US3175373A

US3175373A - Combination trap and baffle for high vacuum systems

Info

Publication number: US3175373A
Application number: US330469A
Authority: US
Inventors: David H Holkeboer; Pagano Frank; Donald J Santeler; Walter F Venneman
Original assignee: Aero Vac Corp
Current assignee: Aero Vac Corp
Priority date: 1963-12-13
Filing date: 1963-12-13
Publication date: 1965-03-30
Anticipated expiration: 1982-03-30

Description

March 30, 1965 n. H, HOLKEBOER ETAL. 3,175,373

COMBINATION TRAP AND BAFFLE FOR HIGH VACUUM SYSTEMS Filed Dec. 15, 1565 s Sheets-Sheet 1 INVENTORS. DAVID H. HOLKEBOER FRANK PAGANO DONALD J. SANTELER WALTER F ViENNEMAN BY g va A TTORNE Y1 March 30, 1965 D. H. HQLKEBOER ETAL COMBINATION TRAP AND BAFFLE FOR HIGH VACUUM SYSTEMS 3 Sheets-Sheet 2 Filed Dec. 13, 1963 m a w wmzmmmmm 03 O E v 9 mm m m w E m a 0 v" WE TW W $55.56 I ON A SE55: m HG CI. 8.83% W .M J 92,: $2 m H. H K W 0 0 g; T 5 5 N mmmm A R 0 D F D w QE BY fim om fu l M rch 30, 1965 D. H. HOLKEBOER ETAL 3,175,373

COMBINATION TRAP AND BAFFLE FOR HIGH VACUUM SYSTEMS 7 Filed Dec. 13, 1963 3 Sheets-Sheet 3 La 1.1 J m i H 1 nhh l f BY %w United States Patent 23,175,373 CGMEINATEON TRAP AND BAFlLE FGR HEGH VAQUUM SYSTEMS David H. Holkeboer and Frank llagano, Schenectady, Donald .l. Santeler, Scotia, and Walter F. Venneman, Schenectady, N.Y., 'assignors to Acre-Vac (Jorporation,

Troy, N.Y., a corporation of New York Filed Dec. 13, 1963, Ser. No. 330,469 3i) Claims. (Cl. 62-468) This invention relates to high vacuum evacuating systems and, more particularlly, to a combination trap and bafile apparatus for use in such systems.

Where a diffusion pump employing materials such as oil or mercury is used in a system to produce a vacuum environment it has been found that the etliciency of the system is substantially affected by the tendency of oil and gas molecules from the pump to make their way back into the vacuum chamber. This backstreaming, which occurs by way of molecular collision and surface migration, contaminates the vacuum chamber and reduces the efiiciency of the pump by increasing back pressure. In the past, attempts have been made to reduce this backstreaming by interposing a trap having an optically tight refrigerated baffle between the pump and vacuum chamber. The cold surfaces of the baffle condense and freeze the molecules in the trap, preventing their entry into the vacuum chamber.

To date, however, those traps which have been proposed have, to our knowledge, been only partially successful. While a bafile which renders the trap optically tight, i.e. no line of sight between the pump and vacuum chamber, will generally assure that most of the molecules will strike at least one cold surface before it enters the vacuum chamber, it has been found that a substantial number will still pass through. This is partially due to the inability of a single cold surface to trap and condense a particular molecule, and partially due to the fact that, by virtue of intermolecular collisions and oil to gas diffusion, a number of the molecules will not strike a cold surface.

One immediate solution to this problem has been to increase the complexity of the baffle so that a molecule will have to strike two or more cold surfaces of the trap. But where this has been done, it has resulted in an undesired restriction of the flow passage with an attendant increase in back pressure and a decrease in the flow conductivity of the unit.

Still another problem which has been encountered in the design of such units is that caused by extreme thermal gradients in the vicinity of the refrigerated and the unrefrigerated vacuum chamber opening. For reasons which will be discussed more fully in the body of this application, it has been found desirable to provide a refrigerated sleeve extending inwardly from the vacuum chamber opening to the baffle. Because of the gradients existing, the contraction and expansion rates of these elements will differ, and there is considerable relative movement between them which, if uncompensated, would weaken the structural strength of the unit. In the past this relative movement has been compensated by inserting a flexible bellows section in the tube carrying refrigerant to the baffie surfaces. However, manufacturing a refrigerant tube having such a bellows section is quite expensive, and has been found to be less than satisfactory.

