FIELD OF THE INVENTION

The field of the invention relates to pumps and in particular to exhaust valves for such pumps.

BACKGROUND

Pumps are used for conveying fluids from an inlet to an outlet. Pumps may act to change the pressure of the gas being pumped. Exhaust valves may be used at the outlet of a pump to impede backflow of any fluids being pumped. Although exhaust valves help to prevent internal leakage, they also act to impede flow and can increase the power requirements of the pump.

The preferred properties of an exhaust valve will depend on the application of the pump. A vacuum pump for example, will achieve a higher ultimate vacuum with a stiffer exhaust valve, however the pumpdown times and power required to attain this ultimate vacuum may be increased with the use of a stiffer exhaust valve.

Conventionally exhaust valves have been configured with the stiffness that is deemed appropriate for the application that they are used in and the properties that are important for that application.

It would be desirable to provide an improved exhaust valve.

SUMMARY

A first aspect provides a pump for pumping a gas from an inlet to an outlet, said pump comprising: an exhaust valve for opening or closing said outlet to allow or impede fluid flow through said outlet; wherein said exhaust valve is configured such that a resistance of said exhaust valve to opening said outlet varies in dependence upon a pressure of said gas being pumped. The inventor of the present invention recognised that the properties of a pump are affected by the resistance to opening or stiffness of the exhaust valve. He also recognised that the properties of the valve which provide improved operation of the pump will change during the pumping process. In particular, a stiffer more resistant exhaust valve may be advantageous in impeding leakage at higher pressure differentials, but may have a significantly detrimental effect at higher fluid flows where it will impede this fluid flow. This impeding of the fluid flow leads to increased power requirements, increased heat generation and a decreased flow rate for the pump.

The inventor recognised that the preferred features of the exhaust valve which change during the pumping process will vary in a similar way to the pressure of the gas being pumped. Thus, were an exhaust valve to be provided with a stiffness that varies in dependence upon the pressure of the gas being pumped, then a pump could be provided where the exhaust valve adapts to the current conditions and allows the operational properties of the pump to be optimised or at least improved across a wide operating range.

In some embodiments, said exhaust valve comprises: a biasing means for biasing said exhaust valve towards a closed position, said exhaust valve being configured such that said biasing force varies in dependence upon said pressure of said gas being pumped.

Although the resistance to the exhaust valve opening may be provided in a number of different ways, in some embodiments, it is provided by a biasing means, the biasing force of the biasing means being controlled in dependence upon the pressure of the gas being pumped. In this way the exhaust valve’s resistance to opening can be controlled such that it varies with and is appropriate to current operating conditions.

In some embodiments, said exhaust valve comprises an actuator configured to constrain said biasing means in dependence upon a pressure of said gas. The biasing force of the biasing means may vary depending on how the biasing means is constrained and thus, can be controlled by using an actuator to constrain the biasing means by different amounts in dependence upon the gas pressure. Where, for example, the biasing means is a spring, holding the spring in a more compressed state will increase the biasing force exerted by the spring. Similarly, where the biasing means is a flexible member, decreasing the length that is free to flex will increase the biasing force required to produce the same amount of flexion.

The actuator may be formed in a number of ways, and in some embodiments, comprises a piston, movement of said piston being triggered by changes in pressure at an inlet of said pump.

A piston, one end of which is linked to the gas being pumped, is a simple way of providing movement that is directly controlled by pressure changes in the gas.

In other embodiments, the pump comprises a pressure sensor for sensing said pressure of said gas, said actuator being configured to constrain said biasing means in dependence upon said pressure sensed by said pressure sensor.

Alternatively the biasing means may be less directly constrained by using a pressure sensor which controls an actuator to move by an amount that depends on the pressure sensed.

