Patent Publication Number: US-6663358-B2

Title: Compressors for providing automatic capacity modulation and heat exchanging system including the same

Description:
RELATED APPLICATIONS 
     The present application is a continuation-in-part of application Ser. No. 09/877,146 filed on Jun. 11, 2001, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to compressors for providing capacity modulation. More particularly, the present invention relates to compressors for providing automatic capacity modulation without any need for external controls, a heat exchanging system including the same, and related capacity modulation methods. 
     Heat exchanging systems, including air-conditioning, refrigeration, and heat-pump systems, utilize compressors to increase the pressure of the fluid flowing through the systems. In response to varying cooling or heating demands, some of these heat exchanging systems modulate their system capacity by varying the capacity of the compressors. These compressors, however, typically rely on external controls for capacity modulation, and therefore, are costly because of additional components required for the external controls. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to improved compressors for providing automatic capacity modulation. The invention is also directed to a heat exchanging system including the improved compressor, and to related capacity modulation methods. The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims. 
     To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, the invention is directed to a variable compressor comprising a compression chamber, a reexpansion area, a flow channel, a valve member, and a control. The flow channel is between the compression chamber and the reexpansion area. The valve member is movable between first and second positions. The valve member in a first position allows flow between the compression chamber and the reexpansion area and in a second position prevents flow between the compression chamber and the reexpansion area, whereby the compressor operates at a first capacity when the valve member is in the first position and at a second, increased capacity when the valve member is in the second position. The control is associated only with the compressor and moves the valve member between the first and second positions as a function of an operating parameter of the compressor, whereby the compressor is automatically modulated based on the operating parameter. 
     In another aspect, the invention is directed to a compressor comprising a compression chamber, a compressing member, a flow passage, a valve member, and a biasing member. The compressing member is movable to compress fluid entering the compression chamber. The flow passage is in fluid communication with the compression chamber at one end and a reexpansion area at the other end. The valve member is associated with the flow passage and is movable between a first position permitting flow through the flow passage and a second position preventing flow through the flow passage. The valve member is continuously subjected to a first operating condition of the fluid such that a first force is continuously exerted on the valve member in a first direction. The valve member is also continuously subjected to a second operating condition of the fluid such that a second force is continuously exerted on the valve member in a second direction opposite to the first direction. The biasing member exerts a biasing force on the valve member in the second direction such that when the first force overcomes the biasing force and the second force combined together, the valve member moves from the first position to the second position and modulates the capacity of the compressor. 
     In yet another aspect, the invention is directed to a heat exchanging system having fluid flowing therethrough in a cycle. The heat exchanging system comprises a condenser, an expansion device, an evaporator, a compressor, and a control. The expansion device is in fluid communication with the condenser. The evaporator is in fluid communication with the expansion device. The compressor is in fluid communication with the evaporator and the condenser. The compressor includes an actuating element. The actuating element is movable between a first position and a second position as a function of an operating parameter of the compressor, such that the compressor operates at a first capacity when the actuating element is in a first position and at a second capacity when the actuating element is in the second position. The control turns the compressor on or off, based on the demand for heating or cooling. 
     In yet another aspect, the invention is directed to a method of operating a variable capacity compressor. The method comprises the steps of: operating the compressor at a first capacity; applying first and second pressures continuously to a movable component in the compressor, the movable component causing the compressor to operate at the first capacity when the movable component is in a first position and at a second increased capacity when the movable component is in a second position; and applying a biasing force to bias the movable component toward the first position, such that the movable component moves to the second position when the relative differential between the first and second pressures reaches a predetermined value, whereby the compressor automatically modulates its capacity based on the relative values of the first and second pressures. 
     In yet another aspect, the invention is directed to a capacity modulation method. The capacity modulation method comprises the steps of: providing a compressor comprising a compression chamber and a compressing member movable to compress fluid entering the compression chamber; providing a flow passage in fluid communication with the compression chamber at one end and a reexpansion area at the other end; providing a valve member associated with the flow passage and movable between a first position permitting flow through the flow passage and a second position preventing flow through the flow passage; subjecting the valve member continuously to a first operating condition of the fluid such that a first force is continuously exerted on the valve member in a first direction; subjecting the valve member continuously to a second operating condition of the fluid such that a second force is continuously exerted on the valve member in a second direction opposite to the first direction; and exerting a biasing force on the valve member in the second direction such that when the first force overcomes the second force and the biasing force combined together, the valve member moves from the first position to the second position and thereby modulates the capacity. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
     FIG. 1 is a sectional view of a compressor incorporating one embodiment of the capacity modulation system of the present invention; 
     FIG. 2 is a partial sectional view on line  2 — 2  of FIG. 1, showing one embodiment of the capacity modulation system of the present invention in a reduced capacity mode; 
     FIG. 3 is a partial sectional view on line  2 — 2  of FIG. 1, showing the embodiment of the capacity modulation system of the present invention shown in FIG. 1 in a full capacity mode; 
     FIG. 4 is a partially schematic partial sectional view on line  2 — 2  of FIG. 1, showing another embodiment of the capacity modulation system of the present invention in a reduced capacity mode; 
     FIG. 5 is a partially schematic partial sectional view on line  2 — 2  of FIG. 1, showing the embodiment of the capacity modulation system of the present invention shown in FIG. 4 in a full capacity mode; 
     FIG. 6 is a partially schematic partial sectional view on line  2 — 2  of FIG. 1, showing yet another embodiment of the capacity modulation system of the present invention in a reduced capacity mode; 
     FIG. 7 is a schematic diagram of a heat exchanging system, such as an air-conditioning, refrigeration, or heat-pump system, having a compressor for providing capacity modulation in accordance with the invention; 
     FIG. 8 is a partial section view of an embodiment of the present invention, incorporated in a reciprocating compressor for an air-conditioning or refrigeration system. In FIG. 8, a valve member of the present invention is shown to be positioned within a reexpansion chamber and in a position to permit flow through a flow passage in fluid communication with a compression chamber and the reexpansion chamber; 
     FIG. 9 is a partial section view of the embodiment of FIG. 8, showing the valve member in a position to prevent flow through the flow passage; 
     FIG. 10 is a partial section view of another embodiment of the present invention, incorporated in a reciprocating compressor for an air-conditioning or refrigeration system. In FIG. 10, a valve member of the present invention is shown to be positioned within a valve chamber and in a position to permit flow through a flow passage in fluid communication with a compression chamber and the reexpansion chamber; 
     FIG. 11 is a partial section view of the embodiment of FIG. 10, showing the valve member in a position to prevent flow through the flow passage; 
     FIG. 12 is a partial section view of another embodiment of the present invention, incorporated in a reciprocating compressor for an air-conditioning or refrigeration system. In FIG. 12, a valve member of the present invention is shown to be in a position to permit flow through a flow passage in fluid communication with a compression chamber and a suction channel; 
     FIG. 13 is a partial section view of the embodiment of FIG. 12, showing the valve member in a position to prevent flow through the flow passage; 
     FIG. 14 is a partial section view of an embodiment of a scroll compressor for an air-conditioning or refrigeration system. As shown, a valve member is movable to permit and prevent flow through a flow passage in fluid communication with a compression chamber and a suction channel; 
     FIG. 15 an enlarged partial section view of the valve member and flow passage shown in FIG. 14, illustrating the valve member in a position permitting flow through the flow passage; and 
     FIG. 16 is an enlarged partial section view of the valve member and flow passage shown in FIG. 14, illustrating the valve member in a position preventing flow through the flow passage; 
     FIG. 17 is a partial section view of yet another embodiment of the present invention, incorporated in a reciprocating compressor for an air-conditioning or refrigeration system. In FIG. 17, a valve member of the present invention is shown to be in a position to permit flow through a flow passage in fluid communication with a compression chamber and a suction channel; 
     FIG. 18 is a partial section view of the embodiment of FIG. 17, showing the valve member in a position to prevent flow through the flow passage; 
     FIG. 19 is a partial section view of yet another embodiment of the present invention, incorporated in a reciprocating compressor for an air-conditioning or refrigeration system. In FIG. 19, a temperature element is applied to a valve member of the present invention and the valve member is shown to be in a position to permit flow through a flow passage in fluid communication with a compression chamber and a suction channel; 
     FIG. 20 is a partial section view of the embodiment of FIG. 19, showing the valve member in a position to prevent flow through the flow passage; 
     FIG. 21 is a partial section view of an embodiment of a temperature element of the present invention applied to a valve member. In FIG. 21, the valve member is shown to be in a position to permit flow through a flow passage in fluid communication with a compression chamber and a suction channel; 
     FIG. 22 is a partial section view of the embodiment of FIG. 21, showing the valve member in a position to prevent flow through the flow passage; 
     FIG. 23 is a partial section view of another embodiment of a temperature element of the present invention applied to a valve member. In FIG. 23, the valve member is shown to be in a position to permit flow through a flow passage in fluid communication with a compression chamber and a suction channel; 
     FIG. 24 is a partial section view of the embodiment of FIG. 23, showing the valve member in a position to prevent flow through the flow passage; 
     FIG. 25 is a partial section view of yet another embodiment of a temperature element of the present invention applied to a valve member. In FIG. 25, the valve member is shown to be in a position to permit flow through a flow passage in fluid communication with a compression chamber and a suction channel; 
     FIG. 26 is a partial section view of the embodiment of FIG. 25, showing the valve member in a position to prevent flow through the flow passage; 
     FIG. 27 is a partial section view of yet another embodiment of the present invention, incorporated in a reciprocating compressor for an air-conditioning or refrigeration system. In FIG. 27, a temperature element applied to a valve member of the present invention is shown to be not exposed to fluid. The valve member is shown to be in a position to permit flow through a flow passage in fluid communication with a compression chamber and a suction channel; and 
     FIG. 28 is a partial section view of the embodiment of FIG. 27, showing the valve member in a position to prevent flow through the flow passage. 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the presently preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     In accordance with the present invention and illustrated in FIG. 7, a heat exchanging system  310 , such as a Heating, Ventilation, and Air-Conditioning (HVAC) or refrigeration system, includes two heat exchangers  312  and  314 , a compressor  316 , and an expansion device  318 . Tubes or pipes connect the heat exchangers  312  and  314 , the compressor  316 , and the expansion device  318 . Fluid at a given pressure flows through the heat exchanger  314 , conventionally called a condenser. While flowing through the condenser  314 , the fluid loses heat. The fluid then flows through the expansion device  318  where its pressure decreases to another level. The fluid then flows through the heat exchanger  312 , conventionally called an evaporator. While flowing though the evaporator  312 , the fluid absorbs heat. Finally, the fluid flows through the compressor  316  where its pressure increases back to the original level. Thus, the fluid flowing through the heat exchanging system  310  forms a cycle. The heat exchangers  312  and  314  are respectively called an evaporator and a condenser because at least a portion of the fluid undergoes a phase change while flowing though them. At least a portion of the fluid changes from liquid to vapor in the evaporator  312  while at least a portion of the fluid changes from vapor to liquid in the condenser  314 . 
