US2386298A - Diffusion pump - Google Patents

Description

J. R. o. DOWNING EI'AL 2,385,298

DIFFUSION PUMP Filed Jan. 30, 1943 2 Shets-She et 1 40 INVENTORS JAMzs 7?. O. D m- 30 WILLIAM 3.4144015;

cfr wi HT TOR/VEV Oct. 9, 1945.

J. R. o. DOWNING ETAL 3% DIFFUSION PUMP 2 Shets-Sheet 2 Filed Jan. 30, 1943 .0 RNS ,V. 0/ NM Mwu JR V H m aB m A M 5A Jw C Y B M 2 Patented Oct. 9, 1945 DIFFUSION PUMPY James R. 0. Downing and William B. Humes, Boston, Mass, assignors to National Research Corporation, Boston, Mass, a corporation of Massachusetts Application January 30, 1943, Serial No. 474,134

6 Claims. (Cl. 230-101) This invention relates to vacuum pumps and particularly to diflusion-or condensation pumps useful for increasing the pumping speeds of mechanical pumps at relatively high backing pressures.

The use of diflusion pumps, particularly in industrial applications, has been restricted by the fact that such pumps, as heretofore constructed, have not operated satisfactorily except under exceedingly low backing pressures, of the order of mm. of mercury, or less. .The cost of producing and maintaining such low backing pressures is prohibitive for most industrial applications of vacuum pumping.

Accordingly, an object of this invention is to provide a vacuum pump of the type referred to embodying novel principles of pump construction by which it is enabled to operate efllciently to produce high pumping speeds at comparatively high backing pressures of from 1 to 3 mm. of mercury, or even higher. A further object is to provide such a pump which is comparatively inexpensive to construct and which is more economical of pumping fluid than prior pumps of its type.

These and other objects and advantages of the invention will be readily understood from the ensuing particular description, in conjunction with the appended drawings, wherein:

Fig. 1 is a vertical section view of a preferre embodiment of pumping apparatus of the invention;

Fig. 2 is a transverse section view on the lines 2-2 of Fig. 1;

Fig. 3 is a vertical'section view of a portion of pumping apparatus, showing modified construction;

Fig. 4 is a transverse section view on the line 4-4- of Fig. 3;

Fig. 5 is a transverse section viewon the line 5-5 of Fig. 3.

Difiusion pumps, as usually constructed, have a pumping chamber interconnected at opposite ends with the system undergoing exhaustion and with the backing pump. Vapors of a pumping fluid are discharged from a jet in the low pressure end of the chamber where they expand, flow toward the high pressure end of the chamber, and condense on its walls. According to generally accepted theory, these vapors perform their pumping function by entraining molecules of gas from the system being exhausted and carrying them along toward the high pressure end of the chamber. In some cases there are several vapor jets arranged in series along the pumping chamber.

In prior pumps of this character, the jets have been of the so-called "umbrella or mushroom type, in which the vapors flow in a substantially by an inclined .shield which confines the stream between itself and this wall of the chamber. Such a jet exposes only one side of the vapor stream to the gases in the chamber, the side away from the chamber wall from which the stream is discharged, as there is no way in which the gases can get between the vapor stream and the wall.

We have discovered that an improved operation is obtained by theuse of a novel jet of the invention by which the vapors are discharged into the chamber in spaced relation from the chamber walls, the jet comprising, at least in part, separate, laterally spaced vapor ducts providing open spaces between them through which the gases can circulate from one side to the other of the jet and thus contact opposite sides of a vapor stream produced by the jet. Preferably,'the ducts are provided with individual, spaced nozzles which discharge separate streams of vapors between the walls of the chamber.

We have also found that the pumping action of such a jet is enhanced by providingin the pumping chamber one or more restricted passages for conducting the vapors from the jet toward the high pressure end of th chamber, such passage or passages preferably diminishing in width between their opp site ends, the jet being so arranged as to discharge the pumping vapors into the mouth or mouths of such passage or passages in spaced relation from the walls of such passage or passages so that the vapors issuing, from the jet are allowed to expand as they enter the passage or passages and are exposed to the gases in the chamber. a

The jet and co-operating restricted passages are preferably preceded in the low pressure end of the chamber by another pumping vapor jet which may be of the conventional umbrella type.

The invention also provides other improvements in the construction of difiusion pumps as will hereinafter appear.