With these and other problems in mind, we have conceived and designed a combination trap and battle in which molecules traveling an uninterrupted straight line not only will strike at least two cold surfaces but in which the flow passage constriction is not appreciably increased. By other unique dimensioning features of our structure, the tendency of the molecules to avoid collision with a cold 3,l75,d73 Patented Mar. 3% 1965 ICC surface, either by intermolecular collision or by oil to gas diffusion, has been minimized. By still other unique structural features, our structure prevents surface migration of oil molecules. With respect to the problem of relative movement betwen the sleeve and refrigerant tubes, our structure provides a flexible mounting which permits relative movement between the elements without adversely affecting their thermal conductivity.

It is therefore an object of this invention to provide a new and improved high vacuum system including a combination trap and baiiie having a double contact configuration without an appreciable restriction of the flow path.

It is another object of this invention to provide such a system including a combination trap and baffle in which the fiow path is dimensioned to minimize the backstreaming caused by intermolecular collision and oil to gas diffusion.

It is another object of this invention to provide such a system including a combination trap and bathe in which all thermal gradient areas are shielded by a cold surface to recondense transiently evaporated condensibles.

It is another object of this invention to provide such a system including a combination trap and bathe in which backstreaming into the vacuum chamber by surface migration is eliminated.

It is another object of this invention to provide such a system including a combination trap and baflle which requires relatively few structural parts, and is reliable and relatively easy to manufacture.

In accomplishing the aforementioned objects, a principal feature resides in the provision of a container having a first opening adapted for attachment to a vacuum chamber and a second opening adapted for attachment to an evacuating means, such as an oil or mercury diffusion pump. A baffie having a plurality of first refrigerated trapping surfaces of conventional design is mounted in the container to extend across the first opening to block the line of sight between the openings. A second refrigerated trapping surface extends along the line of flow through the container and is positioned with respect to the baffle so that no point on either the first or second refrigerated surfaces has line of sight contact with more than one unrefrigerated opening. Depending upon the structure of the surfaces in the bafiie and the positioning of the openings, the second refrigerated surface may be positioned on one side or both sides of the baffle. The spacing between the baffle and the second refrigerated surface is substantially larger than that between adjacent trapping surfaces of the baffle so that two fiow regions having significantly different mean dimensions are provided.

In the preferred construction of the invention, the first and second openings are at right angles to each other, and a reservoir of liquefied gas, such as liquid nitrogen, is used to refrigerate the trapping surfaces. A further feature resides in the provision of additional refrigerated surfaces in connection with a barrier isolating the vacuum chamber from the traps interior except through the baffle passage. These surfaces are a pair of coaxial sleeves, the outer one of which is connected to the container at the vacuum chamber opening to form the barrier. The sleeves extend inwardly and are connected to the bafile by a number of flexible tabs which permit relative movement between the sleeves and the baifie While maintaining thermal conductivity between the elements. The surfaces freeze any migrating oil particles, and the barrier prevents their egress except over the surfaces. All surfaces which are subject to a temperature change so that a molecule condensed on it might boil off are either hidden from the main flow path or are provided with still another refrigerated surface extending over it which recondenses the boiled off molecule.

Other objects, features and advantages of the present invention will become more apparent from the following detailed description taken in Conjunction with the attached drawing in which:

FIGS. 13 are illustrative of conventional trap and baffle structures;

FIGS. 4-7 are schematic representations of trap and baffle structures according to the present invention;

FIG. 8 is a graphic representation of the backstreaming rates through the two trapping regions as a function of pressure with the net backstreaming resulting.

FIG. 9 is a schematic illustration of a high vacuum system employing a combination trap and bathe according to the present invention;

FIG. 10 -is a front view of a combination trap and baffle constructed according to the present invention;

FIG. 11 is a side sectional view of the trap and bafile of FIG. 10 taken along the lines 1111;

FIG. 12 is a top sectional view of the trap and baffle taken along the lines 1212 of FIG. 11 and FIG. 13 is a fragmentary side view showing the connection between the vacuum chamber opening sleeve and the baffle taken along lines 13-13 of FIG. 10.

