US3572973A

US3572973A - Heater and vapor nozzle arrangement for a vacuum diffusion pump

Info

Publication number: US3572973A
Application number: US813565A
Authority: US
Inventors: Joseph A Le Blanc Jr
Original assignee: Precision Scientific Co
Current assignee: Precision Scientific Co
Priority date: 1969-04-04
Filing date: 1969-04-04
Publication date: 1971-03-30
Anticipated expiration: 1988-03-30
Also published as: FR2041979A5; GB1289905A; DE1952952A1

Abstract

An improved vacuum diffusion pump includes a pair of heated caps provided adjacent the pump inlet and forming portions of two vapor jets, or nozzles each of which is designed to minimize the migration of vapor molecules issuing therefrom, toward the pump inlet. An improved diffusion pump also includes a heating element provided in the interior of the pump''s jet assembly in order to minimize vapor condensation on the pump assembly.

Description

United States Patent [72] Inventor Joseph A. Le Blane, Jr.

Chicago, Ill.

[21] Appl. No. 813,565

[22] Filed Apr. 4, 1969 [45] Patented Mar. 30, 1971 [73] Assignee Precision Scientific Co.

Chicago, Ill.

Gomer W. Walters [54] HEATER AND VAPOR NOZZLE ARRANGEMENT FOR A VACUUM DIFFUSION PUMP 10 Claims, 3 Drawing Figs.

ABSTRACT: An improved vacuum diffusion pump includes a pair of heated caps provided adjacent the pump inlet and forming portions of two vapor jets, or nozzles each of which is designed to minimize the migration of vapor molecules issuing therefrom, toward the pump inlet. An improved diffusion pump also includes a heating element provided in the interior of the pumps jet assembly in order to minimize vapor condensation on the pump assembly.

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BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to an improvement in vacuum diffusion pumps. More specifically, this invention is directed to a heater and vapor jet arrangement which improves pump performance by minimizing the migration of vapor particles toward the pump inlet.

2. Description of the Prior Art Vacuum pumps of the diffusion type for producing a high vacuum are of course well-known. Generally, these pumps utilize a normally liquid pumping fluid such as a petroleum based oil which is vaporized in a boiler by suitable heating means and thenfpasses to a vapor jet assembly. The jet assembly comprises one or more vapor nozzles or jets adapted to direct the vapor'in a direction away from the space to be evacuated, which is connected to the pump inlet. The accelerated oil vapor molecules issuing in jet streams from the nozzles, collide with molecules ahead of the nozzle, entraining them and drawing them towardthe pump outlet (which is usually connected to a mechanical backing pump), thus producing a pressure differential between the pump inlet and outlet and causinga pressure decrease in the space to be evacuated. The pumps outer walls are cooled by suitable external cooling means, and upon contacting the interior surface of these walls, the vapor molecules are condensed and the condensate is returned to the boiler for recirculation in the pumping process. v

A problem common to all vapor vacuum diffusion pumps of the typebriefly described above is that of backstreaming (i.e., migration of vapor molecules in the direction of the space to be evacuated). The problem is most proriounced with regard to the jet nearest the pump inlet, or topmost nozzle in the assembly, since errant molecules issuing therefrom are most likely to find their way into the space to be evacuated thereby contaminating this space. Moreover, regardless of whether backstrearning vapor molecules everreach the region to be evacuated, their velocity vector in this direction reduces the speed and efficiency of the pump since it is opposite in direction to the desired rate of flow.

Several different factors have contributed to the backstreaming phenomenon in the vacuum diffusion pumps of the prior art. First, due to the design of the jets the molecules in the vapor jet stream issuing therefrom may acquire a velocity component in the backstreaming direction. Second, due to cooling during travel from the boiler to the topmost vapor jet or in this jet itself, the vapor may condense in the nozzles interior or on its lip. As this condensate is blown out of the nozzle by the high-pressure vapor jet, it enters the lowpressure pumping region between the pump wall and the exterior of the nozzle assembly where it randomly evaporates and thereby contributes to the backstreaming flow. Finally, condensed vapor in the pumping area may come in contact with hot exterior surface of one of the nozzles, thereby causing it to reevapora te a'ndmigrate toward the pump inlet.

