Patent Publication Number: US-10330106-B2

Title: Centrifugal compressor with surge control

Description:
BACKGROUND 
     Field of the Invention 
     The present invention generally relates to a centrifugal compressor. More specifically, the present invention relates to a centrifugal compressor with surge control. 
     Background Information 
     A chiller system is a refrigerating machine or apparatus that removes heat from a medium. Commonly a liquid such as water is used as the medium and the chiller system operates in a vapor-compression refrigeration cycle. This liquid can then be circulated through a heat exchanger to cool air or equipment as required. As a necessary byproduct, refrigeration creates waste heat that must be exhausted to ambient or, for greater efficiency, recovered for heating purposes. A conventional chiller system often utilizes a centrifugal compressor, which is often referred to as a turbo compressor. Thus, such chiller systems can be referred to as turbo chillers. Alternatively, other types of compressors, e.g. a screw compressor, can be utilized. 
     In a conventional (turbo) chiller, refrigerant is compressed in the centrifugal compressor and sent to a heat exchanger in which heat exchange occurs between the refrigerant and a heat exchange medium (liquid). This heat exchanger is referred to as a condenser because the refrigerant condenses in this heat exchanger. As a result, heat is transferred to the medium (liquid) so that the medium is heated. Refrigerant exiting the condenser is expanded by an expansion valve and sent to another heat exchanger in which heat exchange occurs between the refrigerant and a heat exchange medium (liquid). This heat exchanger is referred to as an evaporator because refrigerant is heated (evaporated) in this heat exchanger. As a result, heat is transferred from the medium (liquid) to the refrigerant, and the liquid is chilled. The refrigerant from the evaporator is then returned to the centrifugal compressor and the cycle is repeated. The liquid utilized is often water. 
     A conventional centrifugal compressor basically includes a casing, an inlet guide vane, an impeller, a diffuser, a motor, various sensors and a controller. Refrigerant flows in order through the inlet guide vane, the impeller and the diffuser. Thus, the inlet guide vane is coupled to a gas intake port of the centrifugal compressor while the diffuser is coupled to a gas outlet port of the impeller. The inlet guide vane controls the flow rate of refrigerant gas into the impeller. The impeller increases the velocity of refrigerant gas, generally without changing pressure. The diffuser increases the refrigerant pressure without changing the velocity. The motor rotates the impeller. The controller controls the motor, the inlet guide vane and the expansion valve. In this manner, the refrigerant is compressed in a conventional centrifugal compressor. The inlet guide vane is typically adjustable and the motor speed is typically adjustable to adjust the capacity of the system. In addition, the diffuser may be adjustable to further adjust the capacity of the system. The controller controls the motor, the inlet guide vane and the expansion valve. The controller can further control any additional controllable elements such as the diffuser. 
     When the pressure behind the compressor is higher than the compressor outlet pressure, the fluid tends to reverse or even flow back in the compressor. As a consequence, the pressure will decrease, inlet pressure will increase and the flow reverses again. This phenomenon, called surge, repeats and occurs in cycles. The compressor loses the ability to maintain the peak head when surge occurs and the entire system becomes unstable. A collection of surge points during varying compressor speed or varying inlet guide vane angle is called a surge line. In normal conditions, the compressor operates in the right side of the surge line. However, during startup/emergency shutdown, the operating point will move towards the surge line because flow is reduced. If conditions are such that the operating point approaches the surge line, flow recirculation occurs in the impeller and diffuser. The flow recirculation, which causes flow separation, will eventually cause a decrease in the discharge pressure, and flow from suction to discharge will resume. Surging can cause the compressor to overheat to the point at which the maximum allowable temperature of the unit is exceeded. Also, surging can cause damage to the thrust bearing due to the rotor shifting back and forth from the active to the inactive side. This is defined as the surge cycle of the compressor. 
     Therefore, techniques have been developed to control surge. See for example, Japanese Patent Publication No. 5-263796. 
     SUMMARY 
     In a conventional centrifugal compressor, when surge is predicted by the above technique or any other known technique, a compressor controller can control various parts to control surge. For example, the inlet guide vane and/or the discharge diffuser vane can be controlled or the speed of the compressor can be increased to control surge. While these techniques work relatively well, these systems can require additional components, and thus, increased costs. In addition, these techniques can reduce performance of the compressor. 
