US3154138A - Vacuum vessel - Google Patents

Description

Oct. 27, 1964 c. M. ORR 3,154,138

VACUUM VESSEL Filed April 20, 1961 4 Sheets-Sheet 1 Oct. 27, 1964 c. M. ORR 3,154,138

VACUUM VESSEL Filed April 20, 1961 4 Sheets-rSheet 2 INVENTOR.

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Oct. 27, 1964 c. M. ORR 3,154,138

VACUUM VESSEL Filed April 20, 1961 4 Sheets-Sheet 3 INVENTOR.

Oct. 27, 1964 c, ORR 3,154,138

VACUUM VESSEL Filed April 20, 1961 4 Sheets-Sheet 4 INV EN TOR.

United States Patent 3,154,138 VA'CUUM VESEL Clifiord M. Orr, Western Springs, Ill, assignor to Chicago Bridge 8: Iron Company, Chicago, Ill., a corporation of Illinois Filed Apr. 20, 1961, Ser. No. 104,478 12 Claims. (Cl. l6575) This application relates to a vacuum chamber. It is especially concerned with an improved vacuum chamber for test systems, industrial applications, and other services wherein very low absolute pressures and a variety of temperatures can be produced.

Large size vacuum chambers capable of producing and maintaining pressures of IX torr (torrmm. Hg) or lower have a variety of applications. Such chambers are used in test systems, for example, for basic research in metallurgy, chemistry, solid-state physics such as electronic circuitry and semiconductors and others or other investigatory and test programs wherein equipment and material are tested under environmental conditions simulating actual conditions which will be experienced during operational use of the equipment and material. They also find application as vacuum furnaces, vacuum coat ing process vessels, freeze dryers, as well as other industrial applications. In order to accommodate the test equipment, vessels of this type will have a volume of 10,000 cu. ft. or more.

In vessels of this type a pressure level consistent with the operating requirements must be produced. This requires the use of highly efficient vacuum pumping systems to reach the desired pressure level. Generally the pumping systems are designed to lower the pressure so that the mean free path of the molecules is greater than the boundaries of the chamber where the chambers of this invention are used in test work; this is important because it leaves radiation as the sole means of heat transfer.

According to this invention there is provided a low pressure chamber having at least one pumping manifold which functions as a common inlet for a plurality of vacuum pumps. The manifold arrangement offers greater conductance to the molecules, by permitting radial as well as peripheral movement of the molecules in the manifold toward the vacuum pump inlet, than individual elbow mounting of each pump and facilitates the mounting of the pumps on the test chamber. This makes it possible in many instances to reduce the number of pumps to produce a specified pumping speed.

In the drawings:

FIGURE 1 is a perspective illustration of a low pressure test facility employing the instant invention;

FIGURE 2 is an elevation view of an illustrative test chamber with a partial cross sectional view showing the interior of the chamber and the position of the heat sinks;

FIGURE 3 is a plan view of the test chamber shown in FIGURE 2;

FIGURE 4 is an enlarged fragmentary view of the high conductance pumping manifold;

FIGURES 5 and 6 are alternative manifold systems;

FIGURE 7 is an elevation view of still another embodiment of the instant invention; and

FIGURE 8 is a plan view of the embodiment shown in FIGURE 7.

Although the instant invention has application in a variety of services, in order to facilitate a discussion of the invention it will be specifically described with reference to its use as a low temperature-low pressure test chamber.

In vacuum chambers operating at pressures of 1x l() torr the mean free path of the molecules (the distance one molecule must travel before striking another) is approximately feet and only the random movements of the individual molecules carry them into the pumping portals. Mechanical pumps are used for roughing and diffusion pumps or other high vacuum producing pumps are generally used to bring the chamber to its final pressure. The interior surfaces of such chambers are commonly constructed of stainless steel or other suitable metals polished to minimize outgassing in the vessel walls and to present a relatively smooth and shiny surface to the radiant heat sink panels which in turn have the surfaces exposed to the test article coated black. The interior surface of the chamber, however, is far too rough to reflect an air molecule such that the molecules can be guided or directed in a predictable direction toward the vacuum pump inlet connections which in conventional installations consist of a plurality of ports peripherally spaced around the chamber. Generally, the pump inlet connections are circular openings facing the test article. An elbow from which the diffusion pump vertically depends usually contains a cold trap to reduce backstreaming of oil from the pump. The elbow mounting of a pump combined with a cold trap makes it necessary to provide an opening in the chamber wall which is much larger than the entrance to the diffusion pump. In certain chambers the use of elbow mounting requires that the chamber be larger than otherwise required test space. In other chambers a staggered elbow arrangement may be required to accommodate on individual ports all of the necessary number of vacuum pumps. These disadvantages are obviated by the high conductance pump manifold system which has other advantages as will be apparent from the following detailed description of this invention.

