Patent Publication Number: US-7905102-B2

Title: Control system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 10/683,772, entitled SYSTEM AND METHOD FOR STABILITY CONTROL IN A CENTRIFUGAL COMPRESSOR, filed Oct. 10, 2003. 
    
    
     BACKGROUND 
     The application generally relates to a control system. The application relates more specifically to systems and methods for controlling a variable geometry diffuser mechanism of a centrifugal compressor in response to compressor instability conditions. 
     A centrifugal compressor may encounter instabilities such as surge conditions or stall conditions during the operation of the compressor. Surge or surging is an unstable condition that may occur when a centrifugal compressor is operated at light loads and high pressure ratios. Surge is a transient phenomenon having oscillations in pressures and flow, and, in some cases, the occurrence of a complete flow reversal through the compressor. Surging, if uncontrolled, can cause excessive vibrations in both the rotating and stationary components of the compressor, and may result in permanent compressor damage. One technique to correct or remedy a surge condition may involve the opening of a hot gas bypass valve to return some of the discharge gas of the compressor to the compressor inlet to increase the flow at the compressor inlet. 
     Rotating stall in a centrifugal compressor can occur in the rotating impeller of the compressor or in the stationary diffuser of the compressor downstream from the impeller. In both cases, the presence of rotating stall can adversely affect performance of the compressor and/or system. Mixed flow centrifugal compressors with vaneless radial diffusers can experience diffuser rotating stall during some part, or in some cases, all of their intended operating range. Typically, diffuser rotating stall occurs because the design of the diffuser is unable to accommodate all flows without some of the flow experiencing separation in the diffuser passageway. Diffuser rotating stall results in the creation of low frequency sound energy or pulsations. The pulsations may have high magnitudes in the gas flow passages and may result in the premature failure of the compressor, its controls, or other associated parts/systems. One technique to correct or remedy a stall condition in a centrifugal compressor may involve the closing of the diffuser space in a variable geometry diffuser. Closing of the diffuser space may also enhance the compressor&#39;s ability to resist surge conditions. However, excessive closure of the diffuser gap can reduce the flow rate or capacity through the compressor. 
     SUMMARY 
     The present invention relates to a liquid chiller system having a centrifugal compressor configured to compress a refrigerant vapor. The centrifugal compressor has a compressor inlet to receive uncompressed refrigerant vapor and a compressor exit to discharge compressed refrigerant vapor. Internally, the compressor has a diffuser that has an adjustable diffuser ring to vary the flow passage of the compressed refrigerant vapor through the diffuser. The liquid chiller system also includes an optional hot gas bypass valve connected between the compressor exit and inlet. The optional hot gas bypass valve is configured to permit a portion of the compressed refrigerant vapor to flow to the compressor inlet from the compressor exit, which is used to maintain a minimum refrigerant vapor flow rate through the compressor. The liquid chiller system further includes a stability control system to control the diffuser and the optional hot gas bypass valve to maintain stable operation of the centrifugal compressor. The stability control system has a stall reacting state to control the diffuser ring in response to detecting a stall condition in the centrifugal compressor, a surge reacting state to control the diffuser ring in response to detecting a surge condition in the centrifugal compressor, a hot gas override state to control the optional hot gas bypass valve in response to detecting a second surge condition in the centrifugal compressor, and a probing state to control the diffuser ring to obtain an optimal position for the diffuser ring. 
     The present invention further relates to a chiller system having a compressor, a condenser, and an evaporator connected in a closed refrigerant circuit. The compressor includes a compressor inlet to receive uncompressed refrigerant vapor from the chiller system, a compressor outlet to discharge compressed refrigerant vapor to the chiller system, and a diffuser being disposed adjacent to the compressor outlet. The diffuser having a diffuser space configured to permit passage of compressed refrigerant vapor to the compressor outlet and a diffuser ring adjustably positioned in the diffuser space to vary a size of the diffuser space to control flow of compressed refrigerant vapor through the diffuser space. The chiller system also includes a stability control system to control the position of the diffuser ring in the diffuser space in response to the detection of stall conditions and surge conditions in the compressor to maintain stable operation of the compressor. 
