Abstract:
A compressor capable of cooling an electric motor which drives a compressor impeller even in an environment where no external refrigerant is obtained is to be provided. 
     A compressor includes: an electric motor; an impeller which rotates with rotation of the electric motor; a main flow path formed downstream of the impeller, and a bypass flow path branching off from the main flow path; a heat exchange unit which is arranged upstream of the impeller and where gas upstream of the impeller and gas inside the bypass flow path exchange heat; and a pressure reducing device which is arranged further downstream than the heat exchange unit with respect to the bypass flow path and situated inside a flow path communicating with the inside of the electric motor.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an improvement of an electric motor-integrated compressor or a gas extraction system equipped with a compressor. 
       BACKGROUND ART 
       [0002]    Generally, in an electric rotating machine such as an electric motor, the rotor and the stator need to be cooled and a cooling structure for this purpose is provided. As a typical cooling structure, there is a system to radiate heat to the ambient air from an outer surface of the electric motor. There is also a system for cooling by circulating a cooling medium inside the electric motor. The latter case is divided into an open type in which the cooling medium is taken in from outside and discharged outside after cooling, and a total closed type in which, while the cooling medium is circulated inside the electric motor, a heat exchanger is provided at a part of the circulation route so as to cool the cooling medium. In the case of the total closed type, the internal cooling medium is cooled by the ambient air, cooling water or the like. 
         [0003]    The common cooling systems as described above are cooling systems provided on the assumption that a cooling medium with a lower temperature than the allowable temperature of the electric motor is easily available. As a matter of course, since the temperature of the electric motor cannot be cooled to and below the temperature of the cooling medium, it is difficult to operate the electric motor in an environment where there is no low-temperature refrigerant around the place of installation of the electric motor. 
         [0004]    An electric motor which drives a compressor (hereinafter referred to as an in-well gas compressor) installed inside a well (hereinafter referred to as a gas well) in a gas field is used in such a hard environment where there is no low-temperature refrigerant around the place of installation. The in-well gas compressor is installed for the purpose of increasing the amount of production of natural gas or recovering the amount of production in an old gas field where the pressure in the underground gas reservoir has fallen. If the in-well gas compressor is installed at a depth near the gas reservoir, the amount of production is increased more. However, under the ground, as the depth becomes greater, the surrounding temperature becomes higher. Therefore, the depth at which installation is possible is limited by the allowable temperature of the electric motor. Currently, in general, the upper limit of the intake gas temperature of the compressor is approximately 100° C. However, in many cases, the gas temperature near the gas reservoir, where the compressor should be installed originally, reaches about a hundred and several tens degrees and a cooling system for the electric motor that can endure higher-temperature environments is demanded. 
         [0005]    As one of methods for obtaining a low-temperature refrigerant, the use of a heat pump is conceivable. A method using this for cooling an electric motor is disclosed in PTL 1. According to this related-art technique, in a heat pump made up of a compressor, a first heat exchanger, a first pressure reducing valve and a second heat exchanger, the electric motor is cooled using a refrigerant that is provided by taking out a part of a refrigerant cooled by the first heat exchanger, and then reducing the pressure of the refrigerant with a second pressure reducing valve. 
         [0006]    In this related-art technique, the heat pump system exists first, and the refrigerant of the heat pump is utilized to cool the electric motor that drives the compressor impeller of the heat pump. The first heat exchanger radiates heat to an external refrigerant. Meanwhile, the second heat exchanger absorbs heat from another external refrigerant. Such heat exchange of the refrigerant inside the heat pump system with the external refrigerants (air and cooling water) is an ordinary configuration of a water heater or air conditioner utilizing a heat pump. 
       CITATION LIST 
     Patent Literature 
       [0007]    PTL 1: JP-A-2013-127274 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    The foregoing related art is effective in the case where it is applied to an ordinary heat pump system installed on the ground. However, since the in-well gas compressor to which the invention relates is installed deep under the ground, only high-temperature natural gas exists in the surroundings and no external refrigerant is available. 
