US2818209A

US2818209A - High diffusion pump

Info

Publication number: US2818209A
Application number: US388583A
Authority: US
Inventors: Hiesinger Leopold; Bock Wilhelm
Original assignee: WC Heraus GmbH and Co KG
Current assignee: WC Heraus GmbH and Co KG
Priority date: 1952-11-06
Filing date: 1953-10-27
Publication date: 1957-12-31
Anticipated expiration: 1974-12-31

Description

Dec. 31, 1957 l.. HlEslNGER ETAL 2,818,209

HIGH DIFFUSION PUMP Filed Oct. 27, 1953 v INVENTORS.

LEOPOLD H/Es//VGER W/LHELM BOC/f AT TOR/VE KS United States Patent HIGH 4mrr'uslon PUMP Leopold Hiesinger and Wilhelm Bock, Hanau (Main), Germany, assgnors to W. C. Heraeus, G. m. b. H., Hanau (Main), Germany,-a-German Ebody corporate 'Application October 27, 1953, Serial No. 388,533

Claims priority, application Germany November 46, 1952 4 Claims. '(Cl. 230-1'01) The present invention has for an objectto provide an improved pump for the generation of very low pressures and which is of the kind known as diffusion pumps or condensation pumps and employs mercury or organic oils as the operating liquid.

In such diffusion or condensationpurnps, the operating agent or propellant, constituted by mercury or highboiling organic oils, is heated in a vpot-like enclosed evaporation chamber, and its vapour is conducted by means of chimney-like tubes 'havin-g nozzle outlet elements in the form of Laval nozzles attached thereto, from each tube 'into `a diffusion or condensation chamber which communicates with the receptacle to bel evacuated; from the latter gas molecules enter by diffusion into the jet of the operating agent by which they are carried along and their partial pressure is increased while the operating agent is condensed. This manner of operation will explain both the alternative names diffusion pump and condensation pump.

Preferably the outer Walls ofthe diffusion chambers are cooled in order to enable the jet of propellant vapour to be condensed on `these walls and to be returned into the evaporation chamber in fa suitable manner, thus making continuous operation possible.

In the majority of constructions of such pumps the whole system is accommodated in an upright cylindrical housing having the evaporation chamber at its bottom, while conducting tubes for the propellant vapours are disposed in radial symmetry'to the axis ofthe housing, these conducting tubes being in the case of `separate boiling chambers for fractionating diffusion pumps, nested in each other concentrically. The loutlet nozzles for the propellant vapours are arranged at the upper ends of the individual vapour-conducting tubes or, in the case of pumps operating without fractionation, along the shell of the vapour-conducting tube and at its end. Slots or apertures provided `in the vapour-'conducting tube enable the propellant vapour to enter the nozzles, to be deflected in a downwarddirection and to enter each diffusion or condensation chamber `as an annular jet.

Connected to the upper end of the pump housing is the chamber to be evacuated. The gasmolecules leaving this chamber through the diffusion .gap between the outer edge of the nozzle and the pump housing enter, by diffusion, the propellantjet, are carried along and compressed and thus step by step arrive at the 4connection to the pre-vacuum pump. With lthe use of organic propellants, it is possible to generate pressures even lower than *5 mm. (Hg) in the receptacle (recipient) tobe evacuated.

The present invention is more particularly concerned with the geometric shape and the relative sizes ofthe `diffusion chambers and the diffusion .nozzles for the individual diffusion stages. It has been found that by the choice of suitable `dimensions and by suitable Construction the flow and pressure dynamic conditions can be ksubstantially improved, -and that :more `particularly that in this manner Kthe undesirable back diffusion of Vice 2 propellant vapours from the vapour jet into the chamber to be evacuated can be vreduced and the lsuction power increased.

According to Iour improved construction of diffusion or condensation pumps at least the diffusion or condensation chamber arranged at kthe high-vacuum side is constructed with its cross-section decreasing in ya continuous manner between the `outlet aperture of the associated vapour jet nozzle and the nozzle next in the direction of the jet, the vapourising tube `is surrounded by a funnel-like enveloping sheet, and the volumes of the series-connected diffusion chambers arearranged 'to decrease step by step approximately in yan vinverse vproportion to the desired increase in pressure.

