Patent Publication Number: US-2011056235-A1

Title: Linear electric compressor and refrigerant circuit

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a linear electric compressor and also to a refrigerant circuit including the linear electric compressor. 
     Japanese Patent No. 3953735 discloses a linear electric compressor which includes a double-headed piston including a piston rod and piston heads at the opposite ends of the piston rod and compression chambers formed at the outer end of each piston head. The linear electric compressor further includes permanent magnets provided at positions corresponding to the center of the piston rod of the double-headed piston and to each piston head thereof and coils provided around the piston rod and each piston head. The linear electric compressor still further includes a pair of springs provided inside the double-headed piston. 
     By supplying electric power periodically to energize the coils of the linear electric compressor of the above patent, periodically variable electromagnetic force is generated around the coils and the permanent magnets of the pistons are attracted toward or repelled from the coils by the electromagnetic force. Accordingly, the pistons reciprocate in cylinder bores. The pistons reciprocate also by resonance of natural frequency of the springs. The reciprocating movement of the pistons causes refrigerant gas to be introduced from a suction chamber to a compression chamber, compressed in the compression chamber and discharged into a discharge chamber. Thus, the linear electric compressor can be electrically controlled to compress refrigerant gas and used for an air conditioner for an electric vehicle and the like. 
     Furthermore, this type of linear electric compressor can compress refrigerant gas twice by a single reciprocating movement of the piston and, therefore, the performance of compressing refrigerant gas per unit time can be improved and the compressor be made small as compared with a linear electric compressor having a compression chamber only at one end of the piston. 
     However, the linear electric compressor of the above patent requires a space in the piston for installing the springs. Therefore, the outer diameter of the piston is increased and the inner diameter of the cylinder bore needs to be designed accordingly. This type of linear electric compressor has limitations on downsizing. 
     The present invention is directed to providing a linear electric compressor that can be made small while ensuring the performance of compressing refrigerant gas per unit time and also a refrigerant circuit having the linear electric compressor. 
     SUMMARY OF THE INVENTION 
     A refrigerant circuit includes a linear electric compressor including a housing with a cylinder bore, a pair of end plates, a valve unit, a piston, an urging device for urging the piston, a coil generating electromagnetic force and a permanent magnet. The permanent magnet cooperates with the urging device and the coil to reciprocate the piston in the cylinder bore. The piston further includes a piston rod and the urging device is provided around the piston rod and a pair of piston heads integrally formed at opposite ends of the piston rod. The diameter of the piston rod is smaller than that of the piston head. The permanent magnet is provided on the piston head and the coil surrounds the piston head. The housing further includes a seat located between the pair of piston heads and the urging device is provided between the seat and each of the piston head. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a cross sectional view of a linear electric compressor according to a first embodiment of the present invention; 
         FIG. 2  is a schematic view of a refrigerant circuit using the linear electric compressor of  FIG. 1 ; 
         FIG. 3  is a partially enlarged cross sectional view of the linear electric compressor of  FIG. 1 ; 
         FIG. 4  is a schematic view showing coils and permanent magnets of the linear electric compressor of  FIG. 1 ; 
         FIG. 5  is a schematic view of a refrigerant circuit according to a second embodiment of the present invention; 
         FIG. 6  is an enlarged cross sectional view showing a flow sensor and tubes to which the flow sensor is mounted in the refrigerant circuit of  FIG. 5 ; 
         FIG. 7  is a schematic view of a refrigerant circuit according to a third embodiment of the present invention; and 
         FIG. 8  is an enlarged cross sectional view showing the flow sensor and tubes to which the flow sensor is mounted in the refrigerant circuit of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following will describe the linear electric compressor and the refrigerant circuit according to the first through third embodiments of the present invention with reference to  FIGS. 1 through 8 . The linear electric compressor  100  according to the first embodiment and the refrigerant circuits  140 ,  200 ,  300  according to the first through the third embodiments, respectively, are employed for an air conditioner for a hybrid vehicle or an electric vehicle. 
