Patent Publication Number: US-9845807-B2

Title: Compressor control device, compressor system and compressor control method

Description:
TECHNICAL FIELD 
     The present invention relates to a compressor control device, a compressor system and a compressor control method. 
     Priority is claimed on Japanese Patent Application No. 2012-265642, filed Dec. 4, 2012, the content of which is incorporated herein by reference. 
     BACKGROUND ART 
     A compressor configured to compress a gas and supply the compressed gas to a machine or the like connected to a downstream side thereof is known. As such a compressor, there is a compressor that can control a flow rate. For example, a compressor system includes an inlet guide vane of the compressor installed at an upstream side of an impeller, and introduces a gas to the impeller via the inlet guide vane. Then, the compressor system controls the flow rate of the gas introduced into the impeller by regulating an opening of the inlet guide vane. 
     In addition, the compressor system may include a multi-stage impeller from an upstream side toward a downstream side of a gas flow (for example, see Patent Literature 1). Further, in order to increase a flow rate, there is a compressor system including a plurality of impellers installed at the most upstream side, and configured to join the gas compressed by the plurality of impellers and then introduce the gas into the impeller of the downstream side. In such a compressor system, there is a method of controlling an introduction flow rate to the plurality of impellers connected to the most upstream side in parallel at the same period of the opening degrees of the inlet guide vanes disposed at the upstream side of the impeller (i.e., the opening degrees become equal to each other) and controlling a state of the ejected gas. For example, the compressor system includes the inlet guide vanes installed at the inlet ports of the plurality of impellers at the most upstream side. Then, the compressor system controls the opening degrees of the inlet guide vane to be equal to each other, and controls a state of the ejected gas. 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] Japanese Unexamined Patent Application, First Publication No. H06-88597 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the method of controlling the opening degrees of the inlet guide vanes disposed at the upstream side of the plurality of impellers to be equal to each other and controlling the flow rate in the impeller, when a performance difference is generated between the plurality of impellers due to an individual difference, deterioration with age, or the like, the opening degrees of the inlet guide vanes need to be controlled with reference to the impeller in which performance has decreased. For this reason, in this method, an operable range may be narrowed. In particular, since a flow rate in the impeller in which performance has deteriorated is decreased and approaches the surge region, anti-surge control for protecting the compressor by opening a blowoff valve is considered. In this case, even when there is no need to open the blowoff valve at another impeller, the blowoff valve is opened and a gas flow rate of the compressor is increased, and thus a power need is increased, thereby decreasing efficiency. 
     The present invention provides a compressor control device, a compressor system and a compressor control method that are capable of reducing a decrease in efficiency even when a performance difference is generated between a plurality of impellers. 
     Solution to Problem 
     According to a first aspect of the present invention, a compressor control device configured to control a flow rate of a compressor having a plurality of impellers connected to an outlet port-side flow path in parallel and a flow rate regulation unit configured to regulate the flow rate of each of the impellers, the compressor control device includes a pressure detection unit configured to detect a pressure of the outlet port-side flow path, a flow rate detection unit configured to detect the flow rate of each of the impellers, and a control unit configured to output a flow rate regulation command of each of the impellers to the flow rate regulation unit and control the flow rate regulation unit based on the detection result of the pressure detection unit. The control unit compares a set point set as a lower limit target value of a flow rate with the flow rate of each of the impellers, and corrects a flow rate regulation command of another impeller based on the comparison result. 
     In addition, according to a second aspect of the present invention, in the above-mentioned compressor control device, when a flow rate of a certain impeller is smaller than the set point, the control unit controls the flow rate regulation unit to fix the flow rate of the impeller. 
     In addition, according to a third aspect of the present invention, in the above-mentioned compressor control device, the control unit releases the fixing of the flow rate of the impeller when the set point apart from the flow rate command value more than a predetermined value. 
     In addition, according to a fourth aspect of the present invention, the control unit releases the fixing of the flow rate of the impeller when the flow rate of all of impellers is smaller than the set point. 
     In addition, according to a fifth aspect of the present invention, in the above-mentioned compressor control device, the pressure detection unit detects a pressure of an inlet port-side flow path, and the control unit outputs the flow rate regulation command based on the pressure of the inlet port-side flow path. 
     In addition, according to a sixth aspect of the present invention, a compressor system includes any one of the above-mentioned compressor control devices. 
     In addition, according to a seventh aspect of the present invention, a compressor control method of a compressor control device configured to control a flow rate of a compressor having a plurality of impellers connected to an outlet port-side flow path in parallel, the compressor control method includes a pressure detection step of detecting a pressure of the outlet port-side flow path, a flow rate detection step of detecting a flow rate of each of the impellers, a flow rate regulation step of regulating the flow rate of each of the impellers, and a control step of outputting a flow rate regulation command of each of the impellers in the flow rate regulation step to control the flow rate regulation step based on the detection result in the pressure detection step, wherein, in the control step, a set point set as a lower limit target value of a flow rate and the flow rate of each of the impellers are compared, and a flow rate regulation command of another impeller is corrected based on the comparison result. 
     Advantageous Effects of Invention 
     According to the above-mentioned compressor control device, compressor system and compressor control method, even when a performance difference is generated between the plurality of impellers, a decrease in efficiency can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration view showing a configuration of a compressor system according to a first embodiment of the present invention. 
         FIG. 2  is a schematic configuration view showing a configuration of a compressor system according to a second embodiment of the present invention. 
         FIG. 3  is a view showing a first example of a performance curve of an impeller according to the embodiment. 
         FIG. 4  is a view showing a second example of a performance curve of the impeller according to the embodiment. 
         FIG. 5  is a view for describing an example of an IGV limit control line according to the embodiment. 
         FIG. 6  is a view for describing some components of the compressor system shown in  FIG. 2  according to the embodiment. 
         FIG. 7  is a view for describing some components of the compressor system shown in  FIG. 2  according to the embodiment. 
         FIG. 8  is a view for describing an example of a logical operation in a logical operation unit included in a compressor control device according to the embodiment. 
         FIG. 9  is a schematic configuration view showing a configuration of a compressor system according to a third embodiment of the present invention according to the embodiment. 
         FIG. 10  is a view for describing some components of the compressor system shown in  FIG. 9  according to the embodiment. 
         FIG. 11  is a view for describing some of the components of the compressor system shown in  FIG. 9  according to the embodiment. 
         FIG. 12  is a view for describing an example of correction value tracking performed by a compressor control device according to the embodiment. 
         FIG. 13  is a view for describing some of the components of the compressor system shown in  FIG. 9  according to the embodiment. 
