US2903181A - Oil diffusion pump - Google Patents

Description

Sept. 8, 1959 w. A. GIEPEN OIL DIFFUSION PUMP 3 Shegats-Sheet 1 Filed June 5, 1956 .l'zwexzifor Wm fl. a '29 X2. 721

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OIL DIFFUSION PUMP Filed June 5, 1956 3 Sheets-Sheet 2 ui'wzey Sept. 8, 1959 w. A. GIEPEN 2,903,181

OIL DIFFUSION PUMP Filed June 5, 1956 3 Sheets-Sheet 3 Fig.6.

United States Patent OIL DIFFUSION PUMP William A. Giepen, Hingham, -Mass., assignor to High Vacuum Equipment Corporation, Hingham, Mass., a corporation of Massachusetts Application June 5, 1956, Serial No. 589,452

3 Claims. (Cl. 230-101) This invention relates to vacuum pumps, and particularly to pumps of the oil diifusion'type used for producing an extremely high vacuum,

The usual diifusion pump has a casing with a boiler at its lower end and with an intake opening or duct at or near its upper end for connection to a vessel to be evacuated. The boiler or reservoir is filled with a suitable difiusion fluid, such as one of the special fractionating oils prepared for this purpose, and is heated by internal or external heating coils to vaporize the medium. Between" the boiler and the intake of the pump is mounted a jet structure which consists, in general, of coaxially arranged tubes or chimneys with a corresponding number of outlets surrounded by baflles or umbrellas which direct the vapor downward. Fractionating difiusion pumps operate on the principle of bombardment or entrapment of molecules of gas in the atmosphere by vapor moving at high speed, some times at supersonic velocities, and the degree of vacuum which can be obtained by such a pump is dependent to a considerable extent on the speed of vapor flow which can be obtained. Most of the pumps on the market have a number of sharp inner and outer projections and square corners in the path of the vapor which produce turbulence areas in the vapor flow with consequent loss of energy and reduction in speed of the vapor. In a multi-stage jet thiseffect is particularly critical in the region of the cap which covers the top of the jet and serves as a baffle for the center chimney.

The speed of the vapor flow is dependent to a considerable degree on the rate of evaporation which, in turn, is dependent on the temperature at which the oil in the boiler is maintained. Generally thespeed of the vapor flow will rise as the boiler temperature is raised. It has been observed, however, that when the temperature reaches a certain critical value, which varies for different types of pumps and fluids, a sudden loss of vacuum" will occur, and above this temperature the pump operates very poorly. This phenomenon, which may be referred to as the collapse "or breakdown, is not entirely understood, but is believed to be, at least in part, the result of a suddent widespread shift from laminar toturbulent flow in the's'ystem.

It is, of course, desirable from the standpoint of efiiciency,'to operate the pump below the breakdown temperature, but even more important is the danger of spoilage of the work if a sudden loss of vacuum occurs during certain processes. On the other hand, it is generally desirable to operate the pump at as high a temperature as is possible, without exceeding the critical temperature, for maximum efiiciency.

The principal objects of this invention, accordingly, are to produce a diffusion ptunp in which laminar flow of the vapor can be maintained at high vapor speeds, and whiclr'can be continuously openated in a temperature range-only slightly below the critical temperature, thus achieving maximum efficiency. Otherobjects are to produce' a pump which-is inexpensiveto manufacture, and which can be easily dismantled, cleaned, and reassembled.

. 2 In the drawings illustrating the invention: Fig. 1 is a vertical cross-section of a vacuum pump constructed according to the invention;

Fig. 2 is an enlarged cross-section through the jet assembly along line 2-2 of Fig. 1; t

Fig. 3 is an enlarged cross-section through the jet assembly along line 3-3 of Fig. 1;

Fig. 4 is a vertical enlarged cross-section of a modified form of cap;

Fig. 5 is a cross-section alon the line 5-5 of Fig. 4;

Fig. 6 is a transverse cross-section of one form of heater for the pump boiler;

Fig. 7 is a cross-section of the heater shown in Fig. 1 taken along line 7-7;

Fig. 8 is a transverse cross-section of another form of heater; and

1 designated 14, of a type to be later described. The heating element may be enclosed by a cover 15.