     The present invention also relates to a stability control system for maintaining stable operation of a centrifugal compressor having a compressor inlet, a compressor outlet and a variable geometry diffuser with an adjustable flow passage. The stability control system having a stall reacting state to adjust a flow passage of a variable geometry diffuser in response to detecting a stall condition in a centrifugal compressor and a surge reacting state to adjust a flow passage of a variable geometry diffuser in response to detecting a surge condition in a centrifugal compressor. 
     The present invention further relates to a method of providing stability control in a centrifugal compressor having a variable geometry diffuser with an adjustable flow passage. The method including the steps of repeatedly detecting for a surge condition in a centrifugal compressor during operation of a centrifugal compressor; repeatedly detecting for a stall condition in a centrifugal compressor during operation of a centrifugal compressor; continuously closing a flow passage of a variable geometry diffuser in response to the detection of a surge condition in a centrifugal compressor for a predetermined surge reaction time period; and continuously closing a flow passage of a variable geometry diffuser in response to the detection of a stall condition in a centrifugal compressor until the detected stall condition is corrected or a surge condition is detected. 
     The present invention also relates to a control system to maintain stable operation of a compressor. The control system includes at least one first control state configured to close a flow passage of a diffuser of the compressor in response to detecting one of a stall condition or a surge condition in the compressor. The control system also includes a second control state configured to open the flow passage of the diffuser of the compressor in response to determining an absence of a stall condition or a surge condition. 
     The present invention further relates to method of providing stability control in a centrifugal compressor. The method includes repeatedly detecting for a surge condition during operation of the centrifugal compressor and repeatedly detecting for a stall condition during operation of a centrifugal compressor. The method also includes closing a flow passage of a diffuser of the centrifugal compressor in response to detecting a surge condition or a stall condition in the centrifugal compressor and opening the flow passage of the diffuser of the centrifugal compressor in response to detecting an absence of a stall condition or a surge condition. 
     The present invention also relates to a vapor compression system. The vapor compression system includes a compressor, a first heat exchanger, and a second heat exchanger connected in a closed loop. The compressor includes an inlet to receive uncompressed vapor, an outlet to discharge compressed vapor and a diffuser being disposed near the outlet. The diffuser having a passageway configured to permit flow of compressed vapor to the outlet and a ring adjustably positioned in the passageway to vary a dimension of the passageway to control flow of compressed vapor through the passageway. The vapor compression system also includes a control system to adjust the position of the ring in the passageway in response to one of a presence of stall conditions and surge conditions in the compressor or an absence of stall conditions and surge conditions in the compressor. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  schematically shows an exemplary embodiment of a vapor compression system. 
         FIG. 2  shows a partial sectional view of an exemplary embodiment of a centrifugal compressor and diffuser. 
         FIG. 3  shows an exemplary state diagram for a control system for the vapor compression system of  FIG. 1 . 
         FIG. 4  shows another exemplary state diagram for a control system for the vapor compression system of  FIG. 1 . 
         FIG. 5  schematically shows another exemplary embodiment of a vapor compression system. 
         FIG. 6  shows an exemplary state diagram for a control system for the vapor compression system of  FIG. 5 . 
         FIG. 7  shows another exemplary state diagram for a control system for the vapor compression system of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
       FIG. 1  schematically shows an exemplary vapor compression system that may be used in heating, ventilation and air conditioning (HVAC), refrigeration or liquid chiller systems. Vapor compression system  100  can circulate a fluid, e.g., a refrigerant, through a compressor  108  driven by a motor  152 , a condenser  112 , an expansion device (not shown), and an evaporator  126 . System  100  can also include a control panel  140  that can have an analog to digital (A/D) converter  148 , a microprocessor  150 , a non-volatile memory  144 , and an interface board  146 . Some examples of fluids that may be used as refrigerants in vapor compression system  100  are hydrofluorocarbon (HFC) based refrigerants (e.g., R-410A), carbon dioxide (CO 2 ; R-744), and any other suitable type of refrigerant. 
