Screw Compressor Capacity Control

Abstract

A screw compressor has a housing ( 22; 302 ) having first ( 53; 330 ) and second ( 58; 340 ) ports along a flowpath. A first rotor ( 26; 306 ) has a lobed body. A second rotor ( 28; 308, 310 ) has a lobed body enmeshed with the first rotor body. The rotors and housing cooperate to define a compression path between suction ( 60; 332 ) and discharge ( 62; 342 ) locations along the flowpath. Means ( 100, 110, 120; 200, 210, 220; 370, 380, 390 ) provide relative longitudinal movement between a blocking portion ( 57; 352 ) of the housing and at least one of the first rotor and second rotor between: a first condition wherein a pocket of the first and second rotors is closed by the blocking portion; and a second condition wherein the blocking portion does not close the pocket. To provide capacity control, a control system ( 110; 390 ) is configured to provide duty cycle control of the movement.

1 . A screw compressor comprising:
 a housing ( 22 ;  302 ) having first ( 53 ;  330 ) and second ( 58 ;  340 ) ports along a flowpath;   a first rotor ( 26 ;  306 ) having a lobed body ( 30 ) and an axis ( 500 ) and mounted to the housing for rotation about said first rotor axis; and   a second rotor ( 28 ;  308 ,  310 ) having:
 a lobed body ( 34 ) enmeshed with the first rotor body ( 30 ); and 
 an axis ( 502 ), the second rotor mounted to the housing for rotation about said second rotor axis and cooperating with the first rotor ( 26 ;  306 ) and housing ( 22 ;  302 ) to define a compression path between suction ( 60 ;  332 ) and discharge ( 62 ;  342 ) locations along the flowpath, characterized by: 
   means ( 100 ,  110 ,  120 ;  200 ,  210 ,  220 ;  370 ,  380 ,  390 ) for providing relative longitudinal movement between a blocking portion ( 57 ;  352 ) of the housing and at least one of the first rotor and second rotor between:
 a first condition wherein a pocket of the first and second rotors is closed by the blocking portion; and 
 a second condition wherein the blocking portion does not close the pocket; and 
   a control system ( 110 ;  390 ) configured by one or both of hardware and software to provide duty cycle control of the movement.   
   
   
       2 . The compressor of  claim 1  wherein:
 the means ( 380 ,  390 ) provides longitudinal movement of the blocking portion ( 352 ) relative to a remainder of the housing between a first position associated with the first condition and a second position associated with the second condition.   
   
   
       3 . The compressor of  claim 2  wherein the means comprises:
 a spring ( 370 ) biasing the blocking portion from the second position toward the first position.   
   
   
       4 . The compressor of  claim 1  wherein:
 the means ( 100 ,  110 ,  120 ;  200 ,  210 ,  220 ) provides longitudinal movement of a single one of the first rotor and second rotor relative to the blocking portion ( 57 ).   
   
   
       5 . The compressor of  claim 1  wherein:
 the means provides longitudinal movement of at least a movable rotor ( 28 ) of the first and second rotors between first and second positions; and   the means comprises an actuator ( 100 ;  200 ) coupled to at least the movable rotor of the first and second rotors to shift the movable rotor.   
   
   
       6 . The compressor of  claim 5  wherein:
 a spring ( 120 ;  220 ) biases the movable rotor from the second position toward the first position.   
   
   
       7 . The compressor of  claim 1  wherein:
 the compressor lacks an unloading valve.   
   
   
       8 . The compressor of  claim 1  wherein:
 the compressor lacks variable speed motor control.   
   
   
       9 . The compressor of  claim 1  wherein:
 the control system is configured to vary a frequency of the duty cycle control responsive to a condition of a motor of the compressor.   
   
   
       10 . The compressor of  claim 1  wherein:
 the control system is configured to vary a frequency of the duty cycle control responsive to a condition of a motor of the compressor and a fluctuation in a sensed temperature in a conditioned environment.   
   
   
       11 . A method for operating the compressor of  claim 1 , the method comprising:
 determining a desired loading state; and   duty cycle modulating of the means to provide the desired loading state.   
   
   
       12 . The method of  claim 11  further comprising:
 altering a frequency of the duty cycle modulating responsive to a loading condition of a motor of the compressor.   
   
   
       13 . The method of  claim 11  further comprising:
 altering a frequency of the duty cycle modulating responsive to a sensed temperature fluctuation of a conditioned environment.   
   
   
       14 . The method of  claim 11  further comprising:
 varying a frequency of the duty cycle modulating responsive to a sensed temperature of a motor of the compressor.   
   
   
       15 . A screw compressor comprising:
 a housing ( 302 ) having first ( 330 ) and second ( 340 ) ports along a flowpath;   a first rotor ( 306 ) having a lobed body and an axis ( 510 ) and mounted to the housing for rotation about said first rotor axis; and   a second rotor ( 308 ;  310 ) having:
 a lobed body enmeshed with the first rotor body; and 
 an axis ( 512 ;  514 ), the second rotor mounted to the housing for rotation about said second rotor axis and cooperating with the first rotor ( 306 ) and housing ( 302 ) to define a compression path between suction ( 332 ) and discharge ( 342 ) locations along the flowpath, wherein: 
   a blocking portion ( 352 ) of the housing is mounted for longitudinal movement of the blocking portion ( 352 ) relative to a remainder of the housing between a first position engaging the lobed bodies to define at least one compression pocket and a second position retracted from the first position.   
   
   
       16 . The compressor of  claim 15  further comprising:
 an actuator ( 380 ) coupled to blocking portion to shift the blocking portion between the first and second positions.   
   
   
       17 . The compressor of  claim 15  further comprising:
 means ( 380 ,  390 ) for shifting the blocking portion between the first and second positions.   
   
   
       18 . The compressor of  claim 15  wherein:
 a spring ( 370 ) biases the blocking portion from the second position toward the first position.   
   
   
       19 . The compressor of  claim 15  wherein:
 the first position is relatively toward a section end of the housing; and   a spring ( 370 ) biases the movable rotor from the second position toward the first position.   
   
   
       20 . The compressor of  claim 15  wherein:
 the actuator is fluid-operated.   
   
   
       21 . The compressor of  claim 20  wherein:
 the fluid is refrigerant.   
   
   
       22 . The compressor of  claim 15  further comprising:
 a controller ( 390 ) coupled to the actuator and configured to provide capacity control of the compressor by modulating the blocking portion between the first and second positions.   
   
   
       23 . The compressor of  claim 15  wherein:
 the compressor lacks an unloading valve.   
   
   
       24 . The compressor of  claim 15  wherein:
 the compressor lacks variable speed control.   
   
   
       25 . A method for operating the compressor of  claim 15  comprising:
 determining a desired loading state; and   duty cycle modulating an actuator ( 380 ) of the movement between the first and second positions to provide the desired loading state.   
   
   
       26 . The method of  claim 25  wherein:
 the duty cycle modulating is performed while operating a motor of the compressor at an essentially fixed speed.   
   
   
       27 . The method of  claim 25  wherein:
 the duty cycle modulating is performed via a controller and the actuator fluidically operates to provide the desired loading state.   
   
   
       28 . The method of  claim 25  wherein:
 the modulating of the actuator ( 380 ) operates against a bias spring ( 370 ).