         FIG. 14  is a view for describing an example of a logical operation in a logical operation unit included in a compressor control device according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, while the present invention is described through embodiments of the present invention, the following embodiments do not limit the invention related to the scope of the claims. In addition, not all combinations of features described in the embodiments are necessary for the solutions of the present invention. 
     First Embodiment 
       FIG. 1  is a schematic configuration view showing a configuration of a compressor system according to a first embodiment of the present invention. In  FIG. 1 , a compressor system  1  includes a compressor control device  11 , a compressor  91  and a blowoff valve  811 . The compressor control device  11  includes flow rate sensors  111 A and  111 B, a pressure sensor  121  and a control unit  190 . The compressor  91  includes impellers  911 A and  911 B, and inlet guide vanes (IGV)  921 A and  921 B. 
     The compressor  91  suctions and compresses air, and supplies the compressed air into an instrument disposed downstream from the compressor  91  using the compressed air (hereinafter referred to as “a lower process”). 
     However, a compression target compressed by the compressor  91  is not limited to air. For example, various compressible gases such as a gaseous coolant or the like may be the compression target. 
     The impellers  911 A and  911 B are connected to an outlet port-side flow path W 21  in parallel, compress the air introduced from inlet port-side flow paths W 11 A and W 11 B via the impellers  911 A and  911 B and output the compressed air to the outlet port-side flow path W 21 . However, the number of impellers included in the compressor  91  is not limited to the two shown in  FIG. 1  but may be three or more. 
     The inlet guide vanes (IGV)  921 A and  921 B correspond to one example of a flow rate regulation unit in the embodiment, and regulate a flow rate of each of the impellers. More specifically, the inlet guide vanes  921 A and  921 B are installed at inlet port-sides of the impellers  911 A and  911 B, and regulate the flow rates of the impellers  911 A and  911 B by regulating an IGV opening degree, which is a blade opening degree thereof. However, the flow rate regulation unit according to the embodiment is not limited to the inlet guide vane. For example, the flow rate regulation unit may be a driving rotator installed at each of the impellers  911 A and  911 B, and configured to regulate the flow rate by regulating a speed of the impeller  911 A or  911 B. 
     In addition, while the case in which the flow rate of the inlet port side of the impeller is used as the flow rate of the impeller will be described in the following description, a flow rate of an outlet port side of the impeller may be used as the flow rate of the impeller. 
     The compressor control device  11  controls a flow rate of the compressor  91  based on the flow rate or the measurement value of the pressure in the compressor  91 . 
     The flow rate sensor  111 A detects the flow rate of the impeller  911 A installed at the inlet port-side flow path W 11 A. The flow rate sensor  111 B detects the flow rate of the impeller  911 B installed at the inlet port-side flow path W 11 B. The flow rate sensors  111 A and  111 B correspond to an example of the flow rate detection unit according to the embodiment. 
     However, the flow rate detection unit according to the embodiment is not limited to the flow rate sensor. For example, the flow rate detection unit may be a receiving circuit configured to receive sensing data transmitted from the flow rate sensor. 
     The pressure sensor  121  detects a pressure of the outlet port-side flow path W 21  installed at the outlet port-side flow path W 21 . The pressure sensor  121  corresponds to one example of the pressure detection unit according to the embodiment. 
     However, the pressure detection unit according to the embodiment is not limited to the pressure sensor. For example, the pressure detection unit may be a receiving circuit configured to receive sensing data transmitted from the pressure sensor. 
     The blowoff valve  811  discharges some of the compressed air to the atmosphere in order to prevent surge by securing the flow rate of the impeller and prevent an increase in the compressed air supplied from the compressor  91  when the flow rate flowing through the impeller  911 A or  911 B is reduced. More specifically, when the flow rate flowing through the impeller is lower than a set flow rate value based on output of the pressure sensor  121 , the blowoff valve  811  is opened to prevent generation of the surge. 
     The control unit  190  outputs the IGV opening degree command serving as a flow rate regulation command of each of the impellers to inlet guide vanes  921 A and  921 B and controls the inlet guide vanes  921 A and  921 B based on the detection result of the pressure sensor  121 . 
     In addition, the control unit  190  compares a set point set as a lower limit target value of a flow rate with a flow rate of each impeller, and corrects a flow rate regulation command of another impeller based on the comparison result. 
     Accordingly, when a flow rate of a certain impeller is smaller than a set point, the compressor control device  11  can subtract a flow rate corresponding to a difference between the flow rate and the set point of the impeller from a flow rate target value of the other impeller. Accordingly, the compressor control device  11  can increase the flow rate of the impeller having a flow rate smaller than the set point to approach the set point without increasing the flow rate of all the impellers. 
     In particular, the compressor control device  11  can control all the flow rates while avoiding a situation in which the flow rate of the impeller having a small flow rate is further reduced to open the blowoff valve  811  when a performance difference is generated between the plurality of impellers and causes a difference between the flow rates. In this way, even when a performance difference is generated between the plurality of impellers, the compressor control device  11  can reduce a decrease in efficiency of the compressor  91 . 
     When the flow rate for preventing the surge cannot be secured even by correction of the flow rate regulation command of the other impeller, the control unit  190  prevents the surge by opening the blowoff valve and securing the flow rate. 
     In addition, when a flow rate of a certain impeller is smaller than the set point, the control unit  190  may control the inlet guide vane  921 A or  921 B to fix the flow rate of the impeller. 
     Accordingly, the compressor control device  11  can prevent generation of the surge as the flow rate of the impeller is further reduced. Here, as the flow rate of the other impeller is reduced, the compressor control device  11  can prevent generation of the surge without the necessity of opening the blowoff valve  811  and discharging the compressed air to the atmosphere. 
     In addition, the control unit  190  may release the fixing of the flow rate of the impeller when the set point and the flow rate command value are separated from each other by a predetermined level or more. 
     Accordingly, the compressor control device  11  can vary the flow rate of the impeller and generate the compressed air having a desired flow rate in the compressor  91  when there is no need to increase the flow rate of the impeller and perform the surge prevention control. In particular, the compressor control device  11  can generate a larger amount of compressed air in the compressor  91  by varying the flow rates of the plurality of impellers disposed in parallel. 
     In addition, the control unit  190  may release the fixing of the flow rate of the impeller when a flow rate of every impeller are smaller than the set point. 
