Abstract:
This compressor system is provided with: a driving machine having a rotary-driven first output shaft and a second shaft that is rotary driven so as to reach the same speed as the first output shaft; a working-fluid-compressing first compressor to which the rotation of the first output shaft is transmitted; a working-fluid-compressing second compressor to which the rotation of the second output shaft is transmitted; a variable-speed step-up gear for increasing the speed of the first output shaft and transmitting the increased speed to the first compressor, said step-up gear being capable of varying the increased speed; and a constant-speed step-up gear for increasing the speed of the second output shaft and transmitting the increased speed to the second compressor, said step-up gear keeping the increased speed constant.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a compressor system configured to drive a plurality of compressors using one driving machine. 
       BACKGROUND ART 
       [0002]    As a compressor such as an axial flow compressor, a centrifugal compressor, or the like, configured to generate compressed fluids used as various driving sources, a compressor driven by a driving machine (a motor) is provided (for example, see Patent Literature 1). 
       CITATION LIST 
     Patent Literature 
       [0003]    [Patent Literature 1] 
         [0004]    Published Japanese Translation No. 2000-505525 of the PCT International Publication 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    Incidentally, there is a compressor system in which a high pressure side compressor and a low pressure side compressor are serially connected to one driving machine via a step-up gear or a reduction gear and the two compressors are driven by the driving machine. Then, in the above-mentioned compressor system, in a state in which an outlet pressure of a working fluid delivered from the high pressure side compressor and the low pressure side compressor is constant, the compressor system should be operated in a wide operating range. 
         [0006]    The present invention is directed to provide a compressor system capable of securing a wider range of operating condition and performing a stable operation even when an operation is performed at a constant outlet pressure in a configuration in which a plurality of compressors are driven by one driving machine. 
       Solution to Problem 
       [0007]    According to a first aspect of the present invention, a compressor system includes a driving machine having a first output shaft that is rotatably driven, and a second output shaft that is rotatably driven to have the same rotational speed as the first output shaft; a first compressor configured to receive rotation of the first output shaft and compress a working fluid; a second compressor configured to receive rotation of the second output shaft and compress a working fluid; a variable speed step-up gear configured to increase the rotational speed of the first output shaft and transmit the increased rotational speed to the first compressor and vary the increased rotational speed; and a constant speed step-up gear configured to increase the rotational speed of the second output shaft to transmit the rotational speed to the second compressor and cause the increased rotational speed to be constant. 
         [0008]    According to the above-mentioned configuration, when the compressor system is operated such that a constant outlet pressure is maintained, the compressor system can be operated in a wide operating range. 
         [0009]    According to a second aspect of the present invention, in the compressor system, the first compressor of the compressor system of the first aspect may be a low pressure side compressor, and the second compressor may be a high pressure side compressor. 
         [0010]    According to the above-mentioned configuration, a decrease in outlet pressure of the compressor system can be suppressed, and an output can be stably secured while widening an operating range. 
         [0011]    According to a third aspect of the present invention, in the compressor system of the first or second aspect, the second compressor that compresses the working fluid by rotation may be operated at a circumferential speed of an impeller, which is a Mach number of 0.8 or less. 
         [0012]    According to a fourth aspect of the present invention, in the compressor system of the third aspect, the second compressor may have at least six impellers configured to compress the working fluid by rotation. 
         [0013]    According to the above-mentioned configuration, the second compressor may have a wide operating range that is flatter than that of the first compressor. 
         [0014]    According to the fifth aspect of the present invention, in the compressor system, a head transmitted from the driving machine to the second compressor may occupies 60% or more of a total head transmitted from the driving machine obtained by summing a head transmitted to the first compressor and a head transmitted to the second compressor. 
         [0015]    According to the above-mentioned configuration, when a flow rate is adjusted by the first compressor, a variation can be minimized as much as possible and a stable operation can be performed. 
       Advantageous Effects of Invention 
       [0016]    According to the above-mentioned compressor system, as the rotational speed of the first compressor is varied and the rotational speed of the second compressor is constant, in the configuration in which a plurality of compressors are driven by the one driving machine, even when the operation is performed at a constant outlet pressure, a wider operating condition range can be secured and a stable operation can be performed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  is a view showing a schematic configuration of a compressor system according to an embodiment of the present invention. 
           [0018]      FIG. 2  is a view for describing a plurality of impellers disposed at a high pressure side compressor according to the embodiment of the present invention. 
