GB2602625A

GB2602625A - Method for stopping a vacuum pump

Info

Publication number: GB2602625A
Application number: GB2019818.0A
Authority: GB
Inventors: Glyn Horler Richard; Smerek Petr; Keech Ian
Original assignee: Edwards sro
Current assignee: Edwards sro
Priority date: 2020-12-15
Filing date: 2020-12-15
Publication date: 2022-07-13
Anticipated expiration: 2040-12-15
Also published as: TW202237988A; GB2602625B; WO2022129009A1; EP4264054A1; GB202019818D0; CN116635634A; US20240035472A1; JP2023554413A

Abstract

A method of stopping a rotor 7 of a vacuum pump rotating at an operational RPM from which the rotor cannot be coasted down to zero RPM without a relatively high probability of the rotor and stator 8, 9 clashing, and a vacuum pump comprising a controller 10 configured to carry out the method. The method comprises rotating the rotor at an intermediate RPM at which there is substantially no probability of clashing for a dwell time sufficient for the pump to be at or below a threshold temperature, the intermediate RPM being at or below a threshold RPM which is below the operational RPM, and subsequently coasting down the rotor until cessation. The threshold RPM is the maximum speed at a thermal steady state wherein the rotor can be coasted down until it stops with substantially no probability of the rotor and stator clashing, and the threshold temperature is the temperature of the pump when the rotor is at the threshold RPM at a thermal steady state.

Description

METHOD FOR STOPPING A VACUUM PUMP

Field

[1] The present invention relates to vacuum pumps and, in particular, to a method for ceasing (i.e. stopping) the rotation of a vacuum pump rotor.

Background

[2] \Mien manufacturing a component of a vacuum or compressor pump, much like for all manufactured components, there may be minor variations in the physical dimensions between individual units of the same component. These manufacturing tolerances are generally inherent and unavoidable; but are commonly reduced as much as is practical. A manufacturer may balance the production cost and efficiency of providing components with the smallest manufacturing tolerances, against the benefits that this provides to the final assembled pump or compressor. The acceptable manufacturing tolerance may differ depending on the component and its use.

[003] Reducing manufacturing tolerances is particularly important for components that move relative to other components when the pump or compressor is in use, for example, the rotor blades or lobes of a pump. Reduced tolerances enable the moving components to be arranged with reduced distance therebetween, improving the efficiency of the pump.

[004] This is particularly important in dry vacuum pumps, such as screw pumps, roots pumps, or claw pumps, as there is no lubrication in the main pumping chamber between rotor and stator. The lack of lubrication may be beneficial in reducing contamination of the vacuum system; however, the lack of lubrication requires the clearances between the rotors and stators to be reduced still further to improve pumping efficiency.

[005] As the clearances are reduced, so the chance of a rotor clashing with a stator or another rotor may increase. Such rotor clash may not only stop the pump but may cause damage to the rotor(s) and/or stator(s) involved.

[006] The present inventors have found that in many pumps, for example a multistage roots pump, additional spacing may be required between components in relative motion due to the thermal cycles that the pump undergoes during use. Heat produced by the pump -2 -causes thermal expansion of components of the pump. Different components within the pump comprise different materials, thus the components' thermal expansion rates may differ. An effect of differing thermal expansion of components within the pump is that the spacing between adjacent components of the pump may change with changes in temperature, leading to the effective movement of components relative to one another.

[007] When the pump is "cold-started", i.e. rotation of the rotors is initiated with the vacuum pump at ambient temperature (e.g. 25 °C), the rotors shift in a substantially upstream pump direction. This shift is partially due to rotodynamic effects, and the settling of the bearings upon which the rotor shafts are rotatably mounted. The shift is also partially due to the internal pressure gradient generated within the pump chamber along the pump direction. Typically, the shift may be less than about 100 pm, preferably, less than about 50 pm, for example 20 pm.

[008] Accordingly, to prevent the rotor(s) of each stage clashing with the adjacent upstream partition wall (i.e. stator), upstream axial clearance may be greater than that required for optimum pump efficiency. Typically, therefore, the increased upstream axial clearance is greater than the shift of the rotor when "cold-started".

[009] As discussed above, as a pump is used the temperature of the pump may increase, causing thermal expansion of the components within the pump. It has been found by the present inventors that as the temperature of the pump increases, the rotors may effectively shift in a substantially downstream pump direction relative to the stators. This may be partially due to the differing thermal expansion of the components of the pump caused by differing geometries of the components. Also, this may be partially due to different components comprising different materials having different coefficients of thermal expansion. Typically, the rotors and stators comprise different materials, for example the stators may comprise aluminium and rotors may comprise iron. Thus, the thermal expansion rate of the aluminium stators may be greater than that of the iron rotors.

[010] By way of example, the inventors have found that in some pumps which are operating near their thermal limit, a rotor may shift in a substantially downstream pumping direction relative to a stator by up to about 150 pm or even up to about 250 pm.

[011] "Mien a pump is switched off the rotation rate of the rotors may decrease to until their rotation ceases. This deceleration to standstill may be referred to as coasting down. -3 -

Typically, the rotors free spin until they stop, although braking (e.g. motor braking) may be applied. Coasting down may take less than about 120 seconds, preferably less than about 60 seconds, for example about 20 seconds.

[012] As the pump coasts down, the decrease in rotation rate of the rotors may be non-linear. Immediately after the pump is switched off, the rate of change in the rotational speed of the rotors may initially be small because the pressure gradient is still present within the pump and the rotational inertia of the rotors remains relatively high. However, as the pressure gradient reduces, the rate of decrease in the rotational speed of the rotors may increase due to increased drag on the rotors. Finally, as the rotors near stopping, the rate of decrease in rotation rate of the rotors again reduces.