     Because the fluid flowing through the evaporator  312  absorbs heat, an air-conditioning or refrigeration system results if the evaporator  312  is placed in a space to be cooled. On the other hand, because the fluid flowing through the condenser  314  loses heat, a heat-pump system results if the condenser  314  is placed in a space to be heated. The evaporator  312  and condenser  314  may directly cool or heat a space through air inside). Alternatively, the evaporator  312  and condenser  314  may exchange heat with other heat transfer fluids (e.g., water), which in turn will either cool or heat a space through another heat transfer mechanism. 
     Furthermore, a system that exchanges heat directly with outside air can serve as both an air-conditioning or refrigeration system and a heat-pump system. For example, during the summer, the heat exchanging system  310  shown in FIG. 7 may serve as an air-conditioning or refrigeration system where the evaporator  312  cools inside air by absorbing heat while the condenser  314  rejects heat to outside air. In this air-conditioning or refrigeration system, the fluid flows in a direction designated by the reference number  320 . The reference numbers  315  and  317  respectively designate a suction line in fluid communication with the evaporator  312  and a discharge line in fluid communication with the condenser  317  in this air-conditioning or refrigeration system. During the winter, on the other hand, the flow of the fluid may be reversed as designated by the reference number  322  to transform the air-conditioning or refrigeration system into a heat-pump system. In this heat-pump system, the heat exchanger  312  becomes a condenser, which warms the inside air by rejecting heat thereto, while the heat exchanger  314  becomes an evaporator, which absorbs heat from the outside air. In this heat-pump system, the reference numbers  315  and  317  respectively designate a discharge line in fluid communication with the condenser  312  and a suction line in fluid communication with the evaporator  314 . 
     In accordance with the present invention, the heat exchanging system  310  can modulate its capacity in response to changes in system parameters (e.g., changes in condenser pressure or temperature) or changes in cooling or heating requirements. In other words, the heat exchanging system  310  adjusts its cooling or heating capacity by adjusting the amount of fluid flowing through the system. As described in greater detail below, the compressor  316  of the present invention can automatically modulate its capacity based on changing parameters of the compressor that are in turn applied to change an operating characteristic of the compressor. This automatic modulation of the compressor thereby can affect and modulate the capacity of the heat exchanging system  310  without any need for external controls. Thus, the self-modulating compressor of the present invention can be used in an HVAC system and will self-modulate its capacity as parameters, such as the outside air temperature, change. In such an HVAC system, the compressor can be turned on and off by a standard thermostat control, whenever the desired temperature falls above or below the selected set temperature. Once the compressor is turned on, it will self modulate, depending on the working parameters of the system. 
     The embodiment shown in FIGS. 1-6 illustrates a capacity modulation system  10  of the present invention utilizing a rotary or swing-link compressor  12  of the type used in an air-conditioning or refrigeration system. As described below, however, the capacity modulation system and methods of the present invention can also be incorporated into other types of compressors. Also, the capacity modulation system could be effectively applied in other heat exchanging systems, such as a heat-pump system. 
     As shown in FIG. 1, the compressor  12  includes a housing  14 , a motor  16 , and a rotary compressor unit  18 . The motor  16  turns a shaft  20 , which operates the compressor unit  18 . 
     In operation, the compressor unit  18  draws fluid, such as refrigerant, into the housing  14  through an inlet  22 , at suction pressure through the suction line  315  shown in FIG.  7 . In the compressor shown in FIG. 1, the inlet is proximate to the motor  16 , and the refrigerant cools the motor  16  as it flows to the compressor unit  18 . Alternatively, the inlet  22  can be positioned proximate to the compressor unit  18  in such a manner that the refrigerant does not flow past the motor  16 , but instead is applied directly to the compressor unit  18 . 
     The fluid then passes through the suction channel  24  and enters the compressor unit  18 , where it is compressed. The compressed fluid leaves the compressor unit  18  at discharge pressure through the discharge channel  26 , then passes out of the housing  14  through the outlet  28  to the discharge line  317  shown in FIG.  7 . 
     The fluid is compressed within the compressor unit  18  in a substantially cylindrical compression chamber  30  shown in FIGS. 2-5. The rotatable shaft  20  is disposed within the compression chamber  30 . A cylindrical roller or piston  32  is eccentrically disposed on the shaft  20  within the compression chamber  30  such that it contacts a wall of the compression chamber  30  as the shaft  20  rotates. The roller  32  is free to rotate on an eccentric or crank  34  that is secured to or integral with the shaft  20 . The roller or piston  32  can be any of the types used in conventional rotary or swing link compressors. 
     In the rotary compressor shown in FIGS. 2-5, a partition, or vane  36 , is disposed between the wall of the compression chamber  30  and the roller  32  to define a low pressure portion  38  and a high pressure portion  40  within the compression chamber  30 . As the shaft  20  and the roller  32  rotate from the position shown in FIG. 2, the low pressure portion  38  increases in size as the high pressure portion  40  decreases in size. As a result, the fluid in the high pressure portion  40  is compressed and exits through the discharge port  44 . 
     The vane  36  must be kept in close contact with the roller  32  as the roller  32  moves along the circumference of the compression chamber  30  to insure that the fluid being compressed does not leak back to the low pressure portion  38 . The vane  36  can be spring biased towards the roller  32 , allowing the vane  36  to follow the roller  32  as it moves. Alternatively, the vane  36  can be integral with the roller  32 . Compressors having an integral vane and roller are known as “swing link” compressors. 
     The suction channel  24 , shown in FIGS. 1-5, is in fluid communication with the low pressure portion  38  to provide fluid to the compression chamber  30  at suction pressure. As shown in FIGS. 2-5, the suction channel  24  forms a suction inlet  42  in the wall of the compression chamber  30  adjacent to the vane  36  in the low pressure portion  38 . 
     The discharge channel  26 , shown in FIGS. 1-5, is in fluid communication with the high pressure portion  40  to remove fluid from the compression chamber  30  at discharge pressure. The discharge channel  26  forms a discharge outlet  44  in the wall of the compression chamber  30  adjacent to the vane  36  in the high pressure portion  40 , as shown in FIGS. 2-5. 
     Two embodiments of the capacity modulation system  10  of the present invention are shown in FIGS. 2-5. In both embodiments, a reexpansion chamber  50  is provided adjacent to the compression chamber  30 , with a reexpansion channel  46  providing a flow path between the compression chamber  30  and the reexpansion chamber  50 . The reexpansion channel  46  forms a reexpansion port  48  in the wall of the compression chamber. 