     Accordingly, the compressor control device  11  can reduce the flow rate of each of the impellers from the set point to a surge control line showing a reference flow rate that opens the blowoff valve  811 . That is, the compressor control device  11  can delay the timing at which the blowoff valve  811  is opened by reducing a marginal flow rate formed between the surge control line and the set point, and at this point, can reduce a decrease in efficiency of the compressor  91 . 
     Further, the compressor control device  11 , which is an example of the pressure detection unit according to the embodiment, may further include a pressure sensor configured to detect a pressure of the inlet port-side flow path W 11 A or W 11 B. Then, the control unit  190  may output a flow rate regulation command based on the pressure of the inlet port-side flow path W 11 A or W 11 B. 
     Accordingly, the compressor control device  11  can more accurately generate the compressed air having a desired flow rate even when the pressure of the inlet port-side flow path W 11 A or W 11 B is varied, such as when there is a separate process at the upstream side. 
     Second Embodiment 
     In a second embodiment, an example which further specifies the compressor system  1  according to the first embodiment will be described. 
       FIG. 2  is a schematic configuration view showing a configuration of a compressor system according to the second embodiment of the present invention. In  FIG. 2 , a compressor system  2  includes a compressor control device  12 , a compressor  92 , a blowoff valve  811 , and coolers  821  and  822 . 
     The compressor  92  includes impellers  911 A,  911 B,  912  and  913 , inlet guide vanes  921 A and  921 B, a driving machine  931 , a shaft  941 , and gear boxes  951 ,  952  and  953 . 
     The compressor control device  12  includes flow rate sensors  111 A,  111 B and  112 , pressure sensors  121  and  122 , and a control unit  192 . The control unit  192  includes set point gap storage units  201 A and  201 B, anti-surge control reference point setting units  211 A and  211 B, set point setting units  212 A and  212 B, flow rate control units  213 A,  213 B and  244 , switches  214 A,  214 B and  245 , rate limiters  215 A and  215 B, gain multiplication units  216 A and  216 B, a pressure control unit  221 , function operation units  222 A,  222 B,  242  and  243 , subtraction units  223 A,  223 B,  231 A and  231 B, magnitude determination units  224 A and  224 B, hysteresis units  232 A and  232 B, and a logical operation unit, which will be described below. 
     In  FIG. 2 , components having the same functions corresponding to the components of  FIG. 1  are designated by the same reference numerals  111 A,  111 B,  121 ,  911 A,  911 B,  921 A and  922 B, and description thereof will be omitted. In addition, in  FIG. 2 , a shaft is shown by a chain line, a flow path of air is shown by a broken line, and a flow of data or control information is shown by a solid line. 
     In addition, in  FIG. 2 , “A,” “B,” “C” and “X” surrounded by circles represent input/output with respect to a logical operation unit (to be described below). 
     The impellers  911 A,  911 B,  912  and  913  are constituted by three stages, and the compressed air output from the impellers  911 A and  911 B of a first stage are further compressed by the impeller  912  of a second stage and the impeller  913  of a third stage. 
     Each of the impellers  911 A,  911 B,  912  and  913  is coupled to the driving machine  931  via the shaft  941 . The impellers  911 A and  911 B of the first stage are disposed at one end of the shaft  941 . In addition, the impeller  912  of the second stage and the impeller  913  of the third stage are disposed at the other end of the shaft  941 . The driving machine  931  is connected to the middle of the shaft  941 . Each of the impellers and the driving machine  931  are connected to a shaft  934  via the gear boxes  951 ,  952  and  953 . Further, various instruments that generate a rotational force can be used as the driving machine  931 . For example, the driving machine  931  may be a motor or an engine. In addition, the gear boxes  951 ,  952  and  953  may or may not be included according to disposition or characteristics of a driving machine. For example, a speed-variable driving machine and the impeller may be directed coupled using a shaft and may be configured without using the gear box. 
     In addition, the coolers  821  and  822  are installed between the impeller of the first stage and the impeller of the second stage and between the impeller of the second stage and the impeller of the third stage, and cool the air having a high temperature by compression. 
     The blowoff valve  811  is installed at an outlet port side of the compressor  92 , and the blowoff valve  811  is opened to discharge some of the compressed air generated by the compressor  92  to the atmosphere. 
     The pressure sensor  121  detects a pressure of the outlet port side of the impeller  913  of the third stage. 
     An IGV opening degree command serving as a flow rate regulation command with respect to the inlet guide vane  921 A is generated by the pressure control unit  221  and the function operation unit  222 A based on an outlet port-side pressure of the third stage detected by the pressure sensor  121 . An IGV opening degree command serving as a flow rate regulation command with respect to the inlet guide vane  921 B is generated by the pressure control unit  221  and the function operation unit  222 B based on an outlet port-side pressure of the third stage detected by the pressure sensor  121 . 
     The pressure sensor  122  detects a pressure of the outlet port side of the impellers  911 A and  911 B of the first stage. 
     Both of the anti-surge control reference point setting units  211 A and  211 B open the blowoff valve  811  and set a reference flow rate based on the pressure of the outlet port side of the impellers  911 A and  911 B detected by the pressure sensor  122 . 
     The set point setting units  212 A and  212 B set a set point by adding a set point gap SGp serving as a predetermined margin to the flow rates set by the anti-surge control reference point setting units  211 A and  211 B. The set point is used as a lower limit target value of the flow rates of the impellers  911 A and  911 B. 
     The set point gap storage units  201 A and  201 B store the set point gap serving as a predetermined margin added by the set point setting units  212 A and  212 B. 
     The flow rate control unit  213 A generates a correction value with respect to the IGV opening degree command generated by the pressure control unit  221  and the function operation unit  222 B. That is, the flow rate control unit  213 A generates a correction value with respect to flow rate control of the other impeller  911 B based on a state of the impeller  911 A. In particular, in the following IGV limit control, the flow rate control unit  213 A compares the set point set by the set point setting unit  212 A with the flow rate of each of the impellers, and corrects a flow rate regulation command of the other impeller  911 B based on the comparison result. 
     The flow rate control unit  213 B generates a correction value with respect to the IGV opening command generated by the pressure control unit  221  and the function operation unit  222 A. That is, the flow rate control unit  213 B generates a correction value with respect to the flow rate control of the other impeller  911 A based on a state of the impeller  911 B. In particular, in the following IGV limit control, the flow rate control unit  213 B compares the set point set by the set point setting unit  212 B with the flow rate of each of the impellers, and corrects a flow rate regulation command of the other impeller  911 A based on the comparison result. 
