Patent Publication Number: US-9850902-B2

Title: Compressor with a bypass port

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from and the benefit of U.S. Provisional Application No. 61/163,647, entitled COMPRESSOR, filed Mar. 26, 2009 which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     The application generally relates to positive-displacement compressors. The application relates more specifically to controlling the volume ratio of a screw compressor. 
     In a rotary screw compressor, intake and compression can be accomplished by two tightly-meshing, rotating, helically lobed rotors that alternately draw gas into the threads and compress the gas to a higher pressure. The screw compressor is a positive displacement device with intake and compression cycles similar to a piston/reciprocating compressor. The rotors of the screw compressor can be housed within tightly fitting bores that have built in geometric features that define the inlet and discharge volumes of the compressor to provide for a built in volume ratio of the compressor. The volume ratio of the compressor should be matched to the volume ratio of the system in which the compressor is incorporated, thereby avoiding over or under compression, and the resulting lost work. In a closed loop refrigeration system, the volume ratio of the system is established in the hot and cold side heat exchangers. 
     Fixed volume ratio compressors can be used to avoid the cost and complication of variable volume ratio machines. A screw compressor having fixed inlet and discharge ports built into the housings can be optimized for a specific set of suction and discharge conditions/pressures. However, the system in which the compressor is connected rarely operates at exactly the same conditions hour to hour, especially in an air conditioning application. Nighttime, daytime, and seasonal temperatures can affect the volume ratio of the system and the efficiency with which the compressor operates. In a system where the load varies, the amount of heat being rejected in the condenser fluctuates causing the high side pressure to rise or fall, resulting in a volume ratio for the compressor that deviates from the compressor&#39;s optimum volume ratio. 
     For example, a refrigeration system can include a compressor, condenser, expansion device, and evaporator. The efficiency of the compressor is related to the saturated conditions within the evaporator and condenser. The pressure in the condenser and evaporator can be used to establish the pressure ratio of the system external to the compressor. In the current example, the pressure ratio/compression ratio can be 4. The volume ratio or Vi is linked to the compression ratio by the relation Vi raised to the power of 1/k; k being the ratio of specific heat of the gas or refrigerant being compressed. Using the previous relation, the volume ratio to be built into the compressor geometry for the current example is 3.23 for optimum performance at full load conditions. However, during part load, low ambient conditions, or nighttime, the saturated condition of the condenser in the refrigeration system decreases while evaporator conditions remain relatively constant. To maintain optimum performance of the compressor at part load or low ambient conditions, the Vi for the compressor should be lowered to 2.5. 
     Therefore, what is needed is a system to vary the volume ratio of the compressor at part load or low ambient conditions without using costly and complicated devices such as slide valves. 
     SUMMARY 
     The present invention is directed to a compressor including a compression mechanism. The compression mechanism is configured and positioned to receive vapor from an intake passage and provide compressed vapor to a discharge passage. The compressor also includes a port positioned in the compression mechanism to bypass a portion of the vapor in the compression mechanism to the discharge passage and a valve configured and positioned to control vapor flow through the port. The valve has a first position to permit vapor flow from the compression mechanism to the discharge passage and a second position to prevent vapor flow from the compression mechanism to the discharge passage. The compressor has a first volume ratio in response to the valve being in the second position and a second volume ratio in response to the valve being in the first position. The first volume ratio is greater than the second volume ratio. The valve is controllable in response to predetermined conditions to operate the compressor at the first volume ratio or the second volume ratio. 
     The present invention is also directed to a screw compressor including an intake passage to receive vapor, a discharge passage to supply vapor and a pair of intermeshing rotors. Each rotor of the pair of intermeshing rotors is positioned in a corresponding cylinder. The pair of intermeshing rotors is configured to receive vapor from the intake passage and provide compressed vapor to the discharge passage. The screw compressor also includes a port positioned in at least one rotor cylinder to bypass a portion of the vapor in a compression pocket formed by the pair of intermeshing rotors to the discharge passage and a valve configured and positioned to control vapor flow through the port. The valve has an open position to permit vapor flow from the compression pocket to the discharge passage and a closed position to prevent vapor flow from the compression pocket to the discharge passage. The compressor has a first volume ratio in response to the valve being in the closed position and a second volume ratio in response to the valve being in the open position. The first volume ratio is greater than the second volume ratio. The valve is controllable in response to predetermined conditions to operate the compressor at the first volume ratio or the second volume ratio. 
