Reducing power consumption in vacuum pumps

Abstract

A mechanical vacuum pump 5 comprising a pumping mechanism for pumping a gas from an inlet to an outlet; a motor for driving the pumping mechanism 40, a non-return exhaust valve 12 and a tank 50 provid

Description

FIELD OF THE INVENTION The field of the invention relates to vacuum pumps, and to control of these pumps to reduce power consumption particularly in a steady state standby mode. BACKGROUND In mechanical vacuum pumps such as positive displacement vacuum pumps, total power consumption is a combination of useful power (used to move and / or compress gas) and losses. Losses are mostly associated with electrical and magnetic efficiency (motors and drives) and parasitic losses (gas leakage, cooling fans, friction). Clearly there is potential to reduce losses through improved design, but the power deployed in gas movement / compression is harder to reduce. In many applications, whether evacuating chambers or pumping a steady gas flow at low pressures, vacuum pumps often spend at least some of their operating time maintaining a vacuum and operating with little or no gas throughput. During this mode an exhaust valve is generally closed. ‘Green’ or ‘standby’ modes already exist, and they typically involve a reduction in rotational speed to reduce frictional losses (power = torque x angular velocity, and torque includes bearing frictional losses). Reduced speed results in a different pressure distribution and a higher ultimate pressure, and the power reductions may be quite modest, particularly when compared to the best saving of all, which is to switch the pump off altogether. It would be desirable to be able to reduce the power consumption of a vacuum pump, particularly when operating in standby mode where for much of the time there is little or no throughput of gas and the pump may be pumping at or close to ultimate pressure. SUMMARY According to an aspect there is provided mechanical vacuum pump comprising: a pumping mechanism for pumping a gas from an inlet to an outlet; a motor for driving said pumping mechanism; an exhaust valve; a tank providing a volume in 5 fluid communication with gas within said pumping mechanism; and a controller for controlling operation of said vacuum pump, said controller being configured to: determine that said pump is to enter a standby mode in which said exhaust valve is closed; and boost a speed of said pumping mechanism for a predetermined time prior to said exhaust valve closing to reduce an amount of gas within said 10 pump and said tank. Many mechanical pumps such as positive displacement pumps that operate as primary pumps and exhaust to atmosphere have exhaust valves, which may be controllable exhaust valves, or automatically operated non-return exhaust valves. 15 In steady state conditions with substantially no throughput of gas the valve will be closed. The mode where the valve is closed for some time and there is little or no throughput of gas may be termed a standby mode, during which mode the pump simply maintains a vacuum in a space being pumped. 20 It was recognised that when in standby mode with the exhaust valve closed and with operation of the pump perhaps simply maintaining the pressure within the chamber being pumped, the gas inside the pump may be moved and / or compressed and then expand again with each cycle. This consumes energy. It was also recognised that were the amount of gas within the pump to be reduced 25 during standby mode then the amount of energy required to provide this periodic movement and / or compression of gas within the pump would be reduced. Thus, embodiments provide the pump with a boost in speed prior to entering, or during this, standby mode so that the amount of gas within the pump is reduced and the energy required for the periodic movement and / or compression of the gas within 30 the pump in this mode is correspondingly reduced. There is some increase in power required for the boost in speed and the increase in speed will increase the wear on pump parts such as bearings, however, the increase in speed is only required for a short period of time such that overall power can be saved. The effectiveness of the boost in speed in reducing power consumption depends 5 on how effective it is at reducing the amount of gas within the pump. This is dependent on the configuration of the pump, and for some configurations, where for example, there is a relatively large volume (plenum) between the end of the pumping mechanism and the exhaust valve, then boosting the speed of the pumping mechanism will not affect the gas within this volume and therefore will 10 have a reduced effect on the overall amount of gas reduction in the pump and a corresponding reduced effect on any power savings in standby mode. This has been addressed by providing a tank in fluid communication with the pumping mechanism such that the increase in the pumping speed not only decreases the gas within the pumping mechanism and upstream of