US2722371A

US2722371A - Vacuum pumping apparatus

Info

Publication number: US2722371A
Application number: US308464A
Authority: US
Inventors: Arthur S D Barrett; Power Basil Dixon
Original assignee: Edwards High Vacuum Ltd
Current assignee: Edwards High Vacuum Ltd
Priority date: 1951-09-17
Filing date: 1952-09-08
Publication date: 1955-11-01
Anticipated expiration: 1972-11-01

Description

Nov. 1, 1955 A. s. D. BARRETT Er AL VACUUM PUMPING APPARATUS 2 Sheets-Sheet 1 Filed Sept. 8, 1952 Nov. l, 1955 A. s. D. BARRETT ETAL 2,722,371

VACUUM PNHSING APPARATUS Filed Sept. 8, 1952 2 Sheets-Sheet 2 F/G. Q.

United States Patent() VACUUM PUlVIPING APPARATUS Arthur S. D. Barrett, Shortlands, and Basil Dixon Power, West Wickham, England, assignors to Edwards High Vacuum Limited, Crawley, Sussex, England Application September 8, 1952, Serial No. 308,464

Claims priority, application Great Britain September 17, 1951 4. Claims. (Cl. 230-105) This invention relates to vacuum pump apparatus for use in producing very low pressures as for example in exhausting the envelopes of electronic valves, X-ray tubes and so on.

The mechanical pump, usually of the rotary type, 1s widely used for a variety of high vacuum applications. It is simple to instal and to use and gives long periods of mechanically trouble-free running even when required repeatedly to exhaust containers for atmospheric pressure on quick production cycles. However, this type of pump, while able in the two-stage form to reduce the air pressure in a system to a very low value, can only maintain this performance if its lubricating uid remains uncontaminated by for example water vapour, and exhausts in any case at very reduced speeds at low pressures.

Mechanical pumps are not alone capable of really efficiently producing the desired low pressures for hard vacua devices but they are commonly employed as backing pumps in conjunction with vapour stream pumps (diffusion pumps or booster pumps) which are capable easily of attaining much lower pressures than the mechanical type of pump and can maintain high pumping or exhausting speeds at low pressures. Moreover, the vapour stream pump can be made insensitive to uid contamination during use.

In practice, if it is required repeatedly to exhaust products from atmospheric pressure, the vapour stream pump must be isolated and by-passed by a suitable arrangement of valves until the mechanical pump, working alone, has completed the initial part of the evacuation and only then can the vapour stream pump be safely brought into use. If a valve arrangement is not used and instead, all the gas being pumped is passed through the hot vapour stream pump during each operating cycle, then the working fluid of the pump may be oxidised and decomposed. Further, the rush of air, will during each cycle, carry some of the working fluid of the pump over into the mechanical pump where it will mix with the mechanical pump oil. Eventually therefore, all the vapour stream pump fluid will be lost with resultant lack of economic working. In the case of vapour stream pumps of the booster type a very considerable mass ow of air may be pumped by the vapour stream pump during its normal operation and this air flow may carry with it into the rotary pump considerable quantities of working iluid in the form of mist or vapour.

Pumping fluids of improved chemical stability have recently become available and the risk of serious iluid decomposition can be eliminated in a vapour stream pump of suitable design even if air is passed in quantity over the hot fluids. However, the certainty of severe iluid loss from the pump under such conditions remains.

The object of the present invention is to provide vacuum pump apparatus which presents the advantages of a combined mechanical and vapour pump installation and which is adapted to quick cycle operation without the disadvantages of the consequences of uid loss from thevapour pump or the complication of vacuum valves.

ri 1C@ According to the present invention, in a vacuum pump installation comprising a mechanical and a vapour stream pump, a common working uid is employed which provides lubrication and sealing for the mechanical pump, and the working fluid for the boiler of the diifustion PumP- According to a feature of the invention, the combination of the mechanical pump and the vapour stream pump is such that any iluid loss which may occur from the vapour stream pump to the mechanical pump is replaced by a return feed from the mechanical pump to the vapour stream pump boiler.

Advantageously the common working iluid employed in a pumping installation according to the invention, is of a chemically stable type such that it will not decompose when subjected to the operating temperature of the pumping installation. The said common fluid should also have a low vapour pressure for efiicient operation of the vapour stream pump, and a viscosity and lubricity suitable for the eicient operation of a mechanical pump made of conventional materials. Suitable iluids for use as the common uid in the practice of the present invention include silicone type iluids such as those known as DC 702 or DC 703, or a chlorinated diphenyl iluid, for example Aroclor 1254. Such an installation possesses the ability repeatedly to exhaust products from atmospheric pressure on short production cycles without valves and without trouble due to uid loss. The installation also has the ability to pump to pressures far lower than could a plain mechanical pump and at speeds greatly in excess of those associated with the mechanical pump alone.