Referring to the drawings, in FIGURE 1 an illustrative specific embodiment of the instant invention is shown. The test chamber 10 which for convenience purposes is installed in a subterranean pit 11 comprises a stiffened vertical cylinder 12 supported on tubular columns 13. The cylinder 12 is stiffened by means of rings 12 welded to the cylinder walls. The pit opening is enclosed by hatch covers 14 held in place at the floor level by beams 15. The chamber It) is enclosed at the bottom by a head 16 of torispherical shape. A bottom opening 17 is provided by nozzle 18 and closure plate 19 which is removable. The chamber 13 is enlarged at the top to form a pumping manifold 20. A removable cover 21 is fitted to the chamber top enclosing the top opening 22. Mating flanges 23 and 24, the former mounted on the pumping manifold 20 and the latter comprising the peripheral edge of cover 21, are fitted with conventional double O-ring seals (not shown) or other types of seals reusable for many closures. Cover 21 is designed to support a test platform (not shown) suspended by the four support rods 25 and a suitable test object 26 resting on the platform in addition to the external pressure loading and heat sink cover. The cover is provided with a hoisting harness 27. Suitable stiffeners 28 and 29 are used to stiffen the cover. Stiffeners 28 are radial members several of which are lifting stiffeners 28' and stiffeners 29 are annular rings concentrically mounted on the outer face of cover 21. A personnel access door 30 is located in the cylindrical sidewall 11 near the bottom of the chamber.

The pumping system employed to effect the desired low pressures comprises as the main pumping elements a plurality of fractionating diffusion pumps 35, attached to manifold 20 by nozzles 36. Suspended within the ports are water-cooled traps consisting of radiation shields 38 and optically dense liquid nitrogen-cooled baffles 39 to eliminate backstreaming of the oil from the diffusion pumps into the chamber 10. Backstreaming is reduced by an optically tight liquid nitrogen trap, constructed, for example, of copper and stainless steel, mounted between pump 35 and chamber 10. This trap is constructed to have adequate conductance and has a central heat transfer fluid container 39 surrounded by a conductively cooled drum 40, so that an oil molecule would have to bounce off a cold surface to pass through the trap. A water cooled radiation shield 38 is interposed between the pump and the nitrogen cooled container so that the major portion of any backstreaming oil will be condensed to a liquid and allowed to fall back into the diffusion pump. Thus the pump will not be robbed of oil, in any normal test period, by oil held on the nitrogen trap. The water cooled radiation shield 38 also will protect the liquid nitrogen bafiie 39 from direct heat radiation from the diffusion pump. Automatic liquid nitrogen level controls are used to monitor the cold trap and keep it filled. These pumps with their traps 38 and 39 are arranged around the periphery of the vacuum chamber and are attached to a large bell-shaped manifold forming the top of the chamber. The bell section 20 of the chamber 10 is used as a common pumping manifold for all pumps 35. This arrangement is designed to optimize net speed available at the chamber and offers greater conductance than individual elbowmounting of each pump.

Efficiency of the diffusion pumps is increased substantially by the greater conductivity of the manifold 20 and short attachment nozzles 36 compared with pumps mounted on individual elbows. Increased working space around the outside of the large removable cover is provided by the enlarged top head. Another feature of the large manifold is a capability for the addition of a vacuum shut-off valve at each pumping port if desired. The enlarged manifold 20 also provides improved access to the shroud and liquid nitrogen piping inside as will hereinafter be discussed.

The pump mounting bell is of adequate size and is designed to accommodate additional diffusion pumps if changed conditions at a later date require more pumping capacity.

Backing the diffusion pumps, which are manifolded in two groups by manifold 40, are large mechanical pumps 41 i.e., 400 c.f.m. capacity with mechanical boosters (not shown). These pumps will be used also for roughing the chamber 10 prior to diffusion pumping and are interconnected in such a manner that they can be used flexibly in several combinations to handle the gas flows at various stages of the pumping.