     Therefore, one object of the present invention is to provide a centrifugal compressor that controls surge without reducing performance. 
     Another object of the present invention is to provide a centrifugal compressor that controls surge without overly complicated construction and/or additional parts. 
     One or more of the above objects can basically be attained by providing a centrifugal compressor adapted to be used in a chiller, the centrifugal compressor including: a casing having an inlet portion and an outlet portion; an inlet guide vane disposed in the inlet portion; an impeller disposed downstream of the inlet guide vane, the impeller being rotatable about a rotation axis defining an axial direction, and the impeller being adjustably mounted within the casing along the axial direction between at least a first flow rate position and a second flow rate position; a motor arranged and configured to rotate the impeller; and a diffuser disposed in the outlet portion downstream from the impeller with a discharge port of the outlet portion being disposed between the impeller and the diffuser. 
     These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the attached drawings which form a part of this original disclosure: 
         FIG. 1  illustrates a chiller in accordance with an embodiment of the present invention; 
         FIG. 2  is a perspective view of the centrifugal compressor of the chiller illustrated in  FIG. 1 , with portions broken away and shown in cross-section for the purpose of illustration; 
         FIG. 3  is a longitudinal cross-sectional view of the impeller, motor and magnetic bearing of the centrifugal compressor illustrated in  FIG. 2 ; 
         FIG. 4  is a diagrammatic longitudinal view of part of the bearing, the impeller, casing and diffuser inlet of the centrifugal compressor illustrated in  FIGS. 1-3 , with the impeller in an axial position partially opening (&lt;100%) the diffuser inlet; 
         FIG. 5  is a diagrammatic longitudinal view of part of the bearing, the impeller, casing and diffuser inlet of the centrifugal compressor illustrated in  FIGS. 1-4 , with the impeller in an axial position fully opening (100%) the diffuser inlet; 
         FIG. 6  is an axial view of the shaft of the rotational magnetic bearing illustrating a location of a radial magnetic bearing; 
         FIG. 7  is graph illustrating head as compared to flow rate for three different rpm of the centrifugal compressor, with a surge line illustrated; 
         FIG. 8  is a partial cross-sectional plan view of the magnetic thrust bearing of  FIGS. 2 and 3 ; 
         FIG. 9  is a cutout perspective view of the magnetic thrust bearing of  FIGS. 2, 3, and 8 ; 
         FIG. 10  is a flow chart illustrating a method of increasing operating capacity to control surge; 
         FIG. 11  is a schematic diagram of the chiller controller of the chiller system of  FIGS. 1 and 2 ; and 
         FIG. 12  is a schematic diagram illustrating the relationship between the magnetic bearing assembly, magnetic bearing control section  61 , surge prediction section  62 , and the surge control section  63  of the chiller system of  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
     Selected embodiments will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     Referring initially to  FIG. 1 , a chiller system  10  is illustrated in accordance with an embodiment of the present invention. The chiller system  10  is preferably a water cooled chiller that utilizes cooling water and chiller water in a conventional manner. The chiller system  10  illustrated herein is a single stage chiller system. However, it will be apparent to those skilled in the art from this disclosure that the chiller system  10  could be a multiple stage chiller system. The chiller system  10  basically includes a controller  20 , a compressor  22 , a condenser  24 , an expansion valve  26 , and an evaporator  28  connected together in series to form a loop refrigeration cycle. In addition, various sensors S and T are disposed throughout the circuit as shown in  FIG. 1 . The chiller system  10  is conventional except that the chiller system controls surge in accordance with the present invention. 
     Referring to  FIGS. 1-3 , in the illustrated embodiment, the compressor  22  is a centrifugal compressor. The centrifugal compressor  22  of the illustrated embodiment basically includes a casing,  30 , an inlet guide vane  32 , an impeller  34 , a diffuser  36 , a motor  38  and a magnetic bearing assembly  40  as well as various conventional sensors (only some shown). The controller  20  receives signals from the various sensors and controls the inlet guide vane  32 , the motor  38  and the magnetic bearing assembly  40  in a conventional manner, as explained in more detail below. Refrigerant flows in order through the inlet guide vane  32 , the impeller  34  and the diffuser  36 . The inlet guide vane  32  controls the flow rate of refrigerant gas into the impeller  34  in a conventional manner. The impeller  34  increases the velocity of refrigerant gas, generally without changing pressure. The motor speed determines the amount of increase of the velocity of refrigerant gas. The diffuser  36  increases the refrigerant pressure without changing the velocity. The motor  38  rotates the impeller  34  via a shaft  42 . The magnetic bearing assembly  40  magnetically supports the shaft  42 . In this manner, the refrigerant is compressed in the centrifugal compressor  22 . 
