LUBRICANT-SEALED VACUUM PUMP, LUBRICANT FILTER AND METHOD

Abstract

A lubricant-sealed vacuum pump configured to pump fluid from an inlet to an exhaust, method and filter are disclosed. The lubricant-sealed vacuum pump comprises: a rotor; a filter for filtering lubricant from fluid to be output by the pump; control circuitry for controlling a speed of rotation of the rotor, the control circuitry being configured to control rotation of the rotor, such that the rotor rotates at a reduced speed initially when a pressure at the inlet is high and rotates at a higher operational speed when the pressure at the inlet has reduced.

Claims

1. A lubricant-sealed vacuum pump configured to pump fluid from an inlet to an exhaust, said lubricant-sealed vacuum pump comprising: a rotor; a motor for rotating said rotor; a filter for filtering lubricant from fluid to be output by said pump; control circuitry for controlling a speed of rotation of said rotor, said control circuitry being configured to control rotation of said rotor, such that said rotor rotates at a selected reduced speed initially when a pressure at said inlet is high and rotates at a higher operational speed when said pressure at said inlet has reduced.

2. The lubricant-sealed vacuum pump according to claim 1, wherein said motor comprises a variable speed motor for driving said rotor, said control circuitry being configured to control said speed of rotation of said rotor by controlling a speed of rotation of said motor.

3. The lubricant-sealed vacuum pump according claim 1, wherein said lubricant-sealed vacuum pump comprises at least one sensor for sensing a property of the fluid being pumped, said property being indicative of the pressure at the inlet and said control circuitry comprises a feedback control system for controlling the speed of the rotor in dependence upon the sensed property.

4. The lubricant-sealed vacuum pump according to claim 1, wherein said selected reduced speed is a fixed reduced speed and said control circuitry is configured to control said rotor to rotate at said fixed reduced speed for a predetermined period, and to increase said speed to said higher operational speed after said predetermined period.

5. The lubricant-sealed vacuum pump according to claim 4, wherein said predetermined period comprises a predetermined period of time.

6. The lubricant-sealed vacuum pump according to claim 3, wherein said predetermined period comprises a period while a pressure at one of said inlet or outlet is above a predetermined value.

7. The lubricant-sealed vacuum pump according to claim 3, wherein said predetermined period comprises a period while a flow rate being pumped is greater than a predetermined amount.

8. The lubricant-sealed vacuum pump according to claim 1, wherein said selected reduced speed is a variable reduced speed.

9. The lubricant-sealed vacuum pump according to claim 3, wherein said selected reduced speed is a variable reduced speed and wherein said control circuitry is configured to set said variable reduced speed in dependence upon a signal received from said at least one sensor.

10. The lubricant-sealed vacuum pump according to claim 9, wherein said at least one sensor comprises a flow rate sensor and said control circuitry is configured to set said variable reduced speed to provide a predetermined fluid flow rate.

11. The lubricant-sealed vacuum pump according to claim 9, wherein said at least one sensor comprises a pressure sensor and said control circuitry is configured to set said variable reduced speed in dependence upon a signal from said pressure sensor, said speed being increased in response to said pressure decreasing.

12. The lubricant-sealed vacuum pump according to claim 1, wherein said filter is a reduced sized filter, said filter being sized for filtering a predetermined maximum flow rate of fluid pumped by said lubricant-sealed vacuum pump, said control circuitry being configured to maintain said flow rate of fluid being pumped below said maximum flow rate by controlling said rotor to rotate initially at said selected reduced speed.

13. The lubricant-sealed vacuum pump according to claim 1, wherein said selected initial reduced speed is less than a half of said higher operational speed.

14. A method of evacuating a chamber using a lubricant-sealed pump according to claim 1, said method comprising: rotating a rotor of said pump at a selected initial reduced speed for a predetermined period; and increasing a rotational speed of said rotor to a higher operational speed after a predetermined period.

