GB2034410A - Rotary positive-displacement fluid-machines - Google Patents

Abstract

A vacuum pump with an inlet 8, an exhaust duct, Fig. 1, and a pumping mechanism which includes a working chamber 3 and possibly a rotor 4 and blades 28, has an anti-suck back valve comprising a valve member 22, which closes the inlet 8 when the pump stops and is responsive to the pressure difference between the inlet 8 and the working chamber 3. There may be two rotors disposed in tandem and situated in series-connected chambers, Fig. 1. <IMAGE>

Description

SPECIFICATION Vacuum pumps This invention relates to improvements in or relating to vacuum pumps and particularly relates to such pumps incorporating anti-suck back valves.

A problem frequently associated with vacuum pumps is caused by flow back into the apparatus to which the pump is attached as the pump is stopped, or accidentally breaks down. The majority of two stage vacuum pumps use oil as a lubricant particularly in the low vacuum stage and the net result of stopping the pump is that there is an increase in pressure in the low vacuum stage which causes a corresponding increase in pressure in the high vacuum stage thereby causing a flow back through the pump into the associated apparatus. This flow back can even cause oil to be drawn from the pump contaminating the apparatus.

To overcome this problem it is sometimes the practice to provide vacuum pumps with anti-suck back valves which cut off the inlet pipe when the pump stops thereby ensuring that there is no flow back into the apparatus to which the pump is attached. These valves normally comprise electrically operated solenoid valves associated with the inlet passageway and arranged to close as the power is switched off. Alternative arrangements include centrifugally operated switch mechanisms which activate the valve or separate oil pumps which operate to prevent oil or air entering the high vacuum stage.

Such anti-suck back valves tend to increase the overall cost of the vacuum pump and where electrical solenoid valves are fitted, they do not overcome all modes of failure.

It is with these problems in mind that the present invention has been devised.

In accordance with the present invention there is provided a vacuum pump comprising an inlet, an exhaust and a pump mechanism including a working chamber, an anti-suck back valve associated with said inlet operable to close when the pump stops to prevent flow from the pump to said inlet, said valve including a valve member which is actuatable by a pressure difference between pressure at said inlet and pressure within said working chamber.

In one arrangement the vacuum pump comprises a pump body, an inlet and an exhaust, a pump mechanism which is operable to cause flow from the inlet to the exhaust. The pump mechanism includes a working chamber in operative communication with the inlet and the exhaust. An anti-suck back valve is provided which is adapted to adopt a closed condition when the pump stops, the valve including a valve member which is actuatable in response to a pressure difference between the inlet and the working chamber.

The vacuum pump may include a bleed-off passage in operative communication with the working chamber and one side of the valve member. The other side of the valve member communicates with the inlet the arrangement being such that, in use, the valve member will close the inlet when pressure in the bleed-off passage is greater than pressure at the outlet.

Preferably the inlet includes an inlet passage with a valve seat in or at one end thereof. The valve member may include a piston element and a sealing head arid is movable between and open position and a closed position in which the sealing head seats against the valve seat that movement being in response to pressure difference between the inlet and the working chamber.

Preferably the piston element communicates with the bleed-off passage.

In one form the pump mechanism includes two working chambers which are in communication via a transfer passage one of the working chambers communicating with the inlet and the other working chamber communicating with the exhaust. Preferably the bleed-off passage communicates with said transfer passage.

The valve member may also be a flexible diaphragm which in a closed position is arranged to seal the inlet passageway. It is understood that the present invention is equally applicable to a single stage vacuum pump, the valve being closed by a pressure difference between the inlet passage and the pressure within the working chamber of the single stage pump downstream of the inlet.

One embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a cross-sectional side-on view of a vacuum pump in accordance with the present invention; and Figure 2 is a sectional view taken along the line ll-ll of Fig. 1.

The vacuum pump illustrated in Figs. 1 and 2 comprises a housing 1 defining a first low vacuum chamber 2 and second high vacuum chamber 3 joined by a transfer port 9. The housing also defines an oil reservoir 4 which communicates with the low vacuum stage via an oil feed duct 5. A passageway 10 is also provided between the two chambers 2 and 3 to locate coaxial shafts 1 2 and 11 of two rotors 14 and 1 3 located respectively in the high vacuum and low vacuum working chambers 3 and 2. Alternatively a single shaft may be provided the two rotors being secured thereto. The shafts are keyed together and support annular oil passageways 1 5 and 16, the oil passageway 1 5 on the high vacuum side being serviced by a free standing oil head 26.The rotors are driven by an externally mounted pulley 27 and the rotors are secured within the housing 1 via a pair of end plates 19 and 20.

An inlet pipe 8 extends through the oil chamber 4 and communicates with the working chamber 3 of the high vacuum stage via a port 1 7 and subsequently connects with port 9 illustrated in Fig. 2. The exit from the high vacuum stage to the low vacuum stage is angularly displaced to the inlet port 1 7. The transfer port 9 connects the two chambers 2 and 3 and an exhaust duct 6 connects the low vacuum chamber 2 to the oil reservoir 4 via a one way valve 7.

Fig. 2 illustrates the eccentric mounting of the rotors 1 3 and 1 4 and illustrates the sealing blades 28 which in effect produces the pumping action within the pump. The two stage vacuum pump as described at this stage is a conventional design of vacuum pump and although contributing to the present invention does not constitute the novel features of this invention.