In some embodiments, said exhaust valve comprises a flexible plate for covering said outlet, said flexible plate being fixed to said pump at a fixed point and having a non-fixed free end, movement of said free end away from said outlet opening said outlet, said flexible plate comprising said biasing means and said valve being configured such that a stiffness of said flexible plate varies with said pressure of said gas being pumped. One convenient, reliable and low maintenance way of providing an exhaust valve is with the use of a flexible exhaust plate configured to cover the outlet in the rest position. The flexible plate is fixed to the pump towards one end and the other end is free and able to move. The flexibility of the plate is varied in dependence upon the pressure of the gas pumped to adapt the properties if the exhaust valve to suit the current pumping conditions.

In some embodiments, said exhaust valve further comprises an exhaust valve stop for limiting movement of said flexible plate, said exhaust valve stop being configured to contact said flexible plate at a point between said fixed point and said free end of said flexible plate, a flexibility of said exhaust plate being varied by varying a positon at which said exhaust valve stop contacts said exhaust valve plate.

The flexibility of a flexible plate will vary with the length of plate that is free to flex. Thus, one way of varying the flexibility of such a plate is to change the point at which the plate is constrained, such that the length that is free to flex varies, and thus, the biasing force exerted by the plate also changes.

In some embodiments, said actuator is configured to move said exhaust valve stop such that said contact point changes in dependence upon a pressure of said gas.

In other embodiments it may be the position of the flexible plate itself that is moved. As should be understood it is changes in the relative position of the flexible plate and the valve stop that changes the length of the flexible plate that is free to flex and thus, movement of one relative to the other will provide variations in the stiffness of the exhaust valve.

The exhaust valve stop constrains the movement of the plate at a certain point, thus, movement of the point that the exhaust stop contacts the flexible plate in a direction along the length of the plate between the fixed point and the free end, changes the length of the plate that is free to flex and thus, its resistance to flexing.

In other embodiments, said valve comprises a valve member for closing said outlet and said biasing means comprises a spring for biasing said valve member towards said outlet.

In some embodiments, said exhaust valve comprises a spring stop for limiting movement of said spring, said actuator being configured to move said spring stop such that a compression of said spring when said valve is in said closed position changes in dependence upon a pressure of said gas.

As for flexible members, the biasing force of a spring can change depending on how the spring is constrained. Thus, using a movable stop to constrain one end of the spring by different amounts, makes the compression force exerted by the non-constrained end of the spring vary and allows the biasing force to be controlled.

In some embodiments, said pressure comprises a pressure of said gas at said gas inlet.

As noted previously the pressure of the gas being pumped affects the preferred properties of the exhaust valve. The pressure at the inlet of the pump may be the pressure used to control the exhaust valve. This pressure is generally

straightforward to measure, being away from the moving parts of the pump. Furthermore, many pumps are built with a means for connecting some type of pressure sensor to measure the pressure of the gas at the pump inlet, perhaps on the inlet flange of the pump.

In some embodiments, said exhaust valve is configured such that said resistance to opening said exhaust valve increases as said pressure of said gas being pumped decreases. As noted previously, the preferred properties of an exhaust valve will vary with pressure of the gas being pumped, a stiffer valve impeding flow and thus, being less preferable at higher flow rates, but also impeding leakage and thus, being more preferable at higher pressure differences. In a vacuum pump for example, initially during pump down when gas flows are high and pressure differences low, it is advantageous to have a flexible exhaust valve with a low resistance to opening allowing the fluid to flow in a free manner and reducing the power required and temperature increase associated with the flow and also increasing the pumping speed. As the pressure drops and the fluid flow rate falls, it becomes advantageous to increase the stiffness of the exhaust valve as this allows the ultimate pressure of the vacuum pump to be lower.

It should be noted that the change in stiffness of the valve with pressure may be done continually or it may be done in steps. For example, where the actuator is a piston then the piston may move in response to any change in pressure, such that the flexibility is changed continually as the pressure changes. Alternatively, where a different means of actuating and controlling is used, the change may be done in steps in response to the pressure measured reaching or exceeding particular set pressure thresholds.

In some embodiments, said pump comprises a plurality of outlets and a corresponding plurality of exhaust valves.

