US20060269424A1

US20060269424A1 - Vacuum pump

Info

Publication number: US20060269424A1
Application number: US11/233,681
Authority: US
Inventors: Michael Henry North
Original assignee: BOC Group Ltd
Current assignee: Edwards Ltd
Priority date: 2005-05-27
Filing date: 2005-09-23
Publication date: 2006-11-30
Also published as: GB0510892D0

Abstract

A method for controlling the temperature of a rotor of a vacuum pump comprising said rotor and a stator comprises the steps of flowing a rotor cooling fluid through a rotor cooling circuit to cool the rotor, flowing a stator cooling fluid through a stator cooling circuit to cool the stator, conveying the stator cooling fluid from the stator cooling circuit to a heat exchanger of the rotor cooling circuit to cool the rotor cooling fluid, returning the stator cooling fluid from the heat exchanger of the rotor cooling circuit to the stator cooling circuit, supplying a further coolant to a heat exchanger of the stator cooling circuit to cool the stator cooling fluid, and controlling the supply of further coolant to the heat exchanger of the stator cooling circuit to control the temperature of the stator cooling fluid.

Description

FIELD OF THE INVENTION

The present invention relates to vacuum pumps, and more particularly to a method of cooling of one or more components of a vacuum pump.

BACKGROUND OF THE INVENTION

Dry pumps are widely used in industrial processes to provide a clean and/or low-pressure environment for the manufacture of products. Applications include the pharmaceutical, semiconductor and flat panel manufacturing industries. Such pumps include an essentially dry (or oil free) pumping mechanism, but generally also include some components, such as bearings and transmission gears, for driving the pumping mechanism that require lubrication in order to be effective.
Examples of dry pumps include Roots, Northey (or “claw”) and screw pumps. A typical screw pump mechanism comprises two spaced parallel shafts each carrying externally threaded rotors, the shafts being mounted in a pump body such that the threads of the rotors intermesh. Close tolerances between the rotor threads at the points of intermeshing and with the internal surface of the pump body (which acts as a stator) cause volumes of gas entering an inlet to be trapped between the threads of the rotors and the internal surface and thereby urged towards an outlet of the pump as the rotors rotate.
During use, heat is generated as a result of the compression of the gas by the rotors acting in combination with one another. Consequently, the temperature of the rotors rapidly rises. By comparison, the bulk of the stator is large and heating thereof is somewhat slower. As the rotors of the pump heat up, heat is conducted from the rotors to the shafts. Unless there is rotor cooling or other means of dissipating the heat then the temperature of the rotors will increase until it approaches the pumped gas temperature. There will be very little transfer of heat from the pumped gas through the shaft at steady state conditions since there is not a ready heat path out of the shaft. In view of this, the stator and rotor will undergo differential thermal expansion, especially during transient pumping conditions. This can lead to contact between the stator and the pumping mechanism, which can impair or, in severe conditions, stop the operation of the pump.
Co-pending International patent application PCT/GB20030004415 (Publication Number WO2004/036049) which is assigned to the assignee of the present application is hereby incorporated by reference, describes a number of arrangements for controlling the temperatures of the stator and the rotors of a screw vacuum pump, and thereby inhibit contact between the stator and the rotors during use of the pump. With reference to FIG. 1, a pump 10 comprises a stator 12 defining a pumping chamber 14 housing two intermeshing screw rotors 16. The stator 12 comprises a central inlet portion 18 having an inlet (not shown) for receiving gas to be pumped, and two outlet portions 20 on either side of the inlet portion 18 and each having a respective outlet (not shown) for exhausting pumped gas. Interconnected fluid cavities 30 are provided within the stator 12 for receiving a flow of coolant, typically a mixture of water and anti-freeze, which circulates about the stator 12 to draw heat away from the stator 12.