In the form of apparatus shown in Figs. 1 to 3 of the accompanying drawings, a vertically ar-- ranged hollow cylinder it provides in its interior the main pumping chamber. The cylinder i0 is provided at its upper end with an annular projecting flange i2 by which it may be connected into the system to be exhausted (not shown). The lower end of the cylinder is closed by a bottom wall it. Cylinder i0 is.surrounded by a Jacket I I spaced therefrom, this Jacket being attached at its upper end to the flange l2 and at its lower end being closed by a bottom wall ll spacedfrom the bottom wall l4 the cylinder II. The space between cylinder II and Jacket It receives cooling water through connections hereinafter described.

A conduit 30 opens through the lower part of cylinder l0 into the high pressure end of the main pumping chamber, extending therefrom laterally and upwardly for a considerable 'distance and being provided at its upper end with an annular flange 32 by means or which it may be connected to the suction line of a suitable ally of the cylinder. The upper. end of the chirn ney extends into the low pressure, upper end of the cylinder in and is closed by an end wall 50. An inverted conical shield 52 is attached to the wall 50 by a screw 54 and extends down around the upper end of the chimney which is provided, near its end wall 50 and above the lower edge of shield 52, with a ring of small apertures 56. The shield 52 co-operates with the upper end of the chimney and the apertures 55 to form a conventional umbrella type jet. The annular space between the chimney 48 and the cylinder Ill forms the main pumping chamber.

Within the pumping chamber there is mounted an annular partition 58 which is attached at its outer side to the inner wall of cylinder III and at its inner side to the chimney 48. Through the partition there extend a plurality of passages 60 forming sub-passages in the pumping chamber, there being eight such passages provided in the apparatus illustrated in Figs. 1 and 2 (see Fig. 2). Each of the passages 68, as shown, decreases in cross-section from its upper, inlet end, for a distance somewhat more than half its length, then increases in cross-section to its lower, outlet end, so that a stream of vapor or gas passing through the passage first contracts and then expands.

In operation, the boiler is provided with a quantity of suitable pumping fluid such as mercury or a heat stable'organic compound having low vapor pressure at room temperature. Heat is applied causing vaporization of the fluid, the vapors rising into the chimney 48. Some of these vapors, reaching the top of the chimney, flow outwardly through the apertures 56 and are directed downwardly and outwardly by the shield 52, diffusing into and filling the main pumping chamber above the partition 58 entraining molecules or gas from the system undergoing exhaustion towhich the pumping chamber is connected. Some of .the vapors flow out through the ducts 62, contracting in the passage 1'0, expanding as they proceed along the passage 12 and expanding further as they leave the ducts and enter the mouth of enact the passages 60. These vapors then flow downwardly through the'passages 60, contracting until they reach the narrowest portion of the passage and then expanding as they proceed to the bottom of the passage from which they expand into the portion of the main pumping chamber below the partition 58. The vapor streams from the ducts 62 act to draw the vapors and gases from the upper part of the pumping chamber therewith through the passages 88 and into the lower part of the pumping chamber. As the vapors proceed downwardly they condense on the walls of the pumping chamber and flow downwardly thereon, condensed liquid in the upper part of the chamber flowing downwardly on the walls of the chamber and the walls of the passages 60 into the lower part of the chamber. The gas molecules carried into the lower or high pressure end of the main pumping chamber are drawn oil through the conduit 30 by the suction of the backing pump.

The spaces between the pipes 64 and separat- 40 ing the nozzles 68 from each other, from the chimney 48, which forms one wall of the pumping chamber, and from cylinder III. which forms the other wall 01' the pumping chamber, provide passageways through which the gas and vapor mixture in the upper end of the chamber freely circulates so that the vapor streams issuing from the nozzles are exposed on all sides to the vapor gas mixture. A much more eflicient pumping or entraining action results than is obtainable with a conventional umbrella type jet, such as shown at the upper end of the pumping cham- In the pumping chamber above the partition 58 there is provided a vapor jet comprising a plurality of. individual vapor ducts 62, there being eight such ducts in the illustrated apparatus, arranged radially around the chimney and each co-operating with one of the passages 60. Each of the ducts 62 comprises, as shown, an elbow pipe 64 attached at one end to the chimney 48 and communicating with the interior thereof through a port 66. The opposite end or the elbow pipe 64 extends downwardly and has attached thereto a nozzle 68 extending therefrom to the upper end of one of the passages 68. Nozzles 88 have, as-shown, an inwardly flaring, short inlet passage 10 and a relatively long, gradually outwardly flaring outlet passage 12. The lower end of the nozzles 68 is'of, smaller diameter than the adjacent. upper end portion of the corresponding passage 60, providing space therebetween for vapors and gases to enter the passages ill from the portion of the pumping chamber above the partition 58.