           [0019]      FIG. 3  is a view for describing the impellers used in a low pressure side compressor and a high pressure side compressor used in the embodiment of the present invention. 
           [0020]      FIG. 4A  is a view showing a relation between a flow rate and a head in the low pressure side compressor of the compressor system according to the embodiment of the present invention. 
           [0021]      FIG. 4B  is a view showing a relation between a flow rate and a head in the high pressure side compressor of the compressor system according to the embodiment of the present invention. 
           [0022]      FIG. 4C  is a view showing a relation between a flow rate and an outlet pressure in the compressor system according to the embodiment of the present invention. 
           [0023]      FIG. 5  is a view showing a schematic configuration of a comparative example of the compressor system according to the embodiment of the present invention. 
           [0024]      FIG. 6A  is a view showing a relation between a flow rate and a head in a low pressure side compressor of a comparative example of the compressor system according to the embodiment of the present invention. 
           [0025]      FIG. 6B  is a view showing a relation between a flow rate and a head in a high pressure side compressor of a comparative example of the compressor system according to the embodiment of the present invention. 
           [0026]      FIG. 6C  is a view showing a relation between a flow rate and an outlet pressure of a comparative example of the compressor system according to the embodiment of the present invention. 
           [0027]      FIG. 7  is a view showing a schematic configuration of another comparative example of the compressor system according to the embodiment of the present invention. 
           [0028]      FIG. 8A  is a view showing a relation between a flow rate and a head in a low pressure side compressor of the other comparative example of the compressor system according to the embodiment of the present invention. 
           [0029]      FIG. 8B  is a view showing a relation between a flow rate and a head in a high pressure side compressor of the other comparative example of the compressor system according to the embodiment of the present invention. 
           [0030]      FIG. 8C  is a view showing a relation between a flow rate and an outlet pressure of the other comparative example of the compressor system according to the embodiment of the present invention. 
           [0031]      FIG. 9A  is a view showing a relation between a mass flow rate and a head in a low pressure side compressor of a compressor system according to an example of the present invention. 
           [0032]      FIG. 9B  is a view showing a relation between a mass flow rate and a head in a high pressure side compressor of the compressor system according to the example of the present invention. 
           [0033]      FIG. 9C  is a view showing a state in which a relation between the mass flow rate of the compressor system and heads of the low pressure side compressor and the high pressure side compressor according to the example of the present invention is arrange 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]      FIG. 1  is a view showing a schematic configuration of a compressor system according to an embodiment of the present invention.  FIG. 2  is a view for describing a plurality of impellers disposed at a high pressure side compressor according to the embodiment of the present invention.  FIG. 3  is a view for describing the impellers used in a low pressure side compressor and a high pressure side compressor used in the embodiment of the present invention.  FIG. 4A  is a view showing a relation between a flow rate and a head in the low pressure side compressor of the compressor system according to the embodiment of the present invention.  FIG. 4B  is a view showing a relation between a flow rate and a head in the high pressure side compressor of the compressor system according to the embodiment of the present invention.  FIG. 4C  is a view showing a relation between a flow rate and an outlet pressure in the compressor system according to the embodiment of the present invention. 
         [0035]    As shown in  FIG. 1 , a compressor system  10  of the embodiment includes a driving machine  11 , a low pressure side compressor (a first compressor)  12 , a high pressure side compressor (a second compressor)  13 , a variable speed step-up gear  14 , and a constant speed step-up gear  15 . 
         [0036]    The driving machine  11  serially connects two compressors via a step-up gear or a reduction gear and simultaneously drives the two compressors. The driving machine  11  has a first output shaft  11   a  that is rotatably driven, and a second output shaft  11   b  that is rotatably driven to have the same rotational speed as the first output shaft  11   a . The driving machine  11  of the embodiment is an electric motor and always drives the first output shaft  11   a  and the second output shaft  11   b  at a constant speed. The driving machine  11  is disposed coaxially with the first output shaft  11   a  and the second output shaft  11   b . The first output shaft  11   a  is disposed at an opposite side to the second output shaft  11   b  with a main body of the driving machine  11  interposed therebetween. 
         [0037]    The low pressure side compressor  12  is driven by receiving rotation of the first output shaft  11   a  serving as an output shaft of one side (first side) of the driving machine  11 . The low pressure side compressor  12  of the embodiment compresses a working fluid introduced from the outside and supplies the working fluid to an inlet side of the high pressure side compressor  13 . 