[13] As the pressure gradient within the pump chamber reduces, the rotors may shift in a substantially downstream pumping direction. 15 [14] If the pump has been operating such that the temperature of the pump has caused the rotors to shift in a substantially downstream pumping direction relative to the stators, as the rotor coasts down the rotors may shift further in the downstream direction due to the reduction in pressure gradient. This further shift of the rotors in the downstream direction may cause a rotor to clash with a downstream stator.

[15] When used for certain applications, for example semiconductor manufacturing processes, some pumps may exhibit an accumulation of particulate matter within the pump chamber during use. When the operation of the pump is stopped and the components thermally contract, this particulate matter may be compressed between a rotor and an adjacent stator of the pump chamber. Wien the pump is "cold started" the increased friction between the compressed particulate matter and the rotor may increase the torque required to initiate rotation of the rotor. In some instances, the torque required may exceed the operational torque of the pump, resulting in start-up failure.

[16] To overcome this start-up issue, a variety of mechanisms for purging the particulate matter during shutdown of the pump are known. For example, WO 2004/038222 discloses an automated shutdown sequence comprising ceasing the operation of the pumping mechanism, monitoring the temperature of the pumping mechanism, at a pre-selected temperature interval initiating operation of the pumping mechanism to purge contaminant -4 -particulate matter from within the pump chamber, and finally ceasing operation of the pumping mechanism.

[17] Shutdown via such a method allows deposited particulate matter within the pump chamber to be purged, thereby ensuring that such particulate matter is not compressed between rotors and stators of the pump mechanism. This may avoid the start-up issue.

[18] EP 1900943 (Al) discloses an alternative method for clearing particulate matter from within a pump chamber. The method comprises reducing the rotation speed of the pumping mechanism to below a pre-set threshold speed, retaining the rotation speed for a time period to purge any accumulated particulate matter from between the rotors and adjacent stators, then ceasing rotation of the pumping mechanism. Again, this method prevents accumulation and compaction of particulate matter in the clearances between the rotors and stators. Thus, increased torque requirements during cold-starting of the pump mechanism are avoided.

[19] EP 2048365 (A2) discloses a mechanism for, after the pump stop action has been taken, rotating the pump rotor in a forward and/or reverse direction according to a predetermined timing pattern, before ceasing rotation of the pump. Similarly, this mechanism purges and prevents compaction of particulate matter in the clearance between rotors and stators.

[20] Each of these disclosures is directed towards preventing the increased torque start-up issue for vacuum pumps caused by compaction of particulate matter in the clearance between the rotor and stator. However, none of the disclosures identify the problem of the rotor clashing with the stator during cessation of rotation.

[21] At present, to reduce the risk of clashing during use, the nominal spacing between components accounts for both the tolerance stack and thermal cycling/expansion.

However, increasing the spacing between components, particularly between the rotor and stator, significantly reduces the efficiency of the pump. These issues are particularly relevant when the size of a vacuum pump is reduced, as the efficiency losses have a greater effect on the overall performance of the pump. This may be detrimental to the pumping performance and reduce the ultimate pumping pressure achievable by the pump. -5 -

[22] Accordingly, there is an ongoing need to address the effect of thermal expansion on the efficiency of the pump.

[23] The present invention aims to solve these and other problems with the prior art.

[24] Summary

[25] Accordingly, in a first aspect, the present invention provides a method of ceasing (i.e. stopping) the rotation of a rotor of a vacuum pump. The vacuum pump comprises a pump chamber including the rotor and a stator.

[26] Prior to initiation of the method the rotor is rotating at an operational RPM (rotations per minute). The operational RPM is greater than a threshold RPM, such that the rotor cannot be coasted down from the operational RPM to cessation of rotation (i.e. standstill) without a relatively high probability of the rotor clashing with the stator.

[27] The method comprises the steps of rotating the rotor at an intermediate RPM at which there is substantially no probability of the rotor clashing with the stator for a dwell time sufficient for the vacuum pump to be at or below a threshold temperature. Then, subsequently, the rotor is coasted down until rotation ceases.

[28] The intermediate RPM is at or below the threshold RPM.

[29] Advantageously, the present inventors have found that the method according to the present invention significantly reduces the risk of the rotor and the stator clashing when then the vacuum pump is switched off. Rotating the rotor at an intermediate RPM for a dwell time according to the present method may ensure that sufficient pressure gradient is maintained within the pump whilst the pump temperature is reduced, thus preventing significant downstream shift of the rotor while the components are thermally shifted, which may cause a clash. Moreover, because the effect of this temperature dependent clash mechanism is substantially avoided, the clearances between each rotor and downstream adjacent stator may be reduced, improving pumping efficiency. -6 -

[30] Preferably, use of the method according to the present invention may enable the rotor and stator of the vacuum pump to be arranged such that the minimum distance therebetween (i.e. clearance) is such that if during operation the rotor is rotating above the threshold RPM and coasted down to cessation of rotation, there is a relatively high probability of the rotor clashing with the stator.

[31] For the purposes of the invention, the threshold RPM is the maximum speed of rotation at a thermal steady state wherein the rotor can be coasted down until rotation ceases with substantially no probability of the rotor clashing with the stator.

[32] The threshold temperature is the temperature of the vacuum pump when the rotor is rotating at the threshold RPM in a thermal steady state.

[33] For the purposes of the invention, ceasing rotation of a rotor of a vacuum pump means reducing the rotation speed of the rotor until it is substantially stationary, i.e. rotating at a speed of about 0 RPM. At about 0 RPM rotation is considered to have ceased. Such cessation of rotation may typically be associated with switching the pump off, such as for maintenance or between uses.

[034] Typically, during coasting down the rotor(s) free spin until they stop, although braking may be applied. Generally, a motor will no longer be driving the rotor when coasting down.

