US2839238A - Fractionating oil diffusion pumps - Google Patents

Description

June 17, 1958 w. Bock 2,839,238

FRACTIONATING OIL DIFFUSION PUMPS Filed Oct. 14, 1954 3 Sheets-Sheet 1 INVENTOR.

W/LHELM BOCK A T7'ORNEYS June 17, 1958 w, oc 2,839,238

FRACTIONATING OIL DIFFUSION PUMPS Filed Oct. 14, 1954 3 Sheets-Sheet 2 g g INVENTOR. k W/LHELM BOCK AT TORNEYS June 17, 1958 w. BOQK 2,839,238

FRACTIONATING OIL; DIFFUSION PUMPS Filed Oct. 14, 1954 5 Sheets-Sheet 5 INVENTOR. W/LHELM BOCK A TTORNEVS United States Patent @fitice 2,839,238 Patented June 17, 1958 FRACTIONATING 01L EIFFUSION PUlVlPS Wilhelm Bock, Hanan (Main), Germany, assignor to W. C. Heraeus G. m. b. H., Hanan (Main), Germany, a German body corporate Application October 14, 1954, Serial No. 462,318

Claims priority, application Germany October 21, 1953 13 Claims. (Cl. 230-101) The present invention relates to multi-stage dilfusion pumps adapted to be operated with high-boiling organic oils under fractionation, and more particularly to vertical pumps of this kind, and it has for an object to provide improved pumps of this character in which the fractionating efiect is increased in order to enable a considerably lower end pressure to be attained on the high vacuum side than has been obtainable with hitherto known difiusion pumps of similar size. In addition the invention as hereinafter described also allows an improved continuity of the prevacuum to be attained.

It is known that in multi-stage diffusion pumps operated with organic. oils the fractionation which occurs in the course of the heating, evaporation and condensation of the oil, and by which the oil is divided into components of different volatility or of different vapour pressure, can be utilised for feeding the nozzles of the differ ent diffusion stages between the high vacuum connection and the pre-vacuum connection of the pump with portions of the vapour of the operating medium which in the indicated sequence of stages are progressively richer in highly volatile components. The high-vacuum side nozzle should only be supplied with-that fraction of the organic oil which has the lowest vapour pressure, because the vapour pressure of the operating agent in this stage determines the attainable end value of the total pressure on the high vacuum side. This ideal object has hitherto only been attained to some extent with the help of a certain sub-division of the oil sump which is provided at the bottom of the diffusion pump and is to be heated, into a number of zones arranged side-by-side or concentrically and accordingly extending vertically, more particularly by means of the lower ends of the guiding tubes for the operating-agent vapour, which are arranged to project downwardly into the oil bath, the upper ends of these tubes being arranged generally concentrically with each other and opening at the top into the difiusion nozzles. The condensate of the operating agent, which flows down the cold or cooled outer wall of the pump, and which contains all fractions, reaches in this case successively the various mutually communicating vertical zones of the heated oil bath which are arranged side-by-side and which will respectively, according to their order, yield at their surface vapour bubbles to the individual nozzles of the pre-vacuum stages up to the high-vacuum stages. As a result the more highly volatile components, which again will evaporate first, will be mainly supplied to the nozzles at the pre-vaeuum side, whereas the last vertical zone of the oil bath, which in the case of concentric arrangement is the innermost zone, and which can supply vapour only to the high-vacuum side nozzle, will substantially only be reached by the relatively non volatile component having the lowest vapour pressure.

Now according to all experience the hitherto known sub-division of the oil bath into individual zones arranged side-by-side and communicating with each other, the fractionating effect can never be fully utilised. According to the invention the fractionating operation of the diffusion pump is improved on the basis of the discovery that the heated bath of operating agent will form vapour bubbles having different fractions of the individual components of different volatility and vapour pressure of the operating oil, also in zones which are arranged above each other in a vertical direction, that is to say that so to speak there is also a horizontal fractionating layering. Since the oil bath must have a certain depth, amounting at least to several centimetres, in order to ensure an adequate supply of vapour to the difiusion nozzles, the bottom layers of the oil bath are subject not only to the gas pressure but additionally also to the hydrostatic pressure of the superposed oil layers. The bubbles of the organic oil which are produced in the lower layers after adequate heating to sufiiciently high temperatures must therefore have a high vapour pressure, i. e. in each layer bubbles will develop of those components of the operating agent whose vapour pressure corresponds to the sum of the gas pressure and the hydrostatic pressure at the corresponding level within the oil bath. The lower down the vapour bubbles are formed, the greater a portion they will contain of the more volatile components of higher vapour pressure which ideally should be supplied to the difiusion stages of higher pressure in order to enable the fractionating effect to be fully utilised.