     In the illustrated embodiment, the chiller system  10  predicts surge in a conventional manner. See for example U.S. Pat. No. 5,095,714. However, when surge is predicted, the chiller system  10  controls surge in accordance with the present invention. In particular, the controller  20  controls the current sent to the magnetic bearing assembly  40  to control an axial position of the impeller  34 , as explained in more detail below. 
     Referring to  FIGS. 2-3 , the magnetic bearing assembly  40  is conventional, and thus, will not be discussed and/or illustrated in detail herein, except as related to the present invention. Rather, it will be apparent to those skilled in the art that any suitable magnetic bearing can be used without departing from the present invention. As seen in  FIG. 2 , the magnetic bearing assembly  40  preferably includes a first radial magnetic bearing  44 , a second radial magnetic bearing  46  and an axial (thrust) magnetic bearing  48 . In any case, at least one radial magnetic bearing  44  or  46  rotatably supports the shaft  42 . The thrust magnetic bearing  48  supports the shaft  42  along a rotational axis X by acting on a thrust disk  45 . The thrust magnetic bearing  48  includes the thrust disk  45  which is attached to the shaft  42 . The thrust disk  45  extends radially from the shaft  42  in a direction perpendicular to the rotational axis X, and is fixed relative to the shaft  42 . A position of the shaft  42  along rotational axis X (an axial position) is controlled by an axial position of the thrust disk  45  in accordance with the present invention. The first and second radial magnetic bearings  44  and  46  are disposed on opposite axial ends of the motor  38 , or can be disposed on the same axial end with respect to the motor  38  (not illustrated). Various sensors, discussed in more detail below, sense radial and axial positions of the shaft  42  relative to the magnetic bearings  44 ,  46  and  48 , and send signals to the magnetic bearing control section  61  in a conventional manner. The magnetic bearing control section  61  then controls the electrical current sent to the magnetic bearings  44 ,  46  and  48  in a conventional manner to maintain the shaft  42  in the correct position. Since the operation of magnetic bearings and magnetic bearing assemblies such as magnetic bearings  44 ,  46  and  48  of magnetic bearing assembly  40  are well known in the art, the magnetic bearing assembly  40  will not be explained and/or illustrated in detail herein, except as related to controlling surge in accordance with the present invention. 
     The magnetic bearing assembly  40  is preferably a combination of active magnetic bearings  44 ,  46 , and  48 , which utilizes non-contact position sensors  54 ,  56  and  58  to monitor shaft position and send signals indicative of shaft position to the magnetic bearing control section  61 . Thus, each of the magnetic bearings  44 ,  46  and  48  are preferably active magnetic bearings. A magnetic bearing control section  61  uses this information to adjust the required current to a magnetic actuator to maintain proper rotor position both radially and axially. Active magnetic bearings are well known in the art, and thus, will not be explained and/or illustrated in detail herein, except as related to controlling surge in accordance with the present invention. 
     Referring to  FIGS. 1, 2, and 11 , the controller  20  includes a magnetic bearing control section  61 , a surge prediction section  62 , a surge control section  63 , a variable frequency drive  64 , a motor control section  65 , an inlet guide vane control section  66 , and an expansion valve control section  67 . The magnetic bearing control section  61 , the surge prediction section  62 , the surge control section  63 , the variable frequency drive  64 , the motor control section  65  and the inlet guide vane control section  66  form parts of a centrifugal compressor control portion that is electrically coupled to an I/O interface  50  of the compressor  22 . 
     Because the magnetic bearing control section  61  is connected to several portions of the magnetic bearing assembly  40  and communicates with various sections of the controller  20 , the various sections of the controller  20  can receive signals from the sensors  54 ,  56  and  58  of the compressor  22 , perform calculations and transmit control signals to parts of the compressor  22  such as the magnetic bearing assembly  40 . Similarly, the various sections of the controller  20  can receive signals from the sensors S and T, perform calculations and transmit control signals to the compressor  22  (e.g., the motor) and the expansion valve  26 . The control sections and the variable frequency drive  64  can be separate controllers or can be mere sections of the chiller controller programmed to execute the control of the parts described herein. In other words, it will be apparent to those skilled in the art from this disclosure that the precise number, location and/or structure of the control sections, control portion and/or controller  20  can be changed without departing from the present invention so long as the one or more controllers are programed to execute control of the parts of the chiller system  10  as explained herein. 
