US2753096A - Method of effecting diffusion-type evacuation - Google Patents

Description

2 Sheets-Sheet l Mrs ATToEv.

C. A. KDNER [vETI-IGD OF' EFFECTING DIFFUSION-TYPE EVACUATION gully 3, E955 Filed Jan. 2&3, 1952 Absolue Pressue Absolue Pressue Absolu'e Pressure (Milimicrons of Mercug Juny 3 *i* c. A. KIDNER 2,753,096

METHOD OF EFFECTING DIFFUSIGN-'l YPE EVACUATION Filed Jan. 28, 1952 2 Sheets-Sheet 2 FMA (viliimcrons of Mercury) 2O 25 55 eoMinutes (Millimicons of Mecuy) 2O 25 30 35 40 45 50 55 OMinUBS FIG. 2C

55 60 nutes IN VEN TOR.'

CHARLES A. KIDNER HIS ATTORNEY.

lllnited States Patent METHD F EFFECTING DIFFUSION-TYPE EVACUATIN Charles A. Kidner, Elmwood Park, Ill., assigner to The Rauland Corporation, a corporation of Illinois Application January 2S, 1952, Serial No. 268,479

4 Claims. (Cl. 226--20.2)

This invention relates to a novel process for effecting evacuation or an enclosed chamber through the use of a diffusion-type vacuum system. It is useful in any application in which evacuation of enclosed chambers to very low pressures is desirable and is particularly valuable in connection with the manufacture of electron-discharge devices such as cathode-ray tubes; the description of the invention will therefore be directed to an application of the latter type.

The utilization of dilusion-type pumps for evacuating the envelopes oi electron-discharge devices is a general practice throughout much of the industry. ln operating equipment of this type, the evacuating stack ot the diffusion pump is normally maintained at a temperature below ambient throughout the evacuating process. When this normal mode of operation is employed, the pumps may be subject to contamination due to the condensation ci voiatilizable contaminants on the internal surfaces of the pump stack and on the stack extensions which are frequently used to connect the pump to the envelope being evacuated. The presence of such contaminating elements in the evacuating chamber produces the same eiiect, insofar as effective operation of the pump is concerned, as would be caused by presence of a small hole in the pumping system. The continuing accumulation of contaminants in the evacuating chamber eventually makes it impossible for the pump to attain the desired low pressures and the apparatus must then be removed from service and cleaned. The cleaning process is a difficult and time-consuming project and is doubly disadvantagcous economically in that it necessitates the maintenance oit additional pumping units to replace those requiring service and also necessitates the employment ol additional personnel to perform the cleaning functions. Furthermore, a considerable shrinkage loss in the elcctronsdischarge devices being processed is often oecasioned, since in many installations it is impractical to maintain a continuous check on the pressures achieved in the chamber being evacuated and discovery of poor vacuum conditions is made only upon evidence of faulty operation of the devices after they have been removed from the evacuating system. Moreover, it is usually necessary to discard the difiusion pump oil which is removed during the cleaning process and to operate the pump for a lengthy brealt-in period before it may be returned to service.

l t is an object of this invention, therefore, to provide a new and improved method ot edecting ditfusion-type evacuation ot an enclosed chamber.

lt is an additional object of the invention to reduce shrinkage losses in the ultimate product resulting `from poor evacuation due to the presence ol' contaminants in the evacuating system.

lt is a further object of the invention to provide a method of effecting diffusion-type evacuation which materially reduces maintenance and cleaning costs with respect to the evacuating systems employed.

it is another object of the invention to provide an 'cvacuating process which permits achievement of lower vacuum pressures in less time than is required by known methods.

It is a specific object of the invention to achieve the desired results without the necessity of adding expensive or complicated equipment to existing pump installations7 and simultaneously eiectively to reduce the amount of diffusion pump oil required to replace that lost in the cleaning process.

lt is also an object of the invention to provide an evacuating method which will permit reduction in the required brealoin period tor new oil in a diffusiontype evacuating system.