The jet assembly is made up of three concentrically arranged tubes or chimneys 16, 17 and 18. Tube 16,

j which may be called the first chimney, rests on plate 14. A spun baffle or umbrella 19, which is curved to provide a smooth path for vapor passing out from between tubes 17 and 16, is attached to tube 17, which may be called the second chimney, and overlies the upper n'm 16a of tube 16. This rim is tapered and curved at the top to follow the contour of umbrella 19. Mounted in rim 1611 are three studs 20, whichare preferably rounded so as to produce a minimum of local disturbance in the vapor flow, and which serve as a seat for umbrella 19 to provide a uniform annular opening all around between the umbrella and rim 16a.

The second chimney 17 extends upward beyond umbrella 19, and has a flared downwardly curved rim 17a which is preferably spun integrally with the tube. A second curved umbrella 21 is mounted on chimney 18 and overlies rim 17a of chimney 17. Three round studs 23' are mounted in rim 17a to serve as a seat for umbrella 21. Chimney 18 extends upward beyond umbrella 21 and its top is curved to form a flared downwardly curving rim 18a, similar to rim 1712.

On the upper end of chimney 18 is mounted the cap 22. This cap overhangs rim 18a and has a central stem 22a which projects downward into tube 18. Rim 18a carries studs 24 which serve as spacers and seats for cap 22. Cap 22 is secured in place by means of a rod 25, having a hook-shaped upper end 25a which passes through a hole in the stem 22a, and formed integrally with, or attached to, a tension spring 27 which is securedto the lower end of tube 18 by means of a clip 28f The latter has bent legs 28a which engage under the lower end of tube 18 and which can be drawn together to disengage them from the tube and allow the cap and spring assembly to be lifted out from the top of chimney 18. The three chimneys are secured together by means of three set screws 29 having threaded portions 30 which screw into the heavy walled outer chimney 16, shoulders 31 which bear against the outside of chimney 17, reduced extensions 32 terminating in shoulders 32a which bear against chimney 18, and reduced end portions 33 which pass through holes in chimney 18 a 1 As shown in Fig. 3, umbrella 19 is secured to tube 17 Patented Sept. 8 19 59,-

by means of three blind rivets 34, the center pins of which project to bear against chimney 16. The studs are spaced 120 apart around tube 17 and serve to center chimney 17 with chimney 16 and thusprovideaccurate pacing. etwe n h qh m e i l y, as seenin Fig. 1, umbrella 21 is secured to chimney. 18 by means of three equally spacedblind rivets 35. The center pins of rivets 35 bear on chimney 17, and serve to center chimney 18 with respect to chimney 17. Rivets 34 and 35 are preferably placed somewhat out of line with each other and with screws 29 so as to divide the vapor stream along different'stream lines and thus avoid repeated disturbancet of the Vapor flow along any particular line. Studs 20, 23. and 24 are also oifset circumferentially from each other and from the sets of rivets. The vapor stream will quickly merge again after division around each of these small round obstructions, so that the discharge from each stage of the jetv assembly is substantially uniform all around.

The shape of the cap, as shown in Fig. 1, is of considerable importance. Most caps are flat on the under side and one of the major regions of turbulence is likely to develop in the central part of the upper end of tube 18 where the vapor flowing upward must divide in all directions to-pass out under the rim of the cap. In the present construction the stern 28a is shaped so as to present a stream-lined surface over which the vapor can divide readily in all directions with a minimum of turbulence, and the junction between the cap and the stem is curved on the under side to follow rim 18a so that the vapor passes smoothly out of the upper jet stage.

Another form of cap is shown in Fig. 4. This cap has a. somewhat more elongated pointed stem 37 and carries three wings 38 which may engage the inside of center chimney 18 to provide accurate centering of the cap on the chimney.

The jet assembly fits down over a pair of concentric tubular fins 40 and 41 which are received in grooves in bottom plate 13. These fins not only serve to position the jet on the plate, but are believed also to assist in the fractionating process, in a pump using a fractionating oil. These finsare provided with notches 42 and 43 at the bottom to allow for inward flow of the fluid, and chimney 16 is similarly provided with notches 44 to allow for return flow of fluid condensing around the outside of the jet. In operation the pump is filled with fluid aboutto the level of the tops of fins 40 and 4-1, and this region of the purnpconstitutes the boiler.