The internal heat transfer system or heat sink which absorbs radiant heat from objects Within the chamber employs a selected heat transfer fluid and a radiant panel system. In the illustrative embodiment, a cold wall liquid cooling system is employed so that the shroud wall temperature can be very close to the normal ebullition temperature of liquid nitrogen constantly during cold wall operations. Nitrogen or other selected cryogenic fluids, depending upon the test conditions, are used as heat transfer materials.

The cold wall system also makes provisions for heating the panels to an elevated temperature, e.g., 350- 500 F. for preconditioning bakeout during evacuation, in order to outgas the shroud panels and to energize the air molecules that escape through the pump ports. If low pressures in the 1X10 torr range are desired a complete bakeout at about 500 F., which would include the chamber shell as well as the shroud, might be necessary. If bakeout of the chamber to this extent is performed, no additional pumping capacity should be necessary to reach a pressure of IX 10- torr, providing proper precautions have been taken to avoid injury to the elastomer gaskets and to prevent opening of leaks during bakeout.

In the shroud system shown, the shroud sections preferably are multi-element and consist of a bottom section 56, a cylindrical lower half section 51, a cylindrical upper half section 52, a conical top section 53, and a cover section 54. Liquid feed lines 55 and vapor return lines 56 are connected respectively to the liquid nitrogen storage vessel (not shown) and a vent (not shown). Flexible connectors 57 are used where needed. The panel surfaces of the shroud facing the chamber walls are polished whereas the panel surfaces facing the test objects are heat absorptive, e.g., black.

In one illustrative test chamber installation a chamber 27' in height and having an internal diameter of 19' to provide a 15 feet diameter and 20 feet high clear test area was used. To mount the diffusion pumps a manifold as illustrated in FIGURE 1 and described above having a chamber 286" in diameter was used. Eight diffusion pumps in cooperation with mechanical gas ballast pumps and booster pumps were used to produce and hold a vacuum of l X lO torr.

Roughing the chamber to diffusion pump operating pressure was done automatically by turning on the mechanical gas ballast pumps. These pumps were used to evacuate the chamber to a low pressure level at which point the booster pumps were turned on to augment the pumping speed. The booster pumps provided continuous backing to the diffusion pumps throughout their operatrange. Liquid nitrogen was used as the heat transfer fluid in the heat sink.

To facilitate a description of this invention the various control and instrumentation systems and associated piping, miscellaneous accessories such as heaters, Vaporizers, pumps, valves, etc., the placement of which are obvious to those skilled in the art, are not shown. For example, in the heat sink, controls for the preconditioning bakeout, initial cooling of the shroud with liquid nitrogen, removing liquid nitrogen from the shroud and putting it back into storage for shutting down at the end of a test, and warming the coils if desired at time of shutdown are used. Similarly the pumping system requires suitable control and instrumentation systems.

The subject low pressure (high vacuum) chamber provided with a high conductance pumping manifold has a variety of other uses as a vacuum furnace, vacuum coating process vessel, freeze drying chamber, and others. Obviously suitable accessories must be used in each instance in order to make the chamber suitable for the selected service. For example, heat sources such as areing electrodes or electron beam devices are used in vacu um furnace work; metal Vaporizers are employed in vacuum coating applications, and so forth.

In fabricating the chamber of this invention, although conventional materials of constructions are used, the material should be economical and be selected to provide minimum emissivity and outgassing. Accordingly, 304 stainless steel, cupro-nickel, nickel, or copper materials are preferred. The heat sinks, if required for the vacuum chamber, are constructed from stainless steel, aluminum, or others.

The features of this invention are especially adaptable for relatively large chambers having volumes of about 1,500 to 100,000 cubic feet; however, they also find use in other size installations. The chamber can be cylindrical as shown or can be rectangular, spherical, or other geometrical configurations. The pumping manifold can have a cross sectional configuration similar to the test chamber or it can be different. The manifold can be located at either the upper or lower end of the chamber or intermediate thereto, and one or more manifolds can be used.

The high conductance manifolds can also be used on horizontal vessels. The manifold vertically engirds the vessel and is supplied with suitable outlets radially extending from the manifold to which the vacuum pumps are attached. Horizontal vessels which are especially adaptable for use with high conductance manifolds are those described in Boardman Patents 2,920,784 and 2,- 672,254. The vertical vessels shown therein are also adaptable for use in the instant invention. In these vessels manifolds can be installed at the intersections between the noded spheres.