Several different means have been employed in the vacuum diffusion pumps of the prior art to minimize backstreaming, but these means have attendant disadvantages, which nearly offset the improvements they provide. Generally these have involved attempts to condense backstreaming vapor molecules before they can reach the pump inlet. One such means involves the use of a battle, which may be cooled byexternal means or by conduction from the pump walls, interposed between topmost jet and the space to be evacuated. The baffle is designed to trapand condense backstreaming vapor particles, but it has the disadvantage of having a limited conductance with regard to the flow of gas withdrawn from the region to be evacuatedand this, in turn, greatly reduces the pumping capacity of the pump and baffle combination.

Prior art pumps have also commonly employed cold-caps" to minimize backstreaming. These caps, which are adapted to cover the topmost vapor jet are cooled! by various devices such as Peltier couples or cooling coils surrounding the cap and connected to the coils surrounding the pump walls, and are adapted to condense any vapor molecules that diverge toward the pump inlet from the jet stream issuing from the topmost nozzle. However, to be effective in backstreaming minimization, cold-caps must be of a diameter sufficient to include a substantial portion of the cross-sectional area of the pumping chamber and the greater the area covered, the greater the consequent reduction in pumping capacity caused by a decrease in the throughput of gas withdrawn from the space to be evacuated. Moreover, vapor condensed on a cold-cap" can fall onto a hotter portion of the nozzle assembly positioned therebelow, (especially where the nozzle assembly comprises a plurality of staged nozzles of increasing diameter in the direction of the bottom of the pump) and be reevaporated whereby the resulting vapor may contribute to the backstreaming flow.

Another disadvantage of prior art vacuum diffusion pump relates to the heater nieans employed therein. Commonly, heaters are maintained in the pump boiler located at the bottom of the vapor jet assembly. Heat is transferred to the jets of the assembly only by vapor rising in the assemblys interior and consequently it takes a substantial period of time before the nozzles can be brought to a suitable operating temperature. If the pump is put into operation before the nozzles warm-up sufficiently, vapor will condense in the nozzles, the pump will not operate efficiently, and substantial backstreaming will result.

Apart from warm-up problems, in vacuum diffusion pumps having a nozzle assembly comprising a plurality of staged jets stacked one on top of another, the heat losses from the assembly make it difficult to supply the nozzles with vapor of a sufficiently superheated quality to ensure proper operation of the pump. That is, the heat losses from the jet assembly during pump operation cannot be offset by heat transfer from the vapor rising therein and the assembly may cool to the point where condensation results in the nozzles and consequent backstreaming occurs. Moreover, the vapor itself can cool as it rises in the assembly and condense before reaching the nozzle inlets.

Accordingly it is a primary object of the present invention to provide a heater and vapor nozzle arrangement for utilization in a vacuum diffusion pump that will substantially eliminate the causes of backstrearning, or the migration of oil vapor particles toward the pump inlet.

It is a further object of the present invention to provide a dual jet arrangement replacing the topmost jet in a vacuum diffusion pump in order to prevent the molecules forming the vapor jets streams issuing therefrom from migrating toward the pump inlet.

It is a still further object of the present invention to provide an improved unitary heater arrangement which will supply heat throughout the entire length of thevapor jet assembly in a vacuum diffusion pump in order to bring the assembly to a suitable operating temperature and maintain it at that temperature.

it is yet a further object of the present invention to provide a unitary heater arrangement within the vapor chute of a vacuum diffusion pump to superheat the vapor rising therein.

SUMMARY OF THE INVENTION The foregoing objects, advantages and features of the present invention may be achieved by providing a vacuum pump of the diffusion type, having a staged series of vapor jets, a vapor chute, and an oilsump at one end of the vapor chute, with the improvement comprising: a first cap positioned at the end of the vapor cute chute and forming therewith a first vapor jet, the first cap having a depending skirt adapted to direct oil vapor issuing from the first vapor jet in the general direction of the oil sump, at least one lateral passageway means being provided in the first cap; a second cap covering the first cap, but spaced-apart therefrom and forming, in combination with the first cap and the lateral passageway means, a second vapor jet, the second cap having a depending skirt extending below the skirt of the first cap in the direction of the oil sump in order to direct the vapor jet stream passing from the second jet in the general direction of the sump; and heater means for supplying heat to the first and second caps.