15. A filter for a lubricant-sealed vacuum pump, the lubricant-sealed vacuum pump having an inlet and a rotor that rotates at an initial speed when a pressure at the inlet is high and rotates at an operational speed higher than the initial speed when the pressure at the inlet is reduced, said filter comprising a filtration surface area that is smaller than or equal to a volumetric flow rate of the lubricant-sealed vacuum pump when the rotor rotates at the initial speed divided by a permeability and pressure drop across the filter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

[0044] FIG. 1 schematically shows a vacuum pump according to an embodiment;

[0045] FIG. 2 schematically shows a vacuum pump where the rotational speed is controlled in dependence upon flow rate according to an embodiment;

[0046] FIG. 3 schematically shows a vacuum pump where the rotational speed is controlled in dependence upon pressure according to an embodiment;

[0047] FIG. 4 shows different examples of controlled rotor speed according to an embodiment;

[0048] FIG. 5 shows the flow rate of fluid pumped by rotors rotating at the rotor speeds shown in FIG. 4;

[0049] FIG. 6 shows a comparison of the rotational speed versus increase in pump down time and decrease in filter size;

[0050] FIG. 7 shows a filter according to an embodiment; and

[0051] FIG. 8 shows a flow diagram showing a method of evacuating a chamber according to an embodiment.

DETAILED DESCRIPTION

[0052] Before discussing the embodiments in any more detail, first an overview will be provided.

[0053] In order to reduce the flow passing through a lubricant-sealed pump and filter, the pumping speed is reduced when the pump is started as at this point flow rate is generally at its highest. This is done by varying the rotational speed of the rotor. As an example we can do this in two ways: [0054] 1. Constant limited speed during startup; [0055] 2. Variable ramped-up speed during startup; [0056] 3. Variable speed at startup adjusted in response to detected properties of the fluid being pumped, in order to have a constant flow at exhaust.

[0057] This allows us to substantially reduce the filter size by modifying the starting speed of the pump and in this way the maximum flow rate that passes through by the pump and the filter.

[0058] FIG. 1 shows an oil-sealed pump according to an embodiment. The oil-sealed pump comprises a motor 20 for driving a rotor 10 within a pumping chamber (not shown). The rotor 10 pumps fluid that arrives at an inlet marked by arrow 12 through the pumping chamber to a filter 30 which acts to remove oil mist from the pumped fluid, the fluid being exhausted at exhaust 14.

[0059] The motor 20 is a variable speed motor and the speed of the motor and thus the speed of rotation of the rotor is controlled by control circuitry 22. In this embodiment, control circuitry 22 is configured to control the motor to rotate at an initially reduced speed for a predetermined time and then to accelerate up to full operational speed after that time. In this embodiment, the initial speed is about a quarter of the full operational speed and the pump is configured to operate at this reduced speed during start up for approximately 20 seconds. The result of this is that when the pressure is initially high in the chamber being evacuated the rotational speed is low and thus, the flow rate of the fluid through the pump is reduced compared to a conventional pump. Once the pressure in the chamber has reduced the speed of rotation of the rotor is increased to the normal operational speed. At this point as the pressure in the chamber is reduced, although the rotor starts rotating at the faster speed the flow rate of fluid through the pump is not as high as had the rotor rotated at this higher speed initially. In this way, the maximum flow rate that goes through filter 30 is reduced and the size of the filter can be correspondingly reduced.

[0060] The time to pump down the chamber to the working pressure is increased owing to the initial lower speed however this is generally acceptable as this is only a very small fraction of the time of operation of the pump.

[0061] In the embodiment of FIG. 1 the control circuitry 22 is configured to control the rotor to operate at a constant slower speed for a predetermined time. In other embodiments the control circuitry may be configured to operate at a slower initial speed and to gradually ramp up over time to the higher operational speed. Where the pump is configured to evacuate a chamber with known dimensions or with dimensions within certain known limits then restricting the speed of rotation based on time of operation is acceptable as the pressure reduction that occurs at this time can be estimated, based on the known dimensions and pump speed, allowing the time to be selected such that the operational speed increases when the pressure within the chamber has dropped sufficiently to enable the flow rate not to exceed a certain maximum value that the filter 30 has been configured to support.

[0062] FIG. 2 shows an alternative embodiment where rather than the control circuitry being configured to pump at a reduced speed for a predetermined time the control circuitry is configured to receive signals from a flow rate sensor 26. Flow rate sensor 26 measures the flow rate at the exhaust of the pump and transmits a signal indicative of this flow rate to control circuitry 22. Control circuitry 22 is configured to control the motor to rotate at a speed that allows the flow rate measured by sensor 26 to be substantially constant for the initial period at or close to a maximum flow rate that the filter 30 is configured to support. In this way, the size of the filter can be reduced and yet the pump down time will not be increased unduly.

