US3263434A - High vacuum cryopumps - Google Patents

Description

Aug. 2, 1966 F. x. EDER HIGH VACUUM CRYOPUMPS 2 Sheets-Sheet 1 Filed March 16, 1965 obaaoo @QQQQ INVENTOR. 73% 712114 278 7* United States Patent 3,263,434 HIGH VACUUM CRYOPUMPS Franz X. Eder, Munich, Germany, assignor to Bendix-Balzers Vacuum, Inca, Rochester, N.Y. Filed Mar. 16, 1965, Ser. No. 440,087 Claims priority, application fiwitzerland, Apr. 17, 1964, 4,998/64 7 Claims. (Cl. 6255.5)

Processes to produce high vacuum at a large rate of evacuation by condensation of the gas to be pumped off on deeply cooled surfaces have been known and successfully used for a long time. Heretofore fluid containers with liquid hydrogen or helium have been used on whose outer walls the gas to be pumped off freezes in a solid form. Corresponding to the vapor pressure of air at a temperature of 4.2 to 202 K. an equilibrium vapor pressure of less than torr is established for air or nitrogen as the working gas. In the same way a vacuum of 10* torr may be produced for hydrogen by condensation on a container with boiling helium.

Today the physical principles of this process are for the most part known. It is known, for example, that the sticking probability of gas molecule flying toward the deeply cooled condensation surface is dependent on its temperature and the kinetic energy of the molecule. This makes possible the production of an ultra-high vacuum in a large volume, where the pumping effort used in this case for large rates of evacuation can become considerably smaller than that used by conventional diffusion pumps. The previously known cryopumps consist of a condensation container or a spiral tube, which is charged with liquid hydrogen or helium and built into the vessel to be pumped out. Further equipments are known which allow an optimal regulation of the cooling liquid that are supplied from the storage vessel for the liquid gases through vacuum insulated lines to the actual cooling surface. The availability of a liquefaction apparatus for hydrogen or helium is also known which would permit the cooling liquid to be delivered in large transport vessels to the pumping arrangement.

The subject of the present invention is an arrangement for the production or maintenance of a vacuum in a space in which the gas to be removed is condensed on surfaces at a predetermined low temperature, which is characterized in that it contains a complete gas cooling apparatus consisting of a compressor, a counter current heat exchanger and an expansion machine in a closed circuit, of which at least the parts at the lowest temperature are arranged in a metal casing whose outer walls represent the condensation surface projecting into the closed vessel, which either is to be pumped out or to hold a vacuum.

For large pump capacities, the compressor of the gas cooling machine is installed on the outside of the actual pumping arrangement which contains the heat exchangers that cool down the compressed working gas to a suitable temperature for admission int-o the expansion machine by means of the cool gas fiow from the expansion machine through the counterflow heat exchanger. The working gas expanded from the lowest temperature (that of liquified helium in most cases) flows through the interior of the condensation surface container, while the gas to be pumped off is condensed on its outer surface. The present invention further relates to a pump arrangement in which all parts of the gas cooling apparatus are contained within a closed metal casing projecting into the vessel to be pumped out and for minimizing the size of the compressor, and providing that the heat of compression be removed at a very low temperature, preferably at the temperature of boiling liquid air. The invention also in- Patented August 2, 12966 ice eludes in addition the mechanical coupling between the expansion machine and the compressor so that only a single drive needs to be brought in from the outside through a common shaft, which is operated by a motor installed on the pump.

The advantage of the present invention idea lies in that the high vacuum pump consists only of a single unit which is inserted into the vessel to be evacuated by means of a gas tight flange and is connected to the external compressor by only two gas inlet or gas outlet connections, or in the case of the compressor cooled with liquid air only by current feed throughs with the surroundings. The cooling capacity required for the condensation process corresponds to the enthalpy difference of the gases to be pumped off at room temperature and the temperature of the cooling surface which is relatively small and requires only small gas cooling equipment with a low working capacity even for large evacuation capacities.

FIGURE 1 is a view of the high vacuum pump of the invention.

FIGURE 2 is a view of an embodiment of the high vacuum pump.