The novel features of the present invention are particularly illustrated with reference to Fig. 2. A bleed-off passageway 24 is provided to communicate with the transfer duct 9 between the two chambers 2 and 3. The bleedoff passageway 24 terminates in a working space 21 in which is located the free end 21 a of a valve member 22. The valve member comprises a valve sealing head 23 supported by a valve stem 22 which acts as a piston and is a close sliding fit in a passageway 25 provided within the housing to connect the inlet port 1 7 with the bleed-off passageway 24. The valve sealing head 23 is fitted with a flexible sealing member (not shown) and is arranged to be located within a chamber 1 8 provided at the base of the inlet pipe 8.

In the normal operation of the pump, the valve member 22 assumes the position shown in Fig. 2 and the gas is sucked through the pump via the inlet 8, the chamber 18 into the high vacuum stage 3 and then into the low vacuum stage via the transfer port 9 and to oulet via the outlet passageway 6. Also in normal operation oil for sealing the clearances in chamber 2 is drawn in through passage 5 and along the shaft journal in end plate 1 9.

When the pump stops or breaks down oil will continue to flow into chamber 2 from passageway 5 or from passageway 6 if valve 7 does not seal completely. This has the effect of causing an increase in pressure within the low vacuum stage 2. Normally this increase in pressure causes a subsequent increase in pressure throughout the pump and therefore into the high vacuum chamber via the transfer passageway 9 and eventually into the inlet pipe 8. In certain circumstances this increase in pressure can be such that oil is drawn through the low vacuum chamber into the high vacuum chamber and to enter the inlet pipe 8 and thereby contaminate apparatus to which the pump is attached.

However, in the present invention the increase in pressure also causes an increase in pressure within the bleed-off passageway 24.

Because of the close clearances of rotor 14 and blades 28 in chamber 3 the pressure rises considerably in passageway 24 before a significant pressure increase is communicated to chamber 1 8. Accordingly, the pressure differential between the face of the valve sealing head 23 which is subject to the vacuum within the inlet pipe 8 and the increased pressure within the working space 21 communicated via passageway 24 causes the valve piston 22 to lift so that the valve sealing head 23 engages an annular seat 29 defined by the periphery of the base of the inlet pipe 8. With the valve in this position the inlet pipe 8 is closed from the high vacuum chamber of the pump and therefore there is no possibility of oil or air entering the apparatus to which the pump is coupled.If the pressure within the pump increases further, for instance, by being open to atmosphere, the pressure differential will increase still more, the effect of which merely increases the seal of the valve sealing head on the associated valve seat.

In normal usage, the pressure differential between the base of the inlet pipe 8 and the bleed-off passageway 24 is so small that it will not lift the valve member 22 so the valve member assumes the open position shown in Fig. 2. Furthermore, as the pump is started up, the pressure within the transfer passage 9 drops until the pressure within the bleed-off passageway 24 is so low that it will no longer lift the valve 22 against gravity and therefore the sealing head 23 and valve steam 22 drop to the position shown in Fig. 2 and allow flow through the inlet pipe 8.

The anti-flow back valve described above has the advantage that it is simple in construction and does not require complicated means such as a solenoid or centrifugally operated switches and yet effectively seals the inlet 8 from the pump when the pump ceases to operate. Furthermore, it is quite simple to provide within the casting which forms the housing 1 of the pump, the necessary bleedoff passageway 24 and port 1 7 into which the simple valve member 22 can be located.

The location of the underside of the valve sealing head 23 against the adjacent surface of the port 1 7 as well as the close running fit of the piston 22 in the passageway 25 prevents excessive gas flow between the bleedoff passageway 20 and inlet passageway 8.

Although a piston type member valve is illustrated in Fig. 2, it is understood that within the spirit of this invention a diaphragm or other suitable valve may be used to seal off the inlet passageway. It is also understood that the present invention is not restricted to a two-stage vacuum pump and it is possible to incorporate a similar system in a single stage vacuum pump, the pressure differential being set up between the inlet passageway and a pressure within the working chamber downstream of this passage.

Claims (8)

1. A vacuum pump comprising, a pump body, an inlet and an exhaust, a pump mechanism which is operable to cause flow from said inlet to said exhaust, said pump mechanism including a working chamber in operative communication with said inlet and said exhaust, an anti-suck back valve which is adapted to adopt a closed condition when the pump stops said valve including a valve member which is actuatable in response to a pressure difference between said inlet and said working chamber.

2. A vacuum pump according to claim 1 including a bleed-off passage in operative communication with said working chamber and one side of said valve member the other side of said valve member communicating with said inlet the arrangement being such that, in use, said valve member will close said inlet when pressure in said bleed-off passage is greater than pressure at said inlet.

3. A vacuum pump according to claim 1 or claim 2 wherein said inlet includes an inlet passage with a valve seat in or at one end thereof, said valve member including a piston element and a sealing head and movable between an open position and a closed position in which said sealing head seats against said valve seat said movement being in response to a pressure difference between said inlet and said working chamber.

4. A vacuum pump according to claim 3 when appended to claim 2 wherein said piston element communicates with said bleed-off passage.

5. A vacuum pump according to any preceding claim wherein said pump mechanism includes two working chambers which are in communication via a transfer passage one of said working chambers communicating with said inlet and the other said working chamber communicating with said exhaust.

6. A vacuum pump according to claim 5 when appended to any one of claims 2 to 4 wherein said bleed-off passage communicates with said transfer passage.

7. A vacuum pump comprising an inlet, an exhaust and a pump mechanism including a working chamber, an anti-suck back valve associated with said inlet operable to close when the pump stops to prevent flow from the pump to said inlet, said valve including a valve member which is actuatable by a pressure difference between pressure at said inlet and pressure within said working chamber.

8. A vacuum pump substantially as herein described with reference to the accompanying drawings.