US2585139A - High-vacuum pump - Google Patents

Description

1952 R. B. LAWRANCE ET AL 2,585, 39

HIGH-VACUUM PUMP Filed Oct. 10, 1950 INVENTORS Richard 5. Lawrance John L. Miad/afgn J/r ATTORNEY Patented Feb. 12, 1952 UNITED sTArss rear. OFFICE 2,585,139 HIGH-VACUUM PUMP Application October .10, 1950, Serial .No. 189,444 6 Claims. (Cl. 230101) This invention relates to "a high-vacuum device and more particularly to an improved high vacuum diffusion pump.

A principal object of the present invention is to provide an improved diffusion pump which is rugged in construction and cheap to manufacture.

Another object of the invention is to provide a diffusion pump of the above type which can be readily assembled and disassembled with accurate positioning of the parts thereof for optimum operating conditions.

Still another object of the present invention is to provide a diffusion pump of the above type which can be opened to atmospheric pressure for short periods of time, while the pump is running, without losing appreciable quantities of pump oil.

Still another object of the invention is to provide'a diffusion pumpo'f the above type which is particularly adapted for use on short pumping cycles such as the evacuation of radio tubes on automatic tube-making and tube-sealing equip-- ment.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the construction, combination of elements and. arrangement of parts which are exemplified in the following detailed disclosure. and the scope of the applicationof which will be indicated in the claims.

For afuller understandingof the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein. there is shown a diagrammatic sectional view of one preferred embodiment of the present invention.

In general, the present invention is directed to an improved high-vacuum diffusion pump of high-speed pumping operation and blank-oil pressures 'on the order of 10'' m. Hg abs. In a preferred embodiment of the t invention the diffusion pump comprises a substantially cylindrical pump body and a vapor assembly positioned within the pump body. ThlS jet assembly preferably has a substantially ylindrical portion of a smaller diameter than the pump body which defines' a pumpingchamber with the pump body. At the bottom of the pump body there is provided a boiler for a diffusion manpcil, the bottom of the jet assembly being open to pump oil vapors generated in the boiler. A fcreiineis connected to the pump body adjacent an opening therein which is positioned below the lowest jet on the jet assembly near the bottom of the pumping chamber. A helical member ispositioned between the jet assembly and the pump body, this helical member acting to space the jet assembly concentrically with respect to the pump body and also serving as a long path return for pump oil condensed on the'pump'body during the operation of the pump. This helical member has a sufficiently large transverse di mension so that it substantially completely bridges the space between the outside of the jet assembly and the inside of the pump body in that portion thereof between the pumping chamber and the boiler.

In a preferred embodiment of the invention the helical member comprises a wire coil, the turns of the wire coil being spaced apart by a distance about equal to the diameter of the wire, this spacing permitting the oil condensed in the pump body'to flow in a helical path back to'the boiler.

Referring now more particularly to the draw ing, the pump body is illustrated at it in the form of a cylindrical tube which is open atits upper end ii, and within which a jet assembly 12 is positioned. In the preferred form shown the jet assembly preferably comprises a lower cylindrical portion it which terminates at .its top in a jet 1.5. This lower cylindrical portion It is preferably concentrically positioned with respect to the pump body ill and defines acylindrical lower pumping chamber 18 with the pump body. The jet assembly also includes a "second jet tube .20 which terminates in a top jet 22.. This second jet tube is preferably of smaller diameter than the first jet tube M and definesvan upper pumping chamber with the pump body. A foreline 2 5 is connected to the pump body adjacent an opening 2% in the pump body, this foreline, in operation, being preferably connected to a suitable mechanical vacuum pump. 7

At the bottom of the pump body there is .provided a boiler housing 28 which defines, with a base plate 3!), a relatively large space in which a quantity of pump oil 32 may be confined. The two jet tubes is and 20 are provided with openings at their bottom which communicate with the vapors of the pump oil .32 generated in the boiler. The opening for the outer jet ftubeilfi preferably comprisesholes 3t punched into the lower portion of this tube, while theopening 36 for the inner jet tube 20 may conveniently be the bottom of this jet tube. Connected to the base plate of the pump, in heat-transfer relationship thereto, is a heating element 38 schematically indicated as an electrical resistance heater.

Positioned between the outside of the jet tube l4 and the inside of the pump body I0 is a helical member 40 which preferably extends from a position adjacent the foreline opening to just above the boiler. As shown, this helical member comprises a plurality of turns of wire, the wire diameter being substantially equal to the spacing between the outside of the jet tube l4 and the inside of the pump body III. This helical Wire 40 is preferably tack-welded to the inside of the pump body I 0 and acts to accurately position the jet tube assembly concentrically with respect to the pump body. Since the helical wire 40 substantially completely bridges the space between the jet tube l4 and the inside of the pump body l0, vapors from the boiler cannot travel in a straight line path upwardly towards the pump-v ing chamber l8. However, the vertical Spacing between the turns of the helical wire coil 40 is sufficient to permit the condensed oil to run in a helical path down to the boiler.

