US3126902A

US3126902A - Method and apparatus for producing high vacuum

Info

Publication number: US3126902A
Authority: US
Publication date: 1964-03-31
Anticipated expiration: 1981-03-31

Description

March 31, 1964 w. L. FlTE ETAL 3,126,902-

METHOD AND APPARATUS FOR PRODUCING HIGH VACUUM Filed April 14, 1960 Z7 69 71 v 51 31 J5 3 55\.9 39 All Wow ,kwos -m United States Patent 3,126,902 METHOD AND APPARATUS FOR PRODUCING HIGH VACUUM Wade L. Fife, Encinitas, and Richard T. Brackmann, San

Diego, Calif., assignors to General Dynamics Corporation, New York, N.Y., a corporation of Delaware Filed Apr. 14, 1960, Ser. No. 22,203 6 Claims. (Cl. 137-1) This invention relates toa method and apparatus for producing high vacuum.

Numerous processes of science and technology require conditions of low pressure for their operation. The attainment of pressures of the order of 1x millimeters of mercury or less is ordinarily accomplished by use of oil or mercury diffusion pumps, which operate efficiently only when their discharge is against pressures less than a few tenths of a millimeter of mercury. Therefore, in operation, the diffusion pump discharge is fed into the inlet of a mechanical back-up pump which supplies the necessary intermediate pressure for the outlet of the diffusion pump. To achieve lower pressures down to about 1 10* millimeters of mercury, it is often necessary to employ two or more difiusion pumps series, with a cold trap between the chamber to be evacuated and the pumps in order to prevent migration of the vapors of the diffusion pump fluid into the chamber being evacuated.

It is the principal object of this invention to provide a method whereby pressures below l l0 millimeters of mercury may be obtained without the use of diffusion pumps or their the invention is to provide a vacuum through the use of a method and apparatus which employ generally available laboratory equipment.

Other objects and advantages of the invention will be come known by reference to the following description and the acompanying drawings.

In the drawings:

FIGURE 1 is a diagrammatic representation of one system which embodies various of the features of the invention; and

FIGURE i2 is a diagrammatic representation of another system which embodies various of the features of the invention.

In the method employed for obtaining a high vacuum in the systems illustrated in the drawings, a closed system fiirst partially evacuated to the range of pressure of a few millimeters of mercury. This can be accomplished by any appropriate pumps or pumping systems.

'After the system is partially evacuated, the atmosphere within the system is placed in communication with a sur- I face 'which is cooled to a temperature below the freezing point of water and which preferably roughly corresponds to the boiling point, at atmospheric pressure, of the lowest boiling gas Water vapor is made available in the partially evacuated which is to be removed from the system.

system. The water vapor may be introduced by injecting water or water vapor into the system 'or, in the alternative, water may be present in the system in amounts which will produce the desired amount of water vapor.

As a result of the freezing of the water vapor through contact with the cold surface, the pressure in the system is materially reduced.

'As has been pointed out above, the cold surface in comvmunication with the closed system is preferably maintained at a temperature which roughly corresponds to the boiling point at atmospheric pressure of the lowest boiling gas contained within, and desired to be removed from, the closed system. For example, if the system is initially filled with air, composed primarily of nitrogen and oxygen, surface may conveniently be cooled by contacting the side of the surface opposite the atmosphere in the associated equipment. A further object of the system is exhausted by period of ice chamber with liquid nitrogen or liquid oxygen, or with unfractionated liquid If the system initially contains appreciable quantities of hydrogen and it is desired to remove it, the cold surface may be cooled by contacting it with liquid helium.

The cold surface causes the water vapor in the system to condense and freeze upon the cold surface. The condensation and freezing of the water vapor upon the cold surface is found to be accompanied by a reduction of pressure in the system which is of much greater magnitude than is attributable to removal of the water vapor alone. If a sufficient quantity of water vapor is present within the closed system, its continuing condensation on the cold surface will be acornpanied by a reduction of the mercury;

It should be noted that the final pressure obtainable through the teachings of this invention differs from the initial pressure of about 0.5 to 5 millimeters of mercury by a factor of about 100,000 While the pressure which may be expected from the removal of the water of saturation contained in air differs from the saturated pressure by a factor of only 0.96 to 0.99. It should also be understood that the required area of the cold surface is not sufficient to cool the entire volume of gases in the system, but that its cooling effect is limited to the gases immediately surrounding the surface, leaving a major portion of the contained gases substantially at ambient temperature.

