US3172748A

US3172748A - Sorption pump

Info

Publication number: US3172748A
Authority: US
Publication date: 1965-03-09
Anticipated expiration: 1982-03-09

Description

March 1955 M. FElNLEIB ETAL SORPTION PUMP 2 Sheets-Sheet 1 Filed May 16. 1961 FIG.

D m n a E G a m m W SL U O 7 YRWMRN W 2 wELlfi A N YN ESEAU VIII-ME. NR IRAEY HO MSSR March 1965 M. FEINLEIB ETAL 3,

SORPTION PUMP Filed May 16, 1961 2 Sheets-Sheet 2 E E 23" I! E 5 23 Fl 6.6

TIME

Fl 6.7a 38 v ..r4l i 42 11!, r36

37 I .H 1 "1|? 1 l1 INVENTORS MORRIS FEINLEIB 42 STANLEY L. MERCER J SHERMAN L.RUTHERFORD V ROY E.. HLENBERG 13:? BY T I I I I I I I I I I I, QRNEY 33 7 2,748 Patented Mar. 9, 1965 3,172,748 SORPTION PUMP Morris Feinleib, Los Altos, Stanley L. Mercer, Sunnyvale,

The present invention relates in general to vacuum pumps and, more particularly, to improved vacuum sorption pump structures and to the method of operating vacuum sorption pumps to produce a pressure of 10 mm. of mercury or less.

A clean vacuum system is produced by first reducing the pressure in the system to about mm. of mercury (Hg) with a vacuum sorption forepump and then applying a getter ion pump to further lower the pressure of the system to a working vacuum having a pressure of 10' mm. ofHg or less. The sorption pump operates when the sorption material therein is chilled by inserting it within a massive, heavy, but fragile, dewar vessel or vacuum bottle which is then filled with liquid nitrogen. The temperature of the metal structure of the pump, being a good heat conductor, is reduced rapidly by the liquid nitrogen but, since it has considerable mass, it consumes the major portion of liquid nitrogen. The sorption material being porous, light, only a fraction of the mass of the pump, and a good heat insulator, is slowly cooled but requires less liquid nitrogen. Any improvements made to the sorption pump which decrease the initial chilling time of the sorption material therein would produce a more.

efiicient pump.

In a given system connected to a vacuum sorption pump, the ultimate pressure that can be achieved will decrease as the sizeof the sorption pump increases. However, there is a practical limit to the level of vacuum which may be obtained. Naturally the demand of liquid nitrogen would be greater to chill a larger sorption pump. Other means such as getter ion pumps are used to produce a higher vacuum than sorption pumps can produce (or for extended operation where the sorption pump would become saturated), but a getter ion pump is expensive and requires a skilled operator.

A principal object of this invention is to provide an improved, efiicient, fast operating sorption vacuum pump and an improved process employing these sorption pumps for producing lower pressures efiiciently and faster.

A main feature of this invention is a sorption vacuum pump with the provision of open spaces intermingled with spaces enclosing sorption material wherein the open spaces may be filled with a refrigerant to chill rapidly the sorption material.

Another feature of this invention is a sorption pump having a vessel with tubing disposed therethrough so that refrigerant may flow around said tubing.

Another feature of this invention is a sorption pump having a vessel with tubing protruding therefrom so that refrigerant may flow around said tubing These and other features and advantages will become apparent upon a perusal of the following specification taken in connection with the accompanying drawing wherein,

FIG. 1 illustrates one embodiment of the sorption pump incorporating the present invention,

FIG. 2 is a partial section taken on line 22 of FIG. 1,

FIG. 3 illustrates another embodiment of the sorption P p,

FIG. 4 is a partial section taken on line 4-4 of FIG. 3,

FIG. 5 is another embodiment of the sorption pump,

FIG. 6 is a graph representing the temperature of a series of points in said sorption material with respect to time,

FIG. 7a illustrates a system to be evacuated with two vacuum sorption pumps connected in parallel, and

.FIG. 7b is the same as FIG. 7a with the two pumps connected in series.

