US3781518A

US3781518A - Vacuum pumps

Info

Publication number: US3781518A
Application number: US00267413A
Authority: US
Inventors: B Power; R Oswald; D Tooth
Original assignee: British Oxigen Ltd
Current assignee: BOC Group Ltd; UK Atomic Energy Authority
Priority date: 1972-06-29
Filing date: 1972-06-29
Publication date: 1973-12-25
Anticipated expiration: 1990-12-25

Abstract

An electrically heated vaporiser particularly for a vapour vacuum pump, comprising a porous electrical heating element through which the liquid to be vaporised is pumped and having means for retaining the mechanical integrity of the heating element despite thermal movements and for reducing heat losses.

Description

United States Patent [191 Power et al.

[ Dec. 25, 1973 VACUUM PUMPS Inventors: Basil Dixon Power; Roger Derek Oswald; David Michael Tooth, all of Crawley, England The British Oxygen Company Limited, London, England Filed: June 29, 1972 Appl. No.: 267,413

Assignee:

U.S. Cl 219/271, 219/275, 417/152, 417/208 Int. Cl. F221) 1/28 Field of Search 219/271, 272, 273,

References Cited UNITED STATES PATENTS 8/1972 Rice et al 219/275 X Pope 219/381 x Power 219/275 x Primary Examiner-C. L. Albritton AtmrneyRobert l. Dennison et al.

[ 5 7] ABSTRACT 5 Claims, 1 Drawing Figure VACUUM PUMPS FIELD OF THE INVENTION This invention relates to a vapour generator and more particularly to an electrically heated vaporiser for producing a vapour stream for vacuum pumps.

DESCRIPTION OF THE PRIOR ART In known vapour vacuum pumps the oil or other liquid to be vaporised is heated by a heater immersed in the liquid. This has the disadvantage of having a high thermal inertia i.e. the pump takes a long time to be fully effective after the heater is first energised, and continues to operate for an appreciable period after the heater has been deenergised. In cantradistinction the present invention aims at providing a vapour vacuum pump-of low thermal inertia by the use of a vaporiser positioned adjacent to the vapour nozzles and to which the liquid to be vaporised is pumped from a separate source.

SUMMARY OF THE INVENTION According to one feature of the present invention there is provided an electrically-heated vaporiser including a central support tube down which passes the liquid to be vaporised, the lower end of said tube being gagement with the support tube, the heating element being retained under an axial compression, despite thermalmovements thereof, by spring means positioned between the said slidable collar and an adjacent collar fixed to the support tube.

Preferably the said porous electricql insulation material is made of a ceramic fibre in the form of paper.

According to another feature of the present invention there is provided an electrically-heated vaporiser in which electric current is supplied to one end of a porous heating element through an inlet tube for the liquid to be vaporised, in which the tube is encircled by a longitudinal chamber in communication with the interior of the tube through passages in its walls, the chamber being in communication with the inner surface of the heating element whereby both surfaces of the tube are cooled by the incoming liquid while flowing to the heating element.

According to yet another feature of the present invention there is provided an electrically-heated vaporiser including a tubular porous, heating element having one of its ends connected to a flange, intended to remain at room or other relatively-cold temperature, through a thingauge tubular member of low thermal conductivity, such as stainless steel and in which the flange also supports a second tubular member of like low thermal conductivity supporting a nozzle for the vaporised liquid, the two tubular members being closely spaced apart to inhibit the passage of vapour to the cold flange through the space between the two members.

According to still another feature of the present invention there is provided an electrically-heated vaporiser including a tubular, porous, heating element connected at both ends to metal caps intended to carry the heating current, in which the joints between the heating element and the caps are sealed in a fluid-tight manner by the use of gaskets of flexible electrically conductive material such graphite.

The present invention will now be described by way of example with reference to the accompanying drawing, which is a sectional view of a vaporiser of the present invention, intended for use in a vapour vacuum pump.

The vaporiser includes a central support tube 2, which is secured at its upper end, in a fluid-tight manner, to a mounting member (not shown) and to means for passing the liquid to be vaporised to the interior of the tube. Welded or otherwise secured to the tube is a collar 4 adjacent to a slidable collar 6 of insulating material, the two collars having positioned between them two or more appropriately-directed Schnorr or Belleville washers 8 which function as a spring.

Movable with collar 6 is a sleeve 10, which is sealed against the loss of liquid from the interior of the vaporiser, and is electrically insulated from the tube 2 by means of a layer 12 of plastics material.

Extending from sleeve 10 is a thin-walled tube 14 having its lower end fixed to an annular end cap 16 attached to a short, thin-walled, tube 18 of stainless steel. Tube 18 is connected at its upper end to a support flange 20. One end of an electric cable (not shown) for the heating current is clamped directly to the tube 14.

The cap 16 is in electroconductive contact with the upper end (as viewed) of a porous heating element 22 through an annular gasket 24 of flexible graphite. A particularly-suitable form of graphite is that sold under the trade name Grafoil by Union Carbide Corporation.

A similar gasket 26 is positioned between the lower end of the element and a second end cap 28 which is secured to the lower end of tube 2, by a nut 30, in a manner which does not form part of the subject-matter of the present invention and which is therefore not described in further detail.

Positioned in intimate contact with the inner surface of the heating element 22 are three layers of a porous, temperature;-resistant, material 32. A preferred material is of ceramic fibre in paper form: one particularlysuitable material is that sold under the registered trade mark Fiberfrax by the Carborundum Company Ltd. Because of the necessity that the material be in intimate contact with the inner surface of the heating element 22, a light helical spring (not shown) is positioned within chamber 34 so as to press the fibrous material outwardly into contact with element 22.