In a particular application of the invention, a pump installation consists of a rotary pump, which may be of standard design, as a backing pump for a vapour stream pump as in present practice, but a fluid feed is provided between the reservoir of the rotary pump and the boiler of the vapour stream. The feed is provided by a pipe leading from the reservoir of the rotary pump through a control valve to a trough situated in the inlet pipe of the rotary pump. This trough is connected by a further pipe to the vapour stream pump boiler so that fluid supplied to the trough from the rotary pump reservoir keeps the vapour stream pump boiler filled to the right level. Any excess fluid overows from the trough into the rotary pump mouth and so back to the reservoir.

In an alternative application of the invention to an installation employing a two stage mechanical pump and a vapour pump, the supply to the vapour pump boiler is maintained from a small container associated with a duct between the two stages of the mechanical pump. Some of the iluid flow maintained through the mechanical pump for lubrication is by-passed through the small container to keep it iilled. This method of operation has the advantage that the outgassing of the fluid supplied to the small container does not significantly aifect the fore pressure provided by the mechanical pump for the vapour stream pump-since there is a mechanical pump stage between the container and the fore side of the vapour stream pump.

The particular applications of the invention referred to will now be described in greater detail, as examples, with reference to the accompanying drawings in which:

Figure 1 shows diagrammatically a combination of a diffusion pump with a single stage rotary pump,

Figure 2 shows, also diagrammatically, a combination of a diiusion pump with a two stage rotary pump and 4Figure 3 shows a modication in detail.

Referring to Figure l, the diffusion pump 1 is of conventional two stage design, having a heater 2 for a boiler 3 which is partly lled with a working uid. A single stage rotary pump 4 is housed within a tank 5 containing fluid which is of the same kind as that contained in the boiler 3 and serves to seal and lubricate the pump 4. The interior of the tank 5 is maintained at atmospheric pressure Vby va vent opening 5a in the top ofthe tank. The interior of the pump casing is connected by a duct 5 `.to .the interior -of the diffusion pump 1, the duct thus providing the backing connection. A fluid supply pipe `7 connects lthe interior of the tank 5 with a trough 8 in the duct 6, a valve 9 being provided to control the flow .of fluid from the tank to the trough under the conditions ,about to be described. The trough communicates with `the boiler 3 4through a pipe 10.

In operation, the valve 9 is adjusted so that when the duct 6 is at sub-atmospheric pressure a very slow stream of working fluid is drawn from the tank 5 and runs into the trough S which is normally already full with the result that the extra fluid causes a slow overflow .from the troughS and duct 6 into the interior of the `pump .4. This fluid passes through the pump 4 in the normal way, back into the tank 5 via a valve 11. Under these normal conditions the fluid surface in the boiler 3 is below the fluid surface in the trough 8 because the surface of the fluid in the boiler 3 is depressed by the `boiler pressure above it.

:lf for any reason, such as the passage of a large quantity of air through the pumps, some fluid is lost from `the diffusion pump 1 to the mechanical pump 4, then the level in the boiler 3 drops, the level in the trough 8 also drops and the fluid feed from the pipe 7 serves to rell both the trough 8 and, through the pipe 10, the boiler 3 to their normal levels before the normal overflow of fluid from the trough 8 recommences. It will thus be seen that the diffusion pump boiler never befusion pump 1 is of the same form as shown in Figure 1 and is connected by a duct 6 to the interior of a two stagerotary backing pump 12. An inter-stage transfer duct 13 is provided with a rising pipe 14 which carries fa small sealed container positioned so that the pipe 14 and the container co-operate to constitute a constant head supply tank for working fluid. The container 15 is connected to the boiler 3 of the diffusion pump by a pipe 16 and a further pipe 17 connects the container 15 to a hole in the bush of the end bearing 18 of the rotary pump. A duct 19 connects the well 20 of the bearing 18 with the interior of the tank 21 housing the pump and containing the working fluid which, as in the case of Figure l, is of the same kind as that contained in the boiler 3 of the diffusion pump 1. The interior of the tank 21 is maintained at atmospheric pressure by a vent `opening 21a in the top of the tank.

In normal operation, sub-atmospheric pressure condi- Qtions exist inside both pumps 1 and 12 and a small flow -of fluid is drawn from the tank 21 to the bearing 18 through duct 19. Some of this fluid enters the pump 12 to lubricate it in the usual way and some is drawn up the pipe 17 into the container 15, the rate of fluid ow being controlled by small clearances in the bearing 18. The container 15 thus normally remains full of fluid to the top of the pipe 14 and as fluid enters the container 15 through pipe 17, an equal quantity of fluid over-flows If, for any reason, fluid is lost from the boiler 3, perhaps for example, by being carried over as vapour or mist into the pump 12 due .-to a large rush of air through the pumps, then the levels of fluid in the boiler 3 and also in the container 1S fall and the fluid feed to the container 15 through the pipe `17 serves to refill both the container 15 and the boiler 3 to their normal levels before normal over flow down the pipe 14 recommences.