     The reexpansion chamber  50  can be arranged in locations proximate to the compression chamber  30  and is sized to provide a desired modulation of the compressor capacity, as explained in more detail below. Larger reexpansion chambers will modulate the change in capacity more than will smaller reexpansion chambers. In preferred embodiments, the reexpansion chamber  50  should be sized sufficient to cause the compressor to operate at a lower capacity of 70 to 90% relative to its highest capacity when the reexpansion chamber  50  is closed off from the compression chamber. By means of example only, the reexpansion chamber  50  can be machined as a recess in the cylinder block opposite the compression chamber  30  and connected with the compression chamber  30  by a drilled channel. The open recess can then be enclosed by a cap of the compressor, to provide a sealed reexpansion chamber  50 . 
     As shown in FIGS. 2-5, the reexpansion chamber  50  is connected with a portion of the reexpansion channel  46 . Further, a valve  52  is disposed in the reexpansion channel  46 . The valve  52  is movable between a first position, shown in FIGS. 2 and 4, and a second position, shown in FIGS. 3 and 5. 
     In the first position, the valve  52  allows fluid to flow between the compression chamber  30  and the reexpansion chamber  50 . As described below, the compressor  12  operates in a reduced capacity mode when the valve  52  is in the first position. In the second position, the valve  52  prevents fluid communication between the compression chamber  30  and the reexpansion chamber  50 . As described below, the compressor  12  operates in a full capacity mode when the valve  52  is in the second position. Thus, the valve  52  selectively allows or prevents fluid communication between the compression chamber  30  and the reexpansion chamber  50 . 
     In the embodiment of the capacity modulation system  10  shown in FIGS. 2 and 3, the valve  52  comprises a sliding element  54  biased to the first position by a coil spring  56 . The sliding element  54  has a forward surface  54   a  and a rear surface  54   b . A discharge feed line  58  extends from the discharge channel  26  to the reexpansion channel  46  to expose the rear surface  54   b  of the sliding element  54  to fluid at discharge pressure. 
     When the compressor  12  is initially activated, it is in the reduced capacity mode shown in FIG.  2 . The compression cycle begins as fluid enters the low pressure portion  38  of the compression chamber  30  through the suction channel  24  in advance of the roller  32 . 
     As the roller  32  proceeds along the inner circumference of the compression chamber  30 , the fluid is compressed. Some of this compressed fluid flows through the reexpansion port  48 , along the reexpansion channel  46 , and into the reexpansion chamber  50 . When the roller  32  passes the reexpansion port  48 , the fluid in the reexpansion chamber  50  expands back to the low pressure portion  38  of the compression chamber  30 . Some of this fluid flows back through the suction port  42  into the suction channel  24  until the fluid is at or close to the suction pressure. The remaining fluid in the high pressure portion  40  is further compressed until it is discharged from the compression chamber  30  through the discharge port  44 . 
     Thus, in this mode, not all of the fluid that enters the compression chamber  30  exits through the discharge port  44 . A certain volume of fluid, which is dependent upon the volume of the reexpansion chamber  50 , is allowed to return to the compression chamber  30 . Because not all of the fluid exits the compressor  12 , this operational mode is referred to as the reduced capacity mode. 
     The degree of capacity reduction is determined by a variety of factors, including the volume of the reexpansion chamber  50  and the location of the reexpansion port  48  relative to the suction port  42 . Generally, increasing the volume of the reexpansion chamber  50  provides a greater reduction in the capacity of the compressor  12 . Similarly, locating the reexpansion port  48  farther from the suction port  42  along the roller&#39;s path also provides a greater reduction in capacity. Ultimately, the optimum volume of the reexpansion chamber  50  and location of the reexpansion port  42  for a given application can be determined by a combination of analytical calculations and empirical testing. 
     Referring again to FIG. 2, as the compressor  12  continues to operate, the discharge pressure slowly increases. The force of the fluid on the rear surface  54   b  of the sliding element  54  acts against the biasing force of the spring  56  and the force acting on the forward surface  54   a  of the sliding element  54 . The forward surface of  54   a  is exposed to either the fluid in the low pressure portion  38  or the fluid in the high pressure portion  40 . Accordingly, the forward surface of  54   a  is exposed to at least the suction pressure. In other words, the pressure acting on the forward surface  54   a  of the sliding element  54  varies from the suction pressure and an intermediate pressure achieved in the high pressure portion  40  when the roller  32  reaches the reexpansion port  48 . Eventually, the discharge pressure reaches a predetermined level and overcomes the combined force of the spring force and the force exerted on the forward surface  54   a , causing the sliding element  54  to move to the second position, corresponding to the full capacity mode of the compressor  12 . The predetermined discharge pressure level can be varied by using a biasing means having a different spring constant. The valve  52  of this embodiment, therefore, operates in response to a parameter internal to the compressor  12 . Again, the design of the valve  52  and the selection of a spring  56  for a specific system can be determined through empirical testing. 
     FIG. 3 shows the compressor  12  of this embodiment in the full capacity mode. As shown, the forward surface  54   a  of the sliding element  54  is substantially flush with the wall of the compression chamber  30 . Here, as the roller  32  proceeds around the compression chamber  30 , all of the fluid in the low pressure section  38  is compressed until it is discharged through the discharge port  44 . Thus, in the full capacity mode, each compression stroke of the roller  32  produces a larger volume of high pressure fluid. In this embodiment, the rotary or swing link compressor will operate at the full capacity, in the same manner as conventional rotary and swing link compressors. 
     Although the valve  52  of this embodiment has been described as being a piston-type valve  52  biased with a coil spring  56 , it is noted that other equivalent valve members and biasing devices are considered within the scope of the invention. Examples of suitable biasing means include torsion springs, coil springs, and other springs and elastic elements. 
     In another embodiment, shown in FIGS. 4 and 5, the valve  52  comprises a valve element controlled to open or close in response to a control signal. For example, in FIGS. 4 and 5 the valve includes a sliding element  60  engaged by a solenoid  62 . The sliding element  60  has a forward surface  60   a  and a rear surface  60   b . The solenoid  62  is actuated to move the sliding element  60  in response to a control signal received from a control device  64 . The control device  64  generates the control signal based on input received from one or more sensors  66  located internal or external to the compressor  12 . The valve actuator has been described as a solenoid, but other equivalent actuators, including pneumatic and hydraulic actuators, are considered within the scope of the invention. 
     As shown in FIGS. 4 and 5, the internal sensors  66  can be located in the suction channel  24  and/or the discharge channel  26 . For example, the sensors  66  can be pressure sensors, and the control device  64  can cause the solenoid to move the valve  52  to the closed position when the discharge pressure or the pressure differential reaches a predetermined value. Other sensor locations internal to the compressor  12  are considered within the scope of the invention. For example, temperature sensors could be used. 
     Sensors external to the compressor  12  can also be used and can be located in an any suitable location to measure a desired parameter. One external sensor  66  is shown schematically in FIGS. 4 and 5. 
     Sensors can be used to measure all types of parameters internal and external to the compressor  12 . Examples of parameters internal to the compressor  12  are flow rate, fluid temperature, and fluid pressure. External parameters include air temperature, equipment temperature, humidity, and noise. The valve position, and thus capacity, can be varied as a function of these parameters. Typical control devices used to generate control signals are thermostats, humidistats, and other equivalent devices. Other internal and external parameters and control devices are within the scope of the invention. The control device  64  receives input from the sensors  66  and, guided by internal software or control specifications, actuates the valve  52  to operate the compressor  12  in the full capacity mode or reduced capacity mode to provide optimum capacity and efficiency at given sensed conditions. 
     FIG. 4 shows the compressor  12  of this embodiment in the reduced capacity mode. As described above, when the compressor  12  is operated in this mode, a portion of the fluid is compressed into the reexpansion chamber  50  during each compression cycle. When the roller  32  passes the reexpansion port  48 , the fluid in the reexpansion chamber  50  expands back to the low pressure section  38  of the compression chamber  30 . The remaining fluid in the high pressure section  40  is further compressed until it is discharged from the compression chamber  30  through the discharge port  44 . 
     The compressor  12  operates in the reduced capacity mode until an internal or external parameter is reached, according to the input from one or more sensors  66 . In response to the sensor input, the control device  64  generates a control signal to actuate the solenoid  62 . When the solenoid  62  is actuated, it moves the sliding element  60  from the first position to the second position, thereby putting the compressor  12  into the full capacity mode. The valve  52  of this embodiment, therefore, operates in response to a parameter internal or external to the compressor  12 . 
     FIG. 5 shows the compressor  12  of this embodiment in the full capacity mode. As shown, the forward surface  60   a  of the sliding element  60  is substantially flush with the wall of the compression chamber  30 . As the roller  32  proceeds around the compression chamber  30 , all of the fluid in the low pressure section  38  is compressed until it is discharged through the discharge port  44 . Thus, in the full capacity mode, each compression stroke of the roller  32  produces a larger volume of high pressure fluid. 