[035] For the purposes of the invention, a "relatively high probability of the rotor clashing with the stator" may be defined as there being a greater than about 1% chance of a rotor clashing with a stator, preferably greater than about 5% chance of the rotor clashing with the stator, more preferably greater than about 10% chance of the rotor clashing with the stator, more preferably greater than about 20% chance of the rotor clashing with the stator, more preferably greater than about 30% chance of the rotor clashing with the stator, more preferably greater than about 40% chance of the rotor clashing with the stator, more preferably greater than about 50% chance of the rotor clashing with the stator, more preferably greater than about 60% chance of the rotor clashing with the stator, more preferably greater than about 70% chance of the rotor clashing with the stator, more preferably greater than about 80% chance of the rotor clashing with the stator, more preferably greater than about 90% chance of the rotor clashing with the stator, during a -7 -coast down from that particular RPM. Most preferably there is a greater than about 99% chance of a rotor clashing with a stator during a coast down from that particular RPM.

[36] For the purposes of the invention, "a rotor clashing with a stator" may be defined as direct contact between a rotor and a stator. Typically, the rotor may be rotating when the clash occurs with the stator. During the clash, the friction and impact between the rotor and stator stops the rotation of the rotor. A rotor clashing with a stator may cause damage to the rotor and/or stator. A clash may require repair and/or replacement of one or more components of the pump.

[37] In some embodiments, a clash between a rotor and a stator may result in a locking engagement between the rotor and the stator. Such locking engagement may result from the rotor touching and binding to the stator to cause a seizure of the rotation of the rotor. If a locking engagement occurs during a clash, this may cause irreparable damage to the rotor and/or the stator, and may result in significant downtime for the vacuum pump. Rotors and/or stators comprising aluminium or aluminium alloys may be particularly susceptible to locking engagement during a clash.

[38] "A rotor clashing with a stator" may exclude a rotor contacting particulate matter deposited on a stator.

[39] For the purposes of the invention, "substantially no probability of the rotor clashing with the stator" may be defined as there being less than about 0.1% chance of the rotor clashing with the stator, more preferably less than about 0.01% chance of the rotor clashing with the stator, during a coast down from that particular RPM. Preferably the rotor will not clash with the stator during the lifetime of the vacuum pump or between servicing.

[40] For the purposes of the invention, thermal steady state may be defined as the temperature of the vacuum pump when the rotation of the rotor is held at a specific rotational speed (RPM) until the temperature of the rotor becomes substantially constant, e.g. varies by less than about +/-0.1 °C for at least 1 minute.

[041] The vacuum pump may be a dry vacuum pump. Preferably, the dry vacuum pump may be roots pump, more preferably a multistage roots pump. For example, the vacuum -8 -pump may be an nXRi Dry Multistage Roots Pump or nXLi Dry Multistage Roots Pump as produced by Edwards Vacuum.

[42] The pump chamber may house the one or more rotor(s) and/or stator(s).

Preferably, the pump chamber may be defined by an outer wall which substantially surrounds the one or more rotor(s) and/or stator(s).

[43] The vacuum pump may comprise a plurality of rotors. Preferably the one or more rotors are arranged on one or more rotor shafts. Each rotor may comprise a multi-lobed piston arranged to rotate with the rotor shaft. Preferably, each rotor may comprise a two-, three-, four-, or five-lobed piston. Preferably, each rotor of the vacuum pump has substantially identical dimensions.

[44] Preferably, the rotors of the vacuum pump may be configured in rotor stages. Each rotor stage may comprise a first rotor arranged on a first rotor shaft and a corresponding second rotor arranged on a second rotor shaft. The first and second rotor shafts may be substantially parallel. In use, the first and second rotor shafts may be configured to rotate in opposite directions. The rotational paths of the lobes of the first and second rotors overlap without the first or second rotor touching.

[45] Typically, the outer wall of the pump chamber may comprise an inlet through which fluid may enter the pump chamber. The outer wall of the pump chamber may further comprise an outlet through which fluid may exit the pump chamber. Typically, the pump inlet may be at or towards a first end of the pump chamber, and/or the pump outlet may be at or towards a second end of the pump chamber. Typically, when the pump is in use, the pump inlet may be coupled to a chamber to be evacuated, and/or to a further vacuum pump.

[46] The pump direction may be defined as the direction in which, when the pump is in use, the majority of the fluid flowing through the vacuum pump may flow. Typically, the outlet is positioned substantially downstream in the pump direction from the inlet.

[047] Without departing from the above, the pump inlet may be configured so that during use, fluid enters the main chamber in a substantially perpendicular direction to the pump -9 -direction. Additionally, or alternatively, the pump outlet may be configured so that during use, fluid exits the main chamber in a substantially perpendicular direction to the pump direction. Advantageously, this may enable the size of the pump to be reduced.

[048] Typically, the vacuum pump may further comprise a motor, typically an electric motor, configured to drive rotation of the one or more rotors when in use. Typically, the motor is positioned outside of the pump chamber. The motor may be coupled to each rotor shaft. Alternatively, each rotor shaft may be coupled to a separate motor.

[049] Preferably, the pump may comprise a plurality of rotor stages. The plurality of rotor stages may be arranged along the length of the first and/or second rotor shafts. The rotor stages may be separated by partition walls (i.e. stators). The partition walls may comprise connecting ducts allowing fluid connection between adjacent rotor stages.

[050] Typically, the pump may comprise from about 1 to about 10 rotor stages, preferably from about 2 to about 8. The number of rotor stages may depend on the type of pump.

[051] Typically, the one or more rotor(s) are metallic, for example being made from iron or an alloy thereof.

[52] Typically, the one or more stator(s) are metallic, for example being made from aluminium or an aluminium alloy.

[53] One or more of the stator(s) may be in the form of a partition wall, preferably each stator may be a partition wall. Typically, a partition wall may be positioned between each rotor of adjacent rotor stages.

[54] When the pump is in operation, the operational RPM of the rotor(s) may be from about 5000 RPM to about 16000 RPM, preferably from about 12000 RPM to about 15000 30 RPM.