Accordingly, the invention provides a method ot evacuating an enclosed chamber including an envelope and the exhaust stack of a dilfusion-type pump, the latter having a 'liuid reservoir connected to the exhaust stack, a fluid supply, and a jet-discharge structure connected to the fluid reservoir and extending into the exhaust stack. The inventive method comprises the step of heating the tluid to vapor state to establish a high-velocity vapor discharge through the jet-discharge structure into the exhaust stack, thercb'y lowering the pressure within the stack relative to the remainder ot' the chamber to evacuate the envelope. During a irst portieri of the evacuation process, the exhaust stack is cooled to a temperature below ambient to condense the vapor on the stack walls and effectively remove the vapor from the stack. "the stack is then heated above ambient temperature `for a predetermined period to volatilize any contaminants collccted therein while continuing the vapor discharge into the stael; in order to remove the contaminants from the stack walls. Subsequently, the exhaust stack is recooled, without materially interrupting the vapor discharge, in order to continue eflicient removal of vapor from the stack. Heating and cooling of the stack may be repeated cyclically for optimum results.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention itself, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in which:

Figure l is a schematic diagram, partially in section, of an illustrative example of an evacuating system in which the methods of the invention may be employed;

Figures 2A., 2B, and 2C depict a family of curves in which the eiiects achieved through the application of the invention are compared with the results obtained by the practice of prior-art methods; and

Figure 3 shows a group of curves graphically illustrating the advantages of employing another embodiment of the invention in relation to results achieved by known methods.

The evacua'ting system of Figure l includes two pumping units, a diffusion pump l@ and a mechanical pump ll. Mechanical pump il may be any one of several suitable types and needs no further description here, Diilusion pump 1li includes a heat source lil which is located at the base ot' an oil reservoir 13 in position to apply heat to a localized portion of the reservoir and thus to a supply of oil 14 contained therein, The par-- ticular form of pump illustrated features two operational stages. The low pressure stage includes a riser tube l5 capped by an umbrella-shaped baille plate lo. The base of tube l5 is sealed to the door of oil reservoir above heat source l2; a small opening 17 in the base of tube 1S permits access to that portion of reservoir i3 external to the tube. The high pressure stage of the pump corn prises a risertube 18 which is concentric with and external to 'tube 15 and is capped by a baille plate 19. This stage 'of 4pump 10 is connected to the surrounding yarea of reservoir 13 by opening 17 in line with opening 17 in riser tube 1S, thus providing for a continuous supply of oil 14 to the interior of riser tubes l5 and 18. The high and low pressure stages of the pump occupy the operating portion of an evacuation stack 2li which extends above reservoir 13 in encompassing relation to the aforo mentioned riser tubes and baffle plate struc-tures. A coo-ling coil 22 is wound around evacuation stack 2i. and also surrounds a high-pressure connection stack Stack. 23 connects the high-pressure stage of pump 1u, through the .upper area off `reservoir .13, to auxiliary mechanical pump 11.

The container to Vbe evacuated, shown as electrondischarge ldevice envelope 24, is connected to stack 2l by means oi: a connecting tube 25. When envelope 24 and tube 25 are properly scaled to stack 21 in the usual manner, these three elements `deline an enclosed cham-f ber; evacuation of any portion .of the chamber entails c evacuation of the whole. it should be noted that the connection between tube 25 and envelope 2d and also that between tube 25 and stack 2l must be vacuumtght in ,order for the system to be eilective; the means for accomplishing .this seal is not illustrated, ,as numerous practical and etlicient methods are well known in the art. Envelope 24 is encompassed by the walls of an oven 26. No means have been sho-wn for supplying heat to oven 26, since there are several practical methods of accomplishing this end, e. g., infra-red lamps, radiant heaters, etc., which are matters of general knowledge and form no part of the instant invention.