The rotors 16 are supported at each end thereof by respective shafts 22, each of which is supported in turn by bearings 24 provided in the end plates 26, 28 of the stator 12. One of the shafts 22 is connected to a drive motor (not shown) and is coupled to an adjacent shaft by means of timing gears so that in use the rotors 16 rotate at the same speed but in opposite directions. A cavity 32 is formed in each end of each rotor 16, one end 34 of each shaft 22 extending into a respective cavity 32. Each shaft 22 has a bore housing a coolant supply tube 36 for receiving a rotor coolant, such as oil, and conveying the rotor coolant into the cavity 32. The rotor coolant flows back from the end of the cavity 32 between the walls of the cavity 32 and the end 34 of the shaft 22 and enters the bore of the shaft 22, from which is conveyed back about the supply tube 36 in the opposite direction to which the rotor coolant is conveyed into the cavity 32.
FIGS. 2 and 3 illustrate respective arrangements, as described in WO2004/036049, for controlling the temperatures of the coolants supplied to the stator 12 and the rotors 16 of the pump 10. In the arrangement illustrated in FIG. 2, a stator cooling circuit 40 conveys the stator coolant to, and from, the fluid cavities within the stator 12, and rotor cooling circuits 42, 44, are each provided at respective ends of the pump 10 for conveying the rotor coolant to, and from, the shafts 22 extending from a respective end plate 26, 28 of the pump 10.
The stator cooling circuit 40 comprises a pump 46 for circulating the stator coolant within the stator cooling circuit 40, and a heat exchanger 48. The heat exchanger 48 controls the exchange of heat between the stator coolant and a primary coolant conveyed by a conduit system 50 to the heat exchanger 48 from a primary coolant source 52, typically a mains water supply. A first temperature control valve 54 receives signals indicative of the temperature of the stator 12 from a thermal sensor 56 located at the stator 12, and uses the signals to control the rate of supply of the primary coolant to the heat exchanger 48. This controls the temperature gradient across the heat exchanger 48, thereby controlling the temperature of the stator coolant and thus the temperature of the stator 12. The temperature of the stator 12 can be increased or decreased as required by adjusting the setting of the control valve 54.
Each rotor cooling circuit 42, 44 comprises a pump 58 for circulating the rotor coolant within the second cooling circuit, a coolant filter 60 and a heat exchanger 62. Similar to the heat exchanger 48, the heat exchanger 62 controls the exchange of heat between the rotor coolant and the primary coolant, which is conveyed to each heat exchanger 62 from the primary coolant source 52 by a conduit system 64. A second temperature control valve 66 receives signals from a second thermal sensor indicative of the temperature of the rotor coolant, or of the temperature of process gases within the exhaust stages of the pump, which are in turn indicative of the temperature of the rotors 16. The second valve 66 uses the signals to control the rate of supply of the primary coolant to each heat exchanger 62. This controls the temperature gradient across each heat exchanger 62, thereby controlling the temperature of the rotor coolant and thus the temperature of the rotors 16. The temperature of the rotors 16 can be increased or decreased as required by adjusting the setting of the second valve 66. The conduit systems 50, 64 convey the primary coolant from the heat exchangers 48, 62 to a common outlet 70.
By setting each temperature control valve 54, 66 to a predetermined temperature, a predetermined temperature difference can be established between the stator 12 and the rotors 16. As the running clearance between the stator 12 and the rotors 16 during use of the pump 10 is a function of the temperature difference between the rotors and stator, the running clearance can thus be maintained at or around a predetermined value.
In the arrangement illustrated in FIG. 3, the temperature control valves 54, 66 are replaced by electronically actuated valves 72, 74. A microprocessor 76 monitors the signals output from the sensors 56, 68, and controls the valves 72, 74 in dependence on the received signals so as to maintain the running clearance at or around a predetermined value.
We have found that the supply of a primary coolant, such as water, from a mains supply to the heat exchangers 62 of the rotor cooling circuits 44 can lead to a number of problems. For example, variations in the quality of the water output from the mains supply can lead to blockage of the heat exchangers 62 due to the deposition of suspended solids contained with relatively poor quality water. At relatively low supply rates from the mains, for example due to a burst elsewhere in the mains system, the temperature of the water output from the heat exchangers 62 can increase to an unacceptably high level, which can lead to calcification within the heat exchangers 62. Both of these problems may, in extreme cases, lead to insufficient cooling of the rotors 16 and seizure of the pump 10.
It is an object of the present invention to seek to solve these and other related problems.

