Patent Publication Number: US-2009229301-A1

Title: Refrigeration system

Description:
TECHNICAL FIELD 
     The present invention relates to a refrigeration system including a plurality of compressors connected in parallel. 
     BACKGROUND ART 
     Among conventional refrigeration systems performing refrigeration cycles, there are refrigeration systems having a plurality of compressors connected in parallel so that compressor capacity is varied in a wide range in response to the operating state of a utilization side (e.g., Patent Literature 1). 
     The refrigeration system of Patent Literature 1 includes an indoor unit having an indoor heat exchanger and performing indoor air conditioning, a cold storage unit having a cold-storage heat exchanger and cooling a cold storage showcase, a freezing unit having a freezing heat exchanger and cooling a freezing showcase, and an outdoor unit having an outdoor heat exchanger and three compressors. 
     For cooling only the cold-storage and freezing showcases by the refrigeration system, an inverter compressor and a first non-inverter compressor connected in parallel, which are two of the compressors of the outdoor unit, are operated. In this operation, a refrigerant discharged out of the two compressors is condensed in the outdoor heat exchanger and distributed to the cold storage unit and the freezing unit. The distributed flows of the refrigerant are expanded by expansion valves in the cold storage unit and the freezing unit, respectively, and then evaporate in the corresponding heat exchangers as they absorb heat of the air in the showcases. As a result, the showcases are cooled. Then, the flows of the refrigerant discharged from the cold storage unit and the freezing unit are united and the united flow enters the outdoor unit. After passing through a main suction pipe, the united flow is distributed to suction pipe branches of the compressors to enter the compressors. 
     In the refrigeration system, a discharge pipe in which the flows of the refrigerant discharged from the two compressors are united is provided with an oil separator for separating refrigeration oil from the discharged refrigerant. The refrigeration oil separated by the oil separator is supplied to the main suction pipe through an oil return pipe, and then distributed to the suction pipe branches and supplied to the compressors. 
     Each of the two compressors is provided with an oil equalization pipe connected to part of a casing of the compressor at a certain height at one end and connected to the suction pipe branch of the other compressor at the other end. Each of the oil equalization pipes is provided with a solenoid valve. As the solenoid valves of the oil equalization pipes are opened alternately at predetermined time intervals, the refrigeration oil accumulated in the casing of one of the compressors is supplied to the casing of the other compressor through the oil equalization pipe so as to equalize the amounts of the refrigeration oil in the compressors. 
     Patent Literature 1: Publication of Japanese Patent Application No. 2004-353996 
     DISCLOSURE OF THE INVENTION 
     Problem that the Invention is to Solve 
     According to the refrigeration system of Patent Literature 1, the refrigeration oil separated by the oil separator is merely returned to the main suction pipe. That is, control of the refrigeration oil is not achieved to a sufficient degree. This may lead to a problem of decrease in reliability of the compressor. 
     In view of the foregoing, the present invention has been achieved. With respect to the refrigeration system having a plurality of compressors connected in parallel, an object of the present invention is to control the oil in the compressors in an adequate manner. 
     Means of Solving the Problem 
     According to a first aspect of the present invention, a refrigeration system includes a refrigerant circuit ( 10 ) including a plurality of compressors ( 11   a ,  11   b ,  11   c ) connected in parallel and an oil separator ( 70 ) for separating a refrigeration oil from a refrigerant discharged from the compressors ( 11   a ,  11   b ,  11   c ), the refrigerant circuit ( 10 ) including a main suction pipe ( 55 ) in which a refrigerant to be sucked into the compressors ( 11   a ,  11   b ,  11   c ) flows, suction pipe branches ( 61   a ,  61   b ,  61   c ) for distributing the refrigerant in the main suction pipe ( 55 ) to the compressors ( 11   a ,  11   b ,  11   c ) and an oil return pipe ( 71 ) for returning the refrigeration oil separated by the oil separator ( 70 ) to the main suction pipe ( 55 ), wherein a primary flow biasing element ( 110 ) for biasing the flow of the refrigeration oil is assembled to part of the main suction pipe ( 55 ) downstream of a junction of the main suction pipe ( 55 ) and the oil return pipe ( 71 ) so that a larger amount of the refrigeration oil flows into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) previously chosen from the compressors ( 11   a ,  11   b ,  11   c ). 
     According to the conventional refrigeration system, it is impossible to grasp the amounts of the refrigeration oil returned from the oil separator to the parallel-connected compressors through the main suction pipe. Therefore, in the oil equalizing operation, the unwanted step of opening the solenoid valve of the oil equalization pipe for supplying the refrigeration oil from a casing of the compressor containing enough refrigeration oil to a casing of the compressor lacking the refrigeration oil. Due to the unwanted step, the refrigeration oil is not quickly supplied to the compressor lacking the refrigeration oil. That is, according to the conventional refrigeration system, the unwanted step is performed in addition to the suitable oil equalizing operation to supply the refrigeration oil from the compressor containing a sufficient amount of the refrigeration oil to the compressor containing a small amount of the refrigeration oil through the oil equalization pipe. Therefore, the amount of the refrigeration oil may always be insufficient in a certain compressor. 
     According to a first aspect of the invention, the primary flow biasing element ( 110 ) allows more refrigeration oil to flow into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) so that the largest amount of the refrigeration oil is returned to the first compressor ( 11   a ) among the plurality of compressors ( 11   a ,  11   b ,  11   c ). In this way, the refrigeration oil is surely accumulated in the casing of the first compressor ( 11   a ). The refrigeration oil is supplied from the first compressor ( 11   a ) to the other compressors ( 11   b ,  11   c ) to suitably equalize the amounts of the refrigeration oil in the compressors. 
     According to a second aspect of the invention, a refrigeration system includes a plurality of compressors ( 11   a ,  11   b ,  11   c ) connected in parallel and an oil separator ( 70 ) for separating a refrigeration oil from a refrigerant discharged from the compressors ( 11   a ,  11   b ,  11   c ), the refrigerant circuit ( 10 ) including a main suction pipe ( 55 ) in which a refrigerant to be sucked into the compressors ( 11   a ,  11   b ,  11   c ) flows, suction pipe branches ( 61   a ,  61   b ,  61   c ) for distributing the refrigerant in the main suction pipe ( 55 ) to the compressors ( 11   a ,  11   b ,  11   c ) and an oil return pipe ( 71 ) for returning the refrigeration oil separated by the oil separator ( 70 ) to the main suction pipe ( 55 ), wherein a primary curved portion ( 101 ) and a primary branch element ( 102 ) for branching the main suction pipe ( 55 ) into the suction pipe branches ( 61   a ,  61   b ,  61   c ) are assembled to part of the main suction pipe ( 55 ) downstream of a junction of the main suction pipe ( 55 ) and the oil return pipe ( 71 ), the primary branch element ( 102 ) being located downstream of the primary curved portion ( 101 ), and the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) previously chosen from the compressors ( 11   a ,  11   b ,  11   c ) is at the outermost position in the primary branch element ( 102 ) in the direction of a radius of curvature of the primary curved portion ( 101 ). 