[55] Typically, the intermediate RPM is at or below the threshold RPM. Preferably, the intermediate RPM is about half the rotational speed of the operational RPM, preferably from about 25 % to about 75 % of the operational RPM, preferably from about 40 % to about 60 % of the operational RPM.

-10 - [56] Typically, the vacuum pump comprises a temperature sensor configured to measure the temperature of the vacuum pump. The method may further comprise the step of measuring the temperature of the vacuum pump via the temperature sensor during the dwell time and determining whether the temperature is at or below the threshold temperature. Typically, the temperature of the vacuum pump refers to the temperature of a rotor of the vacuum pump.

[57] Typically, the temperature sensor is configured to measure the temperature of a rotor directly; however, it may also be configured to measure the temperature of a component of the vacuum pump from which the temperature of the rotor can be inferred, for instance a stator and/or a rotor shaft. Preferably, the temperature sensor may be configured to measure the temperature of a stator.

[58] \Mien the pump is in operation, the temperature of the pump (e.g. a rotor) may be from about 50°C to about 150 °C, preferably from about 70°C to about 120 °C. This may be referred to as the operational temperature. The operation temperature will generally be in steady state. The skilled person will appreciate that the temperature of the vacuum pump during operation may depend on the ambient temperature, the rotational speed of the rotor(s), among other factors.

[59] The operational temperature may be the temperature of the pump (e.g. a rotor or stator) when the rotor is rotating at an operational RPM at a thermal steady state. Preferably, when the temperature of the pump is at the operational temperature, the rotor cannot be coasted down from the operational RPM to cessation of rotation without a relatively high probability of the rotor clashing with the stator. For the avoidance of doubt, the relatively high probability of the rotor clashing with the stator is that as referred to hereinbefore.

[60] Typically, the threshold temperature is from about 10 °C to about 70 °C lower that the operational temperature, preferably from about 20 °C to about 50 °C lower.

[61] In embodiments the pump may comprise a plurality of temperature sensors. Each temperature sensor may be configured to measure the temperature of a different component of the vacuum pump. Preferably each temperature sensor may be configured to measure the temperature of a different stage of the vacuum pump. More preferably, each temperature sensor may be configured to measure the temperature of a stator of a different stage of the vacuum pump.

[62] Additionally, or alternatively, the pump may comprise a temperature sensor configured to measure the temperature of the motor or a component thereof.

[63] Typically, the temperature sensor may be a thermistor [64] Advantageously, measuring the temperature of the vacuum pump (rotor) via the temperature sensor may improve the efficiency of the method, as the coasting down of rotation until cessation of rotation may be initiated as soon as the pump is at or below the threshold temperature.

[65] In embodiments, temperature of the vacuum pump (rotor) may be measured via the temperature sensor at predetermined time intervals throughout the dwell time.

Preferably, the predetermined time interval is every second or less. Preferably, the predetermined time interval is less than about 0.5 seconds, more preferably less than about 0.1 seconds. Alternatively, the temperature of the vacuum pump (rotor) may be measure substantially continuously.

[66] Advantageously, measuring the temperature of the vacuum pump via the temperature sensor at predetermined time intervals or substantially continuously reduces the time between the vacuum pump temperature being at or below the threshold temperature and the coast down of the rotation of the rotor to cessation of rotation. This may further improve the efficiency of the method.

[67] The method may further comprise the step of only initiating coast down of the rotation of the rotor from the intermediate RPM to cessation of rotation once the temperature of the vacuum pump as measured by the temperature sensor is less than or equal to the threshold temperature.

[68] Beneficially, by only initiating coast down of the rotation of the rotor from the intermediate RPM to cessation of rotation once the temperature of the vacuum pump is less than or equal to the threshold temperature may ensure that there is substantially no probability of the rotor clashing with the stator, whilst providing a time and energy efficient coast down process. Furthermore, such a method ensures that there is substantially no -12 -probability of the rotor clashing with the stator during cessation of rotation regardless of the operational RPM that the rotor was rotating at during use. Therefore, such a method provides versatility to adapt to the operating conditions of the pump.

[069] Advantageously, measuring the temperature of the vacuum pump, preferably the rotor or via the stator, most preferably via the stator, may enable a more accurate determination of whether the clashing of the rotor with the stator is to be avoided. This is because it is predominantly the differential thermal expansion of the rotor and stator that results in the clashing during cessation of rotation.

[70] Alternatively, the rotor may be rotated at an intermediate RPM for a predetermined dwell time. The predetermined dwell time may be selected according to the specific pump configuration. Preferably, the predetermined dwell time may be determined to ensure that the dwell time is sufficient for the temperature of the vacuum pump to be below the threshold temperature by the end of the predetermined dwell time.

[71] This method may be automatically performed when the vacuum pump is switched off. Accordingly, the user may be unaware that the process is being used. This may apply to other methods according to the invention equally.

[72] The skilled person will appreciate that the threshold temperature and/or threshold RPM, and/or predetermined dwell time may be determined by finite element analysis and/or experimentation.

[073] Rotating the rotor at an intermediate RPM for a predetermined dwell time may advantageously simplify the method. Additionally, operating with a predetermined dwell time may enable no temperature sensor to be required, reducing the cost of the vacuum pump.

[074] Typically, the predetermined dwell time may be from about 0 to about 600 seconds, preferably from about 30 to about 480 seconds.

[075] Typically, the vacuum pump comprises a controller. The method of coasting down the rotor from operational RPM until cessation of rotation may be initiated by a single operator input to the controller. Preferably once initiated the controller may perform the method of the invention automatically.

-13 - [76] Typically, the controller may be coupled to the motor. The controller may be configured to control the rotational speed of the rotor. Preferably, the controller may be coupled to one or more temperature sensors of the vacuum pump.

[77] The controller may be configured to control the rotational speed of the rotor in response to signals generated by the temperature sensor(s). The controller may be configured to compare the temperature measured by the temperature sensor against predetermined temperature values, for example the threshold temperature, and adjust the rotational speed of the rotor accordingly by sending a signal to the motor.