her, in which the vapor stream issuing from the Jet is exposed on one side only to gas molecules being exhausted, in the narrow space between the deflector shield of the jet and the outer wall of the chamber. Moreover, due to the provision of the constricted passages 60' and their relation to the ducts 62 of the jets, the vapors issuing from the ducts expand while they are exposed to the vapors and gases in the chamber above them, then contract and again expand as they flow downwardly through the passages, and this expansion, contraction, expansion of the vapor streams increases their pumping action.

To collect the condensed pumping liquid and return it/to the boiler for re-use, there is provided, in the underside of conduit 30, near the cylinder I 0, a pipe 14 extending downwardly to a T connection It with the boiler. Connection I6 is provided with a bottom cleanout plug 18,

flow thereon by gravity into the bottom of conduit I and thence into the pipe I4.

To promote condensation of the pumping vapors the space between the jacket I6 and cylinder I0 is continuously supplied with cooling water through a pipe connection 8: opening through the bottom wall l8 of the jacket. Also, the. lower part of the chimney 48 is surrounded by an outer wall or jacket 84 extending from the boiler to a point just above the partition 58. Between jacket 14 and the chimney there is provideda small space 08. which is preferably evacuated, and sealed at its opposite ends. We have found that this jacket and evacuated space between it and the chimney efl'ectively insulate the surrounding chamber from the heat which is transmitted to the chimney, particularly in its lower portion, by the heated vapors rising therein, and thus improve the extent of vapor condensation in the chamber. Y

Although the vapor condensation and return system of the pump construction so far described is highly efficient, there is a tendency for a small quantiw of vapors to escape into the system being evacuated or to the backing pump. To prevent such loss of vapors; the invention provides novel water-cooled bafiie units between the low pressure end oi the pump and the system being evacuated and between the high pressure end oi the pump and the backing pump, as will now be described.

Referring to Fig. 1, there is attached to the flange I2 of cylinder I0 a hollow, vertically disposed cylinder 90 of like diameter to the cylinder I0 and'iorming a continuation thereof. Cylinder 00 is provided at its lower end with an annular projecting flange 92 by which it is attached to flange I2 and at its upper end with a like flange 94 by which it may be connected into the system being evacuated (not shown). The cylinder is surrounded by a jacket 90 seated at its opposite ends against the flanges 92 and 94 and providing a space therebetween and the cylinder 90 for cool water which is supplied through a pipe 00 opening at opposite ends through the jacket 96 and the jacket l0 of cylinder I0.

Near the base of cylinder 90, a pipe I00 Drojects inwardly approximately to the axis of the cylinder, thence upwardly along the cylinder axis to near the upper end of the cylinder, thence outwardly through the cylinder wall and its jacket. On the vertical portion of this pipe are fixed a number of bafile plates I02, two such plates of circular shape being shown in the drawings. To the inner surface of the cylinder are fixed a like number of baifle rings I00, alternating with the plates I00 and having central apertures I05 surrounding the pipe 100 of a shape and size corresponding to the shape and size of the plates I02.

The upper end of the conduit 30 is similarly equipped with a jacket H0 providing a water channel therebetween and the outer surface of the conduit 30 which is sealed at opposite ends and supplied with water through a pipe H2 extending into the conduit 30 through the curved lower part thereof and thence upwardly along the axis of the conduit to near its upper end, where a lateral extension thereof opens through the wall of the conduit 30 into the water channel. The pipe H2 is provided with a numberof baliie plates IIl like the baiiie plates I02 and the inner surface of the conduit 30 is provided with a number of baflie rings H6 like the baiile rings I04 and similarly alternating with the bafile plates.

and thence back to the boiler.

In.use, the lower end of pipe 2 is connected to a source of cool water (not shown) which flows upwardly in pipe H2. into the passage be-.

cylinder and its jacket and thence through pipe I00 out of the apparatus where it may be returned for recirculation.

