US2696344A

US2696344A - Vapor vacuum pump

Info

Publication number: US2696344A
Application number: US242812A
Authority: US
Inventors: Power Basil Dixon; Latham Douglas
Original assignee: W Edwards and Company London Ltd
Current assignee: Edwards High Vacuum Ltd
Priority date: 1951-08-21
Filing date: 1951-08-21
Publication date: 1954-12-07
Anticipated expiration: 1971-12-07

Description

Dec. 7, 1954 B. 0. POWER ETAL VAPOR VACUUM PUMP Filed Aug. 21, 1951 appreciated that the limiting United States Patent VAPOR VACUUM PUMP Basil Dixon Power, West Wickham, and Douglas Latham, London, England, assignors to W. Edwards & Co. (London) Limited, London, England Application August 21, 1951, Serial No. 242,812 4 Claims. (Cl. 230-101) This invention relates to vapor vacuum pumps and is more particularly, although not exclusively concerned with pumps of the common vapor vacuum type in which air to be exhausted is entrained in a stream of vapor of a suitable low vapor pressure fluid.

In one form of pump of the kind referred to, the vapor is supplied from a boiler, ascends a central tube and enters an annular jet at the top of the tube from which it issues downwards into a space between the central tube and a cooled outer tube upon which it is condensed and returns to the boiler. The upper end of the outer tube constitutes the inlet to of a pipe connecting the lower part of the annular space between the central and outer tubes to a backing pump. Air exhaustion is effected by the diffusion of the air molecules above the annular jet downwards into the stream of vapor molecules which are travelling in the same direction and at comparable speeds. Once entrained in the vapor stream, the air molecules cannot difluse back upwards in the face of the downward streaming vapor molecules, but are driven downwards and compressed towards the tube leading to the backing pump.

The lowest pressure obtainable with pumps of the kind referred to is limited by various factors which include (a) the vapor pressure of the pure operating fluid where such is used, or the vapor pressure of the most volatile constituent in the case of a mixed operating fluid, b the vapor pressure of impurities which may be present initially or are formed by thermal decomposition in the boiler, (0 gas liberated by thermal decomposition of the flu1d 1n the boiler which gas emerges from the jets into the hlgh vacuum system and (d) gas coming out of solution in the boiler and also emerging from the jets.

In connection with the last mentioned limiting factor,

it is known that when the operating fluid is condensed on the condenser provided in conventional pumps it absorbs gas at the condenser pressure, that is the backing pressure and the condenser temperature and on return of the condensed liquid to the boiler the temperature is raised and the gas is evolved.

' In the design of an efl'lcient diifusion pump, it will be factors referred to must be taken into account. Attempts have been made to reduce the limiting effect due to the vapor pressure of impuritles by the use of multistage pumps of the fractionating type in which each stage is fed from a separate boiler. In such pumps, condensed fluid is returned first to the boiler feeding the highest pressure stage and thence to bo1lers feeding the lower pressure stages. The fluid feeding the lowest pressure stage is thus passed through all the boilers and may thus be considered to have had the most volatile constituents removed. The use of this type of pump also reduces the limitation imposed by gas coming out of solution in the boiler because the fluid is outgassed in boilers feeding the higher pressure stages.

Thermal decomposition of the operating fluid should be avoided as far as possible by correct design and operating procedure but automatic fractionation of the operating fluid is the best guard against impairment of the ultimate vacuum by decomposition products. Decompos1t1on is known to be greatly increased by the presence of air which will be introduced into the boiler when condensate flows back and gas is given off as already described. The quantity of air introduced into the boiler can be reduced by reduction in the backing pressure or more conveniently by using a diflusion pump stage whlch will remove the gases from the returning fluid very thoroughly. It 1s also the pump and the outlet consists ice remove continuously any light fractions device in the backing line. further be reduced by lowdesirable to formed, by using a collecting Thermal decomposition may ering the operating temperature that is to say the boiler pressure and a high degree of performance can still be obtained with low boiler pressure providing a multistage pump is used.

Attempts so far made to fractionate in diffusion pumps have in many cases been limited to heating relatively deep pools of the operating fluid. It is however well known that separation of two liquids of differing volatility Will occur far more readily if the mixture is in the form of a thin film.

An object of the present invention is to provide a pump capable of obtaining optimum results from mixtures of fluids containing constituents, the normal tendency of which is to impair the performance of the pump.