This discovery led to the novel proposal of efiecting a horizontal sub-division of the sump by a substantially horizontal guide surface, in order to feed those vapour bubbles which are produced in the lower layers of the operating-agent to those diffusion nozzles that are to be fed with the fractionating components of higher vapour pressure. Thus the invention not only reduces the amount of low-boiling components of the oil which reaches the high-vacuum nozzle but at the same time meets the requirement of supplying the difiusion stages at the prevacuum'side, that is to say those that work against a high pressure, with a relatively great share of the operating-agent vapour, thus improving the reliable maintenance of the pre-vacuum.

The invention and the particular features serving for its execution will now be described more in detail with reference to the accompanying drawings: v Fig. 1 is an elevation, partly in section, of one embodiment of a fractionating diffusion pump according to the invention, which has been found satisfactory in practice;

Fig. 2 is a top plan view of a guide sheet;

Fig. 2A is a side view of the guide sheet shown partially in cross section along the line 2A2A. in Fig. 2;

Fig. 3 is a side view, shown partially in cross section, of a modified form of the invention;

Fig. 4 is a top plan view of another modification of the present invention; and

Fig. 4A is a side view of the modification illustrated in Fig. 4 shown partially in cross section along line 4A4A of Fig. 4.

The embodiment shown in Fig. l is a three-stage fractionating oil diifusion pump. It comprises a vertical, cylindrical housing 1 of metal, in which three vapourguiding tubes 4, 5 and 6, which each open at their upper end in a mushroom-shaped reversing nozzle 7, 8 or 9, are arranged nested in each other concentrically, so as to project with their lower ends into an operating-agent sump 2 provided at the bottom of the housing 1. This sump is adapted to be heated, for which purpose an immersion heater coil 3 has been provided, and the arrangement is such that the divided volumes of the liquid operating-agent communicate with each other at the lower ends of the tubes.

Connected to a flange 15 of the housing 1 is a communication passage to a chamber to be evacuated; this passage should be as wide as possible. The compressed gases leaving the difiusion pump are conducted through a pipe 16' from the pre-vacuum stage diffusion chamber 12 to a suitable pre-vacuum pump (not shown).

The upper part of thepump housing 1 aswell as the communication pipe 16 are placed in a cooling jacket 18 cooled by a fluid cooling agent, preferably water.

.The fact that the vapour guiding tubes 4, 5 and 6 project into the operating-agent sump. 2 will as is well known, lead to a sub-division of the quantities of vapour vapour bubbles of the operating-agent are produced not only at the surface 26 but above all near the bottom and within the interior of the operating-agent sump 2. V For since. on these vapour bubbles lies the pressure of the operating-agent located above them, they have a higher vapour pressure and, what is most important, for this reason they still contain highervolatility components of the operating agent which ideally should be fed to the pre-vacuum side nozzles 9, or 9 and 8. In view of this it is an object of the invention to control the ratio of the quantifies of vapour flowing to the. individual diffusion nozzles, of the series arrangement in order to ob tain optimum suction effect, a subsidiary object being to. achieve this without having to exchange the set of nozzles with the vapor guiding tubes and more particularly to effect constructional modifications in the pump. In order to attain the stated object it is proposed according to the invention to arrange in the interior of the opcrating-agent sump a suitable guide surface for the rising vapour bubbles produced therein. A substantially horizontally disposed guide surface21 arranged in the interior of the operating-agent sump, preferably in the form of a flat sheet which is convexly curved downwardly so as to have the shape of a spherical calotte over the surface of which moreover apertures 24 may be distributed has been found to be highly effective.

Such a guide sheet has the effect that the vapour bubbles which develop in the central zones and in the interior, particularly at the bottom of the operating-agent sump 2 upon reaching the boiling point will, due to the hydrostatic floating force produced by the oil bath, slide ofif readily outwardly on the surface of the sheet, thus causing a great portion of the vapours produced in the. interior of the operating-agent sump 2 not to proceed to the high-vacuum side nozzles 7, but to the pro-vacuum side nozzle systems 9, or 3 and 9, which experience .has shown to be favourable. The provided perforations or apertures 24 in the guide surface 21 allow part of the vapour bubbles to escape direct upwardly thus ensuring an adequate supply of operating-agent vapour to the central diffusion nozzle 8. V

However, the vapour-bubble guiding sheet 21 may also be shaped in such manner that any one of the nozzles or any one of the diffusion stages is fed with operatingagent vapour to a particularly great extent. The sheet should extend at least over a vertical zone which provides at its surface operating-agent vapour for a diffusion pump stage of relatively low pressure.

