US2078788A - Diffusion vacuum pump - Google Patents

Description

April -27, 1937.

FL E. BANCROFT DIFFUSION VACUUM PUMP Filed Nov. 5, 1935 all Ill;

Ihvehtcjr Frank E. Bancroft by V v is AbtOP'neg Patented I Apr. 27, 1937 aovarss DIFFUSION VACUUM PUMP Frank Edmund Bancroft, Withington, Manchester, England, assignor to Metropolitan- Vickcrs Electrical Company, Limited, a company of Great Britain Application Novembe 5,

1935, Serial No. 48,405

In Great. Britain November 9, 1934 3 Glaims. (Cl. 230-101) This invention relates to improvements in diffusion vacuum pumps of the condensation or difiusion type. It is particularly applicable to pumps of the type described in British, Patent No. 346,293 which are intended for use with a low vapor pressure oil as a working fluid.

One object of the-present invention is to .pro-

vide a high speed high vacuum single jet diffusion pump which is efiicient, reliable and of simple and relatively cheap construction and capable of giving pressures less than 10- mm.

I for use in the continuous, evacuation oflarge I demountable apparatus such as thermionic valves and X-ray tubes. Other contemplated applications include use with induction furnaces,

stills and the like.

To enable the invention to be clearly understood one convenient arrangement thereof by way of example is illustrated in the accompanying drawing and described with reference thereto, the single figure being a sectional side elevation of the vacuum pump.

While my improved pump is by no means limited to use with a particular working fluid, it has been found to possess unusually favorable characteristics for use with the low vapor pressure oil commercially available under the registered trade-mark name, .Apiezon. This com prises a highly refined oil possessing the quality of being able to be heated in vacuum sufficiently to raise its vapor pressure to about 1 or 2 mm. without cracking. Since its vapor pressure becomes extremely low at temperatures not much below room temperature, it is possible to employ it to .obtain a very positive jet action without resorting to excessive cooling for condensation purposes.

Referring to the drawing, l is the outer shell of the. pump, this shell comprising a tubular, porton of cylindrical section and, at its lower end, a conical portion la, the cylindrical portion I having the lateral forevacuum connection pipe 2 near its upper end.'- The smaller diameter of the conical portion la of the shell is approximately half the diameter of the cylindrical portion I of the shell. Secured to the lower end of the conical portion la is the boiler 3 beneath which is disposed the heater which may be a hot plate applied to the flat underside of the boiler. 50 In the arrangement illustrated said heater comprises a plurality, conveniently three, of heater elements, of which one is illustrated at 4, this element being of a known form and comprising a hollow core of insulating material carrying an 55 external wire winding. Each such heating element 4 is adapted to be located in a tube 5 welded beneath the boiler 3 and to be retained in the tube 5 by means of a bayonet fastening indicated by dotted lines at 6a, the slot of the fastening being provided in a detachable cap I, having a bushed perforation 8 through which the electrical leads for the heater 4 pass. The three (or more) tubes 5 may be enclosed in a detachable shell 6 which may be chromiumplated for the purpose of retaining the heat.

Resting upon' the floor of the boiler 3, and engaged with the latter by means of friction studs 9, .is the vapor uptake tube l0 having a diameter only slightly smaller than that of the lower end of the conical portion Ia of the outer shell. The vapor uptake tube I0 is surmounted by the cowl H, and above the latter the molecule baffle l2, these members being secured to a boss 83 which in turnis secured to the centre rod I l,-

-which at its lower end is secured to the centre of the boiler 3. The vapor uptake tube I 0 and the supporting rod It are centred with respect to each other by means of a tube [5 carried by a spider, two of the radial spokes of which are indicated at It.

The condenser tube l'l' depends into and divides the annular space between the main outer shell I and the vapor uptake tube l0. Said condenser tube is attached to or more con- 'veniently, as in the arrangement illustrated, is

integral with the pump intake 11a, The tube Ila is provided with an external flange I8 which is adapted to be demountably vacuum-sealed to the flange i9 provided at the top of the main outer shell I. The vacuum seal is preferably completed'by means of a gasket 20 of appropriate material and an externalv flood of oil in a. channel 2| provided above the flange I9 by the cylindrical flange 22 which may be provided with a drain tap or with the illustrated drain plug 23.

