US3332609A - Vapour vacuum pumps - Google Patents

Description

July 25, 1967 B. D. POWER VAPOUR VACUUM PUMPS Filed June '7, 1965 {VENT/NG CLEARANCE Hmmm IIQUEN Ildlns BAJIL D, Paw/ER' IuvELn-oa -rfrorawsw United States Patent 3,332,609 VAPOUR VACUUM PUMPS Basil Dixon Power, Hursham, England, assignor to Edwards High Vacuum International Limited, Crawley, England, a company of Great Britain Filed .lune 7, 1965, Ser. No. 461,739 Claims priority, application Great Britain, .lune 8, 1964, 23,624/ 64 4 Claims. (Cl. 230-101) This invention relates to vapour vacuum pumps and has for its object to provide a construction of pump which reduces or eliminates certain undesirable effects present in known forms of vapour vacuum pump arising when condensed vapour returns to the boiler of the pump.

In many conventional vapour vacuum pumps employing oily substances as working fluids, the boiling space where working vapour is generated is enclosed by the base of the main pump body and vby a bottom portion of a composite interior hollow structure through which the generated vapour ascends to emerge from the nozzles which -produce the desired pumping action. The bottom portion mentioned will be referred to as the boiler cover.

Having emerged from the nozzles and performed its pumping function, the working vapour condenses on the cooled interior walls of the main pump body and runs down into a gap formed between the body and the boiler cover and from there re-enters the boiling space through apertures at the bottom of the interior structure. The boiler cover thus functions as a wall which separates the hot ascending vapour inside the interior structure from the condensed returning liquid outside that structure and, over large regions of the inside and outside surfaces of the boiler cover hot vapour and condensed liquid respectively are simultaneously owing. Because only the boiler cover wall separates the vapour from the liquid there is danger of undesirably great heat transfer from the vapour to the liquid by conduction through the wall. Moderate heat transfer is acceptable, since it serves t-o pre-heat the returning liquid and also to vapourize any volatile impurity content in the liquid and prevent its return to the boiler. Too great heat transfer causes both copious condensationA of working vapour inside the boiler and excessive vaporization of the returning oondensed liquid. Vapour generated from the returning liquid outside the boiler space opposes the main pumping action and then re-condenses on the cooled pump walls to run down again into the gap between body and interior where it may again be re-vaporized. Considerable heat is thus robbed from the boiler and used to generate vap-our opposing the main pumping action.

It is clearly desirable to limit and control the overall thermal conductivity of the wall of the boiler cover but it is usually convenient to make this portion of a common oonstructional metal, such as copper, aluminium or stainless steel, all of which have too high thermal conductivity to permit easy control merely by selection of material. Other measures which might be adopted involve significant manufacturing cost or other disadvantages.

Thus a double or cavity -boiler cover wall might be used. Ideally such a cavity wall would be sealed against ingress or egress of liquid or vapour, but to ensure adequate sealing for the cavity to be completely vacuum tight, so that it cannot become a source for gas leakage from it into the vacuum system, would be undesirably expensive. Conversely, if the cavity is freely vented circumferentially at the top of the wall so that contained air is rapidly extracted during initial evacuation, oil

3,332,609 Patented July 25, 1967 lCe vapour, will tend to condense inside it and then to reevaporate, providing 4opposing vapour streams and robbing heat from the boiler as discussed earlier.

The present invention provides a simple economical structural arrangement which facilitates achievement of adequate control of the overall thermal conductivity between vapour inside the boiler and liquid in contact with it outside.

Thus, according to the invention there is provided a cavity wall having a cavity or cavities in restricted communication either with the region inside the interior structure or with the region outside the interior structure. The restricted communication permits the reasonably rapid removal of air from the cavity during initial evacuation, and inevitably permits vapour to enter the cavity, where it may condense to liquid, or liquid itself to enter. The communicating passages are not, however, large enough to give rise to significant undersirable vapour streams generated from liquid which has entered the cavities.

It is accepted that the cavities may tend to run full of oily liquid, or many run partly full, containing a mixture of liquid and vapour with some fluid seeping in and some emerging but, in any case, the heat transfer through the cavity can 4be made -very greatly less than than through a solid metal wall. The thermal conductivity of a typical vapour pump oil is 0.0003 gm.Cal/cm.2/ C./ cm., whilst that of a low conductivity metal, like stainless Isteel, may be about 0.035 gm.Cal/cm.2/ C./cm., so that an `oil filled cavity provides equivalent resistance to heat ii-ow to a stainless steel wall over times as thick. Thermal transfer through vapour or by vapour reux within the cavity is likely to be less than transfer through liquid.

Thermal conductivity through the cavity cannot therefore exceed the conductivity through an equivalent area and thickness of the oily liquid used as lpump fluid, and overall conductivity can be controlled by varying the extent of and the cross-dimensions of the lcavity or cavities. Alternative constructions of boiler cover walls embodying the invention will now be described in greater detail by way Iof example wit-h reference to the accompanying drawings in which:

FIGURE 1 is a diagrammatic sectional elevation showing part of a vapour vacuum diffusion pump including one such construction;

FIGURE 1a is a partially cut away exaggerated detail thereof illustrating certain clearances to an enlarged scale;

FIGURE 1b is an enlarged detail of an alternative groove venting construction; and

FiGURES 2 and 3 each show diagrammatically alternative constructions.

