Optimized performance strategy for a multi-stage volumetric expander

Abstract

A multi-stage expansion device having bypass capabilities and a variable speed drive is disclosed. In one example, the multi-stage expansion device has a housing within which a first stage, a second stage, and a third stage are housed. The housing may also be configured with internal working fluid passageways to direct a working fluid from the first stage to the second stage and/or from the second stage to the third stage. Each of the stages may include a pair of non-contacting rotors that are mechanically connected to each other and to a power output device such that energy extracted from the working fluid is converted to mechanical work at the output device. In one example, a bypass line is provided to bypass working fluid around the first stage and a bypass line is provided to bypass working fluid around the second stage.

What is claimed is: 
     
         1 . A multi-stage volumetric fluid expansion device comprising:
 a. a first fluid expansion stage having a first pair of non-contacting rotors disposed between a first inlet and a first outlet, the first fluid expansion stage being configured to generate useful work at the first pair of rotors by expanding a working fluid from a first pressure to a second pressure that is lower than the first pressure;   b. a second fluid expansion stage having a second pair of non-contacting rotors disposed between a second inlet and a second outlet, the second fluid expansion stage being configured to generate useful work at the second pair of rotors by receiving the working fluid from the first fluid expansion stage outlet and expanding the working fluid to a third pressure that is lower than the second pressure;   c. a third fluid expansion stage having a third pair of non-contacting rotors disposed between a third inlet and a third outlet, the third fluid expansion stage being configured to generate useful work at third pair of rotors by receiving the working fluid from the second fluid expansion stage outlet and expanding the working fluid to a fourth pressure that is lower than the third pressure;   d. a first working fluid bypass line extending between the first inlet and first outlet of the first fluid expansion stage, the first working fluid bypass line including a first control valve;   e. a second working fluid bypass line extending between the first inlet and first outlet of the second fluid expansion stage, the second working fluid bypass line including a second control valve;   f. a power output device rotated by the first, second, and second third of rotors.   
     
     
         2 . The multi-stage volumetric fluid expansion device of  claim 1 , further comprising:
 a. a variable speed drive for controlling the rotational speed of the fluid expansion device first, second, and third pairs of rotors, the variable speed drive including a motor connected to the power output device.   
     
     
         3 . The multi-stage volumetric fluid expansion device of  claim 1 , further comprising:
 a. a housing within which the first, second, and third pairs of rotors is disposed, wherein the second outlet and third inlet are joined within the housing to form a continuous working fluid passageway extending between the second inlet and the third outlet, wherein the first bypass line includes a bypass connection tube extending through the housing and into the passageway, wherein the second bypass line includes a bypass connection tube extending through the housing and into the passageway   
     
     
         4 . The multi-stage volumetric fluid expansion device of  claim 1 , wherein:
 a. the first outlet and the second inlet are joined within the housing to form a continuous working fluid passageway extending between the first inlet and the third outlet.   
     
     
         5 . The multi-stage volumetric fluid expansion device of  claim 1 , wherein:
 a. the first pair of rotors have twisted non-contacting lobes, wherein one of the first pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the first pair of rotors;   b. the second pair of rotors have twisted non-contacting lobes, wherein one of the second pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the second pair of rotors; and   c. the third pair of rotors have twisted non-contacting lobes, wherein one of the third pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the third pair of rotors.   
     
     
         6 . A system for generating mechanical work via a closed-loop Rankine cycle, the system comprising:
 a. a power plant that produces a waste heat stream, wherein the power plant has a waste heat outlet through which the waste heat stream exits;   b. at least one heat exchanger in fluid communication with the waste heat stream, the heat exchanger being configured to heat a working fluid;   c. a multi-stage fluid expansion device configured to generate mechanical work at an output device from the working fluid, the expansion device having a housing within which a first stage and a second stage are disposed, the first stage being configured to expand the working fluid, the second stage being configured to receive the working fluid from the first stage and to expand the working fluid;   d. a condenser constructed and arranged to condense the working fluid;   e. a pump constructed and arranged to pump the condensed working fluid to the at least one heat exchanger; and   f. a first working fluid bypass line arranged to bypass at least a portion of the working fluid around the first stage and to the second stage, the first working fluid bypass line including a first control valve.   
     
     
         7 . The system for generating mechanical work of  claim 6 , wherein the multi-stage fluid expansion device housing further includes:
 a. a third stage disposed within the housing that is configured to receive the working fluid from the second stage and to expand the working;   b. a second working fluid bypass line arranged to bypass at least a portion of the working fluid around the second stage and to the third stage, the second working fluid bypass line including a second control valve.   
     
     
         8 . The system for generating mechanical work of  claim 7 , wherein:
 a. The housing defines an internal working fluid pathway within which the working fluid can pass internally from the first stage to the second stage and from the second stage to the third stage.   
     
     
         9 . The system for generating mechanical work of  claim 7 , further comprising:
 a. a variable speed drive for controlling the rotational speed of the fluid expansion device first, second, and third pairs of rotors, the variable speed drive including a motor connected to the power output device.   
     
     
         10 . The system for generating mechanical work of  claim 7 , further comprising:
 a. a housing within which the first, second, and third pairs of rotors is disposed, wherein the second outlet and third inlet are joined within the housing to form a continuous internal working fluid passageway extending between the second inlet and the third outlet, wherein the first bypass line includes a bypass connection tube extending through the housing and into the passageway, wherein the second bypass line includes a bypass connection tube extending through the housing and into the passageway   
     
     
         11 . The multi-stage volumetric fluid expansion device of  claim 8 , wherein:
 a. the first pair of rotors have twisted non-contacting lobes, wherein one of the first pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the first pair of rotors;   b. the second pair of rotors have twisted non-contacting lobes, wherein one of the second pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the second pair of rotors; and   c. the third pair of rotors have twisted non-contacting lobes, wherein one of the third pair of rotors has a number of twisted lobes that equals a number of twisted lobes of the other of the third pair of rotors.