Referring to the drawing and to FIGS. 1 and 2 in particular, there is shown one embodiment of a sorption vacuum pump 11 utilizing the teachings of this invention. The sorption pump 11 has a tubular casing 12 with

end walls

13 and 14. The end walls contain a plurality of aligned openings to which both ends of tubes 16 are fixed. v

The end wall 13 has a center aperture from which a short flanged tube 17 protrudes. The flange on tube 17 has a groove 18 for a vacuum gasket and has bolt holes 19 whereby the sorption pump 11 may be bolted and sealed to the system to be evacuated. The inside of the sorption 1 which otherwise produces an inherently high resistance path to any gas flowing therethrough.

This sorption pump 11 is operated in the usual manner by immersing it in liquid nitrogen (not shown) up to its flange tube 17. The liquid nitrogen not only is disposed around the tubular casing 12 but flows into the tubes 16 located within the casing 12 to provide within a given volume additional exterior wall surface that is capable of contacting the refrigerant such as liquid nitrogen. The exterior wall surface of the vessel has an irregular shape so that a straight line passing through v the casing normal to the axis of the casing pierces the wall at least four times. The sorption material 21 to be effective has small pores comprising almost 50% of its volume thereby making it an inherently good thermal insulator. The thickness of the sorption material between any two cold surfaces must be preferably kept to below a maximum value. This is done by providing more wall surface or heat transfer surface, for the vessel holding the sorption material. The actual wall surface of the vessel is much greater than the largest projected area of the cylindrical outer vessel. The sorption efliciency and rate of cooling of the material is greatly enhanced as the rate .of cooling throughout the sorption material is increased so that the low temperature reaches .uniformity throughout the sorption material more rapidly.

The maximum thickness of sorption material 21 is determined by its thermal insulation characteristics. Referring to FIG. 6, a family of curves 23 illustrates the temperature with respect to time of specific points within the sorption material which points are disposed a specific distance from a cold surface. The curve 23' closest the coordinateaxis represents a point closest to the cold surface while the curve 23" farthest from the coordinate axis represents a point farthest from the cold surface. If a sorption material such as a synthetic zeolite having a formula of 1.0i0.2 CaO: Al O :1.85i0.5 SiO is used in the sorption pump, a point within the material Vs of an inch from the cold surface is foun'cl'to approach a minimum temperature in about 10 minutes or that a curve 23 for that point is almost. parallel to the time ordinate indicating that the temperature gradient within the material between that point and the cold wall is small and the temperature between that point and the wall is uniform. Thus, 'a sorption pump-containing the above sorption materialmust preferablyhave nomore than of. an inchdistance between its cold walls so that all of'the material is at or rapidly reaches a uniform low temperature and is efficiently pumping. This sorption pump will have the ratio of pump-wall-area to sorption-material-volume equal to about 8/3. (Mathematically, this ratio has a dimension unit equivalent to length.) Other sorption material having different thermal insulation properties will require for efiicient operation difier ent spacing between. the cold walls of the sorption pumps in which they are incorporated. This spacing can be determined from a family of curves 23 for that particular material so that the temperature gradient for that particular material is kept low.

Referring to FIGS. 3 and 4, another embodiment of a sorption pump is shown. This embodiment comprises a flat circular vessel 24' which has on one side a flange tube 1 7' similar to the flanged tube 17 in FIGS. 1 and 2, while on the other side of the vessel 24 protrudes a plurality of closeended tubular members 26 which communicate with the flat circular vessel 24. The closed i ends of the tubular members 26 are preferably, for rigidity, fixed to a plate 27. The sorption material 21 is placed within the tubular members 26 so that when the sorption pump is immersed in liquid nitrogen, the liquid nitrogen contacts the outer surface of the tubular mem bers and chills the sorption material 21. Again, in these vessels theexterior'wall surface has an irregular shape wherein a straight line passing through the vessel pierces the walls at least four times, and again, the maximum inside diameter of the tube members is preferably /4 of an inch ifsyntheti'c zeolite is used as the sorption material. To increase the rate of flow of gas to the sorption material, a tubular screen 28 (shown in phantom) can be axially disposed within each member 26 to provide a space free of sorption material.

