US3203624A - High vacuum diffusion pump - Google Patents

Description

' Aug. 31, 1965 SMITH, JR 3,203,624

HIGH VACUUM DIFFUSION PUMP Filed Aug. 6, 1962 2 Sheets-Sheet 1 a -3 F A 20 [Viz 447m 1 6 2 G 4 INVENTOR. Hue P. JM/ Th, /e. BY

o'goxbzw fi; 5:44, 0% W 1965 H. R. SMITH, JR 3,203,624

HIGH VACUUM DIFFUSION PUMP Filed Aug. 6, 1962 2 Sheets-Sheet 2 /vium 77m INVENTOR. G 6 #547 flue/r 2 5mm A United States Patent.

3,203,624 HIGH VACUUM DIFFUSION PUMP Hugh R. Smith, Jr., Piedmont, Calitl, assignor to Temescal Metallurgical Corporation, Berkeley, Calif., a corporation of California Filed Aug. 6, 1962, Ser. No. 215,157 4 Claims. (Cl. 230-101) The present invention relates to improved high vacuum diffusion pumps, having their various elements rearranged to increase their performance and efiiciency.

In prior diffusion pumps it has been the usual practice to arrange jets of vapor directed downwardly and outwardly from a central structure to an outer cylindrical condenser, which may also serve as a casing for the pump. The cylindrical condenser may have an open top, which serves as the mouth or the intake opening of the One deficiency of this prior arrangement is that many of the gas molecules entering the mouth of the pump strike the casing before they are entrained in the vapor jets and rebound back into the vacuum chamber. Thus, prior pumps fall fiar short of ideal performance, which would be to pump out every molecule that enters the mouth of the pump; and, in consequence, the pumping rate actually realized is only a fraction of what should be possible at a given mouth area and pressure.

Another deficiency of the prior-ant pump is that the condenser, which usually serves as the outer casing of the pump, is connected to the hot boiler by a short length of metal, which readily conducts substantial amounts of heat wastefully from the heated boiler to the cooled condenser, resulting in poor thermal efiiciency. Also, the diverging arrangement of the jets necessitates a higher pressure at the nozzles to produce the required vapor density near the relatively large condenser Walls. To produce this higher pressure, the boiler must be hotter, further reducing thermal efficiency, sometimes leading to overheating of the boiler, dissipation of the oil, uneven vapor generation, bumping, poor boiler efficiency, and shorter boiler life. In fact, boiler over-heating is the chief limitation on the capacity of many pumps.

An object of the present invention is to provide an improved, high-vacuum ditfusion pump in which the probability of molecules rebounding from the pump structure back into the vacuum chamber is substantially reduced.

Another object is to increase the thermal efiiciency of a diffusion pump by reducing the wasteful conduction of heat through metal parts between the boil-er and the condenser.

Another object is to reduce the vapor pressure and boiler temperature requirements.

According to the present invention, the vapor jets of the high vacuum diffusion pump are directed downwardly and inwardly to a centrally located condenser. In the preferred structure, an intermediate wall is arranged between an outer casing and the central condenser; and is provided with holes or nozzles that direct vapor jets inwardly and downwardly upon the inner condenser. These jets are supplied with vapor that rises through the space between the outer casing and the intermediate wall from a heated pool of oil, or the like, in a lower part of the pump. The uppermost jets can be quite close to the top of the intermediate wall, which is open to form the mouth of the pump, so that a high percentage of all molecules entering the mouth of the pump are entrained by the jets before they can strike any solid part of the structure.

A further advantage lies in the non-diverging arrangement of the jets, which provides the required amount of vapor in the vicinity of the condenser with less vapor at "ice the nozzles, and thus less vapor pressure in the boiler. A still further advantage lies in the greater thermal efficiency attained in consequence of a smaller central condenser, which is more readily isolated from the boiler through a greater length of smaller cross-section material, which conducts much less heat, wastefully, from the boiler to the condenser than in the case with prior diffusion pumps.

A better understanding of the present invention may be had from the following illustrative description and the accompanying drawings.