     Motor  152  used with compressor  108  can be powered by a variable speed drive (VSD) or can be powered directly from an alternating current (AC) or direct current (DC) power source. A variable speed drive, if used, receives AC power having a particular fixed line voltage and fixed line frequency from the AC power source and provides power having a variable voltage and frequency to the motor. Motor  152  can be any type of electric motor that can be powered by a VSD or directly from an AC or DC power source. For example, motor  152  can be a switched reluctance motor, an induction motor, an electronically commutated permanent magnet motor, or any other suitable motor type. In an alternate embodiment, other drive mechanisms such as steam or gas turbines or engines and associated components can be used to drive compressor  108 . 
     Compressor  108  compresses a refrigerant vapor and delivers the compressed vapor to condenser  112  through a discharge line. In an exemplary embodiment, compressor  108  can be a centrifugal compressor. The refrigerant vapor delivered by compressor  108  to condenser  112  transfers heat to a fluid, e.g., water or air. The refrigerant vapor condenses to a refrigerant liquid in condenser  112  as a result of the heat transfer with the fluid. The liquid refrigerant from condenser  112  flows through an expansion device (not shown) to an evaporator  126 . The liquid refrigerant delivered to evaporator  126  absorbs heat from a fluid, e.g., air or water and undergoes a phase change to a refrigerant vapor. The vapor refrigerant exits evaporator  126  and returns to compressor  108  by a suction line to complete the cycle. 
     In an exemplary embodiment shown in  FIG. 1 , the refrigerant vapor in condenser  112  enters into the heat exchange relationship with water, flowing through a heat-exchanger  116  connected to a cooling tower  122 . The refrigerant vapor in condenser  112  undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the water in heat-exchanger coil. Evaporator  126  can include a heat-exchanger  128  having a supply line  128 S and a return line  128 R connected to a cooling load  130 . Heat-exchanger  128  can include a plurality of tube bundles within evaporator  126 . A secondary liquid, e.g., water, ethylene, calcium chloride brine, sodium chloride brine or any other suitable secondary liquid, travels into evaporator  126  via return line  128 R and exits evaporator  126  via supply line  128 S. The liquid refrigerant in evaporator  126  enters into a heat exchange relationship with the secondary liquid in heat-exchanger  128  to chill the temperature of the secondary liquid in heat-exchanger coil  128 . The refrigerant liquid in evaporator  126  undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the secondary liquid in heat-exchanger coil  128 . 
     At the input or inlet to compressor  108 , there are one or more pre-rotation vanes (PRV) or inlet guide vanes  120  that are used to control the flow of refrigerant to compressor  108 . An actuator is used to open pre-rotation vanes  120  to increase the amount of refrigerant to compressor  108  and thereby increase the capacity of system  100 . Similarly, the actuator is used to close pre-rotation vanes  120  to decrease the amount of refrigerant to compressor  108  and thereby decrease the cooling capacity of system  100 . 
       FIG. 2  shows a partial sectional view of an exemplary embodiment of a centrifugal compressor and diffuser. Compressor  108  includes an impeller  202  for compressing the refrigerant vapor. The compressed vapor then passes through a diffuser  119 . Diffuser  119  can be a vaneless radial diffuser having a variable geometry. The variable geometry diffuser (VGD)  119  has a diffuser space  204  formed between a diffuser plate  206  and a nozzle base plate  208  for the passage of the refrigerant vapor. Nozzle base plate  208  is configured for use with a diffuser ring  210 . Diffuser ring  210  is used to control the velocity of refrigerant vapor that passes through diffuser space or passage  204 . Diffuser ring  210  can be extended into diffuser passage  204  to increase the velocity of the vapor flowing through the passage and can be retracted from diffuser passage  204  to decrease the velocity of the vapor flowing through the passage. Diffuser ring  210  can be extended and retracted using an adjustment mechanism  212  driven by an electric motor to provide the variable geometry of diffuser  119 . A more detailed description of the operation and components of one exemplary variable geometry diffuser is provided in U.S. Pat. No. 6,872,050, issued on Mar. 29, 2005, which patent is hereby incorporated by reference. 