The baiiie plates I02 and H4 are constantly cooled by the water flowing through the pipes I00 and H2 and the baiile rings I04 and H6 are similarly cooled by the water flowing over the surface of cylinder 90 and conduit 30, respectively. These baflie plates are so arranged as to intercept any vapors escaping from the main pumping cylinder and condense them. Condensed vapors on the ballies and walls of cylinder 90 flow down into cylinder I0 and thence downwardly into pipe I4 Condensed vapors on the baiiles and walls of conduit 30 flow downwardly along the conduit walls to the pipe III. A narrow baiile ring I20 in conduit 30just' beyond the pipe I4 has an opening I22 therethrough at the bottom. of the conduit to permit the pumping liquid flowing downwardly along the conduit to pass therethrough to pipe I4.

In the modified pump construction partially shown in Figs. 3 to'5 there is provided a verti- -cally disposed cylinder 200 having a jacket 20I,

like the cylinder I0 and jacket iii of Fig. 1, the cylinder connected, at its upper end (by means not shown), to a system to be evacuated and, at its lower end, to a conduit 202 communicating with a; backing pump. A vapor chimney 204 rises into the interior of the cylinder and is provided at its upper end with an umbrella jet formed by ports 206 in the chimney and an inverted conical shield 208, these parts being also like those shown in Fig. 1.

To the inner wall of the cylinder 200, below the upper end of the chimney, there is fastened a ring H0, and a. second ring M2 is mounted upon,

and surrounds the chimney opposite the ring 2| 0.

These rings 2I0 and 2|2 are thicker in the middle than at their upper and lower ends and form therebetween a restricted passage 2I4 in the pumping chamber having a cross-sectional shape similar to that of a passage 60 of Fig. 1. In this case, however, as shown in Fig. 5, the single passage 2Id is continuous about the chimney. In the chimney 204, above the passage 2 and below the apertures 206, is provided another ring of ports 2I6. Fastened to the chimney above the ports M6 is an inverted, cup-shaped deflector 2H3 which projects laterally from the chimney, then downwardly and outwardly to adjacent the upper end of ring 2 I 0. Also fastened to the chimney, below ports 2I6 is a second deflector 220 projecting laterally from the chimney toward deflector 2I8 thence downwardly and inwardly to adjacent the upper end of ring 2I2. The vertically extending portions of deflectors M8, 220 form therebetween a vapor duct 222 which is continuous about the chimney, this duct being similar in cross-section to a nozzle 68 of Fig. 1 and having its lower end similarly disposed with re, lation to the passage 2I4.

The space between the laterally extending portions of deflectors 2I8, 220 is subdivided into a number of smaller, subpassages between the chimney and duct 222 by means of vertically arranged flues 220 extending through these portions of the deflectors (Figs; 3 and 4) and forming pas- 2il,'deflectors H8, 220 and flues 224, in relation 5 to passage 2, is substantially like the operation .of the Jet comprising ejectors 62 of Fig. 1 in relation topassages 60. The vapors flowing from ports 2li are subdivided by the sides of fiues 224 into separate streams which re-unite as they pass 19 th fines and flow in a single expanding stream from the duct 222, the stream contracting, then expanding as it flows downwardly between the rings 2", 2l2. Vapors and gases in the chamber above the deflector 2l8 flow downwardly between 1 said deflector and the inner wall of cylinder 200 and also, by means of the fiues 224, downwardly into the space between the inner defiector 220 and chimney 20!. Thus both sides of the expanding vapor stream are exposed to the vapors and 20 gases in the pumping chamber and an efiicient pumping action thereof is obtained.

The vapors condensing in the cylinder 200 and duct 202 are returned to the boiler 226 through a pipe 228 and T connection 230.

With apparatus as shown in Fig. 1, there is sometimes encountered a tendency of vapors supplied to the top jet to condense before reaching the passages 60 to a greater extent than is desir-' able. provides a novel boiler construction, of which a form is shown in Fig. 3, which enables heating of the vapors rising from the liquid in the boiler so that they reach the upper let in a-somewhat less readily condensible condition.

