US3686474A

US3686474A - Vacuum pumps

Info

Publication number: US3686474A
Application number: US39565A
Authority: US
Inventors: Basil Dixon Power
Original assignee: British Oxigen Ltd
Current assignee: BOC Group Ltd
Priority date: 1969-05-27
Filing date: 1970-05-21
Publication date: 1972-08-22
Anticipated expiration: 1989-08-22
Also published as: GB1313138A; FR2048853A5; DE2025465A1

Abstract

A vapor vacuum pump includes an electrical vaporiser unit made of thin filaments of electroconductive material arranged to convey between them by capillary action the working liquid of the pump, for vaporization by heat generated by resistance of the filaments to passage of a current of electricity passing therethrough.

Description

United States Patent Power [54] VACUUM PUMPS [72] Inventor: Basil Dixon Power, Horsham, En-

gland v [73] Assignee: The British Oxygen 0 Company Limited, London, England 221 Filed: May21, 1970 21 App1.No.: 39,565

[30] Foreign Application Priority Data May 27, 1969 Great Britain.. ...,....26,829/69 [52] Cl. 219/271, 2l9/275,4l7/52,

[51] Int. Cl ..'.Q ..F22b H28 [58] Field of Search ..219/271, 274, 275, 316, 307, 219/319, 352, 538-539; 417/51, 52,150, 152, 155, 154, 208; 338/208 [56] 7 References Cited I UNITED STATES PATENTS 3,344,257 9/1967 Moeller ..338/208 x 3,346,718. 10/1967 Cooleyetal ..219/319X 115 3,686,474 1451 Aug. 22, 1972 2,140,516 12/1938 Cowan ..219/274 3,344,979 ,10/1967 Chester ..417/15 2,501,276 3/1950 .l-lickman ..417/1s2 FOREIGN PATENTS OR APPLICATIONS 814,547 6/1969 Canada ..219/3o7 96,788 12/1938 016611361616, ..219/274 OTHER-PUBLICATIONS instant Hot Water Flows from Cold-Water Pipe, y 1

Popular Science July, 1961, pp. 44- 46 Primary Examiner-C. L. Albritton Attorney-Dennison, Dennison, Townshend & Meserole ABSTRACT A vapor vacuum pump includes an electrical vaporiser A unit made of thin filaments of electroconductive -material arranged to convey between them by capillary action the working liquid of the pump, for vaporization by heat generated by resistance of the filaments to passage of a current of electricity passing therethrough.

8 Claims, 5 Drawing Figures Patented Aug. 22, 1972 3,686,474

2 Sheets-Sheet 2 VACUUM PUMPS Although described particularly with reference to oil as the working fluid, the vapor vacuum pump of the present invention could be used with water or even mercury as the working fluid, if the problem of the electroconductivity of the liquid could be overcome.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings, in which;

vacuum pump in which the warm-up time is considerably reduced compared with known pumps.

Accordingly the present invention provides a vapor vacuum pump which is as claimed in the appended claims.

By bundle in this specification is meant both a col lection or fascicle of filaments having its shape determined by external constraints, and an assembly of braided or interwoven filaments having a self-defined shape. I

The filaments may all be in parallel with each other and with the longitudinal axis of the bundle, or they may be twisted or interwoven. The bundle may be formed of two ormore layers-of filaments, in which the filaments in each layer are of different composition, structure or assembly. 1

The electroconductive filaments may be made of a resistive metal or of insulation material coatedwith resistive material. The electroconductive filaments may be combined with non-conductive filaments in the same bundle, and the proportion, size or other parameters of the non-conductive and/or electroconductive filaments can be chosen so that the composite bundle presents a'desired resistance of the flow of electrical current along the length of the bundle. By this means, when electric current is passed along the length of the bundle, Joule heat is released which is effective to vaporize liquid flowing along the length of the bundle.

The cross-sectional shape of the bundle is immaterial, although particular shapes and configuration may be desired for some applications.

. Because the filaments composing the bundle are fine, they present a large surface area which is contacted by the liquid, so that when the surfaces of the filaments become heated by Joule heating there is only a relatively small temperature difference between the surfaces of the filaments and the liquid flowing along the bundle. In addition, the closely spaced filaments provide a large number of capillary ducts which extend partially or completely along the length of the bundle so as to result in the rapid distribution through the bundle of the liquid to be vaporized. When capillary forces are relied on, then the degree of compaction of the bundle is such as to provide the required capillary eflect while at the same time permitting the resultant vapor to pass to the exterior of the bundle without applying disruptive forces to the bundle.

By a proper arrangement of the geometry of the bundle of filaments and the rate of supply of the liquid, or other parameters, it is possible to achieve complete vaporization of the liquid within the bundle so that the vapor, in passing through the outer portions of the bundle before leaving the heating element, can be superheated to some extent, which might be advantageous in some applications.