[0063] The flow rate sensor may be a volumetric or mass flow rate sensor. Although the flow rate sensor 26 is shown in this embodiment on the exhaust of the pump it may in other embodiments be located elsewhere within the system.

[0064] FIG. 3 shows an alternative embodiment where control circuitry 22 receives a signal from a pressure sensor 24. In this embodiment the pressure sensor directly measures the pressure of the gas at the input to the pump. In other embodiments pressure may be sensed at a different part of the pump or it may be sensed indirectly by, for example, sensing the torque exerted by the motor on the rotor. Control circuitry 22 controls the speed of the motor and thus, the speed of the rotor in dependence upon the pressure of the fluid being pumped. As noted previously, the filter 30 is configured for a particular maximum flow rate and the flow rate of the gas being pumped will depend on its pressure and the rotational speed of the rotor. Thus, depending on the pressure the rotational speed of the rotor can be controlled to maintain the fluid flow below this maximum flow rate. Again this control of the motor allows effective and accurate control of a flow rate to protect the filter from being overloaded without unduly reducing the initial pump down time.

[0065] FIG. 4 shows examples of different ways in which the rotational speed of a pump’s rotor may be controlled to change over time. Curve 40 shows a constant rotational speed of 1800 rpm and this represents a conventional pump which operates at an operational speed of 1800 rpm from start up until the end of the pumping cycle.

[0066] Curve 42 shows rotor speed variations according to one embodiment, where an initial low speed of 400 rpm is increased over a startup time in response to readings from a sensor, using a feedback loop to provide a substantially constant mass flow rate through the pump during initial the startup time until the maximum operational speed of the pump is reached.

[0067] Curve 44 shows an alternative embodiment where an initial low speed of 400 rpm is provided for a set period of time and is then increased to the operational speed of the pump.

[0068] FIG. 5 shows the impact of the different pumping speeds of FIG. 4 on the flow rate through the pump and on the pump down times. In addition to curves 40, 42 and 44 corresponding to those of FIG. 4, there is also curve 41 which is a theoretical curve for a constant flow rate of a conventional pump which corresponds to curve 40 that shows the measured curve for such a conventional pump. As can be seen from this figure the control of the speed according to curve 42 provides a constant maximum flow rate during the initial startup period which flow rate decreases once the maximum operational speed is reached.

[0069] Curve 44 shows how a constant reduced speed during the startup period provides a flow rate that gradually decreases as the pressure decreases. When the point at which the pump speed is increased to operational maximum speed is reached, there is a sharp increase in flow rate. The point at which this speed is increased is set so that this peak does not rise above the maximum flow rate that is acceptable to the filter of the pump.

[0070] As can be seen the pump down time for the different examples of pumping speeds varies, it being lowest for a conventional pump. The pump down time, shown by curve 42 with the variable reduced speed is lower than the pump down time required for curve 44, where the reduced speed is constant. However, a variable pumping speed such is provided by curve 42 may require a sensor to provide the feedback to maintain the flow rate close to the maximum value that the reduced sized filter can support.

[0071] In the examples of FIGS. 4 and 5, the initial rotor speed is 400 rpm for both example embodiments (42, 44) and this initial speed will set the maximum flow rate and determine the size of filter required. In this example, it is less than a quarter of the operational speed and thus, the filter can be correspondingly reduced in size.

[0072] FIG. 6 shows how both the required size of the filter shown by curve 35 and the time for pump down increases as the maximum flow rate that the pump is configured for is decreased for both mass and volumetric flow rate limits. These reduced maximum flow rates are provided by providing a reduced initial rotational speed of the pump. Curve 46 shows how the time increases where the maximum flow rate is limited by volumetric flow, while curve 48 shows how the time increases where the flow rate is limited by mass flow rate.

[0073] Table 1 below provides this information in table form.