     Control panel  140  has an A/D converter  148  that can receive input signals from system  100  indicative of the performance of system  100 . For example, the input signals received by control panel  140  can include the position of pre-rotation vanes  120 , the temperature of the leaving chilled liquid temperature from evaporator  126 , pressures of evaporator  126  and condenser  112 , and an acoustic or sound pressure measurement in the compressor discharge passage. Control panel  140  also has an interface board  146  to transmit signals to components of system  100  to control the operation of system  100 . For example, control panel  140  can transmit signals to control the position of pre-rotation vanes  120 , to control the position of an optional hot gas bypass valve  134  (see  FIG. 5 ), if present, and to control the position of diffuser ring  210  in variable geometry diffuser  119 . 
     Control panel  140  uses a control algorithm(s) to control operation of system  100  and to determine when to extend and retract diffuser ring  210  in variable geometry diffuser  119  in response to particular compressor conditions in order to maintain system and compressor stability. Control panel  140  can use the control algorithm(s) to open and close the optional, hot gas bypass valve  134  (see  FIGS. 5 through 7 ), if present, in response to particular compressor conditions in order to maintain system and compressor stability. In one embodiment, the control algorithm(s) can be computer programs stored in non-volatile memory  144  having a series of instructions executable by microprocessor  150 . In one exemplary embodiment, the control algorithm is embodied in a computer program(s) and executed by microprocessor  150 . However, it is to be understood that the control algorithm may be implemented and executed using digital and/or analog hardware. If hardware is used to execute the control algorithm, the corresponding configuration of control panel  140  can be changed to incorporate the necessary components and to remove any components that may no longer be required, e.g. A/D converter  148 . 
       FIGS. 3 ,  4 ,  6  and  7  are exemplary state diagram representations of stability control algorithms for maintaining compressor and system stability. The stability control algorithms may be executed as separate programs with respect to the other control algorithms for the system, e.g., an operational control algorithm, or the stability control algorithm can be incorporated into the other control algorithms of the system. As shown in  FIG. 3 , a state diagram  300  for an exemplary embodiment of the stability control algorithm to provide stability control to system  100  of  FIG. 1  can have six control states. The control states include: a startup/shutdown state  302 ; a stall waiting state  304 ; a stall reacting state  306 ; a probing state  308 ; a surge waiting state  310 ; and a surge reacting state  312 . Each control state can include one or more programs or algorithms or other control devices or equipment to execute the corresponding control operations for the particular control state. 
     The startup/shutdown state  302  is the first and last control state in stability control algorithm  300  during operation of system  100 . Upon starting or initiating system  100  from an inactive state, stability control algorithm  300  enters the startup/shutdown state  302 . Similarly, when system  100  is to be stopped or shutdown, startup/shutdown state  302  is entered from any one of the other control states in stability control algorithm  300  in response to a shutdown command from another control algorithm controlling system  100  or stability control algorithm  300 . Stability control algorithm  300  remains in startup/shutdown state  302  until compressor  108  is started. In startup/shutdown state  302 , diffuser ring  210  of variable geometry diffuser  119  is moved to a fully open or retracted position to thereby fully open diffuser space  204 . 