     As shown in  FIG. 1 , the linear electric compressor  100  includes a first and a second cylinder blocks  1 ,  3 , a shell  5  and a center housing  7 , which cooperate to form a housing  9  of the linear electric compressor  100 . The first and the second cylinder blocks  1 ,  3  have formed therein a first and a second cylinder bores  1 A,  3 A, respectively. The first and the second cylinder bores  1 A,  3 A are designed so that they are coaxially formed and have the same diameter. 
     The first and the second cylinder blocks  1 ,  3  have a first and a second flanges  1 B,  3 B around the first and the second cylinder bores  1 A,  3 A, respectively, which are housed in the shell  5  so that the first and the second flanges  1 B,  3 B are located at the opposite ends of the shell  5 . The center housing  7  is provided in the shell  5  between the first and the second cylinder blocks  1 ,  3 . The center housing  7  has formed therethrough in axial direction thereof an accommodation hole  7 A which is coaxial with the first and the second cylinder bores  1 A,  3 A and the diameter of which is substantially the same as those of the first and the second cylinder bores  1 A,  3 A. 
     A first and a second end plates  11 ,  13  are joined to the opposite ends of the shell  5  through a first and a second gaskets  10 ,  12 , respectively. The first and the second end plates  11 ,  13  have formed therein recesses, respectively and a first and a second valve plates  15 ,  17  are held between the first gasket  10  and the first end plate  11  and between the second gasket  12  and the second end plate  13 , respectively. A first and a second discharge chambers  11 A,  13 A are formed by the recesses between the first and the second end plates  11 ,  13  and the first and the second valve plates  15 ,  17 , respectively. The first and the second end plates  11 ,  13  have formed therethrough a first and a second discharge ports  11 B,  13 B, respectively. The first and the second discharge chambers  11 A,  13 A are connected to tubes  101 ,  102  shown in  FIG. 2  through the first and the second discharge ports  11 B,  13 B, respectively. The first and the second discharge chambers  11 A,  13 A form the discharge chamber of the present invention. 
     As shown in  FIG. 3 , the first valve plate  15  has formed therethrough a discharge port  15 A. A reed type discharge valve  19  for opening and closing the discharge port  15 A and a retainer  21  for regulating the opening of the discharge valve  19  are fixed by a rivet  23  to the first valve plate  15  on the first discharge port  11 B side. The first valve plate  15 , the discharge valve  19 , the retainer  21  and the rivet  23  cooperate to form a first valve unit  25 . Similarly, a second valve unit is formed on the second valve plate  17  side. The first and the second valve units  25  are provided between the first and the second cylinder bores  1 A,  3 A and the first and the second end plates  11 ,  13 , respectively. 
     As shown in  FIG. 1 , the first and the second cylinder bores  1 A,  3 A and the accommodation hole  7 A receive therein a reciprocally slidable piston  27 . The piston  27  includes a piston rod  29  and a first and a second piston heads  31 ,  33  which are integrally provided with the piston rod  29  at the opposite ends of the piston rod  29 , respectively and slidable in the first and the second cylinder bores  1 A,  3 A, respectively. 
     As shown in  FIGS. 3 and 4 , the first piston head  31  includes a head  39 , on outer surface of which permanent magnets  35 ,  37  are fixed, and a first and a second spacers  41 ,  43  which are integrally provided with the head  39  and form a space between the inner surface of the first cylinder bore  1 A and the outer surface of the permanent magnets  35 ,  37 . 
     The permanent magnets  35 ,  37  are ring shaped and use a rare-earth magnet. North and south poles of the permanent magnet  35  are located on the outer surface thereof and the inner surface thereof, respectively, and, on the other hand, south and north poles of the permanent magnet  37  are located on the outer surface thereof and the inner surface thereof, respectively. The polar character of the permanent magnets  35 ,  37  may be reversed. 
     In installing the permanent magnets  37 ,  35 , firstly the second spacer  43  is press-fit on the head  39 , the permanent magnets  37 ,  35  are press-fit on the outer surface of the head  39 , and then the first spacer  41  is press-fit on the outer surface of the head  39 , as shown in  FIG. 3 . Thus, the permanent magnets  35 ,  37  are held securely on the outer surface of the head  39  between the first and the second spacers  41 ,  43 . A compression chamber  45  is formed in the first cylinder bore  1 A by the space outward of the first spacer  41  of the first piston head  31 . 
