a diffusion pump having a sealed outer housing and a hollow nozzle assembly located in a vertical position within thehousing, heater means for continuously producing a how of vapor into the nozzle. assemblyfrom a liquid charge in the bottom of the housing, comprising a plurality of rod-like: heaters projecting upwardly i no,"

from the bottom of the housing through the liquid, a plurality of outwardly extending heat-conductive fins secured to each of the heaters for distributing the heat throughout the liquid, and a plurality of hollow cylindrical members, one of said members surrounding each of the heaters and associated fins, the cylindrical members projecting above the surface of the liquid and being formed with an opening in the wall thereof below the surface.

5. Apparatus as defined in claim 4 wherein the cylindrical members each include two sections of different diameter, the uppermost section being of smaller diameter and being concentrically supported from the larger diameter section.

6. In a diffusion pump, having a sealed outer housing and a hollow nozzle assembly located in a vertical position within the housing, heater means for continuously producing a flow of vapor into the nozzle assembly from a liquid charge in the bottom of the housing, comprising at least one heater element having a vertical wall projecting upwardly from the bottom of the housing through the liquid, a plurality of outwardly extending heatconductive fins secured to the heater element for distributing the heat throughout the liquid, and a hollow cylindrical member surrounding the heater element and associated fins, the cylindrical member projecting above the surface of the liquid and being formed with an opening in the wall thereof below the surface.

7. In a difiusion pump having an outer housing and a hollownozzle assembly located in a vertical position within the housing, heater means for continuously producing a flow of vapor into the nozzle assembly from a liquid charge in the bottom of the housing, comprising at least one heater projecting upwardly from the bottom of the housing through the liquid, a plurality of outwardly extending heat-conductive fins secured to the heater for distributing the heat throughout the liquid, and a hollow cylindrical member surrounding the heater and associated fins, the cylindrical member projecting above the surface of the liquid and being formed with an opening in the wall thereof below the surface.

8. In a diffusion pump having an outer housing and a hollow nozzle assembly located in a vertical position within the housing, heater means for continuously producing a flow of vapor into the nozzle assembly from a liquid charge in the bottom of the housing, comprising a plurality of heater elements projecting vertically from the bottom of the housing into the region above the surface of the liquid, and confining cylindrical members surrounding each of the heater elements for confining the liquid and vapors around each of the heating elements, each of said cylindrical members being formed with an opening in the wall thereof below the surface.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain May 6,