FIG. 1 is a schematic, top view of a conventional priorart high vacuum pump;

FIG. 2 is a vertical section taken along the line 2-2 of FIG. 1;

(FIG. .3 is a schematic top view of an improved high vacuum diffusion pump embodying principles of this invention;

FIG. 4 is a vertical section taken along line 4--4 of FIG. 3;

FIG. 5 is a schematic top view of another improved high vacuum diifusion pump embodying principles of this invention;

"BIG. 6 is a vertical section taken along line 66 of FIG. 5.

For purposes of comparison, reference will first be made to a conventional prior-ant, high-vacuum diffusion pump as illustrated in FIGS. 1 and 2. This conventional pump has a cylindrical casing 1 fittedwith a flange 2 at its open top end for connection to the high-vacuum chamber (not shown), which is to be evacuated by the pump. A liquid, usually a special oil, collects in a pool 3 at the bottom of the cylindrical casing 1. This oil is heated by any suitable heating apparatus 4 until vapor is produced. This vapor passes upward and inside the central structure 5, the direction of vapor flow being indicated in the drawing by solid-line arrows. At various places on the central structure 5, there are provided downward and outward facing openings or nozzles 6, 7, 8. [Fr-om these openings, vapor issues in the form of vapor jets directed downward-1y and outwardly, as indicated by the solid-line arrows.

Molecules from the high-vacuum chamber connected to the open top end of the pump randomly enter the region between the cylindrical casing 1 and the central structure 5. Gas molecules entering the paths of the above-mentioned vapor jets are entrained and carried downward, and are eventually expelled through pipe 9, which leads to mechanical pumps for maintaining a partial vacuum at the outlet of the diffusion pump, as is customary.

The walls of the casing are cooled, for example, by cooling pipes 10 attached to the outer surface of easing 1' for receiving a continuous flow of cooling Water, or the like. The vapor jets strike the inner surface of cylindrical casing .1, and the oil vapor condenses on its relatively cold walls; thence the condensed oil runs down the sides of easing 1 into pool 3 at the bottom. From this it can be seen that the oil is continuously recirculated within the vacuum pump, whereas the noncondensible gas molecules entrained by the vapor jets are expelled through the pipe 9.

One deficiency of the prior-art pump described is that molecules leaving the high-vacuum chamber, as indicated by broken line 15, may strike the cylindrical casing 1 before they are entrained by the vapor jets. Since the surface of the cylindrical casing 1 is extremely rough when measured by molecular dimensions, such a molecule may rebound in any direction, and, for example, may either be deflected along path 16 into the vapor jets, or rebound along path 17 back into the vacuum chamber.

Assuming that the probability of rebounding back into the chamber is 50%, each molecule taking a path like path would have only a fifty-fifty chance of being entrained in the jets. Hence, the efficiency of this pump in removing from the system molecules that get into the open top end of the pump is greatly reduced. On the other hand, even if a conventional pump is constructed with a relatively smooth surface this difiiculty is still not avoided; since it may be generally stated that molecules which strike a surface under high-vacuum conditions are re-emitted according to the cosine law, which assumes that very few molecules are specularly reflected but, rather, stick to the surface suificiently long to reach an energy equilibrium with the absorbing surface. Thus, even with a surface which is smooth in relation to molecular dimensions, there is a substantial probability that a molecule will be re-emitted back out of the pump entrance and into the vacuum chamber.

Another deficiency of the prior-art pump will become apparent from inspection of FIG. 1. If the common type of cylindrical pump is employed, the vapor jets diverge while flowing outwardly from the central structure 5 to the condenser or casing 1, thus causing the vapor density to decrease as the radial distance from the center of the pump increases. Because of this, increased vapor pressure must be produced in the interior of the inner structure 5, in order to provide adequate vapor density near the outer casing. Since greater vapor pressure is required, more heat must be applied to increase the temperature within the boiler, reducing the heating efiiciency.