     As shown in  FIG. 3 , a suction port  39 A is formed through the head  39  for fluid communication between the inside of the head  39  and the compression chamber  45 . The first spacer  41  has formed therethrough a valve hole  41 A that is communicable with the suction port  39 A, and houses therein a float-type suction valve  47 . The valve hole  41 A has a stop  41 B formed on the side thereof adjacent to the compression chamber  45 . The suction valve  47  is formed in the outer periphery thereof with a plurality of engagement plates  47 A that are brought into contact with the stop  41 B when the suction port  39 A is opened. A cutout  47 B is formed between any two adjacent engagement plates  47 A. 
     As shown in  FIG. 1 , the first and the second piston heads  31 ,  33  are press-fit on the opposite ends of the piston rod  29 . The diameter of the piston rod  29  is smaller than those of the first and the second piston heads  31 ,  33 . The piston rod  29  has formed therethrough in axial direction thereof a suction passage  29 A. The suction passage  29 A includes also radially extending passages in the center of the piston rod  29  so as to open at the outer peripheral surface of the piston rod  29 . As shown in  FIG. 3 , the suction passage  29 A communicates with the suction port  39 A of the first piston head  31 . The suction passage  29 A, the suction port  39 A, the suction valve  47  and the first spacer  41  cooperate to form a suction valve unit  50 . The suction valve unit on the second piston head  33  side has substantially the same structure. 
     As shown in  FIG. 1 , the center housing  7  has formed on the inner peripheral surface and at the center thereof a seat  7 B that protrudes into the accommodation hole  7 A at the center between the opposite outer end surfaces of the first and the second cylinder blocks  1 ,  3 . It can be also said that the seat  7 B is located between the first and the second piston heads  31 ,  33 . The space between the inner peripheral surface of the accommodation hole  7 A and the outer peripheral surface of the piston rod  29  forms a spring chamber  7 C communicating with the suction passage  29 A. The spring chamber  7 C houses therein a first and a second coil springs  49 ,  51  as an urging device for urging the piston  27 . 
     The first coil spring  49  is preloaded with one end thereof in contact with the seat  7 B and the other end thereof in contact with the second spacer  43  of the first piston head  31 . 
     The center housing  7  and the shell  5  forms therebetween an intermediate chamber  53 . The intermediate chamber  53  may be provided in either the center housing  7  or the shell  5 . A communication hole  7 D that connects the intermediate chamber  53  and the spring chamber  7 C is formed in the center housing  7 . The intermediate chamber  53  is communicable with the first and the second cylinder bores  1 A,  3 A through the suction passage  29 A. Combination of the intermediate chamber  53  and the spring chamber  7 C forms a suction chamber  55 . An inlet  5 A is formed through the shell  5 . The suction chamber  55  is connected to a tube  103  shown in  FIG. 2  through the inlet  5 A. A cover  57  is fixed to the shell  5  for closing the intermediate chamber  53 . Terminals (not shown) that are connected to coils  63 A,  63 B,  65 A,  65 B as will be described hereinafter are fixed to the cover  57 . 
     The coils  63 A,  63 B and  65 A,  65 B are provided between the shell  5  and the first and the second cylinder blocks  1 ,  3 , with the coils  63 A,  63 B and  65 A,  65 B held by a first and a second support members  59 ,  61 , respectively. The coils  63 A,  63 B and  65 A,  65 B are disposed so as to surround the first and the second piston heads  31 ,  33 , respectively. The first and the second cylinder blocks  1 ,  3  and the first and the second support members  59 ,  61  are made of a magnetic material. Alternatively, the first and the second cylinder blocks  1 ,  3  may be made of a nonmagnetic material 
     As shown in  FIG. 2 , the tubes  101 ,  102  connect the linear electric compressor  100  to a tube  104 , which is in turn connected to a condenser  105 . The condenser  105  is connected to an expansion valve  107  and an evaporator  108  through a tube  106 . The evaporator  108  is connected to a tube  103 . The terminals in the intermediate chamber  53  for the coils  63 A,  63 B,  65 A,  65 B are connected to a power supply  110  through a lead wire  109 . The power supply  110  is electrically controlled. The above components cooperate to form the refrigerant circuit  140 . 