     According to the second aspect of the invention, centrifugal force is exerted on the refrigerant and the refrigeration oil as they pass through the primary curved portion ( 101 ) of the main suction pipe ( 55 ). In part of the main suction pipe ( 55 ) downstream of the primary curved portion ( 101 ), the refrigerant flows in an inside region and the refrigeration oil flows in an outside region in the direction of a radius of curvature of the primary curved portion ( 101 ) due to the difference in centrifugal force exerted on the refrigerant and the refrigeration oil. As the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) is at the outermost position in the primary branch element ( 102 ) in the direction of a radius of curvature of the primary curved portion ( 101 ), the refrigeration oil running in the outside region in the main suction pipe ( 55 ) flows into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ). In this way, a larger amount of the refrigeration oil is returned to the first compressor ( 11   a ) among the plurality of the compressors ( 11   a ,  11   b ,  11   c ). The refrigeration oil is supplied from the first compressor ( 11   a ) to the other compressors ( 11   b ,  11   c ) to suitably equalize the amounts of the refrigeration oil in the compressors. 
     According to a third aspect of the invention, a refrigeration system includes a refrigerant circuit ( 10 ) including a plurality of compressors ( 11   a ,  11   b ,  11   c ) connected in parallel and an oil separator ( 70 ) for separating a refrigeration oil from a refrigerant discharged from the compressors ( 11   a ,  11   b ,  11   c ), the refrigerant circuit ( 10 ) including a main suction pipe ( 55 ) in which a refrigerant to be sucked into the compressors ( 11   a ,  11   b ,  11   c ) flows, suction pipe branches ( 61   a ,  61   b ,  61   c ) for distributing the refrigerant in the main suction pipe ( 55 ) to the compressors ( 11   a ,  11   b ,  11   c ) and an oil return pipe ( 71 ) for returning the refrigeration oil separated by the oil separator ( 70 ) to the main suction pipe ( 55 ), wherein the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) previously chosen from the compressors ( 11   a ,  11   b ,  11   c ) is at the bottommost position in a primary branch element ( 102 ) for branching the main suction pipe ( 55 ) into the suction pipe branches ( 61   a ,  61   b ,  61   c ). 
     According to the third aspect of the invention, the refrigerant flows in an upper region in the main suction pipe ( 55 ) and the refrigeration oil flows in a lower region in the main suction pipe ( 55 ) due to the difference in gravity exerted on the refrigerant and the refrigeration oil. As the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) is at the bottommost position in the primary branch element ( 102 ), the refrigeration oil running in the lower region in the main suction pipe ( 55 ) flows into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ). In this way, a larger amount of the refrigeration oil is returned to the first compressor ( 11   a ) among the plurality of compressors ( 11   a ,  11   b ,  11   c ). The refrigeration oil is supplied from the first compressor ( 11   a ) to the other compressors ( 11   b ,  11   c ) to suitably equalize the amounts of the refrigeration oil in the compressors. 
     According to a fourth aspect of the invention, a refrigeration system includes a refrigerant circuit ( 10 ) including a plurality of compressors ( 11   a ,  11   b ,  11   c ) connected in parallel and an oil separator ( 70 ) for separating a refrigeration oil from a refrigerant discharged from the compressors ( 11   a ,  11   b ,  11   c ), the refrigerant circuit ( 10 ) including a main suction pipe ( 55 ) in which a refrigerant to be sucked into the compressors ( 11   a ,  11   b ,  11   c ) flows, suction pipe branches ( 61   a ,  61   h ,  61   c ) for distributing the refrigerant in the main suction pipe ( 55 ) to the compressors ( 11   a ,  11   b ,  11   c ) and an oil return pipe ( 71 ) for returning the refrigeration oil separated by the oil separator ( 70 ) to the main suction pipe ( 55 ), wherein a primary curved portion ( 101 ) and a primary branch element ( 102 ) for branching the main suction pipe ( 55 ) into the suction pipe branches ( 61   a ,  61   b ,  61   c ) are assembled to part of the main suction pipe ( 55 ) downstream of a junction of the main suction pipe ( 55 ) and the oil return pipe ( 71 ), the primary branch element ( 102 ) being located downstream of the primary curved portion ( 101 ), and the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) previously chosen from the compressors ( 11   a ,  11   b ,  11   c ) is at the bottommost position and the outermost position in the primary branch element ( 102 ) in the direction of a radius of curvature of the primary curved portion ( 101 ). 
     That is, according to the fourth aspect of the present invention related to the second aspect of the invention, the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) is at the bottommost position in the primary branch element ( 102 ). 
     According to the fourth aspect of the invention, the refrigerant and the refrigeration oil running through the main suction pipe ( 55 ) experience gravity and centrifugal force caused in the primary curved portion ( 101 ). In part of the main suction pipe ( 55 ) downstream of the primary curved portion ( 101 ), the refrigerant flows in an upper inside region of the main suction pipe ( 55 ) in the direction of a radius of curvature of the primary curved portion ( 101 ), while the refrigeration oil flows in a lower outside region of the main suction pipe ( 55 ) in the direction of a radius of curvature of the primary curved portion ( 101 ). As the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) is at the bottommost position and the outermost position in the primary branch element ( 102 ) in the direction of a radius of curvature of the primary curved portion ( 101 ), the refrigeration oil running in the lower outside region in the main suction pipe ( 55 ) flows into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ). In this way, a larger amount of the refrigeration oil is returned to the first compressor ( 11   a ) among the plurality of compressors ( 11   a ,  11   b ,  11   c ). The refrigeration oil is supplied from the first compressor ( 11   a ) to the other compressors ( 11   b ,  11   c ) to suitably equalize the amounts of the refrigeration oil in the compressors. 
     According to a fifth aspect of the invention related to the first aspect of the invention, the plurality of compressors ( 11   a ,  11   b ,  11   c ) are first to third compressors ( 11   a ,  11   b ,  11   c ), the main suction pipe ( 55 ) is branched into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) and a connecting suction pipe ( 56 ) which is branched into the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) and the suction pipe branch ( 61   c ) of the third compressor ( 11   c ), and a secondary flow biasing element ( 120 ) for biasing the flow of the refrigeration oil in the connecting suction pipe ( 56 ) is provided so that more refrigeration oil flows into the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) than into the suction pipe branch ( 61   c ) of the third compressor ( 11   c ). 
     According to the fifth aspect of the invention, the secondary flow biasing element ( 120 ) allows the second largest amount of the refrigeration oil returning to the second compressor ( 11   b ) among the three compressors ( 11   a ,  11   b ,  11   c ). In this way, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors ( 11   a ,  11   b ,  11   c ), respectively. The refrigeration oil is supplied from the compressors ( 11   a ,  11   b ) containing a larger amount of the refrigeration oil to the compressors ( 11   b ,  11   c ) containing a smaller amount of the refrigeration oil so as to suitably equalize the amounts of the refrigeration oil in the compressors. 