Referring to FIG.. 5, still another embodiment of the invention is shown wherein the transverse shape of the vessel 30 of the sorption pump is substantially a cross and the walls of the pump are irregular so that a straight line passing through the vessel pierces the wall at least four times. This pump, like the previous pumps, has a flange tube 17 so that it may be readily attached to a vacuum system. The cross-shaped vessel 30 is also filled with sorption material 21. The thicknesses of the extending fingers 31 of the vessel from one inside surface tothe opposite inside surface are again made 3 4 of an inch when synthetic zeolite is used as the sorption material 21. Although the pump is shown as a cross with four radially extending fingers 31, a pump can be conceived having three, five, six or any number of radially extended fingers provided that the. thickness of the sorption material between cold walls is maintained at a dimension whereby there is substantially no temperature gradient and rapid cooling occurs in the sorption material.

Any number of vacuum sorption pumps may be attached as appendages to a vacuum system. If two or more pumps are used, there are three possible methods for operating the sorption pumps, as follows:

Method. 1.Chill all the pumps at the same time while in communication with the vacuum system.

Method 2..Chill one pump at a time and have only that pump communicating to the vacuum system while all the other pumps are valved off from the system.

Method 3. Chill one pump at a time with all pumps communicating with the vacuum. system and when the pumping rate of that. pump tapers. off, valve it off from the system and chill the next pump and then valve it off when its pumping rate tapers off, etc.

Referring to FIG. 7a, there is shown a vacuum system 34- to be evacuated by two vacuum sorption pumps 36 and 37 which are attached as appendages to the system 34 with a high vacuum valve 38 disposed between pump 36 and the system and another high vacuum valve 39 disposed between pump 37' and the system. This embodiment is known as parallel pump configuration.

all)

4 The pumps may also be attached in series configuration wherein the pumps 36' and 37' will be attached to the system 34' as shown in FIG. 7b with valve 38' disposed so that pump 36' can be valved off from the system 34' and pump 37' and valve 39 is disposed between pump 37' and the system.

The

pumps

36 and 37 can be chilled together as suggested by method 1, above, by pouring liquid nitrogen in open-

vessels

41 and 42, each disposed around a pump, keeping both valves open. The pressure produced in the system by this process is the same as if one large pump was used. to pump the system and equivalent to the prior art.

The pumps can be chilled as suggested by method 2. First valve 38 is closed, valve 39 is opened, and then pump 37 is chilled. When the pumping rate of pump 37 tapers off, valve 39 is closed, valve 38 is opened, and pump 36 is chilled to further lower the pressure in the system 34. A lower pressure is produced than by method 1 because the pumping rate of a sorption pump is dependent upon the amount of gas which has already been taken up by the sorption material.

The pump can be chilled as suggested by method 3 wherein both

valves

38 and 39 are opened, and only pump 36 is chilled so that it pumps down the pressure in the other pump as well as in the system. Then valve 38 is closed and pump 37 is chilled. This method produces still a lower pressure than either the first or second method.

If the pumps are fixed to the system in series as shown in FIG. 7b, the same procedure is followed as described above.

To mathematically explain the end results let us assume dimensions for the individual items in the system.

Pumps

36 and 37 are large enough to each contain 1000.grams of sorption material. The system 34 contains 0.5 mole of gas. Each pump contains. 0.2 mole of gas at atmospheric pressure.

Let us assume that the sorption material has the fol lowing characteristics:

log y=log x+l.75 (1) when maintained at 195 by liquid nitrogen and has the following characteristics at room temperature (25 C.):

wherein y represents the amount in moles of gas absorbed per 1000 grams of sorption material and x represents the corresponding pressure, in mm. of mercury, in equi librium with the material. The above equations are approximately representative of some sorbents at pressures of the order of one mm. to 10 mm., but are used here for illustrative purposes only; the end result of the various methods of operating two pumps in series or parallel will be the same qualitatively, regardless of the form of the relationship between equilibrium pressure and quantity of gas sorbed.

When the system is pumped by the first method described above, each pump must pump 0.25 mole of gas (/2 of the 0.5 mole in vessel 34) and together with the 0.2 mole it already contains, it has a total of 0.45 mole. Using Equation 1', since the sorption material is at liquid log y=.6 log .x2.4

nitrogentemperature, the pressure in thesystem will be about 8x10 mm. of Hg.