it but also reduces the 15 amount of gas within the tank. Thus, following the boost in speed, and closure of the exhaust valve, the pressure within the tank will be low and where there is pressure equalisation with the higher pressure gas from the plenum flowing back into the pumping mechanism, this gas will also flow into the reduced pressure tank and the amount of gas within the pumping mechanism will be reduced 20 compared to if there had been no tank. The size of the tank may be selected in dependence on the size of the pump, and in particular the size of the volume between the end of the pumping mechanism and the exhaust valve and also on the leakage of gas into the pump and the frequency of repetition of the boost that may be acceptable. 25 In some embodiments, said tank comprises a fluid connector for connecting said tank at a location where when said mechanical vacuum pump is operating at ultimate, said pressure is between 50 to 900 mbars, preferably between 200 and 900 mbars. 30 The location of the tank may advantageously be placed in a position somewhere along the pumping mechanism with a relatively high pressure, such that there is a significant volume of gas within the tank to be moved when the speed of the pumping mechanism is increased. The tank may be physically closer to an outlet of the pumping mechanism than an inlet. 5 In some embodiments, said tank is connected to an inlet port located part way along said pumping mechanism. The pump may have inlet ports providing access to the pumping mechanism, that may serve different purposes and it may be advantageous to reuse one of these 10 inlet ports to attach the tank to the pump. In some embodiments, said inlet port comprises a port for supplying ballast or purge gas to said pump. 15 Mechanical vacuum pumps may have ballast or purge gas inlets allowing for purge or ballast gas to be supplied to the pump. Where such inlet ports exist, then they may be reused to provide the connection for the tank. It should be noted that the inlet will not be used for purge or ballast gas when the tank is connected to the mechanical vacuum pump. 20 In some embodiments, said vacuum pump further comprises an isolating valve for isolating said tank from said vacuum pump. There may be an isolating valve for isolating the tank from the vacuum pump. In 25 some embodiments, the process occurring within the process chamber being evacuated may have different processing steps, some of which may generate dirty or hazardous gasses. Where this is the case, it may be advantageous to have a valve that allows the tank to be isolated from the gas flow while such processing steps are occurring and thereby avoid or at least inhibit the pooling of 30 these gasses within the tank. The valve may be opened when these steps have finished and / or when standby mode is determined to be about to happen and the speed of the motor is to be increased. ln some embodiments, said controller is further configured to reduce said speed of said pumping mechanism and operates said pump in said standby mode with said exhaust valve being closed. 5 In some embodiments, said step of determining that said pump is to enter a standby mode comprises at least one of: receiving a control signal from a user, receiving a control signal from a vacuum system associated with said pump and receiving a control signal from at least one sensor. 10 If the mechanical vacuum pump is to boost the speed prior to entering standby mode, then the vacuum pump needs to be made aware of when standby mode is imminent. The vacuum pump may determine this in dependence upon pressure signals received from one or more pressure sensors sensing the pressure of the 15 gases being pumped, and / or it may determine this in response to a flow sensor indicating that gas flow within the pump is falling or is below a predetermined level, and / or it may determine this from a user control signal or a control signal from the system being pumped or from a related vacuum system. In any case, in response to the signal the controller will boost the speed of the motor to reduce 20 the gas within the tank and the pumping mechanism prior to entering the standby mode. In some case the exhaust is a non-return exhaust valve that closes automatically when the pressure at the exhaust is higher than or equal to the pressure in the pump, in other cases it may be a controlled exhaust valve and in which case when the controller switches the speed of the motor from the boosted 25 speed to a reduced speed it will also close the exhaust valve. In some embodiments, said step of reducing said speed of said pumping mechanism comprises reducing said speed to the pump’s nominal speed. 