     The switch  214 A switches the input into the flow rate control unit  213 A to any one of a closed loop and 0 according to the state of the compressor system  2 . The switch  214 B switches the input into the flow rate control unit  213 B to any one of the closed loop and 0 according to the state of the compressor system  2 . Processing performed by the switch  214 A and  214 B will be described below. 
     The rate limiters  215 A and  215 B performs rate limit processing for suppressing a variation rate within a certain range in order to prevent abrupt variation with respect to the correction values generated by the flow rate control units  213 A and  213 B. 
     Both of the gain multiplication units  216 A and  216 B multiply a gain with respect to a correction value obtained through rate limit processing. 
     The subtraction unit  223 A performs correction of subtracting a correction value from the IGV opening degree command generated by the pressure control unit  221  and the function operation unit  222 A. The subtraction unit  223 B performs correction of subtracting a correction value from the IGV opening degree command generated by the pressure control unit  221  and the function operation unit  222 B. 
     The magnitude determination unit  224 A determines a magnitude relation between the IGV opening degree command after the correction and the maximum/minimum opening degree of the inlet guide vane  921 A, and outputs the opening command or the closing command to the inlet guide vane  921 A according to the determination result. The magnitude determination unit  224 B determines a magnitude relation between the IGV opening degree command after the correction and the maximum/minimum opening degree of the inlet guide vane  921 B, and outputs the opening command or closing command to the inlet guide vane  921 B according to the determination result. 
     The subtraction unit  231 A subtracts the set point set by the set point setting unit  212 A from the flow rate of the impeller  911 A detected by flow rate sensor  111 A. The subtraction unit  231 B subtracts the set point set by the set point setting unit  212 B from the flow rate of the impeller  911 B detected by the flow rate sensor  111 B. 
     The hysteresis unit  232 A determines whether the calculation result of the subtraction unit  231 A is positive or negative. Since the determination result uses mode switching in a logical operation unit (to be described below), in order to avoid frequent occurrence of the mode switching, the hysteresis unit  232 A sets predetermined hysteresis when it is determined whether the calculation result of the subtraction unit  231 A is positive or negative. The hysteresis unit  232 B determines whether the calculation result of the subtraction unit  231 B is positive or negative. Like the case of the hysteresis unit  232 A, the hysteresis unit  232 B sets predetermined hysteresis when it is determined whether the calculation result of the subtraction unit  231 B is positive or negative. 
     The flow rate sensor  112  detects a flow rate of the outlet port side of the impeller  913  of the third stage. 
     The pressure control unit  221 , the function operation unit  242  and  243 , and the flow rate control unit  244  generate control information with respect to the blowoff valve  811  based on the flow rate or the pressure of the outlet port side of the impeller  913  of the third stage. 
     The switch  245  performs switching of the control information with respect to the blowoff valve  811 , and controls opening/closing of the blowoff valve  811  by outputting the control information to the blowoff valve  811 . 
     Here, characteristics and anti-surge control of the impeller will be described with reference to  FIGS. 3 to 5 . 
       FIG. 3  is a view showing a first example of a performance curve of the impeller. In  FIG. 3 , lines L 111 , L 112  and L 113  are pressure P-flow rate F curves in opening degrees of IGV, and in particular, the line L 111  is a pressure P-flow rate F curve when the IGV is a maximum opening degree (entirely opened). In addition, a line L 121  is a surge line, and surging occurs in a region of a left side thereof. More specifically, an air volume is reduced at a region of a left side of the surge line, and a ratio between an impeller inlet port-side pressure and an impeller outlet port-side pressure is increased. For this reason, surges (vibrations) occur when the impeller cannot cause wind to flow toward a wake. When the air volume is increased, the impeller causes the wind to flow toward the wake, and the surge is suppressed. 
     In addition, a line SCL is a surge control line showing a relation between the outlet port-side pressure of the impeller of the first stage and the flow rate control target value in the anti-surge control. As described above, the surge is generated in the region of the left side of the surge line L 121 . For this reason, in a region of a right side of the surge control line SCL having a margin with respect to the surge line L 121 , the anti-surge control for controlling the pressure or the flow rate of the compressor is performed. 
     The anti-surge control is performed by opening the blowoff valve and allowing some of the compressed air to escape to the atmosphere to increase the outlet port flow rate. Since some of the compressed air escapes to the atmosphere, efficiency of the compressor is decreased. 
     In addition, a line L 131  shows the current outlet port-side pressure of the first stage, and a point P 111  shows an example of the outlet port-side pressure and the inlet port-side flow rate according to the current IGV opening degree. 
       FIG. 4  is a view showing a second example of a performance curve of the impeller. The impeller shown in  FIG. 4  has a performance lower than that of the impeller shown in  FIG. 3 . 
     When the performance of the impeller is decreased, there is a tendency for the pressure to decrease with respect to the flow rate. For this reason, when the flow rate control target value is reduced, the flow rate is likely to arrive at the surge control line SCL. When the flow rate of the impeller arrives at the surge control line SCL and the blowoff valve  811  is opened, the compressed air may be discharged to the atmosphere and decrease efficiency of the compressor  92 . 
     Here, the compressor control device  12  sets the IGV limit control line serving as the margin with respect to the surge control line, and performs the IGV limit control using the flow rate in the IGV limit control line as the control target value when the flow rate of the impeller is small. 
       FIG. 5  is a view for describing an example of the IGV limit control line. A performance curve shown in a part (A) of  FIG. 5  is a performance curve of the impeller in which performance is decreased. Meanwhile, a performance curve shown in a part (B) of  FIG. 5  is a performance curve of the impeller in which performance is not decreased. In the description of  FIG. 5 , the performance curve shown in  FIG. 5A  shows performance of the impeller  911 A, and the performance curve shown in  FIG. 5B  shows performance of the impeller  911 B. 
     In addition, in  FIG. 5 , an IGV limit control line ILCL having a margin (a set point gap SGp) corresponding to a flow rate ΔX with respect to the surge control line SCL is shown. In addition, the line L 131  shows the current outlet port-side pressure of the first stage, and a point P 211  shows an example of the outlet port-side pressure and the inlet port-side flow rate according to the current IGV opening degree. 
     Here, an intersection point P 212  between the line L 131  showing the outlet port-side pressure and the surge control line SCL shows a reference flow rate Q SCLA  at which the blowoff valve  811  opens set by the anti-surge control reference point setting unit  211 A. In addition, the flow rate ΔX of the margin between the surge control line SCL and the IGV limit control line ILCL corresponds to the set point gap (SGp) serving as the margin added by the set point setting unit  212 A. Accordingly, an intersection point P 213  between the line L 131  and the IGV limit control line ILCL shows a set point (a flow rate Q ILCLA ) set by the set point setting unit  212 A. 