         [0009]    That is, an object of the invention is to provide a compressor capable of cooling an electric motor which drives a compressor impeller even in an environment where no external refrigerant is obtained, and a gas extraction system using the compressor. 
       Solution to Problem 
       [0010]    In order to solve the foregoing problem, a compressor according to the invention includes: an electric motor; an impeller which rotates with rotation of the electric motor; a main flow path formed downstream of the impeller, and a bypass flow path branching off from the main flow path; a heat exchange unit which is arranged upstream of the impeller and where gas upstream of the impeller and gas inside the bypass flow path exchange heat; and a pressure reducing device which is arranged further downstream than the heat exchange unit with respect to the bypass flow path and situated inside a flow path communicating with the inside of the electric motor. 
         [0011]    Also, in order to solve the foregoing problem, a gas extraction system according to the invention includes: a compressor; a power supply device which is a motive power of the compressor; and a power cable which supplies electric power from the power supply device to the compressor. The compressor is installed inside a gas well which guides gas from an underground gas reservoir to a ground surface. 
       Advantageous Effect of Invention 
       [0012]    According to the invention, it is possible to provide a compressor capable of cooling an electric motor which drives a compressor impeller even in an environment where no external refrigerant is obtained, and a gas extraction system using the compressor. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is an explanatory view of a first example of the compressor according to the invention. 
           [0014]      FIG. 2  is an explanatory view of a second example of the compressor according to the invention. 
           [0015]      FIG. 3  is an explanatory view of a third example of the compressor according to the invention. 
           [0016]      FIG. 4  is an explanatory view of a fourth example of the compressor according to the invention. 
           [0017]      FIG. 5  is an explanatory view of a fifth example of the compressor according to the invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    Hereinafter, examples will be described using the drawings. Needless to say, the examples below are not intended to limit embodiments of the invention and various modifications are possible. In the examples below, a compressor is configured to take out a part of compressed gas downstream of an impeller and to exchange heat with gas on the upstream side of the impeller before compression. 
         [0019]    The gas downstream of the impeller compressed by the compressor increases in pressure and also has a higher temperature than the gas upstream of the impeller due to adiabatic compression. A part of this high-temperature high-pressure gas is guided upstream of the impeller by a bypass flow path, and made to exchange heat with the gas upstream of the impeller via a heat exchange unit. In the heat exchange unit, heat is radiated from the high-temperature gas after compression to the lower-temperature gas before compression. Consequently, the temperature of the gas after compression, split by the bypass flow path, falls. 
         [0020]    Also, the flow rate of the gas split by the bypass flow path may be a minimum rate that is necessary for cooling an electric motor. Therefore, the flow rate of the gas that is split is lower than the flow rate of the gas passing through the impeller (typically, for example, approximately 10 percent or below). Consequently, the split gas can be cooled to around the temperature of the gas upstream of the impeller. 
         [0021]    Since the split gas cooled in the heat exchange unit has high pressure, the gas temperature can be lowered by reducing the pressure of this gas by a pressure reducing device. The range of temperature fall is ideally approximately the same as the range of temperature rise generated at the time of compression by the compressor. Such a configuration enables provision of refrigerant gas with a lower temperature than the ambient environment, that is, the gas temperature upstream of the impeller. The electric motor is cooled with the low-temperature gas that is provided. The gas used for cooling is discharged upstream of the impeller. 
         [0022]    Such double cooling mechanisms are provided, and the generation of the temperature difference of the gas before and after compression is utilized to exchange heat between the same refrigerants, instead of heat exchange with an external refrigerant by an ordinary heat pump system. Also, since only a part of the gas after compression is made to exchange heat with the gas before compression, there is an effect that gas with a lower temperature than the ambient environment is provided. 
         [0023]    It should be noted that this content can be realized by the simple configuration of adding a bypass flow path and a heat exchange unit to the compressor and therefore particularly suitable for installation in a limited space such as a gas well. 