30 In other embodiments, said step of reducing said speed of said pumping mechanism comprises reducing said speed to a reduced speed that is lower than said nominal speed. ln some embodiments, the mechanical vacuum pump further comprises an inlet valve for isolating an inlet of the vacuum pump. In some embodiments, during standby mode the pump may be connected to the vacuum chamber, while in 5 others the vacuum pump may be isolated from the vacuum chamber by an isolation valve. Although the mechanical vacuum pump may have different forms, in some embodiments said mechanical vacuum pump comprises one of: a screw pump, a 10 scroll pump or a multi-stage Roots pump. In some embodiments, said controller is further configured during said standby mode to: determine that said amount of gas within said pump is to be reduced; boost a speed of rotation of said pump for a predetermined time to reduce an 15 amount of gas within said pump; and reduce said rotational speed and return to operating in said standby mode. There may be leakage of gas into the system that renders the pressure within the system higher than is desirable for standby mode with reduced power 20 consumption and thus, the controller may be further configured to boost the speed during standby mode as required. This may be in response to a pressure sensor detecting a rise in pressure or it may be that it is done periodically. A further aspect provides a method of adapting a mechanical vacuum pump for 25 reduced power standby operation, said mechanical vacuum pump comprising a pumping mechanism for pumping gas from an inlet to an outlet and an exhaust valve, said method comprising: attaching a tank via a fluid connector to said mechanical vacuum pump, said fluid connector providing a fluid communication path for gas within said pumping mechanism and said tank; providing a controller 30 for controlling operation of said vacuum pump, said controller being configured to: determine that said pump is to enter a standby mode in which said exhaust valve is closed; and boost a speed of said pumping mechanism for a predetermined time prior to said exhaust valve closing in order to reduce an amount of gas within said pump and said tank. In some embodiments, said step of attaching comprises attaching said fluid 5 connector at a location, where when said mechanical vacuum pump is operating at ultimate, said pressure is between 50 and 900 mbars. In some embodiments, said step of attaching comprises attaching said fluid connector to a gas ballast inlet port. io In some embodiments the method comprises providing a controllable valve between said tank and said pumping mechanism. Another aspect provides a method of operating a mechanical vacuum pump, said 15 mechanical vacuum pump comprising a pumping mechanism for pumping gas from an inlet to an outlet and an exhaust valve, and a tank providing a volume in fluid communication with gas within said pumping mechanism, said method comprising: operating said vacuum pump by driving said pumping mechanism at a nominal speed; determining that said vacuum pump is to enter a standby mode 20 in which said exhaust valve is closed; and boosting a speed of said pumping mechanism for a predetermined time prior to said exhaust valve closing in order to reduce an amount of gas within said pump and said tank. Further particular and preferred aspects are set out in the accompanying 25 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, 30 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 shows a mechanical vacuum pump according to an embodiment; and 5 Figure 2 shows a flow diagram illustrating steps in a method of adapting a mechanical vacuum pump for reduced power standby operation according to an embodiment; and Figure 3 shows a flow diagram schematically illustrating a method of operating a vacuum pump according to an embodiment. io DESCRIPTION OF THE EMBODIMENTS Before discussing the embodiments in any more detail, first an overview will be provided. 15 Many mechanical pumps such as positive displacement pumps that operate as primary pumps and exhaust to atmosphere have exhaust valves, that may be non-return exhaust valves which mitigate the impact of failure modes and prevent or at least impede the pump’s return to atmospheric pressure in a short timescale. In steady state conditions with no throughput of gas the valve will be 20 closed. The mode where the valve is closed for some time and there is little or no throughput of gas may be termed a standby mode during which mode the pump simply maintains a vacuum in a space being pumped. Being able to reduce the pressure within the pump during this standby mode will reduce the power consumption of the pump as there is less gas to be moved and / or 25 compressed with each cycle. Thus, embodiments provide the pump with a boost in speed prior to entering, or during this, standby mode so that the amount of gas within the pump is reduced and the energy required for the periodic movement and / or compression of the 30 gas within the pump in this mode is correspondingly reduced. Reducing the pressure in a vacuum pump by increasing the pumping mechanism speed, may not be as effective at reducing the volume of gas within the pumping mechanism of the vacuum pump for vacuum pumps where the volume in the mechanism is small