Patent Publication Number: US-3876337-A

Title: High vacuum pumping systems

Description:
United States Patent 1 Landfors 1 Apr. 8, 1975 1 HIGH VACUUM PUMPHNG SYSTEMS Arthur A. Landfors, Sharon, Mass.  
 [73] Assignee: Varian Associates, Palo Alto, Calif.  
 [22] Filed: Mar. 15, 1973 [21] Appl. No.: 341,419  
 [75] Inventor:  
 Primary E.\&#39;aminerWilliam L. Freeh Assistant Examiner-Richard E. Gluck 141101710), Agent, or Firm-Stanley Z. Cole; Leon F. Herbert; John J. Morrissey [57] ABSTRACT A trap assembly in a high vacuum system for intercepting back streaming liquid or suspended liquid molecules from a diffusion pump to prevent contamination of the working high vacuum chamber is disclosed. The trap assembly includes a tubular member defining a passageway between the hollow inlet means of the chamber and the inlet orifice of the pump, and a restriction means across the passageway. The tubular member includes an envelope wall physically forming the passageway in the region of the restriction means, and fastener means extending within said passageway inside the confines of said wall to interconnect said wall with said pump and with said hollow inlet means of said chamber to gain maximum internal area for a given overall outer dimensional extent of the tubular member. The fastener means includes mating flanges and a plurality of bolts around the flanges. The bolts may be sealed by sleeves that also bear at least a portion of the tightening load of the bolts and thereby assist the envelope wall in this function. The restriction means in the passageway includes a plate positioned at an angle to allow draining action of condensed liquid, and a cooling tube is effective to cool the plate and the sleeves to increase condensation. The sleeves are cylindrical and only slightly larger than the bolts to provide minimum volume displacement. The radius of the passageway has been increasaed by approximately 50&#39; percent over prior art devices, with a consequent crosssectional area increase in the full region of the passageway of substantially 100 percent. The retained speed of the pump is increased over the prior art capability by approximately 50 percent.  
 6 Claims, 2 Drawing Figures VACUUM v n CHAMBER P/JENTED 8l9?5 3,876,337  
  VACUUM n CHAMBER HIGH VACUUM PUMPING SYSTEMS BACKGROUND OF THE INVENTION The present invention relates to high vacuum pumping systems, and more particularly, to a system having an improved trap assembly for intercepting back streaming liquid.  
  Diffusion or vapor pumps are well known in the art of producing high and ultra-high vacuum conditions. One type, the fractionating diffusion pump, uses separately supplied oil vapor streams of suspended oil molecules ejected in a vertical array of peripheral layers into a tubular pump housing to entrain the gas molecules and thereby create a vacuum in a connected working vacuum chamber, as shown, for example, in my previous patent, Landfors U.S. Pat. No. 3,165,255, issued Ian. 12, 1965. The diffusion pump has the unique capa- One disadvantage of diffusion pumps lies in the fact that, especially with low vapor pressure oils, but also with higher vapor pressure oils, traps are required in the passageway between the pump and the working vacuum chamber in order to eliminate back streaming or back diffusion of oil vapor. In the prior art, trap assemblies including baffles, or a combination of baffles and cold surfaces have been used. Heretofore, the main disadvantage with the use of baffles has been the serious reduction in pumping speed, i.e., the ability to maintain maximum gas flow past the baffles. There has been recognized in the art the need to increase the retained speed of evacuation through a trap assembly while maintaining the efficiency of interception of back streaming vapor.  
 OBJECTS OF THE INVENTION Thus. it is one of the objects of the present invention to provide an improved trap assembly in a high vacuum pumping system that provides maximum internal crosssectional flow area for a given overall outer dimen sional extent of the assembly.  
  It is still another object of the present invention to provide a trap assembly having restriction means in the passageway connecting the pump and the vacuum chamber in a high vacuum pumping system, with the restriction means being surrounded by an envelope wall and with fastener means between the parts of the system being positioned inside the confines of the wall to make maximum use of the space available.  
  It is still another object of the present invention to provide a system wherein existing flanges of the fastening means on a diffusion pump and on the hollow inlet means of a working vacuum chamber can be utilized without alteration to interconnect the improved trap assembly that allows increased flow through the converted system.  
