US3171584A - Diffusion pump assembly - Google Patents

Description

Mrh 2, 1965 cHzKARA HAYAsl-u 3,171,584

DIFFUSION PUMP ASSEMBLY Filad may 14, 1952 ATTORNEY United States Patent O Y 3,171,584 DWFUSEN PUMP ASSEMBLY Chikara Hayashi, Koholm, Yokohama-shi, Slapen, assigner to Nihon Shinku 'Giiutsu Kabushiki Keisha, Yokohamashi, Japan, a hrm Filed May 14, 1952, Ser. No. 194,553 Claims priority, application Japan, May 16, 1961, 3io/16,836; May 2, 1962, 37/17,489 2 Claims. (CI. 23h-4S) This invention relates to an improved diffusion pump provided with a novel means for protecting a vacuum chamber from backstreaming vapor. Backstreaming constitutes a back migration of vapor molecules into the vacuum chamber and is inherent in a diffusion pumping process.

Attainable orders of vacuum or absolute pressure to be established within the vacuum chamber is the sum of the partial pressure of the backstreaming vapor from the pump and the partial pressures of gases evolved from the walls of the vacuum chamber and members mounted therein, and/ or gases introduced therein from the exterior. The partial pressures of gases except that of the backstreaming vapor, if being constituted of a single component, equals to the ratio of the amount of the gases, measured in the unit or" absolute pressure X volume/hour, to the effective pumping speed of the pump. In case the gases consist of several components, the gas pressure is the sum of their respective ratios as above-mentioned.

As seen from the above description, the rising of the orders of vacuum, in other words, the lowering of the absolute pressure Within the vacuum chamber Wholly depends on a lowering of the two partial pressures. As ultra-high vacuum engineering has developed, the emission of gases from the elements or members of the vacuum chamber and/or permeation of gases through the walls of the vacuum chamber have been sufficiently reduced, and manufacturing of pumps having large pumping speed has been facilitated. Therefore, attainment of higher orders of vacuum mainly depends on a lowering of the amount of backstreaming vapors developed from die Working fluid.

Heretofore, to reduce the partial pressure of the backstreaming vapors a cold trap means located between the vacuum chamber and the diffusion pump to catch or hold the backstreaming vapors has been used. The cold trap, however, has disadvantages such that' resistances to the pumping action are increased, consequently the effective pumping speed of the pump will be decreased, and the construction of the pumping assembly will be complicated, and such that when or after the operation of pump is stopped, the condensed volatile component of working uid within the cold trap Will often be reevaporated. Another means is a type of an adsorption trap. The disadvantage of the adsorption trap is that when the adsorption agents are saturated within the vapors of working fluid, the effect of the trap will be decreased. Therefore, some means is urgently required to prevent the baclcstreaming of vapors of the working fluid into the system being pumped by the diffusion pump.

An object of the present invention is to provide a novel and useful diffusion pump provided with means for positively preventing the backstreaming of vapors.

The principle on which the invention is based is Raoults law. For better understanding of the invention, Raoults law is concisely referred to. The Working fluid to be used in the diffusion pump is evaporated and condensed in the system of the pump, and generally consists of many components. For the sake of brevity, however, we may assume that they are divided into two groups, one of which has a low vapor pressure and the other a high vapor pressure at their working temperatures. The former usually has a large molecular weight in comparison with the latter, and constitutes the main part of the Working fluid. Hereinafter, the former is referred to as a main component and the latter as a light distillate, because the latter is of light and quickly evaporated and condensed in comparison with the former by heating and cooling. The existence of the light distillate of high vapor pressure is resulted from the insuilcient purification of material and/or caused by the decomposition of the working fluid due to heat supplied to the pump.

Let Pa and Pb be the vapor pressure of the main component and the light distillate respectively, and MazMb be a molecular ratio of the two components, then the following relations generally hold between them When the Working uid is condensed after evaporation, some part of the condensed product is reevaporated and subiccted to the cyclic process of condensation and evaporation along the pipe line connection the diffusion pump and the vacuum chamber, and finally a part of them arrives at the interior of the vacuum chamber.

According to Raoults law, the ratio of the two components after a first evaporation is PaMazPbMb, and after a second evaporation Pa2Ma:Ph'2Mb and after a nth evapration PanMarPbnMb. Now let Pn be the total vapor pressure over the nth condensed liquid phase, it can be expressed as Then, let lr be the partial pressure of the backstreaming vapor in the vacuum chamber, and P0 be the vapor pressure of the Working fluid condensed with the diffusion pump, then the approximate values of Pr and PD may be derived from Formula l by putting n=n(n l) and n=0,

(2) Pr: nla at f1 1 (3) PoPaMa-kPbMb at 11:0

As mentioned above, MaMb and Ma-l-Mb=1, therefore P0 is sufficiently smaller than Pr.