The invention is more fully explained by reference to FIGURE 1 wherein the high vacuum pump consists of a casing designated by numeral 1 constructed out of a low heat conductivity material which is connected by gas tight means such as a flange to the vessel 2 which is to be evacuated. In the inside of the container 1, which is also closed on the outside with a cover 3, all parts of the gas cooling equipment are accommodated with the exception of the compressor 4 which serves to compress the working gases such as helium, neon or hydrogen. Starting from the surrounding temperature, the equipment consists of the countercurrent heat exchangers 5 and 6, of which the gas in heat exchanger path 5 is provided for the cooling of the high pressure operating gas by the returning cold, already expanded working gas, while in the counter flow path 6 of the other branch high pressure gas is cooled by the evaporating liquid air in the cooler 7. Fill pipe 8 serves for the filling of the cooler 7, and remains closed during operation of the pump. The high pressure gas is cooled in the cooler 7 to about K. and flows in the tube coil 9(a) of the following counter flow path 9 where it is further cooled from the outside by the returning low pressure gas until finally a part of the high pressure flow is conducted via 10 through a pressure equalizing container 11 to the piston expansion machine 12 where it is adiabatically expanded with the extraction of work to a significantly lower temperature. The expanded gas flows back through the pressure equalizing container 13 into the shell of the counter flow path 9 and serves to transfer a part of its heat content to the approaching high pressure gas. In the lowest part of the counter flow path 9(a) a further temperature decrease takes place when the high pressure gas is isoen-t-halpically expanded through the expansion valve 14 whereby the gas approaching valve 14 is cooled. A part of the operating or working gas is liquified by the expansion through valve 14 and is collected in the container 15, whose bottom and side or casing surfaces represent the actual condensation surface. An equilibrium will be estabished between the evaporation of the fluid by the heat generated by the pumping process and the liquifaction thereof. The gas circulation described so far corresponds to the normal gas liquifier except that in accordance with the present invention the bottom and side wall surfaces of the container 15 are also part of the metal shell 1 which projects into the vessel.

If helium is used as the operating gas in such a circuit, then the condensation surface will assume a temperature which corresponds approximately with the boiling temperature of helium, that is 4.2 K. The possibility exists to maintain a pressure in container 15 significantly lower than 1 atmosphere when the gas flow through the expansion valve 14 is adjusted so that a reduced pressure is maintained on the suction side of the compressor 4 and thus also on the low pressure side the heat exchangers 5 and 9. At temperatures below 3.5" K. hydrogen can also be pumped through condensation on the outer walls of container 15.

For the condensation of air, nitrogen and other higher boiling gases the basic embodiment of the invention is modified to the extent that in FIGURE 1 the lower part of the counter flow path 9(a) and the expansion valve 14 are eliminated and the working gas, helium, coming from the expansion machine 12 flows directly into the collecting container 15 and cools it to a temperature of 6 K. to 20 K. The available refrigerating capacity for the condensation process corresponds theoretically to the enthalpy difference of the working gas between the available temperatures at the gas inlet and outlet of the expansion machine 12. In this case, the collecting chamber 15 takes over the role of pressure equalizing container 13. The heat exchange in 15, which may be degraded in this manner of operation, can be improved by the installation of metal baffles in 15.

A further advantage of the present invention is that the condensation surface which is situated at the lowest temperature may be substantially enlarged since the metal shell 1 is also the outer casing for the recirculated low pressure gas of the whole or the lower part of the counter flow path 9 and assumes a temperature which allows the gas to be pumped off to be condensed.

The present invention further provides that the actual condensation surface may be shielded from the vessel by baffle plates 16, which are made of a good heat conducting material having a high radiation reflecting power. Their functions are on one hand to offer the gases to be pumped no flow resistance and on the other hand to reduce the thermal velocity of the incident molecules in that the molecules are being held at a noticeably lower temperature with respect to the temperature of the surroundings and a part of the thermal energy of the gas molecules is being absorbed by impact on the baflie plate. The sticking probability for the condensation is increased in this way. According to the present invention these baffle plates act as a protection against the radiant energy from the walls of the vessel and decrease the temperature of the actual condensation surface under equilibrium. These bafile plates with a latticed structure are in a good thermal contact with the precooled container 7 or with the parts of the counter flow paths 5, 6 and 9 whose temperature is substantially lower than the temperature of the surroundings and lies preferably in that of the boiling liquid air. Because the operating gas is present in the inside of the casing 1 under normal conditions and helium or hydrogen possess good heat conductivity properties, it is suflicient to transfer the heat withdrawn by the bafiie plates through the heat conductance between the casing 1 and the cooler 7 or to the counter flow path through a narrow gas gap.