In a preferred embodiment of the pump illustrated, there is provided a cooling coil 42 which is wrapped around the foreline 24 and around the outside of the pump body II), this cooling coil maintaining the foreline and the pump body sufficiently cool so that the pump oil vapors striking either the pump body or the foreline are condensed thereon. The pump is also preferably provided with a drain 44 and a filling opening 46 so that the pump may have its pump oil drained therefrom, with the addition of a new supply of pump oil, without disassembly of the pump or removal from its associated apparatus.

In the operation of the diffusion pump described above, the upper end ll of the pump body I0 is connected to a vacuum-tight line leading to the apparatus to be evacuated and the foreline 24 is connected to a suitable backing pump such as a usual mechanical vacuum pump. Cooling coil 42 is connected to a suitable source of cooling water, and the boiler is filled with a suitable diffusion pump oil to approximately the level indicated in the drawing. The mechanical vacuum pump is then operated to remove most of the air from the diffusion pump and the system being evacuated. The heater 38 is energized to heat the pump oil to a temperature such that its vapor pressure is on the order of .5-1.00 mm. Hg abs. The vapors generated in the pump boiler travel up the inside of the jet tubes I4 and 20 and are discharged from the jets l6 and 22, respectively, with high velocities, thus pumping air from the opening I l' and forcing it out the foreline 24. The hot oil vapors condensed on the inside of the surface of the cooled pump body l0 run down this pump body until they strike the helical wire coil 40. The condensed oil then travels in a helical path along the helical coil 40 until it reaches the boiler.

It has been found that the helical wire 40 serves several very important functions. In the first place it provides a cheap, simple arrangement for insuring accurate centering of the jet tube assembly with respect to the pump body. Secondly, it presents a high impedance path for pump oil vapors tending to travel upwardly in the diffusion pump between the outside of the jet tube assembly and the inside of the pump body l0. Thirdly, it provides a long path for the condensed pump oil which acts as a partial vapor seal between the hot pump oil in the boiler and any air rushing into the pump body when the top H of the pump body is exposed to a sudden large influx of air. Additionally, the helical wire 40 prevents loss of oil due to pressure bursts within the boiler and jet tube assembly when a radio tube, for example, is being initially evacuated through the diffusion pump by the mechanical backing pump.

In the evacuation of radio tubes, utilizing the present invention, the radio tube is connected to the evacuating system by means of a valved connection. When this valve is opened, air, from the radio tube and connecting parts, rushes into the interior of the diffusion pump. In view of the relatively large volume of the pump and connecting vacuum system, with respect to the radio tube being evacuated, the interior of the diffusion pump initially reaches a pressure somewhat less than atmospheric pressure. However, the vapor pressure within the boiler and jet tube assembly is only about .5-1.00 mm. Hg abs. so that the inrushing air from the radio tube tends to travel to the interior of the jet tube and the boiler. Air can enter the boiler only through two paths. One path is the spiral path defined by the helical wire 40. The inrushing air tends to travel down the spiral path compressing the oil vapors rather than diffusing through these vapors. Due to the high impedance and relatively great length of the path this inrushing air is prevented from reaching the boiler. The other path is through the jet openings and down the jet tubes. A considerable quantity of air does enter the jet tubes in this fashion and the boiler vapors in the jet tubes are compressed by this entering air. The influx of air to the jet tubes reverses when the total pressure in the pumping chamber has been lowered, by the backing pump, to a pressure below the pressure within the jet tubes. This reversal is apt to be quite rapid in view of the relatively high pumping capacity of the usual backing pump. If there were a free escape for this reexpanding air through the boiler portion, a portion of it would rush out through the boiler with sufficient velocity to entrain a considerable amount of oil. However, the construction illustrated presents a high impedance to the passage of air from the jet tubes through the boiler so that this air will pass through the jets instead. Thus this air will not tend to mingle with the hot oil and hot oil vapors in the boiler or in the helical oil-return path. This eliminates, to a large extent, the problem of substantial oil oxidation during the short pumping cycles encountered in the evacuation of radio tubes and eliminates the necessity of by-passing the difl'usion pump during the initial pumpdown of the radio tube. This latter advantage considerably simplifies the piping associated with the installation and eliminates at least two vacuum-tight valves for each installation. I

