US2361245A - Vacuum pump - Google Patents

Description

'Oct. 24,1944. F. w. STALLM ANN 2,361,245

VACUUM PUMP Filed March 27, 1943 2 Sheets-Sheet l WITNESSES: v INVENTOR [Faer/ck 526/057 017/7 v F. w. STALLMANN v VACUUM PUMP Filed March 27, 1943 Oct. 24, 1944.

2 Sheets-Sheet 2 ATTORN Patented Oct. 24, 1944 VACUUM PUMP Frederick W. Stallmann, Wilkinsburg, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsbur- Pennsylvania gh, Pa., a corporation of Application Mar-ch27, 1943, Serial No. 480,723

Claims.

My invention relates to vacuum pumps 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.

Diffusion pumps of the general type illustrated in Patent 1,320,874 to Langmuir, and using oil as a working fluid in accordance with the disclosure of Crawford Patent 1,367,865 have come into wide use in the art of exhausting containers, notably electronic discharge tubes and the like, to pressures of the order of a small fraction of a micron, a micron being one millionth of the pressure of a column of mercury one meter high. Basically, such pumps make use of a stream of some vapor flowin through an orifice from a boiler, the orifice being so positioned that the vapor stream entrains gas molecules flowing from a container in which the vacuum is to be produced, and drives these molecules in the direction of a pipingsystem issuing eventually into the atmosphere, the vapor itself being condensed on a properly cooled surface in the path of outflowing of the gases, and then being returned to the boiler for reuse. It is usually most economical to provide the ofitake piping system with so-called backing-up pumps of the mechanical type which reduce the pressure at the outlet from the diffusion pump proper to a pressure of the order of 50 microns or less; and even in such cases it is usually found more efiicient to design the difiusion pump so that a number of vapor jets operate in tandem or series with each other so that the total pressure rise from the intake to the offtake of the pump comprises a number of steps or stages. The type of pump by which I illustrate the broader principles of my invention herein comprises a plurality of such stages.

Multi-stage diffusion pumps are not in themselves novel as-is shown, for example, by Nelson Patent 2,291,054, but I have devised certain features which greatly improve the operation of these prior art pumps in a way which will be explained more fully below. Typical examples of such features are the shape of nozzle or orifice through which the working vapor is ejected; the spacing between the orifices of successive stages; the division of the surface of the boiling liquid into a number of separate areas having certain specific ratios to each other and individual'to various stages of the pump; and

the mechanical assembly which makes it much out in more detail in connection with the description of the drawings, in which:

Figure 1 is a, view partly in elevation and partly in cross section of one embodiment of my invention,

Fig. 2 is a sectional view taken along the line II-II thereof.

Fig. 3 is a view similar to Fig. 1 of a different an electrical heating element 3 of a type too i well-known 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 l 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 shown) is connected vacuum tight through a flanged connection 5a to the cylinder I.

The bottom portion of the cylinder l 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 ofi-flow of vapor.

Centrally positioned in the cylinder l is a cylindrical vertical pipe II which may, if desired, be of the same material as the cylinder l, and which is preferably flared outward at its lower end to rest on the base i. The cylinder II is likewise provided with an outwardly flared portion at its upper end as will be described in more detail beeasier to dismantle and clean my pump than such prior art pumps as I am familiar with.

The principles of my invention will be pointed of the cap will be discussed in detail below. The

cap I4 is held firmly against the spacers l3 by a nut threaded on the end of the tie rod It.

It will be evident that the upper portion of the cylinder ll together with the cap l4 constitutes what may be term-ed an annular nozzle or orifice from which vapor emitted by the central part of the liquid 9 and flowing through the cylinder II is discharged into the annular space between the is made roughly of the order of of that of the cylinder I in order to provide a suitably 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 ll of a diameter smaller than that just mentioned} such reduction in diameter would not greatly add to the available annular space surrounding the cylinder ll but would materially reduce the pressure of the vapor at the inlet side of the above-mentioned annular orifice. Beyond these general limitations the diameter of the cylinder ii is not a critical dimension but failure to realize the importance of even these general limitations has resulted in inefilciency in many pumps of the prior art, of which I am acquainted.

A hood or umbrella I! of conical form is fastened to the cylinder H at a distance below the lower end of the cap ll which is roughly equal to the mean free path of the molecules of the vapor issuing from the upper end of cylinder ll.

Surrounding the cylinder II is a second and shorter cylinder l8 which may be of the same material as cylinder I l and the lower end of which 'rests on the base 2. The diameter of the cylinder little space for gases to flow from the space above 1 the umbrella l5 to the ofltake conduit 5.