     According to a sixth aspect of the invention related to any one of the second to fourth aspects of the invention, the plurality of compressors ( 11   a ,  11   b ,  11   c ) are first to third compressors ( 11   a ,  11   b ,  11   c ), the main suction pipe ( 55 ) is branched by the primary branch element ( 102 ) into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) and a connecting suction pipe ( 56 ) which is branched by a secondary branch element ( 104 ) into the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) and the suction pipe branch ( 61   c ) of the third compressor ( 11   c ), the connecting suction pipe ( 56 ) is provided with a secondary curved portion ( 103 ) and the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is positioned outside the suction pipe branch ( 61   c ) of the third compressor ( 11   c ) in the secondary branch element ( 104 ) in the direction of a radius of curvature of the secondary curved portion ( 103 ). 
     According to the sixth aspect of the invention, centrifugal force is exerted on the refrigerant and the refrigeration oil as they pass through the secondary curved portion ( 103 ) of the connecting suction pipe ( 56 ). In part of the connecting suction pipe ( 56 ) downstream of the secondary curved portion ( 103 ), the refrigerant flows in an inside region and the refrigeration oil flows in an outside region in the direction of a radius of curvature of the secondary curved portion ( 103 ) due to the difference in centrifugal force exerted on the refrigerant and the refrigeration oil. As the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is positioned outside the suction pipe branch ( 61   c ) of the third compressor ( 11   c ) in the secondary branch element ( 104 ), more refrigeration oil is returned to the second compressor ( 11   b ) than to the third compressor ( 11   c ) from the connecting suction pipe ( 56 ). In this way, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors ( 11   a ,  11   b ,  11   c ), respectively. The refrigeration oil is supplied from the compressors ( 11   a ,  11   b ) containing a larger amount of the refrigeration oil to the compressors ( 11   b ,  11   c ) containing a smaller amount of the refrigeration oil so as to suitably equalize the amounts of the refrigeration oil in the compressors. 
     According to a seventh aspect of the invention related to any one of the second to fourth aspects of the invention, the plurality of compressors ( 11   a ,  11   b ,  11   c ) are first to third compressors ( 11   a ,  11   b ,  11   c ), the main suction pipe ( 55 ) is branched by the primary branch element ( 102 ) into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) and a connecting suction pipe ( 56 ) which is branched by a secondary branch element ( 104 ) into the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) and the suction pipe branch ( 61   c ) of the third compressor ( 11   c ), the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is positioned lower than the suction pipe branch ( 61   c ) of the third compressor ( 11   c ) in the secondary branch element ( 104 ). 
     According to the seventh aspect of the invention, due to the difference in gravity exerted on the refrigerant and the refrigeration oil as they pass through the connecting suction pipe ( 56 ), the refrigerant flows in an upper region and the refrigeration oil flows in a lower region in the connecting suction pipe ( 56 ). As the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is positioned lower than the suction pipe branch ( 61   c ) of the third compressor ( 11   c ) in the secondary branch element ( 104 ), more refrigeration oil is returned to the second compressor ( 11   b ) than to the third compressor ( 11   c ) from the connecting suction pipe ( 56 ). In this way, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors ( 11   a ,  11   b ,  11   c ), respectively. The refrigeration oil is supplied from the compressors ( 11   a ,  11   b ) containing a larger amount of the refrigeration oil to the compressors ( 11   b ,  11   c ) containing a smaller amount of the refrigeration oil so as to suitably equalize the amounts of the refrigeration oil in the compressors. 
     According to an eighth aspect of the invention related to any one of the second to fourth aspects of the invention, the plurality of compressors ( 11   a ,  11   b ,  11   c ) are first to third compressors ( 11   a ,  11   b ,  11   c ), the main suction pipe ( 55 ) is branched by the primary branch element ( 102 ) into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) and a connecting suction pipe ( 56 ) which is branched by a secondary branch element ( 104 ) into the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) and the suction pipe branch ( 61   c ) of the third compressor ( 11   c ), the connecting suction pipe ( 56 ) is provided with a secondary curved portion ( 103 ) and the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is positioned lower than and outside the suction pipe branch ( 61   c ) of the third compressor ( 11   c ) in the secondary branch element ( 104 ) in the direction of a radius of curvature of the secondary curved portion ( 103 ). 
     That is, according to the eighth aspect of the invention related to the sixth aspect of the invention, the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is positioned lower than the suction pipe branch ( 61   c ) of the third compressor ( 11   c ) in the secondary branch element ( 104 ). 
     According to the eighth aspect of the invention, the refrigerant and the refrigeration oil running through the connecting suction pipe ( 56 ) experience gravity and centrifugal force caused in the secondary curved portion ( 103 ). Therefore, in part of the connecting suction pipe ( 56 ) downstream of the secondary curved portion ( 103 ), the refrigerant flows in an upper inside region and the refrigeration oil flows in a lower outside region in the direction of a radius of curvature of the secondary curved portion ( 103 ). As the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is positioned lower than and outside the suction pipe branch ( 61   c ) of the third compressor ( 11   c ) in the secondary branch element ( 104 ) in the direction of a radius of curvature of the secondary curved portion ( 103 ), more refrigeration oil is returned to the second compressor ( 11   b ) than to the third compressor ( 11   c ) from the connecting suction pipe ( 56 ). In this way, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors ( 11   a ,  11   b ,  11   c ), respectively. The refrigeration oil is supplied from the compressors ( 11   a ,  11   b ) containing a larger amount of the refrigeration oil to the compressors refrigeration oil compressors ( 11   b ,  11   c ) containing a smaller amount of the refrigeration oil so as to suitably equalize the amounts of the refrigeration oil in the compressors. 
     According to a ninth aspect of the invention related to the second or fourth aspect of the invention, the plurality of compressors ( 11   a ,  11   b ,  11   c ) are first to third compressors ( 11   a ,  11   b ,  11   c ), the main suction pipe ( 55 ) is branched by the primary branch element ( 102 ) into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) and a connecting suction pipe ( 56 ) which is branched by a secondary branch element ( 104 ) into the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) and the suction pipe branch ( 61   c ) of the third compressor ( 11   c ), and the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is positioned outside the suction pipe branch ( 61   c ) of the third compressor ( 11   c ) in the secondary branch element ( 104 ) in the direction of a radius of curvature of the primary curved portion ( 101 ) of the main suction pipe ( 55 ). 
     According to the ninth aspect of the invention, the refrigerant flows in an inside region and the refrigeration oil flows in an outside region in the connecting suction pipe ( 56 ) in the direction of a radius of curvature of the primary curved portion ( 101 ) due to the difference in centrifugal force exerted on the refrigerant and the refrigeration oil in the connecting suction pipe ( 56 ) as they pass through the primary curved portion ( 101 ) of the main suction pipe ( 55 ). As the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is positioned outside the suction pipe branch ( 61   c ) of the third compressor ( 11   c ) in the secondary branch element ( 104 ) in the direction of a radius of curvature of the primary curved portion ( 101 ) of the main suction pipe ( 55 ), more refrigeration oil is returned to the second compressor ( 11   b ) than to the third compressor ( 11   c ) from the connecting suction pipe ( 56 ). In this way, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors ( 11   a ,  11   b ,  11   c ), respectively. The refrigeration oil is supplied from the compressors ( 11   a ,  11   b ) containing a larger amount of the refrigeration oil to the compressors refrigeration oil compressors ( 11   b ,  11   c ) containing a smaller amount of the refrigeration oil so as to suitably equalize the amounts of the refrigeration oil in the compressors. 