a With the excep'on of Water and carbon dioxide, the dew points of the gases Within the system at pressures in the range of 2X10 millimeters of mercury are considerably below the temperature of the cold surface described above. Thus, it is apparent that the pressure reduction'cannot be explained as being the result of condensation of the residual gases. Instead, the reduction in pressure appears to be effected through the pro'css of freezing the water vapor to ice upon the cold surface described above. While its mechanism is not fully understood, it is thought to possibly involve at least two phenomena. The first may be the physical entrapment of non-condensable gases within the condensing water, and this possibly accounts for a relatively rapid pressure reduction observed during the deposition of ice upon the cold surface. The second phenomenon which may be involved is one of adsorption or absorption of gases upon or within the crystalline ice deposit. This process probably slower, and possibly involves an equilibrium situation, since it is observed that, after the water vapor in freezing, the pressure in the system slowly creeps upward unless the deposit of ice is isolated from the remainder of the system.

It has been empirically determined that, to achieve a pressure in the range of about 2X10 millimeters of mercury, there preferably is present within the system over about 50 molecules of water for every molecule of nonthe system before the deposition of ice begins. Since this amount of water is greater than the proportion which would remain in the .system. after the initial evacuation assuming the system originally contained air saturated with water vapor, it is necessary to supply additional water to the partially evacuated system in order for it to be available during the ice deposition. This can be accomplished either by introducing the required amount Within the system prior to the initial evacuation of the system or by introducing it to the closed system by appropriate means thereafter.

The system illustrated in FIGURE 1, for carrying out the method described above, comprises generally a chamber 11 to be evacuated, a pump 13, a cold trap 15, and an external water reservoir 17. The chamber 11 is adapted to withstand a pressure of one atmosphere. As illustrated, the chamber 11 comprises a generally cylindrical body section 19 whose ends are closed with outwardly dished heads 21. Preferably, the heads are attached to the body section by means of welding or the like to minimize the possibility of gas leakage. As shown in FIG- URE 1, the chamber 11 is supported on a suitable stand which comprises a plurality of legs 23.

An access port 25 is provided to furnish access to the interior of the chamber 11, and to enable placement of specimens or apparatus therein on a specimen stand or rack 27. The port 25 is covered with a suitable cover plate 29 which is designed and adapted to closely conform to the marginal edges of the port 25. A suitable gasket 31 is provided between the cover 29 and the edges of the port 25 to provide a gas tight joint. The cover 29 is secured in place by suitable fasteners which in the illustrated embodiment comprise stud bolts 33 and locking dogs 35.

It is, of course, preferable that unnecessary joints and connections be avoided in the construction of the chamber 11, as well as the remainder of the system, in order to minimize the possibility of air leakage into the vacuum system.

The pump 13 is a pump which is adapted to reduce the pressure in the system to the initial range of between about millimeters of mercury and 1X10" millimeters of mercury. The pump 13 includes an inlet 37 and an outlet (not shown) through which the gas in the system is exhausted. The inlet 37 of the pump is connected to the chamber 11 by means of a pipe 39. In order to isolate the pump from the remainder of the system, the pipe 39 is provided with a shut-01f valve 41.

A vacuum gauge 43 is connected to a fitting 45 in the wall of the chamber 11 by means of a conduit 47. The gauge 43 may constitute a McLeod gauge or other gauge capable of measurement of pressures down to the range of about 1 10 millimeters of mercury.

As illustrated, the cold trap 15 includes a generally cylindrical vessel 48 having openings therein, one of which is connected to the chamber 11 and the other of which is connected to a supply of water. The cold trap 15 is preferably constructed of glass to enable observation of ice deposited in its interior. A water inlet line 49 is provided at the upper end of the cold trap 15. As illustrated, the inlet line 49 is L-shaped and includes a horizontal leg 49a and a vertical leg 4%. The vertical leg is sealed into the water inlet opening in the upper end of the vessel 48 as illustrated.