     The power supply  110  supplies electric power to energize the coils  63 A,  63 B,  65 A,  65 B of the linear electric compressor  100  periodically thereby to generate periodically variable electromagnetic force around the coils  63 A,  63 B,  65 A,  65 B. Referring to  FIG. 4 , when the coil  63 A attracts the permanent magnet  35  of the first piston head  31 , magnetic repulsion is generated between the coil  63 B and the permanent magnet  37  of the first piston head  31 . On the other hand, when magnetic repulsion is generated between the coil  63 A and the permanent magnet  35  of the first piston head  31 , the coil  63 B attracts the permanent magnet  37  of the first piston head  31 . Thus, the piston  27  is caused to reciprocate in the first and the second cylinder bores  1 A,  3 A. The piston  27  also reciprocates by resonance due to natural frequencies of the first and the second coil springs  49 ,  51 . 
     Strokes of suction, compression and discharge of refrigerant gas are accomplished by the reciprocating movement of the piston  27 . The following will describe the operation of the linear electric compressor. The description will focus on the movement of the first piston head  31 . As shown in  FIG. 3 , when the first piston head  31  is in the suction stroke, the pressure in the compression chamber  45  is reduced and, accordingly, the suction valve  47  moves within the valve hole  41 A so as to open the suction port  39 A. Therefore, refrigerant gas in the suction chamber  55  flows from the suction port  39 A into the compression chamber  45  through clearances between the cutouts  47 B of the suction valve  47  and the stop  41 B. Then, the discharge port  15 A is closed by the discharge valve  19 . 
     When the first piston head  31  begins its compression stroke, the suction valve  47  moves within the valve hole  41 A so as to close the suction port  39 A. Accordingly, the pressure in the compression chamber  45  is increased thereby to open the discharge valve  19 . Thus, the first piston head  31  begins its discharge stroke and the compressed refrigerant gas is discharged into the first discharge chamber  11 A through the discharge port  15 A. Though the refrigerant gas in the first discharge chamber  11 A is hot, the first gasket  10  provided between the first end plate  11  and the first cylinder block  1  prevents the piston  27  from being exposed directly to the first discharge chamber  11 A. Therefore, the piston  27  is unsusceptible to the heat of the refrigerant gas in the first discharge chamber  11 A. The same is true of the second piston head  33  side when the second piston head  33  is in the compression stroke. 
     Referring to  FIG. 2 , refrigerant gas flowing out from the evaporator  108  through the tube  103  flows into the compression chamber  45  through the suction chamber  55 . Refrigerant gas is compressed in the compression chamber  45  and then discharged into the first and the second discharge chambers  11 A,  13 A. Refrigerant gas in the first and the second discharge chambers  11 A,  13 A flows out therefrom through the tubes  101 ,  102  to the condenser  105 , the expansion valve  107  and the evaporator  108 . The linear electric compressor  100  which is operable to compress refrigerant gas by electrical control may be used advantageously for air conditioning for an electric vehicle and the like. For example, when the engine of a hybrid vehicle is turned off while the hybrid vehicle is at a stop, comfortable air conditioning can be achieved by the electrically controlled linear electric compressor  100 . 
     The linear electric compressor  100  of the present embodiment can compress refrigerant gas twice by a single reciprocating movement of the piston  27 , thus improving the performance of compressing refrigerant gas per unit time as compared with a linear electric compressor having a compression chamber only at one end of a piston rod. 
     Furthermore, the linear electric compressor  100  includes the first and the second coil springs  49 ,  51  in the center of the double-headed piston  27 . The diameter of the piston rod  29  is smaller than that of the first and the second piston heads  31 ,  33 . The first and the second coil springs  49 ,  51  are provided in the spring chamber  7 C, the diameter of which is substantially the same as that of the first and the second cylinder bores  1 A,  3 A. Therefore, the linear electric compressor  100  dispenses with an urging device in the compression chamber  45  and the compression chamber  45  can be made large. Since the diameter of the first and the second coil springs  49 ,  51  is not larger than that of the first and the second piston heads  31 ,  33 , the inner diameter of the first and the second cylinder bores  1 A,  3 A and the accommodation hole  7 A of the center housing  7  can be designed in accordance with the outer diameter of the first and the second piston heads  31 ,  33 . 