     One advantage of the present application is an improved energy efficiency rating (EER) over a fixed volume ratio compressor due to better part-load performance resulting from the use of a lower volume ratio. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exemplary embodiment for a heating, ventilation and air conditioning system. 
         FIG. 2  shows an isometric view of an exemplary vapor compression system. 
         FIGS. 3 and 4  schematically show exemplary embodiments of a vapor compression system. 
         FIG. 5  shows a partial cut-away view of a compressor having an exemplary embodiment of a volume ratio control system. 
         FIG. 6  shows an enlarged view of a portion of the compressor of  FIG. 5 . 
         FIG. 7  shows a cross sectional view of the compressor of  FIG. 5  configured for a first volume ratio. 
         FIG. 8  shows a cross sectional view of the compressor of  FIG. 5  configured for a second volume ratio. 
         FIG. 9  shows a cross sectional view of the compressor of  FIG. 5  with another exemplary embodiment of a valve body. 
         FIG. 10  shows a chart of force differentials on the valve body for selected saturated discharge temperatures in an exemplary embodiment. 
         FIG. 11  shows a cross sectional view of a compressor having another exemplary embodiment of a volume ratio control system. 
         FIG. 12  shows a cross sectional view of the compressor of  FIG. 11 . 
         FIG. 13  shows an exemplary embodiment of a hole pattern for the compressor of  FIG. 11 . 
         FIG. 14  shows schematically another embodiment of a volume ratio control system that can be used with the compressor of  FIG. 11 . 
         FIG. 15  shows a cross sectional view of a compressor having a further exemplary embodiment of a valve used with the volume ratio control system. 
         FIG. 16  shows a cross sectional view of a compressor having another exemplary embodiment of a volume ratio control system. 
         FIG. 17  shows a cross sectional view of the compressor of  FIG. 16 . 
         FIG. 18  shows a cross sectional view of the compressor of  FIG. 16  with an exemplary hole pattern. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
       FIG. 1  shows an exemplary environment for a heating, ventilation and air conditioning (HVAC) system  10  in a building  12  for a typical commercial setting. System  10  can include a vapor compression system  14  that can supply a chilled liquid which may be used to cool building  12 . System  10  can include a boiler  16  to supply heated liquid that may be used to heat building  12 , and an air distribution system which circulates air through building  12 . The air distribution system can also include an air return duct  18 , an air supply duct  20  and an air handler  22 . Air handler  22  can include a heat exchanger that is connected to boiler  16  and vapor compression system  14  by conduits  24 . The heat exchanger in air handler  22  may receive either heated liquid from boiler  16  or chilled liquid from vapor compression system  14 , depending on the mode of operation of system  10 . System  10  is shown with a separate air handler on each floor of building  12 , but it is appreciated that the components may be shared between or among floors. 
       FIGS. 2 and 3  show an exemplary vapor compression system  14  that can be used in HVAC system  10 . Vapor compression system  14  can circulate a refrigerant through a circuit starting with compressor  32  and including a condenser  34 , expansion valve(s) or device(s)  36 , and an evaporator or liquid chiller  38 . Vapor compression system  14  can also include a control panel  40  that can include an analog to digital (A/D) converter  42 , a microprocessor  44 , a non-volatile memory  46 , and an interface board  48 . Some examples of fluids that may be used as refrigerants in vapor compression system  14  are hydrofluorocarbon (HFC) based refrigerants, for example, R-410A, R-407, R-134a, hydrofluoro olefin (HFO), “natural” refrigerants like ammonia (NH 3 ), R-717, carbon dioxide (CO 2 ), R-744, or hydrocarbon based refrigerants, water vapor or any other suitable type of refrigerant. In an exemplary embodiment, vapor compression system  14  may use one or more of each of variable speed drives (VSDs)  52 , motors  50 , compressors  32 , condensers  34 , expansion valves  36  and/or evaporators  38 . 
     Motor  50  used with compressor  32  can be powered by a variable speed drive (VSD)  52  or can be powered directly from an alternating current (AC) or direct current (DC) power source. VSD  52 , if used, receives AC power having a particular fixed line voltage and fixed line frequency from the AC power source and provides power having a variable voltage and frequency to motor  50 . Motor  50  can include any type of electric motor that can be powered by a VSD or directly from an AC or DC power source. Motor  50  can be any other suitable motor type, for example, a switched reluctance motor, an induction motor, or an electronically commutated permanent magnet motor. In an alternate exemplary embodiment, other drive mechanisms such as steam or gas turbines or engines and associated components can be used to drive compressor  32 . 