     According to a tenth aspect of the invention related to any one of the first to ninth aspects of the invention, the refrigeration system further includes oil equalizers ( 72 ,  73 ) for supplying the refrigeration oil accumulated in a casing of the first compressor ( 11   a ) to the other compressors ( 11   b ,  11   c ). 
     According to the tenth aspect of the invention, the oil equalizers ( 72 ,  73 ) supply the refrigeration oil accumulated in the casing of the first compressor ( 11   a ) to the other compressors ( 11   b ,  11   c ) containing a smaller amount of the refrigeration oil than the first compressor ( 11   a ) so as to suitably equalize the amounts of the refrigeration oil in the compressors. 
     According to an eleventh aspect of the invention related to any one of the first to tenth aspects of the invention, the refrigeration system further includes oil equalizers ( 72 ,  73 ,  74 ) for equalizing the amounts of the refrigeration oil accumulated in casings of the compressors ( 11   a ,  11   b ,  11   c ). 
     According to the eleventh aspect of the invention, the oil equalizers ( 72 ,  73 ,  74 ) equalize the amounts of the refrigeration oil accumulated in the casings of the compressors ( 11   a ,  11   b ,  11   c ). 
     According to a twelfth aspect of the invention related to any one of the fifth, eighth and ninth aspects of the invention, according to a twentieth aspect of the invention related to the sixth aspect of the invention, and according to a twenty-first aspect of the invention related to the seventh aspect of the invention, the refrigeration system further includes a first oil equalization pipe ( 72 ) for supplying the refrigeration oil accumulated in a casing of the first compressor ( 11   a ) to the connecting suction pipe ( 56 ) or the suction pipe branch ( 61   b ) of the second compressor ( 11   b ), a second oil equalization pipe ( 73 ) for supplying the refrigeration oil accumulated in a casing of the second compressor ( 11   b ) to the suction pipe branch ( 61   c ) of the third compressor ( 11   c ) and a third oil equalization pipe ( 74 ) for supplying the refrigeration oil accumulated in a casing of the third compressor ( 11   c ) to the main suction pipe ( 55 ) or the oil return pipe ( 71 ). 
     According to the twelfth, twentieth and twenty-first aspects of the invention, the refrigeration oil is supplied from the first compressor ( 11   a ) containing the largest amount of the refrigeration oil to the second compressor ( 11   b ) containing the second largest amount of the refrigeration oil through the first oil equalization pipe ( 72 ). Thus, the to refrigeration oil is surely accumulated in the second compressor ( 11   b ). As the refrigeration oil is surely accumulated in the second compressor ( 11   b ), the refrigeration oil is supplied from the second compressor ( 11   b ) to the third compressor ( 11   c ) containing the smallest amount of the refrigeration oil through the second oil equalization pipe ( 73 ). Thus, the refrigeration oil is surely accumulated in the third compressor ( 11   c ). A surplus of the refrigeration oil in the third compressor ( 11   c ) is returned to the first compressor ( 11   a ). 
     According to a thirteenth aspect of the invention related to any one of the first to twelfth, twentieth and twenty-first aspects of the invention, the first compressor ( 11   a ) is a capacity invariable compressor ( 11   a ). 
     If the first compressor ( 11   a ) is a capacity variable compressor, the amount of the refrigeration oil returned to the first compressor ( 11   a ) varies as the capacity of the first compressor ( 11   a ) varies, even if more refrigeration oil is allowed to flow into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ). Therefore, according to the thirteenth aspect of the invention, the capacity of the first compressor ( 11   a ) is invariable. As a result, more refrigeration oil is surely returned to the first compressor ( 11   a ) than to the other compressors as long as the first compressor ( 11   a ) is working. In a like manner, according to the fifth to ninth aspects of the invention, in the case where one of the second compressor ( 11   b ) and the third compressor ( 11   c ) is a capacity invariable compressor and the other is a capacity variable compressor, the second compressor ( 11   b ) is configured as the capacity invariable compressor. 
     According to a fourteenth aspect of the invention related to any one of the first to thirteenth, twentieth and twenty-first aspects of the invention, each of the compressors ( 11   a ,  11   b ,  11   c ) is so configured that the refrigeration oil is accumulated in high pressure space in the casing. 
     In low-pressure dome compressors, the refrigeration oil is accumulated in low pressure space in the casings of the compressors. Therefore, the casings (oil storing portions) of the compressors may be connected directly by the oil equalization pipe so as to equalize the amounts of the refrigeration oil. In this case, the oil amount equalization among the low-pressure dome compressors is performed in a suitable manner irrespective of the amounts of the refrigeration oil returned to the compressors. 
     In high-pressure dome compressors or high/low-pressure dome compressors, the refrigeration oil is accumulated in high pressure space in the casings of the compressors. Therefore, the amounts of the refrigeration oil returned to the compressors ( 11   a ,  11   b ,  11   c ) are equalized only by supplying the refrigeration oil from the compressors ( 11   a ,  11   b ,  11   c ) to the suction pipe branches ( 61   a ,  61   b ,  61   c ) of the other compressors ( 11   a ,  11   b ,  11   c ). In this case, there is a need of adjusting the amounts of the refrigeration oil returned to the compressors ( 11   a ,  11   b ,  11   c ) for suitable oil amount equalization. Therefore, according to the fourteenth aspect of the invention, the compressors ( 11   a ,  11   b ,  11   c ) are so configured that the refrigeration oil is accumulated in high pressure space in the casings of the compressors ( 11   a ,  11   b ,  11   c ). 
     According to a fifteenth aspect of the invention related to the first to fourteenth, twentieth and twenty-first aspects of the invention, liquid injection pipes ( 86 ,  86   a ,  86   b ,  86   c ) for introducing a portion of a liquid refrigerant flowing in a liquid pipe ( 84 ) on a high pressure side of the refrigerant circuit ( 10 ) to the suction pipe branches ( 61   a ,  61   b ,  61   c ) of the compressors ( 11   a ,  11   b ,  11   c ) are connected to the suction pipe branches ( 61   a ,  61   b ,  61   c ). 
     When the liquid refrigerant is injected into the main suction pipe ( 55 ), the liquid refrigerant is dissolved in the refrigeration oil so that more liquid refrigerant is supplied to the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) than to the suction pipe branches ( 61   b ) of the other compressors ( 11   b ,  11   c ). Therefore, according to the fifteenth aspect of the invention, the liquid refrigerant is injected to the suction pipe branches ( 61   a ,  61   b ,  61   c ) through the liquid injection pipes ( 86 ,  86   a ,  86   b ,  86   c ) in an individual manner. 