 BRIEF DESCRIPTION OF THE INVENTION A high vacuum pumping system is provided in accordance with the present invention having a trap assembly between the inlet means of the working chamber and the inlet orifice of the diffusion pump that not only has an effective restriction means for intercepting the back streaming liquid molecules, but also has a construction allowing maximum gas flow. These results are attained without necessarily increasing the outer dimensional extent of the tubular member forming the assembly. In brief, this is done by providing a tubular member having an envelope wall that forms the passageway of the assembly, with said wall being positioned at or beyond the outer limit of the connecting flanges of the member thereby greatly increasing the space for gas flow in the region of the restriction means. With the envelope wall extended at least to the outer circumferential limit of the flanges and the fastener means being disposed inside the confines thereof, the effective flow area is substantially doubled over previous prior art arrangements of which I am aware, i.e., prior art wherein the envelope wall is maintained inside the fastener means. With this increase in flow area, I have found that the retained pumping speed through the trap assembly is increased from 40 percent of the unrestricted speed of the pump to 60 percent. In other words, the efficiency ofthe pumping operation in terms of pump speed and flow has been improved by 20 percent, or in terms of an increase over the prior art, an improvement of 50 percent.  
  The fastener means of the apparatus of the invention includes the connecting flanges, and preferably these flanges are the same as those used previously, so that pumping systems now in use can be easily converted to a higher efficiency system. Preferably, a plurality of bolts act to hold the juxtaposed flanges together, and the necessary O-rings or other sealing means may be provided.  
  In the preferred embodiment shown. the number of parts has been held to a minimum by extending single bolts through l the mating flanges between the pump and the trap assembly, and (2) the mating flanges between the inlet means to the working chamber and the trap assembly.  
  Means is provided to seal the passageway adjacent the ends of the bolts. This passageway sealing means may include a cylindrical sleeve or shroud encompassing the body of the bolts, and thereby totally isolating the bolt and bolt holes in the flanges from the internal vacuum condition and thereby simplifying the sealing arrangement. In this arrangement, the sleeves also bear at least a portion of the tightening load of the bolts to assist the envelope wall in this function. The sleeves are cylindrical so as to provide minimum volume displacement.  
  The restriction means within the trap assembly of the present invention includes a flat plate extending across the passageway and preferably slightly larger than the orifice to assure blocking of the backstreaming liquid. The plate may be positioned at an angle with respect to the center line axis of the passageway to allow draining action of condensed liquid. Cooling means in the form of a tube carrying cooling liquid may extend across the passageway and may be in contact with the plate and the sleeves in order to provide effective condensing surfaces.  
  The subcombination trap assembly of the invention may be utilized in the high vacuum system to connect any other two members defining spaced chambers, i.e., any other members besides a pump and the inlet means to the working vacuum chamber. The same advantage of increased efficiency of gas flow with retained efficiency of vapor interception, is realized, as will be obvi ous to those skilled in the art.  
  Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein I have shown and described only the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by me of carrying out my invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictlve.  
 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a cross-sectional view of a high vacuum pumping system constructed in accordance with the principles of the present invention; and  
  FIG. 2 is a section taken through FIG. 1 along line 2-2 and further showing the features of the present invention.  
 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIG. 1 of the drawings, there is shown in cross section and in partial schematic form a high vacuum pumping system having features incorporating the principles of the present invention. Essentially, the pumping system includes a diffusion pump, generally designated by the reference numeral 10, a trap assembly 11 and a working vacuum chamber 12. The diffusion pump is or may be of the type shown in my previous US. Pat. No. 3,165,255, mentioned above, and thus need not be described in full detail here. Suffice it to say that oil vapor passing upwardly through a chimney 13 ejects a peripheral layer or sheet L or oil vapor outwardly against outer housing 14 of the pump. This high velocity vapor entrains the air or other gas molecules in the system above and pumps the gas in the direction of the flow arrow A shown in this figure. The gas enters the pump through an inlet orifice 15 defined by a peripheral flange 16 forming a part of the fastening means of the pump 10 to the trap assembly 11. A sealing O-ring 17 may be provided in a suitable groove to provide a gas tight seal with the trap assembly 11. The housing 14 is permanently attached to the flange 16 by a suitable weld bead 18, for example.  
  The vacuum chamber 12 is connected to the opposite end of the trap assembly 11 via a suitable hollow inlet means or tube 20 that has permanently attached thereto a peripheral flange 21. The gas being evacuated from the chamber 12 flows in the direction of the arrow A in response to the pumping effect of the diffusion pump 10.  
