US2902206A - Vapour vacuum pumps - Google Patents

Description

p 9 B. D POWER 2,902,206

VAPOUR VACUUM PUMPS Filed July 2, 1956' .NVENTOR Baa/Y D Pow r,

ATTORNEY 2,902,206 Patented Sept. 1, 1959 2,902,206 VAPOUR VACUUM PUMPS Basil D. Power, Crawley, England,

High Vacuum Limited, Crawley, England, company assignor to Edwards a British The present invention relates to vacuum pumping systems of the type employing a vapour diffusion pump and has for an object to provide an improved apparatus for, and method of operating a pumping system of the type referred to.

Vapour vacuum pumps, such as the so-called oil or mercury operated diffusion and booster pumps, are frequently used on operating cycles in which the space being pumped must be periodically opened to the atmosphere whilst the pump is at working temperature.

Isolation valves have heretofore been used in such cases, so that the pump can be isolated at low pressure whilst the space being pumped is at atmospheric pressure thereby avoiding danger of damage to the heated pump working fluid which might result from its exposure to the atmosphere. Thus in operation of the system it was necessary to isolate the vapour pump when the system was opened up to atmosphere and then before bringing in the vapour pump to rough the system out by the rotary pump on a by-pass line. This meant three valves, an expensive high vacuum valve at the mouth of the pump, a valve in the backing line and a by-pass valve.

Pump working fluids such as silicone pumping fluids are now, however, available which can withstand exposure to the atmosphere whilst at operating temperature without suffering significant damage, so that, by employing such fluids it has been possible to dispense with the valves mentioned and merely to shut down the mechanical backing pump and switch off the electric heaters of the diffusion pump when it is required to open the system to atmosphere. It is preferred, especially in systems required to operate on rapid repetitive cycles, to employ an isolating valve in the line to the backing pump rather than continuously to stop and start the backing pump and conveniently the valve is operated electro-magnetically so that by a suitable circuit arrangement the electric heaters of the vapour pump are switched off when the valve is closed and switched on when the valve is open, the backing pump being operated continuously.

Thus, during the idle period of the vapour pump, the pump (or parts of the pump if vapour-line-valving is employed), is normally at working temperature and exposed to atmosphere, and is closed off from the backing pump by the valve in the backing line. Under these conditions difiiculties arise in that a vapour or mist of the vapour pump working fluid migrates from the mouth of the vapour pump. Some of this mist may settle on surfaces of a system connected to the pump for evacuation thereby and cause contamination which may in some cases interfere with the effectiveness of the vacuum process, or, if no system or vessel is connected to the pump, the mist may be breathed by, or enter the eyes of the operator causing considerable irritation.

In the case of a quartz crystal frequency calibration plant working on quick cycles and pumped by a silicone oil diffusion pump and a mechanical backing pump operating as described, it was found that satisfactory calibration of the crystals, which involves the deposition on to the crystals of a thin loading vapour metal film, was not possible due to back-migrating oil mist and vapour from the hot diffusion pump contaminating each crystal during the brief periods after placing the crystal in the vacuum chamber and before vacuum was established, and after release of the vacuum at the end of the coating operation but before removal of the crystal from the chamber. Such contamination produces frequency changes after calibration of the crystal and thus invalidates the effectiveness of the operation.

Similarly in the case of aluminizing cathode ray tubes and using the pump operating method described, the oil mist migrating from the mouth of the pump during the changing over from a finished tube to a fresh tube, causes contamination of the tube screen and other parts of the freshly loaded tube before vacuum can be established, thereby adversely affecting the subsequent processing and performance of the tube.

It is an object of the invention to avoid the aforementioned difliculties and according to the invention there is provided in a vacuum pumping system of the type referred to, a means for producing a small, low velocity flow of air into the mouth of the vapour pump while the pump is exposed to the atmosphere. The said flow of air is such that it does not interfere with arrangements being made in the space to be pumped, does not seriously accelerate fluid decomposition in the vapour pump, and does not cause significant fluid carry-over into the backing pump. It is found however that such a small, low velocity of air is completely effective in preventing the aforementioned mist migration from the mouth of the vapour pump. 7

The said flow of air can be produced in a variety of ways, for instance by keeping the backing pump running and closing by a suitable amount a valve interposed between the vapour pump and the backing pump, before admitting air to the space being pumped; or by providing a small auxiliary pump suitably throttled, or a small auxiliary fan, which may be switched on when the backing pump is stopped. In a preferred arrangement according to the invention, in a vacuum pumping system comprising a diifusion pump backed by a mechanical pump and having a backing valve between the two pumps, the backing valve is provided with a fixed bypass so dimensioned as to permit the mechanical pump to provide the desired small, low velocity flow of air during the periods when the pumped space is at atmospheric pressure and the backing valve is closed.