Additionally, the operation of vacuum pumps of thediffusion type may be improved by providing heating means positioned within the vapor chute and extending substantially the length thereof in order minimize start up problems, backstreaming, and condensation; thereby to maximize pump efficiency. Preferably, heating means includes a differential heat distribution arrangement for varying the relative amounts of heat supplied to the various portions of the vapor chute.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view taken substantially through the center of a vacuum diffusion pump incorporating the improvements in accordance with the present invention;

FIG. 2 is a sectional view taken along line 2-2 in FIG. I; and

FIG. 3 is an enlarged fragmentary sectional view of the topmost portion of the pump shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, a vacuum diffusion pump produced in accordance with the present invention comprises an enclosure taking the form of a vertical, hollow, cylindrical pump body 12 having a top i7 and a bottom 14 which is integral therewith and which is adapted to hermetically seal the bottom of pump it). A flange I6 is attached to the top 17 of the pump body 12 and is provided with connecting means 19 adapted to connect the pump 19 to the space to be evacuated. Suitable means (not shown) may also be provided in combination with flange 16 in order to permit gas to be withdrawn from the space to be evacuated into the interior of pump body 12. In the lower portion of the pump body 12, an outlet 18 is provided which connects the interior of pump body I2 with an exhaust conduit 20, which, in turn, normally leads to a mechanical backing pump (not shown).

Pump body 12 and exhaust conduit 20 are surrounded by a coil 22 through which a coolant is pumped by an external pumping means (not shown) during the operation of vacuum diffusion pump it). The flow of coolant through coil 22 serves to cool pump body 12 and exhaust conduit 20 so that oil vapor molecules coming in contact therewith will be condensed.

An oil sump 14 comprising a reservoir of a suitable liquid pumping fluid (such as a petroleum based oil) is maintained in the bottom of pump body 12. For convenience, the fluid will hereinafter be referred to as oil. A vapor jet assembly 26 is centrally positioned within the interior of pump body 12 and extends from its bottom 14 to its top 17, with the interior of the jet assembly forming a vapor chute 28. The vapor jet assembly 26 comprises a series of staged vapor jets or nozzles adapted to direct oil vapor rising from the oil sump 24%, within vapor chute 28, in a direction away from the pump inlet, toward the bottom of the pump.

Still referring to FIG. 1, vapor jet assembly 26 comprises a series of sections 30 (in this case four) stacked one on top of another, Each of these sections 30 comprises a hollow cylindrical chimney 32, having an annular, outwardly directed lip portion 34 attached to the top thereof, adapted to serve as the lip of a corresponding vapor jet or nozzle 36. Connected to the bottom of each chimney 32 (except that of the bottommost section) is an annular depending flange 38 having a generally outwardly directed portion 4d and a generally downwardly projecting portion d2. Each flange 38 is adapted to seat upon the lip portion 34 of the section 30 beneath it, and the downwardly projecting portion 42 of the flange 38 forms the skirt of the corresponding vapor nozzle 36. A plurality of spacers 44 are positioned .on the lip portion 34 at intervals around the circumference, and maintain a spaced relation between the flange 3i; and the portion 34. The space therebetween defines an annular inlet for vapor nozzle 36.

The respective diameters of the chimney, lip, and flange of each given section 30 are smaller than those of the section next beneath it, so that the diameters of the vapor nozzle inlets decrease in the direction toward the top of the pump. The bottommost section of the jet assembly 26 of course does not have a flange attached to its chimney portion, and the chimney rests on the pump bottom 14. Suitable passageway means (not shown) are provided in the walls of the chimney to permit oil vapor condensed on the interior wall of pump body 12 to flow into the sump 24 in the interior of nozzle assembly 26.

To this point, the structure described corresponds to that of prior art vacuum diffusion pumps, and pumps heretofore manufactured in accordance with the above have been designed to function in the' following manner. Heater means have been positioned in the oil sump 24 to evaporate the liquid oil contained therein. The resulting vapor molecules rise in the vapor chute 23 and enter one of the series of vapor nozzles 36. Upon issuing from the nozzle, the jet stream of vapor molecules is deflected by the skirt 42 in a direction away from the pump inlet and the molecules collide with others ahead of the nozzle, entraining them and drawing them toward the bottom of the pump, thereby producing a pressure differential across the pump and decreasing the pressure in the space to be evacuated. The gas withdrawn from the evacuated region passes to the pump outlet 18 and the vapor molecules, after leaving the nozzles, eventually contact the relatively cool inner surface of the pump body 12 and condense thereupon. The condensed vapor thereafter travels down the interior wall of the pump body and returns the oil sump M.