     The controller  20  is conventional, and thus, includes at least one microprocessor or CPU, an Input/output (I/O) interface, Random Access Memory (RAM), Read Only Memory (ROM), a storage device (either temporary or permanent) forming a computer readable medium programmed to execute one or more control programs to control the chiller system  10 . The controller  20  may optionally include an input interface such as a keypad to receive inputs from a user and a display device used to display various parameters to a user. The parts and programming are conventional, except as related to controlling surge, and thus, will not be discussed in detail herein, except as needed to understand the embodiment(s). 
     The magnetic bearing control section  61  normally receives signals from the sensors  54 ,  56  and  58  of the magnetic bearing assembly  40 , and transmits electrical signals to the magnetic bearings  44 ,  46  and  48  to maintain the shaft  42  in the desired position in a conventional manner. More specifically, the magnetic bearing control section  61  is programmed to execute a magnetic bearing control program to maintain the shaft  42  in the desired position in a conventional manner during normal operation when surge is not predicted. However, if surge is predicted, the axial position of the shaft  42  can be adjusted using the surge control section  62  and the axial magnetic bearing  48 . Thus, the axial position of the impeller  34 , which is fixed to the shaft  42 , can be adjusted relative to the diffuser  36 , as explained in more detail below. 
     The variable frequency drive  64  and motor control section  65  receive signals from at least one motor sensor (not shown) and control the rotation speed of the motor  38  to control the capacity of the compressor  22  in a conventional manner. More specifically, the variable frequency drive  64  and motor control section  65  are programmed to execute one or more motor control programs to control the rotation speed of the motor  38  to control the capacity of the compressor  22  in a conventional manner. The inlet guide vane control section  66  receives signals from at least one inlet guide vane sensor (not shown) and controls the position of the inlet guide vane  32  to control the capacity of the compressor  22  in a conventional manner. More specifically, the inlet guide vane control section  66  is programmed to execute an inlet guide vane control program to control the position of the inlet guide vane  32  to control the capacity of the compressor  22  in a conventional manner. The expansion valve control section  67  controls the opening degree of the expansion valve  26  to control the capacity of the chiller system  10  in a conventional manner. More specifically, the expansion valve control section  67  is programmed to execute an expansion valve control program to control the opening degree of the expansion valve  26  to control the capacity of the chiller system  10  in a conventional manner. The motor control section  65  and the inlet guide vane control section  66  work together and with the expansion valve control section  67  to control the overall capacity of the chiller system  10  in a conventional manner. The controller  20  receives signals from the sensors S and optionally T to control the overall capacity in a conventional manner. The optional sensors T are temperature sensors. The sensors S are preferably conventional pressure sensors and/or temperature sensors used in a conventional manner to perform the control. 
     Each the magnetic bearing  44  includes a plurality of actuators  74  and at least one amp  84 . Similarly, each the magnetic bearing  46  includes a plurality of actuators  76  and at least one amplifier  86 . Likewise, Each the magnetic bearing  48  includes a plurality of actuators  78  and at least one amp  88 . The amplifiers  84 ,  86  and  88  of each magnetic bearing  44 ,  46 , and  48  may be a multi-channel amp to control the number actuators thereof, or can include separate amplifiers for each actuator  74 ,  76  and  78 . In either case, the amplifiers  84 ,  86  and  88  are electrically connected to the actuators  74 ,  76  and  78  of each respective magnetic bearing  44 ,  46 , and  48 . 
