US2919061A - Vapour vacuum pumps - Google Patents

Description

Dec. 29, 1959 B. D. POWER 2,919,061

VAPOUR VACUUM PUMPS Filed Aug. 29, 1955 B 16 ower 1 W, flyorne K United States Patent VAPOUR VACUUM PUMPS Basil Dixon Power, Crawley, England, assignor to Edwards High Vacuum Limited, Crawley, England Application August 29, 1955, Serial No. 531,049

Claims priority, application Great Britain August 31, 1954 4 Claims. (Cl. 230-101) The present invention relates to vapour vacuum pumps. In vapour vacuum pumps of the condensation type (the so-called booster and diffusion pumps), and also of the ejector type, the vapour jets emerge from the nozzles, over a wide range of operating conditions, into a region of comparatively low, or extremely low gas pressure.

There is associated with all such vapour jets a phenomenon commonly known as backstreaming which consists of the more or less random backward migration of vapour molecules from the region of the pumping stage, against the pumping direction, and in numbers greater than would be accounted for by the mere reevaporation of vapour molecules from the cooled pump walls at a rate appropriate to wall temperature.

Among the factors which may contribute to backstreaming are the following:

i. The random inter-collision of vapour molecules in the vapour jet resulting in some molecules acquiring velocity components in the backstreaming direction.

ii. Incomplete accommodation of the vapour molecules to the cooled wall temperature of the condensing vapour at first impact (i.e. inefficient condensation).

iii. Random evaporation of liquid from any wet patches on the outside, or lips, of the hot vapour nozzles.

iv. The over expansion or over divergence of the vapour jet as it leaves the nozzle.

Backstreaming constitutes a back migration of vapour molecules into the system being pumped by the vapour pump, at a rate in excess of the theoretical minimum rate i.e. the rate that would provide a vapour pressure just equal to saturation pressure at the temperature of the cooled wall of the pump.

Saturation pressure at system temperature is therefore very likely to be exceeded and heavy condensation of pump fiuid vapour inside the system, resulting in severe system contamination, is likely to result.

Backstreaming, because it constitutes a screen of vapour molecules moving in a direction opposed to the normal pumping direction, might be expected to have an adverse effect on the pumping of gases, by the pump due to collision between the backstreaming vapour molecules and the forward moving gas molecules hindering the migration of the gas molecules into the pumping stage.

This adverse pumping effect is in fact very often negligible, but where severe over-divergence of the jet or jets of the pump is one of the factors contributing to backstreaming, the effect on pumping speed can be very serious, the over-divergent vapour which reaches and condenses on the cooled walls also powerfully contributing to the reverse pumping action.

It is an object of the invention to provide means for controlling backstreaming in a vapour vacuum pump by reducing the divergence of the jet or jets of the pump, and according to the invention, a vapour vacuum pump is provided with means for controlling the shape of the vapour jet of a stage of the pump by removing from the jet most of the vapour molecules thereof which have a velocity component opposed to the direction of pumping.

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Reference will now be made to the accompanying drawings in the respective figures of which the same reference characters designate common parts and in which:

Figs. 1a and 1b illustrate diagrammatically known forms of vapour vacuum pump nozzles,

Fig. 2 illustrates the nozzle of Fig. la together with a cooled wall surface,

Fig. 3 illustrates diagrammatically the desired form of a vapour jet issuing from the nozzle shown in Fig. 2, and

Figs. 4, 5, 6 and 7 show in diagrammatic form vapour pump structure in accordance with the invention.