     According to a sixteenth aspect of the invention related to the first to fifteenth, twentieth and twenty-first aspects of the invention, oil collecting pipes ( 75 ,  76 ,  77 ) connected to the suction pipe branches ( 61   a ,  61   b ,  61   c ) of the compressors ( 11   a ,  11   b ,  11   c ) at one end, respectively, and connected to each other at the other end. 
     The refrigeration oil separated by the oil separator ( 70 ) is returned to the main suction pipe ( 55 ). Therefore, when any one of the compressors (the compressor ( 11   a )) is stopped, the refrigeration oil is accumulated in the suction pipe branch ( 61   a ) of the stopped compressor ( 11   a ). In particular when the first compressor ( 11   a ) is stopped most frequently among the plurality of compressors, a great amount of the refrigeration oil is accumulated in the suction pipe branch ( 61   a ) of the first compressor ( 11   a ). 
     According to the sixteenth aspect of the invention, the refrigeration oil accumulated in the suction pipe branch ( 61   a ) of the stopped compressor ( 11   a ) is sucked into the working compressors ( 11   b ,  11   c ) through the oil collecting pipes ( 75 ,  76 ,  77 ). As a result, the stopped compressor ( 11   a ) does not suck the great amount of the refrigeration oil in the liquid state upon restart. 
     According to a seventeenth aspect of the invention, a refrigeration system includes a refrigerant circuit ( 10 ) including a plurality of compressors ( 11   a ,  11   b ,  11   c ) connected in parallel and an oil separator ( 70 ) for separating a refrigeration oil from a refrigerant discharged from the compressors ( 11   a ,  11   b ,  11   c ), the refrigerant circuit ( 10 ) including a main suction pipe ( 55 ) in which a refrigerant to be sucked into the compressors ( 11   a ,  11   b ,  11   c ) flows, suction pipe branches ( 61   a ,  61   b ,  61   c ) for distributing the refrigerant in the main suction pipe ( 55 ) to the compressors ( 11   a ,  11   b ,  11   c ) and an oil return pipe ( 71 ) for returning the refrigeration oil separated by the oil separator ( 70 ) to the main suction pipe ( 55 ), wherein oil collecting pipes ( 75 ,  76 ,  77 ) connected to the suction pipe branches ( 61   a ,  61   b ,  61   c ) of the compressors ( 11   a ,  11   b ,  11   c ) at one end, respectively, and connected to each other at the other end are provided. 
     More specifically, the refrigeration oil separated by the oil separator ( 70 ) is returned to the main suction pipe ( 55 ). Therefore, for example, when the compressor ( 11   a ) is stopped, the refrigeration oil and the refrigerant are accumulated in the suction pipe branch ( 61   a ) of the stopped compressor ( 11   a ). 
     According to the seventeenth aspect of the invention, the refrigeration oil accumulated in the suction pipe branch ( 61   a ) of the stopped compressor ( 11   a ) is sucked into the working compressors ( 11   b ,  11   c ) through the oil collecting pipes ( 75 ,  76 ,  77 ). As a result, the stopped compressor ( 11   a ) does not suck the great amount of the refrigeration oil in the liquid state upon restart. 
     According to an eighteenth aspect of the invention related to the sixteenth aspect of the invention, and according to a nineteenth aspect of the invention related to the seventeenth aspect of the invention, each of the suction pipe branches ( 61   a ,  61   b ,  61   c ) has an oblique portion ( 59 ) extending obliquely upward in the downstream direction from a certain position of a barrel of the suction pipe branch ( 61   a ,  61   b ,  61   c ) and an oil storing portion ( 58 ) formed upstream of the oblique portion ( 59 ) and the one ends of the oil collecting pipes ( 75 ,  76 ,  77 ) are connected to the oil storing portions ( 58 ). 
     According to the eighteenth and nineteenth aspects of the invention, the oil storing portion ( 58 ) of the suction pipe branch ( 61   a ,  61   b ,  61   c ) is positioned lower than the oblique portion ( 59 ). Therefore, when the compressors ( 11   a ,  11   b ) are stopped, the refrigeration oil is accumulated in the oil storing portions ( 58 ). The one ends of the oil collecting pipes ( 75 ,  76 ,  77 ) are connected to the oil storing portions ( 58 ) of the suction pipe branches ( 61   a ,  61   b ,  61   c ). Therefore, when one compressor ( 11   a ) is stopped, the refrigeration oil accumulated in the suction pipe branch ( 61   a ) of the stopped compressor ( 11   a ) is surely sucked into the working compressors ( 11   b ,  11   c ) through the oil collecting pipes ( 75 ,  76 ,  77 ). 
     EFFECT OF THE INVENTION 
     According to the first aspect of the invention, the largest amount of the refrigeration oil is returned to the first compressor ( 11   a ) among the plurality of compressors ( 11   a ,  11   b ,  11   c ) by the primary flow biasing element ( 110 ). Therefore, the refrigeration oil is surely accumulated in the casing of the first compressor ( 11   a ) and the refrigeration oil in the first compressor ( 11   a ) is distributed to the other compressors ( 11   b ,  11   c ). Thus, the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) are checked with accuracy, thereby improving the reliability of the compressors ( 11   a ,  11   b ,  11   c ). 
     According to the second aspect of the invention, the largest amount of the refrigeration oil is returned to the first compressor ( 11   a ) by making use of the difference in centrifugal force exerted on the refrigerant and the refrigeration oil as they pass through the primary curved portion ( 101 ) of the main suction pipe ( 55 ). Accordingly, the refrigeration oil is surely accumulated in the casing of the first compressor ( 11   a ) and the refrigeration oil in the first compressor ( 11   a ) is distributed to the other compressors ( 11   b ,  11   c ). Thus, the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) are checked with accuracy, thereby improving the reliability of the compressors ( 11   a ,  11   b ,  11   c ). 
     According to the third aspect of the invention, the largest amount of the refrigeration oil is returned to the first compressor ( 11   a ) by making use of the difference in gravity exerted on the refrigerant and the refrigeration oil as they pass through the main suction pipe ( 55 ). Accordingly, the refrigeration oil is surely accumulated in the casing of the first compressor ( 11   a ) and the refrigeration oil in the first compressor ( 11   a ) is distributed to the other compressors ( 11   b ,  11   c ). Thus, the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) are checked with accuracy, thereby improving the reliability of the compressors ( 11   a ,  11   b ,  11   c ). 
     According to the fourth aspect of the invention, the largest amount of the refrigeration oil is returned to the first compressor ( 11   a ) by making use of the difference in gravity exerted on the refrigerant and the refrigeration oil as they pass through the main suction pipe ( 55 ) and the difference in centrifugal force exerted on the refrigerant and the refrigeration oil as they pass through the primary curved portion ( 101 ) of the main suction pipe ( 55 ). Therefore, the refrigeration oil is surely accumulated in the casing of the first compressor ( 11   a ) and the refrigeration oil in the first compressor ( 11   a ) is distributed to the other compressors ( 11   b ,  11   c ). Thus, the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) are checked with accuracy, thereby improving the reliability of the compressors ( 11   a ,  11   b ,  11   c ). 