As shown in FIGURE 5, the manifold 20 (in this i1- lustrative embodiment as well as in the description of the other alternative embodiments the same numerals as previously used will be employed to identify common elements) is positioned at an intermediate point in the wall of test chamber 10. It will be noted in this arrangement that the manifold walls are conical surfaces as opposed to the double curvature walls shown in the test chamber hereinbefore described. Regardless of the position of the manifold, it can be designed to be selfsupporting under test conditions or spaced vertical, structural support members (not shown) can be installed across the manifold opening in order to support the top of the test chamber 10 which includes a removable cover 21. In the latter installation advantages accrue because of the elimination of certain stresses at the location where the supports join the shell of the chamber Ill. Since there is substantially no radial load at the vessel shell caused by pressure on the overhang, there is no requirement for a heavy reinforcing ring at the shell. Only the external pressure on the manifold is carried by the manifold. Radial forces in the manifold may be resisted by external reinforcing members. This is advantageous, especially for stainless steel vessels, since the shell of the manifold can be reinforced by more economical carbon steel members in accordance with conventional structural design practice.

Still another embodiment of the instant invention is shown in FIGURE 6 wherein the vacuum chamber 10 is a spherical vessel formed by segments of spheres of different radii. The bottom portion 60 is formed using one radius of curvature, whereas the upper portion of the vessel 61, which forms a removable top, mounted on flange 62, has a larger radius of curvature. In this instance, the manifold 20 takes the form of a skirt-like wall encompassing the bottom section of the test chamber. Support piers 63 are peripherally spaced about the bottom wall 64- of the manifold 20 and vacuum pump inlet ports 65 are interspersed in the bottom Wall 64 between the support piers for mounting suitable vacuum pumps thereto. The features of this invention also are obtained by the use of a vacuum chamber employing a high conductance manifold arrangement shown in FIGURES 7 and 8. In this instance, the spherical vessel which forms the vacuum chamber 10 is provided with a plurality of high conductance manifolds 20 which interconnect with the interior of the vacuum chamber 10 by means of large circular openings 70. In this alternative embodiment, each manifold is provided with a plurality of vacuum pump inlets 71 to which are dependently and respectively mounted a plurality of vacuum pumps. This assembly is advantageous where it is desirable to add special shutoff valves for the vacuum pumps. With this arrangement, a simple closure diaphragm (not shown) mounted within the interior of the vacuum chamber would permit the high conductance manifolds to be isolated from the vessel interior.

The vacuum chambers are constructed to be fluid tight and leak tight preferably having a leak no greater than about 1 1O' cc./second of air at atmospheric pressure. The number of vacuum producing pumps used such as ion gettering or diffusion pumps will depend upon the test conditions desired which can be as low as l l() torr. Pressures as low as 1 10- torr can be produced with facility.

Although the subject invention has been described with reference to a complete specific embodiment it is apparent that other variations and modifications can be made without departing from the scope of this invention.

What is claimed is:

1. A large size vacuum chamber for low pressure service capable of producing and maintaining pressures of l 10 torr which comprises an enclosed chamber,

means for providing access to the interior of said chamber, a high conductance vacuum pumping manifold having a single inlet directly communicating with said chamber, said manifold having a plurality of outlets being free from interior obstructions to provide access between the interior of said chamber and said inlet to provide means adapted to be common to said chamber and inlet for respedti-vely connecting thereto a plurality of vacuum pumps.

2. A large size vacuum chamber for low pressure service capable of producing and maintaining pressures of 1 10- torr which comprises an enclosed chamber, means for providing access to the interior of said chamber, a high conductance vacuum pumping manifold having a single inlet directly communicating with said chamber, said manifold having a plurality of outlets adapted to be connected to vacuum pump inlets located in the lower extremity of said manifold being free from interior obstructions to provide access between the interior of said chamber and said inlets to provide a manifold common to said chamber and said outlets which are adapted for respectively and dependently mounting therefrom a plurality of vacuum pumps.

3. A vessel in accordance with claim 2 in which said high conductance manifold surrounds said chamber and opens directly thereinto substantially continuously about its peripheral extent.

4. A vessel in accordance with claim 3 in which each of said inlets is provided with a vacuum pump nozzle depending therefrom.