As mentioned hereinbefore, vacuum diffusion pumps which merely comprise the above recited structure have encountered difficulties in operation resulting from the phenomenon of backstreaming, or the migration of oil vapor toward the pump inlet. In accordance with the present invention the causes of backstreaming are substantially eliminated by replacing the topmost nozzle of the jet assembly 26 with a cap arrangement, the preferred embodiment of which is described hereinafter.

Referring to FIG. I, a first or inner cap 46 is provided in order to cover the topmost section 36 of the jet assembly 26. As best shown in FIG. 3, the inner cap 46 comprises a hollow cylindrical body 48 having a top 50 integral therewith adapted to seal the top of body 48. Attached to the bottom of body 48 is an annular, outwardly directed rim 52 adapted to seat upon spacers 54 maintained at intervals around the circumference of the lip 34 of the topmost nozzle section 30. The annular space maintained between the rim 52 and the lip 34 by the spacers 54 defines the inlet of a first or inner vapor jet 56.

Spacers 54 are advantageously formed of music wire of a preselected diameter so that the desired critical spacing may be obtained. Variations in spacing may of course be obtained simply by using spacers of different diameter.

Attached to the end of the rim 52 away from the body 48, is a cylindrical, vertically depending flange 58 which forms the skirt of the inner vapor jet 56. The skirt 58 is adapted to deflect the vapor jet stream issuing from the inner vapor jet 56 inwardly toward the axis of the jet assembly 26, thereby creating a velocity vector in that direction which must be overcome before any vapor molecules can migrate upwardly toward the pump inlet. At least one and preferably a plurality of lateral passageway means 6th are provided in the lower portion of the inner cap 46, but above the inlet of the inner vapor jet 56. The passageway means 64} are adapted to permit the flow of oil vapor, rising in the vapor chute 28, from the interior of the cap 46 to its exterior, Thus, vapor molecules reaching the topmost section of nozzle assembly 2d will either issue from the inner vapor nozzle 56 or pass through the lateral passageway means 60.

As best seen in H6. 3, a second or outer cap 62 is positioned over the inner cap 46. Outer cap62 comprises a hollow cylindrical body portion 6d having means as (i.e., threads) disposed on its interior surface adapted to secure the outer cap 62 to the exterior surface of the upper portion of the body of inner cap 36. The bottom of body as is located above the points where lateral passageway means 60 exit from the inner cap as. Attached to the bottom of body 64 is an outwardly directed rim es paralleling the rim 52 of the first cap as but spaced therefrom. A cylindrical, vertically depending flange 7% is connected to the end of the rim 68 away from the inlet cap body 64. Flange 7i) parallels flange 58 of inner cap 46, but is spacedtherefrom, and extends substantially below the flange 58. The flange 70 forms the skirt of a second or outer jet 72 which comprises an inlet formed by the lateral passageways 60 in the first cap 46, and an outlet defined by the space between the rims 52 and 6b of the inner and outer caps, respectively; Skirt 70 is adapted to deflect the vapor jet issuing from the second nozzle 72 inwardly toward the axis of the jet assembly 26, thereby creating a velocity vector in that direction represented by the arrow A in FIG. 3.

It will be seen that the arrangement of the inner and outer caps 46 and 62, on the topmost jet assembly sectionprovides two vapor jets at this point,,each adapted to impart ainwardly directed velocity vector to the vapor jet stream issuing therefrom which will serve to substantially eliminate a principal cause of backstreaming. Moreover, the jets formed'by the inner and outer caps are adapted so that the respective vapor jet streams issuing therefrom will interact in such a manner that the possibility of the migration of vapor molecules forming either of the jets toward the pump inletwill be minimized.

A further aspect of the present invention involves the provision of a heating arrangement 76 in the vapor chute 28 in order to further minimize backstreaming and startup problems and to promote pump efficiency. Advantageously, this heating arrangement is employed to heat the inner and outer caps as hereinafter described.