FIG. 1 is a diagrammatic view of a vapor vacuum pump of the present invention in which, the bulk of the operating liquid is located remotely from the vaporizer;

FIG. 2 is a diagrammatic view of a large second vapor vacuum pump of the invention;

. siderable advantage that only-a small fraction of the FIG. 3 is a diagrammatic axial view of a helical vaporizer for use in the pump shown in FIG. 2;

FIG. 4 is a diagrammatic sectional view of a form of vaporizer which is an alternative to that form illustrated in FIG. 2 or FIG. 3, and

FIG. 5 represents axial sections of four different forms of vaporizer for use in pumps of the present invention.

In the pump shown in FIG. 1, the vapor condensed on the inner walls of a water-cooled casing 2 flows into a reservoir 4 from which it flows at a controlled rate through a valve 6 to a vaporizer 8 which is adapted to be force-fed (in this case under the force of gravity) with the liquid to be vaporized. In this way a hydrostatic head is provided to carry the operating liquid into the vaporizer 8.

The vapor produced by the vaporizer 8 is emitted from

nozzles

10 and 12 usual with vapor vacuum pumps. The vapor entrains molecules of the gas being evacuated from a system coupled to a flange 14 at the upper end of casing 2. After the vapor has condensed on the inner surfaces of casing 2 the pumped gas leaves thepump by way of an outlet 16, which may be connected to the inlet of a second pump (not shown) connected in cascade and using the same or a different working fluid.

The pumps of the present invention have the conworking liquid is being heated at any one time. This enables the pump to be fully operational shortly after being energized, and to be opened to the atmosphere,-

without excessive degradation of the working fluid, shortly after it is deenergized. This latter facility is particularly important in that it dispenses with the need for the isolation valve which is usually provided for conventional vapor vacuum pumps. This is significant because such valves can cost as much as the pump itself.

In order to cool the pump quickly, the valve 6 is either opened fully to flood the vaporizer 8 with relatively cold liquid after the supply of heating current has been stopped, or closed to stop further liquid reaching the vaporizer, closure taking place shortly before the flow of heating current is stopped. With either mode of shut-down, the vaporizer stops emitting vapor relatively soon after the pump is deenergized.

' In the pump shown in FIG. 2, a vaporizer 8 is incorporated in a vapor nozzle 18. Liquid to be vaporizer is v upper end of the vaporizer 8 passes downwardly through it under both gravitational and capillary forces. The vaporizer 8 is electrically heated, although the cables by which the heating current is supplied are omitted from the drawing for clarity. The resultant vapor issuing from the external surfaces of the vaporizer 8 emerges from the nozzle 18 through a discharge opening 24. In operation, the rate at which liquid is supplied to the nozzle is closely related to the amount of electrical power supplied to it so that vapor of the right quality emerges from the outlet 24.

The vaporizer 8 shown in FIG. 3 may be used to replace the longitudinal filament type shown in FIG. 2. In FIG. 3 the bundle 26 of filaments is of rectangular cross-section and is wrapped in a helical groove in a support 28. The base of the groove is in communication with an axial passage 30 by a series of passages 32. Liquid enters the upper end of the axial passage 30 through an inlet 34 and then passes into the interior of the bundle 26 through the passages 32. Although the passages 32 communicate with the bundle at discrete intervals, capillary forces are effective to saturate the bundle 26 uniformly with the liquid to be vaporized. By means which are not illustrated, current is passed along the length of the bundle 26. Either the bundle 26 is insulated from the support 28 or the support is of insulation material.

The vaporizer 8 shown in FIG. 4 is an alternative to that illustrated in FIG. 3. The main difference is that the cross-section of the bundle of filaments alters along the length thereof. Thus the liquid entering the inlet 34 passes into the splayed-out upper end of the bundle 26. The overall cross-sectional area of the bundle decreases as the liquid descends so that the amount of heat released per unit volume of the bundle increases as the cross-sectional area of the bundle is restricted. When the liquid is vaporized its volume increases considerably and the splayed-out lower end of the bundle 26 enables the vapor to escape easily from the interior of the bundle without exerting disruptive forces thereon. The vapor then emerges from the outlet 24 and functions conventionally. The means by which the bundle 26 is heated are again not illustrated for clarity.

FIG. (a) shows an axial section of one form of vaporizer for a pump of the present invention. It includes two

end plates

36 and 38 between which extends a plurality of parallel longitudinal filaments-40. The bundle of filaments is constrained to be of cylindrical form for the major portion of its length, but towards the bottom of the vaporizer the filaments are splayed out.

' The splayed-out end 42 is provided to increase the spacing between the filaments and thus enable the liquid to be vaporized to enter the interstitial ducts more easily than if the lower end were kept tightly compacted. Although it is difficult to illustrate, all the filaments 40 are intended to be of electroconductive material, the

plates

36 and 38 acting as electrodes by which heating current can be passed along the length of the bundle of filaments.

In the form of heating element shown in FIG. 5(b) (which, like FIGS. 5(a) and 5(d) is a scrap sectional view of the cylindrical portion of a bundle shaped similarly to that illustrated in FIG. 5(a)) the illustrated broken lines 44 are intended to represent filaments of electrical insulation material which are non-conducfilaments 44 which in turn is enclosed by a layer of conductive filaments 40.

