US2390814A - Pump - Google Patents

Description

Dec. 11, 1945. F. w. S'II'ALLYMANNI PUMP Filed June 14, 1944 32 2 Ef M n P; O

INVENTOR freaeri'cA Mjfa/ma nn.

WITNESSES:

ATTORN Y Patented Dec. 11, 1945 UNITED STATES PATENT OFFICE PUMP Frederick W. Stallmann, Wilkinsburg, Pa., assignor to Westinghouse Electric Corporation-East Pittsburgh, Pa., a corporation of Pennsylvania Application June 14, 1944, Serial No. 540,294

2 Claims.

My invention relates to vacuum'pu-mps for producing the high vacua which are needed for certain types of electronic discharge apparatus, and in particular relates to vacuum pumps of the diffusion type. While the principles are of broader application, I illustrate the invention by an embodiment in which the working fluid producing the diffused vapor comprises an oil or other hydrocarbon.

A diffusion pump of this general type is shown in my 'U. S. Patent No. 2,361,245, dated October 24, 1944, which issued on my copending application, Serial No. 480,723, filed March 27, 1943, for Vacuum pumps, and assigned to the Westinghouse Electric & Manufacturing Company, East Pittsbugh, Pennsylvania. The oil diffusion pump shown in that application comprises a plurality of stages in which oil vapors issue from a plurality of nozzles positioned at different heights along the axis of a vertical water-cooled container, the jets of oil vapor issuing from the nozzle exerting a pumping action on gases present in the container by entraining and imparting a translatory velocity to them which carries them to a backing-up pump issuing to the atmosphere. The oil vapors from each jet are, however, condensed on the cool walls of the container, and are then run back through a suitable drain to a boiler which reevaporates the oil,

In accordance with the prior art practice, as illustrated in my above-mentioned application, the boiler is subdivided into a number of sections equal to the number of stages of the pump; the condensed oil is drained into the boiler section connected to the highest pressure stage of the pump, is then passed to the intermediate stage or stages and ends up in the section of the boiler which supplies vapor to the lowest pressure stage of the pump. I have found, however, that higher pumping effectiveness is attained if the condensed oil is drained back into an intermediate stage; specifically in the case of a three-stage pump, the condensed oil should be drained back into the single intermediate stage.

It is, accordingly, an object of my invention to provide a vacuum pump of the diffusion type, employing a fluid having a number of components of different vapor pressures, which shall have higher pumping eificiency than did similar diffusion pumps of the prior art.

It is another object of my invention to produce a structure for diffusion pumps employing a fluid made up of a number of components of different ducing a higher vacuum than was possible with difiusion pumps of the prior art.

It is a still further object of my invention to produce a structure for oil diiTusion pumps which shall enable them to pump against a higher backing-up pressure than pumps of the prior art.

Other objects of my invention will become apparent upon reading the following description, taken in connection with the drawing, in which:

Figure 1 shows an embodiment of my invention in which the various stages of the pump are positioned one above the other; and

Fig. 2 shows an embodiment of my invention in which the, various stages of the pump are'positioned substantially in the same horizontal plane.

Referring in detail to Figure l, the main. body of my pump comprises a vertical cylindrical casing I closed at its lower end by a base 2 having an electrical heating element 3 of a type too .wellknown to need a detailed description in good thermal contact with the floor of the cylinder I. If desired, the elements I and 2 may be made of iron or steel, although either may alternatively comprise aluminum. The upper end of the container I is'provided with a flange 4 to which can be bolted by a vacuum tight connection an intake for gases from the piping system which is to be evacuated. Roughly one fifth of the height of the flange 4 above the base 2, an offtake pipe 5 leading to a backing-pump (not showii), is connected vacuum tight through a flanged connection 5a to the cylinder I.

The bottom portion of the cylinder I contains the working fluid 9 for the pump, the latter being heated by the electrical heater already described to a suflicient temperature to cause a copious offflow of vapor.

Centrally positioned in the cylinder I is a cylindrical vertical pipe II which may, if desired, be of the same material as the cylinder I, and which is preferably flared outward at its lower end to rest on the base 2. The cylinder I I is likewise provided with an outwardly flared portion at its upper end as will be described in more detail below. A tie rod I2 may be positioned in the central axis of the cylinder II, its lower end screwing into the base 2,

Upon the upper end of the cylinder I I I support, preferably by means of three small spacing pieces I3 overlapping its edges, 3. cap I4 roughly in the form of a truncated cone. The precise form of the cap will be discussed in detail below. The cap I4 is held firmly against the spacers I3 by a nut threaded on the end of the tie rod I2.

v large volume for the annular space into which the vapor issues from the above-mentioned annular orifice. While it might be possible to make the cylinder II of a diameter smaller than that just mentioned, such reduction in diameter would not greatly add to the available annular space surrounding the cylinder I I but would materially reduce the pressure of the vapor at the inlet side of the above-mentioned annular orifice. these general limitations the diameter of th cylinder II is not a critical-dimension but failure to realize the importance of even these general limitations has resulted in 'inefficiency in many pumps of the prior artpof Whichil am acquainted.

