US3144756A - Vacuum system cooling trap - Google Patents

Description

T0 VACUUM CHAMBER Aug. 18, 1964 Filed July 25, 1962 K. W. ARNOLD ETAL VACUUM SYSTEM COOLING TRAP 2 Sheets-Sheet 1 T0 DIFFUSION PUMP Aug. 18, 1964 K. w. ARNOLD ETAL 3,144,756

VACUUM SYSTEM COOLING TRAP Filed July 23, 1962 2 Sheets-Sheet 2 ;E; 22 5-5 i -I Fig. 3

United States Patent 3,144,756 VACUUM SYSTEM COGLING TRAP Kenneth W. Arnold, Reading, and Richard B. Britten, Biiierica, Mass, assignors to ion Physics Corporation, Burlington, Mass., a corporation of Delaware Filed . luly 23, 1962, Ser. No. 211,812 4 Claims. ('31. 62-469) This invention relates to vacuum systems employing diffusion pumps and the like, and more particularly to a cooling trap therefor adapted to present a maximum condensation surface to vapors which diffuse onto it from the pump and from the vacuum chamber.

conventionally, a system to be exhausted, such as a high vacuum tank or particle accelerator, is reduced to a rough vacuum by a rotary displacement mechanical pump which discharges into the atmosphere. High vacuum is then obtained by means of a condensation or diffusion pump. This latter type of pump, in essence, comprises a container of a fluid such as mercury, heavy petroleum oil or chlorinated hydrocarbons. The container is heated sufficiently to vaporize the fluid which streams out of jets in the container walls entraining gas from the system before being condensed by the cooled walls of the pump. Diffusion pumps, however, suffer from back-streaming of the operating fluid and are used with a trap cooled by solid carbon dioxide or liquid nitrogen to minimize this. It is toward the improvement of such traps that this invention is directed. Prior art devices of this type are characterized by a metal container which is disposed in the vacuum system between the diffusion pump and the vacuum tank and filled with liquid nitrogen, air, or helium. The upper interior of such a trap is connected through a conduit to the atmosphere to allow disposal of the liquid as it evaporates. Since it is very desirable that such a trap present a maximum condensation surface to vapor diffusing onto it from the pump, the evaporation of the liquid contained within it limits its eifectiveness over any extended period of time. That is, as the liquid evaporates the level in the container drops, thus reducing the total container area in immediate contact with such liquid. Furthermore, conduction through the metal container is of little significance since metals required for vacuum purposes, such as stainless steel, have very poor conductivity characteristics. The problem is still further complicated by the nature of the liquid coolant, which being normally at very low temperatures,

trap.

It is another object of this invention to provide a vacuum system cooling trap of the type described adapted to present a constant maximum condensation surface during the utilization time of each container of liquid coolant.

It is another object of this invention to provide, in a cooling trap of the type described, a liquid coolant container that is of simple hydraulic design and that maintains its coolant contiguous to its entire surface area.

It is another object of this invention to provide a cooling trap of the type described that requires a minimum of maintenance.

It is another object of this invention to provide, in a vacuum system cooling trap, a liquid coolant container having a separate compartment adjacent to the walls and top thereof and means for maintaining a maximum amount of coolant therein.

It is another object of this invention to provide, in

1 3,144,756 Patented Aug. 18, 1964 Ice a vacuum system cooling trap, a liquid coolant container having a separate compartment adjacent the sides and top thereof in combination with a novel self-compensating valve means adapted to maintain coolant therein at a predetermined level.

These together with other objects and features of this invention will become readily apparent from the following detailed description thereof taken in conjunction with the accompanying drawings wherein:

FIGURE 1 is a sectional view of one presently preferred embodiment of the invention;

FIGURE 2 is a detail illustrating a novel self-compensating pressure valve adapted to use in such an embodimerit;

FIGURE 3 is a sectional view of an alternate embodiment of the invention.

Referring now to FIGURE 1, there is illustrated thereby a cooling trap embodying principles of this invention. Vacuum system housing sections 4 and 6 connect the subject cooling trap to the vacuum chamber (not shown) and to [the diffusion pump (not shown) respectively. Section 5, which comprises the housing of the trap is connected therebetween by vacuum seals 31. Container 7 is positioned within section 5 by any convenient means such as tubular members 10 and 11 or by metal struts, should greater mechanical strength be required. Such struts should, however, be fabricated of a metal that has poor heat conduction characteristics and that is readily outgassed for vacuum use. Container 7 is preferably of stainless steel or other metal that is impervoius to mercury vapor and that is acceptable for vacuum use. The geometric configurations and relationship of housing section 5 and container '7 should be such that mercury (or other pump fluid) molecules arriving from the diffusion pump (indicated by arrows 15) must strike the cold surface of container 7 at least once. A metal partition 8, preferably of copper, is provided to establish a compartment 12 within the container that is substantially coextensive with the sides and top thereof. A plurality of apertures 9 are disposed around the lower portion of metal partition as illustrated. The apex of inner compartment 14 is connected to the atmosphere through conduit 10 and pressure valve 16. The apex of compartment 12 is connected to the atmosphere through open conduit 11. Conduits lil and 11 pass through the vacuum system housing by means of vacuum seals 33 and 32 respectively.

