US3478954A

US3478954A - Vacuum pump

Info

Publication number: US3478954A
Application number: US686953A
Authority: US
Inventors: Roy C Gauger
Original assignee: Bendix Corp
Current assignee: Bendix Corp
Priority date: 1967-11-30
Filing date: 1967-11-30
Publication date: 1969-11-18
Anticipated expiration: 1986-11-18
Also published as: GB1201213A; DE1728326A1; FR1579342A

Description

R C. GAUGER Nov. 18, 1969 VACUUM PUMP 2 Sheets-Sheet 1 Filed Nov. 30, 196'? I r II.

1969 R. c. GAUGER 3,478,954

VACUUM PUMP Filed Nov. 30, 1967 2 Sheets-Sheet INVENTOR. flm 6. 62065? United States Patent 3,478,954 VACUUM PUMP Roy C. Gauger, Rochester, N.Y., assignor, by mesne assignments, to The Bendix Corporation, Detroit, Mich, a corporation of Delaware Filed Nov. 30, 1967, Ser. No. 686,953 Int. Cl. F04b 9/02; F28f 13/18 US. Cl. 230-101 8 Claims ABSTRACT OF THE DISCLOSURE An oil diffusion pump is provided with a fluorocarbon material on the outer surface and portions of the interior surface of the first stage nozzle assembly and/ or the interior surface of the portion of the pump casing extending between the vapor jet and the pump inlet. The material may be a plastic of the fluorocarbon resin family which forms a contact angle of 35 or greater with the pump oil to prevent the creep or spread of oil on such surfaces for reducing backstreaming of oil molecules into the container being evacuated.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to vacuum pumps and particularly to oil diffusion pumps. Oil diffusion pumps are widely used for the evacuation of containers or vessels by the entrainment of diffused gas molecules into a supersonic vapor jet. Oil diffusion pumps have an undesirable characteristic called backstreaming which involves the migration of pump oil vapor back into the vessel being evacuated. Backstreaming originates primarily from the following three areas of a conventional diffusion pump:

(1) Re-evaporation of condensed oil molecules on the portion of the pump casing wall which extends above the vapor jet;

(2) Evaporation of the pump oil from the top cap or cowl of the first stage nozzle assembly; and

(3) The diffusion of oil vapor molecules directly back from the main vapor stream.

The present invention is directed to the problem of reducing backstreaming from the first and second sources. Laboratory tests have shown that the rate of backstreaming in oil diffusion pumps may be reduced as much as six fold by the use of the preferred embodiment of this invention.

Description of the prior art In prior art oil diflusion pumps, the casing wall above the vapor jet and the outer surface of the first stage nozzle assembly have been manufactured from materials which have a high surface adhesion with the pumping oil and provide a correspondingly low contact angle with such oil. For example, the surfaces on such prior art pumps may be made from aluminum with or without an oxide coating, stainless steel, or plain carbon steel with a rust preventative coating. The pumping oil which condenses on such surfaces forms a contact angle less than 15 and therefore tends to creep or spread out over the surface. The creep or spread of the oil provides a large surface area per mass of condensed fluid. The rate of evaporation of the pumping fluid is proportional to the surface area at any given temperature. Thus the greater the surface area of condensed pumping fluid above the vapor jet, the greater the rate of backstreaming.

SUMMARY OF THE INVENTION The present invention is directed to the reduction of backstreaming or back migration of oil molecules which ice have condensed on surfaces of the pump extending above the vapor jet. In accordance with the present invention, an oil diffusion pump is provided with an inlet adapted to be connected to a container to be evacuated and an outlet. A boiler is disposed within the pump casing. Nozzle means including at least a first stage nozzle assembly is mounted within the pump casing for directing a jet of oil vapor from the boiler against the casing to entrain gas molecules and thereby provide the pumping action. The nozzle assembly and the portion of the pump casing disposed between the vapor jet and the inlet provide surface areas exposed to the inlet on which molecules tend to condense, re-evaporate and migrate towards the container. An oleophobic material is disposed on a portion of at least one of said areas to reduce the creep or spread of oil vapor condensed thereon and thereby reduce the rate of re-evaporation of the condensed oil molecules.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical cross section through a diffusion pump constructed in accordance with the present invention;

FIG. 2 is a view partly in section of the first stage nozzle assembly of the pump of FIG. 1;

FIG. 3 is a view partly in section of an alternative embodiment of a first stage nozzle assembly in accordance with the present invention which may be utilized in the pump of FIG. 1;

FIG. 4 is a sectional view of a preferred embodiment of a first stage nozzle assembly in accordance with the present invention which may be utilized in the pump of FIG. 1;

FIG. 5 is a fragmentary view partly in section of an alternative embodiment of a first stage nozzle assembly in accordance with the present invention which may be utilized in the pump of FIG. 1;