The upper end of the cylinder I8 is bevelled and spaced slightly away from the umbrella iii to provide an annular orifice fromwhich vapors rising from the surface of the liquid 9 into the space between the cylinders ll and I6 can be elected into the annular space intervening between the cylinders l6 and I.

1 Approximately 1 6 of the length of the cylinder i above the bas 2 is provided a horizontal partition I! which may conveniently be screwed into threads cut on the interior wall of the cylinder l and which covers the space between the cylinder l6 and the outer wall of the cylinder I. This partition I1 is provided with a groove in its lower face which fits the upper end portion of a liner l8 for the interior wall of the lower portion of cylinder l.

The partition I1 is likewise provided with a bevelled hole 2| issuing into a small spout 22 which is intended to dip below the surface of the liquid 9, and to return the latter to the condensate of the vapor which has been condensed on the inner surface of the cylinder l.

A bent cylinder or nozzle Zia is removably inserted in the disc l1 and extends to eject vapor into oiftake 5. a e

The exterior wall of the-casing I and 5 is surrounded by a helical pipe 28 through which water or other cooling fluid flows to maintain the temperature of such walls at a sufliciently low temperature to condense the vapor flowing from the orifices and nozzles already described.

The general mode of operation of the above described pump structure is as follows: The fluid 9 which may be any suitable material described in the art as useable in diffusion pumps, but for which I preferably employ a hydrocarbon of the ring-compound type such as molecular lubricant is heated and th resulting vapor flows up from its surface through the respective cyl nders II, It and 2| a. It will benoted that the ends of the first two of these cylinders cooperate respectively with the cap l4 and the umbrella l5 to form annular orifices through which a stream of vapor flows out into the annular space surrounding such cylinders. These streams of vapor molecules entrain any gases present in the space adjacent their outlets, and the principal component of velocity of these molecules being substantially parallel to the axis of the respective cylinders they impart a similar velocity to any gases which they entrain. Thus the vapor flowing from the orifice at the upper end of the cylinder 2la will entrain any gas in the annular space between its upper end and the wall of pipe 5 and drive it upward in the direction of the arrow appearing above pipe 5a. The backing pump connected to the upper end of the pipe Ia will then carry this gas away and reject it to the atmosphere. However, the vapor flowing from the orifice of cylinder Zla will quickly strike the cold walls of the cylinder 6 and be condensed. It will thereafter gradually travel back down the interior wall of the pipe 5 to the spout 2lb to be reused in the boiler energized by heater 3. Thereupon any gas or vapor present in the annular space between the nozzle Ho and pipe 5 below 'the upper end of nozzle 2Ia will diffuse into the space just described as evacuated by the gas entrained through the efficiency of the vapor jet and hence the orifice at the upper end of nozzle 2la constitutes a pump which continually tends to decrease the pressure within the pipe 5, and the lower portion of cylinder i. In effect, therefore, the pipe 5 may be considered to constitute a backing pump evacuating gas from the lowest portion of the pump cylinder l. l

Similarly, vapor emanating from the portion of the liquid 9 between the cylinders H and iii flows out through the annular orifice between the upper end of cylinder l6 and the umbrella l5; its molecules have a principal velocity component in the downward direction towardthe inlet to pipe 5 and so impart a downward velocity to any gas in the region of the lower edge of the umbrella l5, thereby tending to evacuate this region. The molecules of this vapor stream, however, quickly acquire enough random velocity in the horizontal direction to be projected into contact with the cool wall of cylinder I. They are, accordingly, condensed on the latter and travel down the wall thereof to the partition 11 and vback into the liquid 8 through the spout 2|b.

The orifice about the upper end of the cylinder lli thus constitutes in effect a backing pump for evacuating any permanent gas molecules present in the region of the lower edge of umbrella l5. Consequently, any such gaseous molecules present in the annular space above the umbrella I! will tend to difiuse, into the space below the lower edge of the umbrella l5 and be carried away through the pipe 5 to the outlet of the system.

In a similar manner, the annular spac between the upper end of the cylinder H and the cap it constitutes an annular orifice through which vapor flows with its molecules having a high component of velocity downward parallel to the vaxis of cylinder I., These vapor molecules will impart a similar downward component of velocity to any gas present in the annular space about the lower edge of cap I and tend to drive this gas into the space below the umbrella l5. Gas molecules present in the space above the lower edge of the cap M will, therefore, difluse into the space evacuated by the molecules below the edge of cap I and a continuous stream of gas will consequently the members constituting the flow from the apparatus to be evacuated which is attached to the flange 4. The vapor molecules flowing from the orifice about the upper end of cylinder H will quickly acquire comp nents of horizontal velocity which will 'carry them into contact with the cooled walls of the cylinder I and they will then pass down the sides of the latter to the partition I! and into the boiler through spout 2Ib.