Conduit 11 is made sufiiciently long (dimension h must be equal to or greater than dimension h so that with pressure valve 16 set to maintain the liquid level in compartment 12 at its maximum, no liquid will be forced to spill over the outer end of said conduit 11 regardless of the amount of liquid in the system.

In operation, the device is fieled with a liquid coolant 13 such as liquid nitrogen for instance. This is done conveniently by pouring the liquid nitrogen into conduit 11 until it appears in conduit 10 at valve 16. Pressure valve 16 is then placed over the end of conddit 10. At this point the entire outer surface of the container is very cold and mercury vapor striking it condenses and is thus prevented from passing to the high vacuum chamber. The liquid nitrogen 13, of course, due to the heat created by impinging mercury molecules slowly vaporizes and the vapor passes to the atmosphere through conduit 11. In the conventional cold trap, such evaporation of the liquid coolant results in a steadily declining liquid level and a consequently declining cold surface area. In the present invention, however, the vaporized fluid of compartment 14 cannot escape into the atmosphere due to pressure valve 15, and thus builds up a pressure which forces liquid through apertures 9 to replace liquid of compartment 12 lost by evaporation. The novel container of this invention in this way maintains a constant area ass maximum cold area until substantially all of the liquid coolant has been utilized. Should the pressure in compartment 14 exceed that required to maintain the coolant in compartment 12 at the proper level, pressure valve 16 permits the escape of vapor through conduit in sufiicient quantity to prevent any overflow through conduit 11. A novel pressure valve for use in conjunction with the present invention is illustrated in FIGURE 2. Such a pressure valve is self-compensating and maintains a pressure within the inner chamber adapted to keep the coolant level in outer chamber 12 always at level c. Thus, in combination with the unique container structure, it represents another feature of the invention. Having particular reference to FIGURE 2, the valve comprises vertically disposed conduit 19, compensating rod 21, rigid connecting member 20, and valve cap 18. Valve cap 18 is provided with heat conduction fins to prevent excessive amounts of frost from collecting thereon. It has been determined that the area A of valve cap 18 upon which the pressure of inner compartment 14 acts must be equal to the cross sectional area A of compensating rod 21. Also, the length of compensating rod 21 should coincide with the liquid level difference between full and empty conditions of inner chamber 14 (/1 minus I1 or levels a and b). With these conditions fixed, it is only necessary to determine the combined weight W of compensating rod 21, connecting member and valve cap 18. This is achieved simply in accordance with the relationship W=h P A in which P is the density of the coolant liquid. By way of example, assuming the conditions: [1 cm.; h :2.5 cm.; A=1 crrr and the coolant being liquid nitrogen of density P=0.8; the weight W would equal 25 1 0.8 or 20 grams. The length of the compensating rod would be 22.5 cm. and its volume would be 22.5 cm}. If 11 grams of the weight is in the compensating rod and the rest in the accessories, the density of the rod must be 11/22.5 or 0.49 gram/cc. It would be convenient to make the compensating rod of aluminum plate 0.5 mm. thick, bent into the form of an air-tight cylinder 22.5 cm. long with a 1 cm. cross sectional area. Its weight therefore would be 11.0 grams and the rest of the accessories would weigh 9.0 grams. In practice, to ensure stability of the compensating rod it is necessary that the center of mass thereof is below the center of buoyancy. The valve, in operation acts in the following manner. As the liquid in inner compartment 14 recedes from its full capacity (level b) to empty (level a) increasingly greater pressure is required therein to maintain the liquid in outer compartment 12 at level 0. Because of the particular weight and geometry of the compensating rod and the buoyancy effects of the liquid, valve cap 18 maintains just the right pressure within inner compartment 14 to support the liquid in outer compartment 12. As the liquid level in the inner compartment drops there is less buoyancy effect upon the compensating rod and it effectively becomes heavier thus requiring a higher pressure to activate the valve.