     According to the fifth aspect of the invention, the second largest amount of the refrigeration oil is returned to the second compressor ( 11   b ) among the three compressors ( 11   a ,  11   b ,  11   c ) by the secondary flow biasing element ( 120 ). That is, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors ( 11   a ,  11   b ,  11   c ), respectively. Therefore, the refrigeration oil is supplied from the compressors ( 11   a ,  11   b ) containing a larger amount of the refrigeration oil to the compressors ( 11   b ,  11   c ) containing a smaller amount of the refrigeration oil. Thus, the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) are checked with accuracy, thereby improving the reliability of the compressors ( 11   a ,  11   b ,  11   c ). 
     According to the sixth aspect of the invention, the second largest amount of the refrigeration oil is returned to the second compressor ( 11   b ) among the three compressors ( 11   a ,  11   b ,  11   c ) by making use of the difference in centrifugal force exerted on the refrigerant and the refrigeration oil as they pass through the secondary curved portion ( 103 ) of the connecting suction pipe ( 56 ). That is, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors ( 11   a ,  11   b ,  11   c ), respectively. Therefore, the refrigeration oil is supplied from the compressors ( 11   a ,  11   b ) containing a larger amount of the refrigeration oil to the compressors ( 11   b ,  11   c ) containing a smaller amount of the refrigeration oil. Thus, the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) are checked with accuracy, thereby improving the reliability of the compressors ( 11   a ,  11   b ,  11   c ). 
     According to the seventh aspect of the invention, the second largest amount of the refrigeration oil is returned to the second compressor ( 11   b ) among the three compressors ( 11   a ,  11   b ,  11   c ) by making use of the difference in gravity exerted on the refrigerant and the refrigeration oil as they pass through the connecting suction pipe ( 56 ). That is, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors ( 11   a ,  11   b ,  11   c ), respectively. Therefore, the refrigeration oil is supplied from the compressors ( 11   a ,  11   b ) containing a larger amount of the refrigeration oil to the compressors ( 11   b ,  11   c ) containing a smaller amount of the refrigeration oil. Thus, the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) are checked with accuracy, thereby improving the reliability of the compressors ( 11   a ,  11   b ,  11   c ). 
     According to the eighth aspect of the invention, the second largest amount of the refrigeration oil is returned to the second compressor ( 11   b ) among the three compressors ( 11   a ,  11   b ,  11   c ) by making use of the difference in gravity exerted on the refrigerant and the refrigeration oil as they pass through the connecting suction pipe ( 56 ) and the difference in centrifugal force exerted on the refrigerant and the refrigeration oil as they pass through the secondary curved portion ( 103 ). That is, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors ( 11   a ,  11   b ,  11   c ), respectively. Therefore, the refrigeration oil is supplied from the compressors ( 11   a ,  11   b ) containing a larger amount of the refrigeration oil to the compressors ( 11   b ,  11   c ) containing a smaller amount of the refrigeration oil. Thus, the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) are checked with accuracy, thereby improving the reliability of the compressors ( 11   a ,  11   b ,  11   c ). 
     According to the ninth aspect of the invention, the second largest amount of the refrigeration oil is returned to the second compressor ( 11   b ) among the three compressors ( 11   a ,  11   b ,  11   c ) by making use of the difference in centrifugal force exerted on the refrigerant and the refrigeration oil as they pass through the primary curved portion ( 101 ) of the main suction pipe ( 55 ). That is, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors ( 11   a ,  11   b ,  11   c ), respectively. Therefore, the refrigeration oil is supplied from the compressors ( 11   a ,  11   b ) containing a larger amount of the refrigeration oil to the compressors ( 11   b ,  11   c ) containing a smaller amount of the refrigeration oil. Thus, the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) are checked with accuracy, thereby improving the reliability of the compressors ( 11   a ,  11   b ,  11   c ). 
     According to the tenth aspect of the invention, the refrigeration oil accumulated in the casing of the first compressor ( 11   a ) is supplied to the other compressors ( 11   b ,  11   c ) through the oil equalizers ( 72 ,  73 ) so as to suitably equalize the amounts of the refrigeration oil in the compressors. Therefore, lack of the refrigeration oil in the compressors ( 11   a ,  11   b ,  11   c ) is prevented. 
     According to the eleventh aspect of the invention, the oil equalizers ( 72 ,  73 ,  74 ) make it possible to suitably equalize the amounts of the refrigeration oil accumulated in the casings of the compressors ( 11   a ,  11   b ,  11   c ). Therefore, lack of the refrigeration oil in the compressors ( 11   a ,  11   b ,  11   c ) is prevented. 
     According to the twelfth, twentieth and the twenty-first aspects of the invention, the refrigeration oil accumulated in the casing of the first compressor ( 11   a ) is supplied to the second compressor ( 11   b ) and the refrigeration oil accumulated in the casing of the second compressor ( 11   b ) is supplied to the third compressor ( 11   c ) and a surplus of the refrigeration oil in the third compressor ( 11   c ) is returned to the first compressor ( 11   a ). As a result, the refrigeration oil is sequentially supplied to the compressors ( 11   a ,  11   b ) containing a larger amount of the refrigeration oil to the compressors ( 11   b ,  11   c ) containing a smaller amount of the refrigeration oil so as to suitably equalize the amounts of the refrigeration oil. Further, a surplus of the refrigeration oil is circulated in the compressors ( 11   a ,  11   b ,  11   c ). Therefore, the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) are checked with accuracy. 
     According to the thirteenth aspect of the invention, the first compressor ( 11   a ) is a capacity invariable compressor ( 11   a ). Therefore, when the first compressor ( 11   a ) is working, a larger amount of the refrigeration oil is surely returned to the first compressor ( 11   a ). 
     According to the fourteenth aspect of the invention, each of the compressors ( 11   a ,  11   b ,  11   c ) is so configured that the refrigeration oil is accumulated in high pressure space in the casing. Therefore, the effect of improving the reliability due to the suitable equalization of the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) is expressed more significantly. 
     According to the fifteenth aspect of the invention, liquid injection pipes ( 86 ,  86   a ,  86   b ,  86   c ) are connected to the suction pipe branches ( 61   a ,  61   b ,  61   c ) of the compressors ( 11   a ,  11   b ,  11   c ). Therefore, the liquid refrigerant is surely supplied to the suction pipe branches ( 61   a ,  61   b ,  61   c ). Further, the flows of the refrigerant discharged from the compressors ( 11   a ,  11   b ,  11   c ) are surely reduced in temperature, thereby preventing excessive temperature rise in the compressors ( 11   a ,  11   b ,  11   c ). Thus, the compressors ( 11   a ,  11   b ,  11   c ) are further improved in reliability. 