         [0038]    The high pressure side compressor  13  is driven by receiving rotation of the second output shaft  11   b  serving as an output shaft of the other side (second side) of the driving machine  11 . The high pressure side compressor  13  compresses a working fluid at a higher pressure than that of the low pressure side compressor  12 . The high pressure side compressor  13  of the embodiment further compresses the working fluid compressed by the low pressure side compressor  12 . The high pressure side compressor  13  supplies a working fluid compressed through two steps to a process side via the low pressure side compressor  12 . 
         [0039]    The variable speed step-up gear  14  increases the rotational speed of the first output shaft  11   a  and transmits the rotational speed to the low pressure side compressor  12 . The variable speed step-up gear  14  can vary the increased rotational speed. The variable speed step-up gear  14  of the embodiment is connected to a rotary shaft  12   a  of the low pressure side compressor  12 . The variable speed step-up gear  14  of the embodiment can vary a speed increasing ratio. For example, provided that the driving machine  11  is operated at a constant speed and the rotational speed according to a rated power output serving as the rotational speed after the rotational speed of the first output shaft  11   a  is increased is 100%, the variable speed step-up gear  14  may vary the rotational speed, for example, from about 105% to 70% and transmit the varied rotational speed to the low pressure side compressor  12 . 
         [0040]    The constant speed step-up gear  15  increases the rotational speed of the second output shaft  11   b  and transmits the increased rotational speed to the high pressure side compressor  13 . The constant speed step-up gear  15  of the embodiment is connected to a rotary shaft  13   a  of the high pressure side compressor  13 . The constant speed step-up gear  15  constantly maintains the increased rotational speed. That is, in the constant speed step-up gear  15  of the embodiment, the speed increasing ratio is set to be constant. For example, the constant speed step-up gear  15  may increase the rotational speed of the first output shaft  11   a  to 100% that is the rotational speed according to the rated power output, and transmit the increased rotational speed to the high pressure side compressor  13 . 
         [0041]    In the low pressure side compressor  12  and the high pressure side compressor  13  of the embodiment, a plurality of impellers  20  are attached in parallel to the rotary shaft  12   a  and the rotary shaft  13   a  connected to the variable speed step-up gear  14  or the constant speed step-up gear  15 . 
         [0042]    Specifically, in the low pressure side compressor  12  and the high pressure side compressor  13 , the plurality of impellers  20  are disposed in parallel at intervals in an axial direction in which the rotary shaft  12   a  and the rotary shaft  13   a  extend and are accommodated in a casing (not shown). For example, the high pressure side compressor  13  of the embodiment has, as shown in  FIG. 2 , six impellers  21 ,  22 ,  23 ,  24 ,  25  and  26  from a first stage impeller  21  disposed at the foremost stage of one side (first side) in an axial direction (a left side of the drawing of  FIG. 2 ), in which the working fluid enters, to a sixth stage impeller  26  disposed at the rearmost stage of the other side (second side) in the axial direction (a right side of the drawing of  FIG. 2 ), in which the working fluid exits. 
         [0043]    As shown in  FIG. 3 , each of the impellers  21 ,  22 ,  23 ,  24 ,  25  and  26  has a disk  30  having a substantially circular plate shape, a plurality of blades  40  attached radially to be erected on a surface of the disk  30  and in parallel in a circumferential direction, and a cover  50  attached to cover the plurality of blades  40  in the circumferential direction. Further, any one or all of the impellers  21 ,  22 ,  23 ,  24 ,  25  and  26  may be an open impeller in which the cover  50  is not provided. 
         [0044]    In addition, the high pressure side compressor  13  operated via the constant speed step-up gear  15  having a constant speed may be provided such that a head performance is flat properties that a variation is slight in a wide operating range with respect to a variation of the flow rate. For example, in the high pressure side compressor  13  of the embodiment, circumferential speeds of the impellers  20  may be minimized as possible. Specifically, in the high pressure side compressor  13  of the embodiment, the rotational speed according to the rated power output increased by the constant speed step-up gear  15  may be adjusted such that the impellers  20  are operated at the circumferential speeds of a Mach number of 0.8 or less. 