Another embodiment of this invention is illustrated by FIGURE 3. In this arrangement liquid is pumped from the inner compartment 14 to the outer compartment 26 by means of tube member 25 in combination with conical member 23 as illustrated. The partition 22 which separates inner and outer compartments in this embodiment does not have apertures in the bottom as does the embodiment of FIGURE 1. Conduit 21 provides an exhaust for evaporated liquid from both compartments and no pressure is required for operation. Conical member 23 has a plurality of openings 24 around the bottom to permit the ready flow of liquid thereunder. In operation, a heat fiux generated by the impinging of mercury vapor (indicated by arrows 15) upon the bottom of the container causes bubbles to form within conical member 23. These bubbles congregate and move up the sloping side of the conical member to for ma large bubble which forces liquid up tube 25 and into the outer compartment.

It is to be understood that the above-described arrangements are illustrative of the applications of the principles of this invention. Numerous other arrangements may be devised by those skilled in the art without departing from the scope of the invention.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. In a vacuum system cold trap a container for liquid coolant having a partition therein, said partition being substantially parallel to the sides and top of said container and adapted to divide the interior thereof into first and second compartments, said first compartment being contiguous to the sides and top of said container and having means connected to the apex thereof to permit emission of evaporated coolant formed therein, and said second compartment being contiguous to the bottom of said container and comprising a reservoir for said coolant, and means for pumping coolant from said second compartment to said first compartment.

2. A container for liquid coolant as defined in claim 1 wherein said means for pumping coolant comprises, in combination, a conical member, said conical member having its base substantially co-extensive with and residing on the bottom of said second compartment, and a capillary tube, said capillary tube being connected between the apex of said conical member and the apex of said first compartment and adapted to permit the passage therethrough of said coolant.

3. In a vacuum system cold trap, a liquid coolant con-' tainer having an inner partition adapted to establish a first volume between itself and the top and walls of said container and a second volume between itself and the bottom of said container, said partition having a plurality of apertures therethrough in the vicinity of the bottom of said container, a first vapor escape conduit connecting said first volume to the atmosphere, a second vapor escape conduit connecting said second volume to the atmosphere, and a pressure valve, said pressure valve effecting a closure of said second vapor escape conduit and having a pressure release response adapted to create a vapor pressure within said second volume whereby sufli cient liquid coolant is forced through said partition apertures to maintain said first volume in a full condition.

4. In combination with a vacuum system including a high vacuum diffusion pump and a chamber to be evacuated, a cold trap comprising, a housing member, said housing member being an integral part of the vacuum system and constituting a passage between said diffusion pump and said chamber to be evacuated, and a container for liquid coolant aflixed within said housing member, said container having an inner partition establishing a rst compartment between itself and the top and sides of said container and a second compartment between itself and the bottom of said container, said partition having a plurality of apertures proximate the bottom thereof, a first conduit attached to the top of said container and effecting a communicating relationship between said first compartment and the atmosphere, a second conduit attached to the top of said partition and effecting a communicating relationship between said second compartment and the atmosphere, and a pressure valve, said pressure valve effecting a closure of said second conduit.

References Cited in the file of this patent UNITED STATES PATENTS 2,039,817 MacMillan May 5, 1936 2,052,410 Kucher Aug. 25, 1936 2,293,263 Kornemann Aug. 18, 1942 2,317,814 Schuchmann Apr. 27, 1943 2,389,168 Snyder Nov. 20, 1945 2,462,793 De Groat Mar. 29, 1949 2,853,859 Thompson Sept. 30, 1958 2,985,356 Beecher May 23, 1961 3,009,629 Garin Nov. 21, 1961 3,057,340 Fritts Oct. 4, 1962

Claims (1)

1. IN A VACUUM SYSTEM COLD TRAP A CONTAINER FOR LIQUID COOLANT HAVING A PARTITION THEREIN, SAID PARTITION BEING SUBSTANTIALLY PARALLEL TO THE SIDES AND TOP OF SAID CONTAINER AND ADAPTED TO DIVIDE THE INTERIOR THEREOF INTO FIRST AND SECOND COMPARTMENTS, SAID FIRST COMPARTMENT BEING CONTIGUOUS TO THE SIDES AND TOP OF SAID CONTAINER AND HAVING MEANS CONNECTED TO THE APEX THEREOF TO PERMIT EMISSION OF EVAPORATED COOLANT FORMED THEREIN, AND SAID SECOND COMPARTMENT BEING CONTIGUOUS TO THE BOTTOM OF SAID CONTAINER AND COMPRISING A RESERVOIR FOR SAID COOLANT, AND MEANS FOR PUMPING COOLANT FROM SAID SECOND COMPARTMENT TO SAID FIRST COMPARTMENT.

Images