According to the present invention a vapor vacuum pump comprises in combination a casing, a working fluid boiler, upper and lower jets within said casing and a liquid conveying surface in heat conduction relation with said boiler and in operation of the pump liquid formed by condensation of vapor ejected from an upper jet flows as a thin film down said surface, at least a portion of which is heated sufficiently to effect vaporization from said thin liquid film, the vapor produced from which tends to consist preferentially of the less phlegmatic fractions of said thin liquid film and to include any undesirable high volatile fractions which were contained in the liquid film prior to said vaporization, the vapors from said liquid film being used to feed a lower jet with the result that the return of the less phlegmatic fractions to said boiler is reduced or eliminated.

In particular applications of the invention, a lower jet of the pump may serve to eject any undesirable volatile fractions fed to it completely or substantially completely from the pump. The lower jet is preferably that nearest a backing pump and is so disposed that it discharges into a tube which serves to connect the vacuum pump with a backing pump.

In one construction of the pump according to the invention, the thin liquid film passes through a restricted passage prior to reaching the heated surface, said restricted passage being formed by the inner wall of the pump and an extension of a vapor tube which supplies vapor to said loweret, the restricted passage serving to ensure that, in operation, access of vapor from the region of the heated surface to the region of the pump above said heated surface is wholly or substantially prevented.

The invention may be applied to a multistage vapor vacuum pump having a plurality of jets, the vapors ejected gpnm which are condensed to contribute to the thin liquid In order that the invention may be more clearly understood, a pump having three jets constructed in accordance I therewith will now be described by way of example with reference to the single figure of the accompanying drawmg.

Referring to the drawing, the pump construction is similar in certain respects to known forms in that the boiler 1 has disposed above it a cylindrical casing 2 containmg a central tube 3 and lower and upper annular ets 4 and 5 respectively. Vapor not ejected from the et 4 passes up a further central tube 6 to the jet 5.

The lower end of the central tube 3 is of greater diameter than that of the main length of the tube upon which a skirt-like additional tube '7 is mounted. The tube 7 is shaped to provide a narrow annulus between the points A and B and has an auxiliary jet 8 which discharges into a mixing tube 9 leading to a pipe 10 connected with a rotary backing pump not shown.

In the drawing, double headed arrows are used to indicate vapor flow and the single headed arrows indicate liquid flow. As in normal operation, fluid is heated in the boiler 1 forming vapor which flows up the tubes 3 I and 6 and is ejected from the jets 4 and 5. The vapor is condensed to liquid on the inner wall of the casing 2 and flows downward under the action of gravity. This initial liquid may be a mixture of liquids of various boiling points. The wall of the casing 2 is air or water cooled, not necessarily uniformly, by means not shown, the cooling being applied above the point A. A helical cold water pipe may conveniently beemployed. The liquid continuing its flow, passes through the narrow annulus between A and B, the purpose of which will be described later, and reaches the'r'egion B-C.

The region B C of the wall is heated by conduction from'the-boiler 1 and by heat transfer due to condensation of vapor from the lower part'of the wall. Preferably a radiation shield, not shown, surrounds the lower part of thewall, in order to conserve heat. If desired; the heating referred to can be supplemented by an external heater surrounding the casing as indicated at 17. The distribution 'of temperature from C to B and from B to A, can thus be regulated either by the external heater and/or correct design of the thickness or thermal conductivity of the wall material and'the heat supplied to the boiler l.

When the condensed liquid reaches B'evaporation commences readily since the liquid is in the form of a thin film. Light constituents evaporate more readily and are fed through the duct 11 to the jet S'diS'char'g'ing into the mixing tube 9. It is apparentthat the length BC and thewall temperature may be selected to ensure that only the most phleg'rnatic components are returned to the boiler 1 so that after a period, the boiler 1 will contain a liquid consisting substantially of the least volatile components, and the jets 4 and 5 will be fed only by the vapor of these least volatile components, that is to say, the components which will'least impair the high vacuum in the region 12;

Furthermore, althoughthe ultimate vacuum produced by the jet'8 is not so low as that produced by the jets ti and 5', jet 8 being fed withthe more volatile components which will exert a higher pressure at condenser tem era-1 true, the functions of the jet 8'are to pro'du'c'e'a sufficiently low total'pressure for the adequate operation of jet 4', to produce a sufiiciently low partial pressureof air in the region 13 for removing gases from the fluid film on its way'back to the falling film evaporator and to prevent air at the relatively high backing pressure in the pipe 1% from being absorbed'in the fluid returning to the boiler and subsequently being liberated "in' 'the boiler where it may issue from jets 5 and '4 impairin'g'the vacuum and also giving rise to oxidation in the boiler. A further function of the jet 8 is to entrain and remove from the region 13 any relatively very high vapor pressure components; The mixing tube 9 is cooled only sufiiciently to condense such components as are of value "in the ump operation; Relatively high vapor pressure components either pass over into the rotary backing pump or con dense in'thespace 10 and collect in a trough 14 whence they may be drawn off periodically froma tap 15'.