[78] Preferably, once the method according to the present invention has been initiated, the temperature sensor may deliver signals indicating the temperature of the vacuum pump to the controller. The signals may be delivered substantially continuously, or at predetermined time intervals.

[79] Preferably, the controller is coupled to a display device. The display device may comprise a screen.

[080] Preferably, the controller may be configured such that a user may input commands thereto. The input of commands may occur via switches, touch screen, or other means. Preferably, the method according to the present invention may be initiated by a single user input to the controller, for example a switch. Advantageously, this enables the method to be entirely automated after the initial input by the user. This may reduce the likelihood of user error and may increase the efficiency of the method.

[81] Typically, the vacuum pump comprises a cooling system configured to reduce the temperature of the vacuum pump (e.g. rotor and/or stator). The method may comprise the step of operating the cooling system to reduce the temperature of the vacuum pump.

Preferably the method may further comprise initiating the cooling system, or increasing the cooling performance of the cooling system, when the rotor is rotating at an intermediate RPM.

[82] During operation of the vacuum pump, the motor driving the rotation of the rotor generates heat. This heat may cause thermal expansion of components of the vacuum pump.

-14 - [083] The cooling system may comprise a cooling fan, and/or a fluid cooling system. The fluid cooling system may be a water-cooling system.

[084] The cooling system may be configured to cool the motor during operation.

Additionally, or alternatively, the cooling system may be configured to reduce the temperature of one or more rotors of the vacuum pump.

[85] In embodiments wherein the cooling system comprises a cooling fan, increasing the cooling rate of the cooling system may involve increasing the rotation rate of the cooling fan. In embodiments wherein the cooling system comprises a fluid cooling system, increasing the cooling rate of the cooling system may involve increasing the fluid flow rate.

[86] Typically, during the method according to the present invention, when the rotational speed of the rotor is decreased, the cooling rate of the cooling system may be increased.

Advantageously, the reduction of the temperature of the vacuum pump with the cooling system may decrease the dwell time required for the temperature of the vacuum pump to be below the threshold temperature. This may reduce the time needed to achieve cessation of rotation of the rotor via the method of the present invention.

[87] Typically, the cooling system may be coupled to the controller, such that the operation of the cooling system may be controlled by the controller. Advantageously, this may enable the operation of the cooling system to be automated, and to require no user input beyond initiating the method via the controller.

[88] In embodiments, the cooling effect of the cooling system may be increased whilst rotating the rotor is rotating at a rotational speed that is greater than the threshold RPM, thereby reducing the temperature of the vacuum pump to below the threshold temperature wherein the rotation of the rotor can be coasted down to cessation of rotation.

[89] By reducing the rotation speed of the rotor to an intermediate RPM at or below the threshold RPM, preferably whilst operating the cooling system, the cooling rate of the vacuum pump can be increased. Thus, the method according to the present invention allows for a faster and more energy efficient cessation of rotation of the rotor(s) of the 35 vacuum pump.

-15 - [090] Typically, the threshold RPM is from about 30% to about 70% of the maximum rotational speed of the rotor, preferably from about 40% to about 50% of the maximum rotational speed of the rotor. For example, the threshold RPM may be from about 5000 RPM to about 8000 RPM, preferably from about 6000 RPM to about 7500 RPM. The skilled person will appreciate that the threshold RPM may depend on the specific pump to which the method is being applied, and/or the specific application in which the pump is being used. Moreover, the skilled person will appreciate that the operational RPM may be at or below the maximum rotational speed of the rotor.

[091] In a further aspect, the present invention provides a method of ceasing rotation of a rotor of a vacuum pump. The vacuum pump comprises a pump chamber including the rotor and a stator. The rotor is rotating at an operational RPM and at a thermal steady state.

[092] The method comprises the steps of measuring the temperature of the vacuum pump (rotor), and determining from the temperature of the vacuum pump whether the rotor is rotating at an operational RPM greater than a threshold RPM. If the operational RPM is greater than the threshold RPM rotation of the rotor is ceased according to the method defined in the preceding aspect or, if not, the rotor is coasted down from the operational RPM until cessation without a dwell time at an intermediate RPM.

[93] The threshold RPM is the maximum speed of rotation at a thermal steady state wherein the rotor can be coasted down until cessation of rotation with substantially no probability of the rotor clashing with the stator.

[94] Advantageously, this method may ensure that the method according to the preceding aspect is only used when the rotor is rotating at an operational RPM greater than a threshold RPM. If the rotor is rotating at an operational RPM that is less than the threshold RPM, then there is substantially no probability of the rotor clashing with the stator. Accordingly, there is no requirement for the method as set out in the preceding aspect, and the rotation of the rotor may be ceased without rotating the rotor at an intermediate RPM for a dwell time. Beneficially, this may ensure the cessation of rotation of the rotor of the vacuum pump occurs as efficiently as possible. As discussed above, a cooling system may be used to bring the temperature of the vacuum pump (e.g. rotor) to at or below the threshold temperature.

-16 - [095] Preferably, the temperature of the vacuum pump (e.g. rotor) is measured using one or more temperature sensors.

[096] In a further aspect, the present invention provides a vacuum pump. The vacuum pump comprises a pump chamber including a rotor, a stator, and a controller configured to control the rotational speed of the rotor.

[97] The rotor and stator are arranged such that the distance therebetween ensures that if during operation the rotor is rotating above a threshold RPM and coasted down to cessation of rotation, there is a relatively high probability of the rotor clashing with the stator. The threshold RPM being the maximum speed of rotation at a thermal steady state wherein the rotor can be coasted down until cessation of rotation with substantially no probability of the rotor clashing with the stator.