compared to the volume between the end of the mechanism and the exhaust valve. With such a configuration of pump, the increase in speed of the pumping mechanism will have a reduced impact on the overall pressure within 5 the pump. Embodiments, address this problem by providing an additional volume in fluid communication with gas within the pumping mechanism. With this arrangement acceleration of the pumping mechanism reduces the pressure not only within the pumping mechanism portion of the pump and the portion upstream of the pumping mechanism, but it also reduces the pressure within the 10 additional volume too, such that on closure of the exhaust valve, gas at a higher pressure within the exhaust region of the pump will flow not only into the pumping mechanism but also into the reduced pressure volume, thereby reducing the amount of gas within the pumping mechanism in standby mode. 15 The location of the connection of this additional volume to the pumping mechanism, may advantageously be at a relatively high pressure, i.e. towards the exhaust as this allows an increased amount of gas to be pumped out of the volume by boosting the pump speed. In some embodiments, it may be connected to the gas ballast port such that an additional dedicated port does not need to be 20 added to the vacuum pump. Figure 1 shows a mechanical vacuum pump according to an embodiment. In this embodiment, the mechanical vacuum pump is a screw pump 5. In other embodiments, it may be some other type of pump, such as a multi-stage Roots or 25 scroll pump. Screw pump 5 comprises a pumping mechanism 40 that pumps gas from an inlet 32 to an exhaust 10. There is an isolation valve 30 on the inlet that can selectively isolate the vacuum pump from whatever is being evacuated. 30 The inlet may be connected to a further vacuum pump or to a vacuum chamber. There is a motor 70 for driving pumping mechanism 40 and a controller 60 for controlling the motor and in some cases one or more of valves associated with the pump 5. In this embodiment, there is a tank 50 that is connected to the pumping 5 mechanism 40 at a gas ballast inlet port 54. There is a controllable valve 52 that can selectively isolate the tank from the pumping mechanism. This valve 52 is an optional feature and may not be required in some embodiments. There is an exhaust 10 for exhausting the gas being pumped by pumping mechanism 40 and there is an exhaust valve in this embodiment a non-return exhaust check valve io 12 that opens or closes the exhaust. In other embodiments the exhaust valve may be a controllable valve controlled by controller 60. In this embodiment the exhaust 10 exhausts to atmosphere. Controller 60 is configured to control the motor 70 and to control one or more of 15 the valves. When the pump 5 is to enter a standby mode the controller is configured to boost the rotational speed of the motor 70 such that the pumping mechanism 40 accelerates and reduces the amount of gas within the vacuum pump 5 and also within tank 50. In some embodiments, the controller may be configured to determine whether valve 52 is closed and where so to open it prior 20 to boosting the speed of the motor. The controller may determine that the pump 5 is to enter standby mode in response to a signal from a sensor (not shown). The sensor may be a pressure sensor or a sensor sensing flow rate. Alternatively, the controller may receive a 25 signal from a user or from an associated system, such as an upstream vacuum pump, or a processing system being evacuated. The vacuum pump may then enter standby mode and the exhaust check valve 12 is closed and in some cases the isolation inlet valve 30 may be closed by 30 controller 60. The speed of the motor 70 is then reduced by controller 60 to a standby speed which may be the pump’s nominal speed or may be a further reduced speed and pump 5 continues to operate but with no or very little gas flow. As the amount of gas in the pump has been reduced prior to entering the standby mode the amount of power required for this operation is reduced and thus, the power expended in the standby mode is also correspondingly reduced. 5 In this regard, the boosted pumping speed reduces the pressure within the pumping mechanism 40 and tank 50 and thus, in standby mode when there is some pressure equalisation within the pump, higher pressure gas from within the volume between the pumping mechanism and the exhaust check valve 12 flows back into the pumping mechanism 40, but also into the tank and this reduces the io amount of gas within the pumping mechanism 40 itself and reduces the power required for standby operation. The provision of a tank 50 is particularly applicable to vacuum pumps which have a relatively large plenum (volume between the pumping mechanism and exhaust valve) which volume of gas is unaffected by the boost in speed. Providing a tank that is connected to the 15 pumping mechanism allows the pressure within the tank to be reduced when the speed is boosted and if the volume