     The capacity modulation system  10  of this embodiment may also be utilized so that the compressor  12  begins operation in the full capacity mode and transitions to the reduced capacity mode in response to the measurement of an internal or external parameter. 
     In an alternative embodiment, the valve  52  can be manually controlled using a switch  68  connected to the control device  64 , as shown in FIGS. 4 and 5. With the switch  68 , a user can change the operational mode of the compressor  12  between the full capacity mode and the reduced capacity mode, as desired. 
     Although the valves  52  of the above-described embodiments have been described as comprising a sliding element  54 ,  60 , a variety of other mechanisms can be applied according to the principles of the present invention. Examples of suitable valves include ball valves, gate valves, globe valves, butterfly valves, and check valves. These valves can be positioned along the reexpansion channel  46  between the compression chamber  30  and the reexpansion chamber  50 . Further, the valves can be designed to open and permit fluid flow between the chambers when the compressor  12  is to be operated in the reduced capacity mode, and to close and prevent, or significantly limit, flow when the compressor  12  is to be operated in the full capacity mode. More generally, such valves or other flow control devices are arranged to increase or decrease the capacity of the compressor, as a function of one or more operating parameters. Preferably, the valves or flow control devices vary the capacity of the compressor, as a function of the compressor itself, so that no external controls are required. 
     The specific embodiments of FIGS. 1-5 discussed above provide a rotary or swing link compressor with a dual capacity. However, the principles of the invention can be applied to provide a compressor  12  having three or more differential capacities by providing more than one reexpansion chamber  50 . 
     In a further embodiment of the capacity modulation system  10  of the present invention shown in FIG. 6, two separate reexpansion chambers  150 ,  250  and reexpansion channels  146 ,  246  are provided to selectively communicate with the compression chamber  30  under desired conditions. In this embodiment, the general elements and valve systems described above are used for each reexpansion chamber  150 ,  250 . 
     In operation, the control device  64  of this embodiment opens both valves  152 ,  252  to allow flow between the compression chamber  30  and both reexpansion chambers  150 ,  250  to operate the compressor at a maximum level of capacity reduction. Two intermediate levels of capacity reduction are achieved by selectively opening the first valve  152  and closing the second valve  252 , then closing the first valve  152  and opening the second valve  252 . When both valves  152 ,  252  are closed, the compressor  12  operates at full capacity. The control device  64  can select the proper valve configuration to optimize the operation of the compressor  12  under a given set of conditions. Alternatively, as shown in FIG. 6, a switch  68  may be provided to allow manual control over the capacity of the compressor  12 . Compressors utilizing more than two reexpansion chambers are within the scope of the invention. 
     In a further embodiment, a portion of a single reexpansion chamber can be designed so that the volume exposed to the compressed fluid can be varied by valves or other means. 
     FIGS. 8 and 9 illustrate another embodiment of a compressor of the present invention for an air-conditioning or refrigeration system. In the illustrated embodiment, compressor  316  is a reciprocating compressor. The reciprocating compressor  316  includes a crankcase  330  and a manifold  324 . The manifold  324  includes a suction channel  328  in fluid communication with the suction line  315  (FIG. 7) to receive the fluid from the evaporator  312  at a suction pressure. The manifold  324  also includes a discharge channel  326  in fluid communication with the discharge line  317  (FIG. 7) to discharge the fluid at a discharge pressure to the condenser  314 . A compression chamber  332  formed in the crankcase  330  is in fluid communication with the suction channel  328  and receives the fluid therefrom at the suction pressure. The compression chamber  332  is also in fluid communication with the discharge channel  326  and the fluid is discharged to the discharge channel  326  at the discharge pressure. 
     The reciprocating compressor  316  includes a reciprocating piston  336  positioned and movable within the compression chamber  332  to compress fluid (e.g., refrigerant) entering the compression chamber  332  through the suction channel  328  and to discharge the fluid to the discharge channel  326 . A valve plate  338  mounted on the crankcase  330  has an inlet  340  and an outlet  342 . An inlet valve  344  opens and closes the inlet  340  to control the flow of the fluid into the compression chamber  332  from the suction channel  328 . Similarly, an outlet valve  346  opens and closes the outlet  342  to control the flow of the fluid out of the compression chamber  332  to the discharge channel  326 . A variety of different known valves and valve systems, such as those now commercially used, can be applied to control the flow into and out of the compression chamber  332 . 
     When the reciprocating piston  336  moves in a suction stroke  348 , the inlet valve  344  opens and the fluid at the suction pressure enters the compression chamber  332  from the suction channel  328  through the inlet  340 . The outlet valve  346  remains closed while the reciprocating piston  336  moves in the suction stroke  348 . On the other hand, the reciprocating piston  336  moving in a compression stroke  350  compresses the fluid within the compression chamber  332 . When the pressure differential across the outlet valve  346  (i.e., the difference between the pressure within the compression chamber  332  and the discharge pressure in the discharge channel  326 ) reaches a predetermined value, the outlet valve  346  opens and discharges the fluid to the discharge channel  326  at the discharge pressure. In other words, when the reciprocating piston  336  increases the fluid pressure within the compression chamber  332  over the discharge pressure in the discharge channel  326  by the predetermined value, the outlet valve  346  opens to discharge the fluid to the discharge channel  326 , which is in fluid communication with the condenser  314  (FIG. 7) through the discharge line  317 . The inlet valve  344  remains closed while the reciprocating piston  336  moves in the compression stroke  350 . 
     The reciprocating compressor  316  further includes a reexpansion chamber  334 . This reexpansion chamber can be in a variety of forms and can be sized to achieve the desired variation between a first compressor capacity and a second compressor capacity. Preferably, the reexpansion chamber is machined into the block or the crankcase of the compressor and sized such that the reduce compressor capacity is 70 to 90% of the full capacity. The reexpansion chamber  334  is in fluid communication with the compression chamber  332  through a flow passage  354 . In the embodiment shown in FIG. 8, the flow passage  354  is defined by the valve plate  338  and a recess formed in the crankcase  330 . A flow passage formed in the crankcase  330 , rather than defined by valve plate  338  and a recess formed in the crankcase  330 , is also within the scope of the present invention. 
     A valve member  356  positioned within the reexpansion chamber  334  controls the flow of the fluid between the compression chamber  332  and the reexpansion chamber  334  by permitting and preventing flow through the flow passage  354 . As explained below, the valve member  356  operates similarly to the sliding element  54  of the rotary compressor shown in FIGS. 2 and 3. The valve member  356  is movable between a first position permitting flow through the flow passage  354  (FIG. 8) and a second position preventing flow through the flow passage  354  (FIG.  9 ). 
     In the embodiment shown in FIGS. 8 and 9, the valve member  356  includes a head portion  358 , a tail portion  360 , and a stem portion  364  connecting the head and tail portions  358  and  360 . The side surfaces of the head and tail portions  358  and  360  respectively have sealing members  359  and  361 , such as o-rings, provided therein for a sealing contact with the inner surface of the reexpansion chamber  334 . In this embodiment, the head and tail portions  358  and  360  and the reexpansion chamber  334  are circular in shape and are sized to have a close fit between the opposed surfaces. 
     As designated by the reference number  368  in FIGS. 8 and 9, the head portion  358  of the valve member  356  is exposed continuously to the discharge pressure of the fluid through an opening  366  in fluid communication with the discharge channel  326 . Accordingly, the fluid at the discharge pressure continuously acts on the head portion  358  and continuously exerts a force on valve member  356  in a direction tending to seat the bottom of the tail portion  360  against the bottom of the reexpansion chamber  334  and prevent flow through the flow passage  354 , as shown in FIG.  9 . 
     An annular projection  357  formed on the head portion  358  abuts the top surface of the reexpansion chamber  334  when the valve member  356  is the first position permitting flow through the flow passage  354 , as shown in FIG.  8 . The surface area of the annular projection  357  may be adjusted to vary the area of the head portion  358  exposed to the discharge pressure. For example, if a substantially constant area exposed to the discharge pressure is desired regardless of the position of the valve member  356 , the surface area of the annular projection  357  may be minimized. Alternatively, instead of the annular projection  357 , projections spaced apart from each other may be provided if a substantially constant area exposed to the discharge pressure is desired. On the other hand, if the surface area of the annular projection  357  is substantial, the area exposed to the discharge pressure may be significantly increased when the valve member  356  begins to move from the first position shown in FIG. 8 to the second position shown in FIG.  9 . Instead of the annular projections  357  formed on the head portion  358  of the valve member  356 , a recess may be formed around the opening  366  for the same purpose. 