TABLE-US-00001 Maximum flow rate Size of the filter (%) time (s) Pumpdown time(s) time increase (%) Lim Volume flow Temps (s) Time increase (%) Lim mass flow 120 100% 17,8 17,8 0% 17,8 0% 110 92% 17,8 18,0 1% 18,0 1% 100 83% 17,8 18,0 1% 18,0 1% 90 75% 17,8 18,2 2% 18,0 1% 80 67% 17,8 18,6 4% 18,2 2% 70 58% 17,8 19,0 7% 18,4 3% 60 50% 17,8 19,8 11% 18,8 6% 50 42% 17,8 21,2 19% 19,4 9% 40 33% 17,8 23,8 34% 20,4 15% 30 25% 17,8 29,2 64% 22,4 26% 20 17% 17,8 42,2 137% 26,6 49% 10 8% 17,8 84,0 372% 41,2 131%

[0074] As can be seen from the graph of FIG. 6 and from table 1, where the maximum flow rate is set to 120, this corresponds to the flow rate of a conventional pump and the filter required is that of the conventional pump, and this is set as 100%. The pump down time is the pump down time of a conventional pump which is in this case 17.8 seconds. Where the maximum flow rate is decreased from 120 to 110, so by 8% the size of the filter is correspondingly reduced by 8% whereas the pump down time increases by 1% for both volumetric and mass flow rate. As the maximum flow rate continues to decrease so the pump down times increase and the size of the filter decreases. As can be seen from FIG. 6 there is an optimal point where the size of the filter has decreased significantly and yet the pump down times have not increased significantly. This occurs at a flow rate of about 30 which is quarter of the maximum flow rate, and beyond this the pump down times increase significantly. This reduced flow rate requires a size of filter that is about a quarter of the size of the standard filter for the conventional pump.

[0075] FIG. 7 shows filter 30 according to an embodiment.

[0076] FIG. 8 shows a flow diagram illustrating steps in a method for evacuating a chamber according to an embodiment. At an initial step S10, the rotor is rotated at an initial speed. The initial speed is set so that the maximum air flow is less than a predetermined value. This maximum air flow determines the size of the filter. There is a feedback loop in the method whereby the flow rate is monitored and the rotor speed increased in response to the flow rate being detected as falling. This feedback loop involves determination at S15 of whether the flow rate has dropped below a fixed value and if it has the rotor speed is increased by a fixed amount delta at step S20. In this way the rotor speed is maintained substantially constant. When the rotor speed is determined to have reached the maximum operational speed of the pump at step S25, that is the operational speed during the normal pumping process the control process for adjusting the speed is stopped and the rotational speed of the rotor is maintained at step S30, at this operational maximum speed during the rest of the pumping process.

[0077] In summary, the initial speed of rotation of the rotor is constrained to reduce the maximum air flow and this in turn reduces the size of lubricant filter required to clean the lubricant from the fluid output by the pump.

[0078] In this regard the size of the filter required is related to the maximum flow rate of the fluid being pumped by the equation: [0079] S = Q / (permeability × P ) [0080] Where S : is the filtration surface in m.sup.2 (for a cylindrical filter S = πr.sup.2L) [0081] Where L : (m) length of the filter, and r : (m) radius of the filter [0082] P : is the acceptable pressure drop across the filter [0083] Q : air flow ( m.sup.3/s ) [0084] Permeability : is a filter parameter m.sup.3/ (m.sup.2 × Pa × s)

[0085] The pressure drop across the filter and the permeability of the filter are properties of the filter and thus, setting the maximum flow rate of the pump to a size for the filter. Reducing the maximum flow rate, which is the initial flow rate to less than half of the conventional initial flow rate by reducing the speed of rotation of the rotor allows the size of the filter to be reduced correspondingly by more than a half.

[0086] Different speed control modes can be used to control the initial rotational speed of the rotor and thereby the initial flow of the fluid. These include: [0087] 1. limitation of the Initial speed [0088] 2. the initial Speed is ramped-up from an initial low value, the slope of the ramp depending on the vessel size being evacuated [0089] 3. the speed may have an initial low value for a predetermined time dependent on the vessel size being evacuated [0090] 4. the initial speed may be regulated by a loop feedback control that is dependent on the air flow, measured in some embodiments at the exhaust [0091] 5. the initial speed can be regulated by a loop feedback control dependent on the pressure measured in some embodiments at the inlet of the pump

[0092] Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

[0093] 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.

[0094] 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.