     The set point (the flow rate Q ILCLA ) is used as a lower limit target value of the flow rate of the impeller  911 A in the IGV limit control. The IGV limit control is control for suppressing the blowoff valve from being opened even when any one of the impellers of the first stage arrives at the surge control line and there is a margin from the surge control line in the flow rate of the other impeller of the first stage. 
     In an example of the part (A) of  FIG. 5 , an inlet port-side flow rate of the impeller  911 A shown at a point P 211  is disposed at a left side of the IGV limit control line ILCL showing the set point of the IGV limit control, and the inlet port-side flow rate of the impeller  911 A is smaller than the set point (the flow rate Q ILCLA ). In this case, the compressor control device  12  controls the inlet port-side flow rate of the impeller  911 A to approach the set point (the flow rate Q ILCLA ) in the IGV limit control. 
     Here, the compressor control device  12  regulates the entire flow rate of the first stage by reducing the target flow rate according to the margin of the flow rate when there is a need to reduce the flow rate. In the example of  FIG. 5 , as shown in the part (B) of  FIG. 5 , the compressor control device  12  reduces the flow rate of the impeller  911 B having the margin of the flow rate. 
     Next, processing performed by the compressor control device  12  in the IGV limit control will be described with reference to  FIG. 6 . 
       FIG. 6  is a view for describing some components of the compressor system  2  shown in  FIG. 2 . In  FIG. 6 , among the components shown in  FIG. 2 , the impellers  911 A and  911 B, the inlet guide vanes  921 A and  921 B, the flow rate sensors  111 A and  111 B, the anti-surge control reference point setting unit  211 A, the set point setting unit  212 A, the flow rate control unit  213 A, the rate limiter  215 B, the gain multiplication unit  216 B, the pressure control unit  221 , the function operation unit  222 B, the subtraction unit  223 B and the magnitude determination unit  224 B are shown. 
     For example, when the flow rate of the impeller  911 A is smaller than the set point (the flow rate Q ILCLA  in the example of  FIG. 5 ) of the IGV limit control, the compressor control device  12  performs the IGV limit control and causes the flow rate of the impeller  911 A to approach the set point. 
     Specifically, the flow rate control unit  213 A calculates a target flow rate in proportional integral control (PI control) in order to match the flow rate of the impeller  911 A detected by the flow rate sensor  111 A with the set point of the IGV limit control set by the set point setting unit  212 A. 
     Further, in the following description, the set point of the IGV limit control is simply referred to as “a set point.” 
     Then, the subtraction unit  223 B subtracts the flow rate obtained through fairing such as rate limit processing, gain multiplication, or the like, with respect to the target flow rate calculated by the flow rate control unit  213 A from the target flow rate of the impeller  911 B. That is, the compressor control device  12  applies offset to the impeller  911 B to reduce only an incremental difference of the flow rate in the impeller  911 A from an original target flow rate. 
     As the impeller  911 B reduces the flow rate, the flow rate command value output from the pressure control unit  221  is increased, and as a result, the flow rate of the impeller  911 A approaches the set point. 
     Meanwhile, when the flow rate is larger than the set point at both of the impellers  911 A and  911 B, the compressor control device  12  does not perform the IGV limit control, and the flow rate control unit  213 A or  213 B performs tracking while constantly holding the correction value. This will be described with reference to  FIG. 7 . 
       FIG. 7  is a view for describing some components of the compressor system  2  shown in  FIG. 2 . In  FIG. 7 , among the components shown in  FIG. 2 , the impellers  911 A and  911 B, the inlet guide vanes  921 A and  921 B, the flow rate sensors  111 A and  111 B, the flow rate control unit  213 A, the switch  214 A, the rate limiter  215 B, the gain multiplication unit  216 B, the pressure control unit  221 , the function operation unit  222 B, the subtraction unit  223 B and the magnitude determination unit  224 B are shown. 
     When the flow rate is larger than the set point at both of the impellers  911 A and  911 B, the flow rate control unit  213 A or  213 B is set to a manual mode, which is a mode in which the IGV limit control is not performed. In this case, the flow rate control unit  213 A or  213 B tracks a correction value set immediately before switching to a manual mode from an auto mode. In the case of  FIG. 7 , the flow rate control unit  213 A receives the correction value directly output from the flow rate control unit  213 A and outputs the correction value again in a closed loop constituted by the switch  214 A. 
     In this way, as the flow rate control unit  213 A or  213 B tracks the correction value set immediately before switching to the manual mode from the auto mode, the compressor control device  12  performs correction of the target flow rate according to a performance difference between the impellers  911 A and  911 B. Specifically, the compressor control device  12  performs correction to reduce the flow rate of the impeller as the performance is improved. Accordingly, because the margin between the flow rate of the impeller and the surge control line is increased as the performance is deteriorated, a breadth in which the compressor  92  is controlled by the compressor control device  12  is increased without opening the blowoff valve. 
     Further, in a state in which an environment for performing the IGV limit control is not prepared, for example, when the driving machine  931  is stopped, the anti-surge control becomes manual, or the like, the flow rate control unit  213 A or  213 B sets the tracking value to zero. 
     In the configuration shown in  FIG. 7 , the switch  214 A connects a constant value “0.0” and the flow rate control unit  213 A, and the flow rate control unit  213 A outputs the constant value. 
       FIG. 8  is a view for describing an example of a logical operation of the logical operation unit included in the compressor control device  12 . The logical operation unit calculates control information with respect to the switch  214 A or  214 B, or control information with respect to a mode in the flow rate control unit  213 A or  213 B. 
     In the logical operation shown in  FIG. 8 , the logical operation unit outputs control information of instructing connection to the closed loop side with respect to the switch  214 A or  214 B when the driving machine  931  is in operation or the anti-surging control is in the auto mode. On the other hand, when the driving machine  931  is stopped or the anti-surging control is in the manual mode, the logical operation unit outputs control information of instructing connection to a constant zero side with respect to the switch  214 A or  214 B. 
     In addition, the logical operation unit receives a logical product of three conditions under a condition in which a mode of the flow rate control unit  213 A or  213 B is set to auto. First, like the control of the switch  214 A or  214 B, the driving machine  931  is in operation, and the anti-surging control becomes the auto mode. Second, the pressure control is the auto mode, i.e., the pressure control unit  221  controls the flow rate of the impeller  911 A or  911 B through the pressure control. Third, in one of the impellers  911 A and  911 B, a difference between the set point and the inlet port flow rate measurement value is negative, and in the other impeller, a difference between the set point and the inlet port flow rate measurement value is positive. That is, one of the impellers  911 A and  911 B is in a state in which the IGV limit control is to be performed, and the other impeller is in a state in which there is a margin from the set point. 