     The tail portion  360  of the valve member  356  has a recessed portion  362 . A biasing member  370  is positioned in the recessed portion  362  and exerts a biasing force in a direction to abut the annular projection  357  against the top surface of the reexpansion chamber  334  and permit flow through the flow passage  354 , as shown in FIG.  8 . The biasing force, therefore, opposes the force exerted on the valve member  356  by the discharge pressure. In addition, the recessed portion  362  is exposed to the suction pressure of the fluid through an opening  372 , which is in fluid communication with the suction channel  328 . Thus, as designated by the reference number  374  in FIGS. 8 and 9, the suction pressure acts on the recessed portion  362  to exert a force on the valve member  356  in the same direction of the biasing force. Accordingly, the biasing force of biasing member  370  and the force exerted by the suction pressure combine to oppose the force exerted by the discharge pressure. 
     When the force exerted by the discharge pressure is less than the combined force (i.e., the biasing force of the biasing member  370  plus the force exerted by the suction pressure), the valve member  356  is in the first position permitting flow through the flow passage  354 , as illustrated in FIG.  8 . When the valve member  356  is in the first position permitting flow through the flow passage  354 , the reciprocating compressor  316  operates in a reduced capacity mode because some of the fluid entering and exiting the compression chamber  332  through the inlet  340  and outlet  342  flows into and out of the reexpansion chamber  334 . 
     As the reciprocating piston  336  moves in the compression stroke  350  with the valve member  356  in the first position illustrated in FIG. 8, some of the fluid within the compression chamber  332  flows through the flow passage  354  into the reexpansion chamber  334  as designated by the solid arrows  380 . Subsequently, as the reciprocating piston  336  moves in the suction stoke  348  with the valve member  356  in the first position illustrated in FIG. 8, the fluid in the reexpansion chamber  334  expands and flows back into the compression chamber  332  as designated by the dashed arrows  382 . Accordingly, the reciprocating compressor  316  operates in a reduced capacity mode because the amount of the fluid entering and exiting the compression chamber  332  through the inlet  340  and outlet  342  is less when the valve member  356  is in the first position permitting flow through the flow passage  354  than when the valve member  356  is in the second position preventing flow through the flow passage  354 . 
     When the discharge pressure reaches a predetermined level, however, the force exerted by the discharge pressure overcomes the combined force (i.e., the biasing force of the biasing member  370  plus the force exerted by the suction pressure) and moves the valve member  356  to the second position preventing flow through the flow passage  354 , as illustrated in FIG.  9 . When the valve member  356  is in the second position preventing flow through the flow passage  354 , the reciprocating compressor  316  operates in a full capacity mode because the fluid entering and exiting the compression chamber  332  through the inlet  340  and outlet  342  does not flow into and out of the reexpansion chamber  334 . 
     The degree of capacity modulation is determined by a variety of factors, including the volume of the reexpansion chamber  334  available to the fluid and the location of the flow passage  354 . Generally, increasing the volume of the reexpansion chamber  334  available to the fluid results in a greater capacity modulation. Also, locating the flow passage  354  closer to the top of the compression chamber  332  results in a greater capacity modulation. A desired capacity modulation can therefore be controlled by adjusting the volume of the reexpansion chamber  334  available to the fluid and the location of the flow passage  354 . Preferably, the volume of the reexpansion chamber  334  available to the fluid and the location of the flow passage  354  are adjusted such that the reduced capacity is 70 to 90% of the full capacity. 
     Similarly, the level of the discharge pressure at which the valve member  356  prevents flow through the flow passage  354  is determined by a variety of factors, including the biasing force exerted by the biasing member  370  and the suction pressure. A desired level of the discharge pressure at which valve member  356  prevents flow through the flow passage  354  can therefore be controlled by adjusting the combined force exerted by the biasing member  370  and the suction pressure. The suction pressure, however, is a system parameter, which cannot be readily adjusted. The biasing force, on the other hand, can be readily adjusted. Accordingly, a desired level of the discharge pressure at which the valve member  356  prevents flow through the flow passage  354  can be most readily controlled by adjusting the biasing force. For example, a biasing member having a different spring constant can be selected to control the level of the discharge pressure at which the valve member  356  prevents flow through the flow passage  354 . A variety of suitable springs and other elastic elements may be used for the biasing member. Examples of suitable springs include, among other springs, coil springs and torsion springs. 
     The embodiment illustrated in FIGS. 8 and 9 may also be modified so that the valve member is not positioned within the reexpansion chamber. For example, as illustrated in FIGS. 10 and 11, the valve member  356  may be positioned within a valve chamber  384  formed in the crankcase  330  between the compression chamber  332  and the reexpansion chamber  334 . Instead of moving within the reexpansion chamber  334 , the valve member  356  moves within the valve chamber  384  between a first position permitting flow through the flow passage  354  (FIG. 10) and a second position preventing flow through the flow passage  354  (FIG.  11 ). As shown, the structure and operation of the valve member  356  shown in FIGS. 10 and 11 are essentially the same as those of the embodiment shown in FIGS. 8 and 9. 
     In these embodiments, as in the embodiment applied to the rotary compressor, two or more reexpansion chambers and associated flow passages and valves can be incorporated into the compressor to allow more than two different capacities. In any embodiment, the valve arrangement preferably provides for automatic modulation of the compressor capacity based solely on the compressor and the reexpansion chambers, valves, and flow passages incorporated into the compressor. Thus, the compressor will automatically regulate itself, as the discharge pressure reaches a predetermined value relative to the suction pressure. 
     As explained above, the degree of capacity modulation and the level of the discharge pressure at which the valve member  356  prevents flow through the flow passage  354  are two parameters that can be controlled to optimize a given heat exchanging system. The optimum combination of the degree of capacity modulation and the level of the discharge pressure at which the valve member  356  prevents flow through the flow passage  354  may be determined through analytical calculations, empirical testing, or a combination of both. The optimum combination, of course, changes for different heat exchanging systems having different design characteristics and operating conditions. 
     For an air-conditioning or refrigeration system, the system efficiency can be improved by operating the compressor in a reduced capacity mode. The system efficiency of an air-conditioning or refrigeration system increases as the temperature and pressure in a condenser decrease. The temperature and pressure in the condenser, on the other hand, decrease as the capacity of the system decreases. Accordingly, the system efficiency of an air-conditioning or refrigeration system improves if the capacity of the system is reduced. 
     An air-conditioning or refrigeration system, however, needs to provide a certain cooling capacity at a certain condition even if the system efficiency suffers as a consequence. For example, to maintain a space at a comfortable temperature, an air-conditioning system needs to operate in a full capacity mode during a hot summer day even if doing so decreases the system efficiency. 
     For many air-conditioning or refrigeration systems, a condenser is customarily located outdoor to reject heat to outside air. The Seasonal Energy Efficiency Ratio (SEER) is a parameter indicating how efficiently such systems operate. The SEER value for such systems is determined by a weighted average of the system efficiencies at different capacities. Because the condenser is subjected to varying outside air temperatures, the weights given to the system efficiencies at different capacities are calculated based on the most common building types and their operating hours using average weather data in the United States. 
     To determine a SEER value using these calculated weights, the Air-Conditioning &amp; Refrigeration Institute (ARI) requires that the system efficiencies at different capacities be measured at specified air temperatures. For example, the ARI requires that the system efficiency at 100% capacity can be measured at an ambient (outside) air temperature of 95° F. This system efficiency, however, contributes minimally to the SEER value because the number of hours that a condenser is subjected to an outside air temperature of 95° F. is limited. Instead, the system efficiency at a reduced outside air temperature contributes more to the SEER value. 
     Accordingly, the degree of capacity modulation and the level of the discharge pressure at which the valve member  356  prevents flow through the flow passage  354  can be optimized to increase the SEER value for an air-conditioning or refrigeration system. For example, the spring constant of the biasing member  370  can be selected such that the valve member  356  prevents flow through the flow passage  354  when the outside air temperature is greater than a predetermined value. Also, for a compressor having a plurality of compression chambers and corresponding reexpansion chambers, each reexpansion chamber may utilize a biasing member with a different spring constant in order to provide one or more intermediate capacity modulation depending on the outside air temperature. 
     As is well known, the pressure and temperature in the condenser  314  increase as the outside air temperature increases and the compressor discharge pressure and temperature increase as the pressure and temperature in the condenser  314  increase. Accordingly, the spring constant of the biasing member  370  can be selected such that when the outside air temperature is greater than a predetermined value, the compressor discharge pressure increases to the level at which the valve member  356  prevents flow through the flow passage  354 . The predetermined value of the outside air temperature should be selected to maximize the SEER value for a given air-conditioning or refrigeration system. By way of example only, an outside air temperature in the range of 74 to 94° F. can be used as the predetermined value above which the valve member  356  prevents flow through the flow passage  354 . In other words, a given air-conditioning or refrigeration system operates in a reduced capacity mode unless an outside air temperature is greater than a predetermined value in the range of 75 to 94° F. 