         [0045]    As another configuration to obtain a head performance having flat properties in which a variation is slight in a wide operating range with respect to a variation of the flow rate, for example, in the high pressure side compressor  13  of the embodiment, the number of the impellers  20  may be increased as much as possible. Specifically, in the high pressure side compressor  13  of the embodiment, the number of the impellers  20  may be at least six or more with respect to one rotary shaft  13   a.    
         [0046]    In addition, in the compressor system  10  of the embodiment, the head transmitted from the driving machine  11  is not evenly distributed with respect to the low pressure side compressor  12  and the high pressure side compressor  13 , and the head transmitted to the high pressure side compressor  13  rotated at a constant speed may be supplied from the driving machine  11  such that the head is larger than the head transmitted to the low pressure side compressor  12 . In the embodiment, the head transmitted to the high pressure side compressor  13  may occupy 60% or more with respect to the entire head of the compressor system  10  transmitted from the driving machine  11  obtained by summing the head transmitted to the low pressure side compressor  12  and the head transmitted to the high pressure side compressor  13 . 
         [0047]    In the above-mentioned compressor system  10 , as shown in  FIG. 4A , the low pressure side compressor  12  can be operated with a variable rotational speed by varying the speed increasing ratio in the variable speed step-up gear  14 . In the embodiment, the low pressure side compressor  12  is operated at the rotational speed of, for example, 70% to 100% with respect to the rated power output by varying the speed increasing ratio in the variable speed step-up gear  14 . 
         [0048]      FIG. 4A  shows a relation between a flow rate of a working fluid and a head in the low pressure side compressor  12  when the low pressure side compressor  12  is operated at the rotational speed of, for example, 70% to 100% with respect to the rated power output. As shown in  FIG. 4A , when the low pressure side compressor  12  is operated between a line L 1  of 70% and a line L 2  of 100% within a range of a surge line Ls or less, a relation between a flow rate and a head serving as a pressure difference between the inlet side and the outlet side is indicated by a range D 1  having a predetermined region. 
         [0049]    In addition, as shown in  FIG. 4B , since the high pressure side compressor  13  is operated at a constant rotational speed because the speed increasing ratio of the constant speed step-up gear  15  is set to be constant. The high pressure side compressor  13  may be operated at a rotational speed as close to the rated power output as possible in order to maintain a high output (an outlet pressure) in the compressor system  10 . In the embodiment, the high pressure side compressor  13  is operated at a rotational speed of, for example, 100% with respect to the rated power output. 
         [0050]      FIG. 4B  shows a relation between a flow rate of a working fluid and a head in the high pressure side compressor  13  when the high pressure side compressor  13  is operated at the rotational speed of, for example, 100% with respect to the rated power output. As shown in  FIG. 4B , when the high pressure side compressor  13  is operated on a line L 3  of 100% within a range of the surge line Ls or less, a relation between a flow rate and a head serving as a pressure difference between the inlet side and the outlet side is indicated by a range D 2  shown in a linear shape. 
         [0051]    In the compressor system  10 , the working fluid compressed by the low pressure side compressor  12  and the high pressure side compressor  13  is output toward the process side. Here, when the low pressure side compressor  12  is operated within the range D 1  shown in  FIG. 4A  and the high pressure side compressor  13  is operated within the range D 2  shown in  FIG. 4B , a relation between the flow rate and the outlet pressure of the working fluid output from the compressor system  10  is indicated by a range D 3  shown in  FIG. 4C . For this reason, in the compressor system  10 , when the outlet pressure is maintained at a constant pressure P 1 , a line on which the range D 3  and the pressure P 1  overlap each other becomes an operating range D 3   a.    
         [0052]    Here, provisionally, comparison between a case in which the step-up gear of the low pressure side compressor  12  side has a constant speed and a case in which the driving machine  11  has a variable speed is performed. 
       Comparative Example 1 
       [0053]      FIG. 5  is a view showing a schematic configuration of Comparative example 1 of the compressor system according to the embodiment of the present invention.  FIG. 6A  is a view showing a relation between a flow rate and a head in the low pressure side compressor  12  of Comparative example 1 of the compressor system according to the embodiment of the present invention.  FIG. 6B  is a view showing a relation between a flow rate and a head in the high pressure side compressor  13  of Comparative example 1 of the compressor system according to the embodiment of the present invention.  FIG. 6C  is a view showing a relation between a flow rate and an outlet pressure of Comparative example 1 of the compressor system according to the embodiment of the present invention. 