     Compressor  32  compresses a refrigerant vapor and delivers the vapor to condenser  34  through a discharge passage. Compressor  32  can be a screw compressor in one exemplary embodiment. The refrigerant vapor delivered by compressor  32  to condenser  34  transfers heat to a fluid, for example, water or air. The refrigerant vapor condenses to a refrigerant liquid in condenser  34  as a result of the heat transfer with the fluid. The liquid refrigerant from condenser  34  flows through expansion device  36  to evaporator  38 . In the exemplary embodiment shown in  FIG. 3 , condenser  34  is water cooled and includes a tube bundle  54  connected to a cooling tower  56 . 
     The liquid refrigerant delivered to evaporator  38  absorbs heat from another fluid, which may or may not be the same type of fluid used for condenser  34 , and undergoes a phase change to a refrigerant vapor. In the exemplary embodiment shown in  FIG. 3 , evaporator  38  includes a tube bundle having a supply line  60 S and a return line  60 R connected to a cooling load  62 . A process fluid, for example, water, ethylene glycol, calcium chloride brine, sodium chloride brine, or any other suitable liquid, enters evaporator  38  via return line  60 R and exits evaporator  38  via supply line  60 S. Evaporator  38  chills the temperature of the process fluid in the tubes. The tube bundle in evaporator  38  can include a plurality of tubes and a plurality of tube bundles. The vapor refrigerant exits evaporator  38  and returns to compressor  32  by a suction line to complete the cycle. 
       FIG. 4 , which is similar to  FIG. 3 , shows the vapor compression system  14  with an intermediate circuit  64  incorporated between condenser  34  and expansion device  36 . Intermediate circuit  64  has an inlet line  68  that can be either connected directly to or can be in fluid communication with condenser  34 . As shown, inlet line  68  includes an expansion device  66  positioned upstream of an intermediate vessel  70 . Intermediate vessel  70  can be a flash tank, also referred to as a flash intercooler, in an exemplary embodiment. In an alternate exemplary embodiment, intermediate vessel  70  can be configured as a heat exchanger or a “surface economizer.” In the configuration shown in  FIG. 4 , i.e., the intermediate vessel  70  is used as a flash tank, a first expansion device  66  operates to lower the pressure of the liquid received from condenser  34 . During the expansion process, a portion of the liquid vaporizes. Intermediate vessel  70  may be used to separate the vapor from the liquid received from first expansion device  66  and may also permit further expansion of the liquid. The vapor may be drawn by compressor  32  from intermediate vessel  70  through a line  74  to the suction inlet, a port at a pressure intermediate between suction and discharge or an intermediate stage of compression. The liquid that collects in the intermediate vessel  70  is at a lower enthalpy from the expansion process. The liquid from intermediate vessel  70  flows in line  72  through a second expansion device  36  to evaporator  38 . 
     In an exemplary embodiment, compressor  32  can include a compressor housing that contains the working parts of compressor  32 . Vapor from evaporator  38  can be directed to an intake passage of compressor  32 . Compressor  32  compresses the vapor with a compression mechanism and delivers the compressed vapor to condenser  34  through a discharge passage. Motor  50  may be connected to the compression mechanism of compressor  32  by a drive shaft. 
     Vapor flows from the intake passage of compressor  32  and enters a compression pocket of the compression mechanism. The compression pocket is reduced in size by the operation of the compression mechanism to compress the vapor. The compressed vapor can be discharged into the discharge passage. For example, for a screw compressor, the compression pocket is defined between the surfaces of the rotors of the compressor. As the rotors of the compressor engage one another, the compression pockets between the rotors of the compressor, also referred to as lobes, are reduced in size and are axially displaced to a discharge side of the compressor. 
     As the vapor travels in the compression pocket, a port can be positioned in the compression mechanism prior to the discharge end. The port can provide a flow path for the vapor in the compression pocket from an intermediate point in the compression mechanism to the discharge passage. A valve can be used to open (completely or partially) and close the flow path provided by the port. In an exemplary embodiment, the valve can be used to control the volume ratio of compressor  32  by enabling or disabling the flow of vapor from the port to the discharge passage. The valve can provide two (or more) predetermined volume ratios for compressor  32  depending on the position of the valve. 