     Stall waiting state  304  is entered after compressor  108  has started. Stall waiting state  304  can be entered following the correction of a stall condition in stall reacting state  306 . The stability control algorithm  300  remains in stall waiting state  304  until one of the following conditions occurs: a predetermined stall waiting period expires; a surge condition is detected; a stall condition is detected; or pre-rotation vanes  120  are moved more than a predetermined PRV offset amount. The movement of pre-rotation vanes  120  can be an indicator that compressor conditions (e.g., flow and/or head) are changing and may require adjustment of variable geometry diffuser  119 . According to an exemplary embodiment, the predetermined stall waiting period can range from about 0.5 minutes to about 15 minutes, and can be about 10 minutes, and the predetermined PRV offset amount can range from 0% to about 5% of the range of pre-rotation vane motion, and can be about 3%. In stall waiting state  304 , diffuser ring  210  of variable geometry diffuser  119  is held or maintained in the same position that diffuser ring  210  of variable geometry diffuser  119  had in the previous state to thereby hold or maintain the opening in diffuser space  204 . 
     Stall reacting state  306  is entered in response to the detection of stall in compressor  108  in either stall waiting state  304  or probing state  308 . A more detailed description of the process and components for an exemplary technique for detecting stall in a compressor is provided in U.S. Pat. No. 6,857,845, issued on Feb. 22, 2005, which patent is hereby incorporated by reference. However, it is to be understood that any suitable stall detection technique can be used to detect stall in the system. Stability control algorithm  300  remains in stall reacting state  306  until the stall condition that is detected in compressor  108  is corrected or remedied or until a surge condition is detected in compressor  108 . According to an exemplary embodiment, the stall condition is considered corrected or remedied in response to a corresponding stall sensor voltage being less than a predetermined stall minimum threshold voltage, which predetermined stall minimum threshold voltage can range from about 0.4 V to about 0.8 V, and can be about 0.6 V. In stall reacting state  306 , diffuser ring  210  of variable geometry diffuser  119  is continuously extended toward a closed position to thereby close the opening in diffuser space  204  until the stall condition that has been detected in compressor  108  is corrected or remedied. Upon correcting or remedying the stall condition in stall reacting state  306 , stability control algorithm  300  returns to stall waiting state  304 . 
     Probing state  308  is entered in response to the expiration of the predetermined stall waiting period or the movement of pre-rotation vanes  120  by more than the predetermined PRV offset amount in stall waiting state  304 . Probing state  308  can be entered following the expiration of a predetermined surge waiting period in surge waiting state  310 . Stability control algorithm  300  remains in probing state  308  until a stall condition or a surge condition is detected in compressor  108 . According to an exemplary embodiment, the stall condition is detected in response to a corresponding stall sensor voltage being greater than a predetermined stall maximum threshold voltage, which predetermined stall maximum threshold voltage can range from about 0.6 V to about 1.2 V, and can be about 0.8 V. In probing state  308 , diffuser ring  210  of variable geometry diffuser  119  is opened or retracted to thereby increase the opening in diffuser space  204  until a surge condition or stall condition is detected in compressor  108 . According to an exemplary embodiment, diffuser ring  210  of variable geometry diffuser  119  is opened or retracted in incremental amounts or steps triggered by pulses having a predetermined pulse interval that can range from about 0.5 seconds to about 5 seconds and can be about 1 or 2 seconds. At lower compressor loads, e.g., less than 70% of compressor capacity, a stall condition is typically detected and controlled before a surge condition can occur. However, at higher compressor loads, e.g., more than 70% of compressor capacity and very high heads or lifts, a surge condition can occur while in probing state  308 , which may be momentary in nature and not detected as stall noise. 
     Surge reacting state  312  is entered in response to the detection of surge in compressor  108  in either stall waiting state  304 , stall reacting state  306  or probing state  308 . A more detailed description of the process and components for an exemplary technique for detecting surge in compressor  108  is provided in U.S. Pat. No. 6,427,464, which patent is hereby incorporated by reference. However, it is to be understood that any suitable surge detection technique can be used with the system. Stability control algorithm  300  remains in surge reacting state  312  until a predetermined surge reaction time has expired. According to an exemplary embodiment, the predetermined surge reaction time can range from about 1 second to about 30 seconds, and can be about 5 seconds. In surge reacting state  312 , diffuser ring  210  of variable geometry diffuser  119  is continuously extended toward a closed position over the predetermined surge reaction time period to thereby reduce diffuser space or gap  204  to provide a more stable compressor operating capacity. The surge reaction time period can vary depending on overall speed of variable geometry diffuser ring mechanism  212  and drive actuator motor, and the desired VGD ring  210  movement needed to achieve surge stability. 