Referring now to FIGS. 3 and 4, an improved pump embodying the present invention has a cylindrical casing 29 provided with an open top end fitted with a flange 21 for connection to the high-vacuum chamber (not shown) which is to be evacuated. Oil is collected in a pool 22 at the bottom of the casing 20, and the heat necessary to vaporize the oil in the pool 22 is supplied by any suitable heating means such as gas or electric heater 23. Pipes or tubes 24, through which cooling water may be circulated when desired, may be provided within the boiler, adjacent to the wall heated by the heater, to cool the oil pool when the pump is to be shut down, thereby facilitating rapid shut-down.

The oil vapor passes up through the space between the outer cylindrical casing and an intermediate cylindrical wall 25, which is provided with a number of openings or nozzles 26, 27, and 23, directed downwardly and inwardly, as shown. Through these openings, jets of vapor issue as indicated by the solid-line arrows. The vapor jets strike an inner condenser structure 29, which is kept relatively cold by conventional means such as the circulation of water at ambient or cooler temperatures through cooling pipes 30. Vapor condenses on structure 29 and runs down its outside surface into pool 22, thus providing a continuous circulation of oil. The jets entrain any gas molecules that enter their paths, and eventually such gas molecules are expelled through a pipe 31 leading to a conventional mechanical pump.

The returning oil droplets fall back into the pool 22 at a point which is at a higher elevation than the bulk of the oil because of the difference in pressure between the high-vacuum space inside intermediate wall and the space between wall 25 and casing 20. The lower part of wall 25 extends down into the oil pool to form a trap so that this pressure, created by the vaporization of oil in the boiler, can be maintained, whereby the vapor is forced through nozzles 26, 27, and 28 to form jets.

The casing 20, preferably is surrounded by gas or electric heaters 32 which keep the casing sufiiciently hot so that vapor will not condense thereon and fall wastefully back into the pool 22. Also, these heaters may be used to superheat the vapor somewhat, if desired. Insulation 33 reduces heat loss to the surrounding atmosphere.

Comparing the new pump, FIGS. 3 and 4, with the conventional prior-art pump, FIGS. 1 and 2, it is apparent that the present invention provides a pump that presents, to entering molecules, a much higher proportion of vapor jet area to solid surface area. The uppermost jets issue from points near the top of the casing 20, at the mouth of the pump, and shield most of the solid parts of the pump from molecules entering the mouth of the pump from the vacuum chamber. Since the top portion of condenser 29 is small, a relatively small number of molecules strike this solid surface and rebound back into the vacuum chamber from it. For this reason, gas molecules entering the top of the pump have a higher probability of entrainment by the jets before striking any solid part of the apparatus and, hence, have a low probability of rebounding back into the vacuum chamber. Furthermore, condensing structure 29 is small, and is centrally located, so that the absorption of heat from sources other than the condensing vapor is minimized providing a significant increase in the cooling efliciency. It should be noted also that the cold condenser is connected to the hot boiler by a part 34, preferably made up of thin sheet metal, which provides a relatively long, small cross-section path for the conduction of heat. Thus, the wasteful flow of heat between the hot boiler and the cold condenser, through the metal parts connecting them, is much less in the improved pump than it is in conventional, prior-art pumps.

When the improved pump is cylindrical, the vapor jets flow radially inward, as shown in FIG. 3, from the intermediate wall 25 to converge on the central condenser structure 29. Hence the vapor density near the condenser is at least as great as that near the nozzles, and the required vapor density at the nozzles is minimized, thereby alleviating the need for high vapor pressures in the space between the outer casing 20 and the intermediate wall 25. Because adequate vapor density near the condenser surface is thus maintained with a lower vapor pressure in the boiler than is required in prior-art pumps, boiler temperatures ar reduced, thermal efliciency is increased, less heat input is required, and boiler life is lengthened.

It should be noted that the present invention is not limited to cylindrical embodiments, and that other shapes and configurations may be used when desired. For example, certain advantages are present with rectangular pumps, as illustrated by FIGS. 5 and 6. These rectangular pumps may be made long and narrow, fitting into areas which would be inaccessible to a circular pump of the same capacity. Also, when a plurality of pumps are desired, they may be banked or stacked side-by-side or back-to-back without loss of valuable space.