In a high vacuum system employing a diffusion pump there is a tendency for some of the pump fluid molecules to work their way back into the vacuum chamber. This backstreaming is undesirable for several reasons, some of the most important being contamination of the vacuum chamber, impairment of the vacuum in the chamber, and an increase in pump back pressure. Some of the 'backstreaming occurs as a result of gaseous molecular flow, while some of it is the result of surface migration of condensed molecules.

One solution which has been proposed to alleviate this problem is the insertion of a trap having a refrigerated baffle in the flow path between the pump and the vacuum chamber. This baffle interrupts any straight line path by which a molecule may pass through the trap region and presents a refrigerated surface upon which a gaseous molecule may condense and freeze, thus preventing its passage. FIG. 1 illustrates one conventional type of battle 1 in which a plurality of surfaces 2 and 2' are disposed in a chevron or V arrangement and extend across flow path 3 of a trap 4. Molecules are prevented from bypassing baffle 1 by a barrier 5 which isolates the inlet and outlet regions.

An alternative arrangement of conventional baffle structure is shown in FIG. 2 in which a baflle 6 consists of a plurality of spaced overlapping surfaces 7 which extend perpendicularly to flow path 8 and are connected by an additional pair of surfaces 9 which extend in the same direction as flow path 8.

While these types of baflles are partially effective, it is known that a certain percentage of molecules will not condense and freeze upon a single collision with a refrigerated surface. In addition, another percentage of molecules will not strike any cold surface, either because of collision with other molecules, or because of the erratic paths caused when a fluid diffuses into its gaseous state. This type of diffusion is likely to occur because of the pressure differential between the vacuum chamber and the pump region. It has been found that a simple optically tight geometry of the type illustrated in FIGS. 1 and 2 has only partial success in preventing passage by these three phenomena, and where an extremely high vacuum is desired, the presence of only a few molecules in the vacuum chamber can be very critical.

One solution which has been proposed has been to increase the incidence of collision with a cold surface by increasing the complexity of the battle to a point where a molecule traveling an uninterrupted straight line path will strike at least two cold surfaces. This type of arrangement is illustrated in FIG. 3 where the chevron type baffie of FIG. 1 is modified by adding a third set of surfaces 2 so that surface 2 is in effect a surface common to two oppositely disposed chevron arrangements 10 and Ill. With this arrangement, no point on baffle 12 has line of sight contact with more than one trap opening, and this type of configuration defines a double contact bafile.

This more complex type of baffle decreases the likelihood of a molecules passing through without striking any refrigerated surface, but at the same time the added restriction in the flow path increases the back pressure and has a significant effect on the efficiency of the system.

Unlike the configurations heretofore described, the baffle arrangement according to our invention achieves this double contact result without adversely affecting system efficiency, and in addition produces further advantages, which will be subsequently described, which the conventional baffle, either single or double contact, does not offer.

In constructing a trap according to the invention, we have provided a conventional bafile extending across the flow path which has a plurality of first refrigerated surfaces arranged to present an optically tight geometry. We have then added a second refrigerated surface which extends in the direction of the flow path. By positioning this second surface with respect to the baffle so that no point on either the first or second surfaces has line of sight contact with more than one unrefrigerated opening in the trap, a double contact configuration is achieved, but since the second surface extends along the direction of flow, it does not impede the flow, and there is no undesired added constriction in the trap. It should be pointed out at this point that in the sense used herein opening refers to what could be considered either the inlet or outlet of the trap. However, since there is a flow from the vacuum chamber to the pump, and an opposite flow, or backstreaming, from the pump to the vacuum chamber, it is not practical to describe any one opening as being either an inlet or outlet. Instead, the openings will be referred to as vacuum chamber or pump where a distinction between the two is required.

FIGS. 4-7 illustrate several ways in which the configuration according to our invention can be achieved; FIGS. 4 and 5 show traps of the type illustrated in FIGS. 1 and 2, while FIGS. 6 and 7 are traps of a different design.