A gas lubricated piston machine with or without control through mechanically or electrically operated valves for the inlet or outlet of gas is preferably used as the expansion machine 12 and the work output thereof is absorbed and carried away by an electric generator 17. A well known purifier 18 is inserted between the compressor 4 and the pump at the beginning of pumping to exclude impurities in the working gas. The purifier is by-passed in the continuous operation.

The present invention provides further that through the shut down of the expansion machine and the discharge of the precooled bath from cooler 7 the pump container 1 very quickly reaches higher temperatures and the required bake out for the attainment of an ultrahigh vacuum can be performed without any special additional precautions. Since the Joule-Thomson effect for helium and also for hydrogen is negative at room temperature, the working gas which reaches the expansion valve 14 without precooling and expands there is heated and finally reaches a temperature of several hundred degrees centigrade.

It has already been mentioned that the cooling capacity necessary for the condensation of the gas to be pumped is relatively small and that small gas cooling equipment is sufficient for large pumping capacities of such apparatus. These advantages are provided by the present invention.

Another embodiment of the present invention provides that not only the refrigeration part of the gas cooling device is accommodated inside the actual pump chamber but also the primary compressor.

An example of this arrangement is shown schematically in FIGURE 2. The pump includes again a jacket or casing 19, preferably of stainless steel, having low heat conductivity which is fastened vacuum tight by a flange in the inside of the vessel 20 to be pumped. Within the pump container 19 is placed a closed circuit gas cooling apparatus, which for example consists out of gas lubricated piston machines for the compression and adiabatic expansion of the working gas and the counter current heat exchangers. In FIGURE 2 the compressor 21 is provided with a differential piston 22 and is built together with the expansion machine 24 in a thin walled metal tube 23 of poor heat conducting material in such a way that any gas leakage collected in 23 can be led into the inside of the pump container 19 or through the connection 25 toward the intake side of the compressor 21. The embodiment shown in FIGURE 2 also has the advantage that the mechanical work performed by the expansion machine 24 directly provides a part of the compression work. In addition, in this embodiment, a piston rod is provided within tube 23 which mechanically couples the working pistons 21 and 27 through two ball and socket joints without impairing the lateral guidance of the gas lubricated parts. The common operation of both piston machines 21 and 24 occurs through a further piston rod 28 supported by universal joints and connected to a crosshead 29 at its upper end. Crosshead 29 is operated by a connecting rod 30 from a crankshaft 31 which is supported, gas tight, in the piston casing 32. The electric motor 33 which acts the common prime mover for machines 21 and 24 theoretically has only to carry the difference between the compressor capacity and the expansion capacity. The compressor 21 is fastened on the cover flange of the casing 32 through a tube 34, which is preferably made out of stainless steel the same as tube 23.

The thermodynamic circuit of the gas cooling apparatus in this embodiment is different from the first embodiment shown in FIGURE 1 and from all other known apparatus because of the quasi-isothermal compression of the working gas at a very low temperature, preferably that of liquid air or liquid nitrogen, and thus the piston displacement becomes smaller in proportion to the corresponding temperatures by approximately a factor of 1:4 as compared to the corresponding water cooled machine. Thus, it becomes possible to accommodate the compressor 21 in the inside of the pump container 19. The cooler 35 which has preferably been cooled with liquid air together with the tube coil 36 to which the working gas is being supplied through the pressure valve 37 of the compressor 21 serve to remove the heat of compression. The cooler 35 is filled with liquid air through a filler tube 38 which also serves as an evaporation relief tube. The compressed working gas cooled in 35 enters the counter current heat exchanger 39 which is represented in FIGURE 2 as a cross counter flow path and consists of the tube coil 40 and two cylindrical sheet metal covers. The high pressure gas is discharged from the lower, cold end of 39 into the pressure equalizing reservoir 41 and from there into the expansion machine 24 where it is adiabatically expanded to a lower pressure and strongly cooled. This cold low pressure gas enters through tube 42 into container 43 which is formed between the bottom 44 of the pump container 19 and the intermediate bottom 45. The bottom and side surfaces of 43 form the condensation surfaces etfective for pumping and are at the lowest temperature of the cooling circuit. After absorbing the heat of condensation during pumping, the working gas leaves the container or space 43 flowing back through the low pressure part of the heat exchanger 39 where it is heated and returns to compressor 21 through the suction valve 46.