Mounted on the outside of casing 10, next to the boiler region, is a thermostat 45. This thermostat is connected in the energizing. circuit of the electric heating element 14. Ordinarily an electric coil or similar heating unit is used to heat the boiler of a diffusion pump. Such units are usually spaced away from the bottom of the boiler to avoid local overheating which might crack the oil.

The electric heating units of the heater elements here employed are preferably made in the type of a rod used as burner elements in electric stoves. This rod material consists. of a heat-resistant metal casing with a mica jacketed core of highly conductive metal. The composite rod can be formed into a variety of shapes. These electric heating units are embedded in a cast block of a metal which has good heat conduction properties, for example aluminum or copper. This block is mounted in direct contact with the bottom plate 13, thus effecting rapid heat transfer. The block also distributes the heat evenly so that the danger of local overheating is greatly reduced. As shown in Fig. 6, the heater element is formed as a small piece with diverging legs 46 and 47, and three parallel loops 48, 49 and 50. This element is cast into a cylindrical block 56.

In Fig. 7 the heater is made in the form of a single circular loop 51, cast into a ring-shaped block 57. Fig.

8 shows a tandem type of heater in the form of two independent loops 52 and 53, having reversely bent tip portions 52:: and 53a, respectively, cast into cylindrical block 56. Fig. 9 shows a heater similar to that of Fig. 6 but having only two loops S4 and 55 cast into ringshaped block 57. In all cases, the block is mounted immediately below, and in direct contact with, bottom plate 13 which forms the bottom of the :boiler.

The thermostat is designed to maintain the temperature within the optimum operating range, which depends on the size of the pump, and the type of fluid used. For example, a range of 380 to 410 F. is appropriate for a two inch diameter pump of this type using diethylhexylphthalate. For a six inch pump 375 F. is about the maximum eflicient operating temperature. The close control made possible by the use of heaters of the type here described, in conjunction with the thermostat, allows the pump to be operated in its range of highest efficiency without danger of a breakdown.

The entire jet assembly can be readily dismantled in a few minutes, for example, by releasing the clip 28 and removing the shouldered screws 29. The cap may then be lifted out and the chimneys separated from each other. The parts are. easy to-clean, as they have very few projections and crevices in'which contaminating mate-. rials can collect. To re-assemble the jet it is only necessary to set the chimneys in place, insert the three screws 29, and secure the cap by means of clip 28. The chimneys and cap will be aligned properly because of the studs 20, 23, 24, and rivets 34 and 35. An additional advantage of thisconstruction is its stability under the considerable vibration which is set up by the boiling of the diffusion medium. The three set screws are the only parts which can possibly be loosened, and these usually remain tight because the upward force on umbrellas 19 and 21 is transmitted through chimneys 18-v and 18a to the screws and tends to produce a gripping eflect on the screws.

The chimneys and umbrellas may be easily manufactured by spinning from aluminum or other suitable metal tubing. The cap may be cast or forged from similar material.

What is claimed is:

1. In a diffusion pump of the type having a casing with an intake opening and a discharge opening, and a vapor jet disposed intermediate said openings; a jet structure comprising a tubular chimney-having an outwardly and downwardly curved upper rim, a number of projections in said rim, a cap overlying said chimney and having a curved under surface portion overlying said rim, said cap. being seated on said projections and having a substantially pointed central stem extending downward into said chimney, and a spring connected to said stem and engageable with said chimney to provide a resilient retaining means for said cap, said spring being manually disengageable from said chimney to permit removal of said cap.

2. A jet structure as described in claim 1, said stem being of circular cross-section throughout, and spaced allaround from said chimney.

3. A jet structure as described in claim 1, said stem having spaced wings engaged with said chimney.

References Cited in the file of this patent UNITED STATES PATENTS 1,734,135 Kobel Nov. 5, 1929. 2,063,665 Edwards Dec. 8, 1936 2,336,764 Abbott Dec. 14, 1943 2,489,05 8 Stoltenberg Nov. 22, 1949 2,639,085 Johnson May 19, 1953 2,639,086 Dayton May 19, 1953 2,684,195 Warren July 20, 1954 2,818,209 Hiesinger, Dec. 31, 1957

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