Although a pump may have a single outlet port and a corresponding exhaust valve, in some embodiments, there are multiple outlets and correspondingly multiple valves. Each valve has a means for closing the outlets that is biased by a biasing means. The multiple valves may share actuating means and in some cases, biasing means.

Although embodiments may be used for compressors, in some embodiments, said pump comprises a vacuum pump. In some embodiments, said pump comprises a rotary vane oil-sealed vacuum pump.

A rotary vane oil-sealed vacuum pump lends itself particularly well to this technique. Even at very low pressures the oil being circulated will be pushed against the valve, so a stiff exhaust valve is opened even at low pressures by the oil pushing against the valve. This allows the stiffness of the exhaust valve to be increased at low pressures, where such a stiff valve increases the ultimate vacuum attainable by the pump.

A second aspect provides a method of varying a resistance to opening of an exhaust valve of a pump, said method comprising: constraining a biasing means for biasing said exhaust valve to a closed position, said constraining of said biasing means changing in dependence upon a pressure of said gas being pumped, such that a biasing force exerted by said biasing means varies in dependence upon a pressure of said gas being pumped.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

Figure 1 schematically shows a rotary oil sealed vacuum pump with a variable stiffness exhaust valve according to an embodiment; Figure 2 shows the exhaust valve of the pump of Figure 1 in more detail;

Figure 3 shows an alternative embodiment of a variable stiffness exhaust valve for a pump according to an embodiment;

Figure 4 shows a portion of the exhaust valve of Figure 3;

Figure 5 shows a further embodiment of a variable stiffness exhaust valve for use in a pump according to an embodiment; and

Figure 6 shows a flow diagram illustrating steps in a method of varying a stiffness of an exhaust valve according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided.

Embodiments provide a pump with an inlet, an outlet, a pumping chamber for moving gas from the inlet to the outlet and an exhaust valve. The exhaust valve is configured to have a controllable resistance to opening, such that this can be changed in dependence upon a current operating state of the pump. In embodiments the resistance is varied in dependence upon a pressure of a gas being pumped. In some embodiments, the control is such that the variation changes continually in response to changes in pressure, while in other embodiments, the variation is controlled to change the resistance in steps, such that predefined changes in pressure trigger a certain change in the resistance of the exhaust valve.

Providing a pump with an exhaust valve that is configured such that the properties of the exhaust valve and in particular, its resistance to opening, vary in dependence upon the pressure of a gas being pumped allows the pump to operate with improved properties across a range of operating pressures. In particular, an improved ultimate pressure along with reduced pumping power, reduced temperature increases and improved pumpdown speeds may be achieved. Changing the resistance to opening of the exhaust valve can be done by varying a biasing force of a biasing means configured to bias the exhaust valve towards its closed position. An actuator can be used to move a member to constrain the biasing means by different amounts and change the biasing force in that way.

For example, the biasing means may be a flexible finger or plate and changing the length of the plate that can flex will change the force required to flex the plate and thus, the biasing force biasing the plate to its rest closed position. A valve stop means may be moved to constrain the flexible plate at different positions along its length allowing the length of the flexible portion to change and thereby change the stiffness of the valve.

Alternatively the biasing means may be a spring and this can be held in a compressed state at rest, the amount of compression being dependent on a position of the valve stop constraining the valve, the valve stop being movable to change its position and thereby the biasing force.

In this way the biasing force exerted by exhaust valves to resist the valve being opened can be changed. Doing so in response to the pressure of the gas being pumped allows the pump to be adapted to particular operating conditions and leads to improved performance.

Where pumps are used to continually cycle the pressure within a chamber between a starting atmospheric pressure and a desired pressure, being able to change the stiffness of the exhaust valve in dependence upon the pressure of the gas being pumped will have a considerable effect on performance. Providing a reduced stiffness valve where there is a high flow rate at the beginning of any pumping procedure when pressure differences are low reduces power

consumption, while providing an increased valve stiffness at higher pressure differences improves ultimate performance. Where the pump is a vacuum pump, then it is advantageous if the exhaust valve stiffness is increased with decreasing pressure where flow rates are reduced and the pressure differentials are higher. Where the pump is a compressor then it is advantageous if the exhaust valve stiffness is increased with increasing pressure where flow rates are reduced and the pressure differentials are higher.

