GB2119443A

GB2119443A - An oil sealed pump

Info

Publication number: GB2119443A
Application number: GB08211916A
Authority: GB
Inventors: William Bruce Trainer; John Alfred Smith; John Wilson
Original assignee: General Engineering Co Ltd
Current assignee: General Engineering Co Ltd
Priority date: 1982-04-24
Filing date: 1982-04-24
Publication date: 1983-11-16

Abstract

An oil-sealed gas-pump 11 for with an oil separator 13 downstream of the pumping chamber 16 and an oil-return path leading back from the oil outlet 21 of the filter to the said chamber. The pump may drive gas through a catalytic convertor 14 and may be of the rotary sliding-vane type (as shown). <IMAGE>

Description

SPECIFICATION An oil sealed pump The present invention relates to an oil sealed pump.

Such oil sealed pumps are used, for example, in the gas circuits of gas lasers in order to draw off a proportion of the main gas flow and pass it through a catalytic converter in order to restore the gas mixture. A problem with such pumps is that oil is entrained in the gas flow through the pump and -this oil, if not removed, contaminates not ooly the convertor but other parts of the circuit such as mirrors. A thin film of oil on the surface of a mirror can drastically alter the optical characteristics of the mirror and render it all but useless for the purpose intended. The resultant cleaning of the mirrors and convertor which is necessary results in wasteful and expensive downtime of the laser.

According to one aspect of the present invention, there is provided an oil sealed pump having an oil cleaning circuit, the oil cleaning circuit comprising an oil filter disposed on the downstream side of the pump and an oil return path leading back from the filter to the pump chamber whereby oil entrained in the fluid flow through the puinp during operation thereof is filtered out of that flow and returned to the pump chamber.

A preferred embodiment of the pump may include one or more of the following advantageous features: (a) the oil sealed pump is a vane pump, (b) the oil sealed pump is a vacuum pump, (c) the vane of the pump of (a) is eccentrically mounted and extendable, (d) the cross section of the, or a part of the, oil return path should be such as to permit not more than 30% and preferably not more than 10% of the rated pumping speed of the pump measured at atmospheric pressure to flow therethrough (rated pumping speed is defined as volume per unit time), (e) the gas outlet from the oil filter leads to the inlet of a catalytic convertor forming part of a gas restoration sub-circuit of a gas laser and the gas inlet to the oil sealed pump is connected to the main gas circuit of the laser, (f) a solenoid operable valve is connected in the oil return path, (g) a non-return valve is connected in the oil return path, (h) a manually operable valve is disposed in the oil return path, (i) the oil filter comprises a porous filtration element disposed in a surrounding container, which defines a gas inlet and outlet and an oil outlet, the gas inlet leading to one side of the porous filtration element.

According to another aspect of the present invention, there is provided a gas circuit for a laser comprising a convertor for restoring the properties of the working gas during operation, an oil sealed pump for pumping the gas through the convertor and an oil filter associated with the pump on the downstream side thereof for removing oil entrained in the gas flow through the pump.

In order that the invention may be more clearly understood, one embodiment thereof will now be described by way of example, with reference to the accompanying drawing, in which: Figure 1 shows a simplified gas circuit diagram of a gas laser, and Figure 2 shows a more detailed view of a part of the circuit diagram of Figure 1.

Referring to Figure 1, the gas laser comprises a gas circuit incorporating four glass tubes through which a gas is drawn at sonic speed. Two of-the glass tubes are reference 1 in Figure 1 , the remaining two being disposed directly behind these two. In a preferred form of laser, the gas comprises a mixture of 6% carbon dioxide, 14% nitrogen and 80% helium gases. A high voltage is applied to the tubes and the molecules of nitrogen are caused to vibrate rapidly and which in turn causes the molecules of carbon dioxide to vibrate as well. When the molecules relax to a low energy state the energy is released in the form of protons of infra-red energy having a characteristic wavelength of 10.6 Mm. Some of this energy is also converted into heat which is conducted away by the helium gas.The protons travelling along the tubes are reflected back and forth by mirrors 2 and .3 disposed at the outer end of the tubes 1. vane of the mirrors 3 is partly transparent to allow transmission of the radiation at 10.6 ,um. This transmittion radiation is released as a beam of infra-red energy which can be focused and employed for laser machining.

The gas mixture is continually circulated in the direction indicated by the arrows through the tubes 1 by means of a Rootes pump 4 which is driven by an electric motor 5. Heat exchangers 6 and 7 for the gas mixture are disposed on the upstream and downstream sides respectively of the pump 4. The main gas circuit comprises right and left hand branches 8 and 9 which respectively conduct the gas mixture from the downstream side of the pump 4 to the extreme free ends of the right and left glass tubes 1. Connected to the main gas circuit is a subsidiary gas circuit indicated generally by the reference numeral 1 0. The purpose of this circuit is to process a proportion of the main gas mixture flow to restore, at least partially, the properties of the gas mixture necessary to satisfactory operation of the laser.

Referring to Figure 2, the subsidiary gas circuit comprises a rotary vane vacuum pump 11 , driven by an electric motor 12 (see Fig. 1), an oil filter 13 and a catalytic convertor 1 4. The convertor comprises three banks 1 4a of catalytic filters connected in series and a "tell-tale" activated alumina filter 1 4b at the input of the first bank which indicates the state of the banks 1 4a of catalytic filters and in particular when cleaning thereof is required. Initially the gas circuit of the system is evacuated via a three way exhaust valve 30 by means of the pump 11 and is subsequently filled with the required gas mixture from a gas mixture supply 31 via a valve 32 (see Figure 1).