Referring now to FIG. 4, it is seen that trap 13 has a flow path 14 interrupted by a simple refrigerated chevron or V baffle 15 extending across it. The open ends of in dividual baffie elements 16 face to the right, and it can be seen that points a and b on the interior surfaces of elements -16 would have line of sight contact with both openings. In order to block these points off, a second refrigerated surface is positioned on the right hand side of trap 13 and extends in the direction of flow path 14, along both sides of baffle 15. Surface 17 extends along fiow path 14 following the interior configuration of trap if: until it intersects the plane 18 in which left hand element 16' lies and consequently intersects the planes of all other elements 16. Thus in the example shown where the interior configuration of trap 43 is cylindrical, surface 17 takes the form of a cylinder truncated by planes 18. Of course, surface 17 may take the form of a complete cylinder if desired.

A similar result is achieved in FIG. 5 with a different form of bathe structure. in this case bafile 19 is made up of overlapping spaced parallel plates 2%} with inner walls 21 extending in the direction of flow path 22. As can be seen, points 0 on walls 21 would have visual contact with both openings and it is thus necessary to extend a second surface '23 from both sides of baffle 19 which will extend completely around the interior surface of trap 24 which is conventionally cylindrical. However, as was done with trap 13 of FIG. 4, surface 23 may be partially truncated beyond lines a extending from points 0.

Another modification is shown in FIGS, 6 and 7 in which the trap openings are at right angles to each other. In FIG. 6, chevron or V baffle extends across both

openings

26 and 27 at a 45 angle so that each leg 28 of an individual bathe element 29 extends at right angles to its respective flow path. A right angled cylindrical second surface 31 extends from bathe 25 towards

openings

26, 27 and terminates at the intersection with planes 2 and f in which legs 28' of bottom bathe element 29 lie.

In FIG. 7, chevron or V bathe 33 is positioned in trap 34 with its major axis (defined by a line drawn through the apex of individual baffle elements 35) extending at right angles to the longitudinal axis of first opening 36 and parallel to the longitudinal axis of second opening 37. Second surface 38 is in the form of a fragmentary partially closed cylinder having a top 39 and a side 40 which extends around the interior of trap 34 slightly overlapping the free ends of inner legs 41 of baffle elements 35 which are pointed upwardly so that their planes intersect surface 38. It should be noted that while top 39 and side 40 of second surface 38 are at right angles, they both extend in the direction of the flow path by virtue of the change in flow direction caused by the angular relationship between openings 36 and -37.

Further reduction in the backstreaming rate is achieved in the traps illustrated by virtue of the fact that each trap has in effect two trapping regions with significantly different dimensions. One region is defined by the mean distance between the second refrigerated surface and the refrigerated baffle, and the second region is defined as the distance between the adjacent refrigerated surfaces of the bathe. As was mentioned earlier, one method by which pump fluid molecules will escape condensation on a cold surface is where they are intercepted and deflected by another molecule before they can strike such a surface. The average distance which a molecule will travel before it will collide with another molecule is inversely proportional to pressure, ie, at low pressure the mean distance or free path will be longer than it is at high pressure. However, where the pressure is high, the backstreaming is offset by the tendency of the residual gas molecules being pumped from the vacuum chamber to sweep the pump fluid molecules out of the trap. Similarly, at low pressure, backstreaming is offset by the tendency of a pump fluid molecule to strike a cold surface before it collides with another molecule.

As is illustrated by the graph of FIG. 8, it has been found that the collision rate, and hence the backstreaming caused by the phenomenon will reach a peak at an intermediate pressure where the mean free path is of the same order or size of its trap region dimensions. Thus, by providing two trapping regions A and B (shown in FIG. 11) having signifiicantly different dimensions it is possible to minimize the effect of a peak backstreaming rate in one region by the significantly lower backstreaming rate in the second region. Conversely, where the backstreaming rate reaches its peak in the second region, it will be offset by the lower backstreaming rate in the first region. Since the trapping effect of both regions is cumulative, it is possible to provide a net backstreaming rate which is lower than that provided by a single trapping region and which is not adversely affected by pressure variations.