5. A large size vacuum chamber for low pressure service capable of producing and maintaining pressures of 1 10 torr which comprises an enclosed chamber, means for providing access to the interior of said chamber, a high conductance vacuum pumping manifold having a single inlet directly communicating with said chamber, said manifold having a plurality of outlets adapted to be connected to vacuum pump inlets for mounting therefrom a plurality of vacuum pumps and being free from interior obstructions to provide access between the interior of said chamber and said inlets, said manifold being located at a terminal end of said chamber.

6. A large size vacuum chamber for low pressure service capable of producing and maintaining pressures of 1 10- torr which comprises an enclosed chamber, means for providing access to the interior of said chamber, a high conductance vacuum pumping manifold having a single inlet directly communicating with said chamber, said manifold having a plurality of outlets adapted to be connected to vacuum pump inlets located in the lower extremity of said manifold and being free from interior obstructions to provide access between the interior of said chamber and said inlets to provide a common manifold for dependently mounting therefrom a plurality of vacuum pumps, said manifold being located at a terminal end of said chamber.

7. A vessel in accordance with claim 6 in which said chamber is cylindrical.

8. A large size test vessel for high vacuum testing capable of producing and maintaining pressures of 1 1O- torr which comprises a chamber defining a test space, access means communicating with the interior of said chamber, a high conductance pumping manifold opening through a single outlet into said chamber and having disposed therein a plurality of outlets adapted to be connected to a plurality of vacuum pump inlets to provide a common manifold for connecting a plurality of vacuum pumps therefrom and being free from interior obstructions to provide access between the interior of said chamber and said inlets, a plurality of vacuum pumps and a heat sink located within said chamber comprising a plurality of panels having means for circulating a heat transfer fiuid therethrough for absorbing radiant heat from objects within said chamber.

9. A large size test vessel for high vacuum testing capable of producing and maintaining pressures of 1 10 torr which comprises a chamber defining a test space, access means communicating with the interior of said chamber, a high conductance pumping manifold opening through a single outlet into said chamber and having disposed in the lower extremity thereof a plurality of outlets to provide a common manifold for dependently mounting a plurality of vacuum pumps therefrom and being free from interior obstructions to provide access between the interior of said chamber and said inlets, a plurality of vacuum pumps and a heat sink located within said chamber comprising a plurality of panels having means for circulating a heat transfer fluid theretbrough for absorbing radiant heat from objects within said chamber. 7

10. A vessel in accordance with claim 9 in which said high conductance manifold surrounds said chamber and opens directly thereinto substantially continuously about its peripheral extent.

11. A test vessel for high vacuum testing in accordance with claim 10 in which said manifold is located at a terminal end of said chamber and encloses said terminal extremity.

12. A large size test vessel for high vacuum testing capable of producing and maintaining pressures of 1 10- torr which comprises a chamber defining a test space, a high conductance pumping manifold surrounding said chamber and opening through a single inlet directly thereinto substantially continuously about its peripheral extent and a plurality of vacuum pump nozzles depending from said manifold and being free from interior obstructions to provide access between the interior of said chamber and said inlets, said chamber being located at the upper terminal end of said test chamber enclosing said chamber end and being provided with access means communicating with the interior of said chamber, a removable fluid-tight closure enclosing said access means and a heat sink located Within said chamber comprising a plurality of panels having means for circulating a heat transfer fluid therethrough for absorbing radiant heat from objects Within said chamber.

References Cited in the file of this patent UNITED STATES PATENTS 2,088,012 Rector July 27, 1937 2,916,536 Gruber et a1. Dec. 8, 1959 2,984,876 Garmy May 23, 1961

Claims (1)

1. A LARGE SIZE VACUUM CHAMBER FOR LOW PRESSURE SERVICE CAPABLE OF PRODUCING AND MAINTAINING PRESSURES OF 1X10-4 TORR WHICH COMPRISES AN ENCLOSED CHAMBER, MEANS FOR PROVIDING ACCESS TO THE INTERIOR OF SAID CHAMBER, A HIGH CONDUCTANCE VACUUM PUMPING MANIFOLD HAVING A SINGLE INLET DIRECTLY COMMUNICATING WITH SAID CHAMBER, SAID MANIFOLD HAVING A PLURALITY OF OUTLETS BEING FREE FROM INTERIOR OBSTRUCTIONS TO PROVIDE ACCESS BETWEEN THE INTERIOR OF SAID CHAMBER AND SAID INLET TO PROVIDE MEANS ADAPTED TO BE COMMON TO SAID CHAMBER AND INLET FOR RESPECTIVELY CONNECTING THERETO, A PLURALITY OF VACUUM PUMPS.

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