METHOD FOR STOPPING A VACUUM PUMP

Abstract

The present invention provides a method of ceasing rotation of a rotor of a vacuum pump. 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, and subsequently coasting down the rotation of the rotor until cessation of rotation.

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 throughout the dwell time, preferably wherein the predetermined time interval is every second or less.

4. The method according to claim 2, 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 claim 2, wherein the temperature sensor is configured to measure the temperature of the rotor or the stator, preferably the rotor.

6. 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 claim 1, 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 claim 1, 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 claim 1, wherein the threshold RPM is from about 5000 RPM to about 8000 RPM, preferably from about 6000 RPM to about 7500 RPM.

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 claim 1; 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 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 claim 12, 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 claim 12, wherein the pump is a multi-stage vacuum pump, preferably a multi-stage roots pump.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0115] Preferred features of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0116] FIG. 1(a) shows a schematic of the arrangement of the rotor stage (1) when the pump is switched off under the prior art.

[0117] FIG. 1(b) shows a schematic of the rotor stage when the pump has been switched on, i.e. during the first 20 seconds of use under the prior art.

[0118] FIG. 1(c) shows a schematic of the rotor stage (1) when the pump has been running and the temperature of the pump (rotor) has increased to an operational temperature under the prior art.

[0119] FIG. 1(d) shows a schematic of the rotor stage (1) when the pump (rotor) is at a temperature above 70 C., when rotation of the rotor (2) is ceased under the prior art.

[0120] FIG. 2(a) shows a schematic of the arrangement of the rotor stage (1) when the pump is switched off under one embodiment.

[0121] FIG. 2(b) shows a schematic of the rotor stage when the pump has been switched on, i.e. during the first 20 seconds of use under one embodiment.

[0122] FIG. 2(c) shows a schematic of the rotor stage (1) when the pump has been running and the temperature of the pump (rotor) has increased to an operational temperature under one embodiment.

[0123] FIG. 2(d) shows a schematic of the rotor stage (1) when the pump (rotor) is at a temperature above 70 C., when rotation of the rotor (2) is under one embodiment.

[0124] FIG. 3 illustrates a flow chart of a method of operation of a pump according to the present invention.

DETAILED DESCRIPTION

[0125] FIG. 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.

[0126] 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) 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.

[0127] FIG. 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.

[0128] 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).

[0129] 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.

[0130] 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).

[0131] FIG. 1(b) shows the rotor stage when the pump has been switched on, i.e. during the first 20 seconds of use. When the pump is switched on, the rotor (2) rotates upon its rotational axis (Z) relative to the stators (4,5).

[0132] 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).

[0133] 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).

[0134] 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 FIG. 1(a).

[0135] 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).

[0136] FIG. 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.

[0137] 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).

[0138] 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 FIG. 1(b).

[0139] FIG. 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.

[0140] 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).

[0141] This clash may result in damage to the rotor (2) and/or stator (5), along with machine downtime.

[0142] FIGS. 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.

[0143] FIGS. 2(a) and 2(b) are substantially the same as those of FIGS. 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).

[0144] FIG. 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).

[0145] 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.

[0146] 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.

[0147] 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 70 C.), then drive to the rotor (7) may be removed such that the rotor slows to a standstill (e.g. coasted down).

[0148] FIG. 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.

[0149] There is a small reduction in the downstream clearance (y) between FIGS. 2(c) and 2(d), but not enough to cause a clash between the rotor (7) and the stator (9).

[0150] As the pump cools further, the pump will return towards the configuration shown in FIG. 2(a) reaching said original configuration at about ambient temperature (e.g. 20 C.).

[0151] FIG. 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.

[0152] The method then comprises the step of rotating the rotor at an intermediate RPM (13). When 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.

[0153] 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.

[0154] 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.

[0155] The method comprises the step of coasting down the rotation of the rotor until cessation of rotation (16).

[0156] 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.

[0157] Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.

[0158] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.