The effect of this shaping feature is Aassisted if according to a preferred feature, the diffusion lchamber at the high-vacuum side 'is substantially larger as compared with the diffusion chambers of the subsequent pressure stages, and ratio of the distance from the rst'diffusion nozzle to the next following nozzle to the distance from the second diffusion nozzle to a third diffusion nozzle is at least 2.5 :l and preferably greater than 3:1.

In order that this invention may be more readily understood, a three-stage diffusion pump incorporating the invention is illustrated by way of example in the accompanying drawing.

An upright cylindrical housing, preferably of metal, contains in its lower portion the evaporation vessel 2 for the propellant, the so-called oil sump, which is pro* vided with a heating coil 3, by means of 'which the propellant is adapted to be heated.

When the apparatus is constructed as a three-stage fractionating diffusion pump as illustrated, three vapour rising or vapour-conducting tubes 4, 5 and 6 are provided nested coaxially in each other, their diameters being outwardly flared in a cone shape at the lower ends. These tubes lead the propellant vapours of the three fractional stages respectively to the associated

diffusion nozzles

7, 8, 9 of mushroom or annular shape and formed as Laval nozzles, these nozzles being arranged at the upper end of each tube. At least the uppermost diffusion chamber 10 is, in accordance with the present invention, formed with a cross-section which decreases continuously in the direction of evacuation due to provision of a funnel-shaped enveloping sheet 13 of straight or slightly curved profile which commences at the inner lip 20 of the diffusion nozzle, wholly envelops the innermost vapour-conducting tube 6, and extends approximately to the outer lip of the next diffusion nozzle. In the pump as illustrated, the second diffusion chamber 11 is also formed with a gradually decreasing cross-section due to the provision of a further enveloping sheet 14 interposed between the apertures of the

nozzles

8 and 9. No enveloping sheet for enclosing the associated vapourising tube is generally provided for the diffusion chamber 12 at the pre-vacuum side. The enveloping sheets may be connected to the cooled pump housing 1 by means of heat-conducting metal bridges 22.

Connected to the pump `by means of a flange 15 is the communication passage -leading'to the recipient, which is made as wide as possible, and a tube 16 serves for conducting the gases compressed by the diffusion pump from `the pre-vacuum side 'of the diffusion chamber 12 to the pre-vacuum pump. The upper portion of the pump housing and the connecting tube 16 are Yarranged in a cooling sleeve 18 through which a flowable cooling means, preferably water will be circulated. It will be readily appreciated that the annular high vacuum side diffusion chamber 10 is substantially larger than the subsequent diffusion chambers 11 and n12, and that, in accordance with a feature vof the present invention,v thedistancebetween the first vnozzle rand :the 'second' nozzle is very much greater than that between the second and third nozzles.

Another feature of considerable importance in a pump according to the invention is also the relation of the area sizes of the diffusion gaps, such as the annular gap between the outer lip of a diffusion nozzle, for example, of the outer lip 19 of the high-vacuum side nozzle, and the pump housing 1 adjacent this nozzle. It has been found advantageous that the ratio or ratios of the cross-sections of the diffusion gaps of series-connected stages should in a two-stage arrangement be 100:10 and in a threestage arrangement approximately between 100:10.1 and 70:15:1-the most favourable ratios having been found to be 80:1211.

The cross-section of the ditfusion gaps has a decisie inuence upon the magnitude to gas volume passing out of the high vacuum chamber or of a preceding diffusion chamber and entering by diffusion the gas jet in order to be compressed. The feature proposed by the present invention ensures that the vapour jets leaving the nozzles do not wobble or oscillate and assume a smooth shape approximately of a cone shell since, due to the elimination of any dead pockets, they will follow the shape of the funnelshaped enveloping sheets 13 or 14, thus oiiering a large diffusion area to the gas to be compressed. The enveloping sheets also screen the hot vapourising tube and prevent direct contact of the vapour jets with the hot outer surface of these tubes so that the vapour jet will be uni* formly condensed at the cooled inner wall of the housing without re-vapourisation. With this object in view it is also of advantage for the inner side of the enveloping sheets themselves to be cooled. The molecules of the vapour are further prevented from impinging perpendicularly upon any surfaces, for example upon the upper side of the next following nozzle, being there reliected, and thus causing a diffusion of the propellant vapours into the recipient in a direction opposite to that o-f evacuation.