A vapour jet issuing from the nozzle of a vacuum vapour pump into aregion of low pressure is liable to diverge so as to have a large included angle especially when the pressure ratio is very large, and this is particularly the case with the known organic pumping fluids where the vapour molecules are large and complicated so that the ratio of specific heats at constant pressure and constant volume approaches unity. The angle of divergence may indeed be more than in many cases as is shown diagrammatically in Fig. 1(a) and Fig. 1(b) illustrating the vapour jet J (dash lines) from the nozzles D and D' of a condensation pump and an ejector pump respectively. The nozzle of Fig. 1a will be referred to more fully hereinafter for illustrating the teachings of the invention and comprises a frusto-conical cap 14 mounted over the outlet 16 of a vapour tube or chimney 18 supplied at its lower end from a boiler or generator (not shown). The nozzle has a discharge end 20 with a lip 22. Much of the over-divergent part of the vapour jet reaches the cooled pump walls (not shown) and is condens'ed in the normal way, but in severe cases, some over-divergent vapour may escape from the pump mouth and contribute to backstreaming. It will be apparent from a consideration of Figs. la and 1b that those vapour molecules of the jet which are first encountered by the gas being pumped as it approaches and enters the pumping stage have components of velocity in a direction opposed to the pumping direction. The proportion of vapour molecules with such unfavourable velocity components is still further increased if the condensing wall of the pump is of frusto-conical shape as shown at A in Fig. 2 (cooling means being represented by coils 24) as this unfavourably modifies the mean direction of the gas being pumped, indicated by the arrows B, with respect to the over-divergent part of the jet. It may in fact be stated that part of the jet J working above a line C perpendicular to the pump wall and passing through the nozzle lip 22 exerts its pumping action in the reverse direction.

In considering the pumping action as the gas molecules migrate towards the pumping stage, even with a well formed jet it is found that a proportion of the gas molecules will make unfavourable collisions with vapour molecules and will be driven back from the pumping stage. With an over-divergent jet, this proportion will be considerably increased. However most of the gas molecules will migrate a substantial distance into the vapour jet and will be driven largely in the direction of vapour flow i.e. towards the wall of the pump. Thus the gas molecules carried by the vapour jet against the wall A will tend to be driven along it, either in the direction of or against the direction of pumping according to whether they are on the downstream side or upstream side of the line C. Those driven against the direction of pumping will emerge at the top of the pumping stage and will have to be repumped.

If the over-divergent part of the jet could be removed, the pumping action could be speeded up, and also the risk of part of the jet emerging from the pump mouth would be substantially eliminated. It would in fact be desirable to remove rather more of the jet than the part upstream of the line C, as shown in Fig. 3 so as to ensure that all the jet molecules have positive components of velocity in the pumping direction along the pump body wall. I

' The aforementioned means for controlling the shape of the vapour jet of a stage of a vapour vacuum pump consists of a nozzle assembly in which an adequately -cooled guard ring surrounds the nozzle of the jet and is posishown in Fig. 4 in which a water cooled guardring E is arranged around the nozzle D of a condensing vapour vacuum pump. The ring may be cooled by conduction cooling, or alternatively, refrigerative cooling may be employed (cooling coil 26 being illustrative). The ring E extends below the lip of the nozzle D and effectively condenses the grossly over divergent portion of the jet,

and other backward moving vapour molecules in the immediate vicinity of the nozzle D.

The guard ring E is preferably of frusto-conical form as shown in Fig. 4, the inclination of the inner wall of the ring with the respect to the axis of the nozzle D (i.e. the length of chimney 18) being such that any gas entrained in the vapour intercepted and condensed by the guard ring is driven along the inner wall of the ring by the vapour bombardment, in the direction of pumping.

In an alternative construction, the upper edge portion of a guard ring E extends upwardly and over the cap of the nozzle D as shown in Fig. 6 to intercept any random evaporation from the exterior or from the lip of the cap. In such case the ring B may be in the form of a second cap superposed upon the cap of the nozzle in spaced relation therewith, the inner surface of the second cap being bright and polished to reduce radiation loss from the nozzle.