Heating arrangement 76 comprises a unitary, replaceable, cylindrical cartridge heater 78 which extends along the axis of the vapor chute 28 substantially throughout the length thereof. The lower end of the heater 78 extends through the pump bottom M and is connected to electrical leads 80 at a point beneath the bottom of the pump. The upper end of the cylindrical cartridge heater 78 extends into the hollow interior of the inner cap 46 and is in a heat conducting relationship therewith. it will be seen that the heatenwhich is of the resistance variety, is adapted to supply heat along the entire length of the vapor chute and directly to the inner and outer caps, 46 and 62, forming inner and outer topmost vapor jets in the jet assembly 26. The lower portion of the cartridge heater '78 is located in the oil sump 24 and is adapted to vaporize the liquid pumping fluid for the pumping operation.

Surrounding the entire length of the unitary cartridge heater 78 is a hollow tubular casing 82. The tubular casing 82 is formed of a suitable heat-conducting material and has a plurality of radial fins 84 attached to its exterior surface at intervals about its circumference. The radial fins 84 are also constructed of a suitable heat-conducting material and are maintained in a vertical plane between the pump bottom and the bottom of the topmost section of the jet assembly 26. The radial fins as extend outwardly form casing 82 to a point adjacent the interior surfaces of the sections 30 of the jet assembly 2s, and the outer vertical edges of the radial fins 84 are connected by a heating cylinder 86. The height of the heating cylinder 4% is the same as that of the radial fins 84, and together, the fins and cylinder provide a means for conducting the heat supplied by the cartridge heater 78m the sections 3%) forming the nozzle assembly 26.

Because of the dimensions of the elements of heater 78 heat is distributed throughout the interior of vapor chute 28 in a manner such that undesireable vapor condensation is minimized. More particularly, because of the substantially larger diameter and surface area provided in the lower portion of chute 28, and because of the larger diameter of this portion of the chute, relatively smaller amounts of heat per unit volume are radiated in this area, whereas relatively larger amounts are given off in the upper reaches of chute 2b where condensation problems and backstreaming are a particular problem. Furthermore, because of the direct connection of the heater 78 to the caps db, b2, condensation and backstreaming are minimized and pump efficiency is promoted.

in using a vacuum diffusion pump comprising the heating arrangement described above, electricity is supplied through leads 80 to the resistance cartridge heater '78. The cartridge heater immediately begins to supply heat 1 to the liquid pumping fluid maintained in the oil sump 24', 2 to the jet assembly 26, by conductance through the radial fins 8d and heating ring 86, and 3 directly to the inner and outer caps, 46 and 62, forming the first and second topmost jets in the jet assembly. By the time sufficient heat is transferred to the pumping fluid in the sump 2d to initiate the evaporation thereof, all the nozzles of the assembly 26 have been sufficiently heated by the heater arrangement 76 to insure proper pump operation, and evacuation of the region connected to the pump inlet can begin. Moreover during the pumping operation, any heat loss by radiation, fromthe nozzle assembly 26, or from the caps comprising the topmost nozzles thereof will be offset by heat supplied by the heating arrangement 76 in accordance with the subject invention.

The heating means of the present invention thus eliminate the problems heretofore encountered in prior art vacuum diffusion pumps with regard to heat loss from the jet assembly and the cooling of oil vapor as it rises therein. As in the case of the prior art pumps, the heating means of the subject invention are adapted to evaporate the normally liquid pumping fluid maintained in the oil sump. However, in contrast to the teachings of the prior art, the unitary cartridge heater 78 and its associated radial fins 84, and heating ring as, supply heat to the jet assembly to offset any radiation losses encountered thereby during the pumping operation and therefore ensure efficient pump operation. That is, the heat supplied by the cartridge heater is conducted from the heater casing outwardly, through the radial fins to the heating ring adjacent the interior surface of the jet assembly, and maintains the assembly at a temperature where condensation of oil vapor will not occur in the jets of assembly. Thus, the present invention does not rely on heat transfer from the oil vapor, itself, to initially bring the nozzle assembly to its operating temperature or to malre up the heat loss suffered by the assembly during the pumps operation. Moreover, the heating arrangement of the present invention serves to superheat the vapor chute 23, ensuring that the vapor supplied to the various nozzles will not cool while'travelling thereto and will be of the quality necessary for efficient pump operation.