     FIGS. 12 and 13 illustrate another embodiment of a reciprocating compressor of the present invention. In the illustrated embodiment, a reciprocating compressor  416  includes a flow passage  454  in fluid communication with the suction channel  328 . The flow passage  454  is also in fluid communication with the compression chamber  332  through an opening  484  formed on a side surface of the compression chamber  332 . The opening  484  is formed between a bottom dead center position and a top dead center position of the reciprocating piston  336 . The top of the opening  484  is formed a predetermined distance D away from the top surface of the reciprocating piston  336  in its bottom dead center position. 
     The reciprocating compressor  416  further includes a valve mechanism  461 . The valve mechanism  461  includes a cap  467  and a valve member  464 . The cap is fittingly engaged (e.g., threaded engagement) with a hole  469  formed in the crankcase  330 . The valve member  464  positioned within the cap  467  and the hole  469  controls the flow of the fluid between the compression chamber  332  and the suction channel  328  by permitting and preventing flow through the flow passage  454 . The valve member  464  is movable between a first position permitting flow through the flow passage  454  (FIG. 12) and a second position preventing flow through the flow passage  454  (FIG.  13 ). 
     The valve member  464  includes a head portion  463  and a stem portion  465 . As illustrated in FIGS. 12 and 13, the head portion  463  of the valve member  464  is exposed continuously to the discharge pressure of the fluid through a feed line  486 , which is in fluid communication with the discharge channel  326 . Accordingly, the fluid at the discharge pressure continuously acts on the head portion  463  and continuously exerts a force in a direction such that the valve member  464  prevents flow through the flow passage  454 . 
     The valve mechanism  461  further includes a biasing member  470 , such as a coil spring, exerting a biasing force in a direction such that the valve member  464  permits flow through the flow passage  454 . In addition, the front surface of the stem portion  465  is continuously exposed to the pressure within the compression chamber  332 . Accordingly, at least the suction pressure continuously acts on the front surface of the stem portion  465  to exert a force on the valve member  464  in the same direction of the biasing force. Accordingly, the biasing force of the biasing member  470  and the force exerted by the suction pressure combine to oppose the force exerted by the discharge pressure. 
     As illustrated in FIG. 12, when the force exerted by the discharge pressure is less than the combined force (i.e., the biasing force of the biasing member  470  plus the force exerted by the suction pressure), the valve member  464  is in the first position and permits flow through the opening  484  and the flow passage  454  to the suction channel  328 . When the valve member  464  is in the first position opening the flow passage  454 , the reciprocating compressor  416  operates in a reduced capacity mode. In this mode, the fluid in the compression chamber  332  flows back through the opening  484 , into flow passage  454 , and even into the suction channel  328  in the manifold  324 . Similar to the reexpansion chamber described above, these elements are in effect combined to provide a reexpansion area in fluid communication with the compression chamber. In effect, the fluid in the compression chamber is not compressed beyond the suction pressure, until the reciprocating piston travels beyond the opening  484 . 
     As the reciprocating piston  336  moves in the compression stroke  350  from its bottom dead center position toward its top dead center position, the fluid within the compression chamber  332  is discharged to the suction channel  328  through the opening  484  and flow passage  454 . This discharge to the suction channel  328  continues until the top surface of the reciprocating piston  336  reaches the top of the opening  484  and closes the opening  484 . In other words, until the reciprocating piston  336  moves the predetermined distance D from its bottom dead center position, no or little compression results. After the top surface of the reciprocating piston the top of the opening  484  and closes the opening  484 , significant compression begins. Accordingly, the reciprocating compressor  416  effectively reduces the stroke length of the reciprocating piston  336  and therefore operates in a reduced capacity mode. 
     As illustrated in FIG. 13, however, when the discharge pressure reaches a predetermined level, the force exerted by the discharge pressure overcomes the combined force (i.e., the biasing force of the biasing member  470  plus the force exerted by the suction pressure) and moves the valve member  464  to the second position and the stem portion  465  prevents flow through the flow passage  454 . When the valve member  464  is in the second position preventing flow through the flow passage  454 , the reciprocating compressor  416  operates in a full capacity mode because no fluid exits the compression chamber  332  through the flow passage  454 . In other words, the full stroke length of the reciprocating piston  336  is utilized to compress the fluid entering and exiting the compression chamber  332  through the inlet  340  and outlet  342 . 
     When the valve member  464  is in the second position preventing flow through the flow passage  454 , the front surface of the stem portion  465  is exposed to at least the suction pressure. In other words, the pressure acting on the front surface of the stem portion  465  varies from the suction pressure and an intermediate pressure achieved when the reciprocating piston  336  reaches the opening  484  from its bottom dead center position. To ensure that the valve member  464  does not experience a transitional phase where the valve member  464  flutters due to this increase in pressure within the compression chamber  332 , the surface area of an annular projection  459  formed on the head portion  463  may be adjusted. As explained above, by increasing the surface area of the annular projection  459 , the area exposed to the discharge pressure may be significantly increased when the valve member  464  begins to move from the first position (FIG. 12) to the second position (FIG.  13 ). Accordingly, the surface area of the annular projection  459  may be adjusted to offset the increase in pressure within the compression chamber  332 . 
     Thus, by adjusting the location of the opening  484  relative to the bottom dead center position of the reciprocating piston  336 , the reciprocating compressor  416  achieves a desired capacity modulation. Also, by adjusting the biasing force exerted by the biasing member  470 , the reciprocating compressor  416  controls the discharge pressure at which valve member  464  prevents flow through the flow passage  454 . Accordingly, as explained in relation to the embodiments illustrated in FIGS. 8-11, the system efficiency of an air-conditioning or refrigeration system can be improved by optimizing the combination of the degree of capacity modulation and the pressure at which the valve member  464  prevents flow through the flow passage  454 . Preferably, the location of the opening  484  is adjusted such that the reduced capacity is 70 to 90% of the full capacity. Also, for example, an outside air temperature in the range of 75 to 94° F. may be utilized as the predetermined value above which the valve member  464  prevents flow through the flow passage  454 . 
     FIG. 14 illustrates another embodiment of a compressor of the present invention. In the illustrated embodiment, the compressor is a scroll compressor  516 . The scroll compressor  516  includes a fixed scroll member  518  and a scroll member  520  movable in orbiting motion relative to the fixed scroll member  518 . As is known in the art, the fixed and movable scroll members  518  and  520  are involute wraps intermeshed with each other and define one or more moving compression chambers. The moving compression chambers progressively decrease in size as the movable scroll member  520  orbits. The moving compression chambers travel from an outer inlet in fluid communication with a suction channel  528  to a center outlet in fluid communication with a discharge channel  526 . The reference number  532  designates the outermost compression chamber. 
     The scroll compressor  516  of the present invention includes a flow passage  554  formed in the fixed scroll member  518 . The flow passage  554  is in fluid communication with the suction channel  528 . The flow passage  554  is also in fluid communication with the outermost compression chamber  532  through an opening  584  formed in the fixed scroll member  518 . The scroll compressor  516  further includes a valve member  564 . The valve member  564  controls the flow of the fluid between the outermost compression chamber  532  and the suction channel  528  by permitting and preventing flow through the flow passage  554 . The valve member  564  is movable between a first position permitting flow through the flow passage  554  (FIG. 15) and a second position preventing flow through the flow passage  554  (FIG.  16 ). 
     As illustrated in FIGS. 15 and 16, the valve member  564  includes a head portion  563  and a stem portion  565 . As designated by the reference number  568 , the head portion  563  of the valve member  564  is exposed continuously to the discharge pressure of the fluid in the discharge channel  526 . Accordingly, the fluid at the discharge pressure continuously acts on the head portion  563  and continuously exerts a force on valve member  564  in a direction such that the valve member  564  prevents flow through the flow passage  554 . 
     A biasing member  570 , such as a coil spring, is positioned between the head portion  563  and the top surface of the fixed scroll member  518 . The biasing member  570  exerts a biasing force in a direction such that the valve member  564  permits flow through the flow passage  554 . In addition, the front surface of the stem portion  565  is exposed to the pressure within the outermost compression chamber  532 , in effect the suction pressure. Accordingly, at least the suction pressure continuously acts on the front surface of the stem portion  565  to exert a force on the valve member  564  in the same direction of the biasing force. Accordingly, the biasing force of the biasing member  570  and the force exerted by the suction pressure combine to oppose the force exerted by the discharge pressure. 
     As illustrated in FIG. 15, when the force exerted by the discharge pressure is less than the combined force (i.e., the biasing force of the biasing member  570  plus the force exerted by the suction pressure), the valve member  564  is in the first position and permits flow through an annular recess  591  and the flow passage  554  to the suction channel  528 . When the valve member  564  is in the first position permitting flow through the flow passage  554 , the scroll compressor  416  operates in a reduced capacity mode. 