FIG. 6 is a sectional view of another embodiment of a baifle cap in accordance with the present invention which may be utilized in the first stage nozzle assembly shown in FIG. 4; and

FIG. 7 is a sectional view of another embodiment of a baflle cap in accordance with the present invention which may be utilized in the first stage nozzle assembly shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and in particular to FIG. 1, there is shown an oil diffusion pump generally indicated by the reference numeral 10 which includes a hollow cylindrical casing 12 with an integral bottom wall 14. The pump casing includes an inlet opening 17 which may be connected by means of flanges 16 and 19 to a vessel 18 to be evacuated. The pump 10 includes an outlet opening 20 which may be connected to a suitable backing pump (not shown) by a flange 21. Suitable heaters 23 are disposed in a boiler compartment 25 at the bottom of the pump casing to heat the pump fluid and cause vaporization thereof.

A hollow multi-stage nozzle assembly indicated generally at 26 is centrally arranged within the pump casing 12 above the boiler compartment and comprises four jet nozzles indicated at 27, 28, 29 and 30. Vapors rising within the interior of the multi-stage nozzle due to evaporation of the pump fluid are directed by the respective nozzles downwardly and outwardly against the inner wall of the pump casing 12. A cooling coil 24 is disposed around the casing 12 to condense the oil vapors from the jet nozzles. The condensed oil vapor is returned to the boiler compartment 25 by gravity.

The main core of the vapor jet from the first stage nozzle asembly 27 is indicated by the dashed lines 32. The first stage nozzle assembly 27 includes a chimney 33 and a cowl 34 disposed over the chimney to provide an inverted nozzle having a throat at 35 as is illustrated in FIG. 2. The cowl 34 is suitably secured to a vertical post 36 coaxially arranged within the chimney 33. The cowl 34 includes a downwardly depending skirt 38. An oleophobic material 40 is disposed over the outer or top surface of the cowl 34 to prevent pumping oil which has condensed on the interior surface of the cowl 34 from creeping around the lower edge of the skirt 38 and along the exterior surface thereof. In conventional oil diffusion pumps the outer surface of the cowl is provided with a metallic surface which may or may not have an oxide coating. Such a surface is wetted by the pumping oil causing the pumping oil to creep up the exterior surface of the cowl and be re-evaporated, thereby contributing to the back-streaming phenomena. The oleophobic coating 40 reduces the tendency of the condensed oil vapor to creep over the exterior surface of the cowl thereby reducing backstreaming. The coating 40 need not extend over the entire exterior surface of the cowl 34 but should extend over the portion of such surface that is adjacent the lower edge of the cowl. The oleophobic coating 40 may also extend over the bottom and the lower inside portion of the skirt 38 as shown in FIG. 3. This coating on the inside and lower edge of the cowl 40 causes any oil condensed inside the nozzle to form only small drops which tend to fall off from the inside surface of the cowl. This reduces the tendency for large drops to form on the lower edge of the cowl where such drops are optically exposed to the pump inlet.

A coating of oleophobic material 42 is also disposed on the interior surface of the pump casing 12 between the pump inlet and the vapor jet 32 as shown in FIG. 1. The coating 42 prevents the spreading of condensed oil vapor thereon and thereby reduces backstreaming. Pumping oil which condenses on the coating 42 also tends to form oil drops which fall from the coating by gravity through the vapor jet 32 to the bottom of the pump where the oil is returned to the boiler compartment.

The term oleophobic material is defined herein as including any material which has a contact angle with the pump oil of 35 or greater. Pump oils for diffusion pumps include refined hydrocarbon oils, esters, chlorinated biphenyls, and semi-organic silicone oils. The oleophobic material may be a plastic material in the form of a fluorocarbon resin such as polytetrafluoroethylene (TFE) or a copolymer of tetrafluoroethylene and hexafluoropropylene (FEP). These resins are distributed under the trademark Teflon. A third grouping in the family of fluorocarbon resins is commonly referred to as a polychloralethylene (CTFE). Tests have shown that the contact angle between a solid surface consisting of Teflon and silicone pumping oils distributed commercially under the designations DC 704 and 705 provide a contact angle in excess of 60 where the Teflon surface is clean. The contact angle between a hydrocarbon oil distributed under the trademark Convoil-2O and a Teflon surface is greater than 50 whereas the contact angle between a polyphenyl ether pumping fluid distributed under the trademark Convalex-lO and a Teflon surface exceeds 85. Tests have also shown that the contact angle between such pump fluids and a plain or anodized aluminum cap is less than 15".

Referring now to FIG. 4, there is illustrated a preferred top jet assembly in accordance with the present invention in which a baflle cap 50 surrounds the cowl 34 to intercept a portion of the backstream oil molecules from the vapor jet. The baffle cap 50 is spaced from the cowl 34 and is made of solid Teflon. The cap 50 is secured to the cowl 34 by means of a pair of ceramic washers 52 and 53 and a suitable bolt 54 which threadably engages a nut 55 as shown.