A hood or umbrella '15 of conical form is :fastened to the cylinder II 'at a distance below the lower end of the cap I'd which is roughly equal to the 'mean free path of the molecules of the vapor issuing from the upper endof cylinder EI I.

isurrounding the "cylinder H is a second and shorter cylinder I6 which may be of the :same material as cylinder 11 and the lower end of which rests on the base .2. The diameter of the cylinder :I 6 should be inth'eneighborho'od of twothirds that of the cylinder I, the principal limitation on this dimension being that, if too small, it will reduce the vapor pressure at the orifice issuing from its upper end, and that if too large it will leave too little space for gases to flow from the space above the umbrella IE to the oiTta-ke conduit 5.

Th upper end of the cylinder I6 is bevelledand spacedslightly awayfrom the umbrella I15 to provide an annular orifice from which vapors rising from the surface of the liquid "9 into the :space between the cylinders II and I6 "can be ejected into the annular space intervening between the "cylinders I6 and I.

Approximately one-tenth of the length of the cylinder Iabove the base 2 is provided a horizontal partition I! which may conveniently be screwed into threads cut on the interior wall of thecylinder I and which covers the space between thecy-linder I6 and the outer wall of the cylinder 1. This partition I1 is provided with a groove in its lower face which fits th upper end portion of a liner I8 for the interior wall of the lower portion of cylinder .I. p

'The partition I1 i likewise provided with a bevelled hole 2I issuing into a small .duct ZIb which connects with a hole in the cylinder [6 below the surface of the oil therein. By this arrangement, the condensed oil is returned-to the portion of the boiler immediately below, and which issues vapor into, 'theinterior of the cylinder I6. This vapor issuing through the orifice at the upper end of the cylinder I6 "constitutes the intermediate stage of the pump,

A bent cylinder or nozzle '2-'-I-a 1s rem'ovably inserted in the disc '=I-I and extends to eject vapor into-off-take'fia.

The exterior wall of the casing 'I and 5 is-surrounded by a helical pipe i28-through which water or other cooling fluid flows tomain'tazin the temperature of such walls at a sufficiently low tem- Beyond perature to condense th vapor fiOWiIlg from the orifices and nozzles already described.

The general mode of operation of the abovedescribed pump structure is as follows: The fluid 9 which may be any suitable material described in the art as useable in difiusion pumps, but for which I preferably employ a hydrocarbon of the ring-compound type such as molecular lubricant is heated and the resulting vapor flows up from its surface through the respective cylinders II, Hi and 2 Ia.

It will be evident from the foregoing that the vapor jets flowing respectively from the upper .end of cylinders I'I, -I6 and 2Ia constitute various stages of a pump for evacuating any containers connected to the flange 4; that the stream flowing from cylinder II constitutes the first, or lowest pressure, or highest vacuum stage of this pumping system; that the vapor flowing from the upper end of cylinder I6 constitutes an intermediate stage; and that the vapor flowing firom the upper end of cylinder '2Ia constitutes the highest pressure or lowest vacuum stage.

In actual practice, it is found impossible to obtain pumping fluids which are literally and absolutely devoid of impurities; and in particular, while the hydrocarbon fluids such a I have proposed using have nearly uniform constitution, they are nevertheless found to have components of slightly different vapor pressures at any given temperature. The result of this is that 'the components having higher vapor pressures tend to boil away first and leave the components of lower vapor pressure behind. "The ultimate vacuum obtainable about the outlet of the orifices in any stage of the pump is no greater than the vapor pressure of the material issuingfrom the orifice, and hence it isobviously -desirable that the higher vapor pressure materials shall issue from the orifice at the top of the cylinder I6 rather than from the higher vacuum stage constituted by the orifice at the top of cylinder ='I'I. With this in mind, I havearranged the cylinder Hi to have an opening in its *lower wall beneath the level of the liquid 9 atone sideof the pump, and the cylinder H to have a similar opening at a different point. The duct 21b and the two openings just mentioned may, for instance, be positioned degrees apart about axial rod I2. -In this way any liquid flowing-down the walls of cylinder to oluct -2 I b 'mus't first flow into the space within cylinder I6 and 'be heated there before it can-flow through the respective openings into the interior of cylinder 'I- I and into the annular space communicating with nozzle 2 Ia. I

The umbrella i5 is made to extend a considerable distance below the upper end of the cylinder I'G and in designing a :pump ior use of the hydrocarbon molecular lubricant, :I have "found it desirable to flare the umbrella I'5 :atan angle of approximately 15 to the vertical. Iihaverfound a width of about one-sixteenth :of .an inch for the throat of this oiiifice' to give good results. However, the form and idimensions of the orifice constituted by the cylinder I6 and umbrella I5 are .notiso criti'cal a matter as is the shaping 10f the members constituting the lorificeat the upper endof -cylinder .I:I.