A gas inlet conduit 51 is sealed at one end in the wall of the vessel 48 and its other end is connected by a connector 53 to a pipe 55 which is fitted into the wall of the chamber 11. In order to isolate the cold trap 15 from the chamber 11 the pipe 55 is provided with a shut-off valve 57.

It should be noted that a relatively unrestricted flow of gases from the chamber 11 to the interior of the cold trap 15 will result in a more rapid reduction of pressure in the chamber 11. Therefore, it is preferable that the passageways through the pipe 53, the shut-off valve 57, and the gas inlet connection 51 to the cold trap 15, be of relatively large diameter.

The lower portion of the cold trap 15 is in a Dewar flask 63, or similar container for holding liquid gases at low temperatures. The interior diameter of the Dewar flask 63 is large enough to provide a space between the Dewar flask 63 and the exterior surface of the cold trap 15 and this space is filled with liquefied gas for maintaining a predetermined area of the interior surface 65 of the cold trap 15 at the desired temperature.

Thus, it will be seen that one surface, the interior surface, of the vessel 48 of the cold trap 15 is in contact with gases contained within the chamber and the other surface, the exterior surface, of the vessel 48 of the cold trap 15 is arranged to be contacted with cold liquefied gas.

As illustrated in FIGURE 1, the Water reservoir 17 comprises a graduated burette 66, including an outlet tube 69 having a stopcock 67 by means of which the amount of water to be admitted to the'vacuumized system may be controlled. The tube 69 is sealed in the open end of the horizontal leg 49a of the Water inlet line 49 of the cold trap 15. Of course, a suitable sealing member 71 is provided to effect a gas-tight connection between the inlet line 49 and the burette tube 69. It is preferable that the tip 73 of the tube 69 is drawn out to a relatively small diameter in order to provide a fine stream of water which is more readily vaporizable. This results in a more rapid dispersion of water vapor and thus a more rapid reduction of pressure within the system.

The apparatus illustrated in FIGURE 1 may be operated to produce a high vacuum in the following manner. The chamber 11 and cold trap 15 are initially evacuated to a pressure of from about 5 to 1 10- millimeters of mercury by the pump 13, with the shut-off

valves

41 and 57 in open positions and the stopcock 67 in a closed position. Upon completion of the initial evacuation, the pump 13 is isolated from the remainder of the closed system by the valve 41. The Dewar flask 63 is then filled with sufiicient liquid air or other cold liquid to a height suflicient to immerse the lower end of the cold trap vessel 48 in the liquid air. The stopcock 67 is then opened so as to admit into the partially evacuated system a small amount of water. When the desired vacuum is attained, as indicated by the vacuum gauge 43, admission of water is discontinued, and the cold trap 15 is isolated from the remainder of the system by the valve 57. If the pressure within the chamber 11 should subsequently increase to an undesirable level, it may again be reduced to the desired level by opening the valve 57 and admitting additional Water from the burette 66.

It has been found that when a chamber having a volume of about 400 liters is initially evacuated to a pressure in the range of 0.5 millimeter of mercury, and when liquid nitrogen is added to the Dewar flask so that about 500 square centimeters of cold surface is exposed to the atmosphere contained in the system, that introduction of approximately one milliliter of liquid water into the system will reduce the pressure in the system to about 2 10- millimeters of mercury. Prior to the addition of the Water there is but a very slight reduction in pressure in the system due to the liquid nitrogen contacting the cold surface. Thus the addition of the water effects a marked reduction in the pressure within the system.

FIGURE 2 shows a system in accordance with the invention adapted for use with laboratory equipment. The vacuum chamber in the system comprises a bell jar 81 which is supported upon a suitable supporting plate 83. As illustrated, the vacuum is drawn from the bell jar 81 through a conduit 85, one end 85a of which is sealed in the supporting plate and the other end 85b of which is connected to a conduit 87 with a T connection 89. One end of the conduit 87 is connected to a vacuum gauge 91 to indicate the pressure within the bell jar, the gauge consisting of a McLeod gauge or other device suitable for measurement of pressures down to the range of about 1 10 millimeters of mercury.

In order to isolate the bell jar 81 from the remainder of the system, the other end of the conduit 87 is connected to one side of a stopcock 93, whose other side is connected to a cold trap 95 by means of a conduit 97.