The rectangular pump is essentially the same as the circular pump, except for the change from circular to rectangular geometry. FIG. 6 shows a vertical section wherein an outer casing 5i with gas or electric heaters 51 attached thereto, is surrounded by insulation 52. As before, the use of heaters on the outside wall will increase efliciency by adding thermal energy to the vapor, which might otherwise condense on the casing and drop back down into the oil pool. The intermediate wall 53 has openings 54, 5-5, and 56 in it, forming orifices or nozzles to allow jets of vapor to flow downwardly and inwardly toward the central condensing structure 57. These openings may be in the form of elongated, rectangular slits, forming jets that are wide, thin, sheets of vapor. The inner condensing structure 57 is centrally located within the intermediate wall 53, and may comprise two parallel metal plates, forming a thin, flat, hollow wall, filled with flowing water 58 or other liquid supplied at ambient or lower temperature through pipes 59 connected to each end of the condenser structure; or it may comprise a single plate to which copper cooling-coils are attached for the circulation of the cooling fluid. The condensing structure is supported and separated from oil pool 61, and oil pool heaters 62, by a thin supporting flange 63, providing a relatively long, small cross-section, and therefore highimpedance, path for the conduction of heat. This flange provides the necessary insulation between relatively cold inner condenser 57 and relatively hot oil pool 61 and heaters 62. Cooling-water pipes 64 are provided in the oil pool for rapid cooling at shutdown.

The exhaust pipes 65 and 66, which lead to conventional mechanical pumps, run parallel to condenser 57 and are shielded from the oil flowing down condenser 57 by shields 67 and 68 of semicircular cross-section, as shown.

Referring to FIG. 5, it will be seen that all vapor jets are either parallel or converging; thus, the substantial decrease in vapor density as the distance from the jet openings increases, characteristic of prior-art pumps, is avoided in the present invention. Hence, the vapor pressure required at the nozzles to maintain adequate vapor pressure near condenser 57 is not as high as in prior-art pumps, and less heat, particularly a lower boiler temperature, is required to maintain such vapor pressure.

This invention in its broader aspects is not limited to the specific examples herein illustrated and described, and the following claims are intended to cover all changes and modifications that do not depart from the true spirit and scope of this invention.

What is claimed is:

1. A high vacuum diffusion pump comprising an elongated hollow conduit having an inlet at its upper end adapted to be placed in communication with a chamber to be evacuated and having an outlet spaced downwardly from the inlet and adapted to be connected to an exhausting pump, nozzle-defining means disposed around the periphery of said inlet and capable of providing downwardly and inwardly directed vapor, a casing encircling said conduit and including a wall spaced outwardly from the wall of said conduit so as to define a passageway between the conduit wall and the adjacent wall of the casing, said passageway being in communication with said nozzledefining means and being adapted to supply vapor from a vapor source to said nozzle-defining means, said conduit and said casing being interconnected at their upper ends so as to preclude the passage of gas molecules downwardly therebetween, and a condenser surface arranged within said conduit and positioned in the path of vapor provided by said nozzle-defining means.

2. A high vacuum difiusion pump comprising an elongated hollow conduit having an inlet at its upper end adapted to be placed in communication with a chamber to be evacuated and having an outlet spaced downwardly from the inlet and adapted to be connected to an exhausting pump, nozzle-defining means disposed around the periphery of said inlet and capable of providing clownwardly and inwardly directed vapor, a casing encircling said conduit and including a wall spaced outwardly from the wall of said conduit so as to define a passageway between the conduit wall and the adjacent wall of the casing, said passageway being in communication with said nozzle-defining means and being adapted to supply vapor from the vapor source to said nozzle-defining means, said conduit and said casing being interconnected at their upper ends so as to preclude the passage of gas molecules downwardly therebetween, means for heating the walls of said casing which define said passageway so as to prevent condensation thereon of vapor conducted through said passageway, and a condenser surface arranged within said conduit and positioned in the path of vapor provided by said nozzle-defining means.