  The trap assembly 11 constructed in accordance with the teachings of the present invention comprises a tubular member defining a passageway between the inlet means 20 and the inlet orifice 15, as shown in FIG. 1. The tubular member includes an outer peripheral envelope wall 30 and sealing flanges 31, 32 at opposite ends thereof. As shown in this figure, the flanges 31, 32 cooperate in juxtaposition with the flanges 16, 21 so as to, in effect, form a closed pumping system. A suitable O- ring 33 can of course be provided in the flange 31 to seal the joint between the flanges .21, 31.  
  It will be noted that extending through the flanges are bolts 35 that, in effect, form a part of the fastening means and clamp the joints between the flanges. A nut 36 when tightened will compress the O-rings 17, 33 until air tight seal is provided and the passageway is isolated from ambient conditions. The envelope wall 30 bears the tightening load of the bolt and nut combinations 35, 36. By using such a combination, the number of parts is reduced over that required, for example, if separate bolts and nuts were used to clamp the individualjoints between the flanges 16, 32 and the flanges 21, 31.  
  Centered along the axis of the passage through the trap assembly 11 is a restriction means in the form of a blocking plate 40. This blocking plate is slightly larger than the orifice 15 and the inlet tube 20 so that said plate, in effect, intercepts any oil from the ejected layer L or from a sheet of oil that has condensed on the inside surface of the housing 14. This back streaming oil vapor made up of suspended oil molecules is physically intercepted and/or condensed on the underneath side of the plate 40 and, due to a slight tilt or angle, the oil is encouraged to drain to the lower side by gravity eventually causing the oil to form a droplet sufficient in size to drop down through the pump housing 14 for recirculation in the normal manner. The dashed line 41 shows the effectiveness of the blocking plate 40 in capturing oil vapor escaping from the inner circle of the layer L, and the dashed line 42 defines the normal boundary for oil vapor escaping upwardly from the outer circle of the layer L and pump 10, i.e., oil evaporating from the outermost point of the layer L and evaporated oil from the inside surface of the housing 14.  
  In accordance with an important feature of the present invention, it will be noted in FIG. 1 that the envelope wall 30 is extended out or positioned on the outer limit of the flange 31 so as to provide a maximum internal cross-sectional flow area within the passageway in the region of the plate 40. This feature allows maximum gas flow past the restriction means or blocking plate 40. It will be realized that if the limiting wall of the passageway were positioned inside the bolts 35, as in the prior art, the area around the edges of the plate 40 would be greatly restricted. Thus, by positioning the fastening means including the bolt and nut combination 35, 36 and the flanges 31, 32 within the confines of the outer envelope wall 30, the flow area has been increased substantially by the amount as shown in the areas A A A A, shown in FIG. 2. It will be understood that the increased flow areas A -A extend along the full height of the wall 30, and thus four curved parallelepiped-like volumes are generated in the region of the plate 40 to improve the flow characteristics.  
  In an example, with a prior art envelope wall positioned inside the fastening means at a radius of four units, 40 percent retained speed of the pump is realized. The envelope 30 at the outer limit of the flanges 31, 32 was built in accordance with the present invention at a radius of six units (50 percent increase), and the retained speed was increased to substantially 60 percent. Thus, an approximately 50 percent increase over prior capacity was gained. The enabling factor in this advantageous increase is an actual increase in available flow area of substantially percent. This can be seen by comparing 36 square units to 16 square units plus the blocked or unusable area occupied by the fastening means, that is, the area outside the areas A A, (see FIG. 2).  
  The sealing means for the region of the bolts 35 is preferably provided by a sleeve or shroud 45 that is slightly larger in diameter than the bolt 35. This sleeve is suitably welded at both ends to the aperture through &#39;the flange 31 and thereby prevents contamination of the internal evacuated passageway. The sleeve 45 also is thus functional to bear at least a portion of the tightening load ofthe bolts 35 to assist the envelope wall 30. The sealing by this shroud system is also desirable in that the removal of a bolt or the failure of complete tightening of one of the bolts does not allow a leak into the system so long as the O-rings 17, 33 remain effective.  