     As described above, the control unit  192  (in particular, the pressure control unit  221 ) outputs the IGV opening degree command serving as the flow rate regulation command of each impeller to the inlet guide vanes  921 A and  921 B and controls them based on the detection result of the pressure sensor  121 . 
     In addition, the control unit  192  (in particular, the flow rate control unit  213 A and  213 B) compares the set point set as the lower limit target value of the flow rate with the flow rate of each impeller, and corrects the flow rate regulation command of the other impeller based on the comparison result. 
     Accordingly, when the flow rate of the certain impeller is smaller than the set point, the compressor control device  12  can subtract the flow rate corresponding to the difference between the flow rate and the set point of the impeller from the flow rate target value of the other impeller. Accordingly, the compressor control device  12  can increase the flow rate of the impeller having a smaller flow rate than the set point to approach the set point without increasing the entire flow rate of the impeller. 
     In particular, the compressor control device  12  can control the entire flow rate while avoiding the situation in which the flow rate of the impeller having a small flow rate is further reduced to open the blowoff valve, when the performance difference is generated between the plurality of impellers and generates a difference in flow rate. In this way, the compressor control device  11  can reduce a decrease in efficiency of the compressor  92  even when the performance difference is generated between the plurality of impellers. 
     Third Embodiment 
     In the second embodiment, another example which further specifies the compressor system  1  according to the first embodiment will be described. 
       FIG. 9  is a schematic configuration view showing a configuration of a compressor system according to a third embodiment of the present invention. In  FIG. 9 , a compressor system  3  includes a compressor control device  13 , a compressor  92 , a blowoff valve  811 , and coolers  821  and  822 . 
     The compressor  92  includes impellers  911 A,  911 B,  912  and  913 , inlet guide vanes  921 A and  921 B, a driving machine  931 , a shaft  941 , and gear boxes  951 ,  952  and  953 . 
     The compressor control device  13  includes flow rate sensors  111 A,  111 B and  112 , pressure sensors  121  and  122 , and a control unit  193 . The control unit  193  includes anti-surge control reference point setting units  211 A and  211 B, set point setting units  212 A and  212 B, flow rate control units  213 A,  213 B and  244 , switches  214 A,  214 B,  245 ,  311 A,  311 B,  331 A and  331 B, rate limiters  215 A and  215 B, gain multiplication units  216 A and  216 B, a pressure control unit  221 , function operation units  222 A,  222 B,  242  and  243 , subtraction units  223 A,  223 B,  231 A,  231 B,  321 A and  321 B, magnitude determination units  224 A and  224 B, hysteresis units  232 A,  232 B,  322 A and  322 B, and a logical operation unit, which will be described below. 
     In  FIG. 9 , portions having the same functions corresponding to the components of  FIG. 2  are designated by the same reference numerals  111 A,  111 B,  112 ,  121 ,  122 ,  201 A,  201 B,  211 A,  211 B,  212 A,  212 B,  213 A,  213 B,  244 ,  214 A,  214 B,  245 ,  215 A,  215 B,  216 A,  216 B,  219 A,  219 B,  221 ,  222 A,  222 B,  242 ,  243 ,  223 A,  223 B,  231 A,  231 B,  224 A,  224 B,  232 A,  232 B,  811 ,  821 ,  822 ,  92 ,  911 A,  911 B,  912 ,  913 ,  921 A,  922 B,  931 ,  941 , and  951  to  953 , and description thereof will be omitted. In addition, in  FIG. 9 , a shaft is shown by a chain line, a flow path of air is shown by broken lines, and a flow of data or control information is shown by solid lines. 
     In addition, “A1,” “A2,” “A3,” “B1,” “B2,” “B3,” “X” and “Y” in circles in  FIG. 9  show input/output with respect to the logical operation unit, which will be described below. 
     When the set point and the flow rate of the impeller are compared, cases in which (1) the flow rate of the impeller is large at both of the impellers  911 A and  911 B, (2) the flow rate of one of the impellers  911 A and  911 B is smaller than the set point, and (3) the flow rate of the impeller is smaller than the set point at both of the impellers  911 A and  911 B, are considered. The compressor control device  13  performs control of the compressor  92  at an operation mode corresponding each of the three cases. 
     In order to perform these operation modes, the subtraction units  321 A and  321 B and hysteresis units  322 A and  322 B generate a signal showing whether divergence between the IGV opening degree and the command value is large or not at each of the inlet guide vanes  921 A and  921 B as the input into the logical operation unit. 
     The switches  331 A and  331 B switch fixing/non-fixing of the IGV opening degree. 
     (1) When the flow rate of the impeller is large at both of the impellers  911 A and  911 B, the flow rate control units  213 A and  213 B are set to the auto mode. When the compressor flow rate is sufficiently larger than the IGV limit control line, the correction value by the IGV limit control becomes zero. 
     Meanwhile, when the flow rate of the impeller is reduced to approach the IGV limit control line, the flow rate control unit  213 A or  213 B performs the PI control serving as the IGV limit control, and outputs the correction signal with respect to the flow rate command value of the opposite impeller. 
       FIG. 10  is a view for describing some of the components of the compressor system  3  shown in  FIG. 9 . In  FIG. 10 , among the components shown in  FIG. 9 , the impellers  911 A and  911 B, the inlet guide vanes  921 A and  921 B, the flow rate sensors  111 A and  111 B, the anti-surge control reference point setting unit  21  IA, the set point setting unit  212 A, the flow rate control unit  213 A, the rate limiter  215 B, the gain multiplication unit  216 B, the pressure control unit  221 , the function operation unit  222 B, the subtraction unit  223 B, the switch  331 B and the magnitude determination unit  224 B are shown. 
     According to the above-mentioned configuration, like the case of  FIG. 6 , the flow rate control unit  213 A performs the IGV limit control. 
     (2) When the flow rate of any one of the impellers  911 A and  911 B is smaller than the set point, the compressor control device  13  fixes the IGV opening degree of the inlet guide vane of the impeller side having the flow rate smaller than the set point. 