     The volume ratio for compressor  32  can be calculated by dividing the volume of vapor entering the intake passage (or the volume of vapor in the compression pocket before compression of the vapor begins) by the volume of vapor discharged from the discharge passage (or the volume of vapor obtained from the compression pocket after the compression of the vapor). Since the port is positioned prior to or upstream from the discharge end of the compression mechanism, vapor flow from the port to the discharge passage can increase the volume of vapor at the discharge passage because partially compressed vapor having a greater volume from the port is being mixed with completely or fully compressed vapor from the discharge end of the compression mechanism having a smaller volume. The volume of vapor from the port is greater than the volume of vapor from the discharge end of the compression mechanism because pressure and volume are inversely related, thus lower pressure vapor would have a correspondingly larger volume than higher pressure vapor. Thus, the volume ratio for compressor  32  can be adjusted based on whether or not vapor is permitted to flow from the port. When the valve is in the closed position, i.e., the valve prevents vapor flow from the port, compressor  32  operates at a full-load volume ratio. When the valve is in an open position, i.e., the valve permits vapor flow from the port, the compressor operates at a part-load volume ratio that is less than the full-load volume ratio. In an exemplary embodiment, there are several factors that can determine the difference between full-load volume ratio and part-load volume ratio, for example, the number and location of the ports and the amount of vapor flow permitted through the ports by the valve can all be used to adjust the part-load volume ratio for compressor  32 . In an another exemplary embodiment, the configuration or shape of the ports  88  can be used to adjust the part-load volume ratio of compressor  32 . 
       FIGS. 5 and 6  show an exemplary embodiment of a compressor. Compressor  132  includes a compressor housing  76  that contains the working parts of compressor  132 . Compressor housing  76  includes an intake housing  78  and a rotor housing  80 . Vapor from evaporator  38  can be directed to an intake passage  84  of compressor  132 . Compressor  132  compresses the vapor and delivers the compressed vapor to condenser  34  through a discharge passage  82 . Motor  50  may be connected to rotors of compressor  132  by a drive shaft. The rotors of compressor  132  can matingly engage with each other via intermeshing lands and grooves. Each of the rotors of compressor  132  can revolve in an accurately machined cylinder  86  within rotor housing  80 . 
     In the exemplary embodiment shown in  FIGS. 5-8 , a port  88  can be positioned in cylinder  86  prior to the discharge end of the rotors. Port  88  can provide a flow path for the vapor in the compression pocket from an intermediate point in the rotors to discharge passage  82 . A valve  90  can be used to open (completely or partially) and close the flow path provided by port  88 . Valve  90  can be positioned below the rotors and extend across compressor  132  substantially perpendicular to the flow of vapor. In an exemplary embodiment, valve  90  can automatically control the volume ratio of compressor  132  by enabling or disabling the flow of vapor from port  88  to discharge passage  82 . Valve  90  can provide two (or more) predetermined volume ratios for compressor  132  depending on the position of valve  90 . Port(s)  88  can extend through cylinder  86  in the portions of cylinder  86  associated with the male rotor and/or the female rotor. In an exemplary embodiment, the size of port(s)  88  associated the male rotor may differ from the size of port(s)  88  associated with the female rotor. Discharge passage  82  may partially extend below valve  90  and ports  88  may include channels fluidly connected to discharge passage  82 . 
       FIGS. 7 and 8  show valve  90  in an open position and a closed position, respectively, to either permit or prevent vapor flow from port  88  to discharge passage  82 . In  FIG. 7 , valve  90  is positioned in a closed position, thereby preventing or blocking the vapor flow from port  88  to discharge passage  82 . With valve  90  in the closed position, compression of vapor by the rotors in compressor  132  can occur through reduction of the volume by the rotors as the vapor travels axially to discharge passage  82  which results in the full-load volume ratio for compressor  132 . 
     In  FIG. 8 , valve  90  is positioned in an open position, thereby permitting the vapor flow from port  88  to discharge passage  82 . With valve  90  in the open position, compression of vapor by the rotors in compressor  132  can occur through reduction of the volume by the rotors as the vapor travels axially toward the discharge passage  82 . However, some of the vapor can flow into port  88  and then to discharge passage  82 . Stated another way, a portion of the vapor in the compression pocket can bypass a portion of the rotors by traveling through port  88  to discharge passage  82  when valve  90  is in an open position. The vapor in discharge passage  82  from the discharge end of the rotors and the vapor from port  88  results in a greater volume of vapor at discharge and the part-load compression ratio for compressor  132 . 