Figure 1 shows a rotary vane oil-sealed vacuum pump having an exhaust valve according to an embodiment. Rotary vane pump 5 comprises an inlet 10 providing access to a pumping chamber formed by a rotor 12 mounted to rotate eccentrically within stator 14. In this embodiment, rotor 12 comprises vanes 13 slidably mounted within rotor 12 and able to change their length as the rotor rotates eccentrically.

The stator 14 comprises an outlet or exhaust port 16. The outlet 16 is closed by a flexible exhaust plate 20 which in this embodiment is configured to flex in response to the rotor blades pushing fluid against the plate allowing fluid to exit via the exhaust. As this pump in this example is an oil sealed pump in addition to the gas being pumped, there is also some oil present within the pumping chamber and this oil will also be forced out through the outlet 16 by rotation of rotor 12.

The exhaust plate 20 is fixed to the stator 14 at a fixed point 21 , while the end adjacent to the outlet 16 is not fixed and is free to move.

The exhaust valve further comprises a valve stop 26 which limits the movement of the exhaust plate 20. The exhaust plate 20 and valve stop 26 are mounted within a fixing guide 28, the exhaust plate being fixed and the valve stop being mounted such that it can move longitudinally within the guide. In this

embodiment the valve stop 26 has a straight portion up until point 26a at which point it curves away from the flexible plate 20. Thus, the portion of valve stop below and up to point 26a hold the exhaust plate 20 in position against stator 14, while the upper curved portion allows the exhaust plate to flex and move away from the stator.

The valve stop 26 is mounted to slide in a longitudinal direction supported within fixing guide 28 such that point 26a and the portion of the valve stop 26 contacting the valve plate 20 moves between the fixed point 21 and the free end of the exhaust plate thereby changing the length of the flexible exhaust plate 20 that is free to flex and in this way the biasing force required to flex the plate into an open position and thus, the stiffness of the exhaust valve.

In this embodiment the longitudinal movement of the valve stop is achieved via a piston 22 which is driven by gas received from the inlet of the pump via a connecting passage or conduit 30. Piston 22 is biased towards an upper position by spring 24, and is connected at its lower end to valve stop 26. Increases in pressure of the gas at the inlet 10 of the pump increases the force exerted by the gas on the piston head 22 and causes it to press against spring 24 and move downwards, thereby moving the valve stop 26 in a corresponding manner. This increases the length of the exhaust plate 20 able to flex and thus, the flexibility of the plate, thereby reducing the stiffness of the exhaust valve at higher inlet pressures when fluid flow is higher. When the pressure at the inlet of the vacuum pump falls, the spring 24 pushes the piston head 22 upwards and the valve stop moves in a corresponding direction. This decreases the length of the exhaust plate 20 that can flex and increases the stiffness of the exhaust valve.

In this way the flexibility of the exhaust valve varies with the inlet pressure of the gas being pumped, the flexibility increasing with higher pressures, which in a vacuum pump relate to higher flow rates and decreasing with decreased pressures.

Figure 2 shows the valve stop 26, fixing guide plate 28 and piston 22 in further detail. As can be seen the fixing guide plate 28 has side extensions for retaining the valve stop 26 laterally, while allowing it to slide in a longitudinal direction. In this embodiment, the valve stop has three fingers 26’ 26” and 26”’, which correspond to three flexible fingers on the exhaust plate 20 which cover three outlets. The valve stop 26 is mounted to slide in a direction perpendicular to the lateral walls of fixing guide plate 28 in response to movement of the piston 22 as shown by the arrows. It should be noted that although in this embodiment three outlets and a corresponding three flexible fingers are shown, the skilled person would understand that the number of outlets and corresponding exhaust plate fingers can vary from a single finger and corresponding single outlet to multiple of each, depending on the embodiment.