     Referring to  FIGS. 11 and 12 , the magnetic bearing control section  61  is electrically connected to the surge control section  63 , and receives signals from the surge control section  63 . The magnetic bearing control section  61  can adjust the desired axial position of the shaft  42  to be any point within a shiftable range of the magnetic bearing  48 . The magnetic bearing control section  61  is programed to adjust the electrical signal to the amplifier  88  of the magnetic bearing  48  to adjust the axial position of the shaft  42 . The magnetic bearing  48  may include an amplifier  88  with two channels to independently control each actuator  78  of the magnetic bearing  48  respectively, or each actuator  78  of the magnetic bearing  48  may have a unique corresponding amplifier  88 . The actuators  78  of the magnetic bearing  48  act on the thrust disk  45  by exerting a magnetic force. The actuators  78  of the magnetic bearing  48  generate a magnetic force which is based upon an electrical current. Thus, the magnetic force can be variably controlled by controlling the amount of current supplied to each actuator  78 , as will be explained in further detail below. 
     In the illustrated embodiment, the magnetic bearing  48  includes the thrust disk  45 , two actuators  78  disposed on opposite sides of the thrust disk  45 , two position sensors  58  disposed on opposite sides of the thrust disk  45 , an amplifier  88  electrically connected to the two actuators  78 , and the magnetic bearing control section  61 . The magnetic bearing control section  61  is electrically connected to the amplifier  88 , the position sensors  58 , and the other portions of the controller  20 . Each actuator  78  receives a respective current from the amplifier  88 , and each current being determined by the magnetic bearing control section  61  and communicated to the amplifier  88  by a signal. The actuators  78  of the magnetic bearing  48  bias the thrust disk  45  to an axial position in which the net force of the two actuators  78  reach an equilibrium. During normal operation, the shaft  42  will be disposed at an axial position in which the flow rate is 100% as illustrated in  FIG. 5 . 
     The magnetic bearing control section  61  of the present invention differs from a conventional magnetic bearing controller in that it is arranged to receive at least one external signal. The at least one external signal is an adjustment signal which indicates an adjustment to the desired axial position, which is needed in response to surge being predicted. The magnetic bearing control section  61  is programed to receive the adjustment signal and adjust the signal output to the amplifier  88  of the magnetic bearing  48  that indicates the amount of current to be supplied to the actuators  78  of magnetic bearing  48 . In other words, the magnetic bearing control section  61  of the present invention will adjust the position of the shaft  42  in the axial direction based on an adjustment signal received. 
     The axial position of the impeller  34  relative to the inlet will determine the flow rate of the refrigerant and the velocity of the flow of refrigerant out of the impeller  34  when all other aspects of the chiller  10  remain constant. The flow rate of the refrigerant will also affect the capacity of the compressor  22 . Because shaft  42  is shiftable to any point within the shiftable range of magnetic bearing  48 , and the impeller  34  is attached to the shaft  42 , the impeller  34  is also shiftable to an infinite number of positions in the axial direction. Each axial position of the impeller results in a unique flow rate and unique velocity. Thus, the flow rate and velocity of the refrigerant from the impeller  34  of the compressor may be infinitely adjusted.  FIG. 4  illustrates an axial position of the impeller  34  in which the flow rate is less than 100%, which may be any point within the shiftable range that is not the closest to the diffuser  36  (shown in  FIG. 5 ).  FIG. 5  illustrates an axial position of the impeller  34  in which the flow rate is 100% and the impeller  34  is disposed at the point of the shiftable range closest to diffuser  36 . 
     The surge control section  63  is programmed to control surge upon receiving a signal from the surge prediction section  62 . The signal from the surge prediction section  62  indicates that surge is predicted to occur. The surge prediction section  62  may predict surge in a conventional manner, such as those set forth in U.S. Pat. No. 5,095,714, or using any other technique without departing from the scope of this invention, as would be apparent in light of this disclosure. However, in the illustrated embodiment, the surge control section  63  controls surge by adjusting the axial position of the impeller  34  (moving the impeller toward the right in the views shown herein), i.e., from the 100% flow rate position shown in  FIG. 5  toward a less open &lt;100% flow rate position (only one shown in  FIG. 4 ). If the full axial position adjustment of the impeller  34  is insufficient to eliminate surge being predicted by the surge prediction section  62 , optionally other conventional techniques, such as increasing rotation speed of the motor  38  and/or adjusting the inlet guide vane, can be used in addition to the technique discussed and illustrated herein. However, by using the surge control achieved from axial position adjustment of the impeller  34  disclosed herein, one or more conventional surge control techniques can be avoided and/or eliminated. For example surge control using a diffuser vane could be eliminated. 