     Valve  90  can include a valve body or shuttle  102  snugly positioned in a bore  104  to avoid unnecessary leakage. Valve body  102  can also include one or more gaskets or seals to prevent the leakage of fluids. Valve body  102  can have a varying diameters including a larger diameter portion  106  and a smaller diameter portion  108 . In one exemplary embodiment as shown in  FIG. 9 , valve body  102  can have a large diameter portion  106  corresponding to each port  88  in cylinder  86 . In one exemplary embodiment, the ends of bore  104  can be sealed and portions or volumes of bore  104  can be pressurized or vented with a fluid to move valve body  102  back and forth in bore  104 . When the valve body  102  is positioned in the closed position (see  FIGS. 7 and 9 ), larger diameter portion(s)  106  of valve body  102  block or close off ports  88 . When the valve body  102  is positioned in the open position (see  FIG. 8 ), smaller diameter portion  108  of valve body  102  is positioned near port  88  to permit flow of vapor from port  88  around smaller diameter portion  108  to discharge passage  82 . 
     In an exemplary embodiment, valve  90  can be opened or closed automatically in response to suction pressure, e.g., the pressure of vapor entering intake passage  84 , and discharge pressure, e.g., the pressure of vapor discharged from discharge passage  82 . For example, suction pressure may be applied to larger diameter portion  106  located at one end of valve body  102  and discharge pressure may be applied to smaller diameter portion  108  located at the other end of valve body  102 . Fluid at suction pressure can be provided to bore  104  and larger diameter portion  106  through internal or external piping to create a first force on valve body  102 . The first force applied to valve body  102  can be equal to the fluid pressure (suction pressure) multiplied by the area of larger diameter portion  106 . Similarly, fluid at discharge pressure can be provided to bore  104  and smaller diameter portion  108  through internal or external piping to create a second force on valve body  102  opposing the first force on valve body  102 . The second force applied to valve body  102  can be equal to the fluid pressure (discharge pressure) multiplied by the area of smaller diameter portion  108 . 
     When the first force equals the second force, valve body  102  can remain in a substantially stationary position. When the first force exceeds the second force, valve body  102  can be urged or moved in bore  104  to position valve  90  in either the open position or the closed position. In the exemplary embodiment shown in  FIG. 7 , the first force would move valve body  102  toward the closed position. In contrast, when the second force is greater than the first force, valve body  102  can be urged or moved in bore  104  to position valve  90  in the opposite position from the positioned obtained when the first force is larger. In the exemplary embodiment shown in  FIG. 8 , the second force would move valve body  102  toward the open position.  FIG. 10  is a chart showing force differentials between the first force and the second force on valve body  102  (and corresponding valve positions) for selected saturated discharge temperatures in an exemplary embodiment and gives an example of a specific switch point for valve body  102 . The switch point can be moved by adjusting the pressures or spring force acting on valve body  102 . 
     In an exemplary embodiment, the sizing of larger diameter portion  106  and smaller diameter portion  108  may permit automatic movement of valve body  102  when the suction and discharge pressures reach a predetermined point. For example, the predetermined point may correlate with a preselected compression ratio or a preselected volume ratio. In another exemplary embodiment, valve  90  can include a mechanical stop, for example a shoulder positioned in bore  104 , to limit the movement of valve body  102  to two positions (for example, closed and open). In another exemplary embodiment, valve body  102  can be moved to an intermediate position between the open and closed position that permits partial flow of vapor from port  88  to obtain another volume ratio for compressor  132 . In a further exemplary embodiment, valve body  102  can have several portions of varying diameters to obtain different volume ratios for compressor  132  based on the amount of vapor flow from port  88  each varying diameter permits. 
     In another exemplary embodiment, a spring can be positioned in bore  104  near larger diameter portion  106  to supplement the first force. The use of the spring can smooth the transition between the closed position and the open position and can avoid frequent switching between positions if the force differential remains near the switching point. In another exemplary embodiment, a spring can also be positioned in bore  104  near smaller diameter portion  108  to supplement the second force. 
