Dry Pumps

Abstract

A screw pump comprises a pair of rotors ( 92, 94 ) each carrying an external screw thread ( 96, 98 ), the pair of rotors being rotatably mounted in a stator ( 72 ) and arranged such that, in operation, the screw threads of the rotors intermesh as the rotors rotate in opposing directions. Means ( 100, 102 ) are provided for actively controlling the axial position of the rotors within the stator during use of the pump.

1 . A dry pump comprising a stator housing first and second intermeshing rotors adapted for counter-rotation within the stator, and means for effecting axial movement of the rotors within the stator to vary at least one clearance between the rotors and the stator during use of the pump.  
     
     
         2 . The pump according to  claim 1  wherein the means for effecting axial movement of the rotors is configured to effect axial movement of the rotors in response to an axial load generated in the rotors during operation of the pump.  
     
     
         3 . The pump according to  claim 2  wherein each rotor is mounted on, a respective shaft rotatably mounted within the pump.  
     
     
         4 . The pump according to  claim 3  comprising a bearing assembly for rotatably supporting the shafts relative to the stator, and wherein the means for effecting axial movement of the rotors is configured to move the bearing assembly relative to the stator.  
     
     
         5 . The pump according to  claim 4  wherein the bearing assembly is free to move in an axial direction within a housing, and the means for effecting axial movement of the rotors comprises a spring mechanism arranged with respect to a rotor such that when the rotor is subjected to an axial load, the spring mechanism compresses or extends causing an axial reactive load, so as to vary an axial position of the rotor over time.  
     
     
         6 . The pump according to  claim 5  wherein the spring mechanism comprises a setting spring positioned in the housing between the bearing assembly and an end surface of the housing.  
     
     
         7 . The pump according to  claim 5  wherein the housing is a cylindrical housing having an end surface extending radially inwardly toward the rotor.  
     
     
         8 . The pump according to  claim 5  wherein the spring mechanism is selected such that the maximum axial load to which a rotor is subjected does not exceed the elastic limit of the spring mechanism.  
     
     
         9 . The pump according to  claim 1  comprising control means for actively controlling operation of the means for effecting axial movement of the rotors.  
     
     
         10 . The pump according to  claim 9  wherein the means for effecting axial movement of the rotors comprises an actuator, and the control means is configured to control actuation of the actuator and thereby control the axial position of the rotors.  
     
     
         11 . The pump according to  claim 10  wherein each rotor is mounted on a respective shaft rotatably mounted within the pump, the actuator being configured to axially move the shafts so as to effect axial movement of the rotors.  
     
     
         12 . The pump according to  claim 10  wherein the means for effecting axial movement of the rotors comprises a motor for rotating a drive shaft engaging the actuator, and the control means is configured to control operation of the motor so as to axially move the actuator relative to the stator with rotation of the drive shaft.  
     
     
         13 . The pump according to  claim 12  wherein the drive shaft comprises a lead screw passing through a conformingly-threaded aperture in the actuator.  
     
     
         14 . The pump according to  claim 11  further comprising a bearing assembly for rotatably supporting the shafts relative to the stator, the actuator being configured to move the bearing assembly relative to the stator.  
     
     
         15 . The pump according to  claim 14  wherein the actuator comprises part of a housing for the bearing assembly.  
     
     
         16 . The pump according to  claim 15  wherein the housing includes an internal pump sealing mechanism.  
     
     
         17 . The pump according to  claim 15  wherein the housing defines an end surface of the stator.  
     
     
         18 . The pump according to  claim 14  wherein the bearing assembly supports one end of each of the rotors, and further comprising a second bearing assembly for supporting a second end of each of the rotors.  
     
     
         19 . The pump according to  claim 18  wherein the second bearing assembly is arranged to move axially with axial movement of the rotors.  
     
     
         20 . The pump according to  claim 19  wherein a housing of the second bearing assembly includes an internal pump sealing mechanism.  
     
     
         21 . The pump according to  claim 19  wherein a housing of the second bearing assembly defines an end surface of the stator.  
     
     
         22 . The pump according to  claim 19  wherein the control means comprises means for detecting the value of an operational parameter of the pump, and is configured to control the means for effecting axial movement of the rotors in response to the detected value of the operational parameter.  
     
     
         23 . The pump according to  claim 22  wherein the operational parameter comprises one of the temperature of the stator, the temperature within the stator, backpressure, the power consumption of the pump and the pressure at the inlet of the pump.  
     
     
         24 . The pump according to claims  9  wherein the control means comprises a sensor for detecting the size of the clearance between the rotors and the stator, and is configured to control the means for effecting axial movement of the rotors in response to the detected size of the clearance.  
     
     
         25 . The pump according to  claim 9  wherein the control means comprises a sensor for detecting the rate of change of the clearance between the rotors and the stator, and is configured to control the means for effecting axial movement of the rotors in response to the detected rate.  
     
     
         26 . The pump according to  claim 24  wherein the sensor comprises a Hall effect sensor.  
     
     
         27 . The pump according to  claim 9  wherein the control means is configured to control the means for effecting axial movement of the rotors to sequentially increase and decrease the size of the clearance during use of the pump.  
     
     
         28 . The pump according to  claim 9  wherein the control means is configured to control operation of the means for effecting axial movement of the rotors such that the rate of increase of the size of the clearance is different to the rate of decrease of the size of the clearance.  
     
     
         29 . The pump according to  claim 1 , wherein at least part of each rotor has an outer diameter that tapers decreasingly in a direction from a pump inlet to a pump outlet.  
     
     
         30 . The pump according to  claim 29  wherein only part of each rotor has an outer diameter that tapers towards the pump outlet.  
     
     
         31 . The pump according to  claim 1  wherein the rotors have a Roots profile.  
     
     
         32 . The pump according to  claim 1  wherein the rotors have a Northey profile.  
     
     
         33 . The pump according to  claim 1  wherein the rotors are externally threaded rotors.  
     
     
         34 . A method of controlling operation of a pump comprising a stator housing first and second intermeshing rotors adapted for counter-rotation within the stator, the method comprising the steps of axially moving the rotors relative to the stator to increase an axial clearance between the rotors and the stator when rotors are stationary, starting rotor rotation, and axially moving the rotors relative to the stator during rotor rotation to decrease the axial clearance between the rotors and the stator.  
     
     
         35 . The method according to  claim 34 , further comprising the steps of increasing and decreasing the axial clearance during use of the pump to remove deposits from the axial clearance.  
     
     
         36 . The method according to  claim 35  wherein the axial clearance is periodically increased and decreased during use of the pump.  
     
     
         37 . A method of controlling operation of a pump comprising a stator housing first and second intermeshing rotors adapted for counter-rotation within the stator, the method comprising the steps of sequentially axially moving the rotors in opposite directions relative to the stator to periodically vary an axial clearance between the rotors and the stator to remove deposits from the axial clearance.  
     
     
         38 . The pump according to  claim 25  wherein the sensor comprises a Hall effect sensor.