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawing, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A high-vacuum diffusion pump comprising a substantially cylindrical pump body, a substantially cylindrical vapor jet assembly positioned within said pump body, said jet assembly having a smaller diameter than said pump body and defining a pumping chamber with said pump body, at least one jet on said jet assembly arranged to discharge a jet of difiusion pump oil vapors downwardly into said pumping chamber, a boiler for vaporizing diflusion pump oil at the bottom of said pump body, the bottom of said jet assembly being open to vapors generated in said boiler, a ioreline connected to said pump body adjacent an opening therein which is positioned at the bottom of said pumping chamber, and a member coiled in the form of a helix positioned between said jet assembly and said pump body below said pumping chamber, said helical member having a suificient transverse dimension to substantially completely bridge the space between the outside of said jet assembly and the inside of said pump body below the pumping chamber so as to block the straight line path between said boiler and said pumping chamber, the turns of said helical member being spaced apart to permit oil condensed on said pump body to flow in a helical path into said boiler.

2. The pump of claim 1 wherein said helical path is several times as long as the straight line path from said foreline to said boiler.

3. A high-vacuum diffusion pump comprising a substantially cylindrical pump body, a substantially cylindrical vapor jet assembly positioned within said pump body, said jet assembly having a smaller diameter than said pump body and defining a pumping chamber with said pump body, at least one jet on said jet assembly arranged to discharge a jet of diffusion pump oil vapors downwardly into said pumping chamber, a boiler for vaporizing diffusion pump oil at the bottom of said pump body, the bottom of said jet assembly being open to vapors generated in said boiler, a foreline connected to said pump body adjacent an openingtherein which is positioned at the bottom of said pumping chamber, and a member coiled in the form of a helix positioned between said jet assembly and said pump body below said pumping chamber, said helical member being secured to the inside of said pump body and being arranged to engage said jet assembly and to position said jet assembly accurately with respect to said pump body and also to block the straight line path between said boiler and said pumping chamber, the turns of said helical member being spaced apart to permit oil condensed on said pump body to flow in a helical path into said boiler.

4. A high-vacuum diffusion pump comprising a substantially cylindrical pump body, a substantially cylindrical vapor jet assembly positioned within said pump body, said jet assembly having a smaller diameter than said pump body and defining a pumping chamber with said pump body, at least one jet on said jet assembly arranged to discharge a jet of diffusion pump oil vapors downwardly into said pumping chamber, a boiler for vaporizing difiusion pump oil at the bottom of said pump body, the bottom of said jet assembly being open to vapors generated in said boiler, a foreline connected to said pump body adjacent an opening therein which is positioned at the bottom of said pumping chamber, and a member coiled in the form of a helix positioned between said jet assembly and said pump body below said pumping chamber, said helical member having a sumcient transverse dimension to substantially completely bridge the space between the outside of said jet assembly and the inside of said pump body below the pumping chamber, said helical member forming a high impedance path for vapors and gases between the boiler and said pumping chamber and forming a low impedance long path for liquid running back to the boiler.

5. A high-vacuum diffusion pump comprising a substantially cylindrical pump body, a substantially cylindrical vapor jet assembly positioned within said pump body, said jet assembly having a smaller diameter than said pump body and defining a, pumping chamber with said pump body, at least one jet on said jet assembly arranged to discharge a jet of diffusion pump oil vapors downwardly into said pumping chamber, a boiler for vaporizing .diffusion pump oil at the bottom of said pump body, the bottom of said jet assembly being open to vapors generated in said boiler, a foreline connected to said pump body adjacent an opening therein which is positioned at the bottom of said pumping chamber, and a member coiled around said jet assembly, said member bridging the space between the outside of said jet assembly and the inside of said pump body below the pumping chamber so as to block the straight line path between said boiler and said pumping chamber, the turns of said coiled member being spaced apart to permit oil condensed on said pump body to flow in a helical path into said boiler.

6. A high-vacuum diffusion pump comprising a substantially cylindrical pump body, a substantially cylindrical vapor jet assembly positioned within said pump body, said jet assembly having a smaller diameter than said pump body and defining a pumping chamber with said pump body, at least one jet on said jet assembly arranged to discharge a jet of diffusion pump oil vapors downwardly into said pumping chamber, a boiler for vaporizing diffusion pump oil at the bottom of said pump body, the bottom of said jet assembly being open to vapors generated in said boiler, a foreline connected to said pump body adjacent an opening therein which is positioned at the bottom of said pumping chamber, and an elongated member positioned between and in contact with said jet assembly and said pump body below said pumping chamber, said member being several times as long as the straight line path between said pumping cham-. her and said boiler and blocking said straight line path, adjacent portions of said member being spaced vertically to define an elongated space therebetween in which condensed pump oil can return to said boiler by a long path which has high impedance to the passage of air and vapors.

RICHARD B. LAWRANCE. JOHN L. MIDDLETON, JR.

No references cited.

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