Ideally, the dimensional difference between the trapping regions should be as large as possible. Practically it has been found that a mean distance of between five and ten inches between the second surface and the bathe, and a mean distance of one-half inch between adjacent surfaces of the bathe has been satisfactory.

A high vacuum system in which applicants invention may be utilized is illustrated in FIG. 9 in which a vacuum chamber 42 is initially evacuated by a mechanical pump 43. When the limit of pump 43 is reached, a diffusion pump 44 employing a suitable medium as oil or mercury is then switched in for the final evacuation process. A combination trap and bathe 45 is interposed between cham- 5 her 42 and diffusion pump 44 and operates in the manner heretofore and subsequently described.

A specific design of a combination trap and bathe according to our invention is illustrated in FIGS. 10-13 and embodies additional features which add to its effectiveness.

Trap 46 illustrated in FIGS. 10-13 employs a two collision configuration similar to that shown in FIG. 7, and comprises a cylindrical casing 47 of stainless steel having a first opening 48 in its side fitted with a sleeve 49 and a flange 59 adapted for attachment to a vacuum chamber. A second sleeve 52 extends inwardly from opening 48 and is Welded to sleeve 49 via a sealing ring 53. A third sleeve 54, coaxial with, but of lesser diameter than, sleeve 52 extends inwardly from opening 48. Both sleeves 52 and 54 are bolted to a rectangular plate 64. Plate 64 has a central opening 65 which provides access to a bathe 55 extending across opening 48 and a pair of flanges 66 extending around bathe 55 to provide an optically tight passage from the vacuum chamber to the interior of trap 46. The assembly of sleeves 52 and 54 and plate 64 is thermally connected to bathe 55 by a number of twisted tabs 67 which form a flexible coupling.

Bathe 55 consists of a plurality of pairs of vertically spaced

planar surface elements

56 and 57 extending across opening 48 and connected at their ends by mounting plates 68. The major surfaces of elements 56 and 5'7 lie in intersecting planes at an angle to the horizontal. The row of elements 56 is slightly vertically offset from the row of elements 57 so that the elements of each pair are spaced to form an incomplete chevron or V. However, each element 56 slightly overlaps its adjacent element 57 so that a substantially chevron or V arrangement is obtained, and the configuration is optically tight. This substantially V configuration has been found to be preferable to the completely closed V because, while maintaining optical tightness, it provides a more conductive flow path by reducing the probability that a molecule will be deflected back into the vacuum chamber rather than into the trap interior.

Each of the

elements

56 and 57 is mounted on a pair of hollow vertical tubes 53 which is attached to a refrigerant reservoir 59 mounted in the top of casing 47. Tubes 5% supply refrigerant to

elements

56 and 57 and chill them as well as sleeves 52, 54 by conduction.

Reservoir 59 extends inwardly into casing 47 with a slight spacing from its interior wall. In the trap illustrated, liquid nitrogen is used as the refrigerant agent, but any similar cooling medium capable of condensing and freezing pump fluid molecules may be employed. The bottom 60 of reservoir 59 forms part of the second tr apping surface in conjunction with a semi-cylindrical shroud 61 which is welded at one end to the side wall of reservoir 55 and extends into casing 47 with its concave surface facing bathe 55 and extending parallel to the direction of flow. Shroud 61 is conductively refrigerated by the liquified gas in reservoir 59 and extends around the interior wall of casing 4-7 until it slightly overlaps bathe 55. Inner elements 56 of bafhe 55 are pointed upwardly so that the plane of their major surfaces intersect the elements 60, 61 of the second trapping surface.