At the topmost jet wherein the problems of backstreaming are most pronounced in the pumps of the prior art, the heating arrangement of the present invention, in combination with the inner and outer caps as, 62, of the present invention, provides an arrangement designed to substantially eliminate this backstreaming problem. Heat is directly conducted from the cartridge heater to the inner cap and, in turn, to the outer cap which is in contact therewith. This arrangement ensures that the two topmost nozzles formed by the first and second caps will be maintained at a temperature sufficient to prevent condensation of oil vapor therein, and therefore to prevent one of the recognized causes of backstreaming. in the pumps of the prior art, the topmost nozzle is the most difficult to bring to, and maintain at, a suitable operating temperature because it is farthest removed from the heater positioned in the oil sump at the bottom of the pump, and this difficulty is obviated in accord-ance with the arrangement of the present invention.

lclaim:

l. in a vacuum diffusion pump including a jet assembly comprising a series of staged vapor jets and a vapor chute, a

heated oil sump at one end of the vapor chute away from the space to be evacuated, the improvement comprising:

a first cap positioned at the end of the vapor chute away from the oil sump and forming with the vapor chute a first vapor jet, the first vapor jet, the first cap having a depending skirt adapted to direct oil vapor issuing from said first vapor jet in the general direction of the oil sump, and the first cap being provided with at least one lateral passageway means to permit the flow of oil vapor therethrough;

a second cap covering said first cap'and in heat conducting relationship therewith but spaced-apart therefrom and forming, in combination with said first cap and said lateral passageway means, a second vapor jet for oil vapor passing through said passageway means in said first cap, the second cap having a depending skirt extending below the skirt of said first cap in the direction of the oil sump in order to direct the oil vapor issuing from said second vapor jet in the general direction of the oil sump;

heater means adapted to supply heat to said first and second caps; and

whereby backstreaming of oil vapor in the pump is minimized.

2. An improvement, as claimed in claim 1, wherein the first cap comprises a body with an outwardly directed rim positioned at it bottom, the rim being adapted to seat upon the end of the vapor chute in spaced relation thereto, the lateral passageway means being provided in the body; wherein the depending skirt of the first cap is positioned on the rim; and wherein the first jet takes the form of a nozzle formed between the rim and the end of the vapor chute, whereby oil vapor passing through the jet is deflected generally in the direction of the oil sump.

3. An improved heater and vapor nozzle arrangement as claimed in claim 2; wherein said second cap comprises a body positioned adjacent to, and in contact with, the upper portion of the body of the first cap; wherein the depending skirt of the second cap is attached to the body of said second cap so as to surround the skirt of said first cap, but spaced therefrom, the space between the depending skirts forming, in combination with the lateral passageway means in the body of the first cap, the second vapor jet.

4. An improvement, as claimed in claim 3, wherein said heater means comprises cartridge-heating means positioned within the vapor chute having a first end maintained in the oil sump and a second end in heat conducting relationship with the first and second caps, whereby heat is supplied by to the oil sump, to the jet assembly, and directly to the first cap.

5. An improvement as claimed in claim 4, wherein the cartridge-heating means comprises:

a unitary cartridge-heater passing longitudinally throughout substantially the length of the vapor chute;

a casing of heat-conducting material, positioned within the vapor chute, surrounding the cartridge heater;

a plurality of fins, of a heat-conducting material, attached to the casing and extending outwardly therefrom;

a heating member of a heat-conducting material connecting the free ends of the fins; and

whereby the heat supplied by said cartridge heater is uniformly distributed to the vapor jet assembly.

6. An improvement as claimed in claim 1, wherein the jet assembly is generally annular in cross section and wherein the first cap comprises a hollow cylindrical body having a top integral therewith and having an annular, outwardly extending rim attached to its bottom, said said rim being positioned adjacent the end of the vapor chute opposite the oil sump, but spaced therefrom, the annular space therebetween defining the first vapor jet, the lateral passageway means being provided in the lower portion of the cylindrical body, with the de pending skirt comprising a vertically depending flange attached to the end of said annular rim opposite the cap body, whereby oil vapor passing through the first vapor jet is deflected generally in the direction of the oil sump.

An improvement, as claimed in claim 6, wherein the second cap comprises a hollow cylindrical body, having an interior surface and an exterior surface positioned so that the interior surface thereof surrounds and contacts the upper portion of the exterior surface of the body of the first cap, the depending skirt taking the form of a vertically depending flange attached to the bottom of the hollow cylindrical body, and extending first outwardly and then downwardly therefrom, the flange on the second cap being positioned so as to surround the flange of the first cap, but spaced therefrom, the space between the flange forming, in combination with said lateral passageway means in the body of the first cap, the second vapor jet; whereby the oil vapor passing through the first and second vapor jets is deflected in the direction of the oil sump.