[98] During operation when the rotor is coasting down from an operational RPM that is greater than the threshold RPM, the controller is configured to reduce the rotation speed of the rotor to an intermediate RPM, at which there is substantially no probability of the rotor clashing with the stator, and retain the rotational speed at said intermediate RPM for a dwell time sufficient for the vacuum pump to be at or below a threshold temperature. The intermediate RPM is at or below the threshold RPM. The controller is configured to, after the dwell time, coast down the rotation of the rotor until cessation of rotation.

[99] The threshold temperature is the temperature of the vacuum pump (e.g. rotor) when the rotor is rotating at the threshold RPM at a thermal steady state.

[100] The vacuum pump may further comprise a cooling system configured to reduce the temperature of the vacuum pump.

[101] The cooling system may comprise a cooling fan, and/or a fluid cooling system. The fluid cooling system may be a water-cooling system. Preferably, the cooling system comprises a cooling fan.

[102] The cooling system may be configured to cool the motor during operation.

Advantageously, the reduction of the temperature of the vacuum pump using the cooling system may decrease the dwell time required for the temperature of the vacuum pump to -17 -be below the threshold temperature. In turn, this may reduce the time required to achieve cessation of rotation of the rotor.

[103] Typically, the cooling system may be coupled to the controller, such that the controller may control the operation of the cooling system. Advantageously, this may enable the operation of the cooling fan to be automated, and to require no user input beyond initiating the cessation of rotation via the controller, e.g. pressing an "off" command.

[104] Typically, the pump is a multi-stage vacuum pump. Preferably, the pump is a multi-stage roots pump. For example, the pump may be an nXLi Dry Multistage Roots Pump or an nXRi Dry Multistage Roots Pump as produced by Edwards Vacuum.

[105] In a further aspect, the present invention provides a vacuum pump comprising a pump chamber including a rotor and a stator, and a controller configured to control the rotational speed of the rotor. The rotor and stator are arranged such that the minimum distance therebetween is such that if during operation the rotor is rotating above a threshold RPM and coasted down to cessation of rotation, there is a relatively high probability of the rotor clashing with the stator.

[106] The threshold RPM is the maximum speed of rotation at a thermal steady state wherein the rotor can be coasted down until rotation ceases with substantially no probability of the rotor clashing with the stator. The pump may comprise a temperature sensor configured to measure the temperature of the vacuum pump. The controller may be coupled to the temperature sensor.

[107] During operation when the rotor is coasting down from an operational RPM that is greater than the threshold RPM, the controller is configured to reduce the rotation speed of the rotor at a rate such that there is substantially no probability of the rotor clashing with the stator. This may be achieved by the temperature sensor sending temperature signals at predetermined times, or substantially continuously, to the controller.

-18 - [108] For the avoidance of doubt, a "relatively high probability of the rotor clashing with the stator, and "substantially no chance of the rotor clashing with the stator" is as defined hereinbefore.

[109] The controller may compare the temperature signal received from the temperature sensor to a predetermined rotor rotational speed vs pump threshold temperature. The maximum rotor rotational speed vs temperature threshold may determine the maximum permissible thermal steady state pump temperature at each rotor rotation speed from which the rotor rotation speed may be coasted down to cessation of rotation with substantially no probability of the rotor clashing with the stator. Preferably, if the measured temperature of the vacuum pump is above the predetermined rotor rotational speed vs pump temperature threshold then the controller will slow the rate of decrease of the rotation of the rotor.

[110] For the avoidance of doubt, features of aspects and embodiments described herein may be combined, and still fall within the scope of the present invention.

Brief Description of Figures

Preferred features of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1(a-d) illustrates a schematic of operation of a pump according to the prior art.

Figure 2(a-d) illustrates a schematic of operation of a pump according to the present invention.

Figure 3 illustrates a flow chart of a method of operation of a pump according to the present invention.

Detailed Description

[111] Figure 1(a-d) show a schematic of a pump according to the prior art. The figures show a cross-sectional view of a rotor stage (1) of a vacuum pump [112] The rotor stage (1) comprises a rotor (2) mounted on a rotor shaft (3). The rotor shaft (3) may be substantially parallel to a pump direction (A). The pump direction (A) -19 -defines the direction of bulk fluid flow during use of the pump. The pump direction (A) may define the direction between a pump inlet (not shown) and a pump outlet (not shown). The rotor stage (1) further comprises a pair of stators (4,5). The pair of stators include an upstream stator (4) and a downstream stator (5). The terms "upstream" and "downstream" define the position of each stator relative to a specific rotor. The upstream stator (4) and downstream stator (5) may each be in the form of an interstage partition wall.

[113] Figure 1(a) shows the arrangement of the rotor stage (1) when the pump is switched off, i.e. the rotor (2) is not rotating relative to the pair of stators (4,5). This shows the configuration of the rotor (2) within the rotor stage (1) and the clearances within the pump when it is at rest.

[114] Between the rotor (2) and the upstream stator (4), there is an upstream rotor clearance (x), defining the minimum distance between the rotor (2) and the upstream stator (4). Between the rotor (2) and the downstream stator (5), there is a downstream rotor clearance (y), defining the minimum distance between the rotor (2) and the downstream stator (5). Typically, the upstream rotor clearance (x) is less than the downstream rotor clearance (y) when the rotor (2) is not rotating relative to the pair of stators (4,5).

[115] The pump is configured so that the upstream rotor clearance (x) and downstream rotor clearance (y) are relatively small whilst ensuring that the rotor does not touch a stator, taking into account manufacturing tolerances of the components. This may reduce fluid leakage between the stages of the vacuum pump.

[116] When the rotor (2) is at rest, the fluid pressure within the rotor stage is at equilibrium, i.e. there is substantially no difference between the fluid pressure at the upstream end of the rotor stage (1) and the downstream end of the rotor stage (1). The upstream end of the rotor stage (1) may be defined as between the upstream stator (4) and the rotor (2). The downstream end of the rotor stage (1) may be defined as between the rotor (2) and the downstream stator (5).