The inventor has perceived this point and invented a novel means able to reduce Pl. to P0 which corresponds to the partial pressure appearing in the vacuum chamber in case there is no backstreaming, in other words, means able to reduce the amount of the backstreaming to zero.

It is to be noted that the above description is made in reference to a light group composed of a single component, but the same principle can be applied to a group composed of several components.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

The single figure is a diagrammatic sectional view of the invention. I

Referring t'o the single figure there is shown one ernbodiment of the invention where reference numeral 10 indicates a backstrearning check device which serves to prevent the backstrearning vapor of the workingfluid to tend toward a vacuum chamber to be evacuated and which is able to serve as a high vacuum side diffusion pumping device. The backstreaming check device 10 comprises an upright cylindrical casing l2 provided with a head cover il and a boiler 13 mounted Within and at the bottom of the casing l2. The boiler 13 comprises a heater 14, and outer cylinder 15, a cylindrical ejector coaxially disposed with the outer cylinder 15. The interior of the outer cylinder is divided by both a horizontal partition 17 and the ejector lo into three chambers such as an upper chamber 1?, a lower chamber 19 and an evaporation chamber 2t), and the lower chamber 19 is communicated with the evaporation chamber 20 through opening 21 formed in the lower portion of ejector 16. Around the upper portion 22 of the outer cylinder above the partition 17 is providing a cooling means 23. The cylindrical ejector 16 is formed to have at its upper portion a flare adapted for an effective ejection of the evaporated working fluid, and at its lower la diametrically enlarged section to increase the amount of the vapor of the working fluid. By providing an inclined plate 24 Within and at the upper portion of the casing 12, there is formed a jet chamber 25. A cooling means 26 is mounted to cover the most part of the casing 12 under the inclined plate 25. An inlet port 27 is formed at the lower portion of the casing to connect the backstreaming check device 10 to a vacuum chamber C shown in imaginary line. The casing 12 is communicated with the lower chamber 19 of the boiler 13 by a return tube 28 which extends outward from the bottom of the casing 12 and terminates at a portion adjacent the bottom of the boiler 13.

The backstreaming check device 10 is connected to a 'diffusion pump 4t), which is described hereinafter, by a connecting pipe 30 horizontally extending from a portion of the casing 12 underneath the upper edge of the inclined plate 24, a distillation pipe 33 extending upward from the lower portion of the outer cylinder 15 above the liquid level of the working fluid passing a heater 31, turning downward and passing through a cooling means 32, and other connecting pipe 34 extending from a portion of the outer cylinder 15 under the liquid level of the working fluid.

A difusion pump generally indicated by reference numeral 4) is a conventional diitusion pump of the distillation type comprising a casing 41 yand a boiler unit 42 enclosed therein. The casing 41 provided with a cooling means 43 and an outlet port 44 to be connected with a back pump (not shown). The boiler unit 42 includes an outer cylinder 45, an inner cylinder 46 coaxially disposed With the outer cylinder 45 and a heating means 47. The working uid ofV liquid phase in the diffusion pump is substantially separated by the walls of the outer and inner cylinders and 46, but partially communicated With each other through openings 55 and 56 formed in their respective walls. The working fluid stored at the bottom of the diiusion pump 4i) is heated to its evaporation temperature by a heating means 47. The vapor produced from the working fluid within an outer chamber 50 is downwardly ejected through `an annular opening 51 into a pumping chamber 57 deiined by the casing 41 and the boiler 42. The other vapor produced within the inner cylinder 46 is similarly ejected through openings 53 vand 54. These ejected vapor Vstreams effect to catch the gas molecules coming from the vacuum chamber C, through the connecting pipe 3), into the diitusion pumpV 40, and the most part of the ejected'vapors is condensed on the inside surface of the casing 41 being cooled by the cooling means 43, zand then flows down along the Wall of the casing 41 and inally is collected at the bottom of the pumping chamber 57, While a part of the ejected vapors is drawn, by the pumping action of the back pump, through the outlet port 44 into the side of the backing pump. The condensation of Working fluid accumulated at the bottom of the pumping chamber S7 should arrive `at the inner chamber 52 after passing through a roundabout passage which is formed only by provision of the openings 55 and 56. The working iluid is subjected to fractional distillation while passing the passage, and as a result the working iluid arrived at the inner cylinder 52 has a 10W vapor pressure at the working temperature and a large molecular Weight. This low vapor pressure working fluid is supplied into the lower chamber 19 of the boiler 13 by means of the connecting pi'pe 34.