It will be evident from the foregoing that the vapor jets flowing respectively from the upper end of cylinders I I, I6 and 2 la 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 from the upper end of cylinder 2Ia constitutes the highest pressure or lowest vacuum stage.

In actual practice, it is found impossible toobtain pumping fluids which are literally and absolutely devoid of impurities; and in particular, while the hydrocarbon fluids such as I have proposed using have nearly uniform constitution, they are nevertheless found to have components of slightly different vapor pressures at anygiven 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 ori- I beneath the level of the liquid 9 at one side of the pump, and the cylinder II to have a similar ,opening at a diametrical point.' In this way what may be termed a labyrinth is formed between the cylinders II and I6 below the surface of the liquid 9 so that any liquid flowing down the wall of cylinder I to spout Zlb must first flow into the. space between cylinders II and IG'and be heated there before it can flow around and through the other opening into the interior of cylinder II. Before the liquid 9 reaches the cylinder II, therefore, the higher vapor pressure components are boiled away and the vapor used in the second stage of the pump is accordingly made up largely of the higherpressure components. On the other hand,.the vapor flowing through and from the upper end of cylinder II to constitute the first and highest vacuum stage of the pump is predominantly the low pressure component.

The umbrella I6 is made to extend-a considerable distance below the upper end of the I cylinder I 6 and'in designing a pump for use of the hydrocarbon molecular lubricant, I have found it desirable to flare the umbrella I5 at an angle of approximately 15 to the vertical. I have found a width of about of an inch for the throat of this orifice to give good results.

Howeven'the form anddimens'ions. of the orifice constituted by the cylinder-I6 and umbrella I5 is not so critical a matter is the shaping of "ifice at the upper end of cylinder I I.

In the case of the last mentioned orifice, I have effected a great improvement over prior art devices by flaring the upper end of the cylinder II outwardly and also flaring the lower end of the cap I4, each at an angle of approximately 12 to the vertical. I have found that with the particular hydrocarbon present mentioned above, an annular spacing of about of an inch between the end of cylinder II and cap I4 gives good results, and that the distance from the throat of this 1 space to the lower edge of the cap I4 may suitably be made about 1". In

this way, it will be noted that the central line of th vapor jet is substantially parallel to the axis of tube I.

While I do not wish to be limited by such a theory, it is my present belief that it is highly important in attaining pumping eiiiciency to minimize turbulence in the stream of vapor outfiowing from the orifice just mentioned, and that such turbulence is greatly reduced, at the pressures prevailing with the materials I have mentioned by proportioning the structure as I have just described. WhileI doubt if the matter the upper face of the passage above the narrow dismantled for cleaning.

gap between cylinders II and M. I

'Another feature of importance, probably also effecting a decrease of turbulence in the vapor stream is that of making the distance between the lower edge of the cap I4 and the umbrella I5 of the order of the mean free path of the vapor molecules flowing from the lowest pressure orifice. The mean free path of moleculesof 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 4" pump above mentioned, I have found this mean free path to be of the order of 5 or 6".

Another feature of importance tending to promote efficiency in the operation ofsuch a diffusion pump as is the proper proportioning of the relative areas of the surface of liquid 8 which are included respectively inside the cylinder II and between the cylinders II and I6,

and between cylinder I6 and the'wall I. While- I suspect that this may vary with the composition of the working fluid used, I have found that for the fluid I have specifically described, the areas above mentioned should be respectively in the ratios /2:1:2.

Another feature of importance in practical operation of pumps is that they shall be easily The pump which I have shown in Fig. 1 can.' very readily. be dismantled by'merely unbolting the flange 4 and then after removing cylindrical nozzle 5a, unscrewing the partition II and the nut at the top of the tie rod I2. All parts in the interior of the cylinder I can then be readily lifted out'and the entire assemblage subjected to cleaning operations. '0 J While I have described the cylinder l as, in

efiect, backed up by a second pump comprising the nozzle a, it will be recognized that other forms of backing pumps may be substituted at the outlet of the ofitake pipe 5, and that the nozzle 5a and its parts are merely illustrative of one convenient form of backing pump.

Figs. 3 and 4 show a modification of my invention in which the lowest vacuum stage nozzle 5a is replaced by an auxiliary pumping cylinder 6 somewhat similar, in general, but diiferent in detail, from the cylinder l. The cylinder 6 is provided at its lower end with a base I and an auxiliary heater 8 of the same general character already described in connection with cylinder l.

The upper end of the cylinder 6 is provided with a flange to which a vacuum tight connection from a backing-up pump, for example, of the mechanical type, may be connected.