     As the movable scroll member  520  moves and decreases the volume within the outermost compression chamber  532 , the fluid within the outermost compression chamber  532  is discharged therefrom through the annular recess  591  and the flow passage  554  to the suction channel  528 . These therefore serve as a reexpansion area. After a predetermined amount of the fluid within the outermost compression chamber  532  is discharged, the movable scroll member  520  covers the opening  584  and stops further discharge. As the movable scroll member  520  further orbits, it again uncovers the opening  584  to discharge the fluid within the outermost compression chamber  532  to the suction channel  528 . 
     As illustrated in FIG. 16, however, when the discharge pressure reaches a predetermined level, the force exerted by the discharge pressure overcomes the combined force (i.e., the biasing force of the biasing member  570  plus the force exerted by the suction pressure) and moves the valve member  564  to the second position and the stem portion  565  prevents flow through the flow passage  554 . When the valve member  564  is in the second position preventing flow through the flow passage  554 , the scroll compressor  516  operates in a full capacity mode because no fluid exits the outermost compression chamber  532  through the flow passage  554 . 
     When the valve member  564  is in the second position preventing flow through the flow passage  554 , the front surface of the stem portion  565  is exposed to at least the suction pressure. In other words, the pressure acting on the front surface of the stem portion  565  varies from the suction pressure and an intermediate pressure achieved when the movable scroll member  520  begins to cover the opening  584 . To ensure that the valve member  564  does not experience a transitional phase where the valve member  564  flutters due to this increase in pressure within the outermost compression chamber  532 , the surface area of an annular projection  559  as well as the surface area of an annular shoulder portion  593  may be adjusted. By increasing the surface area of the annular projection  559  and the surface area of the annular shoulder portion, the area exposed to the pressure within the outermost compression chamber  532  may be significantly reduced when the valve member  564  reaches the second position. Accordingly, the surface area of the annular projection  559  and the surface area of the annular shoulder portion  593  may be adjusted to offset the increase in pressure within the outermost compression chamber  532 . 
     Therefore, by adjusting the location of the opening  584 , the scroll compressor  516  achieves a desired capacity modulation. Also, by adjusting the biasing force exerted by the biasing member  570 , the scroll compressor  516  controls the discharge pressure at which valve member  564  prevents flow through the flow passage  554 . Accordingly, as explained in relation to the embodiments illustrated in FIGS. 8-11, the system efficiency of an air-conditioning or refrigeration system can be improved by optimizing the combination of the degree of capacity modulation and the pressure at which the valve member  564  prevents flow through the flow passage  554 . Preferably, the location of the opening  584  is adjusted such that the reduced capacity is 70 to 90% of the full capacity. Also, for example, an outside air temperature of in the range of 75 to 94° F. may be utilized as the predetermined value above which the valve member  564  closes the flow passage  554 . 
     FIGS. 17 and 18 illustrate yet another embodiment of a reciprocating compressor of the present invention. In the illustrated embodiment, a reciprocating compressor  616  includes a valve chamber  660  formed in the crankcase  330  next to the compression chamber  332 . The valve chamber  660  is in fluid communication with the compression chamber  332  through the opening  484  formed on a side surface of the compression chamber  332 . The valve chamber  660  is also in fluid communication with the suction channel  328  through a flow passage  654 . Thus, the flow passage  654  is in fluid communication with the compression chamber  332  through the opening  484 . The opening  484  is formed between a bottom dead center position and a top dead center position of the reciprocating piston  336 . The top of the opening  484  is formed a predetermined distance D away from the top surface of the reciprocating piston  336  in its bottom dead center position. 
     A valve member  664  is disposed within the valve chamber  660 . The valve member  664  controls the flow of the fluid between the compression chamber  332  and the suction channel  328  by permitting and preventing flow through the flow passage  654 . The valve member  664  is movable between a first position permitting flow through the flow passage  654  (FIG. 17) and a second position preventing flow through the flow passage  654  (FIG.  18 ). 
     The valve member  664  includes a head portion  663 , a tail portion  666 , and a stem portion  665  connecting the head and tail portion  663  and  666 . Preferably, the head portion  663 , the tail portion  666 , and the stem portion  665  are circular in cross section and have the same diameter. The side surfaces of the head and tail portions  663  and  665  respectively have sealing members  659  and  661 , such as o-rings or flip seals, provided therein for a sealing contact with the inner surface of the valve chamber  660 . 
     As illustrated in FIGS. 17 and 18, the head portion  663  of the valve member  664  is exposed continuously to the discharge pressure of the fluid through a flow passage  686 , which is in fluid communication with the discharge channel  326 . Accordingly, the fluid at the discharge pressure continuously acts on the head portion  663  and continuously exerts a force in a direction such that the valve member  664  prevents flow through the flow passage  654 . 
     As illustrated in FIG. 17, when the valve member  664  is in the first position permitting flow through the flow passage  654 , the tail portion  666  of the valve member  664  is exposed continuously to the suction pressure of the fluid through the flow passage  684 , which is in fluid communication with the suction channel  328 , as well as through the opening  484 , which is in fluid communication with the compression chamber  332 . However, as illustrated in FIG. 18, when the valve member  664  is in the second position preventing flow through the flow passage  654 , the tail portion  666  of the valve member  664  is exposed continuously to the suction pressure of the fluid only through the flow passage  654 . In both configurations, the fluid at the suction pressure continuously acts on the tail portion  666  and continuously exerts a force in a direction such that the valve member  664  permits flow through the flow passage  654 . 
     In addition, a biasing member  670 , such as a coil spring, is provided to exerts a biasing force in a direction such that the valve member  664  permits flow through the flow passage  654 . Accordingly, the biasing force of the biasing member  670  and the force exerted by the suction pressure combine to oppose the force exerted by the discharge pressure. 
     As illustrated in FIG. 17, when the force exerted by the discharge pressure is less than the combined force (i.e., the biasing force of the biasing member  670  plus the force exerted by the suction pressure), the valve member  664  is in the first position and permits flow through the opening  484  and the flow passage  654  to the suction channel  328 . When the valve member  664  is in the first position, the reciprocating compressor  616  operates in a reduced capacity mode. In this mode, the fluid in the compression chamber  332  flows back through the opening  484 , into flow passage  654 , and even into the suction channel  328  in the manifold  324 . Similar to the reexpansion chamber described with regard to the embodiments illustrated in FIGS. 8-11, these elements are in effect combined to provide a reexpansion area in fluid communication with the compression chamber. In effect, the fluid in the compression chamber is not compressed beyond the suction pressure, until the reciprocating piston travels beyond the opening  484 . 
     As the reciprocating piston  336  moves in the compression stroke  350  from its bottom dead center position toward its top dead center position, the fluid within the compression chamber  332  is discharged to the suction channel  328  through the opening  484 , valve chamber  660 , and flow passage  654 . This discharge to the suction channel  328  continues until the top surface of the reciprocating piston  336  reaches the top of the opening  484  and closes the opening  484 . In other words, until the reciprocating piston  336  moves the predetermined distance D from its bottom dead center position, no or little compression results. After the top surface of the reciprocating piston  336  reaches the top of the opening  484  and closes the opening  484 , significant compression begins. Accordingly, the reciprocating compressor  616  effectively reduces the stroke length of the reciprocating piston  336  and therefore operates in a reduced capacity mode. 
     As illustrated in FIG. 18, however, when the discharge pressure reaches a predetermined level, the force exerted by the discharge pressure overcomes the combined force (i.e., the biasing force of the biasing member  770  plus the force exerted by the suction pressure) and moves the valve member  664  to the second position. When the valve member  664  is in the second position preventing flow through the flow passage  654 , the reciprocating compressor  616  operates in a full capacity mode because no fluid exits the compression chamber  332  through the flow passage  654 . In other words, the full stroke length of the reciprocating piston  336  is utilized to compress the fluid entering and exiting the compression chamber  332  through the inlet  340  and outlet  342 . 
     As illustrated in FIG. 18, when the valve member  664  is in the second position, the stem portion  665  blocks the opening  484  and prevents flow through the opening  484  and the flow passage  654  to the suction channel  328 . Compared with the embodiment illustrated in FIGS. 12 and 13 where the valve member  464  moves perpendicular to the movement of the reciprocating piston  336 , the valve member  664  in the embodiment illustrated in FIGS. 17 and 18 moves parallel with the movement of the reciprocating piston  336 . In the embodiment illustrated in FIGS. 12 and 13, the pressure in the compression chamber  332  exerts a net force on the front surface of the stem portion  465  when the valve member  464  is in the second position. That net force, which is exerted in the direction of the movement of the valve member  464 , changes as the pressure in the compression chamber  332  varies between the suction pressure and an intermediate pressure achieved when the reciprocating piston  336  reaches the opening  484  from its bottom dead center position. However, in the embodiment illustrated in FIGS. 17 and 18, when the valve member  664  is in the second position, the pressure in the compression chamber  332  exerts no net force on the valve member  664  in the direction of the movement of the valve member  664 . In other words, when the valve member  664  is in the second position, the increase in pressure from the suction pressure to the intermediate pressure achieved when the reciprocating piston  336  reaches the opening  484  has no impact on the valve member  664  because the valve member  664  moves parallel with the movement of the reciprocating piston  336 . Accordingly, the embodiment illustrated in FIGS. 17 and 18 eliminates any instability problem that may exist in the embodiment illustrated in FIGS. 12 and 13. 