3. A high vacuum diffusion pump comprising a casing closed at its lower end and having an upper end adapted to be placed in communication with a chamber to be evacuated, said lower end of said casing being adapted to contain a body of pump fluid, a generally cylindrical conduit disposed within said casing and having a lower end positioned below the surface of said body of pump fluid and having an upper end defining the inlet to said pump, adjacent walls of said conduit and said casing defining a passageway therebetween for the upward flow of vapors from said body of pump fluid and being interconnected at their upper ends to prevent entry of gas molecules into said passageway, nozzle-defining means located around the periphery of said inlet defined by said conduit and being adapted to direct vapor rising in said passageway downwardly and inwardly of the interior of the conduit, further nozzle-defining means spaced downwardly from said first-mentioned nozzle-defining means and adapted to direct vapor downwardly and inwardly of the interior of the conduit, means for heating that portion of the walls of said casing defining said passageway, a generally cylindrical condenser arranged centrally within said conduit in the path of vapors provided by said nozzle-defining means, said condenser being of a relatively narrow diameter with respect to the diameter of said conduit and defining an outlet at its lower end adapted to be connected to an exhausting pump.

4. A high vacuum difiusion pump comprising, an elongated hollow conduit defining at its upper end an unobstructed pump inlet adapted to be placed in communication with a chamber to be evacuated and communicating with a pump outlet spaced downwardly from the inlet and adapted to be connected to an exhausting pump, nozzle-defining means disposed around the periphery of said inlet capable of providing converging downwardly and inwardly directed vapor streams, means connected to said nozzle-defining means for supplying vapor thereto, a generally cylindrical condenser surface within said conduit intermediate said pump inlet and said pump outlet and positioned in the path of converging vapor provided by said nozzle-defining means, said generally cylindrical condenser surface having a relatively narrow diameter with respect to the diameter of said conduit and being arranged within said conduit such that the vapor streams provided by said nozzle-defining means converge thereon, thereby providing a greater vapor density at said surface than adjacent the periphery of the inlet, and means connected to said condenser surface for isolating said pump outlet from the vapor source.

References Cited by the Examiner UNITED STATES PATENTS 2,819,011 1/58 Winkler 230-101 3,096,928 7/63 Bukata 230l01 FOREIGN PATENTS 1,188,768 3/59 France.

LAURENCE V. EFNER, Primary Examiner.

WARREN COLEMAN, Examiner.

Claims (1)

1. A HIGH VACUUM DIFFUSION PUMP COMPRISING AN ELON GATED HOLLOW CONDUIT HAVING AN INLET AT ITS UPPER END ADAPTED TO BE PLACED IN COMMUNICATION WITH A CHAMBER TO BE EVACUATED AND HAVING AN OUTLET SPACED DOWNWARDLY FROM THE INLET AND ADAPTED TO BE CONNECTED TO AN EXHAUSTING PUMP, NOZZLE-DEFINING MEANS DISPOSED AROUND THE PERIPHERY OF SAID INLET AND CAPABLE OF PROVIDING DOWNWARDLY AND INWARDLY DIRECTED VAPOR, A CASING ENCIRCLING SAID CONDUIT AND INCLUDING A WALL SAPCED OUTWARDLY FROM THE WALL OF SAID CONDUIT SO AS TO DEFINE A PASSAGEWAY BETWEEN THE CONDUIT WALL AND THE ADJACENT WALL OF THE CASING, SAID PASSAGEWAY BEING IN COMMUNICATION WITH SAID NOZZLEDEFINING MEANS AND BEING ADAPTED TO SUPPLY VAPOR FROM A VAPOR SOURCE TO SAID NOZZLE-DEFINING MEANS, SAID CONDUIT AND SAID CASING BEING INTERCONNECTED TO THEIR UPPER ENDS SO AS TO PRELUDE THE PASSAGE OF GAS MOLECULES DOWNWARDLY THEREBETWEEN, AND A CONDENSER SURFACE ARRANGED WITHIN SAID CONDUIT AND POSITIONED IN THE PATH OF VAPOR PROVIDED BY SAID NOZZLE-DEFINING MEANS.

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