The condenser tube H has a water. jacket, which in the illustrated arrangement comprises a tube 24 arranged in double or re-entrant helical form around the outer surface of the tube I1, and connected thereto such as by welding or soldering in a manner to provide good thermal contact. It will be obvious that the water jacket might be alternatively provided by means of an additional shell secured coaxially to the tube l1.

The ends of the tube 24, which is preferably of flattened section as shown, are carried through perforations in the flange l8, as indicated at.24a. Within the condenser tube I1 is flxed the molecule baflie 25 in the form of an internalfiange, this bafiie in conjunction with the baflle I2 serving to prevent molecules of working fluid vapor passing into the evacuating system, as described for example in British Patent No. 346,293.

The lower edge of the condenser tube I1 is bevelled, as indicated at Hb, to facilitate the dripping of the condensed working fluid therefrom clear of the conical baiiie Illa which is secured towards the lower end of the vapor uptake tube In within the conical portion In of the outer shell, said bailie Ilia having a diameter slightly less than that of the condenser tube H. The tube 10 is formed at its lower end with holes [b to permit the condensed working fluid to flow into the boiler.

It will be seen that the pump inlet of the condenser and its securing means comprises a unit which can readily be connected with the apparatus to be evacuated either direct or by means of a pipe 26 which may be secured in a dismountably vacuum-tight manner, conveniently by an arrangement which may be generally similar to the seal already described comprising the items l8--23.

The floor of the boiler 3 has a ribbed upper surface, the ribs being arranged as two concentric circles, as' indicated at 3a and radial channels (which are not visible in the drawing) are provided, namely, in order that a uniform fluid level may be maintained within the boiler in case the fluid level should become insuflicient to flood the ribs 3a. Conveniently there are three such radial communicating channels arranged at an angle of 120 with respect to each other.

The pump above described'and shown in the- .drawing is conveniently made of mild steel with the exception of the vapor uptake tube l0, its cowl ll, baille l2, and centre supporting rod H, which parts are preferably of copper or dther metal having a relatively high thermal conductivity, for the purpose set forth in the aforesaid British Patent No. 346,293.

The pump as above described and shown in the drawing has actually been constructed and successfully operated, and the vertical height of the condenser tube l1, excluding the thickness of the illustrated flange at its upper end, was 11 inches, the accompanying drawing being otherwise substantially to scale. The normal working level of the oil in the boiler 3 is indicated at 21, and the capacity at this level is approximately 600 cc.

It will be understood that various minor modifications may be made in the arrangement illustrated, all within the scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States, is

1. A single jet diifusion pump comprising a substantially cylindrical outer casing having an outlet for pumped gases adjacent the top thereof, said casing being inwardly tapered at the bottom to join with a circular heating member of substantially less diameter than said casing, a vapor uptake tube comprising a metal cylinder of substantially the same diameter as said member and projecting upwardly therefrom, a working fluid injection orifice adjacent the top of said tube, a cylindrical condenser unit projecting downwardly into the annular space between said outer casing and said uptake tube, an inlet at the top of said pump assembly for admitting pumped gases downwardly within said condenser unit, an external flange projecting laterally from said condenser unit for supporting the same, and demountable means hermetically sealing said flange to the upper portion of said casing.

2. A single jet diffusion pump comprising an outer casing having an outlet for pumped gases adjacent the top thereof, said casing being inwardly tapered at the bottom to join with a heating member of approximately one-half the diameter of the casing, a vapor uptake tube of approximately the same diameter as said member and joined thereto, a working fluid injection oriflce adjacent the top of said tube, a condenser including a metal shell projecting downwardly into the space between said outer casing and said uptake tube, said condenser shell being outwardly beveled at its lower edge, a conical baffle of less diameter than the condenser shell secured to the vapor-uptake beneath the lower edge of the condenser shell and an inlet at the top ofsaid pump for admitting pumped gases downwardly within said condenser shell.

3. A diffusion pump comprising an outer casing having an outlet for pumped gases adjacent the top thereof and being inwardly tapered at the bottom to join with a heating member of substantially less diameter than the casing, said heating member being provided with ribbed heatexchang'ing surfaces, a vapor uptake tube of substantially the same diameter as said member and projecting upwardly therefrom, a working fluid injection orifice adjacent the top of said tube, a condenser unit projecting downwardly into the space between said outer casing and said uptake tube, an inlet at the top of said pump for admitting pumped gases downwardly within said condenser unit, an external flange projecting latfor supplying cooling fluid to said condenser unit.

FRANK EDMUND BANCROFI.

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