Referring to Fig. 3, the boiler 226 is shown as provided with bottom plate 232 having a central opening 234 capped by an inverted tube 236 which rises into the lower part of the chimney 20! and To remedy this condition, the invention a system to be evacuated and a second end portion adapted for connection to a pressure reducing means, said chamber comprising at least one passage tor the how of vapors from said first end portion to said second end portion of the chamber of restricted cross-section relative to said first end portion of the chamber. a first let for discharging pumping vapors into said first end of the chamber, a second jet for discharging pumping vapors into said chamber comprising a plurality of vapor ducts within said first end portion of the chamber arranged in laterally spaced relation about the axis of the chamber and providing openings therebetween through said jet, each said duct provided with a nozzle for directing vapors therefrom into the mouth of a said passage, and means for supplying pumping vapors to both of said jets.

2. A difiusion pump having, in combination, conduit means forming a pumping chamber having a first end portion adapted for connection to a system to be evacuated and a second endporfrom said first end portion to said second end.

streams of pumping vapors into the mouths of said sub-passages, and means for supplying pumping vapors to both of said Jets.

3. In a diffusion pump, the combination, with means forming a pumping chamber adapted for is closed at its upper end. Within this tube 238 411 connection at one end to a system to be evacuthere is contained an electric heating element 238. Similar elements 2 are dis osed immediately below the plate 232. In operation, the pumping liquid in the boiler vaporized by the heat from elements 248 and the vapors, as they rise in the chimney, are heated byheat from element 238 radiating from tube 2". A jacket 242 enclosing an evacuated space 2 about the chimney, similar to the jacket 84 of Fig. 1, prevents any substantial heating of the pumping chamber by the element 238.

An additional heating element may, if desired, be provided about the lower part of the chimney between the base 01' the cylinder and the top of the boiler, or may be used in place of the interior heating element 238 and tube 233 shown. However, we have found that the arrangement shown is generally suflicient of itself and is superior to an exterior heating of the base of the vapor chimney both because the latter arrangement has a 6 greater tendency to heat the pumping chamber and because, with the arrangement shown, the

vapors receiving the most heat are at the center of the chimney and have a tendency to rise past the ports 2 l 6, which receive the less heated vapors nearer the walls of the chimney, as is desirable.

It will be understood that we have illustrated and described herein preferred embodiments only of apparatus of the invention and that the invention is to be considered as limited thereto only in so far as may be required by the appended claims.

We claim: 7 1. A diffusion pump having. in combination,

conduit means forming a pumping chamber having a first end portion adapted for connection to open to the chamber, and means for supplying pumping vapors to the space between said deflectors.

4. A diffusion pump having, in combination, conduit means forming the outer wall of a pumping passage, tubular means in said conduit means having opposite walls inwardly sloped from each end to form a portion of said passage of diminishing cross-section between its ends, jet means including a nozzle for directing a stream of pumping vapor into the inlet end of said passage portion formed by said tubular means, said nozzle providing a vapor outlet passage of increasing cross-section toward its outer end and having its outlet end directed axially of the pumping passage portion formed by said tubular means and of substantially smaller crosssection than the inlet end of said passage portion, jet means for diffusing pumping vapor into said pumping passage between said first named jet means and the 'inlet end of said conduit means, and means for simultaneously supplying pumping vapor to each of said jet means.

5. A diflusion pump having, in combination,

a casing forming the exterior wall of a pumping chamber adapted for connection at one end to a system to be exhausted and at the other end to a, pressure reducing means, a vapor chimney extending into said chamber, jetmeans for dising a pumping chamber for connection between a system to be evacuated and a low pressure producing system and means for diflusing condensible pump operating vapors into said chamber, means for conducting gases between .said chamber and one of said systems and for preventing escape of uncondensed vapors from said chamber therethrough comprising a conduit connected at one end into said chamber and adapted for connection at the opposite end into said one of said systems, a fluid conductive pipe extending longitudinally of the interior of said conduit, a first set of baifies mounted in longitudinally spaced relation in said-pipe and projecting laterally therefrom toward the inner walls of said conduit, asecond set of bafiles secured to the inner wall of said conduit in alternate relation with the baflles of said first set, the battles of said second set each projecting laterally toward said pipe and substantially surrounding the pipe with their inner edges spaced from said pipe, means for circulating cooling fluid through said pipe,- means for circulating cooling fluid over the outer surface of said conduit, and means for collecting the liquid of condensed pumping vapors condensing in said conduit.

JAMES R. O DOWNING. WILLIAM B. HUMES.

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