     By adjusting the location of the opening  484  relative to the bottom dead center position of the reciprocating piston  336 , the reciprocating compressor  616  achieves a desired capacity modulation. Also, by adjusting the biasing force exerted by the biasing member  670 , the reciprocating compressor  616  controls the discharge pressure at which valve member  664  prevents flow through the flow passage  654 . Accordingly, as explained in relation to the embodiments illustrated in FIGS. 8-11, the system efficiency of an air-conditioning or refrigeration system can be improved by optimizing the combination of the degree of capacity modulation and the pressure at which the valve member  664  prevents flow through the flow passage  654 . Preferably, the location of the opening  484  is adjusted such that the reduced capacity is 70 to 90% of the full capacity. Also, for example, an outside air temperature in the range of 75 to 94° F. may be utilized as the predetermined value above which the valve member  664  prevents flow through the flow passage  654 . 
     FIGS. 19 and 20 illustrate yet another embodiment of a reciprocating compressor of the present invention. In the illustrated embodiment, a reciprocating compressor  716  includes a valve mechanism  761 . The valve mechanism  761  includes a temperature element  775 . For the purposes of the following description, the term “temperature element” refers to a material or a combination of materials that changes volume or shape as a function of temperature. 
     Compared with the embodiment illustrated in FIGS. 12 and 13 where the pressure controls the capacity modulation, temperature controls the capacity modulation in the embodiment illustrated in FIGS. 19 and 20. As illustrated in FIGS. 19 and 20, in addition to the structures included in valve mechanism  461  illustrated in FIGS. 12 and 13, the valve mechanism  761  includes the temperature element  775 . The temperature element  775  is positioned between the head portion  463  of the valve member  464  and the cap  467  and is exposed continuously to the discharge temperature of the fluid through the feed line  486 . As the temperature of element  775  changes, it exerts a varying force on the head portion  463  of the valve member  464 , as it expands/contracts or changes its shape. Therefore, a thermal force, which varies in magnitude as a function of the discharge temperature, is exerted on the head portion  463  of the valve member  464  in addition to the force exerted by the discharge pressure on the valve member  464 . 
     In the embodiment illustrated in FIGS. 19 and 20, the spring constant of the biasing member  470  is adjusted such that, at a predetermined operating condition of the fluid, the force exerted by the discharge pressure alone is not enough to overcome the combined force (i.e., the biasing force of the biasing member  470  plus the force exerted by the suction pressure). However, at the predetermined operating condition of the fluid, the thermal force of the temperature element  775  combined with the force exerted by the discharge pressure overcomes the opposing force to move the valve member  464  to the second position illustrated in FIG.  20 . Accordingly, when the fluid reaches the predetermined operating condition, the discharge pressure and temperature cause the valve member  464  to move from the first position (FIG. 19) to the second position (FIG.  20 ). The temperature element  775  may be secured to the head portion  463  and the cap  467 . Alternatively, the temperature element  775  may be positioned to abut the head portion  463  and the cap  467  without being secured thereto. 
     As illustrated in FIGS. 21 and 22, the temperature element  775  may be a wax  777  or other material that changes volume as the temperature changes. Preferably, the wax material  777  is annular in shape. Alternatively, as illustrated in FIGS. 23 and 24, the temperature element  775  may be a bladder  779 . The bladder  779  has a hollow enclosure  781  filled with gas. As the temperature changes, the gas within the hollow enclosure  781  expands or contracts to exert a thermal force on the head portion  463 . Preferably, the bladder  779  is toroidal (i.e., donut-like) in shape and has a refrigerant as the gas that fills the hollow enclosure  781 . 
     Alternatively, as illustrated in FIGS. 25 and 26, the temperature element  775  may be a bi-metal disk  783 . As the temperature changes, the bi-metal disk  783  changes its shape and changes the magnitude of the thermal force exerted on the head portion  463 . For example, when the fluid reaches the predetermined operating condition having a predetermined temperature, the bi-metal disk  783  snaps to provide the thermal force necessary to move the valve member  464  from the first position (FIG. 25) to the second position (FIG.  26 ). A plurality of disks may be stacked together to provide the necessary thermal force when the fluid reaches the predetermined operating condition. 
     Preferably, as illustrated in FIGS. 19-26, the temperature element  775  is in direct contact with the fluid at the discharge temperature for heat transfer therebetween. Alternatively, however, as illustrated in FIGS. 27 and 28, a valve mechanism  861  may include a cap  867 , which has no opening. Because the cap  867  has no opening, a direct contact between the fluid at the discharge temperature and a temperature element  875  is not permitted. In this embodiment, the heat transfer between the temperature element  875  and the fluid at the discharge temperature occurs indirectly through the cap  867 . In this embodiment, to assist the heat transfer between the fluid and the temperature element  875  through the cap  867 , the feed line  486  may have an enlarged opening  863  where the feed line  486  connects to the cap  867 . The enlarged opening  863  increases the heat transfer surface and thereby increases the heat transfer between the temperature element  875  and the fluid at the discharge temperature. 
     In the embodiment illustrated in FIGS. 27 and 28, the spring constant of the biasing member  470  is adjusted such that, at a predetermined operating condition of the fluid, the thermal force of the temperature element  875  alone is sufficient to overcome the opposing force (i.e., the biasing force of the biasing member  470  plus the force exerted by the suction pressure) to move the valve member  464  to the second position illustrated in FIG.  28 . Accordingly, when the fluid reaches the predetermined operating condition, the discharge temperature alone causes the valve member  464  to move from the first position (FIG. 27) to the second position (FIG.  28 ). 
     For the temperature element  875 , the embodiments illustrated in FIGS. 21-26 may be used. The temperature element  875  may be different in shape from the temperature elements illustrated in FIGS. 21-26 because the fluid need not directly contact the head portion  463  of the valve member  464 . For example, the temperature element  875  may be circular in shape. 
     FIGS. 19 through 28 illustrate temperature elements applied to an embodiment of a reciprocating compressor described in FIGS. 12 and 13. However, the temperature elements illustrated in FIGS. 19 through 28 may also be applied to other embodiments of the compressors described in FIGS. 1-6,  8 - 11 , and  14 - 18 . 
     In several of the embodiments illustrated above, the valve member is subjected to the suction and discharge pressures of the compressor. Alternatively, however, the valve member may be subjected to one or more intermediate pressures between the suction and discharge pressures. In other words, the valve member may be subjected to (1) the suction pressure on one side and an intermediate pressure on the other side, (2) an intermediate pressure on one side and the discharge pressure on the other side, or (3) two different intermediate pressures on opposite sides. In a reciprocating compressor, an intermediate pressure may be obtained from a compression chamber through an opening formed between the bottom and top dead center positions of the reciprocating piston. Similarly, in a rotary compressor, an intermediate pressure may be obtained from a compression chamber through an opening formed between a suction inlet and a discharge outlet. In a scroll compressor, an intermediate pressure may be obtained through an opening formed in the fixed scroll member and aligned with any of the moving compression chambers. 
     In summary, the present invention may be applied to a variety of different compressors, including but not limited to rotary, reciprocating, or scroll compressors. In each instance, the invention can provide a compressor that will automatically self-adjust or modulate its capacity from a first capacity to a second capacity, based on operating parameters of the compressor and/or the HVAC system, and without any controls outside the compressor. The compressor preferably is incorporated in an HVAC system and is turned on or off by a standard thermostat. Once the compressor is turned on, it will self modulate its capacity, as conditions change. Whenever, the desired conditioning of the served space is achieved, the thermostat will turn the compressor off. 
     In several of the embodiments, the invention includes a valve member that is subjected to a first operating condition of the fluid on one side and a second operating condition of the fluid on the other side. The changes in the first and second operating conditions of the fluid cause the valve member to move between a first position and a second position. When the valve member is in the first position, the compressor operates at a reduced capacity, because fluid is allowed to bleed off to a reexpansion area or chamber. When the valve member is in the second position, the compressor operates at an increased, or a maximum, capacity. By using two or more valve members and associated reexpansion areas or chambers, the present invention can provide an automatically modulated compressor having more than two capacities. Varying the positioning of the opening(s) served by the valve member(s) or the size and/or shape of the reexamination chamber can vary the degree of difference between one capacity and another. 
     In yet other embodiment, the self modulation of the compressor is achieved by incorporating a temperature sensitive element in the compressor, that will change in size or shape as operating temperatures of the compressor changes. This change in size and shape is then applied to open or close a valve, or otherwise actuate an element, to vary the capacity of the compressor. 
     It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the scope or spirit of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.