     Therefore, the linear electric compressor  100  can be made smaller while achieving high performance of compressing refrigerant gas per unit time. The refrigerant circuit  140  employing the linear electric compressor  100  can be also made small while maintaining high compression performance. 
     In the linear electric compressor  100  of the present embodiment wherein the permanent magnets  35 ,  37  are provided in the first and the second piston heads  31 ,  33  and the coils  63 A,  63 B and  65 A,  65 B are provided around the first and the second piston heads  31 ,  33 , respectively, the electromagnetic force and the permanent magnets  35 ,  37  operate each other at the opposite ends of the double-headed piston  27 . Therefore, it is hard for the ends of the piston  27  to deflect in radial direction of the piston  27 , which makes it difficult for the first and the second piston heads  31 ,  33  to interfere with the inner surface of the first and the second cylinder bores  1 A,  3 A, respectively. 
     Since the housing  9  of the linear electric compressor  100  includes the first and the second cylinder blocks  1 ,  3  and the shell  5 , it is easy to install the coils  63 A,  63 B and  65 A,  65 B between the shell  5  and the respective first and the second cylinder blocks  1 ,  3 , thus facilitating manufacturing of the linear electric compressor  100 . 
     The intermediate chamber  53  of the linear electric compressor  100  is formed by the shell  5  and the center housing  7  between the first and the second cylinder blocks  1 ,  3 . The first and the second discharge chambers  11 A,  13 A are formed in the first and the second end plates  11 ,  13  by providing the valve units  25 , respectively, and the suction valve units  50  are provided in the first and the second piston heads  31 ,  33 , respectively. Moreover, the spring chamber  7 C and the suction passage  29 A both serving also as a part of the suction chamber  55  are formed in the piston  27 . This structure makes it possible for the linear electric compressor  100  to be made small and light while achieving high performance of compressing refrigerant gas. 
     Now referring to  FIG. 5 , the refrigerant circuit  200  according to the second embodiment is made by modifying a part of the refrigerant circuit  140  ( FIG. 2 ) according to the first embodiment. As shown in  FIG. 5 , the tube  104  of the first embodiment is replaced by a tube  150 . The tube  150  is provided between the condenser  105  and the respective first and the second discharge chambers  11 A,  13 A ( FIG. 1 ). The refrigerant circuit  200  includes a flow sensor  111  as a detecting device and a control device  112 . 
     As shown in  FIG. 6 , a throttle  70  is provided inside the tube  150 . Reference symbols  150 A and  150 B designate first and second positions in the refrigerant circuit  200  that are upstream and downstream of the throttle  70 , respectively, with respect to the flowing direction of refrigerant gas in the tube  150  indicated by arrow. An upstream tube  120  is connected to the first position  150 A and a downstream tube  121  is connected to the second position  150 B, respectively. 
     The flow sensor  111  is provided in the tube  150  for detecting a pressure difference between the first pressure P 1  and the second pressure P 2  of refrigerant gas flowing through the first position  150 A and the second position  150 B, respectively. The flow sensor  111  includes a sensor body  71  and a hall device  73  as a magnetic force detecting device. 
     The sensor body  71  houses a spool  75  that is movable in vertical direction. A moving permanent magnet  77  is fixed to the spool  75 . A spring seat  79  is fixed to lower end of the sensor body  71  and a first spring  81  is provided between the spring seat  79  and the spool  75  for urging the spool  75  upward as viewed in the drawing. A second spring  83  is provided between upper inner surface of the sensor body  71  and the spool  75  for urging the spool  75  downward. 
     The upstream tube  120  is connected to the sensor body  71  at a position that is higher than that of the spool  75  and the downstream tube  121  is connected to the spring seat  79 , as shown in  FIG. 6 . When the second pressure P 2  is higher than the first pressure P 1 , the spool  75  is moved upward against the urging force of the second spring  83  in the sensor body  71 . When the second pressure P 2  is lower than the first pressure P 1 , on the other hand, the spool  75  is moved downward against the urging force of the first spring  81  in the sensor body  71 . 