SUMMARY OF THE INVENTION

The present invention provides a method of controlling the temperature of a rotor of a vacuum pump comprising said rotor and a stator, the method comprising the steps of flowing a rotor cooling fluid through a rotor cooling circuit to cool the rotor, flowing a stator cooling fluid through a stator cooling circuit to cool the stator, conveying the stator cooling fluid from the stator cooling circuit to a heat exchanger of the rotor cooling circuit to cool the rotor cooling fluid, returning the stato cooling fluid from the heat exchanger of the rotor cooling circuit to the stator cooling circuit, supplying a further coolant to a heat exchanger of the stator cooling circuit to cool the stator cooling fluid, and controlling the supply of further coolant to the heat exchanger of the stator cooling circuit to control the temperature of the stator cooling fluid.
The present invention also provides apparatus for controlling the temperature of a rotor of a vacuum pump comprising said rotor and a stator, the apparatus comprising a rotor cooling circuit, means for circulating a rotor cooling fluid within the rotor cooling circuit to cool the rotor, a stator cooling circuit, means for circulating the stator cooling fluid within the stator cooling circuit to cool the stator, means for conveying stator cooling fluid from the stator cooling circuit to a heat exchanger of the rotor cooling circuit to cool the rotor cooling fluid, and for subsequently returning the supplied stator cooling fluid to the stator cooling circuit, means for supplying a further coolant to a heat exchanger of the stator cooling circuit to cool the stator cooling fluid, and means for controlling the supply of further coolant to the heat exchanger of the stator cooling circuit to control the temperature of the stator cooling fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred features of the present invention will now be described with reference to the accompanying drawings, in which:
FIG. 1 illustrates a cross-section through part of a screw pump;
FIG. 2 illustrates an arrangement for cooling the rotors and stator of the pump of FIG. 1;
FIG. 3 illustrates a second arrangement for cooling the rotors and stator of the pump of FIG. 1;
FIG. 4 illustrates a cross-section through part of a screw pump in accordance with the present invention; and
FIG. 5 illustrates an arrangement according to the present invention for cooling the rotors and stator of the pump of FIG. 4.
With reference to FIG. 4, a screw vacuum pump 100 comprises a stator 102 defining a pumping chamber 104 housing two intermeshing screw rotors 106. The stator 102 comprises a central inlet portion 108 having an inlet (not shown) for receiving gas to be pumped, and two outlet portions 110 on either side of the inlet portion 108 and each having a respective outlet (not shown) for exhausting pumped gas.
The rotors 106 are supported at each end thereof by respective shafts 112, each of which is supported in turn by bearings 114 provided in the end plates 116, 118 of the stator 102. One of the shafts 112 is connected to a drive motor (not shown) and is coupled to an adjacent shaft by means of timing gears so that in use the rotors 106 rotate at the same speed but in opposite directions.
Interconnected fluid cavities 120 are provided within the stator 102 and extending about the pumping chamber 104. With reference also to FIG. 5, the fluid cavities 120 form part of a stator cooling circuit 122 for cooling the stator. A flow of a stator cooling fluid, typically a mixture of water and anti-freeze, is circulated within the stator cooling circuit 122 by a water pump 124. The flow of stator cooling fluid enters the cavities 120 at one end of the pump 100 and circulates along and about the pumping chamber 104 before being exhaust from the cavities 120 at the other end of the pump 100, thereby drawing heat away from the stator 102.
Returning to FIG. 4, a cavity 126 is formed in each end of each rotor 106, one end 128 of each shaft 112 extending into a respective cavity 126. Each shaft 112 has a bore housing a supply tube 130 for receiving a rotor cooling fluid, such as oil, and conveying the rotor cooling fluid into the cavity 126. The rotor cooling fluid flows back from the end of the cavity 126 between the walls of the cavity 126 and the end 128 of the shaft 112 and enters the bore of the shaft 112, from which is conveyed back about the supply tube 130 in the opposite direction to which the rotor cooling fluid is conveyed into the cavity 126 within the supply tube 130.
With reference again to FIG. 5, the two shafts 112 and supply tubes 130 located at each end of the pump 100 form part of respective rotor cooling circuits 132, 134. A flow of rotor cooling fluid is circulated within each rotor cooling circuit 132, 134 by a respective oil pump 136. Each rotor cooling circuit 132, 134 also includes a filter 138 for filtering the rotor cooling fluid before it is returned to the supply tubes 130.
During use of the pump, heat is generated as a result of the compression of the gas by the rotors 106. Consequently, the temperature of the rotors 106 rapidly rises. By comparison, the bulk of the stator 102 is large and heating thereof is somewhat slower. This creates a temperature difference between the stator 102 and the rotors 106. As the temperature difference between the stator and the rotors increases during use of the pump, this leads to a reduction in the running clearance between the stator and the rotors, which, if allowed to decrease unchecked, could lead to contact between the stators and the rotors.
In order to maintain the temperature difference between the rotors and the stator at or around a predetermined value, for example around 20° C., the temperatures of the rotor cooling fluid and the stator cooling fluid are controlled. As illustrated in FIG. 5, each rotor cooling circuit 132, 134 includes a heat exchanger 140. A first conduit system 142 conveys a flow of relatively cool stator cooling fluid from the stator cooling circuit 122 to the heat exchangers 140 in order to cool the relatively warmer rotor cooling fluid within the rotor cooling circuits, and returns the stator cooling fluid from the heat exchangers 140 to the stator cooling circuit 122. In order to control the temperature of the stator cooling fluid, and thus, in turn, control the temperature of the rotor cooling fluid, the stator cooling circuit 122 also includes a heat exchanger 144. A second conduit system 146 conveys a flow of a further coolant, in this example water from a mains supply 148, to the heat exchanger 144 to cool the stator cooling fluid.
The temperature of the stator cooling fluid is controlled by controlling the supply of the further coolant to the heat exchanger 144. In this example, the flow rate of the further coolant to the heat exchanger 144 is controlled by a variable flow control device 150 located within the conduit system 146 and upstream from the heat exchanger 144. As illustrated, the variable flow control device 150 may be provided by a temperature control valve, which receives signals indicative of the temperature of the stator 102 from a thermal sensor 152 located, for example, on the external surface of the stator 102. The valve 150 has a conductance that varies in dependence on, preferably in proportion to, the temperature of the stator 102, so that as the temperature of the stator 102 increases, the valve 150 increases the supply of further coolant to the heat exchanger 144. Due to the increased quantity of further coolant flowing through the heat exchanger 144, the temperature of the stator cooling fluid is reduced, which reduces the temperature of the stator 102. In addition, as the temperature of the stator cooling fluid conveyed to the rotor cooling circuits 132, 134 also reduces, this in turn reduces the temperature of the rotor cooling fluid and thus reduces the temperature of the rotors 106.
The temperature of the rotors 106 can thus closely follow the temperature of the stator 102. For example, by controlling the size of the heat exchangers 140 and/or by adjusting the speed of the pump to control the rate at which the stator cooling fluid is supplied to the heat exchangers 140, a substantially constant temperature difference between the stator 102 and the rotors 106 can be achieved, irrespective of ambient temperatures, and the temperature pressure and flow rate of the further coolant. By adjusting the setting of the control valve 150, preferably manually, the temperatures of both the stator and the rotors can be readily adjusted.
In comparison to the arrangements illustrated in FIGS. 2 and 3, according to the present invention only one control valve and one thermal sensor are required to control the temperatures of both the stator and the rotors, thereby reducing the number of components and the complexity of the arrangements. Furthermore, as the composition and the flow rate of the stator cooling fluid can be controlled, there are no problems associated with calcification or suspension deposition within the heat exchangers 140 of the rotor cooling circuits 132, 134, thereby increasing the reliability of the rotor cooling.
While the invention has been described in relation to a dual-outlet screw pump, the invention is equally applicable to other forms of dry pump. Where the pump has a single gas outlet, a single rotor cooling circuit may be provided.
While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the true spirit and scope of the present invention.