     In still another exemplary embodiment, the position of valve body  102  can be controlled with one or more solenoid valves to vary the pressures at each end of valve body  102 . The solenoid valve can be controlled by sensing suction and discharge pressures outside or exterior of compressor  132  and then adjusting the pressures on each end of the valve body  102 . 
     In the exemplary embodiment shown in  FIGS. 11-14 , ports  288  can be positioned in cylinder  286  prior to the discharge end of the rotors. Ports  288  can provide a flow path for the vapor in the compression pocket from an intermediate point in the rotors to discharge passage  282 . Valves  290  can be used to open (completely or partially) and close the flow path provided by ports  288 . Valves  290  can be positioned below the rotors and extend substantially parallel to the flow of vapor in compressor  232 . In an exemplary embodiment, valves  290  can control the volume ratio of compressor  232  by enabling or disabling the flow of vapor from ports  288  to discharge passage  282  in response to system conditions. Valves  290  can provide two (or more) predetermined volume ratios for compressor  232  depending on the position of valves  290 . Ports  288  can extend through cylinder  286  in the portions of cylinder  286  associated with the male rotor and/or the female rotor. In an exemplary embodiment, the size of ports  288  associated the male rotor may differ from the size of ports  288  associated with the female rotor. Discharge passage  282  may partially extend below valves  290  and ports  288  may include channels fluidly connected to discharge passage  282 . 
       FIG. 12  shows valve  290 A positioned in a closed position, thereby preventing or blocking the vapor flow from port  288  to discharge passage  282  and shows valve  290 B positioned in an open position thereby permitting the vapor flow from port  288  to discharge passage  282 . With valve  290 A in the closed position and valve  290 B in the open position, compression of vapor by the rotors in compressor  232  can occur through reduction of the volume by the rotors as the vapor travels axially toward the discharge passage  282  for both valves  290 A and  290 B. However, some of the vapor can flow into ports  288  associated with valve  290 B and then to discharge passage  282 . The vapor in discharge passage  282  from the discharge end of the rotors and the vapor from ports  288  associated with valve  290 B results in a greater volume of vapor at discharge and a first part-load compression ratio for compressor  232 . 
     When both valves  290 A and  290 B are in the closed position, compression of vapor by the rotors in compressor  232  can occur through reduction of the volume by the rotors as the vapor travels axially to discharge passage  282  which results in the full-load volume ratio for compressor  232 . When both valves  290 A and  290 B are in the open position, compression of vapor by the rotors in compressor  232  can occur through reduction of the volume by the rotors as the vapor travels axially toward the discharge passage  282 . However, some of the vapor can flow into ports  288  and then to discharge passage  282 . Stated another way, a portion of the vapor in the compression pocket can bypass a portion of the rotors by traveling through ports  288  to discharge passage  282  when valves  290 A and  290 B are in an open position. The vapor in discharge passage  282  from the discharge end of the rotors and the vapor from ports  288  results in a greater volume of vapor at discharge and a second part-load compression ratio for compressor  132  that is lower than the first part-load compression ratio. 
     Valves  290  can include a valve body  202  snugly positioned in a bore  204  to avoid unnecessary leakage. Valve body  202  can also include one or more gaskets or seals to prevent the leakage of fluids. Valve body  202  can have a substantially uniform diameter. In one exemplary embodiment, one end of bore  204  can be sealed and a fluid connection  206  can be provided near the sealed end of bore  204 . The other end of bore  204  can be exposed to fluid at discharge pressure. Fluid connection  206  can be used to adjust the magnitude of the fluid pressure in the sealed end of bore  204 , i.e., pressurize or vent the sealed end of bore  204 , to move valve body  202  back and forth in bore  204 . Fluid connection  206  can be connected to a valve  208  (see  FIG. 14 ), for example a proportional valve or 3-way valve, that is used to supply fluids of different pressures to the sealed end of bore  204  through fluid connection  206 . Valve  208  can permit fluid at discharge pressure (P D ), fluid at a reference pressure less than discharge pressure (P REF ), or a mixture of fluid at the discharge pressure and the reference pressure to flow into fluid connection  206 . In one exemplary embodiment, the reference pressure can be equal to or greater than the suction pressure. In another exemplary embodiment, valve  208  can be operated with oil from the lubrication system. In still another exemplary embodiment, more than one valve can be used to supply fluid to fluid connection  206 . Valve  208  can be controlled by a control system based on measured system parameters, such as discharge pressure, suction pressure, evaporating temperature, condensing temperature or other suitable parameters. When the valve body  202  is positioned in the closed position, valve body  202  blocks or closes off ports  288 . When the valve body  202  is positioned in the open position, valve body  202  is at least partially moved away from the ports  288  to permit flow of vapor from ports  288  to discharge passage  282 . The vapor can flow from ports  288  to discharge passage  282  because the pressure in the compression pocket is at a higher pressure than the discharge pressure. Once the vapor enters ports  288  there can be a pressure drop in the vapor because of the expansion of the vapor into bore  204 . 