         [0054]    As shown in  FIG. 5  and similar to the configuration of the embodiment shown in  FIG. 1 , a compressor system  1 A serving as Comparative example 1 includes the driving machine  11 , the low pressure side compressor  12 , the high pressure side compressor  13  and the constant speed step-up gear  15 . The compressor system  1 A includes a constant speed step-up gear  4  whose speed increasing ratio is set to be constant and which is disposed between the driving machine  11  and the low pressure side compressor  12 , instead of the variable speed step-up gear  14 . 
         [0055]    In the compressor system  1 A having the above-mentioned configuration, as shown in  FIG. 6A , since the speed increasing ratio of the constant speed step-up gear  4  is set to be constant, the compressor is operated at the constant rotational speed. For example, when the low pressure side compressor  12  is operated at the rotational speed of, for example, 100% with respect to the rated power output within the range of the surge line Ls or less, a relation between a flow rate and a head serving as a pressure difference between the inlet side and the outlet side is indicated by a range D 11  shown in a linear shape on a line L 11  of 100%. 
         [0056]    In addition, as shown in  FIG. 6B , the high pressure side compressor  13  is operated at the constant rotational speed because the speed increasing ratio of the constant speed step-up gear  15  is set to be constant. For example, when the high pressure side compressor  13  is operated at the rotational speed of, for example, the rated power output of 100% within a range of the surge line Ls or less, a relation between a flow rate and a head serving as a pressure difference between the inlet side and the outlet side is indicated by a range D 12  shown in a linear shape on a line L 12  of 100%. 
         [0057]    Here, when the low pressure side compressor  12  is operated within the range D 11  shown in  FIG. 6A  and the high pressure side compressor  13  is operated within the range D 12  shown in  FIG. 6B , a relation between the flow rate and the outlet pressure of the working fluid output from the compressor system  1 A is indicated by a range D 13  shown in  FIG. 6C . 
         [0058]    For this reason, as shown in  FIG. 6C , when the outlet pressure is maintained at the constant pressure P 1 , the compressor system  1 A may be operated at only the operating point D 13   a  at which the range D 13  and the pressure P 1  overlap each other. In this way, in the compressor system  1 A in which the driving machine  11 , the low pressure side compressor  12  and the high pressure side compressor  13  are operated at only the constant rotational speed, when the outlet pressure is maintained at the constant pressure P 1 , the operating range is simply one point of an operating point D 13   a.    
       Comparative Example 2 
       [0059]      FIG. 7  is a view showing a schematic configuration of Comparative example 2 of the compressor system according to the embodiment of the present invention.  FIG. 8A  is a view showing a relation between a flow rate and a head in a low pressure side compressor of Comparative example 2 of the compressor system according to the embodiment of the present invention.  FIG. 8B  is a view showing a relation between a flow rate and a head in a high pressure side compressor of Comparative example 2 of the compressor system according to the embodiment of the present invention.  FIG. 8C  is a view showing a relation between a flow rate and an outlet pressure of Comparative example 2 of the compressor system according to the embodiment of the present invention. 
         [0060]    As shown in  FIG. 7 , a compressor system  1 B as Comparative example 2 includes a variable speed driving machine  2  constituted by a variable speed motor, the low pressure side compressor  12 , the high pressure side compressor  13 , and the constant speed step-up gears  4  and  15  whose speed increasing ratio is set to be constant. 
         [0061]    In the above-mentioned compressor system  1 B, as the rotational speed of the variable speed driving machine  2  constituted by the variable speed motor is varied, the numbers of revolutions of the low pressure side compressor  12  and the high pressure side compressor  13  are synchronized and varied. 
         [0062]    In the compressor system  1 B having the above-mentioned configuration, as shown in  FIG. 8A , the low pressure side compressor  12  can be operated at the rotational speed of, for example, 70% to 100% with respect to the rated power output according to a variation of the rotational speed of the variable speed driving machine  2 .  FIG. 8A  shows a relation between a flow rate of a working fluid and a head in the low pressure side compressor  12  when the low pressure side compressor  12  is operated at the rotational speed of, for example, 70% to 100% with respect to the rated power output within the range of the surge line Ls or less. The relation between the flow rate and the head serving as the pressure difference between the inlet side and the outlet side in the low pressure side compressor  12  is indicated by a range D 21  having a predetermined range between a line L 21  of 70% and a line L 22  of 100%. 