The advantage of the present invention can be seen in that gas cooling apparatus is arranged in the inside of the pump where it is hermetically sealed from the outside and in consequence of the special construction of the compressor and expansion machine, impurities of the working gas are excluded. It therefore does not require any purification apparatus, because it always operates with the same gas charge. According to the invention the Vessel can in addition be pumped from the outside to a higher selected vacuum, in this way the cooling capacity which is approximately proportional to the pressure can be easily changed and adjusted to the pumping requirements. After the shut off of the cooling apparatus its elements are heated and through this the gas pressure is increased because of the limited gas supply the equilibrium pressure established is not very high and can be reduced by using a pressure equalizing container on the outside of the pump. The fact that the pump according to the present invention is not fixed but portable and requires only an electrical supply of energy for its operation is a great advantage of this form. The lowest attainable condensation temperatures lie at 6 K.; by a partial by-pass of the compressor and decreasing of the number of revolutions, arbitrary higher temperatures can be established and maintained. The example of the construction shown in FIGURE 2 is only a variation in which the helium gas is not liquified. Naturally the possibility exists here, as well as in the previous embodiment to add cooled battle plates.

The explanation of the present invention is very specific in the example in FIGURE 2. The invention naturally provides for the gas compressor and the expansion machine to be separate units operated by a common crankshaft from the outside but without requiring equal piston strokes.

I claim:

1. A cryopump for removing gas from a vessel comprising: a compressor, a pump casing extending into the vessel to be evacuated, a countercurrent heat exchanger in said casing, and connecting means forming a closed circuit for connecting said compressor to said heat exchanger and said heat exchanger to said machine and said machine to said compressor, a portion of the outer wall of said pump casing forming the condensation surface for said cryopump.

2. A cryopump according to claim 1 wherein the pump casing is formed with a container for liquified gas adjacent the outer wall Which forms the condensation surface of the cryopump.

3. A cryopump according to claim 1 wherein the compressor is mounted inside the pump casing and a common shaft means connects said compressor to said expansion machine.

4-. A cryopump according to claim 1 wherein a cooler is included in said closed circuit by said connecting means for carrying away the heat of compression generated by said compressor.

5. A cryopump according to claim 1 wherein the heat exchanger is of the cross flow counter current type having a shell at the lowest temperature in the heat exchanger, said shell forming conduit means for back-flowing low pressure gas from said expansion machine.

6. A cryopump according to claim 1 wherein a plurality of baffle plates are thermally connected to said heat exchanger and provide radiation protection for said condensation surface.

7. A cryopump according to claim 1 wherein a bypass connection is provided in said connecting means to selectively by-pass said expansion machine whereby the casing may be heated by operation of said compressor.

References Cited by the Examiner UNITED STATES PATENTS 3,081,068 3/1963 Milleron 62-55.5 3,103,108 9/1963 Santeler 6255.5 3,144,200 8/1964 Taylor 62-55.5

LLOYD L. KING, Primary Examiner.

Claims (1)

1. A CRYOPUMP FOR REMOVING GAS FROM A VESSEL COMPRISING: A COMPRESSOR, A PUMP CASING EXTENDING INTO THE VESSEL TO BE EVACUATED, A COUNTERCURRENT HEAT EXCHANGER IN SAID CASING, AND CONNECTING MEANS FORMING A CLOSED CIRCUIT FOR CONNECTING SAID COMPRESSOR TO SAID HEAT EXCHANGER AND SAID HEAT EXCHANGER TO SAID MACHINE AND SAID MACHINE TO SAID COMPRESSOR, A PORTION OF THE OUTER WALL OF SAID PUMP CASING FORMING THE CONDENSATION SURFACE FOR SAID CRYOPUMP.

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