     Surge waiting state  310  is entered upon the correcting or remedying of a surge condition in compressor  108  in surge reacting state  312 . The stability control algorithm  300  remains in surge waiting state  310  until a predetermined surge waiting period expires or compressor  108  enters into another surge condition. According to an exemplary embodiment, the predetermined surge waiting period can range from about 0.5 minutes to about 15 minutes, and can be about 10 minutes. In surge waiting state  310 , diffuser ring  210  of variable geometry diffuser  119  is held or maintained in the same position that diffuser ring  210  of variable geometry diffuser  119  had in the previous state to thereby hold or maintain the opening in diffuser space  204 . In an exemplary embodiment, stability control algorithm  300  may re-enter surge reacting state  312  in response to the detection of another surge condition in surge waiting state  310 . Alternatively, another control algorithm may be used in response to the detection of another surge condition in surge waiting state  310 . The surge events may be counted independently or as part of the control algorithm to determine when to shutdown compressor  108 . In the event of continued surges in a short time period, stability control algorithm  300  or another control algorithm may provide alarms or shutdown protection of compressor  108  to avoid damaging compressor  108 . Otherwise, stability control algorithm  300  enters probing state  308  in response to the expiration of the predetermined surge waiting period in surge waiting state  310 . 
       FIG. 4  shows another exemplary state diagram for a control system similar to the state control diagram of  FIG. 3  except that stability control algorithm  300  remains in surge waiting state  310  until a predetermined surge waiting period expires, a stall condition is detected or compressor  108  enters into another surge condition and stability control algorithm  300  remains in stall reacting state  306  until the stall condition that is detected in compressor  108  (either from surge waiting state  310 , probing state  308  or stall waiting state  304 ) is corrected or remedied or until a surge condition is detected in compressor  108 . If a stall condition occurs while in surge waiting state  310 , stability control algorithm  300  pauses or suspends the timer for the surge waiting period in surge waiting state  310  and enters stall reacting state  306 . Stability control algorithm  300  remains in stall reacting state  306  until the stall condition that is detected in compressor  108  from surge waiting state  310  is corrected or remedied or until a surge condition is detected in compressor  108 . When the stall condition that is detected in compressor  108  from surge waiting state  310  is corrected or remedied, stability control algorithm  300  re-enters surge waiting state  310  and resumes the timer for the surge waiting period in surge waiting state  310 . In another exemplary embodiment, when stability control algorithm  300  re-enters surge waiting state  310 , the timer for the surge waiting period can be restarted to remain in surge waiting state  310  for the full time period. 
       FIG. 5  schematically shows another exemplary embodiment of a vapor compression system. The vapor compression system  200  illustrated in  FIG. 5  is similar to the vapor compression system  100  illustrated in  FIG. 1  except that a hot gas bypass line  132  and a hot gas bypass (HGBP) valve  134  are connected between the outlet or discharge of compressor  108  and the inlet of pre-rotation vanes  120  to permit compressed refrigerant from the compressor discharge to be diverted or recycled back to the inlet of compressor  108 , when HGBP valve  134  is open, in response to the presence of a surge condition. The position of HGBP valve  134  is controlled to regulate the amount of compressed refrigerant, if any, which is provided to compressor  108 . A description of an exemplary control process for a HGBP valve is provided in U.S. Pat. No. 6,427,464, which patent is hereby incorporated by reference. However, it is to be understood that any suitable HGBP valve and corresponding control process can be used with the system. 