of the tank is chosen appropriately then this reduction in pressure can significantly compensate for the lack of pressure reduction in the plenum. The size of the tank may be selected in dependence upon space and size considerations for the pump, the volume of the plenum and 20 the amount of outgassing or gas leakage expected during standby mode, and also the acceptable time period for periodic re-boosting of the pump. Once the pump has entered standby mode, then the controller may be configured as the pressure rises within the pump, to boost the speed again to reduce the 25 pressure. When the speed is boosted the exhaust check valve will open. The controller may determine to perform this additional boost in response to a signal received from a sensor such as a pressure sensor, or it may do it periodically. The speed of the motor and pumping mechanism during a boost may be at least 30 10% higher than usual operational speed. In some embodiments, between 10 and 50% higher. ln some embodiments, the controller 60 may also control the valve 52 on the line connecting tank 50 to gas ballast inlet port 54. In some cases, this valve 52 may be used to inhibit the pooling of certain process gases within the tank that may be generated by a semiconductor process. Where the pump 5 is evacuating a 5 semiconductor processing chamber then during certain steps of the semiconductor process hazardous and / or dirty gases may be pumped and if the tank is connected to the pump during these times, then these gases may collect within the tank. To address this potential there may be certain time periods during operation where it is advantageous if the tank is isolated from the pumping io mechanism to avoid or at least inhibit the pooling of these particles or hazardous gases within the tank. Figure 2 shows steps in a method of adapting a mechanical vacuum pump for reduced power standby operation. In a first step S10 a tank is attached via a fluid 15 connector to a mechanical vacuum pump. It may be attached to a ballast gas inlet port or to some other port. In any case it will be attached so that it is in fluid communication with the gas within the pumping mechanism. At step S20 a controller is provided for controlling operation of the vacuum pump. 20 The controller will control the speed of the motor and may control operation of one or more valves, such as an isolation valve between the tank and pumping mechanism, an inlet valve and in some cases the exhaust valve. In other cases the exhaust valve may be a non-return check valve. 25 The controller will be configured to control operation of the vacuum pump, so that on determining that the pump is to enter a standby mode in which the exhaust valve is closed it will boost the speed of the motor driving the pumping mechanism for a predetermined time before closure of the exhaust valve and in some cases the inlet valve. It will then reduce the speed of the motor and 30 operate in standby mode. The controller may then periodically control the motor to boost the speed again as the pressure in the pump rises due to leakage or outgassing. Figure 3 shows steps in a method for operating a pump in standby mode according to an embodiment. These steps may be steps that controller 60 controls pump 5 to perform, or they may be steps performed by an operator. 5 Initially at step S100 the pump may be operational and driven at a nominal speed. There may then be a step S110 where it is determined whether the pump is to enter a standby mode, this may be determined in response to a user input, an input from a related vacuum system, or a signal from a sensor such as a pressure sensor. If it is determined at step S110 that the pump 5 is to entre io standby mode, then at step S120 tank isolating valve 52 may be opened (if it was previously closed) and in some cases inlet valve 30 may also be closed. At step S130 the speed of the motor is increased for a predetermined time and then at step S140 the speed is reduced to a standby speed and exhaust valve 12 closes. 15 In some embodiments, a step D5 may be performed during standby mode, in which it is determined whether the pressure in the pump should be reduced again and if so step S130 of boosting the speed of the motor is performed again. In this embodiment, step D5 is performed based on the amount of gas in the pump, generally determined from a pressure sensor, in other embodiments, it may 20 rather be that the repetition of step S130 is performed periodically. The boost in speed may increase the rotational speed by more than 10% in some cases by up to 60%. 25 The reduced standby speed may be reduced by more than 10% compared to the nominal rated operation speed of the pump, in some cases by up to 80% . Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the 30 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 vacuum pump 10 exhaust 12 exhaust valve 5 30 inlet valve 32 inlet 40 pumping mechanism 50 tank 52 tank isolating valve io 54 gas ballast port 60 controller 70 motor