Y In operation of the diffusion pump assembly, the inlet port 27 of the backstrearning check device 10 is connected to the vacuum chamber C, the outlet port 44 of the diiusion pump 44B is connected to the back pump (not shown) and then the back pump is operated. When the interiors of the vacuum chamber C, the backstreaming check device 1t? and the diusion pump 40 are evacuated by the back pump to a vacuum adapted for starting of the diffusion pump 40, cooling mediums of appropriate temperatures are supplied from ia source or their respective sources, such as refrigerating machines or cooling pumps, to the-cooling means 26, 23, 32 and 43 respectively; the heating means 14 and 47 are energized by an electric source(s) to evaporate the working iluids within the backstreaming check device 10 and the ditusion pump 49 and at the same the heater 31 is energized to accelerate Flowing of the vapor in the distillation pipe 33; and thus the backstrearning check device 10 and the diffusion pump 46 are set in operation. By pumping actions of the diffusion pump 4i) and the back pump, the gas molecules within the vacuum chamber are drawn into the ejection chamber 25 and directed to the connection pipe 36 by the vapor upwardly ejected from the flared opening of the ejection cylinder 18, and' introduced into the diffusion pump 41B by way of the connection pipe 36, and subjected to the pumping action of the ditusion pump and finally drawn through the outlet port 44 into the system of :the back pump.

It is to be noted that the vapor ejected from the flared opening of the ejection cylinder 16 Consists of a vapor of low vapor pressure and large molecular Weight, because the working huid from which the ejected vapor is produced, has already been subjected to a fractional distillation by heat before arriving at the ejection chamber 16. The main part of the ejected vapor shall be condensed on the inclined plate 24 which, if necessary, should be kept at an appropriate temperature to effect further removal of the trace of lighter component in the condensate. The lighter component in the condensate on the plate 24 is subjected to reevaporation and migrates through the tube 30 .to the diiusion pump 40. AS a whole the plate 24 expresses a simple means of purication ot the condensate. Therefore its effect can be amplied or intensified by using an additional distillation tower or equivalents. Thus the puried condensate which is composed of enriched low vapor pressure component iiows down along the casing 12, suiciently cooled by a cooling means 26 and arrives at the bottom of the ejection chamber 25 without being reevaporized, and arrives at the lower outer chamber 19 of the boiler unit 13 by means of the return pipe 28 and ows through the opening 21 into the original chamber or evaporation chamber 20.

As is apparent from the above description, the most part of the ejected vapor consists of the vapor having a low vapor pressure at the working temperature and a large molecular weight, but yet it involves a small portion of the vapor having a high vapor pressure at the Working temperature, which vapor is produced from the light distillate.

The upper portion 22 of the outer cylinder 15 including the cooling means 23 serves as a heat insulator for the casing 12 and to condense the vapors impinged thereon.

Referring now to evaporation, condensation and evaporation of the working iluid to be circulated between the backstreaming device 10 and the diffusion pump 40, when the working fluid (liquid) within the lower, outer chamber 19 is heated, the component involved therein and having a high vapor pressure is first evaporated, and the working uid becomes rich in the component having a low vapor pressure.

The working tluid rich in the main component flows through the opening 21 into the evaporation chamber Z0 of the cylindrical ejector 16, and is converted by the eating means l into the vapor to be ejected from the lared opening of the cylindrical ejector 16. The ejected vapor may act as a iine side vapor jet of a diffusion pump. The ejector' i6 may therefore be equipped with .a multistage jet assembly which is more elective as a pumping means. Thus the baclrstrearning check device lil itself is a diit'usion pump. rEhe ejected vapor returns to the original chamber after being subjected to the fractional distillation, as above mentioned. The vapor produced by the fractional distillation of lthe working liuid, and having a high vapor pressure rises in the light distillate pipe 33 and is heated by the heater 3l to accelerate the migra ion of the lighter component to the portion 32 where they are condensed by the cooling means 3;, and flo-w into the diffusion pump 4@ and is collected at the bottom of the pumping chamber 57. As described hereinbefore, the vapor oi the light distillate in the boiler fr? and the gas molecules evolved in 19 introduced through the connecting pipe Si@ linto the diffusion pump llt?, and the vapor produced from the main working iuid and ejected from the openings 51, 53 and d also are condensed and collected at the bottom of the pumping chamber S7. Therefore, the Working fluid in the pumping chamber 57 contains a relatively large amount of the light distillate, but this light distillate may be evaporated during passing its roundabout passage, and the working fluid becomes rich in the main component when it arrived at the inner chamber 52. The resultant Working iiuid is returned to the lower, outer chamber i@ by means of the connecting pipe 34 and subjected to another fractional distillation as described hereinbefore. T ius, both the baclrstrearning checl: device 1t) and the diiusion pump eiiect the fractional distillations of the common working tiuid and act the pumping actions in conjunction with each other. Now that the inside Wall of the lower part of the casing l2 is always kept cool and is covered with a fresh condensate flowing down from the inclined plate 24 to the return tube 28. The fresh condensate is mostly composed of the low vapor pressure iiuid which will not have any appreciable migration velocity to the chamber C. A lighter component backstreaming from the vapor jet from 16 lis eiectively caught by the cold low vapor pressure condensate and its migration to the chamber C is kept minimum as proved by the Raon ts theorem.