       FIG. 6  shows an exemplary state diagram for a control system for the vapor compression system of  FIG. 5 . As shown in  FIG. 6 , state diagram  500  for an embodiment of the stability control algorithm for providing stability control to system  200  of  FIG. 5  is similar to the state diagram for stability control algorithm  300  illustrated in  FIG. 3  and described in detail above except for the addition of a seventh control state, a hot gas override state  314  and the corresponding intra-connections to hot gas override state  314 . 
     Hot gas override state  314  is entered in response to compressor  108  experiencing a second surge condition while in surge waiting state  310  instead of possibly returning to surge reacting state  312  or using another control algorithm in response to the detection of another surge condition as described above with respect to stability control algorithm  300 . Stability control algorithm  500  can enter hot gas override state  314  from stall waiting state  304 , stall reacting state  306  or probing state  308  in response to the detection of a HGBP valve open command from another control algorithm controlling the system. The HGBP valve open command can be generated as described in U.S. Pat. No. 6,427,464, which patent is hereby incorporated by reference, or using any other suitable HGBP valve control process. The stability control algorithm  500  remains in hot gas override state  314  until HGBP valve  134  returns to a closed position. In hot gas override state  314 , diffuser ring  210  of variable geometry diffuser  119  is held or fixed in position whenever HGBP valve  134  is in an open position to thereby hold or fix the opening in diffuser space  204  in order to keep variable geometry diffuser  119  at a position of similar surge stability when the system head is later lowered and HGBP valve  134  is closed. Upon the closing of HGBP valve  134  in hot gas override state  314 , stability control algorithm  500  enters stall waiting state  304 . 
       FIG. 7  shows another exemplary state diagram for a control system similar to  FIG. 6  except that stability control algorithm  500  remains in surge waiting state  310  until a predetermined surge waiting period expires, a stall condition is detected or compressor  108  enters into another surge condition and stability control algorithm  500  remains in stall reacting state  306  until the stall condition that is detected in compressor  108  (either from surge waiting state  310 , probing state  308  or stall waiting state  304 ) is corrected or remedied or until a surge condition is detected in compressor  108 . If a stall condition occurs while in surge waiting state  310 , stability control algorithm  500  pauses or suspends the timer for the surge waiting period in surge waiting state  310  and enters stall reacting state  306 . Stability control algorithm  500  remains in stall reacting state  306  until the stall condition that is detected in compressor  108  from surge waiting state  310  is corrected or remedied or until a surge condition is detected in compressor  108 . When the stall condition that is detected in compressor  108  from surge waiting state  310  is corrected or remedied, stability control algorithm  500  re-enters surge waiting state  310  and resumes the timer for the surge waiting period in surge waiting state  310 . In another exemplary embodiment, when stability control algorithm  500  re-enters surge waiting state  310 , the timer for the surge waiting period can be restarted to remain in surge waiting state  310  for the full time period. 
     In an exemplary embodiment, motor  152  is connected to a variable speed drive (not shown) that varies the speed of motor  152 . The varying of the speed of the compressor by the variable speed drive (VSD) affects both the refrigerant vapor flow rate through the system and the compressor&#39;s stability relative to surge conditions. Stability control algorithms  300 ,  500  may be used in conjunction with a variable speed drive. When a variable speed drive is used, adaptive capacity control logic utilizing system operating parameters and compressor PRV position information can be used to operate the compressor at a faster speed when a surge is detected while stability control algorithms  300 ,  500  are in surge reacting state  312 . Past performance parameters can be mapped and stored in memory to avoid future surge conditions by the adaptive capacity control logic. A description of an exemplary adaptive capacity control process is provided in U.S. Pat. No. 4,608,833 which patent is hereby incorporated by reference. However, it is to be understood that any suitable adaptive capacity control process can be used with the system. 
     While only certain features and embodiments of the invention have been illustrated and described, many modifications and changes may occur to those skilled in the art (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (e.g., temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited in the claims. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described (i.e., those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the claimed invention). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.