During operation of the laser, a small proportion of carbon dioxide and nitrogen is destroyed and the convertor restores this thereby enabling expensive helium gas to be conserved. The rotary vane vacuum pump 11 comprises a extendable vane .15 eccentrically mounted in a circular section pump chamber 1 6. A seal between the pump chamber wall and the vane is maintained by means of a film of oil. During operation of the pump 11, oil is entrained in the gas mixture flowing through it and would, if not filtered out, contaminate the convertor 1 4 and the remainder of the gas circuit.

Even a thin film of oil on the mirrors of the circuit impairs their normal operation and results in diversion of the photon beam. The oil filter 13 therefore acts to remove entrained oil before it reaches the convertor 1 4. The oil filter 1 3 comprises a felt sock 1 7 disposed in a surrounding container 1 8. The interior of the sock 1 7 is connected to the gas inlet 1 9 and the gas mixture flows outwardly from the interior leaving the oil in the felt and exits via a gas outlet 20. The oil then drips from the felt and collects in the base of the container. An oil outlet 21 leads from the container base back via a non-return valve 22, to an inlet 24 into the chamber 1 6 of the pump.The cross sectional dimensions of the, or a part of the, oil return path should be such as to permit not more than 30% and preferably not more than 10%, of the rated pumping speed of the pump measured at atmospheric pressure to flow therethrough (rated pumping speed is defined as volume per unit time). Thus the oil itself is effectively recirculated so that it may once again perform its normal function.

As the vane 1 5 rotates in the pump the inlet 24 is alternately subjected to pressure and vacuum.

When subjected to vacuum oil is effectively sucked out of the filter 13 back into the chamber 1 6 of the pump 11. When subjected to pressure, any tendency of the oil to be pushed back into the filter 1 3 is prevented by means of a non-return valve 25. Pressures which may be experienced during operation of the laser at various points in the circuit 10 are 6 to 7 psi gauge (1070/1 120 Torr) at points A and B, 380 Torr at point C and 100 Torr at point D.

In rotary vane vacuum pumps, a gas ballast inlet duct is usually provided leading to the chamber of the pump. Such an inlet duct is shown in Figure 2 referenced 26 and has been tapped into to provide for the oil return to the chamber 16 of the pump. The normal function of the gas ballast inlet, which is to allow air to be drawn into the chamber 1 6 to reduce or eliminate the tendency of vapour to condense in the chamber and contaminate the oil, may be retained.

With the above described arrangement downtime of the laser previously necessary either to clean the convertors or the mirrors or both is considerably reduced. This in turn leads to increased efficiency and reduced operating and maintenance costs.

It will be appreciated that the above embodiment has been described by way of example only and that many variations are possible without departing from the scope of the invention. For example a manually or solenoid operated valve could be placed in the oil return path in addition to, or instead of, non-return valve 22.

Claims

1. A oil sealed pump having an oil cleaning circuit, the oil cleaning circuit comprising an oil filter disposed on-the downstream side of the pump and an oil return path leading back from the filter to the pump chamber whereby oil entrained in the fluid flow through the pump during operation thereof is filtered out of that flow and returned to the pump chamber.

2. An oil sealed pump as claimed in claim 1, in which the oil sealed pump is a vane pump.

3. An oil sealed pump as claimed in claim 1 or 2, in which the oil sealed pump is a vacuum pump.

4. An oil-sealed pump as claimed in claim 2 in which the vane of the pump is eccentrically mounted and extendable.

5. An oil sealed pump as claimed in any preceding claim, in which the cross section of the, or a part of the, oil return path should be such as to permit not more than 30% and preferably not more than 10% of the rated pumping speed of the pump measured at atmospheric pressure toflow therethrough, rated pumping speed being defined as volume per unit time.

6. An oil sealed pump as claimed in any preceding claim, in which the gas outlet from the oil filter leads to the inlet of a catalytic converter forming part of a gas restoration sub-circuit of a gas laser and the gas inlet to the oil sealed pump is connected to the main gas circuit of the laser.

7. An oil sealed pump as claimed in any preceding claim, in which a solenoid operable valve is connected to the oil return path.

8. An oil sealed pump as claimed in any of claims 1 to 6, in which a non-return valve is connected in the oil return path.

9. An oil sealed pump as claimed in any of claims 1 to 6, in which a manually operable valve is disposed in the oil return path.

10. An oil sealed pump as claimed in any preceding claim, in which the oil filter comprises a porous filtration element disposed in a surrounding container, which defines a gas inlet and outlet and an oil outlet, the gas inlet leading to one side of the porous filtration element.

11. A gas circuit for a laser comprising a convertor for restoring the properties of the working gas during operation, an oil sealed pump for pumping the gas through the convertor and an oil filter associated with the pump on the downstream side thereof for removing oil entrained in the gas flow through the pump.

12. An oil sealed pump having an oil cleaning circuit substantially as hereinbefore described with reference to the accompanying drawings.

13. A gas circuit for a laser substantially as hereinbefore described with reference to the accompanying drawings.