-20 - [117] Figure 1(b) shows the rotor stage when the pump has been switched on, i.e. during the first 20 seconds of use. Wien the pump is switched on, the rotor (2) rotates upon its rotational axis (Z) relative to the stators (4,5).

[118] When the rotor (2) is rotated, a pressure gradient is established between an upstream end and a downstream end of the rotor stage (1). The fluid pressure may be lower at the upstream end of the stage and higher at the downstream end of the stage. Accordingly, this may bias the position of the rotor (2) towards an upstream end of the stage (1).

[119] Furthermore, rotodynamic effects and settling of the bearings (not shown) upon which the rotor shaft (3) is rotatably mounted, contribute to the shifting of the rotor towards the upstream end of the rotor stage (1).

[120] The shift of the rotor (2) towards the upstream end of the rotor stage (1) may reduce the upstream clearance (x) and increase the downstream clearance (y) in comparison to Figure 1(a).

[121] This upstream shift of the rotor is also a factor that may be taken into account when the pump is being designed, to minimise the risk of the rotor (2) clashing with the stator (4).

[122] Figure 1(c) shows the rotor stage (1) when the pump has been running and the temperature of the pump (rotor) has increased to an operational temperature, for example 85 °C. The operational temperature is greater than the threshold temperature.

[123] The temperature of the vacuum pump has increased and caused differential thermal expansion of the rotor (2) and stators (4,5), respectively, because they are made from different materials. This effectively causes a shift of the rotor (2) towards the downstream end of the rotor stage (1).

[124] The shift of the rotor (2) towards the downstream end of the rotor stage (1) may increase the upstream clearance (x) and decrease the downstream clearance (y) in comparison to Figure 1(b).

-21 - [125] Figure 1(d) shows the rotor stage (1) when the pump (rotor) is at a temperature above 70 °C, when rotation of the rotor (2) is ceased. The temperature of the vacuum pump being above the threshold temperature.

[126] When the rotation of the rotor (2) is ceased, the pressure gradient between the upstream and downstream ends of the rotor stage (1) reduces. This removes the bias on the position of the rotor (2) towards the upstream end of the rotor stage (1), causing a further shift of the rotor (2) towards the downstream end of the rotor stage (1). As shown, the further shift causes the rotor (2) to clash with the downstream stator (5). The clash is because the downstream clearance (y) effectively becoming zero, resulting in direct contact between the rotor (2) and the stator (5).

[127] This clash may result in damage to the rotor (2) and/or stator (5), along with machine downtime.

[128] Figures 2(a-d) show a schematic of operation of a pump according to the present invention. The figures show a cross-sectional view of a rotor stage (6) of a pump according to the present invention.

[129] Figures 2(a) and 2(b) are substantially the same as those of Figure 1(a) and (b), the conditions and processes within the pump are and so the description will not be repeated. The rotor stage (6) comprises a controller (10) configured to control the rotation speed of the rotor (7).

[130] Figure 2(c) shows the rotor stage (6) when the pump has been running and the temperature of the pump has increased above ambient temperature (e.g. 20 °C) to an operational temperature, for example 85 °C. The operational temperature is greater than the threshold temperature. As the rotor and stators are made from different materials each with different coefficients of thermal expansion, their heating effectively causes a shift of the rotor towards the downstream end of the rotor stage (6).

-22 - [131] When the pump is switched off using a method according to the invention, the rotation speed of the rotor (7) is reduced from the operational RPM to an intermediate RPM. The intermediate RPM is less than the threshold RPM.

[132] The rotor stage (6) further comprises a temperature sensor (11) configured to measure the temperature of the stator (8,9) during operation. The temperature of the rotor may be inferred from the temperature of the stator.

[133] The rotation speed of the rotor (7) is held at the intermediate RPM for a dwell time.

During the dwell time, the temperature sensor (11) is continuously measuring the temperature of the stator (8,9). The controller (10) may compare the temperature signal received from the temperature sensor (11) to the threshold temperature. When the temperature of the stator (8,9) indicates that the rotor has dropped to at or below the threshold temperature (e.g. at or below 700C), then drive to the rotor (7) may be removed such that the rotor slows to a standstill (e.g. coasted down).

[134] Figure 2(d) shows the rotor stage (6) when the rotation of the rotor (7) has ceased. As is shown, there is no clash between the rotor (7) and the stators (8,9) during coast down of rotation of the rotor (7) to cessation of rotation [135] There is a small reduction in the downstream clearance (y) between Figure 2(c) and 2(d), but not enough to cause a clash between the rotor (7) and the stator (9).

[136] As the pump cools further, the pump will return towards the configuration shown in Figure 2(a) reaching said original configuration at about ambient temperature (e.g. 20 °C).

[137] Figure 3 shows a flow chart of a method of operation of a pump according to the present invention. Initially, prior to initiation of the method of ceasing rotation of the rotor of the vacuum pump, the rotor is rotating at an operational RPM (12). The operational RPM is greater than a threshold RPM, such that the rotor cannot be coasted down from the operational RPM to cessation of rotation without a relatively high probability of the rotor clashing with the stator. -23 -

[138] The method then comprises the step of rotating the rotor at an intermediate RPM (13). Wien rotating at the intermediate RPM there is substantially no probability of the rotor clashing with the stator. The intermediate RPM is at or below a threshold RPM. The threshold temperature is the temperature of the vacuum pump when the rotor is rotating at the threshold RPM at a thermal steady state. The rotor is rotated at the intermediate RPM for a dwell time sufficient for the vacuum pump to be at or below a threshold temperature.

[139] The method may further comprise the step of initiating a cooling system (14). The cooling system may be configured to reduce the temperature of the vacuum pump.

Preferably, the temperature sensor may be configured to measure the temperature of the rotor or the stator.

[140] The method may further comprise the step of measuring the temperature of the vacuum pump via a temperature sensor (15) during the dwell time. The temperature may be measured via the temperature sensor at predetermined time intervals throughout the dwell time. Preferably, the predetermined time intervals may be every second or less.