8. An improvement, as claimed in claim 7, wherein said heater means comprises cylindrical cartridgeheating means positioned within the vapor chute and having a first end maintained in the oil sump and a second end maintained within the hollow cylindrical body of the first cap, whereby heat is supplied to the oil in the oil sump to the jet assembly, and directly to the first cap.

9. An improvement, as claimed in claim 8, wherein the cartridge-heating means comprises:

a unitary cylindrical cartridge heater passing longitudinally throughout substantially the length of the vapor chute;

a hollow tubular casing of a heat-conducting material, positioned within the vapor chute and enclosing the entire length of said cylindrical cartridge heater;

a plurality of vertical fins of a heat-conducting material positioned within the vapor chute, one end of each of said fins being attached to the exterior of said casing, and said fins extending radially outward therefrom;

a hollow-heating cylinder of a heat-conducting material positioned within the vapor chute, attached, at points on its interior surface, to the end of each of said fins opposite the casing; and

whereby the heat supplied by said cartridge heater is distributed throughout caps; and interior of the vapor chute.

E0. in a vacuum diffusion pump having an inlet, an exhaust,

an oil sump, and a nozzle assembly formed by a vapor chute and a series of staged vapor jets between the inlets and the oil sump, the improvement comprising:

a cylindrical cartridge heater positioned within the vapor chute and having a first end positioned in the oil sump and a second end positioned adjacent the vapor jet nearest the pump inlet, the cartridge heater being adapted to supply heat throughout its length; and

heat distribution means in association with the cartridge heater, adapted to distribute the heat from the cartridge heater substantially uniformly throughout the vapor chute, comprising:

a tubular casing of heat conducting material, positioned within the vapor chute, surrounding substantially the entire length of said cartridge heater;

a hollow-heating cylinder of heat conducting material, positioned in the vapor chute, the axis of said cylinder being parallel with the axis of the chute, and said hollow cylinder surrounding the lower portion of the casing; and

a plurality of radial fins of a heat-conducting material, connecting the exterior surface of said casing and the interior surface of said heating cylinder, whereby substantial uniformity of heat distribution is achieved.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 572, 973 Dated March 30, 1.971

In ent flsc) Joseph A. LeBlanc, Jr,

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, after line 61, insert the following paragraph:

--It is yet a further object of the present inveni to provide an improved unitary heater arrangement in a vacul diffusion pump in order to supply heat directly to the topme jet of the pump to prevent the condensation of vapor molecul therein. ----7 Column 2, line 73, delete "cute";

Column 5, line 64, delete "form" and substitute --from-;

Column 5, line 74, cancel "undesireable" and substitute --undesirable--;

Column 7, line 5, after "vapor jet," delete "the first vapor jet":

Column 7, line 26, "it" should be --its--;

Column 7, line 65, delete "said" (first instance) Column 8, line 16, "flange" should be -flanges--;

Column 8, line 43, cancel "caps; and" and substitt -the--.

Signed and sealed this 9th day of November 1971..

(SEAL) Attest:

EDWARD M.F'LETCHER,JR. ROBERT GO'ITSGHALK Attesting Officer Acting Commissioner of Paten FORM PO-105O (O-69) sc -nc 503w

Claims

1. In a vacuum diffusion pump including a jet assembly comprising a series of staged vapor jets and a vapor chute, a heated oil sump at one end of the vapor chute away from the space to be evacuated, the improvement comprising: a first cap positioned at the end of the vapor chute away from the oil sump and forming with the vapor chute a first vapor jet, the first vapor jet, the first cap having a depending skirt adapted to direct oil vapor issuing from said first vapor jet in the general direction of the oil sump, and the first cap being provided with at least one lateral passageway means to permit the flow of oil vapor therethrough; a second cap covering said first cap and in heat conducting relationship therewith but spaced-apart therefrom and forming, in combination with said first cap and said lateral passageway means, a second vapor jet for oil vapor passing through said passageway means in said first cap, the second cap having a depending skirt extending below the skirt of said first cap in the direction of the oil sump in order to direct the oil vapor issuing from said second vapor jet in the general direction of the oil sump; heater means adapted to supply heat to said first and second caps; and whereby backstreaming of oil vapor in the pump is minimized.