It will be seen that the sleeve 10, tube 14, end cap 16, porous material 32 and end cap 28 define the chamber 34. The liquid to be vaporised enters this chamber through passages 36 formed in the walls of tube 2. Thus the tube has its inner and outer surfaces contacted by a stream of the incoming liquid to be cooled thereby. Because the tube 2 functions as an input lead for the heating current, any Joule heat released in the tube by the passage of this current serves to preheat the liquid and which also serves to prevent or reduce the occurrence of local hot-spots in the tube 2.

Surrounding the heating element 22, and spaced from it, is a casing 38 leading to a nozzle 40 arranged to direct the emitted vapour into the interior of a vapour vacuum pump, of which the other components are conventional and are therefore not shown in the drawing. A feature to note is that the inner end of nozzle 40 extends above the end wall 42 of casing 38. This is to ensure that any unvaporised liquid which passes through the element 22, and any condensate produced in the interior of casing 38, flow through a drip tube 44 in preference to passing down the nozzle 40. The drip tube 44 preferably extends to the exterior of the pump housing, or to adjacent the exterior water-cooled surfaces thereof, so that the liquid from the tube does not interfere with operation of the pump.

The casing 38 is supported from a flange 46 which is secured to flange 20 in a fluid-tight manner and forms a connection with heating element 22. The connection is through a thin-walled tube 48 of stainless steel which is spaced radially by only a short distance from tube 18.

This spacing leaves only a small gap intended to inhibit I the passage of vapour through the gap and into contact with the cold flange 20. This thus reduces the effective loss of vapour caused by condensation thereof internally of the vaporiser.

Also secured to flange 46, by means of an intermediate tapered support 50, is a radiation shield 52. This is provided primarily to keep the enclosed boiler unit hot and to reduce recondensation of vapour inside the boiler unit, and only secondarily to prevent radiation from the heated parts of the vaporiser from falling directly on the inner surfaces of the vacuum pump. This is important because, under operating conditions, the evacuated interior of the pump favours the transference of heat by radiation rather than by either conduction or convection, so that the shield 52 acts to main tain the temperature differential between-the vaporiser and the pump housing.

' In operation it is intended that the illustrated vaporiser would be usedwith its axis vertical and oriented as shown. For other applications it may be desirable to mount the vaporiser at an acute angle to the horizontal. In such a case the basic components of the vaporiser would be unchanged, but minor changes might have to be made to the nozzle and drip tube construction to ensure, as far as possible, that no liquid issues from nozzle 40, but only vapour.

When the pump is to be operated electric current is passed down tube 2 and then flows along the length of element 22 through the annular end cap 16 to tube 14. Element 22 has the highest electrical resistance, so that it dissipates the largest amount of Joule heat. After a relatively-short time element 22 reaches such a temperature that when liquid is forced radially outwardly through its pores it is vaporised before or as it reaches the outer surface of element 22. To prevent the element 22 from being overheated it is normally arranged for the flow of heating current to be started only after starting of the pump which supplies liquid to the interior of the vaporiser. Because of the small mass of the material to be heated, the vaporiser hasonly a short warming-up period before it reaches its operating conditions. The same applies when thepump is to be stopped, for the supply of heating current is stopped a short time before the liquid pump is stopped. This ensures that the element 22 is cooled quickly by fresh liquid which is pumped through it until it is cool enough to vaporise only a negligible amount, if any, of the liquid passing through it. When this has happened the supply of liquid is stopped.

It will thus be seen that the present invention provides a vaporiser with very low thermal inertia useful for vaporising oils or other liquids which are nonconductors of electricity.

We claim:

1. An electrically heated vaporizer comprising an elongated support tube through which the liquid to be vaporized passes, a casing enclosing the lower portion of said tube, a tubular, porous electrical heating element within said casing and encircling said support tube at the lower portion thereof, means placing said electrical heating element under axial compression, at least one layer of porous insulating material in intimate contact with the inner surface of said heating element and radially spaced from said tube at the lower end thereof, a first collar member welded to said tube portion that extends outside said casing, a second collar in sliding, fluid tight engagement with a portion of said tube that extends outside said casing and below and adjacent said first collar, means connecting said electrical heating element to said second collar, and spring means between said first collar and said second collar to retain said heating element under axial compression during thermal movements.

2. An electrically heated vaporiser as claimed in claim 1 wherein said porous electrical insulation material is made of a ceramic fibre in the form of paper.

3. The vaporizer set forth in claim 1 in which said support tube comprises an inlet tube for the liquid to be vaporized, said tube having passages formed in the walls thereof for the passage of liquid therethrough, means forming a chamber encircling said tube and having a portion extending within said casing, said portion within said casing disposed within said heating element, whereby said chamber is in communication with the inner surface of said heating element to thereby cool the inner and outer surfaces of said tube as liquid is passed to said heating element.

4. The vaporizer of claim 1 and further including a metal cap secured to said tubular, porous heating element at each end thereof in electroconductive contact therewith, means for sealing the joints between the respective ends of said tubular, porous heating element and the caps secured thereto, said sealing means comprising an annular gasket of flexible electrically conductive material between each cap and the respective ends of said tubular element.

5. The vaporizer of claim 1 and further including a flange outside said casing and exposed to ambient temperature to be thereby maintained relatively cool, means interconnecting said flange to said tubular, porous electrical heating element, said interconnecting means comprising a first tubular member of low thermal conductivity connected at one end to said flange and at the other end to said heating element, and a second tubular member of low thermal conductivity connected at one end to said flange and closely spaced from said first tubular member at the flange connected end and forms a supporting means at the other end for a nozzle through which vaporized liquid is passed, said close spacing between the respective ends of the first and second tubular members inhibiting the passage of vapor to the relatively cool flange.