[141] The method comprises the step of coasting down the rotation of the rotor until cessation of rotation (16).

[142] It will be appreciated that various modifications may be made to the embodiments shown without departing from the spirit and scope of the invention as defined by the accompanying claims as interpreted under patent law.

-24 -Reference Key

1. Rotor stage (prior art)

2. Rotor (prior art)

3. Rotor shaft (prior art)

4. Stator (prior art)

5. Stator (prior art)

6. Rotor stage 7. Rotor 8. Stator 9. Stator 10. Controller 11. Temperature Sensor 12. Rotor rotating at an operational RPM 13. Rotor rotating at an intermediate RPM 14. Initiating a cooling system 15. Measuring temperature of the vacuum pump via a temperature sensor 16. Coasting down rotation of the rotor until cessation of rotation -25 -

Claims

Claims 1. A method of ceasing rotation of a rotor of a vacuum pump, the vacuum pump comprising a pump chamber including the rotor and a stator; wherein prior to initiation of the method the rotor is rotating at an operational RPM, wherein the operational RPM is greater than a threshold RPM, such that the rotor cannot be coasted down from the operational RPM to cessation of rotation without a relatively high probability of the rotor clashing with the stator; the method comprising the steps of: a) rotating the rotor at an intermediate RPM, at which there is substantially no probability of the rotor clashing with the stator, for a dwell time sufficient for the vacuum pump to be at or below a threshold temperature, wherein the intermediate RPM is at or below the threshold RPM; b) subsequently coasting down the rotation of the rotor until cessation of rotation; wherein the threshold RPM is the maximum speed of rotation at a thermal steady state wherein the rotor can be coasted down until cessation of rotation with substantially no probability of the rotor clashing with the stator; and wherein the threshold temperature is the temperature of the vacuum pump when the rotor is rotating at the threshold RPM at a thermal steady state.
2. The method according to claim 1, wherein the vacuum pump comprises a temperature sensor configured to measure the temperature of the vacuum pump, the method further comprising the step of measuring the temperature of the vacuum pump via the temperature sensor during the dwell time and determining whether the temperature is at or below the threshold temperature.
3. The method according to claim 2, comprising measuring the temperature of the vacuum pump via the temperature sensor at predetermined time intervals -26 -throughout the dwell time, preferably wherein the predetermined time interval is every second or less.
4. The method according to either claim 2 or claim 3, comprising the step of only initiating coast down of the rotation of the rotor from the intermediate RPM until cessation of rotation once the temperature of the vacuum pump as measured by the temperature sensor is less than or equal to the threshold temperature.
5. The method according to any of claims 2 to claim 4, wherein the temperature sensor is configured to measure the temperature of the rotor or the stator, preferably the rotor.
The method according to claim 1, wherein during step (a), the rotor is rotated at an intermediate RPM for a predetermined dwell time.
7. The method according to claim 6, wherein the predetermined dwell time is less than or equal to about 600 seconds, preferably from about 30 seconds to about 480 seconds.
8. The method according to any preceding claim, wherein the vacuum pump comprises a controller, and wherein the method is initiated by a single user input to the controller.
9. The method according to any preceding claim, wherein the vacuum pump comprises a cooling system configured to reduce the temperature of the vacuum pump, wherein the method comprises the step of operating the cooling system to reduce the temperature of the vacuum pump, preferably further comprising increasing the cooling performance of the cooling system when the rotor is rotating at the intermediate RPM.
10. The method according to any preceding claim, wherein the threshold RPM is from about 5000 RPM to about 8000 RPM, preferably from about 6000 RPM to about 7500 RPM. -27 -
11. A method of ceasing rotation of a rotor of a vacuum pump, the vacuum pump comprising a pump chamber including the rotor and a stator, wherein the rotor is rotating and at a thermal steady state; the method comprising the steps of: a) measuring the temperature of the vacuum pump, preferably the temperature of the rotor and/or the stator; b) determining whether the rotor is rotating at an operational RPM greater than a threshold RPM; c) if so, ceasing rotation of the rotor according to the method defined in any of claims 1 to 10; wherein the threshold RPM is the maximum speed of rotation at a thermal steady state wherein the rotor can be coasted down until cessation of rotation with substantially no probability of the rotor clashing with the stator.
12. A vacuum pump comprising a pump chamber including a rotor, a stator, and a controller configured to control the rotational speed of the rotor; wherein the rotor and stator are arranged such that the minimum distance therebetween is such that if during operation the rotor is rotating above a threshold RPM and coasted down to cessation of rotation, there is a relatively high probability of the rotor clashing with the stator; wherein the threshold RPM is the maximum speed of rotation at a thermal steady state wherein the rotor can be coasted down until cessation of rotation with substantially no probability of the rotor clashing with the stator, wherein during operation when the rotational speed of the rotor is reduced from an operational RPM that is greater than the threshold RPM, the controller is configured to reduce the rotational speed of the rotor to an intermediate RPM, at which there is substantially no probability of the rotor clashing with the stator, and retain the rotation speed at said intermediate RPM for a dwell time -28 -sufficient for the vacuum pump to be at or below a threshold temperature, wherein the intermediate RPM is at or below the threshold RPM; the controller being configured to, after the dwell time, coast down the rotation of the rotor until cessation of rotation; wherein the threshold temperature is the temperature of the vacuum pump when the rotor is rotating at the threshold RPM at a thermal steady state.
13. The vacuum pump according to claim 12, further comprising a temperature sensor configured to measure the temperature of the rotor and/or stator.
14. The vacuum pump according to any of claims 12 or claim 13, further comprising a cooling system configured to reduce the temperature of the rotor, preferably wherein the cooling system comprises a fan.
15. The vacuum pump according to either claim 12 to 14, wherein the pump is a multi-stage vacuum pump, preferably a multi-stage roots pump.