Due to the smooth and aerodynamically favourable shape of the housing, the propellant nozzles, and the deflector surfaces, a reduction of the llow resistance for the gas to be evacuated is also obtained. In order to obtain this favourable characteristic preferably the upper side of the high-vacuum side nozzle 7 should be approximately of mushroom shape, and the inner nozzle passages of the various diffusion nozzles should, for the same reason, be so shaped as to offer as little ilow resistance as possible. Furthermore, the various conducting or deliector elements for the vapour jet, which preferably are made of aluminum, may be, by anodic oxidation, rendered mechanically and chemically resistant. smoothing may also be obtained by coating the surfaces with synthetic resin or with enamel, an enamel coating also having the advantage of not permitting any catalytically decomposing action upon organic propellants, such as is exerted by many metals.

The sizes of the individual diffusion apertures also require careful dimensioning. It has been found to be particularly advantageous in the case of the high vacuum side nozzle for the ratio of the area of the annular nozzle aperture to the area of the corresponding annular diffusion gap to be not more than 1:14 while in the case of the next following nozzle this ratio should not exceed 1: 1.6 and in the case of the pre-vacuum side nozzle not more than 1:02. Thus, the increase of the high vacuum side diffusion gap is particularly remarkable.

The pumping elfect is finally also substantially affected by the quantity of the propellant vapour which passes through the individual nozzles. The relation of these quantities is determined by the magnitude of the individual vapour-supplying areas in the propellant sump as sub-divided by the immersed walls of the vapour conducting tubes. The ratio of the vapour supplying area of the `high vacuum side stage to the corresponding area of the .intermediate and the pre-vacuum side stages will in a favourable case be between 1:0.7:2 and 1:O.9:1.5 and Compacting and preferably about 1:O.8: 1.7. It will be noticed that a very considerable portion of the available vapour-supplying areas must be allocated to the pre-vacuum side stage, which stage in pumps operating according to the fractionating principle is mainly operated with the lower boiling components of the propellant.

While the proposed arrangement, in which the vapourconducting tubes are surrounded by enveloping sheets, will favour condensation of the propellent vapour in the diffusion chambers, it is nevertheless desired that as little loss by condensation as possible should occur within the vapour-conducting tubes. This is ensured to a large extent by a further feature of the invention according to which a rod 17 of high-heat-conductivity metal extends along the axis of the vapour guiding tube associated with the high vacuum nozzle 7,' one end of this rod being connected to the bottom of the evaporation vessel and its other end being connected to the high vacuum nozzle. By the heat conduction into vapour guiding tube and to he high vacuum nozzle ensured in this manner, heat losses and condensation of the propellant component having particularly low volatility are couteracted. In the case of the other vapour-conducting tubes it is suicient for these tubes to be made of a material of good heat conductivity, preferably of aluminum.

The feature of the individual diffusion chambers decreasing gradually in cross-section may, as an alternative to the proposed feature of the enveloping of the vapourconducting tubes, also be attained by the pump housing being flared in la funnel-like manner, say from the point `of the pre-vacuum connection to the high vacuum connection. The enveloping sheets may be cooled similarly to the outer wall, which may be effected in a simple manner by the provision of metal bridges 22 having good heat conductivity.

The combination and construction of the pump elements of a diffusion pump according to the invention may similarly be realised in pumps having a downwardly extending or horizontally disposed nozzle insertion and also in pumps in which the nozzle insertion surrounds concentrically the internally disposed diffusion chambers. Such modications will be readily available to those skilled in the art, and it should be understood that such modifications are within the scope of our invention.