Tests have shown that the se of a T flon bafile cap 50 reduces the backstreaming rate by a factor of three or more as compared with the use of an aluminum baflle cap 50 having an anodized exterior surface.

' If desired the cap 50 may be manufactured from a suitable metal such as aluminum and provided with a coating of an oleophobic material on the interior and exterior surfaces thereof. It is not necessary, however, to provide a coating of oleophobic material on the interior surface of the baffle cap 50. Such a coating may be placed on the exterior surface of the cap 50 and may be limited to an annular area thereof adjacent the lower edge 51 to prevent the creep of oil from the lip over the adjacent exterior surface of the cap.

FIG. 5 illustrates an alternative top jet assembly in which a cap 56 surrounds the cowl 34 in spaced relation thereto. The cap 56 is made of a suitable metal and provided with an oleophobic coating 58 on the exterior surface thereof, along the bottom edge and on the lower portion of the inside surface of the cap. The coating 58 on the inside surface of the cap 56 adjacent the lower edge thereof functions to reduce the tendency for oil drops of appreciable size to form on the lower edge of the cap Where such drops would be optically exposed to the pump inlet and provide a source for backstreaming.

FIG. 6 illustrates a baffle cap 60 which may be utilized in the top stage nozzle assembly of FIG. 4 instead of the cap 50. In this embodiment the baffle cap 60 is made of Teflon and provided with a thick lower edge portion 62 to prevent oil drops 63 forming on the inner edge of the cap from being exposed to the pump inlet.

FIG. 7 illustrates a baflle cap 70 which may be made of Teflon and utilized in the top stage nozzle assembly of FIG. 4 instead of the cap 50. The bafiie cap 70 includes a pair of downwardly extending annular lip portions 72 and 74 to provide the cap 70 with a bottom edge having a concave cross section. The double edged lip configuration of the cap 70 is particularly advantageous in preventing any oil drops which form on the inner lip portion 72 from tending to creep to the outside lip 74. This tends to prevent any oil drops which form on the bottom of the cap from being optically exposed to the pump inlet where such drops could contribute to the backstreaming phenomena.

'Additional baflles with an oleophobic coating may also be disposed between the first stage nozzle assembly and the pump inlet to reduce backstreaming.

In other embodiments within the scope of this invention the top stage nozzle assembly may have the nozzle cowl made from solid oleophobic material and in the form shown in FIG. 6 or in the form shown in FIG. 7.

' I claim:

1. In an oil diffusion pump for evacuating a container, the combination comprising:

(a) a pump casing having an inlet adapted to be connected to the container and an outlet;

(b) a boiler disposed within the pump casing;

(c) nozzle means mounted in the pump casing for providing a vapor jet stream;

((1) the nozzle means including a first stage nozzle assembly comprising a chimney and a cowl having a skirt disposed over the chimney between which the jet of vapor is expelled for pumping action;

(e) baflle means disposed between the nozzle and the casing inlet for intercepting a portion of the backstreaming oil molecules from the vapor jet; and

(f) a fluorocarbon resin selected from the grou consisting-of polytetrafluoroethylene, polychloroalethylene and a copolymer of tetrafluoroethylene and hexafluoropropylene disposed on at least a portion of the surface of the baffle means to reduce the creep of oil vapor over the said surface and thereby reduce the rate of re-evaporation of oil molecules condensed on the baflie :means.

2. The combination as defined in claim 1 wherein th baflle means forms a cap surrounding and spaced from the cowl, and wherein the fluorocarbon resin is disposed on the exterior surface of the cap at least adjacent the lower edge thereof to reduce the creep of oil vapor'over the exterior surface of the cap.

3. The combination as defined in claim 2 wherein a fluorocarbon resin is disposed over the interior and exterior surfaces of the baflle cap.

4. The combination as defined in claim 3 wherein the baflle cap comprises said fluorocarbon resin.

5. The combination as defined in claim 2 wherein the fluorocarbon resin extends along the lower edge of the baflle cap to provide a thick annular edge portion to prevent oil drops forming on the edge from being exposed to the pump inlet.

6. The combination as defined in claim 2 wherein the bafile cap includes a pair of downwardly extending spaced lip portions on the lower edge thereof and wherein the fluorocarbon resin extends over the lower edge to prevent oil drops forming on the inner lip portion from being exposed to the pump inlet.

7. The combination as defined in claim 1 wherein said fluorocarbon resin is polytetrafluoroethylene.

8. The combination as defined in claim 1 wherein said fluorocarbon resin is further disposed on at least a portion of the interior surface of the pump casing between the pump inlet and the vapor jet.

References Cited UNITED STATES PATENTS DONLEY J. STOCKING, Primary Examiner WARREN J. KRAUSS, Assistant Examiner U.S. Cl. X.R. -133