It is to be noted that the guard ring condenses only vapour which exerts little or no forward pumping action, i.e. only vapour detrimental to the performance of the pump. Moreover in many pumps which are called on to handle a high mass throughput of gas, the jet is not sufficiently divergent while'the high throughput is being pumped to require the control of the guard ring; in fact the jet may not impinge upon the guard ring. The jet mayonly diverge excessively when a low pressure has been attained abovethe pump and under such low pres sure conditions, pumping speed increases of over 60% have been obtained by the use of the guard ring, anda reduction of up to 98% in the rate of back-streaming'has been achieved.

Fig. 5 shows a guard ring applied according to the invention to each of the first and second stages 27 and 28 of a multi-stage pump. The nozzle D of stage 28 is substantially identical to the nozzle of Fig. 4, having the same frusto-conical cap 14 and guard ring E. The chimney has successive sections 18a and 18b, the outlet 16' of section 18a being surrounded by the frusto-conical lower part 14' of section 18b to form a nozzle D. The lip 22 of part 14 is surrounded by another guard ring E. It will be apparent that the final shape of the jet is determined by the shape, and positioning of the guard ring and it is possible by suitably shaping the jet, to position the nozzle above the mouth of the pump and intercept and condense by means Of t e guard ring all those vapour molecules which would normally flow from the jet over the top of the condensing wall of the pump.

A guard ring as describedis most beneficial in vapour pumps employing vapours with complicated molecules which give rise to the most divergent jets. It may be applied however, with beneficial ..results to all forms of vapour vacuum pumps including mercury and steam operated pum'ps.

v In order to intercept theover divergent portion of the jet, the guard ring extends below' the lower end of the nozzle and thus performs a function quite distinct from that of the conventional bafiles which are provided in order to prevent the random backward migration of molecules.

The guard ring according to the invention may be used quite independently ofany system of bafiles but, when it is used in a pump provided with conventional baflles, it tends greatly to reduce the load on the baflles.

In a furthermodification of the construction shown in Figure 4, the vapour tube 18 is flared at the nozzle D" (as shown'at 30 in Fig. 7) which has'a cylindrical cap 14 and the ring E of Figure 4 takes the form ofa second cylindrical cap E" superposed on the nozzle cap in spaced relation therewith. This arrangement makes it possible to locate the nozzle outside the pump mouth.

I claim:

- 1.-'In a vapour vacuum pump, means for producing a vapour jet, a cooled wall surface in the path of said jet for condensing 'vapour from said jet, said wall having an inlet and-an outlet, said outlet being down-stream of the jet, said jet producing means comprising a nozzle assembly which includes a nozzle, said nozzle having a discharge end for discharging a jet of vapour, said discharge end having a lip, a guard ring surrounding the nozzle in closely spaced relation to the lip, and means for cooling the guard ring, said guard ring extending in the direction of the jet a substantial distance beyond said lip and into the'jet of vapour, thereby to condense the undesirably over-divergent portion of the vapour jet as it emerges from under the nozzle lip and also to condense molecules emerging at random from under the nozzle lip, with the result that undesirable constituents of the vapour jet are removed almost completely or to a substantial, extent from the jet before it leaves the nozzle assembly. I

2. In the pump of claim 1, said nozzle having an outwardly inclined wall with a discharge end at which said lip is located, said guard ring closely surrounding said wall and having a wall outwardly inclined and in opposed spaced relation with the nozzle wall.

3. In the pump of claim 2, said nozzle wall and said guard ring wall being frusto-conical.

4. In the pump of claim 1, said nozzle assembly including an upwardly extending vapour tube having an open upper end, and said nozzle including a cap sup; ported over the upper end of said vapour tube in spaced relation therewith, said cap having a downwardly extending wall with a lower discharge end surrounding and spaced from said vapour tube and forming said lip, said guard ring extending over said nozzle cap to form a second capspaced from the nozzle cap.

References Cited in the file of this patent UNITED STATES PATENTS 2,237,806 Bancroft Apr. 8, 1941 2,406,017 Hickman Aug. 20, 1946 FOREIGN PATENTS 1,096,377 France June 20, 1955

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