       FIG. 11  is a view for describing some of the components of the compressor system  3  shown in  FIG. 9 . In  FIG. 11 , among the components shown in  FIG. 9 , the impellers  911 A and  911 B, the inlet guide vanes  921 A and  921 B, the flow rate sensors  111 A and  111 B, the anti-surge control reference point setting units  211 A and  211 B, the set point setting units  212 A and  212 B, the flow rate control units  213 A and  213 B, the switches  214 A,  214 B,  311 A,  311 B,  331 A and  331 B, the rate limiter  215 A, the gain multiplication unit  216 A, the pressure control unit  221 , the function operation units  222 A and  222 B, the subtraction unit  223 A and the magnitude determination units  224 A and  224 B are shown. 
     For example, when the flow rate of the impeller  911 B is smaller than the set point, the switch  331 B configures a loop to hold the IGV opening degree command value of the inlet guide vane  921 B. In addition, the switches  214 B and  311 B configure a loop to hold the correction value in the IGV opening degree command value. 
     In this way, as the compressor control device  13  fixes the flow rate of the impeller, the flow rate of the impeller can be further reduced to prevent generation of the surge. Here, as the flow rate of the other impeller is reduced, the compressor control device  13  can open the blowoff valve to prevent generation of the surge without necessity of discharging the compressed air to the atmosphere. 
     Further, when the IGV opening degree is fixed, the compressor control device  13  performs tracking of the correction value such that the IGV opening degree is not abruptly varied when the fixing of the IGV opening degree is released. 
       FIG. 12  is a view for describing an example of correction value tracking performed by the compressor control device  13 . 
     In  FIG. 12 , among the components shown in  FIG. 9 , the flow rate control unit  213 A, the pressure control unit  221 , the subtraction units  223 B and  321 B, the switch  331 B, the impeller  911 B and the inlet guide vane  921 B are shown. Further, description of some of the components on a path of a signal will also be omitted in order to simplify the drawings. 
     In the example shown in  FIG. 12 , when the IGV opening degree of the inlet guide vane  921 B is fixed, the IGV opening degree command from the pressure control unit  221  is 30%, and the correction value generated by the flow rate control unit  213 A is 10%. Here, in a state in which the subtraction unit  223 B calculates the IGV opening degree command after correction as 20% and outputs the command to the magnitude determination unit  224 B, the switch  331 B configures the closed loop and the IGV opening degree of 20% is held. 
     After that, in the case in which the IGV opening degree command value from the pressure control unit  221  is reduced to 25%, provisionally, when the flow rate control unit  213 A continues to output the correction value of 10%, the IGV opening degree command after correction becomes 15%, and the fixed value of the IGV opening degree is varied. Like this, when the switch  331 B varies connection to the subtraction unit  223 B side to release the fixing of the IGV opening degree, the IGV opening degree may abruptly vary from 20% to 15%. 
     Here, the subtraction unit  321 B calculates a difference between the opening degree command from the pressure control unit  221  and the fixed value of the IGV opening degree, and varies the correction value output from the flow rate control unit  213 A. 
     In the example of  FIG. 12 , when the IGV opening degree command from the pressure control unit  221  is varied to 25%, the subtraction unit  321 B subtracts the IGV opening degree having the fixed value of 20% from the IGV opening degree command of 25% to calculate 5%. Then, the flow rate control unit  213 A outputs 5% calculated by the subtraction unit  321 B as the correction value. 
     Accordingly, the fixed value of the IGV opening degree and the IGV opening degree command after correction become the same value, and abrupt variation of the IGV opening degree is not generated when the switch  331 B releases the fixing of the IGV opening degree. 
     (3) When the flow rate of the impeller is smaller than the set point at both of the impellers  911 A and  911 B, the compressor control device  13  can release the fixing of the IGV opening degree, and both of the impellers  911 A and  911 B can also vary the flow rate. Here, the compressor control device  13  tracks the correction value immediately before switching to the state (3). 
       FIG. 13  is a view for describing some of the components of the compressor system  3  shown in  FIG. 9 . In  FIG. 11 , among the components shown in  FIG. 9 , the impellers  911 A and  911 B, the inlet guide vanes  921 A and  921 B, the flow rate sensors  111 A and  111 B, the flow rate control units  213 A and  213 B, the switches  214 A,  214 B,  311 A,  311 B,  331 A and  331 B, the rate limiters  215 A and  215 B, the gain multiplication units  216 A and  216 B, the pressure control unit  221 , the function operation units  222 A and  222 B, the subtraction units  223 A and  223 B, and the magnitude determination units  224 A and  224 B are shown. 
     In  FIG. 13 , the switches  214 A and  311 A configure the closed loop, and the flow rate control unit  213 A holds the correction value at the closed loop. The switches  214 B and  311 B and the flow rate control unit  213 B are also the same as above. 
     Then, the subtraction unit  223 A subtracts the correction value from the flow rate command from the pressure control unit  221 , and outputs the flow rate command after correction to the inlet guide vane  921 A. The impeller  911 B is also the same as above. 
     In this way, when the flow rates of both of the impellers  911 A and  911 B is smaller than the set point, the switches  331 A and  331 B release the fixing of the flow rate of the impeller. 
     Accordingly, the compressor control device  13  can reduce the flow rate of each of the impellers from the set point to the surge control line showing the reference flow rate of opening the blowoff valve  811 . That is, the compressor control device  13  can delay the timing of opening the blowoff valve  811  and thus can reduce a decrease in efficiency of the compressor  92  by reducing the flow rate according to the margin formed between the surge control line and the set point. 
     In addition, the compressor control device  13  performs correction of the target flow rate according to the performance difference between the impellers  911 A and  911 B by tracking the correction value immediately before switching to the mode of (3). Specifically, the compressor control device  13  performs the correction such that the flow rate of the impeller is reduced as the performance is increased. Accordingly, because a margin between the flow rate of the impeller at which the performance is deteriorated and the surge control line is increased, a width in which the compressor control device  13  controls the compressor  92  is increased without opening the blowoff valve. 
       FIG. 14  is a view for describing an example of a logical operation in the logical operation unit included in the compressor control device  13 . The logical operation unit calculates the control information with respect to the switch  214 A,  214 B,  311 A or  311 B, and the control information with respect to a mode in the flow rate control unit  213 A or  213 B. 
     In the logical operation shown in  FIG. 14 , the logical operation unit performs the IGV limit control when the driving machine  931  is in operation and the anti-surging control is in the auto mode. In addition, the case in which the logical operation unit automatically sets the IGV limit control is the case of the above-mentioned (1). Specifically, the logical operation unit obtains a logical product of three conditions under a condition in which the IGV limit control is automatically set. 