     The surge control section  63  is electrically connected to the bearing control section  61 . The surge control section  63  sends an adjustment signal to the magnetic bearing control section  61  to control surge. More specifically, the surge control section  63  controls surge by shifting the shaft  42  in the axial direction. More specifically, the surge control section  63  is programmed to output an adjustment signal indicating an adjustment to the axial position of the impeller  34 . The adjustment corresponds to a portion of the adjustable range. For example, each adjustment can be 5%, 10%, or 15% of the adjustable range. Thus, the surge control section  63  is programed to control surge by adjusting the flow rate of the compressor  22  which occurs when the impeller  34  is shifted in increments. 
     The surge control section  63  is programmed to adjust the axial position of the impeller  34  from a normal operating position (illustrated in  FIG. 5 ) to numerous adjusted positions (only one illustrated in  FIG. 4 ). Incremental adjustment as mentioned above is merely one example of how the axial position of the impeller may be adjusted in accordance with this disclosure. Alternatively, the adjustment signal may indicate a single amount of adjustment to be sent from the surge control section  63  to the magnetic bearing control section  61  based on a determination of how much of a shift must be made to control the predicted surge as calculated by the surge control section  63 , or based on predetermined values such as a map as will be further explained in detail below. 
     The surge control section  63  is programmed to determine the amount of adjustment of the position of impeller  34 . The surge control section  63  is programmed to determine the amount of adjustment based on at least one operating parameter of the compressor  22 . More specifically, the surge control section  63  is programmed to determine a target flow rate based on the predicted surge, as would be apparent in light of this disclosure. For example, the target flow rate may be determined based on at least one of the pressure of the refrigerant at the inlet of the impeller  34  and the pressure of the refrigerant within the diffuser. Once the surge control section  63  has determined the target flow rate, the surge control section  63  then calculates an adjustment to the axial position of the impeller  34  that would result in the target flow rate. The surge control section  63  then sends an adjustment signal to the magnetic bearing control section  61  indicating the adjustment to the axial position of the impeller  34 . By non-limiting example, surge may be controlled by increasing velocity of the coolant. Increasing velocity of the coolant expands the operation range. Thus, the surge control section  63  may generate an adjustment signal corresponding to a portion of the adjustable range. For Example, each adjustment resulting from the adjustment signal can be 5%, 10%, or 15% of the adjustable range. 
     In response to the adjustment signal, the magnetic bearing control section  61  shifts the impeller in the axial direction from the normal operating position to the adjusted position. The normal operation position has a first flow rate, and an adjusted position has a second flow rate. By non-limiting example, the first flow rate is a peak flow rate (100%) of the compressor  22  as illustrated in  FIG. 5 , while the second flow rate is less than the peak flow rate of the compressor  22  as illustrated in  FIG. 4 . The adjustment signal may also depend on different flow rates as determined based upon the method of controlling surge to which the surge control section  63  is programmed to execute. It would be apparent to one of ordinary skill in the art, in light of this disclosure, that various methods of calculating the amount of adjustment necessary based on a prediction of surge may be used. 
     Referring to  FIGS. 4 and 5 , the flow rate will affect the velocity of the coolant exiting the impeller  34 . In a normal operating position of the impeller  34 , the clearance C is small, and the gap G from which coolant exits the impeller is large. In  FIGS. 4-5 , the clearance and the structure of the compressor are greatly simplified for the sake of understanding. In this normal arrangement ( FIG. 5 ), the flow rate of the coolant exiting the impeller  32  is normal, and the velocity is normal. After the impeller  34  is shifted in response to a prediction that surge will occur, as illustrated in  FIG. 4 , the gap G is smaller, relative to the normal operating position. In the adjusted arrangement, the flow rate of the coolant exiting the impeller  32  is less than the flow rate of the coolant in the normal arrangement, and the velocity of the coolant is greater than the velocity of the coolant in the normal arrangement. The clearance C also grows, but as understood from  FIG. 2 , the clearance C will not have an impact on the flow rate or velocity of coolant leaving the impeller  34  because the clearance C is preferable seal from the inlet guide vane supplying coolant to the impeller. The differences in flow rate and velocity of the coolant are a result of the gap G narrowing in the adjusted arrangement. Generally, the changes to clearance C do not interfere with the changes to the flow rate and velocity of the coolant, as would be understood in light of this disclosure and as mentioned above. 