     The hail device  73  is fixed to top surface of the sensor body  71 . The hall device  73  detects the magnetic flux density that is variable in accordance with the vertical movement of the spool  75  with the moving permanent magnet  77  toward and away from the hall device  73 . As shown in  FIG. 5 , the hall device  73  is electrically connected to the control device  112  through a first control circuit  130 . The hall device  73  generates to the control device  112  a control signal representing the detected magnetic flux density. 
     The control device  112  includes a stroke computing part  113  and a voltage-frequency controlling part  114 . The control device  112  is electrically connected to the power supply  110  through a second control circuit  131 . 
     The stroke computing part  113  of the control device  112  computes the present position of the piston  27  ( FIG. 1 ) based on the control signal received from the hall device  73 , i.e., the flow rate of refrigerant gas flowing through the tube  150 . The position of the piston  27  represents to the state quantity of the linear electric compressor  100 . The state quantity of the linear electric compressor  100  can be a physical quantity influenced by the current position of the piston  27  as indicated above, a pressure or a temperature of refrigerant gas being discharged from the linear electric compressor  100  or flowing thereinto, or a combination of these state quantities. In other words, the physical quantity can be determined indirectly by measuring the flow rate of refrigerant gas flowing through the linear electric compressor  100  or any tube in the refrigerant circuit  200  with the aid of a flow meter. In the case of the present embodiment, the physical quantity should preferably be the pressure difference between the first pressure P 1  in the first position  150 A and the second pressure P 2  in the second position  1508  that is located downstream of the first position  150 A. The stroke computing part  113  determines the current position of the piston  27  by backward calculation of the drive frequency of the piston  27 . The stroke computing part  113  shown in  FIG. 5  generates to the voltage-frequency controlling part  114  a control signal representing the state quantity of the linear electric compressor  100 . 
     The voltage-frequency controlling part  114  of the control device  112  controls the voltage, the current and the current frequency of electric power supplied from the power supply  110  to the linear electric compressor  100 , based on the control signal that is received from the stroke computing part  113 . The voltage-frequency controlling part  114  can control independently the voltage, the current and the current frequency of electric power which is supplied from the power supply  110  to the linear electric compressor  100 , i.e., the voltage, the current and the cycle length of current of electric power which is supplied to coils  63 A,  63 B,  65 A,  65 B. The same reference numerals are used for the common elements or components of the refrigerant circuit  200  and the refrigerant circuit  140  according to the first embodiment, and the description of such elements or components for the second embodiment will be omitted. 
     The power supply  110  in the refrigerant circuit  200  according to the second embodiment supplies electric power to the linear electric compressor  100  based on the state quantity. The state quantity of the linear electric compressor  100  in the refrigerant circuit  200  is determined by detecting the physical quantity influenced by the position of the piston  27  ( FIG. 1 ) in the linear electric compressor  100 . Thus, the electric power supplied to the linear electric compressor  100  is controlled based on the state quantity. Not only when the voltage and the current which are supplied to the coils  63 A,  63 B,  65 A,  65 B, respectively increase but also when the periodic voltage and current are kept constant, the thermal load and the pressure of refrigerant gas in the compression chamber  45  of the linear electric compressor  100  changes. Then, the distance of reciprocating movement of the piston  27 , i.e., the piston stroke, caused by the electromagnetic force generated by the predetermined electric power also changes. For example, when the thermal load is low and the pressure of refrigerant gas in the compression chamber  45  decreases, the first and the second pistons may collide against their corresponding valve units and the collision causes noise and vibration in the linear electric compressor  100 . This may decrease the durability of the linear electric compressor  100 . Therefore, when the thermal load decreases and the pressure of the refrigerant gas in the compression chamber  45  of the linear electric compressor  100  also decreases, the collision of the first and the second piston heads  31 ,  33  against the first and the second valve plates  15 ,  17 , respectively, can be prevented by the refrigerant circuit  200  according to the second embodiment having the above-mentioned control based on the state quantity. Accordingly, the durability of the linear electric compressor  100  in the refrigerant circuit  200  can be also improved. 
     The flow sensor  111  in the refrigerant circuit  200  can detects the pressure difference between the first pressure P 1  and the second pressure P 2  in the tube  150  by detecting the change of the magnetic flux density. Therefore, the flow rate of refrigerant gas flowing through the tube  150  can be detected precisely at a moderate cost. The flow sensor  111  which is provided in the tube  150  away from the linear electric compressor  100  is free from the influence of the magnetic flux generated by the coils  63 A,  63 B,  65 A,  65 B of the linear electric compressor  100 . 