The cold trap 95 illustrated, comprises a doubled-walled container 99 having an open topped inner vessel 101 which is adapted to contain liquefied gas. An outer vessel 103 extends around the bottom of the vessel 101 and their upper ends are sealed together with a gas-tight seal. As illustrated, the gases evacuated from the bell jar 81 are conducted through the jacket formed by the inner and

outer vessels

101 and 103. In this connection the conduit 97 is sealed in the outercontainer 103 so as to conduct gas from the bell jar into the hollow jacket.

A water aspirator 105 is provided to effect the initial reduction of pressure in the system. As illustrated, the operation of the aspirator is controlled by a water valve 107. The vacuum connection 109 of the aspirator 105 15 connected by means of rubber tubing 111 or the like, to a stopcock 113 which, in turn, is connected by a conduit 115 to one leg of a U-tube 117. The other leg of the U-tube 117 is connected by a conduit 119 to the hollow jacket between the

vessels

101 and 103.

The inner vessel 101 is adapted to receive liquefied gas at the desired temperature in order that a predetermined area of the exterior surface of the vessel 101 is exposed to the atmosphere within the closed system.

The operation of the apparatus illustrated in FIGURE 2 consists first of introducing about 1 milliliter of water in the U-tube 117 and assembling the various components, leaving the

stopcocks

93 and 113 in the open position. The system is then partially evacuated by means of the aspirator 105 through appropriate control of the valve 107. During this portion of the operation, observation of the water contained in the U-tube 117 affords an indication of the extent of evacuation, in that a cessation of air bubbles through the water indicates that the pressure in the system is at the minimum attainable with the aspirator. When bubbling stops or becomes very slow, the stopcock 113 is closed, and liquid air is introduced into the inner vessel 101. The water contained in the U-tube 117 supplies suificient water vapor for final evacuation to a pressure in the range of 2 l millimeters of mercury, without need for further attention by the operator. When the pressure in the system has been reduced to the desired degrees, the stopcock 93 is closed to isolate the bell jar 81.

The economic advantage of the use of the described embodiments of the invention over the use of a diffusion pump to produce pressures as low as about 2 1(] millimeters of mercury is apparent. The components comprising the various embodiments of the invention may be assembled at a nominal cost compared to the cost of a diffusion pump and its accessories. Further advantages which may be noted are the absence of a potentially contaminatory substance such as the mercury or oil in a diffusion pump; the relatively simple technique of operation, and the adaptability to compact installation and varied applications.

Various of the features of the invention are in the following claims.

What is claimed is:

1. Apparatus for producing a high vacuum comprising means defining a gas-tight chamber, means for evacuating said chamber to a pressure of between about 5 millimeters of mercury and 1X10 millimeter of mercury, a first conduit connecting said evacuating means to said chamber, means on said first conduit for selectively isolating said evacuating means from said chamber, means defining a surface which is cooled below the freezing point of water so as to condense and freeze water vapor contacting said surface, a second conduit connecting said surface to said chamber, means on said second conduit for selectively isolating said chamber from said surface, and a water vapor source which provides sufiicient water vapor in said chamber to aiford over about 50 molecules of water vapor for each molecule of non-condensible gas remaining in said chamber after said chamber is evacuated by said evacuating means.

2. Apparatus for producing a high vacuum, comprising means defining a gas-tight chamber, means for evacuating said chamber to a pressure of between about 5 millimeters of mercury and 1X10" millimeter of mercury, a first conduit connecting said evacuating means to said chamber, means on said first conduit for selectively isolating said evacuating means from said chamber, means defining a surface which is cooled to about the boiling point at atmospheric pressure of the lowest boiling gas to be removed from the chamber, a second conduit conset forth necting said surface to said chamber, means on said second conduit for selectively isolating said chamber from said surface, and a water vapor source which provides sufiicient water vapor in said chamber to afford over about 50 molecules of water vapor for each molecule of non-condensible gas remaining in said chamber after said chamber is evacuated by said evacuating means.

3. Apparatus for producing a high vacuum comprising means defining a gas-tight chamber, a pump for evacuating said chamber to a pressure of between about 5 millimeters of mercury and 1X10" millimeter of mercury, a first conduit connecting said pump to said chamber, a valve in said first conduit for selectively isolating said pump from said chamber, a source of water which provides sufficient water vapor in said chamber to afford over about 50 molecules of water vapor for each molecule of non-condensible gas in said chamber after said chamber is evacuated by said pump, a second conduit connecting said source to said chamber, means for cooling a portion of said second conduit to about the boiling point at atmospheric pressure of the lowest boiling gas to be removed from the chamber, and a valve in said second conduit means for selectively isolating said chamber from said cooled portion of said second conduit.