In each case a very considerable increase of the suction power is obtained, which may be as high as 60% as compared with known forms of pump requiring the same power input and having the same pump cross-section and the same number of nozzles.

We claim:

1. A Vapor-operated vacuum pump comprising a pump casing having an inlet and an outlet, a boiler section positioned in the lower portion of said pump for holding a pool of liquid, heating means for vaporizing the liquid, tirst and second jet nozzles in the casing and adapted to direct vapor to ow from the casing inlet toward the outlet, the first jet nozzle being nearer the casing inlet than the second, vapor supply means within the casing and connected to the jet nozzles for supplying vapor from the boiler to the jet nozzles, and a shield spaced from and surrounding the vapor supply means, the shield beginning -at the rst jet nozzle and terminating at the second jet nozzle, the shield and casing interior being shaped to form a dilusion chamber of continuously decreasing transverse cross section in the direction leading away from the casing inlet, the shield preventing direct contact between the vapor supply means and vapor in the diffusion chamber.

2. A pump according to claim 1 which includes metal bridging means of good heat conductivity connecting the shield with the pump casing.

3. A vapor-operated vacuum pump comprising a pump casing having an inlet and an outlet, a boiler section positioned in the lower portion of said pump for holding a pool of liquid, heating means for vaporizing the liquid, at least two coaxally disposed vapor chimneys in the casing and having intake and discharge ends, the chimneys opening at their intake ends into the boiler section, the discharge end of one of the chimneys terminating nearer the casing inlet than the discharge end of the other chimney, a first jet nozzle on the discharge end of the chimney nearer the casing inlet and adapted to direct vapor to flow from the casing inlet toward the outlet, a second jet nozzle on the discharge end of the other chimney and adapted to direct vapor to ow from the casing inlet toward the outn let, each jet nozzle being spaced from the casing interior so that a respective diffusion gap is between each jet nozzle and the adjacent portion of the casing interior, anda shield spaced from and surrounding the chimney connected to the first jet nozzle, the shield beginning at the rst jet nozzle and terminating at the second jet nozzle, the shield and casing interior being shaped to form a diffusion chamber of continuously decreasing transverse cross section in the direction leading away from the casing inlet, the shield preventing direct Contact between the chimney it surrounds and vapor in the diffusion chamber.

4. A vapor-operated vacuum pump comprising a pump casing having `an inlet and an outlet, a boiler section positioned in the lower portion of said pump for holding a pool of liquid, heating means for vaporizing the liquid, at least three coaxially disposed vapor chimneys in the casing and having intake and discharge ends, the chimneys opening at their intake ends into the boiler section, the discharge ends of one of the chimneys terminating at different distances from the casing inlet, an annular rst jet nozzle on the discharge end of the chimney nearest the casing inlet, an annular second jet nozzle on the discharge end of the chimney next nearest the casing inlet, an annular third jet nozzle on the discharge end of the chimney farthest from the casing inlet, each jet nozzle being adapted to direct vapor to ow from the casing inlet toward the outlet, each jet nozzle being spaced from the casing interior so that a respective annular dilusion gap is formed between each jet nozzle and the adjacent portion `of the casing interior, and a shield spaced from and surrounding the chimney having its discharge end nearest the casing inlet, the shield beginning at the rst jet nozzle and terminating `at the second jet nozzle, the shield and casing interior being shaped to form a diffusion chamber of continuously decreasing transverse cross section in the direction leading away from the casing inlet, the shield preventing direct contact between the chimney it surrounds and vapor in the dilusion chamber, and the ratio of the distances between the rst and second jet nozzles, and the second and third jet nozzles being at least 5:2.

References Cited in the file of this patent UNTED STATES PATENTS 2,249,450 Bancroft July 15, 1941 2,361,245 Stallman Oct. 24, 1944 2,379,152 Hickman June 26, 1945 2,432,226 Cox Dec. 9, 1947 2,435,686 Kuipers Feb. l0, 1948 2,437,897 Stoltenberg Mar. 16, 1948