     In an exemplary embodiment, valves  290  can be opened or closed in response to the supply or withdrawal of fluid from the sealed end of bore  204 . To move valve body  202  into the closed position, fluid at discharge pressure is provided to fluid connection  206  by valve  208 . The fluid at discharge pressure moves valve body  202  away from the sealed end of bore  204  to close or seal ports  288  by overcoming the force applied to the opposite side of valve body  202 . In contrast, to move valve body  202  into the open position, fluid at reference pressure is provided to fluid connection  206  by valve  208 . The fluid at reference pressure enables valve body  202  to move towards the sealed end of bore  204  to open or uncover ports  288  since the force applied to the opposite side of valve body  202  is greater than the force applied to valve body  202  at the sealed end of bore  204 . The use of valve  208  to adjust the magnitude of the fluid pressure in the sealed end of bore  204  permits valves  290  to be opened and closed in response to specific system conditions. 
     In another exemplary embodiment, a spring can be positioned in the sealed end of bore  204  to supplement the force of the fluid used to close the valve. The use of the spring can smooth the transition between the closed position and the open position and can avoid frequent switching between positions if the force differential remains near the switching point. 
     In a further exemplary embodiment, the valves  290  can be independently controlled to permit one valve  290  to be opened, while closing the other valve  290 . When the valves  290  are independently controlled, each valve  290  can have a corresponding valve  208  that is independently controlled to supply fluid to valve  290  as determined by system conditions. In another exemplary embodiment, the valves  290  can be jointly controlled to have both valves opened or closed at the same time. When the valves are jointly controlled a single valve  208  can be used to supply fluid to the valves  290 . However, each valve  290  may have a corresponding valve  208  that receives common or joint control signals to open or close the valves  290 . 
     In still another exemplary embodiment shown in  FIG. 15 , the bores  204  may be connected to discharge passage  282  by channels  210 . Channels  210  may be used when the size of bore  204  does not permit a direct fluid connection between bore  204  and discharge passage  282 . Channels  210  can have any suitable size or shape to permit fluid flow from bore  204  to discharge passage  282 . 
     In the exemplary embodiment shown in  FIGS. 16-18 , ports  388  can be positioned in cylinder  386  prior to the discharge end of the rotors. Ports  388  can provide a flow path for the vapor in the compression pocket from an intermediate point in the rotors to discharge passage  382 . Valve  390  can be used to open (completely or partially) and close the flow path provided by ports  388 . Valve  390  can be positioned below the rotors at a position substantially centered between the rotors and extend substantially parallel to the flow of vapor in compressor  332 . In an exemplary embodiment, valve  390  can control the volume ratio of compressor  332  by enabling or disabling the flow of vapor from ports  388  to discharge passage  382  in response to system conditions. Valve  390  can provide two (or more) predetermined volume ratios for compressor  332  depending on the position of valve  390 . Ports  388  can extend through cylinder  386  in the portions of cylinder  386  associated with the male rotor and/or the female rotor. In an exemplary embodiment, the size of ports  388  associated the male rotor may differ from the size of ports  388  associated with the female rotor. 
       FIG. 16  shows valve  390  positioned in a closed position, thereby preventing or blocking the vapor flow from ports  388  to discharge passage  382 . When valve  390  is in the closed position, compression of vapor by the rotors in compressor  332  can occur through reduction of the volume by the rotors as the vapor travels axially to discharge passage  382  which results in the full-load volume ratio for compressor  332 .  FIG. 17  shows valve  390  positioned in an open position thereby permitting the vapor flow from ports  388  to discharge passage  382 . When valve  390  is in the open position, compression of vapor by the rotors in compressor  332  can occur through reduction of the volume by the rotors as the vapor travels axially toward the discharge passage  382 . However, some of the vapor can flow into ports  388  and then to discharge passage  382 . Stated another way, a portion of the vapor in the compression pocket can bypass a portion of the rotors by traveling through ports  388  to discharge passage  382  when valve  390  is in an open position. The vapor in discharge passage  382  from the discharge end of the rotors and the vapor from ports  388  results in a greater volume of vapor at discharge and a part-load compression ratio for compressor  332  that is lower than the full-load compression ratio. 