One form of apparatus suitable for carrying the invention into effect will now be described with reference to the sole figure of the accompanying drawing illustrating in diagrammatic form a vacuum pumping system according to the invention for the aluminizing of cathoderay tubes.

In the drawing there is shown a diffusion pump 1 arranged for evacuating the cathode ray tubes one of which is shown at 2 fitted to the mouth of the pump 1. A backing pump means shown as a mechanical backing pump 3 has its suction end connected to the discharge outlet of the diffusion pump 1 by means of a pipe line 4 in which is included an electro-magnetically operated valve means 5 for isolating the diffusion pump from the backing pump when it is desired to interrupt the pumping action during the removal of an aluminized tube 2' and replacement by a fresh tube to be evacuated. Conveniently the electric circuit of the valve 5 is coupled with the circuit of the electric heaters (not shown) of the boiler of the diffusion pump 1 so that when the valve is closed to obstruct the flow of gas the heaters are turned off.

The connection between the backing pump means and the diffusion pump means also includes a fluid econo- .3 miser -6 adapted to intercept droplets of the dilfusion pump operating liquid which may be carried over in the stream of gas during operation of the backing pump.

Conveniently the said economiser is of the form described in our British Patent No. 748,376 which comprises an elongated housing through which the stream of as is caused to flow, the said housing having a plurality of pairs of apertured plates extending across the path of the gas and spaced lengthwise of the housing. Each pair of plates comprises a first apertured plate for dividing the stream of gas into a plurality of accelerated streams, and a second apertured plate spaced from the said first apertured plate in the direction of flow and having its apertures angularly off-set with respect to the apertures of the first plate thereby providing surfaces upon which the said accelerated streams impinge so as sharply to change their direction of flow and release therefrom droplets of pump fluid entrained in the gas.

The connection between the backing pump and the diffusion pump furthermore includes a by-pass for the valve in the form of a narrow bore tube 7 connected at one end to the outlet from the economiser 6 and at the other end to the inlet to the backing pump 3. in this manner, when the valve 5 is closed and the heaters of the diifusion pump are turned off, the tube 7 ensures that a gentle current of air is drawn in through the mouth of the diffusion pump by the backing pump acting through the tube 7, which currentof air is sufiicient to prevent the migration of vapour or mist of the diifusion pump liquid from the mouth of the pump into the freshly placed cathode ray tube before pumping is started.

The length and bore of the tube 7 will depend upon a number of factors such as for example the size and type of the diffusion pump, the design of any baflle system or connecting piece between the diffusion pump and the vessel to be evacuated, and the displacement of the backing pump. If necessary the tube can be coiled as at 8 in order to accommodate the required length.

I claim:

1. A vacuum pumping system'comprising vapour diffusion pump means having a mouth for connection to a chamber to be evacuated and a discharge outlet for discharging gases evacuated from said chamber, backing pump means having a suction end connected to said discharge outlet, valve means closable for obstructing flow of gases from said discharge outlet to said backing pump means, and means providing a restricted passage having a'pre-determinedimpedance between said discharge out let and the suction end, of said backing pump means when the said valve means is closed, thereby to producea low velocity flow of air into said diffusion pump means through said mouth while said mouth is exposed to atmosphere with said valve means closed.

2. A vacuum pumping system comprising a vapour diffusion pump having a mouth for connection to a chamber to be evacuated and a discharge outlet for discharging gases evacuated from said chamber, a backing pump having a suction end connected to said discharge outlet, a valve operable to a closed position for obstructe ing flow of gases from said discharge outlet to said backing pump, and a small bore tube connecting the dis charge outlet of said diflfusion pump to the suction end of said backing pump thereby to provide a restricted passage having a predetermined impedance between said discharge outlet and said suction end of the backing pump when the valve is operated to the closed position so that there is produced a low velocity flow of air into said month while the mouth is exposed to atmosphere and the said valve is closed.

References Cited in the file of this patent UNITED STATES PATENTS

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