     First, the driving machine  931  is in operation and the anti-surging control is in the auto mode. Second, the pressure control is the auto mode, i.e., the pressure control unit  221  controls the flow rate of the impeller  911 A or  911 B through the pressure control. Third, divergence between the IGV opening degree and the opening degree command value of the inlet guide vane  921 A is increased and the flow rate of the impeller  911 A is smaller than the IGV limit control line, or the flow rates of both of the impellers  911 A and  911 B are increased to be larger than the IGV limit control line, or divergence between the IGV opening degree and the opening degree command value of the inlet guide vane  921 B is increased and the flow rate of the impeller  911 B is smaller than the IGV limit control line. 
     Further, the condition in which the divergence between the IGV opening degree and opening degree command value of the inlet guide vane  921 A is increased and the flow rate of the impeller  911 A is smaller than the IGV limit control line is a condition for transition from (2) to (1). The condition in which the divergence between the IGV opening degree and the opening degree command value of the inlet guide vane  921 B is increased and the flow rate of the impeller  911 B is smaller than the IGV limit control line is also the same as above. 
     In addition, the condition under which the logical operation unit fixes the IGV opening degree of the inlet guide vane  921 A is that the flow rate of the impeller  911 B be larger than the IGV limit control line, the flow rate of the impeller  911 A be smaller than the IGV limit control line, and the divergence between the IGV opening degree and the opening degree command value of the inlet guide vane  921 A not be increased. 
     In addition, the condition under which the logical operation unit fixes the IGV opening degree of the inlet guide vane  921 B is that the flow rate of the impeller  911 A be increased to be larger than the IGV limit control line, the flow rate of the impeller  911 B be smaller than the IGV limit control line, and the divergence between the IGV opening degree and the opening degree command value of the inlet guide vane  921 B not be increased. 
     That is, the logical operation unit fixes the IGV opening degree of the inlet guide vane applied to the impeller when the flow rate of any one of the impellers  911 A and  911 B is smaller than the IGV limit control line and the divergence between the flow rate and the flow rate command value of the impeller having the flow rate smaller than the IGV limit control line is larger than a predetermined value. 
     As described above, the control unit  193  controls a corresponding one of the inlet guide vanes  921 A and  921 B to fix the flow rate of the impeller when the flow rate of the impeller  911 A or  911 B is smaller than the set point. 
     Accordingly, the compressor control device  13  can prevent the flow rate of the impeller from being further reduced and the surge from being generated. Here, as the flow rate of the other impeller is reduced, the compressor control device  13  can prevent generation of the surge without necessity of opening the blowoff valve  811  and discharging the compressed air to the atmosphere. 
     In addition, the control unit  193  releases the fixing of the flow rate of the impeller when there is a predetermined interval or more between the set point and the flow rate command value. 
     Accordingly, the compressor control device  13  can vary the flow rate of the impeller to generate the compressed air having a desired flow rate in the compressor  92  when there is no necessity to increase the flow rate of the impeller and perform the surge prevention control. In particular, the compressor control device  13  can generate a larger amount of compressed air in the compressor  92  by varying the flow rate of the plurality of impellers disposed in parallel. 
     In addition, the control unit  193  releases the fixing of the flow rate of the impeller when the flow rates of both of the impellers are smaller than the set point. 
     Accordingly, the compressor control device  13  can reduce the flow rate of each of the impellers from the set point to the surge control line that opens the blowoff valve and shows the reference flow rate. That is, the compressor control device  13  can delay the timing of opening the blowoff valve and thus reducing a decrease in efficiency of the compressor  92  by reducing the flow rate according to the margin formed between the surge control line and the set point. 
     In this way, the compressor control device  13  can perform finer processing than the compressor control device  12 . Meanwhile, the compressor control device  12  is more simply controlled than the compressor control device  13 , and thus maintenance or alteration may be easily performed. 
     Further, the compressor control device  13  may further include a pressure sensor configured to detect a pressure in the inlet port-side flow path, as an example of the pressure detection unit according to the embodiment. Then, the control unit  193  may be configured to output a flow rate regulation command based on the pressure of the inlet port-side flow path. 
     Accordingly, the compressor control device  13  can more precisely generate the compressed air having a desired flow rate even when the pressure in the inlet port-side flow path is varied, such as when there is a separate process at the upstream side. 
     Further, processing of each part may be performed by recording a program for realizing functions of all or some of the compressor control devices  11 ,  12  and  13  on a computer-readable recording medium, reading the program recorded on the recording medium using a computer system, and performing the program. Further, the “computer system” described above includes an OS or hardware such as peripheral devices, or the like. 
     In addition, the “computer system” also includes a homepage providing environment (or a display environment) when a WWW system is used. 
     In addition, the “computer-readable recording medium” is referred to as a portable medium such as a flexible disk, a magneto-optical disc, a ROM, a CD-ROM, or the like, and a storage device such as a hard disk or the like installed in the computer system. Further, the “computer-readable recording medium” also includes an object that holds a program for a certain time such as an object for dynamically holding a program for a short time like a communication wire when the program is transmitted via a network such as the Internet or the like or a communication line such as a telephone line or the like, and like a volatile memory in the computer system which serves as a server or a client in this case. In addition, the program may be configured to realize some of the above-mentioned functions, and further, the above-mentioned functions may be realized in combination with the program recorded in the computer system. 
     As described above, while the embodiments of the present invention have been described in detail with reference to the accompanying drawings, a specific configuration is not limited to the embodiments and design changes that do not depart from the spirit of the present invention are also included in the present invention. 
     INDUSTRIAL APPLICABILITY 
     The present invention relates to a compressor control device configured to control a flow rate of a compressor having a plurality of impellers connected to an outlet port-side flow path in parallel and a flow rate regulation unit configured to regulate a flow rate of each of the impellers, the compressor control device including: a pressure detection unit configured to detect a pressure of the outlet port-side flow path; a flow rate detection unit configured to detect the flow rate of each of the impellers; and a control unit configured to output a flow rate regulation command of each of the impellers to the flow rate regulation unit and control the flow rate regulation unit based on the detection result of the pressure detection unit, wherein the control unit compares a set point set as a lower limit target value of the flow rate with the flow rate of each of the impellers, and corrects a flow rate regulation command of the other impeller based on the comparison result. 
     According to the present invention, a decrease in efficiency can be reduced even when a performance difference is generated between the plurality of impellers. 
     REFERENCE SIGNS LIST 
     
         
           11  compressor control device 
           91  compressor 
           111 A,  111 B flow rate sensor 
           121  pressure sensor 
           190  control unit 
           811  blowoff valve 
           911 A,  911 B impeller 
           921 A,  921 B inlet guide vane