Claims

1. A method of controlling the temperature of a rotor of a vacuum pump comprising said rotor and a stator, the method comprising the steps of flowing a rotor cooling fluid through a rotor cooling circuit to cool the rotor proximate a first end, flowing a stator cooling fluid through a stator cooling circuit to cool the stator, conveying the stator cooling fluid from the stator cooling circuit to a heat exchanger of the rotor cooling circuit to cool the rotor cooling fluid, returning the stator cooling fluid from the heat exchanger of the rotor cooling circuit to the stator cooling circuit, supplying a further coolant to a heat exchanger of the stator cooling circuit to cool the stator cooling fluid, and controlling the supply of further coolant to the heat exchanger of the stator cooling circuit to control the temperature of the stator cooling fluid.

2. The method according to claim 1, wherein the supply of further coolant to the heat exchanger of the stator cooling circuit is controlled as a function of the temperature of the stator.

3. The method according to claim 1, wherein the supply of further coolant to the heat exchanger of the stator cooling circuit is controlled using a variable flow control device.

4. The method according to claim 3, wherein the variable flow control device has a conductance which is varied as a function of the temperature of the stator to control the rate of supply of the further coolant to the heat exchanger of the stator cooling circuit.

5. The method according to claim 4, wherein the conductance of the variable flow control device is controlled as a function of a signal output from a sensor for monitoring the temperature of the stator.

6. The method according to claim 3, wherein the variable flow control device has a setting that is variable to control the respective temperatures of both the stator and the rotor.

7. The method according to claim 3, wherein the variable flow control device is a manually controlled valve.

8. The method according to claim 1, wherein the first cooling fluid comprises an oil.

9. The method according to claim 1, wherein the second cooling fluid comprises water.

10. The method according to claim 1, wherein the further coolant is supplied to the heat exchanger of the stator cooling circuit from a mains water supply.

11. The method according to claim 1, wherein the stator cooling circuit extends about the stator.

12. The method according to claim 1, wherein the rotor cooling circuit is configured to supply the rotor cooling fluid to one end of the rotor.

13. The method according to claim 12, wherein a second rotor cooling circuit is provided for cooling the rotor proximate a second end, the method further comprising the steps of flowing a rotor cooling fluid through the second rotor cooling circuit, conveying the stator cooling fluid from the stator cooling circuit to a heat exchanger of the second rotor cooling circuit to cool the rotor cooling fluid therein, and returning the stator cooling fluid from the heat exchanger of the second rotor cooling circuit to the stator cooling circuit.

14. The method according to claim 1, wherein the vacuum pump is a dry vacuum pump.

15. The method according to claim 1, wherein the vacuum pump is a screw vacuum pump.