     According to the sixteenth aspect of the invention, the oil collecting pipes ( 75 ,  76 ,  77 ) are provided. Therefore, when a certain compressor ( 11   a ) among the plurality of the compressors ( 11   a ,  11   b ,  11   c ) is stopped, the refrigeration oil accumulated in the suction pipe branch ( 61   a ) of the stopped compressor ( 11   a ) is sucked into the other working compressors ( 11   h ,  11   c ). Therefore, the stopped compressor ( 11   a ) is prevented from sucking a large amount of the refrigeration oil in the liquid state and performing the liquid compression upon restart. As a result, the compressor ( 11   a ) is further improved in reliability. 
     In particular when the first compressor ( 11   a ) is stopped most frequently among the plurality of the compressors, the effect of improving the reliability due to the suitable equalization of the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) is expressed more significantly. 
     According to the seventeenth aspect of the invention, the oil collecting pipes ( 75 ,  76 ,  77 ) are provided. Therefore, when a certain compressor ( 11   a ) among the plurality of the compressors ( 11   a ,  11   b ,  11   c ) is stopped, the refrigeration oil accumulated in the suction pipe branch ( 61   a ) of the stopped compressor ( 11   a ) is sucked into the other working compressors ( 11   b ,  11   c ). Therefore, the stopped compressor ( 11   a ) is prevented from sucking a large amount of the refrigeration oil in the liquid state and performing the liquid compression upon restart. As a result, the compressor ( 11   a ) is further improved in reliability. 
     According to the eighteenth and nineteenth aspects of the invention, the oil collecting pipes ( 75 ,  76 ,  77 ) are connected to the oil storing portions ( 58 ) of the suction pipe branches ( 61   a ,  61   b ,  61   c ). Therefore, the refrigeration oil accumulated in the suction pipe branch ( 61   a ) of the stopped compressor ( 11   a ) is surely sucked into the other working compressors ( 11   b ,  11   c ). 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       [FIG. 1] 
         FIG. 1  is a piping diagram of a refrigerant circuit in a refrigeration system according to Embodiment 1. 
       [FIG. 2] 
         FIG. 2  is a schematic perspective view of refrigerant pipes on a suction side of compressors according to Embodiment 1. 
       [FIG. 3] 
         FIG. 3  is a piping diagram illustrating the direction of refrigerant circulation in the refrigeration system according to Embodiment 1 during a cooling operation. 
       [FIG. 4] 
         FIG. 4  is a schematic perspective view of refrigerant pipes on a suction side of compressors according to Embodiment 2. 
       [FIG. 5] 
         FIG. 5  is a schematic view illustrating refrigerant pipes on a suction side of compressors according to Embodiment 3. 
     
    
    
     EXPLANATION OF REFERENCE NUMERALS 
     
         
         
           
               1  Refrigeration system 
               10  Refrigerant circuit 
               11   a  First compressor 
               11   b  Second compressor 
               11   c  Third compressor 
               55  Main suction pipe 
               56  Connecting suction pipe 
               58  Oil storing portion 
               59  Oblique portion 
               61   a  First suction pipe branch (suction pipe branch) 
               61   b  Second suction pipe branch (suction pipe branch) 
               61   c  Third suction pipe branch (suction pipe branch) 
               70  Oil separator 
               71  Oil return pipe 
               72  First oil equalization pipe 
               73  Second oil equalization pipe 
               74  Third oil equalization pipe 
               84  Fourth liquid pipe (liquid pipe) 
               101  Primary curved portion 
               102  Primary branch element 
               103  Secondary curved portion 
               104  Secondary branch element 
               110  Primary flow biasing element 
               120  Secondary flow biasing element 
           
         
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will now be described with reference to the drawings. 
     Embodiment 1 
     According to Embodiment 1 of the present invention, a refrigeration system ( 1 ) for performing an operation of cooling a cooling chamber includes an outdoor unit ( 2 ), a cold storage unit ( 3 ) and a controller ( 100 ) as shown in  FIG. 1 . 
     In the refrigeration system ( 1 ), the outdoor unit ( 2 ) includes an outdoor circuit ( 20 ) and the cold storage unit ( 3 ) includes a cold-storage circuit ( 30 ). In the refrigeration system ( 1 ), a gas end of the outdoor circuit ( 20 ) is connected to a gas end of the cold-storage circuit ( 30 ) by a gas-end connecting pipe ( 22 ). A liquid end of the outdoor circuit ( 20 ) is connected to a liquid end of the cold-storage circuit ( 30 ) by a liquid-end connecting pipe ( 21 ). In this way, a refrigerant circuit ( 10 ) for performing vapor compression refrigerating cycles is formed. 
     (Outdoor Unit) 
     The outdoor circuit ( 20 ) of the outdoor unit ( 2 ) includes three compressors ( 11   a ,  11   b ,  11   c ), an outdoor heat exchanger ( 13 ), a receiver ( 14 ), a refrigerant heat exchanger ( 50 ), a first expansion valve ( 45 ), a second expansion valve ( 46 ) and a third expansion valve ( 47 ). The outdoor circuit ( 20 ) further includes a four-way switch valve ( 12 ), a liquid-end stop valve ( 53 ) and a gas-end stop valve ( 54 ). In the outdoor circuit ( 20 ), one end of the liquid-end connecting pipe ( 21 ) is connected to the liquid-end stop valve ( 53 ) and one end of the gas-end connecting pipe ( 22 ) is connected to the gas-end stop valve ( 54 ). 
     The three compressors ( 11   a ,  11   b ,  11   c ) are connected in parallel in the refrigerant circuit ( 10 ). Each of the three compressors ( 11   a ,  11   b ,  11   c ) is a high-pressure dome scroll compressor. A first compressor ( 11   a ) and a second compressor ( 11   b ) are capacity invariable compressors, while a third compressor ( 11   c ) is a capacity variable compressor to which power is supplied through an inverter and which is capable of varying the operation capacity by changing the output frequency of the inverter. When the refrigeration system ( 1 ) is working, the third compressor ( 11   c ) is predominantly driven among the three compressors ( 11   a ,  11   b ,  11   c ). Then, in response to the operating state of a utilization side of the refrigeration system ( 1 ), the second compressor ( 11   b ) and the first compressor ( 11   a ) are sequentially driven in this order. 
     To the suction sides of the first to third compressors ( 11   a ,  11   b ,  11   c ), a main suction pipe ( 55 ) is connected via suction pipe branches ( 61   a ,  61   b ,  61   c ). More specifically, the main suction pipe ( 55 ) is connected to the four-way switch valve ( 12 ) at one end and to a primary branch element ( 102 ) at the other end. One end of the first suction pipe branch ( 61   a ) and one end of a connecting suction pipe ( 56 ) are connected to the primary branch element ( 102 ) of the main suction pipe ( 55 ), and the other end of the first suction pipe branch ( 61   a ) is connected to the suction side of the first compressor ( 11   a ). The connecting suction pipe ( 56 ) has a secondary branch element ( 104 ) connected to the other end thereof. To the secondary branch element ( 104 ), one end of a second suction pipe branch ( 61   b ) and one end of a third suction pipe branch ( 61   c ) are connected. The other end of the second suction pipe branch ( 61   b ) is connected to the suction side of the second compressor ( 11   b ). The other end of the third suction pipe branch ( 61   c ) is connected to the suction side of the third compressor ( 11   c ). As a feature of the present invention, the main suction pipe ( 55 ) is provided with a primary flow biasing element ( 110 ) and the connecting suction pipe ( 56 ) is provided with a secondary flow biasing element ( 120 ). Their specific structures are described later in more detail with reference to  FIG. 2 . 
