US2404022A - Vapor vacuum pump of the double jet type - Google Patents

Description

P. ALEXANDER'ETAL.

VAPOR'VACUUM-PUMP OF THE'DOUBLE JET TYPE Filed May 26, 1945 Patented July 16, 1946 UNITED STATES PATENT OFFICE Paul Alexander, Berkhamsted, and Cecil Whiley, London, England Application May 26, 1945, Serial No. 596,035 In Great Britain April 21, 1945 1 claim. l

rhis invention relates to vapor vacuum pumps of the double jet type.

Mercury is the liquid usually vaporized in Vapor pumps but other liquids, such as oils, are employed. In pumps of this type mercury vapor from a mercury boiler ascends a central tube, the upper jet being xed at the top of the tube, while the lower jet is xed about halfway up the tube; both jets are supplied with vapor from the central tube, and the vapor issues from the jets into the annular space between the central tube and the outer tube constituting the body of the pump. The efficiency of the pump for a given vacuum depends largely on the density' of the vapor issuing from the two jets and on the downward velocity of the molecules in this vapor.

Now it has been found that, in the described arrangement in which the two jets are supplied with vapor from the same source, it is generally impracticable to design the two jets so as to produce vapor jets with the desired density and velocity. There is necessarily a fall in vapor pressure from the bottom to the top of the central tube to produce a flow up the tube, and the pressure at one of the jets depends on the rate of ilow through the other jet. The two jets therefore are interdependent.

According to the invention, the two jets are supplied with vapor through two non-communicating passages from two non-communicating evaporating surfaces of two communicating masses of liquid. The upper jet may be supplied with vapor through a central tube descending below the level of the liquid in the boiler and separating `the liquid in the boiler into a central portion within the tube and an annular portion outside the tube, and by the lower jet being supplied with vapor from the annular portion of the liquid through a tube between the central tube and the pumping space, and holes in the central tube below the level of the liquid giving communication between the two portions. rPhe tube through which vapor is supplied to the lower jet may be connected with the outer wall of the pumping space through a liquid seal, through which liquid condensed in the pumping space returns to the annular portion of the liquid in the boiler.

In the accompanying drawing, which shows in a single igure by way of example a form of pump embodying the invention in vertical section, a mercury boiler I, containing mercury normally up to the level of the dotted line 2, with heater 3, supports the central vapor tube 4 and the outer tube 5 forming the body of the pump, which terminates above in the inlet 6 of the pump. The upper annular jet l, with annular orice 3 is fixed on the top of the central tube 4. The vapor from the mercury in the boiler, passing up the tube 4 enters the jet 'l through holes I0 in the central body of the jet and issues from the orifice 8 into the pumping space II between the tubes 4 and 5.

The lower jet has its outer member I2 xed to the tube 4, While its inner member i3 is fixed to the top of a tube I4 attached to the outer tube 5 at I5.

The mercury in the boiler is separated into two portions, a. central portion inside the tube 4 and an annular portion between the tubes 4 and 5.V The two portions are in communication lthlrough the holes I9 in the tube 4 below the mercury level 2, and the two portions are heated together by an electric heater in the space 3. Mercury evaporated from the annular portion ascends into the space I6 between the tubes Il and I4 and issues from the lower jets I I3. The evaporation from this portion is independent of that from the central portion, and the vapor pressure in the tube 4 leading to the upper jet may diier from that in the space i8 leading to the lower jet, any diierence in vapor pressure being compensated by a difference of level of the mercury inside and outside the tube 4.

The upper jet, therefore, operates independently of the lower jet, and the quantity of vapor issuing from each jet depends only on the dimensions of the jet and the temperature at which the mercury is maintained.

The outer tube 5 is surrounded as is customary by a water jacket 25, and the outlet 2l of the pump leads from the bottom of the space li to the backing pump. The mercury vapor from the two jets is condensed by the cooled wall 5 and falls into the space 22 between the tubes I4 and 5. A mercury seal is formed at this point by the channel 23, and the condensed mercury passes through holes 24 in the tube I and, overflowing the channel 23, returns to the boiler.

As shown in the drawing, the outer tube 5 pref erably has parallel walls at its upper part surrounding the jet l, the annular space between the tube 5 and jet I of width A determining in large measure the rate of evacuation of the pump.

As described and claimed in co-pending application No. 596,034 the tube 5 diminishes in diameter by the inclusion of a conical wall just below the jet l, which conical wall joins up with the parallel walls of the lower part of the tube 5, so that the tube 5 diminishes in diameter at a point near jet orifice level so that the length of vapor jet measured along a line B with angle 15 degrees from the general pumping direction, indicated by the line C, does not exceed 2.6 times the width A of the annular space at jet orice level.

Further as described and claimed in co-pending application No. 596,036 a water jacket 25 may surround, but not touch, the tube 4, for a length just below the jet orice 3Y downwards, the jacket having inlet and outlet pipes 26 by which it is continuously supplied with cold water.

As fully explained in the aforesaid co-pending application No. 596,036, the eciency of the pump is further increased by the employment of this inner jacket since vapor molecules are thereby prevented from striking the hot tube A and achieving a random velocity which enables such molecules to collide with molecules issuing from the jet 8 and diminish the downward component of their velocity, 't

4 We claim: A vapor vacuum pump comprising a boiler for the vapor, a central tube depending into said boiler and having apertures in its depending part,

' an outer tube Vforming with said central tube an annular space in said boiler, an intermediate tube between said central and outer tubes forming inner and outer annular spaces respectively therebetween, upper and lower annular jets supplied with vapor from the boiler respectively through the central tube and the inner annular space, and

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