       Example 1 
       [0024]    A first example of the electric rotating machine according to the invention will be described on the basis of  FIG. 1 . 
         [0025]    The compressor in this example has an electric motor  1  as a motor. An impeller  2  is mounted on the rotation axis of the electric motor  1 . This impeller rotates with the rotation of the electric motor. In this example, the impeller  2  is directly connected or integrated with the rotation axis of the electric motor  1 . However, the case where a speed change mechanism is provided between the two is not excluded. Also, while the impeller  2  represents a centrifugal-type example, an axial-flow or diagonal-flow impeller may also be employed. As the impeller  2  rotates, gas  3  upstream of the impeller is pressurized and compressed by the impeller and sent out downstream of the impeller. The pressurized and compressed gas passes through a main flow path  4  and is sent under pressure toward the ground surface. 
         [0026]    In this example, a bypass flow path  5  which extracts a part of the gas from the main flow path  4  downstream of the impeller  2  is provided in the basic configuration of the compressor as described above. The gas split by the bypass flow path  5  is guided to a heat exchange unit  6  installed on the upstream side of the impeller and exchanges heat with the gas  3  on the upstream side of the impeller. The bypass flow path  5  downstream from the heat exchange unit  6  communicates with the electric motor  1 . Also, a pressure reducing device  7  is installed on the downstream side from the heat exchange unit  6  in the bypass flow path  5 . The pressure reducing device  7  is situated inside the flow path communicating with the inside of the electric motor  1 . As the pressure reducing device  7 , for example, a pressure reducing valve or orifice can be used. The gas reduced in pressure by the pressure reducing device  7  drops in temperature due to (adiabatic) expansion. The electric motor  1  is cooled, using this gas with the temperature drop. In this example, the gas is circulated inside the electric motor  1  (specifically, the gap between the rotor and the stator) so as to perform cooling. In view of cooling, the configuration as in this example of directly cooling the inside of the electric motor  1  is effective. However, the outer surface of the electric motor  1  may be cooled. The gas, after cooling the electric motor  1 , is discharged upstream of the impeller. 
         [0027]    According to the configuration as described above, even in an environment where external air, cooling water or the like cannot be obtained, cooling gas with a lower temperature than the surroundings can be provided and the cooling of the electric motor can be enhanced. Also, since a part of the functions of the compressor is used, the cooling can be enhanced by the simple configuration of adding the bypass flow path, the heat exchange unit and the pressure reducing device. 
         [0028]    It should be noted that while this example describes the case of using the bypass flow path, any flow splitting measure that extracts a part of gas from the main flow path  4  may be employed. The extracted split gas is made to exchange heat in the heat exchange unit. 
       Example 2 
       [0029]    A second example of the electric rotating machine according to the invention will be described on the basis of  FIG. 2 . The schematic configuration of this example is substantially the same as Example 1. Explanation of the overlapping parts is omitted. 
         [0030]    In this example, a filtering device  8  is provided inside the bypass flow path  5 . The filtering device can use, for example, a structure including stacked unwoven fabrics, screen meshes or the like. 
         [0031]    It is desirable that the filtering device  8  is installed on the downstream side from the heat exchange unit  6  in the bypass flow path  5 . The reason is as follows. Since pressure loss is generated also in the filtering device, the gas flowing through the bypass flow path  5  is subject to pressure-reducing expansion when passing through the filtering device  8  as well. Therefore, if the filtering device  8  is installed upstream from the heat exchange unit  6 , the gas temperature drops before heat exchange, and heat exchange efficiency falls. Thus, by installing the filtering device  8  downstream of the heat exchange unit  6 , heat exchange can be carried out more effectively in the heat exchange unit  6 . 