         [0063]    In the meantime, in the compressor system  1 B, as shown in  FIG. 8B , the high pressure side compressor  13  can be operated at the rotational speed of, for example, 70% to 100% with respect to the rated power output within a range of the surge line Ls or less according to a variation in the rotational speed of the variable speed driving machine  2 .  FIG. 8B  shows a relation between a flow rate of a working fluid and a head in the low pressure side compressor  12  when the high pressure side compressor  13  is operated at the rotational speed of, for example, 70% to 100% with respect to the rated power output. The relation between the flow rate and the head serving as the pressure difference between the inlet side and the outlet side in the high pressure side compressor  13  is indicated by a range D 22  having a predetermined region between a line L 31  of 70% and a line L 32  of 100%. 
         [0064]    In the compressor system  1 B, when the low pressure side compressor  12  is operated within the range D 21  shown in  FIG. 8A  and the high pressure side compressor  13  is operated within the range D 22  shown in  FIG. 8B , the relation between the flow rate and the outlet pressure of the working fluid output from the compressor system  1 B is indicated by a range D 23  shown in  FIG. 8C . 
         [0065]    In the above-mentioned compressor system  1 B, when the outlet pressure is maintained at the constant pressure P 1 , a line on which the range D 23  and the pressure P 1  overlap each other is an operating range D 23   a.    
       (Comparison Between Compressor System  10  of Embodiment and Comparative Examples 1 and 2) 
       [0066]    As shown in  FIG. 6C , in the compressor system  1 A of Comparative example 1 in which the driving machine  11  and the constant speed step-up gears  4  and  15  are at a constant speed, when the outlet pressure is maintained at the constant pressure P 1 , the operation can be performed only at the one operating point D 13   a . However, a load occurs due to a circumstance of the process side in which the working fluid supplied from the high pressure side compressor  13  and the low pressure side compressor  12  is used, and operating conditions such as a flow rate or the like vary. When variation occurs, the constant outlet pressure cannot be maintained but deviates from the operating point D 13   a . Accordingly, it is difficult to stably operate the compressor system  1 A while maintaining a constant outlet pressure according to requirements of the process side in the first place. 
         [0067]    In the meantime, as shown in  FIG. 8C , in the compressor system  1 B of Comparative example 2 in which the speed of the variable speed driving machine  2  is varied, when the outlet pressure is maintained at the constant pressure P 1 , the compressor system can be operated within the operating range D 23   a . However, when the rotational speed of the variable speed driving machine  2  is varied to adjust the flow rate, the numbers of revolutions of both of the low pressure side compressor  12  and the high pressure side compressor  13  vary and the outlet pressure varies greatly. In particular, when the rotational speed is reduced to, for example, about 70% with respect to the rated power output, the heads of the low pressure side compressor  12  and the high pressure side compressor  13  are simultaneously decreased. As a result, the outlet pressure of the compressor system  1 B decreases greatly. Accordingly, the operating range D 23   a  in which the compressor system  1 B can be operated while maintaining the outlet pressure at a constant value according to requirements of the process side is narrower than the operating range D 3   a  of the compressor system  10 , which will be described below. 
         [0068]    With respect to Comparative examples 1 and 2, as shown in  FIG. 4C , in the compressor system  10  of the embodiment in which the driving machine  11  has a constant speed and the step-up gear of the low pressure side compressor  12  serves as the variable speed step-up gear  14 , the compressor is operated within the range D 3 . In this configuration, even when the speed increasing ratio of the variable speed step-up gear  14  is varied to adjust the flow rate, the rotational speed of the high pressure side compressor  13  does not vary. For this reason, the high pressure side compressor  13  can compress the working fluid having a small flow rate while maintaining the rotational speed, and the head can be increased. Accordingly, the operating range in which the compressor can be operated while the outlet pressure is held at the constant value according to requirements of the process side may be the operating range D 3   a  that is wider than this. 
         [0069]    Specifically, a case in which the numbers of revolutions of the compressor system  1 B of Comparative example 2 and the compressor system  10  of the embodiment are simply varied in a state in which the systems are operated within the ranges D 23  and D 3  will be exemplarily described. 