In the arrangement shown in FIG. 5(d) the hollow A core 46 isv enclosed by a hollow cylindrical layer of conductive filaments 40 which is enclosed by a layer of non-conductive elements 44. Both the arrangements shown in FIGS. 5(c) and 5(d) provide a passage by which liquid to be vaporized can conveniently be introduced into the interior of the bundle so that complete reliance is not placed on capillary forces to introduce the liquid into the vaporizer. V

For ease of illustration the

filaments

40 and 44 in the vaporizers illustrated in FIG. 5 have been shown as being parallel to each other and to the axis of the bundle. In practice the filaments could be arranged in'helically twisted layers of opposite hand, or given a simple helical twist throughout, or be of interwoven construction. Any suitable arrangement can be used to retain the desired physical integrity of the bundle while providing a conductor of the desired cross-sectional area and electrical resistivity.

One suitable material for the conductive filaments 40 is an electrical resistance wire such as Nichrome (trade mark). Suitable non-conductive filaments can be of glass or other insulation material.

Any of the illustrated forms of vaporizer may be connected electrically in series, but the inlet and outlet of the vaporizers are preferably connected in parallel between a-common source of liquid to be vaporized and a plenum chamber or parallel arrangement of vapor nozzles. Such an arrangement permits a wide choice of systems for supplying heating current to the vaporizer while ensuring that each vaporizer functions as intended.

Pumps like that illustrated in FIG. 2 can .be arranged in many different ways which do not in themselves form part of the subject matter of this invention. In some cases more than one nozzle can be supplied from a single vaporizer. In some cases it is preferable to use separate vaporizers (and possibly separate reservoirs) for pumping stages which may be connected to pump in series. This makes it possible to use different pumping liquids in the various stages so that, for instance, the stage nearest the system being evacuated may employ less-volatile liquid (which is advantageous for. the

production of the lowest possible pressure), whereas the stage nearest the pump outlet may employ a-morevolatile liquid, since this is advantageous for compressing the pumped gases to the greatest possible ex- I tent before they are discharged from the system.

An advantage of the present invention is that it is necessary only for the element itself and for a small quantity of liquid to be raised to boiling temperature. Because of the small mass to be heated, very rapid warm-up is possible. Pumps can be designed so that only a low-mass nozzle structure has to be heated by the vapor generated. Pumps can also be designed to stop emitting vapor very rapidly, by either continuing to supply liquid after the heating current is stopped or stopping the flow of the liquid before the heating current is stopped.

Because of the very large surface area presented by the filaments, the temperature difference between the filaments and the liquid flowing along the capillary ducts formed by the interstitial spaces between the filaments can be very small, while still enabling the required amount of energy to be transferred to the liquid. The heating elements can also be designed quite conveniently to provide a degree of superheating of the vapor by suitable choice of the various parameters of the heating element.

Iclaim:

l. A vapor vacuum pump including an electrically heated vaporizer positioned remotely from a reservoir of working liquid and adapted to be supplied with the liquid under pressure, in which the vaporizer includes a bundle of thin filaments which are electro-conductive so that the bundle can vaporize liquid by Joule heat released over a very large surface area.

2. A vapor vacuum pump as claimed in claim 1 in which the filaments are compacted to define interstitial spaces therebetween and so that said interstitial spaces are longitudinally extending and are of size small enough for the liquid to be vaporized to be drawn into the interior of the vaporizer at least partially by capillary forces.

3. A vapor vacuum pump as claimed in claim 2, in which the vaporizer includes separate and contiguous layers of conductive and non-conductive filaments.

4. A vapor vacuum pump as claimed in claim 3, in which the layers of filaments are circular or annular in cross-section.

5. A vapor vacuum pump as claimed in claim 3, in which the cross-sectional area of the bundle varies along its length.

6. A vapor vacuum pump as claimed in claim 1, in which the bundle has a hollow core into which is introduced the liquid to be vaporized.

7. A vapor vacuum pump as claimed in claim '1, in which the bundle of filaments is positioned in a helical groove in the surface of a support of insulation material, the bundle being in communication at a plurality of points spaced along its length with a hollow passage in the interior of the support for the liquid to be vaporized.

8. A vapor vacuum pump as claimed in claim 1, in which the bundle of filaments is positioned in a housing having an inlet for liquid at one end, and having an outlet for vapor at its other end.

Claims

1. A vapor vacuum pump including an electrically heated vaporizer positioned remotely from a reservoir of working liquid and adapted to be supplied with the liquid under pressure, in which the vaporizer includes a bundle of thin filaments which are electro-conductive so that the bundle can vaporize liquid by Joule heat released over a very large surface area.

7. A vapor vacuum pump as claimed in claim 1, in which the bundle of filaments is positioned in a helical groove in the surface of a support of insulation material, the bundle being in communication at a plurality of points spaced along its length with a hollow passage in the interior of the support for the liquid to be vaporized.