in .normal operation, at the start Iof the evacuating process, envelope 2li .is connected -to pump l@ by means of tube 25 which links the envelope with stack 21. Source l2 is activated, locally lheating reservoir 13 and thus oil 14; after suiiicient .interval `the oil is vapor-ized and forced upward through tubes l and 1S with considerable velocity. In .the iirst, or low pressure stage of the pump, the vaporized oil emerging from tube strikes baille plate 16 and is guided by that plate in -t'he path indicated by the arrows A; the conguration of tube l5 and plate i6 concentrates the stream of vapori-Zed oil into a jet as it emerges from the tube. The oil jet impinges upon the internal surface of stack 21 and condenses thereon, returning to oil reservoir 13. The particles of vaporized oil, moving yrapidly downward through s'taclc 2l, tend to collide with any gas molecules which are present in the stack and accelerate them toward reservoir i3. The gas molecules enter reservoir 13 and are .removed vtherefrom by mechanical pump lll. The net result of this operation is a lowering of the pressure at ythe level of `baille pla te i6 in comparison to that prevailing in the remainder' ol' the chamber dened by stack 2l, 'tube 25, and envelope 7.4; ythe evacuation `ot' that chamber is thus initiated. `ln order to insure rapid condensation ol the oil, a `coolant such as cold water (approximately lO-ZO degreets centigrade) is circulated through coil 2210 maintain the normal vtemperature of stack 2l at or below the ambient temperature in which the system is operated, usually about 2O to 24 vdegrees centigrade, A similar process takes place at the point of exit of the vaporizcd oil from riser- 'tube 13 as directed by baille plate i9. Re turn of the condensed oil 14 to the riser tube, where it is again vaporized, is .permitted 'by the openings 17 and 17' connecting the 'riser 'tubes to the surrounding area of oil reservoir i3. The lighter 'and more volatile fractions of oil i4 are vaporized before they reach the internal arca of riser tube 15; it is Athese lighter oil fractions which normally operate through the -hi'gh pressure stage cornprising tube 1S `and baille plate 19. ln order to lmaintain a sutiiciently low back pressure to permit diffusion pump l!) to operate efectively and reduce the pump-down time for the evacuating process, mechanical pump lll is maintained in continuous operation throughout ythe process.

The removal from envelope 24 of volatilizable materials which 'might 4be vaporized when the envelope is later utilized in its normal operation is effected by heat supplied to the envelope by oven 26 to subject the envelope to a temperature substantially higher than `its normal operating temperature and thus vaporize and drive ott those materials, which are then removed by pump itl. This balteeou-t voperation .is normally carried out only vduring the early stages of the evacuating process.

As indicated in the previous description of the evacuating process, stack 21 is normally maintained at a temperature suflciently 'low to provide for rapid condensation of the vaporized oil. This temperature is ordinarily somewhat below ambient, depending on the temperature of the coolant circulated through coil 22; the coolant temperaturc is usually approximately 16 degrees centigrade. As with most mechanical apparatus, this type of pump is not perfect in its operation, and some of the more volatile fractions -of diffusion pump oil 14 escape .during operation of the pump into the upper regions of stack 21 above baffle plate 16, and are then condensed on the wall of stack 21. In the initial `part of the evacuating process, when the pressure in the stack is still relatively high, these contaminating oil fractions do not adversely affect the process, since they tend to remain condensed on the stack wall. However, when lower pressures are reached in the stack, these materials again start to volatilize.