3. An improved heater and vapor nozzle arrangement as claimed in claim 2 wherein said second cap comprises a body positioned adjacent to, and in contact with, the upper portion of the body of the first cap; wherein the depending skirt of the second cap is aTtached to the body of said second cap so as to surround the skirt of said first cap, but spaced therefrom, the space between the depending skirts forming, in combination with the lateral passageway means in the body of the first cap, the second vapor jet.

5. An improvement as claimed in claim 4, wherein the cartridge-heating means comprises: a unitary cartridge-heater passing longitudinally throughout substantially the length of the vapor chute; a casing of heat-conducting material, positioned within the vapor chute, surrounding the cartridge heater; a plurality of fins, of a heat-conducting material, attached to the casing and extending outwardly therefrom; a heating member of a heat-conducting material connecting the free ends of the fins; and whereby the heat supplied by said cartridge heater is uniformly distributed to the vapor jet assembly.

6. An improvement as claimed in claim 1, wherein the jet assembly is generally annular in cross section and wherein the first cap comprises a hollow cylindrical body having a top integral therewith and having an annular, outwardly extending rim attached to its bottom, said said rim being positioned adjacent the end of the vapor chute opposite the oil sump, but spaced therefrom, the annular space therebetween defining the first vapor jet, the lateral passageway means being provided in the lower portion of the cylindrical body, with the depending skirt comprising a vertically depending flange attached to the end of said annular rim opposite the cap body, whereby oil vapor passing through the first vapor jet is deflected generally in the direction of the oil sump.

7. An improvement, as claimed in claim 6, wherein the second cap comprises a hollow cylindrical body, having an interior surface and an exterior surface positioned so that the interior surface thereof surrounds and contacts the upper portion of the exterior surface of the body of the first cap, the depending skirt taking the form of a vertically depending flange attached to the bottom of the hollow cylindrical body, and extending first outwardly and then downwardly therefrom, the flange on the second cap being positioned so as to surround the flange of the first cap, but spaced therefrom, the space between the flange forming, in combination with said lateral passageway means in the body of the first cap, the second vapor jet; whereby the oil vapor passing through the first and second vapor jets is deflected in the direction of the oil sump.

8. An improvement, as claimed in claim 7, wherein said heater means comprises cylindrical cartridge-heating means positioned within the vapor chute and having a first end maintained in the oil sump and a second end maintained within the hollow cylindrical body of the first cap, whereby heat is supplied to the oil in the oil sump to the jet assembly, and directly to the first cap.

9. An improvement, as claimed in claim 8, wherein the cartridge-heating means comprises: a unitary cylindrical cartridge heater passing longitudinally throughout substantially the length of the vapor chute; a hollow tubular casing of a heat-conducting material, positioned within the vapor chute and enclosing the entire length of said cylindrical cartridge heater; a plurality of vertical fins of a heat-conducting material positioned within the vapor chute, one end of each of said fins being attached to the exterior of said casing, and said fins extending radially outward therefrom; a hollow-heating cylinder of a heat-conducting material positioned within the vapor chute, attached, at points on its interior surface, to the end of each of said fins oPposite the casing; and whereby the heat supplied by said cartridge heater is distributed throughout caps; and interior of the vapor chute.

10. In a vacuum diffusion pump having an inlet, an exhaust, an oil sump, and a nozzle assembly formed by a vapor chute and a series of staged vapor jets between the inlets and the oil sump, the improvement comprising: a cylindrical cartridge heater positioned within the vapor chute and having a first end positioned in the oil sump and a second end positioned adjacent the vapor jet nearest the pump inlet, the cartridge heater being adapted to supply heat throughout its length; and heat distribution means in association with the cartridge heater, adapted to distribute the heat from the cartridge heater substantially uniformly throughout the vapor chute, comprising: a tubular casing of heat conducting material, positioned within the vapor chute, surrounding substantially the entire length of said cartridge heater; a hollow-heating cylinder of heat conducting material, positioned in the vapor chute, the axis of said cylinder being parallel with the axis of the chute, and said hollow cylinder surrounding the lower portion of the casing; and a plurality of radial fins of a heat-conducting material, connecting the exterior surface of said casing and the interior surface of said heating cylinder, whereby substantial uniformity of heat distribution is achieved.