     The second flow rate and second velocity (the adjusted position of the impeller  34 ) may be determined according to several techniques. In one embodiment, the surge control section  63  may incrementally adjust the flow rate. For example, if the surge control section  63  receives a signal from the surge prediction section  62 , the surge control section may adjust the flow rate by 5% by adjusting the position of the impeller  34 . Should the surge prediction section  62  predict surge after the surge control section  63  has adjusted the flow rate by 5%, the surge control section  63  would adjust the flow rate by 10% by adjusting the position of the impeller  34 . This cycle of incrementally adjusting the flow rate would continue until no surge is predicted by the surge prediction section  62 , or the surge control section  63  has reached a maximum amount of adjustment. 
     Alternatively, the second flow rate and second velocity (the adjusted position of the impeller  34 ) may be determined by the surge control section  63  based on a predicted amount of surge. In other words, if surge prediction section  62  predicts a surge of amount X, the surge control section  63  may be programmed to determine an adjustment amount to account for a surge of amount X. Based on the adjustment amount to account for a surge of amount X, the surge control section can generate an adjustment signal based on the amount of adjustment, and adjust the position of the impeller  34 . 
     Moreover, the second flow rate and second velocity (the adjusted position of the impeller  34 ) may be determined by the surge control section  63  based on a predetermined amount. For example, the amount of adjustment may be a static value, or based on a predetermined map. The surge control section  63  may default to a predetermined static adjustment amount during each instance the surge control section  63  receives a signal predicting surge and adjust the position of the impeller  34  to a predetermined position. Alternatively, the surge control section  63  may determine the amount of adjustment based on a predetermined map. The predetermined map may indicate an adjustment amount respective to a time or a duration which the surge prediction section  63  has predicted surge, and adjust the position of the impeller  34  to a position determined based on the predetermined map. Such a predetermined map is usually generated from experiments and programmed into the controller  20 . 
     Conventionally, the inlet guide vane control section  66  controls the flow rate of refrigerant gas into the impeller by controlling the inlet guide vane  32 . For example, the guide vane control section may determine a target capacity of the system, determine the amount of adjustment to the guide vane  32  necessary to reach the target capacity, and control the guide vane  32  to achieve the target capacity to control surge. However, an adjustable guide vane  32  increases the complexity of a conventional chiller system, and are a point of failure for conventional chiller systems so equipped. Likewise, some centrifugal compressors utilize an adjustable diffuser vane, which can be eliminated. 
     By controlling surge using the techniques described herein, the chiller system  10  is no longer limited to controlling surge via the inlet guide vane/guide vane control section, and/or an adjustable diffuser guide vane. In addition other adjustment structures may possibly be eliminated or made unnecessary. In other words, the diffuser may have no diffuser vanes (adjustable diffuser vanes) (not illustrated). Alternatively, the inlet guide vane may be fixed, and not adjustable (not illustrated). By foregoing the guide vane  32 , the reliability of chiller system  10  may be increased, and the cost may be decreased. 
     Referring to  FIG. 7 , surge is the complete breakdown of steady flow in the compressor, which typically occurs at a low flow rate.  FIG. 7  illustrates a surge line SL, which connects the surge points S 1 , S 2 , and S 3  at rpm 1 , rpm 2 , and rpm 3 , respectively. These points are the peak points in which pressure generated by the compressor is less than the pipe pressure downstream of the compressor. These points illustrate initiation of the surge cycle. Broken line PA illustrates a surge control line. The distance between line PA and SL show the inefficiency of surge control methods. By reducing the difference between a surge control line PA and surge line SL, the compressor  22  can be controlled to be more efficient. One advantage of the aforementioned surge control methods is that it provides a novel methods of controlling surge; thus the surge control line PA may be closer to surge line SL when compared to previous methods. 
     General Interpretation of Terms 
     In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. 
     The term “detect” as used herein to describe an operation or function carried out by a component, a section, a device or the like includes a component, a section, a device or the like that does not require physical detection, but rather includes determining, measuring, modeling, predicting or computing or the like to carry out the operation or function. 
     The term “configured” as used herein to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function. 
     The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. 
     While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. For example, the size, shape, location or orientation of the various components can be changed as needed and/or desired. Components that are shown directly connected or contacting each other can have intermediate structures disposed between them. The functions of one element can be performed by two, and vice versa. The structures and functions of one embodiment can be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.