Operating the system by' the second method described above, when the first pump is chilled and! the second is valved off, the first pump will sorbthe 0.5-mole in the system and the 0.2 mole it already contains for a total of 0.7 mole. Using Equation 1, the pressure in the system will be about 12x10" mm. of mercury. Then the first pump is valved. off and the valve of the second pump is opened and the pump chilled. The amount of gas left in vessel 34 is negligible in comparison withthe 0.2 mole originally contained in the second pump. and which must now be sorbed by this pump. Again, using Equa tion 1. the pressure will be 36x10 mm. of mercury. The ultimate pressure of the system is reduced by a factor of two compared to the pressure produced by the first method.

Operating the system using the third method, the first pump is chilled thereby absorbing the 0.5 mole in vessel 34, the 0.2 mole in the other pump, and its own 0.2 mole for a total of 0.9 mole. Again, using the above Equation 1, the pressure in the system will be about 1.6x 10- mm. of mercury. At this pressure, the other pump, which was not chilled and is at room temperature, contains only about 3.54 10- moles of gas as calculated from Equation 2. Then the first pump is valved off and the second pump is chilled. Again, since the moles of gas absorbed by the second pump from the vessel is negligible, in comparison with the 3.54X 10 moles, the pressure is calculated to be about 7.45 10" mm. of mercury, using Equation 1. If three or four pumps were used in order, the resulting pressure would be correspondingly less.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A vacuum sorption pump apparatus comprising: a hollow casing made of heat conductive, impervious material, having a cylindrical side wall and top and bottom walls, said topwall having a gas inlet aperture, said casing being vacuumtight except for said gas inlet aperture; a plurality of vertical, heat conductive, impervious tubes extending into and through said casing from the top wall to the bottom wall of said casing and coextensive with said cylindrical side wall; and, sorption material disposed within said casing and contacting said casing and said vertical tubes.

2. The apparatus according to claim 1 including a hollow, vertical, apertured, cylindrical member disposed within said casing to define a sorption material receiving region between, said apertured, cylindrical member and the side wall of said casing, said sorption material being disposed within said sorption material receiving region and contacting said tubes and said casing, and said apertured, cylindrical memberproviding a low resistance gas path from said gas inlet aperture to said sorption material receiving region.

pervious material, each having a cylindrical side wall and an impervious bottom wall and communicating at their top with said casing through a respective one of said apertures in said casing bottom wall, said casing and tubular members being vacuum tight except for said gas inlet aperture; and, sorption material disposed within said tubular members and contacting the walls of said tubular members.

4. The apparatus according to claim 3 including a plurality of hollow, vertical, apertured, cylindrical members each disposed within a respective one of said tubular members to define a sorption material receiving region between each of said apertured, cylindrical members and the side wall of the respective tubular member, said sorption material being disposed within said sorption material receiving regions and contacting the walls of said tubular members, said apertured cylindrical members providing low resistance gas paths from said gas inlet aperture to said sorption material receiving regions.

5. The apparatus according to claim 4 wherein said tubular members are parallel to each other, and including a rigid plate connecting the closed ends of said tubular members.

References Cited by the Examiner UNITED STATES PATENTS REUBEN FRIEDMAN, Primary Examiner.

HARRY B. THORNTON, Examiner.

Claims

1. A VACUUM SORPTION PUMP APPARATUS COMPRISING: A HOLLOW CASING MADE OF HEAT CONDUCTIVE, IMPERVIOUS MATERIAL, HAVING A CYLINDRICAL SIDE WALL AND TOP AND BOTTOM WALLS, SAID TOP WALL HAVING A GAS INLET APERTURE, SAID CASING BEING VACUUM TIGHT EXCEPT FOR SAID GAS INLET APERTURE; A PLURALITY OF VERTICAL, HEAT CONDUCTIVE, IMPERVIOUS TUBES EXTENDING INTO AND THROUGH SAID CASING FROM THE TOP WALL TO THE BOTTOM WALL OF SAID CASING AND COEXTENSIVE WITH SAID CYLINDRICAL SIDE WALL; AND, SORPTION MATERIAL DISPOSED WITHIN SAID CASING AND CONTACTING SAID CASING AND SAID VERTICAL TUBES.