A practical example is listed to show the merit of the invention.

Vacuum in torr: Pumping speed in liter second The above data were obtained by using a diffusion pump assembly of the type shown in FIG. l, and having the following particulars:

Backstreaming check Cooling means Working tluid Rate of backstream at the pump inlet 'Less than 0.601 cc./hr.

As is apparent from the practical data above-mentioned, the diffusion pump in accordance with the present invention, such advantages that, it is to obtain higher vacua than those obtainable by previous diffusion pumps and there are no diiiiculties such as a backstreaming of vapor which diiculties are inherent in diusion pumps.

It is to be noted that a coolant to be used in the cooling means may be water trom a water source or cooling medium directly supplied from a refrigerator or brine cooled by the cooling medium from a refrigerator, A thermoelectric cooling element can also be used as a cooling means.

The back pump referred in the description may be a rotary pump or a device employing a molecular sieve or any other auxiliary pump.

Si ce certain changes may be made in the above apparatus and process Without departing from the scope 0f the invention herein involved, 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 i claim is:

l. A diiusion pump assembly comprising a first diffusion pump in communication with a chamber to be evacuated and a second ditusion pump adapted to be placed in communication with a backing pump, said pumps being multi-cha mbered and in series communication with each other, each pump having means for vaporizing the organic working liquid employed therein to catch backstreaming gases, high vapor pressure component in said liquid being fractionated out in said rst pump in one chamber thereof, a second chamber in said iirst pump for vaporizing the low pressure component of said liquid into the main gas stream, means for conducting said thus separated high vapor pressure component to said second pump into the main gas stream Without contaminating the main stream in the first pump, a chamber in said second pump for first fractionating out the high vapor pressure component from the liquid therein into the main gas stream, a second chamber in said second pump for then vaporizing the low vapor pressure component into the main gas stream, means for conducting liquid from said second chamber of said second pump to said iirst mentioned chamber of said irst pump without contaminating the main gas stream.

2. A device in accordance with claim l wherein cooling means are employed at various connecting points in the system to recondense vapor in these regions and prevent the backstreaming of gases.

LAURENCE V. EFNER, Primary Examiner. WARREN E. COLEMAN, Examiner.

Claims (1)

1. A DIFFUSION PUMP ASSEMBLY COMPRISING A FIRST DIFFUSION PUMP IN COMMUNICATION WITH A CHAMBER TO BE EVACUATED AND A SECOND DIFFUSION PUMP ADAPTED TO BE PLACED IN COMMUNICATION WITHA BACKING PUMP, SAID PUMPS BEING MULTI-CHAMBERED AND IN SERIES COMMUNICATION WITH EACH OTHER, EACH PUMP HAVING MEANS FOR VAPORIZING THE ORGANIC WORKING LIQUID EMPLOYED THEREIN TO CATCH BACKSTREAMING GASES, HIGH VAPOR PRESSURE COMPONENT IN SID LIQUID BEING FRACTIONATED OUT IN SAID FIRST PUMP IN ONE CHAMBER THEREOF, A SECOND CHAMBER IN SAID FIRST PUMP FOR VAPORIZING THE LOW PRESSURE COMPONENT OF SAID LIQUID INTO THE MAIN GAS STREAM, MEANS FOR CONDUCTING SAID THUS SEPARATED HIGH VAPOR PRESSURE COMPONENT TO SAID SECOND PUMP INTO THE MAIN GAS STREAM WITHOUT CONTAMINATING THE MAIN STREAM IN THE FIRST PUMP, A CHAMBER IN SAID SECOND PUMP FOR FIRST FRACTIONATING OUT THE HIGH VAPOR PRESSURE COMPONENT FROM THE LIQUID THEREIN INTO THE MAIN GAS STREAM, A SECOND CHAMBER IN SAID SECOND PUMP FOR THEN VAPORIZING THE LOW VAPOR PRESSURE COMPONENT INTO THE MAIN GAS STREAM, MEANS FOR CONDUCTING LIQUID FROM SAID SECOND CHAMBER OF SAID SECOND PUMP TO SAID FIRST MENTIONED CHAMBER OF SAID FIRST PUMP WITHOUT CONTAMINATING THE MAIN GAS STREAM.

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