In the case of the last-mentioned -ori-fioe,.. I have effected a great improvement over prior :art devices by flaring the upper :end' of zthe 'cylinder :I-I outwardly :and also flaring :the lower end :of the cap I4, teach :at .an :angle :of approximately 12% to the vertical. I have found that withth'e particular hydrocarbon present mentioned above, an annular spacing of about one-sixteenth of an inch between the end of cylinder H and cap l4 gives good results, and that the distance from the throat of this one-sixteenth space to the lower edge of the cap l4 may suitably be made about one inch. In this Way, it will be noted that the central line of the vapor jet is substantially parallel to the axis of tube l.

Another feature of importance, probably also effecting a decrease of turbulance in the vapor stream is that of making the distance between the lower edge of the cap 14 and the umbrella l of the order of the mean free path of the vapor molecules flowing from the lowest pressure orifice. The mean free path of molecules of the vapor for the pressure corresponding to its vapor pressure at the temperature of the wall of cylinder I can readily be calculated from known data of physical chemistry. In the case of the four-inch pump above mentioned, I have found this mean free path to be of the order of five or six inches.

Referring now to the modification of my invention shown in Fig. 2, a cylinder 3! which may be of steel has a connection 32 at one end, suitable for attachment of an enclosure to be evacuated. The cylinder is preferably surrounded by a pipe 33 for cooling water similar to the pipe 28 in Fig. l; and at its opposite end is provided with a connection for a duct 34 leading to a backing-up pump analogous to that attached to the ofitake pipe 5a in Fig. 1.

Projecting through one side of the cylinder 3! are a series of ducts 35, 36, 31, each of which is bent over and provided with an expanded portion to form a nozzle having an axis substantially coincidental with the axis of the cylindrical casing 3!. The lower ends of the pipe 35, 36, 3! connect to a series of chambers 38, 39, 4!, adapted to contain oil or other working fluid for the diffusion pump, and respectively provided on their lower surfaces with heaters 42. 43, 44 which are analogous to the heater 2 in Fig. 1. The ducts 35, 3B, 31 will be seen to provide a series of nozzles which will act by projecting vapor of the working fluid, as three stages of a diffusion pump analogous to the vertically disposed stages of the pump in Fig. 1. The fluid vapor issuing from the three nozzles are condensed upon striking the cooled wall of the cylinder; and will thereafter run by gravity to a duct 45 projecting from the lower wall of the cylinder 3| and connected to the chamber 39 which supplies vapor to the intermediate nozzle 35. The chamber 39 has an opening beneath the surface of the oil therein which has branches running respectively to the chambers 38 and 4| by pipe lines which are maintained everywhere below the lever of the oil surface in the chamber 39. In order to facilitate the draining of the condensed oil from the walls of the cylinder 3| through the duct 45, the walls of the cylinder may be deformed slightly so that the outlet to the duct 45 is at the lowest point in the wall of chamber 3|.

It will be evident that in the modification shown in Fig. 2, the condensed oil is first returned to the boiler section which supplies vapor to the intermediate stage of the pump, and then flows out therefrom to the boiler sections supplying the high-pressure and low-pressure stages of the pump.

While I do not wish to be bound by any theory as to mode of operation, it is my present belief that the embodiments of my invention shown in Figs. 1 and 2 have the following advantages over prior art difiusion pumping systems:

1. Since the oil is returned to a jet of intermediate pressure, the fractionation is done with the aid of the pumping action of the previous stage. This leads to better fractionation.

2. The pumping of the oil vapor by the first stage gives less condensation of the high vapor pressure fractions of the oil since it is being pumped by a jet having low vapor pressure fractions of the oil.

3. While the intermediate jet will operate at slightly reduced efiiciency :the following jet or the one pumping against the forevac operates at increased efficiency due to the higher quality oil vapor supplied to it and so the overall efliciency of the pump will not be decreased.

4. The lower vapor pressure constituents supplied to the last jet will actually increase its effilciency to the point where it will pump against a higher backing pressure than conventional pumps.

While in accordance with the patent statutes. I have described a particular embodiment of the principles of my invention, those principles are capable of realization in various alternative arrangements which will be evident to those skilled in the art.

I claim as my invention:

1. A difiusion pump of the type having at least three stages comprising, a boiler subdivided into a number of sections equal :to the number of said stages, means defining an orifice for projecting vapor from the boiler section of each stage, a common collection-space for condensed vapor from said stages, and a drain from said space directly to the boiler section which supplies vapor to an intermediate stage.

2. A dilfusion pump of :the type having at least three stages comprising, a boiler subdivided into a number of sections equal to the number of said stages, means defining an orifice for projecting vapor from the boiler section of each stage, a common collection-space for condensed vapor from said stages, a drain from said space directly to the FREDERICK W. STALLMANN.

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