This action has the same effect on the operation of the system as if there were a small leak present in the upper area of the stack, since the continuous vaporization of .these contaminants prevents the reduction of pressure in the stack, and therefore within envelope 24, to the desired point. Further, some of the contaminating materials which are .present in envelope 24 and are vaporized -by the heat from oven 26 in the early stages -of the process may also `condense in the upper portion of stack 21 with the same effect on later y.stages of the process as that exhibited in the ,case of the lighter oil fraction strays. Eventually, the accumulation of these two diierent types of contaminants in the ychamber being evacuated effectively negates `the usefulness of the system and it becomes necessary to remove the pump from operation for clean* ing. AS previously noted, this is a time-consuming and expensive process which, when the pumps are operated in the normal manner, is -necessary approximately every three months, although this period varies considerably. Since the condition of the final product is often the only practical method Vof vdetermining the eiiiciency of a given pumping installation, substantial shrinkage losses are often occasioned through the Vdefective operation of the contaminated pumps. .In addition, where the pump stack is cooled `throughout the baking-out stage of the evacuation process, the y.oil 14 tends to absorb a part of the contaminants driven off from envelope 24 during bakeout, which results in deterioration of the operating properties vof the oil. It is therefore normally impractical `to attempt to reuse the yoil 14 which is removed from the pump in the Vcleaning process. Since this oil is relatively expensive and is `sometimes diiicult to obtain, any reduction in this loss is highly desirable.

The instant invention provides a method whereby the contaminants of both classes previously outlined may be continuously removed from pump 10 during the evacuat ing process without requiring shutdowns for servicing. This is accomplished by subjecting the evacuating stack 21 of pump 10 to a temperature substantially above ambient `temperature during a fractional part of the evacuating process. The application `of heat to stack 21 accelerates volatilization of any `contaminants which may be collected in the stack and prevents further condensation of undesirable materials in the stack. Applicant has found that one very Ieconomical yand. effective means of carrying out thisheat .treatment is fthe circulation of hot water or other liquid through lcoil 22 during ya given fractional portion of the evacuating process. The effects and advantages derived through the application of two embodiments of the invention will be discussed more completely in connection with Figures 2A, 2B, 2C and 3. Briefly, the application of su'licient heat to stack 21 during a portion of the evacuating process eiectively volatilizes and drives oit" any contaminants present in the stack, effecting their removal prior to the linal low pressure pumping achieved near the end of the process and thus avoids the deleterious effects prevailing when these materials are permitted to collect in the stack during the early stages of the process.

Figures 2A, 2B, and 2C illustrate the results of various test runs utilizing the method of the invention, and compare those results with the eiects achieved through the operation of the same equipment under similar conditions in the manner of the prior art. ln each of the figures, the dotted line curve indicates the results of operation according to the teaching of the prior art, while the solid line curve shows the effects of the invention under otherwise identical conditions. Curve 2A was run on a system in which the oil had been used for some time prior to the performance of the tests. The dotted line shows the resulting decrease in pressure plotted against the time of operation of the pump where cooling of the pump stack was carried out continuously throughout the evacuating process. ln accordance with the invention, the second or solid line curve was obtained by subjecting stack 21 of the pump to a temperature considerably in excess of ambient during a irst fractional part of the evacuating process; in this instance the temperature increase was achieved by circulation of Water of approximately 73 degrees centigrade through coil 22 during that portion of the evacuating process. Heating of the stack was continued for the iirst 10 minutes of the evacuating process, at which time the application of heat to the stack was discontinued, and the circulation of cold water through coil 22 was commenced, resulting in a lowering of the stack temperature during the final stages of the process. Only the last hour of each test run is shown, the periods of operation for the two tests being identical. The cold water used in these tests was at approximately 16 degrees centigrade.