  The plate 40 may be supported in the blocking position by a bifurcated water tube 50 extending across the passageway. This tube is, of course, for the purpose of increasing the condensing ability of the plate 40 upon passage of cooling water or other liquid therethrough. The tube 50 may contact the adjacent sleeves 45 in order to provide an additional area of heat conductance so as to, in turn, provide a greater condensing function on the surface of the sleeves 45. The sleeves 45 are preferably cylindrical so as to take up the minimum amount of space and thereby keep the increased flow area A -A, at a maximum; however, the additional surfaces for forming condensate are secondarily impor tant in that an increase in the condensing efficiency of the trap assembly 11 is increased. The ends of the tube 50 are, of course, sealed (by suitable welding or other means) around their periphery at the point of projection throught the envelope wall 30.  
  In summary, it can be seen that a vacuum pumping system l-12 and a subcombination trap assembly 11 has been provided that can afford a significant increase in the efficiency of operation in such a system. The envelope wall 30 of the trap assembly 11 is positioned at the outer limits of the flanges 31, 32, or to put it another way, the bolt and nut combinations 35, 36 are placed within the confines of said wall. This gives structure improving the gas flow characteristics of the trap assembly without adding dimensionally to the system. The preferred embodiment of the invention can, by virtue of the use of the flanges 31, 32, be directly substituted for a more restricted trap asembly in a pumping system presently in use, thereby allowing present users to convert to higher efficiency operation with a minimum cost. The sleeves 45 isolate the bolts 35, minimize the interference with the increased flow areas A A,, provide additional support against the tightening force of the bolts 35 and provide additional condensing surface. Thus, not only are the gas flow characteristics of the system utilizing the present invention improved, but also the liquid condensing and trapping efficiency is improved.  
  In this disclosure, there is shown and described only the preferred embodiment of the invention, but, as aforementioned, it is to be understood that the invention is capable of changes or modifications within the scope of the inventive concept as expressed herein.  
  What is claimed is: 1. In a high vacuum pumping system comprising a working vacuum chamber, a diffusion pump for pumping gas by liquid vapor ejection, hollow inlet means to said working vacuum chamber, and an inlet opening to said pump, the improvement comprising a tubular member defining a passageway connecting said inlet means of said chamber and said inlet opening of said pump, restriction means in said passageway for intercepting back streaming liquid from said pump, said tubular member comprising an envelope wall forming said passageway in the region of said restriction means, a connecting flange affixed on each end of said envelope wall and extending inwardly from said envelope wall, a first mating flange affixed on an end of said pump adjacent said opening for connection with the connecting flange on one end of said envelope wall, a second mating flange affixed to said hollow inlet means for connection with the connecting flange on the other end of said envelope wall, a plurality of sleeves spaced apart within said envelope Wall, each of said sleeves being sealed at its ends to said connecting flanges on each end of said envelope wall, and bolts through said sleeves and connecting said flanges on the envelope wall to the adjacent ones of said first and second mating flanges, whereby maximum internal cross-sectional flow area within said passageway is realized for maxi mum gas flow past said restriction means for a given overall outer dimensional extent of said tubular member.  
  2. The vacuum pumping system of claim 1 wherein said sleeves are cylindrical and slightly larger than said bolts to provide minimum volume displacement.  
  3. The vacuum pumping system of claim 2 further comprising cooling means in contact with one of said sleeves.  
  4. The vacuum pumping system of claim 1 wherein said restriction means comprises a stationary plate extending across said passageway, said plate being slightly larger than said inlet opening; to said pump to assure blocking of said back streaming liquid.  
  5. The vacuum pumping system of claim 4 wherein said plate is positioned nonperpendicularly with respect to the longitudinal axis of said tubular member, whereby to allow draining action of condensed liquid.  
  6. A trap assembly for use in connecting two spaced chambers in a high vacuum system, said trap assembly allowing gas flow from one of said chambers to the other but intercepting suspended liquid molecules, said trap assembly comprising a tubular member defining a passageway, restriction means disposed in said passageway for intercepting said suspended liquid particles, said tubular member comprising an envelope wall forming said passageway in the region of said restriction means, a connecting flange affixed on each end of said envelope wall and extending inwardly from said envelope wall, a plurality of sleeves spaced apart within said envelope wall, each of said sleeves being sealed at its ends to said connecting flanges on each end of said envelope wall, and bolts receivable through said sleeves, whereby maximum internal crosssectional flow area within said passageway is realized for maximum gas flow past said restriction means for a given 7 overall outer dimensional extent of said trap assembly.