         [0070]    As shown in  FIGS. 8A, 8B and 8C , in the compressor system  1 B of Comparative example 2, the operation transitions from the state of the rotational speed of 100% to the state of the rotational speed of 70%, and the operation states of both of the low pressure side compressor  12  and the high pressure side compressor  13  are changed from operating points AL 11  and AH 11  to operating points AL 12  and AH 12  along a process resistance wire R. In this way, in the compressor system  1 B of Comparative example 2, since the head is also decreased in the high pressure side compressor  13  together with the low pressure side compressor  12 , the outlet pressure of the compressor system  1 B decreases greatly. 
         [0071]    On the other hand, in the compressor system  10  of the embodiment, as shown in  FIGS. 4A and 4B , when the operation state is transferred from the state of the rotational speed of 100% to the rotational speed of 70%, the operation state of the low pressure side compressor  12  is changed from an operating point AL 1  to an operating point AL 2  along the process resistance wire R. In the meantime, in the high pressure side compressor  13 , since the rotational speed is maintained at the rotational speed of 100%, the rotational speed changes on the line L 3  of 100%. Specifically, as the rotational speed is varied and the flow rate is decreased in the low pressure side compressor  12 , the rotational speed is transferred from an operating point AH 1  to an operating point AH 2  on the line L 3 , and the head is increased. Accordingly, the outlet pressure of the compressor system  10  can be prevented from greatly decreasing. 
         [0072]    According to the above-mentioned configuration, when the rotational speed of the low pressure side compressor  12  is varied by the variable speed step-up gear  14  while driving the high pressure side compressor  13  at the constant rotational speed and the compressor is operated such that the constant outlet pressure is maintained, the compressor system  10  can be operated within the wide operating range D 3   a . Accordingly, even when the compressor is operated while the outlet pressure is constant, a wider operating range D 3   a  can be secured and a stable operation can be performed. 
         [0073]    In addition, a decrease in outlet pressure of the compressor system  10  can be suppressed and the output can be stably held while widening the operating range by holding the constant rotational speed of the high pressure side compressor  13  in a state in which the rotational speed of the low pressure side compressor  12  is varied. 
         [0074]    Further, as the head by the high pressure side compressor  13  operated via the constant speed step-up gear  15  occupies 60% or more of the entire head, when the flow rate is adjusted by the low pressure side compressor  12 , the variation can be extremely suppressed and the operation can be stably performed. 
         [0075]    In addition, in the high pressure side compressor  13  operated via the constant speed step-up gear  15 , as the circumferential speed of the impellers  20  is a Mach number of 0.8 or less and the number of impellers  20  is six (six stages) or more, a wide operating range that is flatter than that of the low pressure side compressor  12  may be provided. 
       Example 
       [0076]    Hereinafter, while the embodiment of the present invention according to the example has been described in detail, the embodiment of the present invention is not limited by the following description. 
         [0077]      FIG. 9A  is a view showing a relation between a mass flow rate and a head in a low pressure side compressor of a compressor system according to an example of the present invention.  FIG. 9B  is a view showing a relation between a mass flow rate and a head in a high pressure side compressor of the compressor system according to the example of the present invention.  FIG. 9C  is a view showing a state in which a relation between the mass flow rate of the compressor system and heads of the low pressure side compressor and the high pressure side compressor according to the example of the present invention is arranged. 
         [0078]    In the compressor system of the example, as shown in  FIG. 9A , the low pressure side compressor can be operated at the rotational speed of 75% to 105% with respect to the rated power output according to the variation in rotational speed of the driving machine.  FIG. 9A  shows a relation between the mass flow rate of the working fluid and the head in the low pressure side compressor when the low pressure side compressor is operated at the rotational speed of, for example, 75% to 105% with respect to the rated power output within a range of the surge line Ls or less. 
         [0079]    In addition, as shown in  FIG. 9B , the high pressure side compressor is operated at the constant rotational speed because the speed increasing ratio of the constant speed step-up gear is fixed. In the example, the high pressure side compressor is operated at the rotational speed of 100% with respect to the rated power output.  FIG. 9B  shows a relation between the mass flow rate of the working fluid and the head in the high pressure side compressor when the high pressure side compressor is operated at the rotational speed of 100% with respect to the rated power output. 
         [0080]    Here,  FIG. 9C  shows a state in which a relation between the head of the low pressure side compressor and the head of the high pressure side compressor with respect to the mass flow rate is arranged. Specifically,  FIG. 9C  is a view in which  FIG. 9B  that showing the relation between the mass flow rate and the head of the working fluid of the high pressure side compressor is reversed to overlap  FIG. 9A  showing the relation between the mass flow rate and the head of the working fluid of the low pressure side compressor to match the values of the mass flow rate serving as a lateral axis. 