In the embodiment illustrated a second bathe 62 extends across opening 5i. Unlike baffle 55, baffle 62 is water cooled, or otherwise refrigerated, by pipe 69 leading to a fluid source (not shown) and may be inserted in the trap to perform an initial condensing operation on fluids escaping from the diffusion pump as well as providing a thermal barrier between the pump and the trap interior. Addition of bathe 62 reduces nitrogen consumption and, more importantly, reduces the quantity of fluid condensed and frozen in the trap interior. This result is desirable since a build up of fluid on the trapping surfaces will tend to insulate them, reducing efliciency. Because of a lower a 3,1 ave build up of pump fluid, the system can be run for longer eriods of time without the necessity of shutting down for purposes of cleaning,

An additional problem which contributes to bachstreaming is the evaporation of condensed molecules when the surfaces upon which they are condensed are subject to warming. This occurs in those sections of the trap where one end of a surface is connected to a refrigerated source, and the other end is connected to an unrefrigerated source. The resulting thermal gradient along the surface is such thatthe condensing point will be constantly shifting, depending upon the temperature differential between the sources. In the trap illustrated in- FIGURES -13, this occurs in two principal areas. When reservoir 59 is filled the immediately adjacent portion of casing 47 will be chilled to a point where it will condense gas on its surface. However, as the nitrogen evel lowers, the area in the vicinity will warm, and the gas condensed thereon will tend to boil off. More significantly, because of its location adjacent the vacuum chamber, one end of second sleeve 52 is conductively refrigerated by its attachment to plate 64, while its opposite end is connected to sleeve 49 which is unrefrigerated. Where gas has condensed on this surface, a slight shift in the thermal gradient along sleeve 52 may cause rapid boil 01f and diffusion into the vacuum chamber.

In both these cases a second refrigerated surface is provided which is slightly spaced from the thermal gradient surface and which will trap and refreeze the boiled off molecule. With respect to side wall 63 this is accomplished by the side walls of reservoir 59. Sleeve 52 is protected by third sleeve 54 which is conductively refrigerated by its thermal connection with bafile 55, but is not connected to any warmer source at its free end.

As was indicated earlier, the thermal gradient in the region of baffle 55 and second and third sleeves 52 and 54- raises the problem of relative movement caused by unequal expansion and contraction. In the trap illustrated, this problem has been overcome by independently rigidly mounting bafiie 55 and sleeves 52 and 54 and then thermally connecting them with a plurality of tabs 67 which are twisted to provide a flexible connection. The optical tightness of the assembly is maintained by plate (54 to which sleeves 52 and 54 are attached and which has a pair of flanges 66 extending around mounting plates 68.

It should also be noted that sealing ring 53 blocks passage of any gas except through baffie 55, and the combination of refrigerated surfaces 60, 61, baffle 55 and third sleeve 54 provide extended refrigerated surfaces over which a migrating molecule must pass before entering the vacuum chamber. These surfaces virtually eliminate the possibility of surface migration.

The trap of FIGS. 1013 also has the significantly different trapping region dimensions previously described. As can be seen from the drawings, it is apparent that the distance B between surfaces 60, 61 and batlle 55 is substantially greater than the distance A between vertically adjacent elements of baffle 55.

While certain preferred embodiments of the invention have been described and illustrated, it is to be understood that the invention is not restricted soley thereto, but that it is intended to cover all modifications which would be apparent to one skilled in the art, and which come within the spirit and scope of the invention.

We claim:

1. For use in a high vacuum system, a combination trap and baffie apparatus comprising a casing having a first opening adapted for attachment to a vacuum chamber and a second opening adapted for attachment to an evacuating means, a bafiie having first refrigerated trapping means mounted in said casing to block the line of sight between said openings, and second refrigerated trapping means mounted in said casing. said second trapping means having major dimensions extending along the direction of flow through said openings, any point on said second trapping means having line of sight contact with no more than one of said openings.

2. Combination trap and baffie apparatus according to claim 1 wherein said second trapping means has line of sight contact with only a portion of said first trapping means.

3. Combination trap and baffie apparatus according to claim 1' wherein any point on said first trapping means has line of sight contact with no more than one of said openings.

4. Combination trap and bafile apparatus according to claim 1 wherein said first trapping means comprises a first and second plurality of planar surfaces extending across at least one of said openings, said first and second planar surfaces forming a substantially V section; and said second trapping means has a surface intersecting the planes of at least said first plurality of planar surfaces.

5. Combination trap and baffle apparatus according to claim 4 wherein the mean distance between said first trapping means and said second trapping means is substantially greater than the mean distance between adjacent ones of said V sections.

6. Combination trap and baffle apparatus according to claim 4 wherein said first and second openings are at an angle to each other.