It is advisable to leave in the centre of the guide sheet 21 a fairly large-area aperture 23 of more or less circular shape in order to allow the low-volatility components of the operating agent which accumulate there during the fractionating operation, to move after their evaporation direct into the high-vacuum side vapour- "guiding tube 6 or 6a.. The vapour bubbles of the components of greater volatility on the other hand, which form during the course of the flow from the edge of the boiling sump towards the centre, will be prevented by the guide sheet from moving to the high-vacuum side nozzle.

By providing a set of guide sheets of different shape for one single pump and for one single set of nozzles the invention may be utilised to'obtain a choice of various quite different courses of the evacuation process. It is even possible to vary the temperature characteristics during the operation of the pump for example by pro-. viding constructional features which permit raising or lowering of the guide sheet 21 in the operating-agent sump or variation of the distribution and size of the passage apertures on its surface. All that is required for the latter purpose is to place two thin guide sheets on top of each other, each having radial perforations and to make 1 them mutually rotatable, thereby enabling the size of the resultant passage apertures to be considerably varied. Even complete blocking is possible, in whichcase all vapour bubbles rising from below are fedto the, pre-.

vacuum side nozzle system.

Fig. 3 illustrates a modificationv of the invention providing a conventional mechanical linkage for the vertical adjustment of the metal guide sheet 21. The mechanical linkage adjusts, the guide sheet 21 from outside of the pump casing and includes a toothed rack 27 mounted in the bottom of the housing and vertically movable bymeans of pinion 29 mounted on shaft 28 which is rotated by a knob-31. The shaft 28 is supported by brackets 30.

The modification shown inFig. 4 illustrates two guide sheets 21a, 21b connected by a head on the external periphery of the guide'sheet 21b. The bead along outer edge of the guide sheetinterlocks with the annular edge of guide sheet 21a to connect the guide sheets for relative movement. The relative movement of the guide sheets 21:: and 2112. provides large openings by coin-. cidence of apertures-24a in respective guide sheets, smaller openings through coincidence of apertures 24 and 24a; and closure of all openings upon rotation of the guide sheets to close off apertures 24 and 24a.

Referring-again to Fig. 1, it will be noted that-at least 7 the uppermost diffusion chamber 10 is constricted continuously in the direction of evacuation by means of a funnel-shaped or slightly curved enveloping sheet 13 which starts at the inner lip 20 of the diffusion nozzle, wholly envelops the inner vapour-guiding tube 6, and extends ap proximately to the outer lip to the next following diffusion nozzle. In general no enveloping sheet is provided for the pro-vacuum stage diffusion chamber 12 to envelop the associated vapour-rising tube. The enveloping sheets may be connected to the cooled pump housing 1 with the help of heat-conducting metal bridges 22.

The annular diffusion chamber 10 at the high-vacuum side is advantageously made substantially larger than the following diffusion chambers 11 and 12, and the first nozzle 7 should preferably be spaced by a very much greater distance from the secondnozzle 8 than the latter is from the third nozzle 9.

This ensures that the vapour jets emitted by the nozzles will not vacillate, and that they will assume a smooth form, substantially similar to the surface of a cone, because, owing to the removal of dead spaces, they will keep closely to the surface of the funnel-shaped enveloping sheets 13 or 14 and thus oflfer a great diffusion area to, the gas to be compressed. Theenveloping sheets also screen off the hot vapour-rising tubes and prevent direct contact of the vapour jets with the hot external surface of these tubes so that the vapour jet will be condensed, uniformly and without re-evaporation by contactwith the cooled inner wall of the housing. For this purpose it is also advantageous for the inner side of the enveloping sheets themselves to. be cooled. The molecules of the vapour are also being prevented from striking vertically upon any surfaces, for example upon t.e upper side of the next following nozzle, and being there reflected, thus causing an undesired diffusion of the, operating-agent vapourinto the recipient, i. e. in a direction opposite to that of the evac nation. The smooth and flow-dynamically favourable shape of the housing, of the operating-agent nozzles, more particularly of the mushroom-shaped outer lip 19 of the high-vacuum side nozzle 7, and of the guiding surfaces also results in a reduction of the flow resistance met by the gas to be evacuated.

Losses by condensation of operating-agent vapours within the vapour-guiding tubes may be reduced by the provision of a rod 17, made of a metal having good heat conductivity, which is arranged in the axis of the inner vapour-guiding tube 6, said rod being connected at its lower end with the bottom of the evaporation vessel and at its upper end with the hood 19 of the high-vacuum side nozzle. By this means so much heat is conducted from the evaporation sump into the vapour rising tubes that the latter are prevented from cooling below the evaporation temperature.