         [0081]    As shown in  FIG. 9C , in the compressor system of the example, for example, provided that the head of the entire compressor system is uniformly 41000 [kg-m/kg] with respect to the mass flow rate while the outlet pressure of the compressor system is at a constant value, an operating line Le on which the constant head of the entire compressor system is held at 41000 [kg-m/kg] is obtained. 
         [0082]    In the above-mentioned compressor system, for example, when the rotational speed of the low pressure side compressor is decreased from 100% to 90%, the head is reduced from about 15000 [kg-m/kg] to about 13000 [kg-m/kg]. Here, the mass flow rate of the compressor system is decreased from about 27000 [kg/h] to about 20000 [kg/h]. Incidentally, as the mass flow rate is decreased to about 20000 [kg/h], the head of the high pressure side compressor is increased from about 26000 [kg-m/kg] to about 28000 [kg-m/kg]. 
         [0083]    That is, the compressor system of the example can increase the head to an extent decreased by the high pressure side compressor even when the rotational speed of the low pressure side compressor is varied to decrease the mass flow rate as the compressor is operated on the operation line Le. Accordingly, in the entire compressor system, a decrease in outlet pressure can be suppressed and the compressor can be operated while maintaining the constant outlet pressure at 41000 [kg-m/kg] while the operating range is widened by varying the rotational speed of the low pressure side compressor. 
         [0084]    Accordingly, in the compressor system of the example, as shown in  FIG. 9C , the operating range can be increased to a wide region of about 15000 [kg/h] to about 28000 [kg/h]. 
         [0085]    In addition, as described in the compressor system of the example, the head of the high pressure side compressor operated while the rotational speed is constant is 60% of the head of the entire compressor system. Specifically, a maximum mass flow rate when the compressor system of the example is operated while the outlet pressure is held at 41000 [kg-m/kg] is about 28000 [kg/h] in the operating range. The head of the high pressure side compressor at this time is about 24600 [kg-m/kg] from  FIG. 9C . Accordingly, the head of the high pressure side compressor operated while the rotational speed is constant occupies 60% of the head of the entire compressor system. 
       OTHER EMBODIMENTS 
       [0086]    Further, the present invention is not limited to the above-mentioned embodiment and design changes may be made without departing from the spirit of the present invention. 
         [0087]    For example, in the embodiment, while the rotational speed of the low pressure side compressor  12  is varied by the variable speed step-up gear  14 , instead of the low pressure side compressor  12 , the variable speed step-up gear  14  may be provided at the high pressure side compressor  13  and the rotational speed of the high pressure side compressor  13  may be varied. 
         [0088]    In addition, in the embodiment, while the variable speed step-up gear  14  is provided at the low pressure side compressor  12  side and the constant speed step-up gear  15  is provided at the high pressure side compressor  13 , at least one of these may not be the step-up gear but may be a reduction gear. 
         [0089]    In addition, in the configuration in which the low pressure side compressor  12  and the high pressure side compressor  13  are driven in the one driving machine  11 , an inlet guide vane (IGV) may be used at an inlet of the compressor in order to vary the rotational speed of only one of the low pressure side compressor  12  and the high pressure side compressor  13 . However, according to the configuration of the embodiment, in comparison with the case in which only the IGV is installed, a wider operating range can be obtained while an operating efficiency is improved in not only the planned operating point but also an operating zone other than the planned operating point. 
       INDUSTRIAL APPLICABILITY 
       [0090]    In a compressor system for driving a first compressor and a second compressor using a driving machine, as a variable speed step-up gear is installed between the first compressor and the driving machine and a constant speed step-up gear is installed between the second compressor and the driving machine, a wider operating condition range can be secured and a stable operation can be performed. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               10  Compressor system 
               11  Driving machine 
               11   a  First output shaft 
               11   b  Second output shaft 
               12  Low pressure side compressor (first compressor) 
               13  High pressure side compressor (second compressor) 
               12   a ,  13   a  Rotary shaft 
               14  Variable speed step-up gear 
               15  Constant speed step-up gear 
               20 ,  21 ,  22 ,  23 ,  24 ,  25 ,  26  Impeller 
               30  Disk 
               40  Blade 
               50  Cover 
               2  Variable speed driving machine