7. Combination trap and bafiie apparatus according. to claim 1 wherein said first trapping means comprises a first and second plurality of planar surfaces extending across at least one of said openings, said first and second planar surfaces forming a substantially V section; and said second trapping means has surfaces intersecting the planes of said first and second plurality of planar surfaces.

8. Combination trap and bafiie apparatus according to claim 7 wherein the mean distance between said first trapping means and said second trapping means is substantially greater than the mean distance between adjacent ones of said V sections.

9. Combination trap and baffle apparatus according to claim 7 wherein said first and second openings are at an angle to each other. I

10. Combination trap and bathe apparatus according to claim 9 wherein said first and second planar surfaces extend across both of said openings.

11. Combination trap and baffle apparatus according to claim 1 wherein said first and second openings are in line with each other; said first refrigerated trapping means comprises a plurality of spaced surfaces extending partially across the flow path through said container, said surfaces partially overlapping each other whereby the line of sight between said first and second openings is blocked; and said second refrigerated trapping means comprises a surface extending along said flow path on either side or" said first trapping means.

12. Combination trap and bafiie apparatus according to claim 11 wherein the mean distance between said first trapping means and said second trapping means is substantially greater than the mean distance between adjacent ones of said spaced surfaces.

13. Combination trap and bafile apparatus according to claim 11 wherein said surface of said second refrigerated trapping means extends from either side of said first trapping means a sufficient distance to intersect an uninterrupted straight line drawn from any interior point of said first trapping means otherwise having line of sight contact with both the said openings.

14. Combination trap and baffle apparatus according to claim 1 wherein the mean distance between said first trapping means and said second trapping means is substantially larger than the mean distance between elements of said first trapping means.

15. Combination trap and baffle apparatus according to claim 1 wherein said first trapping means comprises a plurality of spaced planar surface pairs extending across at least one of said openings, the elements of said pairs lying in intersecting planes but spaced from each other so that said elements form an incomplete V configuration.

16. Combination trap and baffle apparatus according to claim wherein said elements of said pairs overlap each other to provide an optically tight arrangement.

17. For use in a high vacuum system, a combination trap and batlle comprising a container having a first opening adapted for connection to a vacuum chamber and a second opening at right angles to said first opening adapted for connection to an evacuating means, the space between said first and second openings defining a flow path; a baffle in said flow path extending across said first opening to block the line of sight between said first and second openings, said baffle including first refrigerated trapping surfaces for condensing and trapping gaseous particles tending to flow into the vacuum chamber; a second refrigerated trapping surface extending along the line of flow between said first and second openings; and means for refrigerating said first and second trapping surfaces.

18. Combination trap and baffle apparatus according to claim 17 wherein said first refrigerated trapping surfaces comprise a plurality of spaced pairs of planar surface elements, each mounted on a tube connected to said refrigerating means and forming a substantially V section bafile whereof the plane of each element surface intersects said second refrigerated trapping surface.

19. Combination trap and baffie apparatus according to claim 18 wherein the planar surface elements of said pairs are transverse to said fiow path to and from said openings, respectively.

20. Combination trap and baffle apparatus according to claim 18 wherein the planar surface elements of said pairs are in an overlapping spaced relationship whereby an incomplete V is formed.

21. Combination trap and baffle apparatus according to claim 17 wherein said means for refrigerating comprises a liquified gas reservoir having a flow tube in connection with said first refrigerated trapping surfaces, and said second refrigerated trapping surface is in thermal connection with said reservoir.

22. Combination trap and battle apparatus according to claim 21 wherein said liquifled gas reservoir is mounted in said container and the external surface of said reservoir augments said second refrigerated trapping surface.

23. Combination trap and bafiie apparatus according to claim 17 wherein said second refrigerated trapping surface comprises a semicylindrical sheet having its interior surface facing towards said bafile.

24. Combination trap and baffle apparatus according to claim 17 further comprising a second refrigerated bafile extending across said second opening.

25. Combination trap and battle apparatus according to claim 24 wherein said second bafile extending across said second opening is maintained at a higher temperature than said first and second trapping surfaces.