Centrally positioned in the cylindrical casing 6 is a cylindrical pipe 22 flared at its lower end and resting on the base I to which it may be aflixed by wedge or screw clips or other suitable means. The upper end of the cylinder 22 is flared outwardly and extends to within a short distance of the-cylindrical wall 6. Uniformly spaced from the upper flared end of the cylinder 22 and conveniently separated therefrom by spacing members 23 so as to leave an annular channel for outflow of gases is a member 24 having the form of a truncated cone. Similarly supported upon and spaced from the member 24 is a conical member 25 which extends to leave a narrow annular channel between its rim and the cylinder wall 6. A partition 26 separates the liquid containing chamber in the base I from the interior of the cylinder 6 and this partition is provided with a spout 21 through which liquid condensed on the walls of the cylinder 6 may flow back into the boiler chamber.

I have, however, found that with such a form of backing pump as cylinder 6, it is possible to discharge relatively large volumes of gas even though the pressure at the offtake side of pump 6 rises as high as 500 microns instead of the 50 microns backing pressure which I mentioned above. The pumping cylinder I has a very high pumping capacity even at extremely low pressures by reason of the large cross section of the annular vapor flows in its highest vacuum and intermediate vacuum stages. A backing pump of the type shown in cylinder 6 is able to discharge such large volumes of gas even against a pressure of 500 microns at its outlet side provided the distance between the upper edges of the cylinders 22 andelements 24 and 25 is of the order of {e of an inch and the spacing of member 24 from elements 25 and 22 likewise about 1% of an inch.

Fig. 6 shows a slightly different modification of my pump in which the auxiliary pump ii is replaced by a pair of pump elements. Thus the cylinder I may be similar to Fig. 1 except as follows. The partition I! is provided at the side adjacent the outlet pipe with a conduit issuing into an orifice 32 having its upper end flared outwardly. The orifice 32 traversed by vapors boiling oif from the surface of the fluid ll constitutes what may be termed a third stage for backing up the first two stages embodied in cylinder l as already described in connection with Fig. 1. The conduit 32 is made readily detachable from the partition I! so that it may be removed through the flanged opening of the ofitake pipe'Bd when it is desired to dismantle the pump section i by unscrewing partition H. The flanged pipe 34 is provided for connecting an offtake pipe 35 which is provided with the elements of a backing pump similar to that already described in connection with cylinder 6 of Fig. 1. This backing pump in pipe 35 draws vapor from the fluid 9 by an auxiliary conduit 36, 31 of the form shown. In effect, therefore, the pump shown in Fig. 4 is of the 4-stage type.

While I have described particular embodiments illustrating my invention, it will be recognized that the principles thereof are of broad application in arrangements which will be evident to those skilled in the art.

I claim as my invention:

1. A diffusion pump comprising a container having'a liquid at its lower end, a first cylinder having its lower end extending below the surface of said liquid and having an annular orifice about its upper end, a second cylinder surrounding said first cylinder which is shorter and of larger diameter than said first cylinder and having an annular orifice about its upper end, the distance between the first mentioned orifice and the second mentioned orifice being of the order of the mean free path of molecules of the vapor of said liquid at the operating temperature of said container.

2. A diffusion pump comprising a container having a liquid at its lower end, a cylinder having its lower end extending below the surface of said liquid and having an annular orifice about its upper end, the upper end of said cylinder being flared outwardly, a cap in the form of a truncated cone extending downwardly over said flared end and enclosing the same, both said flared end and said cone flaring at an angle of about 12 /2" relative to the axis of said cylinder.

3. A diffusion pump comprising a container having a liquid at its lower end, a first cylinder having its lower end extending below the surface of said liquid and having an annular orifice about its upper end. the upper end of said cylinder being flared outwardly, a cap in the form of a truncated cone extending downwardly over said flared end and enclosing the same, both said flared end and said cone flaring at about the same angle relative to the axis of said cylinder, so as to direct a vapor stream substantially parallel to said axis.

4. A diffusion pump comprising a cylindrical casing closed at its bottom end, a first cylinder axially disposed in said casing, a second cylinder shorter and of larger diameter than said first cylinder surrounding the latter, the lower ends of said cylinders resting upon the lower end of said casing, the area of annular s acing between said casing and said outer cylinder being abou twice the area of the annular space between said first cylinder and said second cylinder. and the area of the space within said first cylinder being about V the area of the annular space hetiween said second cylinder and said first cylin- 5. A vacuum pump comprising a cylindrical outer casing having its lower end closed by a first transverse partition, a first cylinder axially disposed within said casing, a second shorter cylinder of larger diameter surrounding said first cylinder, both said cylinders having one end resting upon said partition and a detachable second partition enclosing a space around the lower ends of said cylinders. and detachable means extending to the region of the upper end of said casing for securing said cylinders to said first transverse partition.

FREDERICK W. STALLMAN.

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