4. Apparatus for producing a high vacuum comprising means defining a gas-tight chamber, an aspirating type pump for evacuating said chamber to a pressure of between about 5 millimeters of mercury and 1X10" millimeter of mercury, a conduit connecting said pump to said chamber, a first valve in said conduit for selectively isolating said pump from said chamber, a source of water in said conduit between said valve and said chamber, said source of water providing sufiicient water vapor in said chamber to afford over about 50 molecules of water vapor for each molecule of non-condensi'ble gas in said chamber after said chamber is evacuated by said pump, means for cooling a portion of said conduit disposed between said source of water and said chamber to about the boiling point at atmospheric pressure of the lowest boiling gas to be removed from the chamber, and a valve in said conduit between said cooled portion and said chamber for selectively isolating said chamber from said cooled portion of said conduit.

5. A method of producing high vacuum comprising evacuating a closed system to a pressure in the range from about 5 millimeters of mercury to 1 10- millimeter of mercury by an evacuating means, isolating said evacuating means from said closed system after said closed system is evacuated, providing in said closed system a quan tity of water vapor suificient to afford over about 50 molecules of water vapor for every molecule of noncondensible gas remaining in said closed system after said system is evacuated, exposing the gases in said closed system to a surface which is maintained at a temperature below the freezing point of Water whereby water vapor in said closed system is condensed and frozen to thereby reduce the pressure in said system, and isolating said surface from said closed system after the pressure is reduced in said system.

6. A method of producing high vacuum comprising evacuating a closed system to a pressure in the range from about 5 millimeters of mercury to 1X10 millimeter of mercury by an evacuating means, isolating said evacuating means from said closed system after said closed system is evacuated, providing in said closed system a quantity of water vapor sufficient to aiford over about 50 molecules of water vapor for every molecule of non-condensible gas remaining in said closed system after said system is evacuated, exposing the gases within said closed system to a surface which is maintained at a temperature of about the boiling point at atmospheric pressure of the lowest boiling gas to be removed from said closed system, and then isolating said surface from said closed system.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Folsom Nov. 19, 1946 Hickman Feb. 24, 1948 8 Lander July 18, 1950 Jenkins Oct. 23, 1951 Bancroft Jan. 24, 1956 Holleman Jan. 7, 1958 Alpert Apr. 22, 1958 Kohler June 3, 1958 Copson Nov. 11, 1958

Claims

1. APPARATUS FOR PRODUCING A HIGH VACUUM COMPRISING MEANS DEFINING A GAS-TIGHT CHAMBER, MEANS FOR EVACUATING SAID CHAMBER TO A PRESSURE OF BETWEEN ABOUT 5 MILLIMETERS OF MERCURY AND 1X10**-2 MILLIMETER OF MERCURY, A FIRST CONDUIT CONNECTING SAID EVACUATING MEANS TO SAID CHAMBER, MEANS ONSAID FIRST CONDUIT FOR SELECTIVELY ISOLATING SAID EVACUATING MEANS FROM SAID CHAMBER, MEANS DEFINING A SURFACE WHICH IS COOLED BELOW THE FREEZING POINT OF WATER SO AS TO CONDENSE AND FREEZE WATER VAPOR CONTACTING SAID SURFACE, A SECOND CONDUIT CONNECTING SAID SURFACE TO SAID CHAMBER, MEANS ON SAID SECOND CONDUIT FOR SELECTIVELY ISOLATING SAID CHAMBER FROM SAID SURFACE, AND A WATER VAPOR SOURCE WHICH PROVIDES SUFFICIENT WATER VAPOR IN SAID CHAMBER TO AFFORD OVER ABOUT 50 MOLECULES OF WATER VAPOR FOR EACH MOLECULE OF NON-CONDENSIBLE GAS REMAINING IN SAID CHAMBER AFTER SAID CHAMBER IS EVACUATED BY SAID EVACUATING MEDANS.