Figure 3 shows an alternative embodiment, where the exhaust valve 40 provides a sealing portion that is biased towards a closed position in which it obstructs the outlet by biasing means 42, which in this embodiment comprises a helical spring. Spring 42 is held at one end - the end that is remote from the end contacting exhaust valve 40 by spring stop means 44. Spring stop means 44 is movably mounted such that it can slide in a horizontal direction as shown by the arrows. It is connected to piston 22 such that changes in inlet pressure Pi cause piston head 22 to slide and spring stop 44 to move accordingly, thereby changing the biasing force exerted by spring 42. Piston head 22 is biased towards spring stop 44 by spring 24. Increases in inlet pressure cause piston 22 to move away from the pump exhaust and thus decrease the biasing force exerted by spring 42 on the exhaust valve 40. Decreases in inlet pressure Pi cause the piston 22 to move towards the pump exhaust and to increase the force exerted by biasing means 42 on the exhaust valve 40.

Figure 4 shows how the spring stop 44 is slidably mounted in guides 46 and is pushed laterally by piston head 22. In this case there are three springs mounted in parallel between the spring stop 44 and exhaust valve 40. The exhaust valve 40 may be configured to obstruct a single outlet from the pump or it may be configured to obstruct multiple outlets.

Figure 5 shows an embodiment, where the actuator is a motor 54 that is controlled by signals received from pressure sensor 56. Pressure sensor 56 is mounted on the inlet flange of the pump and monitors the inlet pressure of gas being pumped by the pump. Signals from the pressure sensor are directed via relay 52 to motor 54 which is configured to laterally slide spring stop 44 in a similar way to the embodiment of Figure 3 and thereby change the biasing force exerted by helical spring 42 on exhaust valve 40.

This embodiment may be configured such that the motor 54 makes step changes in the position of spring stop 44 in response to the pressure sensor indicating changes in pressure above a predetermined value.

It should be noted that although the pressure sensor and motor are shown in conjunction with the helical spring and spring stop of the embodiment of Figure 3, they could also be used to move the valve stop of figures 1 and 2 and thereby restrain the flexible exhaust plate of these embodiments.

Figure 6 shows a flow diagram schematically showing a method of varying a stiffness of the vacuum pump exhaust valve of Figure 5. The method comprises measuring the inlet pressure of a gas input to the pump. The change in pressure of the inlet gas is determined and when it has changed by more than a

predetermined amount then the spring stop is moved. If the inlet pressure has decreased then the spring stop is moved towards the pump exhaust thereby increasing the stiffness of the exhaust valve, whereas if it has increased then the spring stop is moved away from exhaust valve thereby decreasing the stiffness of the exhaust valve.

The amount that the spring stop is moved will depend on the change in pressure. In some embodiments the motor may be a step motor and the number of steps taken by the motor will be dependent on the change in pressure. The

predetermined value provides a movement of one step and multiples of the predetermined value providing multiple steps. This method is slightly different to a method performed in conjunction with the embodiments of Figures 1 to 4, where any change in inlet pressure is fed automatically via the piston to the constraining means for constraining the biasing means as opposed to in a step wise fashion where predetermined values are exceeded before any change is triggered.

For a vacuum pump the biasing means is constrained to exert a higher biasing force in response to a decrease in pressure and a lower biasing force in response to an increase in pressure. Where the exhaust valve is on a compressor then the apparatus and method will be configured such that the constraining means constrains the biasing means to exert a higher biasing force as pressure increases and flow rates drop.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

REFERENCE SIGNS

5 pump

10 pump inlet

12 rotor

13 vanes

14 stator

16 pump exhaust

20 flexible exhaust plate

21 fixed attachment point

22 piston

24 biasing spring

26 valve stop

26a point of contact between valve stop and exhaust plate 28 fixing guide plate

30 conduit

40 exhaust valve

42 helical biasing spring

44 spring stop

46 spring stop guide

52 relay

54 motor

56 pressure sensor