26. Combination trap and battle apparatus comprising a container having a first opening adapted for connection to a vacuum chamber and a second opening at right angles to said first opening adapted for connection to an evacuating means, the space between said openings defining a flow path; a first sleeve rigidly attached to said container and extending inwardly into said container from said first opening; a second sleeve coaxial with and of lesser diameter than said first sleeve extending into said container and rigidly attached at its inner end to said first sleeve; a bafile mounted in said container interiorly of said sleeves extending across said first opening to block the line of sight between the vacuum chamber and the interior of said container; thermally conductive 10 flexible means connecting said baffie with said first and second sleeves to permit relative movement between said bafile and said sleeves caused by unequal contraction and expansion; and means to refrigerate said batlle and said sleeves.

27. Combination trap and baffle apparatus according to claim 26 wherein the interior ends of said sleeves are mounted on a plate extending across said first opening, said plate having flanges surrounding the ends of said baffle and said thermally conductive flexible means cornprise a plurality of flexible metallic tabs attached to said plate and said baffle.

28. Combination trap and battle apparatus comprising a container having a first opening adapted for connection to a vacuum chamber and a second opening at right angles to said first opening adapted for connection to an evacuating means, the space between said openings defining a flow path;

a first sleeve rigidly attached to said container and ex tending inwardly into said container from said first opening;

a second sleeve coaxial with and of a lesser diameter than said first sleeve extending into said container;

a plate rigidly attached to the inner ends of said sleeves and extending across said first opening, said plate having an opening for passage of gas therethrough;

a refrigerated bafile rigidly mounted in said container interiorly of said plate extending across said first opening and said plate opening to block the line of sight between said first opening and the interior of said container, said baffle including a plurality of first refrigerated trapping surfaces comprising a plurality of vertically spaced planar surface pairs connected by hollow tubes, the elements of said pair lying in intersecting planes but spaced from and overlapping each other so that a series of incomplete \ls is formed,

said plate having flanges overlying the ends of said baffle;

a plurality of flexible metallic tabs thermally connecting said bairle to said plate to permit relative movement therebetween;

a liquified gas reservoir mounted in the top of said container, said tubes being connected to said reservoir for providing liquified gas to refrigerate said baffle and said sleeves;

and a semi-cylindrical shroud attached to said reservoir inside said container, said shroud having its concave major surface extending towards said second opening, the bottom of said reservoir and said shroud surface defining a second refrigerated trapping surface, said shroud extending around the interior of said container and overlapping the ends of said bafiie.

29. Combination trap and bafile apparatus according to claim 28 further comprising a second refrigerated bafile extending across said second opening, said second baffle being maintained at a higher temperature than said first and second refrigerated surfaces.

30. Combination trap and bafile apparatus according to claim 28 wherein the mean distance between said second trapping surface and said bafile is substantially larger than the mean distance between adjacent ones of said first trapping surfaces.

References Cited in the file of this patent UNITED STATES PATENTS 2,934,257 Power Apr. 26, 1960 3,081,068 Milleron Mar. 12, 1963 3,103,108 Santeler Sept. 10, 1963 3,122,896 Hickey Mar. 3, 1964

Claims

1. FOR USE IN A HIGH VACUUM SYSTEM, A COMBINATION ITRAP OF BAFFLE APPARATUS COMPRISING A CASING HAVING A FIRST OPENING ADAPTED FOR ATTACHMENT TO A VACCUUM CHAMBER AND A SECOND OPENING ADAPTED FOR ATTACHMENT TO AN EVACUATING MEANS, A BAFFLE HAVING FIRST REFRIGERATED TRAPPING MEANS MOUNTED IN SAID CASING TO BLOCK THE LINE OF SIGHT BETWEEN SAID OPENINGS, AND SECOND REGRIGERATRD TRAPPING MEANS MOUNTED IN SAID CASING, SAID SECOND TRAPPING MEANS HAVING MAJOR DIMENSONS EXTENDING ALONG THE DIRECTION OF FLOW THROUGH SAID OPENINGS, ANY POINT ON SAID SECOND TRAPPING MEANS HAVING LINE OF SIGHT CON TACT WITH NO MORE THAN ONE OF SAID OPENINGS.