     Valve  390  can include a valve body  302  snugly positioned in a bore  304  to avoid unnecessary leakage. Valve body  302  can also include one or more gaskets or seals to prevent the leakage of fluids. Valve body  302  can have a substantially uniform diameter. In one exemplary embodiment, one end of bore  304  can be sealed and a fluid connection  306  can be provided near the sealed end of bore  304 . The other end of the bore can be exposed to fluid at discharge pressure. Fluid connection  306  can be used to adjust the magnitude of the fluid pressure in the sealed end of bore  204 , i.e., pressurize or vent the sealed end of bore  204 , to move valve body  302  back and forth in bore  304 . Fluid connection  306  can be connected to a valve, for example a proportional valve or 3-way valve, that is used to supply fluids of different pressures to the sealed end of bore  304  through fluid connection  306 . Fluid at discharge pressure (P D ), fluid at a reference pressure less than the discharge pressure (P REF ), or a mixture of fluid at discharge pressure and reference pressure can flow into fluid connection  306 . In another exemplary embodiment, more than one valve can be used to supply fluid to fluid connection  306 . The valve supplying fluid connection  306  can be controlled by a control system based on measured system parameters, such as discharge pressure, suction pressure, evaporating temperature, condensing temperature or other suitable parameters. When the valve body  302  is positioned in the closed position, valve body  302  blocks or closes off ports  388 . When the valve body  302  is positioned in the open position, valve body  302  is moved from the ports  388  to permit flow of vapor from ports  388  to discharge passage  382 . 
     In an exemplary embodiment, valve  390  can be opened or closed in response to the supply or withdrawal of fluid from the sealed end of bore  304 . To move valve body  302  into the closed position, fluid at discharge pressure is provided to fluid connection  306 . The fluid at discharge pressure moves valve body  302  away from the sealed end of bore  304  to close or seal ports  388  by overcoming the force on the opposite side of valve body  302 . In contrast, to move valve body  302  into the open position, fluid at reference pressure is provided to fluid connection  306 . The fluid at reference pressure enables valve body  302  to move towards the sealed end of bore  304  to open or uncover ports  388  since the force applied to the opposite side of valve body  302  is greater than the force applied to valve body  302  at the sealed end of bore  304 . The pressurizing or venting of the sealed end of bore  304 , permits valve  390  to be opened and closed in response to specific conditions. 
     In another exemplary embodiment, a spring can be positioned in the sealed end of bore  304  to supplement the force of the fluid used to close the valve. The use of the spring can smooth the transition between the closed position and the open position. 
     In exemplary embodiments, the ports and/or the valves of the volume ratio control system can be used to adjust the volume ratio of the compressor by adjusting the size of the ports and/or the valves, and/or the positioning of the ports and/or the valves with respect to the rotors and/or the discharge path. By increasing the size of the ports, a larger volume of the vapor can pass through ports. Similarly, by decreasing the size of the ports, a smaller volume of the vapor can pass through the ports. Additionally or alternatively, including multiple ports with respect to one valve can increase the volume of the vapor. By positioning the ports and valves closer to the discharge end of the rotors, the difference in volume of the vapor traveling through the ports can be lower. Similarly, by positioning the ports and valves farther from the discharge end of the rotors, the difference in volume of the vapor traveling through the ports can be higher. 
     In other exemplary embodiments, the bores and the valve bodies used in the valves can have standard shapes that are easily manufactured. For example, the bores can have a cylindrical shape, including a right circular cylindrical shape, and the valve bodies can have a corresponding cylindrical or piston shape, including a right circular cylindrical shape. However, the bores and valve bodies can have any suitable shape that can open and close the ports in the cylinder as required. 
     In another exemplary embodiment, a slide valve and corresponding controls can be used with the volume ratio control system. The use of a slide valve with the volume ratio control system can provide a smoother Vi vs. capacity curve. 
     While only certain features and embodiments of the invention have been shown and described, many modifications and changes may occur to those skilled in the art (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (e.g., temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described (i.e., those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the claimed invention). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.