The fluid entering the container 15 normally contains dissolved gas which is liberated under low pressure in the container. As however, a stage of the mechanical pump 12 is operating between the container 15 and the duct 6, gas liberated in the container 15 will cause only a very limited pressure rise in the duct 6 and the pressure in that duct will usually remain adequately low for effective backing of the diffusion pump 1.

A modification of a detail of the arrangement shown of Figure 2 is illustrated in Figure 3.

In Figure 3 it will be seen that the container 15 of Figure 2 has been eliminated and :the three

pipes

14, 16 and 17 form a junction such that fluid entering the junction along pipe 17 will normally flow down pipe 14 to the inter-stage duct of the mechanical pump, pipe 16 remaining full to the junction, of static fluid.

Any fluid loss from the boiler 3 of the vapour stream pump will cause the pipe 16 to become partially empty of fluid, and fluid supplied along pipe 17 will now enter pipe 16 until boiler 3 and pipe 16 are relled to their normal levels.

Also in Figure 3 the pipe 16 is shown leading into the bottom of boiler 3 and `continued up for some way inside the boiler 3. This is to ensure that any sudden surges of pressure which may occur, tending to drive fluid out of the boiler 3 along the pipe 16 will be unable to empty boiler 3 below the level of the end of the pipe 16 in the boiler. Thus, sufficient fluid will in such cases, be left in the boiler to operate the pump until the boiler can be refilled along pipe 16.

It will, of course, be appreciated that the invention Vmay find application to combined mechanical and vapour stream pump installations other than those specifically mentioned and it will be clear that other forms of installation may be designed in which, in utilising the present invention, a common main reservoir for working fluid is arranged so that the fluid circulating through the mechanical pump and any fluid lost from the vapour stream pump is returned to the common reservoir from which the working fluid is supplied to both the mechanical or vapour stream pumps as required. The return feed of the fluid may be effected automatically as in the particular forms of installation described, but in installations in which .the loss of working fluid from the boiler is not great, the return feed may be effected periodically under pressure supplied by a small hand pump, or by suction induced by the mechanical pump itself and via a control tap in cases in which the diffusion pump is at a higher level than the mechanical pump. Gravity feed under tap control may be used in cases in which the reservoir of the mechanical pump is at a higher level than the boiler of the diffusion pump.

We claim:

1. Vacuum pump apparatus comprising a vapour stream pump having a boiler in which the working fluid of the pump is vaporised, a fluid-sealed and lubricated mechanical backing pump connected to the outlet of the vapour pump, a reservoir adapted to contain the sealing and lubricating fluid of the backing pump, and means communicating the said reservoir with the boiler of the vapour pump whereby a common fluid is employed for `the working fluid of the vapour pump and for the sealing and lubricating fluid of the mechanical backing pump.

2. Vacuum pump apparatus comprising a vapour stream pump having a boiler in which the working fluid of the pump is vaporised. an inlet and an outlet to said vapour pump, a mechanical backing pump of the` rotary fluid-sealed and lubricated type having an inlet port and an outlet port, a reservoir adapted to contain the sealing and lubricating fluid of the backing pump at atmospheric pressure, a duct connecting the outlet of the vapour pump to the inlet port of the mechanical backing pump, a trough formed in the wall of said duct intermediate the length thereof, a pipe connecting the said reservoir with the trough whereby fluid ilows from the reservoir to the trough when the said duct is at sub-atmospheric pressure, valve means in said pipe for controlling the said flow of fluid therethrough, and a further pipe connecting the said trough to the boiler of the vapour pump whereby fluid delivered to said trough from the reservoir is conveyed to the boiler of the vapour pump.

3. Vacuum pump apparatus comprising a vapour stream pump having a boiler in which the Working fluid of the pump is vaporised, an inlet and an outlet to said vapour pump, a two stage rotary mechanical backing pump of the fluid sealed and lubricated type, a transfer duct connecting the first stage of said backing pump to the second stage thereof, a duct connecting the outlet of the vapour pump to the inlet of said rst stage of the backing pump, a reservoir adapted to contain uid at atmospheric pressure for sealing and lubricating the said backing pump, a pipe connecting the said reservoir to a closed fluid container in communication with the said transfer duct whereby when the said transfer duct is at subatmospheric pressure, fluid is drawn into the said chamber from the reservoir, and a further pipe connecting the said chamber to the boiler of the vapour pump for delivering to the boiler the fluid drawn into the chamber from the reservoir.

4. Vacuum pump apparatus as claimed n claim 3 wherein the mechanical backing pump is submerged in the fluid container in the reservoir and the said pipe connecting the reservoir to the closed chamber is connected to the lubricant feed duct of a bearing of the said mechanical pump.

References Cited in the ile of this patent UNITED STATES PATENTS 1,346,408 Lovell July 13, 1920 2,206,093 Hickman July 2, 1940 FOREIGN PATENTS 213,460 Switzerland May 1, 1941