     In the refrigerant circuit  200  according to the second embodiment, the throttle  70  is provided in the tube  150  in which high-pressure refrigerant gas flows. Therefore, pressure loss of refrigerant gas incurred in the throttle  70  does not decrease the performance of the refrigerant circuit  200 . The other advantageous effects are the same as those in the refrigerant circuit according to the first embodiment. 
     Referring to  FIG. 7 , the refrigerant circuit  300  according to the third embodiment differs from the refrigerant circuit  200  of the second embodiment in that the flow sensor  111  is provided in a bend  90  of the tube  103 . Accordingly, in the refrigerant circuit  300  of the third embodiment, the tube  104  that is used in the refrigerant circuit  140  of the first embodiment is used in place of the tube  150  in the refrigerant circuit  200  of the second embodiment. No throttle such as  70  is provided in the refrigerant circuit  300 . 
     Referring to  FIG. 8 , reference symbols  130 A,  130 B designate first and second positions in the tube  103  of the refrigerant circuit  300  that are upstream and downstream of the bend  90 , respectively, with respect to the flowing direction of refrigerant gas in the tube  103  indicated by bent arrow. The upstream tube  120  is connected to the first position  103 A and the downstream tube  121  is connected to the second position  1038 . The same reference numerals are used for the common elements or components of the refrigerant circuits  200  and  300  according to the second and the third embodiments, and the description of such elements or components for the third embodiment will be omitted. 
     The flow sensor  111  in the refrigerant circuit  300  can detect the pressure difference between the first pressure P 1  and the second pressure P 2  of refrigerant gas flowing through the first position  103 A and the second position  103 B, respectively, based on the flow passage resistance caused by the bend  90  through which refrigerant gas flows. By installing the flow sensor  111  to the bend  90  of the tube  103  which is inevitably formed for mounting of the refrigerant circuit  300  to the vehicle, the flow rate of refrigerant gas flowing through the tube  103  is detected easily and efficiently. The bend  90  may not necessarily be formed by bending the tube  103  at almost a right angle. As long as a resistance is generated against the refrigerant gas flowing through the bend  90 , the bend  90  may be formed by bending the tube  103  at any angle other than a right angle. Additionally, the bend  90  may be provided at a position in which high-pressure refrigerant gas flows, e.g., at a position anywhere in the tube  104 . The other advantageous effects are the same as those in the refrigerant circuit  200  according to the second embodiment. 
     The present invention is not limited to the first through third embodiments, but may be modified within the effects of the present invention. 
     The linear electric compressor  100  according to the first embodiment is used alone, but it may be used in combination with any other compressor. This is also applicable to the second and the third embodiments. 
     In the first through third embodiments, the first and the second discharge chambers  11 A,  13 A are formed on the first and the second end plate  11 ,  13  sides, respectively and a suction chamber  55  is formed in the piston  27 . However, suction chambers may be formed on the first and the second end plate  11 ,  13  sides, respectively and a discharge chamber may be formed in the piston  27 . 
     The first and the second spacers  41 ,  43  may be made of fluororesin such as PTFE. In this case, the piston  27  reciprocates suitably in the first and the second cylinder bores  1 A,  3 A. 
     The suction valve unit  50  may be of a reed type. 
     As the detecting device for detecting the piston  27 , any suitable sensor may be used, including a position sensor using laser or magnetic flux, a differential transformer and a proximity switch. 
     A plurality of flow sensors  111  may be provided in the tubes  101 - 104  ( 150 ),  106 . For detecting the state of refrigerant gas flowing through the linear electric compressor  100  and the tubes  101 - 104  ( 105 ),  106  with an increased accuracy, a pressure sensor and a temperature sensor may be used in place of the flow sensor  111 . In this case, the stroke computing part  113  can compute the physical quantity more accurately. 
     The refrigerant circuit according to the present invention may be used for a hybrid vehicle and an electric vehicle using an electric motor. It is also applicable to a vehicle equipped with an engine.