Claims

1. A mechanical vacuum pump comprising:a pumping mechanism for pumping a gas from an inlet to an outlet;5 a motor for driving said pumping mechanism;a non-return exhaust valve;a tank providing a volume in fluid communication with gas within said pumping mechanism; anda controller for controlling operation of said vacuum pump, said controller io being configured to:determine that said pump is to enter a standby mode in which said exhaust valve is closed; andboost a speed of said pumping mechanism for a predetermined time prior to said exhaust valve closing to reduce an amount of gas within 15 said pump and said tank.

2. A mechanical vacuum pump according to claim 1, wherein said tank comprises a fluid connector for connecting said tank at a location where when said mechanical vacuum pump is operating at ultimate, said pressure is between 20 50 to 900mbars, preferably between 200 and 900mbars.

3. A mechanical vacuum pump according to any preceding claim, wherein said tank is connected to an inlet port located part way along said pumping mechanism.

254. A mechanical vacuum pump according to claim 3, wherein said inlet port comprises a port for supplying ballast or purge gas to said pump.

5. A mechanical vacuum pump according to any preceding claim, said 30 vacuum pump further comprising an isolating valve for isolating said tank from said vacuum pump.

6. A mechanical vacuum pump according to any preceding claim, said controller being further configured to reduce said speed of said pumping mechanism and operate said pump in said standby mode with said exhaust valve being closed.

57. A mechanical vacuum pump according to claim 6, wherein said step of determining that said pump is to enter a standby mode comprises at least one of: receiving a control signal from a user, receiving a control signal from a vacuum system associated with said pump and receiving a control signal from at least io one sensor.

8. A mechanical vacuum pump according to claim 6 or 7, wherein said step of reducing said speed of said pumping mechanism comprises reducing said speed to the pump’s nominal speed.159 A mechanical vacuum pump according to claim 6 or 7, wherein said step of reducing said speed of said pumping mechanism comprises reducing said speed to a reduced speed that is lower than said nominal speed.20 10. A mechanical vacuum pump according to any preceding claim, furthercomprising an inlet valve for isolating an inlet of said vacuum pump.

11. A mechanical vacuum pump according to any preceding claim, wherein said mechanical vacuum pump comprises a screw pump, a scroll pump or a 25 multi-stage Roots pump.

12. A mechanical vacuum pump according to any preceding claim, wherein said controller is further configured during said standby mode to:determine that said amount of gas within said pump is to be reduced;30 boost a speed of rotation of said pump for a predetermined time to reducean amount of gas within said pump; andreduce said rotational speed and return to operating in said standby mode.

13. A method of adapting a mechanical vacuum pump for reduced power standby operation, said mechanical vacuum pump comprising a pumping mechanism for pumping gas from an inlet to an outlet and an exhaust valve, said 5 method comprisingattaching a tank via a fluid connector to said mechanical vacuum pump, said fluid connector providing a fluid communication path for gas within said pumping mechanism and said tank;providing a controller for controlling operation of said vacuum pump, said io controller being configured to:determine that said pump is to enter a standby mode in which said exhaust valve is closed; andboost a speed of said pumping mechanism for a predetermined time prior to said exhaust valve closing in order to reduce an amount of 15 gas within said pump and said tank.

14. A method according to claim 13, wherein said step of attaching comprises attaching said fluid connector at a location where when said mechanical vacuum pump is operating at ultimate, said pressure is between 50 and 900mbars.2015. A method according to any one of claims 13 or 14, comprising providing a controllable valve between said tank and said pumping mechanism.

16. A method of operating a mechanical vacuum pump, said mechanical25 vacuum pump comprising a pumping mechanism for pumping gas from an inlet to an outlet and an exhaust valve, and a tank providing a volume in fluid communication with gas within said pumping mechanism, said method comprising:operating said vacuum pump by driving said pumping mechanism at a 30 nominal speed;determining that said vacuum pump is to enter a standby mode in which said exhaust valve is closed; andboosting a speed of said pumping mechanism for a predetermined time prior to said exhaust valve closing in order to reduce an amount of gas within said pump and said tank.

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