The use of the guide sheet for the vapour bubbles that are produced in the interior of the heated operating-agent bath, and consist of operating-agent components having the higher vapour pressure, i. e. the feature which constitutes the core of the present invention, is not restricted to diffusion pumps of the vertical type. On the contrary in any diffusion pump operated with organic oils substantially horizontally disposed guide sheets, which preferably are, slightly curved downwardly, may be provided with similar effect for the interior of the operating-agents baths. This basic discovery will enable any expert to adapt without difficulty the features necessary for this apparatus, without having to add anything of importance to the idea of the invention.

The construction of diffusion pumps according to the invention involves considerable progress in this branch of the art inasmuch as it will ensure that at the high-vacuum side nozzle as far as possible only fractions of the operating agent with the lowest vapour pressure will work, the consistency of the pre-vacuum being also increased by an appreciable amount of pressure.

I claim:

1. In a multi-stage oil diffusion pump comprising a vacuum-tight housing forming the outer wall of a series of interconnected oil diffusion chambers including a high vacuum stage, a fore-vacuum stage and at least one intermediate pressure stage, a sump containing an organic oil bath in the bottom part of the housing, heating means mounted in said sump and adapted to produce oil vapors constituting the operating medium of the pump, diffusion nozzles, and a plurality of oil vapor guide tubes each terminating in a diiiusion nozzle in a respective one of said stages, said tubes being nested in each other and in said housing whereby the end portions thereof are immersed in the oil bath dividing the oil bath into adjacent and intercommunicating vertical zones: a substantially horizontally arranged dividing element in said sump for separating the oil bath into superposed horizontal layers, said element forming a baflie means deflecting vapor bubbles produced by said heating means in the horizontal oil bath layer below the element to at least one of said oil vapor guide tubes other than the guide tube leading to the high vacuum stage.

2. The oil difiusion pump of claim 1, wherein said housing is a vertical cylinder and said tubes and diffusion nozzles are arranged coaxially with and concentrically within the cylindrical housing.

3. The oil difiusion pump of claim 1, wherein said dividing element has a substantially horizontal guide surface extending at least over one of said vertical oil bath zones supplying surface oil vapor bubbles to one of said stages other than the fore-vacuum stage.

4. The oil difiusion pump of claim 1, wherein said dividing element is a convexly shaped metal sheet curved towards the bottom of the housing, said element having a plurality of apertures, said metal sheet extending at least over one of said vertical oil bath zone supplying surface oil vapor bubbles to one of said stages other than the fore-vacuum stage.

5. The oil diffusion pump of claim 1, wherein said dividing element is a metal sheet so shaped and arranged that oil vapor bubbles produced during heating of the oil in the interior and at the bottom of the bath are fed to predetermined ones of said vertical bath zones and, subsequently, to predetermined ones of said pressure stages.

6. The oil difiusion pump of claim 5, wherein said metal sheet has an approximately circular center aperture of substantial diameter.

7. The oil dilfusion pump of claim 1, comprising means coupled to the dividing element for vertically adjusting the position of the element in said bath.

8. The oil diffusion pump of claim 1, wherein said dividing element consists of two perforated metal sheets, means connected to said pump for assembling said sheets in superposed relatively rotatable relationship, the perforation of each sheet being of such size and so distributed that the resultant passages through the element are varied by relative rotation of said sheets.

9. The oil diifusion pump of claim 8, wherein said metal sheets have radially extending perforations.

10. In a multi-stage oil diffusion pump comprising a vacuum-tight cylindrical housing forming the outer wall of a series of interconnected oil diffusion chambers including a high-vacuum stage, an intermediate pressure stage and a fore-vacuum stage, a sump containing an organic oil bath in the bottom part of the housing, an electric heating coil mounted in the sump and adapted to produce oil vapors constituting the operating medium of the pump, a plurality of oil vapor guide tubes arranged coaxially with and concentrically Within the housing and immersed with one of their ends in the oil bath, the immersed ends being spaced from the bottom of the housing to form concentrically adjacent inter-communicating vertical bath zones, and a difiusion nozzle at the other end of each guide tube, each nozzle being arranged to discharge oil vapors emanating from a respective one of said vertical bath zones into a respective one of said pressure stages, the outermost concentric bath zone being adapted to feed oil vapors to the nozzle in the pie-vacuum stage and the innermost bath zone adapted to feed oil vapors to the nozzle in the high-vacuum stage: a substantially horizontal bafile member centrally mounted in said sump and spaced from the bottom thereof, the bafiie member extending at least over a portion of the innermost vertical bath zone.

11. The oil diffusion pump of claim 10 wherein said baflie member extends also over at least a portion of the vertical bath zone adjacent to the innermost zone.

12. The oil diffusion pump of claim 10, wherein said baffle member is a convexly-shaped perforated metal disc curved toward the bottom of the housing.

13. The oil diffusion pump of claim 10, wherein said bafiie member is a convexly-shaped perforated metal annulus curved toward the bottom of the housing.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain Mar. 14, 1956

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