Referring to the drawings, a vapour vacuum pump indicated generally at 1 has a boiler containing a working uid 2. The main wall 3 of the pump is provided with a water cooling jacket 4 and a duct 5 leads to a backing pump in the usual manner. The interior structure or main vapour tube 6 is ared at its lower end 7 and terminates in a skirt or boiler cover portion indicated generally at 8. Condensate returning to the boiler is shown at 9.

In applying the present invention, the outer surface of the boiler cover wall 8 is formed with a series of wide shallow circumferential grooves 10. A thin tight fitting tube 11 is fitted over the outer surface 12 of the wall 8 so that the grooves 10 become almost completely enclosed cavities. The construction is, however, such that adequate venting of the cavities to the exterior of the wall 8 is provided and for this purpose the it of the tube 11 is made such that suficient clearances are pro- 3 vided to ensure the desired venting, as illustrated in FIGURE la. Alternatively, small vents may be formed in the portions of the tube covering the grooves, as illustrated at 16, FIGURE lb. The function of the tube may be served by a thin sheet of metal tightly wrapped around the surface 12 and spot Welded in position, as illustrated at 15, FIGURE la.

Referring now to FIGURE 2, the cavities produced by the grooves 1) and tube 11 are formed in a manner generally similar to that described with reference to FIGURE 1 in this case the cavities are vented towards the interior of the Wall 8. In the further modication shown in FIGURE 3 a spun or shaped outer cover 13 co-operates with a similarly shaped boiler cover 8 to provide cavities of which one extends up and over the shoulder of the boiler cover indicated generally at 14. This shoulder may, in operation of the pump be washed by oily liquid returning to the pump boiler.

Iclaim:

1. A vacuum vapour diifusion pum-p -of the type comprising in combination:

(a) an outer pump body;

(b) an interior vapour tube for passage of pumping fluid toat least one pump jet; and

(c) a boiler delined 4by a base portion of said pump body, a lower portion of said interior vapour tube and a substantially cylindrical portion depending from said lower portion to allow access of pumping uid under said cylindrical portion from an annular region outside said cylindrical portion to said boiler;

said pump particularly characterized in that said cylindrical portion is formed with at least one cavity in restricted communication with a region adjacent one of its surfaces only to allow reasonably rapid removal of air from the cavity during initial operation of the pump and thereafter to permit limited entry of pumping iiuid to said cavity from said region.

2. A pump according to claim 1 in which said region adjacent one of the surfaces of said substantially cylindrical portion is said annular region.

3. A pump according to claim 1 in which said substantially cylindrical portion comprises:

(l) a rst tubular member provided with at least one circumferential groove, and

(2) a second tubular member attached to said irst member to cover said groove; said restricted communication being provided between said first and second members along their juxtaposed surfaces.

4. A pump according to claim 3 in which said rst and second members are each formed with an upper portion inclined with respect t-o the axis of said vapour tube.

References Cited UNITED STATES PATENTS 2,249,450 7/1941 Bancroft et al. 230--101 2,404,021 7/1946 Alexander et al. 230-101 2,438,395 3/1948 Hopper 230-101 FOREIGN PATENTS 664,121 8/1929' France. 684,470 12/1952 Great Britain.

DONLEY I. STOCKING, Primary Examiner.

W. J. KRAUSS, Assistant Examiner.

Claims (1)

1. A VACUUM VAPOR DIFFUSION PUMP OF THE TYPE COMPRISING IN COMBINATION: (A) AN OUTER PUMP BODY; (B) AN INTERIOR VAPOUR TUBE FOR PASSAGE OF PUMPING FLUID TO AT LEAST ONE PUMP JET; AND (C) A BOILER DEFINED BY A BASE PORTION OF SAID PUMP BODY, A LOWER PORTION OF SAID INTERIOR VAPOUR TUBE AND A SUBSTANTIALLY CYLINDRICAL PORTION DEPENDING FROM SAID LOWER PORTION TO ALLOW ACCESS OF PUMPING FLUID UNDER SAID CYLINDRICAL PORTION FROM AN ANNULAR REGION OUTSIDE CYLINDRICAL PORTION TO SAID BOILER; SAID PUMP PARTICULARLY CHARACTERIZED IN THAT SAID CYLINDRICAL PORTION IS FORMED WITH AT LEAST ONE CAVITY IN RESTRICTED COMMUNICATION WITH A REGION ADJACENT ONE OF ITS SURFACES ONLY TO ALLOW REASONABLY RAPID REMOVAL OF AIR FROM THE CAVITY DURING INITIAL OPERATION OF THE PUMP AND THEREAFTER TO PERMIT LIMITED ENTRY OF PUMPING FLUID TO SAID CAVITY FROM SAID REGION.

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