in Figure 2B, the results of a test similar to that of Figure 2A are shown, the evacuating process in this case being carried out with fresh unworked oil. Figure 2C similarly shows comparative results of the utilization of the invention and operation according to known methods where the oil employed has been subjected to an implosion, an implosion in this case signifying an opening of the diffusion-type evacuating system to the atmosphere while the pump is operating, whether accidental or otherwise. it will be noted that the curves of Figures 2B and 2C are quite similar and that neither test quite achieves the results shown in Figure 2A. This is due to the fact that the unworked and imploded oils absorb a considerable amount of gas due to their exposure to the atmosphere. The results of this absorption of gas into the oil makes the initial stages of operation of the process extremely turbulent and therefore less eiiicient. It should also be noted that the difference between normal operation and operation according to the invention under these two sets of conditions is even more pronounced than the variation apparent where the oil has been previously worked-in. As a practical consideration, this means that a pump which is newly placed in service or which resumes operation after an implosion becomes effective in its operation after a considerably shorter working-in period than is the case where only the normal methods of operation are employed. Accordingly, the working-in period may be virtually eliminated, since, as indicated by the curves, practical Working pressures of 10 millimicrons or less are achieved after a period only slightly in excess of one hour whereas with normal operation the same equipment requires a run-down period of between 5 and 6 hours. While the results shown in all of the Figures 2A, 2B, and 2C apply to one particular type of pump, corresponding effects have been obtained with. varying pump models and types. Examination of the graphs of these figures reveals that the methods of the invention permit a substantial savingr in time in pumping down to any given pressure within the system capacity, regardless of the condition ofthe oil being used.

Figure 3 graphically represents the operation of a diffusion-type evacuating system according to a second embodiment of the invention. ln obtaining this data, the evacuating stack was subjected to increased temperatures in a cyclical fashion; that is, the stack was heated for the first 15 minutes, cooled during the subsequent 30 minutes, again subjected to an increased temperature for 15 minutes, again cooled, etc. ln order to save space in presenting the results of this operational procedure, and to provide a more intelligible picture thereof, the earlier portion of the test run has been omitted, the iirst values shown occurring in the rst cold portion of the process. The curve W, shown as a dotted line, indicates the pressures achieved when the pump stack was kept at a temperature no greater than ambient throughout the evacuating process, in accordance with the teaching of the prior art. Curves, X, Y, and Z depict the results of methods of the invention; the pump stack was subjected to varying degrees or' temperature increase during the hot portions of the process. All of these tests were run on the same pump and under identical conditions with respect to the oil; this was achieved by running all of the tests with fresh unworked oil drawn from a common batch. Curve X, shown in dash outline, shows the effect of operating temperatures of approximately 34 degrees centigrade during the hot portions of the process cycle; curve Y, in dash-dot line, indicates the etect of an operating temperature of approximately 49 degrees centigrade during the hot portions of the cycle; and curve Z shows the results of heating the stack to approximately 66 degrees centigrade during the hot portions of the process. As indicated by each curve, the application of heat to the stack causes an immediate and precipitous rise in pressure within the evacuation chamber which rapidly reaches a peak and commences to drop ott" again shortly thereafter. The pressure drop tends to level out if the application of heat is continued; lowering the temperature of the stack at the beginning of the cold part of the process causes an immediate sharp drop in the pressure. lf one considers that the objective with respect to the ultimate pressure in the evacuated chamber is of the order of 3 to 5 millimicrons, it is readily apparent that the time required for the evacuating process may be considerably reduced where a suiiciently high temperature is applied during the he portion of the process. As shown in curve Z, a pressure of 4 millimicrons was achieved after approximately 72 minutes of operation, whereas with the temperature used in obtaining curves X and Y, a period of ,approximately 112 to 114 minutes is required. If a pressure of 3 millimicrons is demanded, the omission of the last heat portion of the cycle would probably result in this pressure after approximately to 100 minutes of operation under the conditions of curve Z, whereas a time somewhat in excess of minutes is necessary where the heating of stack 21 is limited to the temperatures of 34 degrees centigrade and 49 degrees centigrade used in obtaining curves X and Y. While it is not readily apparent how long it would take, employing the prior art methods represented by curve W, to achieve either of these pressures, experience indicates that at least 6 hours is necessary.