         [0032]    As described above, since pressure loss is generated in the filtering device  8 , the pressure reducing function can be achieved partly. Also, the magnitude of the pressure loss can be adjusted, for example, with the number of filters that form the filtering device, or the like. Thus, by adjusting the pressure loss of the filter so as to be precisely equal to the amount of pressure reduction that is necessary in the pressure reducing device  7 , the filtering device can achieve the function of the pressure reducing device as well. In this case, there is an advantage that the pressure reducing device  7  can be omitted. Of course, the use of the two devices together is not excluded. 
       Example 3 
       [0033]    A third example of the electric rotating machine according to the invention will be described on the basis of  FIG. 3 . The schematic configuration of this example is substantially the same as Example 1. Explanation of the parts overlapping with Example 1 is omitted. 
         [0034]    In this example, a cyclone separator  9  is used as a filtering device. The cyclone separator is a kind of filtering device that generates a strong revolving stream, causes a foreign matter with a higher density than the gas to move to the outer peripheral part of the revolving stream, allows only the gas in the center part of the revolving stream to pass, and thus eliminates the foreign matter. Using the cyclone separator as a filtering device has an advantage that the possibility of clogging as in a filtering device that uses ordinary filters is reduced, thus enabling reduction in the time and effort for maintenance. If the pressure loss in the cyclone separator  9  is properly adjusted, it can also be used as the pressure reducing device  7  similarly as described in Example 2. 
       Example 4 
       [0035]    A fourth example of the electric rotating machine according to the invention will be described on the basis of  FIG. 4 . The schematic configuration of this example is substantially the same as Example 1. Explanation of the parts overlapping with Example 1 is omitted. 
         [0036]    In this example, a part of the pipe forming the bypass flow path  5  is wound spirally to form a heat exchange unit  10 . According to such a configuration, the heat exchange function can be provided without separately using a heat exchanger or the like. Therefore, the structure can be simplified and the cost can be reduced. 
         [0037]    It should be noted that, while this example describes the case where the heat exchange unit is formed by the structure in which the pipe is spirally wound on an outer casing  11  covering the outer peripheral side of the impeller, a structure in which the pipe is simply wound spirally without being wound on the outer casing  11  may be employed. However, if the structure in which the pipe is spirally wound on the outer casing is used, as in this example, the structural stability of the pipe is enhanced and the heat transfer area is increased by contact. Therefore, cooling performance can be improved further. Also, while this example describes the structure in which the pipe is wound on the outer peripheral side of the outer casing  11 , the pipe may be spirally formed on the inner side of the outer casing  11  and fixed by welding or brazing to the outer casing  11 . 
       Example 5 
       [0038]    A fifth example of the electric rotating machine according to the invention will be described on the basis of  FIG. 5 . 
         [0039]    This example illustrates a gas extraction system for natural gas having the compressor in which cooling is strengthened, described in the foregoing Examples 1 to 4. A compressor  12  is installed at a depth near an underground gas reservoir  14 , inside a gas well  13  which guides gas from the underground gas reservoir to the ground surface. A power supply device  16  is installed on a ground surface  15 . The power supply device  16  may be a power receiving and transforming facilities if an ordinary power grid is available, and may be a power generator using an engine or gas turbine for motive power if a power grid is unavailable. Also, in some cases, power conversion facilities and control facilities corresponding to the form of the electric motor may be provided as well. The electric power of the electric motor is supplied from the power supply device  16  to the electric motor via a power cable  17 . Using the gas extraction system of the configuration as in this example, the compressor can be installed at a great depth with a higher temperature than in the related art and therefore the amount of production of natural gas can be increased. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  electric motor 
           2  impeller 
           3  gas upstream of impeller 
           4  main flow path of gas downstream of impeller 
           5  bypass flow path 
           6  heat exchange unit 
           7  pressure reducing device 
           8  filtering device 
           9  cyclone separator 
           10  heat exchange unit with spirally formed pipe 
           11  outer casing of impeller 
           12  compressor 
           13  gas well 
           14  gas reservoir 
           15  ground surface 
           16  power supply device 
           17  power cable