Owing to the evaporation occurring from the portion B-C of the wall there exists in the region 11 a pressure of vapor greater than exists in region 16. This is due in the described form of the application of the invention, to the provision and shape of the skirt member 7 i The vapor produced by evaporation from the portion BC of the wall is used to feed the jet 8'. Any tendency'for the'vapor to flow from B-A against the liquid flow from A-B is pre-' vented by a proper selectionof the distance d andthe length AB so that the passage AB is always sealed with liquid. The high vapor pressure components of the vapor evaporated from the portion BC are partially entrained by the jet 8 and partially condensed on the part of the condensing surface surrounding the region 16'. Condensation may be controlled by allowing this part of the condensing surface to run sufiiciently warm to condense only those low vapor pressure components of value to the operation of the pump.

A particular advantage of the invention is that it can be applied to produce effective fractionation in pumps of small bore in which the method of fractionation by allowing the fluid to flow in a substantially horizontal direction across the boiler is ineffective owing to the short path involved.

'Although particular methods of use of the invention have been described, it will be apparent that it may be carried out in other ways and is capable of application to forms of pump other than those specifically referred to.

We claim:

1. A vapor vacuum pump comprising in combination a working fluid boiler, a casing extending upwardly there from in heat conducting relation therewith and having an upper wall portion and a lower wall portion, said lower wall portion being in heat conducting relation with said boiler, an upper jet and a lower jet, a tube within said casing in spaced relation therewith extending upwardly from said boiler to said upper jet, a second tube surrounding said first tube intermediate the length thereof and spaced therefrom to define an annular channel surrounding the first tube and leading to the 'said lower jet,

the said second tube being connected at itsuppjer' end to the external peripheryof the first tube and having at its lower end an annular portion uniformly spaced from the casing intermediate the length thereof to define a restricted annular through-passage of uniform width throughout its entire height communicating said upper wall portion with said lower wall portion, the said upper jet being adapted to dischargevapor downwardly against the saidupper wallportion of the casing to be condensed thereon, the condensate descending the said upper wall portion in the form of a thin film of liquid which extends through said restricted passage to form a seal between the upper and lower wallportions of the casing, and over the lower wall portion to be vaporized therefrom by heat conducted thereto from the boiler, the vapor thus produced passing through said annular channel to the said lower jet, which vapor includes the lessphlegmatic fractions of the liquid film and any undesirable high volatile fractions contained in the film'whereby the return of the said less phlegrnatic fractions to the boiler is reduced or substantially eliminated.

2. A vapor vacuum pump as claimed in claim 1 wherein an auxiliary heater is positioned adjacent the said lower wall portion of the casing to supplement the heat conducted thereto from the boiler of the pump.

3. A vaporvacuum pump comprising a fluid boiler, a casing extending upwardly therefrom having an upper wall portion, said casing having a lower wall portion in thermal contact with'the boiler and being of substantial height'and in heat conducting relation with said boiler, an upper jet, a lower jet, a tube extending upwardly from thebo'iler to the upper jet in spacedrelation to the inner face of the casing, a' second tube surrounding said first tube intermediate the length thereof and spaced therefrom to define an annular channel surrounding the first tube and leading to said lower jet, said second tube having the lower portion thereof imperforate and uniformly spaced from and cooperating with the inner face of the casing to define a vertical restricted annular throughpassage of uniform cross sectional area throughout its height communicating'said upper and lower wall portions, the said upper jet being adapted to discharge vapor downwardly against the said upper wall portion of the casing to he condensed thereon, the condensate descending the saidu'pp'er wall portion in the form' of a thin film of liquid which extends through said restricted passage to form 'a seal between the upper and lower wall portions of the casing, and over the lower wall portion to be vaporized therefrom by heat conducted thereto from the boiler, the vapor thus produced passing through said annular channel to the said lower jet, which vapor includes the less phlegmatic fractions of the liquid film and any undesirhigh volatile fractions contained 'in the film whereby the return of the said less phlegr'natic fractions to the boiler is reduced or substantially eliminated.

4. A vapor vacuum pump as claimed in claim 3 provided with an auxiliary heater surrounding said lower wall portion of the casing.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,291,054 Nelson July 28, 1942 2,361,245 Stallmann Oct. 24, 1944 2,505,953 Flosdorf May 2, 1950