The improvements in the evacuation process achieved through operation in accordance with the invention are not limited to the particular embodiments described in connection with Figures 2A, 2B, 2C, and 3i. Somewhat higher temperatures may be employed during the portions of the process in which the stack is subjected to an increase in temperature; the above-outlined operations were limited only by the equipment available and the particular method chosen for applying heat to the pump.

v.af/tienes Furthermore, in cyclical operation according to the invention, it is not necessary'that the temperatures achieved during the alternate hot :and cold portions of the `cycle be identical with respect to prior analogous portions, nor 'is it necessary `to maintain the relationship between the duration of the hot and cold portions of the cycle shown in Figure 3. Tests -have been run wherein the heated portion or portions of the process exceeded the cooled portions in length of time; the results, while not vquite so satisfactory `as Vthose indicated herein, still show a marked limprovement over the prior art methods.

In sum, the invention provides a method of effectively evacuating an enclosed chamber which materially reduces `the number of times -that the evacuating equipment must be removed from-service for maintenance purposes. Operation in accordance with the methods of the invention effectively reduces shrinkage -losses in the ultimate product occasioned 'by ineicient operation of the evacuating system. It eliminates a substantial portion of the losses occasioned by the necessity of discarding used ditlusionpump oil and permits achievement of lower pressures in the evacuated chamber in less time than is required by known -methods of operation. Virtually no new equipment is yrequired to practice the process.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without ldeparting from this invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within -the true Spirit and scope of this invention.

I claim:

l. The method of evacuating an enclosed chamber including an envelope yand the exhaust stack of a ditn fusion-type pump, the latter having a fluid reservoir connected to said exhaust stack, a fluid supply, and a jetdischarge `structure connected to said yuid reservoir and extending into said exhaust stack, said method comprising .the following steps: heatingsaid fluid to vapor state to establish .a :high-velocity vapor discharge through said jet-discharge .structure into said exhaust stack, thereby `lowering 4the pressure within Aa portion of said stack relative yto the remainder of said enclosed chamber to evacuate said envelope; cooling said 4exhaust stack below ambient temperature to condense said vapor yon the walls thereof and remove said vapor vfrom .said stack; heating said exhaust stack above ambient temperature for a predetermined period to volatilize any contaminants collected therein while continuing said vapor discharge into said stack, thereby removing said contaminants from said stack; and subsequently re-cooling said exhaust stack without materially interrupting said vapor discharge into said stack to continue removal of said vapor from said stack. v

2. The method of evacuating an enclosed chamber according to claim l in which the last two steps are repeated cycl-ically during Kevacuation of the chamber.

3. The method of evacuating Aan enclosed chamber according to 4claim yl in which said exhaust stack is heated to a temperature above 430" C, during the .penultimate step of the 1recited evacuation process.

4. The method of evacuating a-n enclosed chamber according to claim l in which said envelope constitutes the envelope for an electron-discharge device a-nd in which said envelope is heated to a temperature above the normal operating 'temperature of said device during the initial portion of the evacuation process.

References Cited in the file of this patent UNITED STATES PATENTS l845,670 Thomas r Feb. 26, 1907

Claims (1)

1. THE METHOD OF EVACUATING AN ENCLOSED CHAMBER INCLUDING AN ENVELOPE AND THE EXHAUST STACK OF A DIFFUSION-TYPE PUMP, THE LATTER HAVING A FLUID RESERVOIR CONNECTED TO SAID EXHAUST STACK, A FLUID SUPPLY, AND A JETDISCHARGE STRUCTURE CONNECTED TO SAID FLUID RESERVOIR AND EXTENDING INTO SAID EXHAUST STACK, SAID METHOD COMPRISING THE FOLLOWING STEPS: HEATING SAID FLUID TO VAPOR STATE TO ESTABLISH A HIGH-VELOCITY VAPOR DISCHARGE THROUGH SAID JET-DISCHARGE STRUCTURE INTO SAID EXHAUST STACK, THEREBY LOWERING THE PRESSURE WITHIN A PORTION OF SAID STACK RELATIVE TO THE REMAINDER OF SAID ENCLOSED CHAMBER TO EVACUATE SAID ENVELOPE; COOLING SAID EXHAUST STACK BELOW AMBIENT

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