     To the discharge sides of the three compressors ( 11   a ,  11   b ,  11   c ), a main discharge pipe ( 64 ) is connected. More specifically, one end of the main discharge pipe ( 64 ) is connected to the four-way switch valve ( 12 ) and the other end thereof is branched into a first discharge pipe branch ( 64   a ), a second discharge pipe branch ( 64   b ) and a third discharge pipe branch ( 64   c ). The first discharge pipe branch ( 64   a ) is connected to the discharge side of the first compressor ( 11   a ). The second discharge pipe branch ( 64   b ) is connected to the discharge side of the second compressor ( 11   b ). The third discharge pipe branch ( 64   c ) is connected to the discharge side of the third compressor ( 11   c ). The discharge pipe branches ( 64   a ,  64   b ,  64   c ) have check valves (CV- 1 , CV- 2 , CV- 3 ), respectively. Each of the check valves passes only a refrigerant flow traveling from the corresponding compressor ( 11   a ,  11   b ,  11   c ) to the four-way switch valve ( 12 ). 
     The outdoor heat exchanger ( 13 ) is a cross-fin type fin-and-tube heat exchanger which performs heat exchange between the refrigerant and outdoor air. One end of the outdoor heat exchanger ( 13 ) is connected to the four-way switch valve ( 12 ) and the other end thereof is connected to the top of the receiver ( 14 ) via a first liquid pipe ( 81 ). The first liquid pipe ( 81 ) is provided with a check valve (CV- 4 ) which passes only a refrigerant flow traveling from the outdoor heat exchanger ( 13 ) to the receiver ( 14 ). One end of a second liquid pipe ( 82 ) is connected to the bottom of the receiver ( 14 ). 
     The refrigerant heat exchanger ( 50 ) is a plate-shaped heat exchanger and performs heat exchange between refrigerants. The refrigerant heat exchanger ( 50 ) includes a first flow path ( 50   a ) and a second flow path ( 50   b ). One end of the first flow path ( 50   a ) of the refrigerant heat exchanger ( 50 ) is connected to the other end of the second liquid pipe ( 82 ) and the other end of the first flow path ( 50   a ) is connected to one end of a third liquid pipe ( 83 ). The other end of the third liquid pipe ( 83 ) is connected to one end of the liquid-end connecting pipe ( 21 ) by the liquid-end stop valve ( 53 ). The third liquid pipe ( 83 ) is provided with a check valve (CV- 5 ) which passes only a refrigerant flow traveling from the other end of the first flow path ( 50   a ) to the liquid-end stop valve ( 53 ). 
     One end of a fourth liquid pipe ( 84 ) is connected to part of the third liquid pipe ( 83 ) upstream of the check valve (CV- 5 ) and the other end of the fourth liquid pipe ( 84 ) is connected to one end of the second flow path ( 50   b ) of the refrigerant heat exchanger ( 50 ). The fourth liquid pipe ( 84 ) is provided with a second expansion valve ( 46 ). The second expansion valve ( 46 ) is an electronic expansion valve whose degree of opening is adjustable. 
     The other end of the second flow path ( 50   b ) of the refrigerant heat exchanger ( 50 ) is connected to the barrel of the main suction pipe ( 55 ) by a gas injection pipe ( 85 ). The gas injection pipe ( 85 ) functions to inject refrigerant gas to the suction side of the compressor ( 11   a ,  11   b ,  11   c ). 
     One end of a fifth liquid pipe ( 88 ) is connected to part of the third liquid pipe ( 83 ) between the check valve (CV- 5 ) and the liquid-end stop valve ( 53 ). The other end of the fifth liquid pipe ( 88 ) is connected to part of the first liquid pipe ( 81 ) between the check valve (CV- 4 ) and the receiver ( 14 ). The fifth liquid pipe ( 88 ) is provided with a check valve (CV- 6 ) which passes only a refrigerant flow traveling from the one end to the other end of the fifth liquid pipe ( 88 ). 
     One end of a sixth liquid pipe ( 89 ) is connected to part of the fourth liquid pipe ( 84 ) between the one end of the fourth liquid pipe ( 84 ) and the second expansion valve ( 46 ). The other end of the sixth liquid pipe ( 89 ) is connected to part of the first liquid pipe ( 81 ) between the other end of the outdoor heat exchanger ( 13 ) and the check valve (CV- 4 ). The sixth liquid pipe ( 89 ) is provided with a first expansion valve ( 45 ). The first expansion valve ( 45 ) is an electronic expansion valve whose degree of opening is adjustable. 
     One end of a communicating pipe ( 78 ) is connected to part of the first liquid pipe ( 81 ) between the check valve (CV- 4 ) and the junction with the fifth liquid pipe ( 88 ). The other end of the communicating pipe ( 78 ) is connected to the main discharge pipe ( 64 ). The communicating pipe ( 78 ) is provided with a check valve (CV- 7 ) which passes only a refrigerant flow traveling from the receiver ( 14 ) to the main discharge pipe ( 64 ). 
     The four-way switch valve ( 12 ) is configured so that a first port is connected to the main discharge pipe ( 64 ), a second port is connected to the main suction pipe ( 55 ), a third port is connected to the one end of the outdoor heat exchanger ( 13 ) and a fourth port is connected to the gas-end stop valve ( 54 ). The four-way switch valve ( 12 ) is switchable between a first state where the first and third ports communicate with each other and the second and fourth ports communicate with each other (a state depicted by a solid line in  FIG. 1 ) and a second state where the first and fourth ports communicate with each other and the second and third ports communicate with each other (a state depicted by a broken line in  FIG. 1 ). 
     The outdoor circuit ( 20 ) includes the oil separator ( 70 ). As a feature of the present invention, the outdoor circuit ( 20 ) further includes three oil equalization pipes ( 72 ,  73 ,  74 ), liquid injection pipes ( 86 ,  86   a ,  86   b ,  86   c ) and three oil collecting pipes ( 75 ,  76 ,  77 ). 
     The oil separator ( 70 ) is connected to the main discharge pipe ( 64 ) to separate refrigeration oil from the flows of the refrigerant discharged from the compressors ( 11   a ,  11   b ,  11   c ). The oil separator ( 70 ) is connected to part of the main suction pipe ( 55 ) downstream of the junction with the gas injection pipe ( 85 ) by an oil return pipe ( 71 ). The oil return pipe ( 71 ) is provided with a solenoid valve (SV- 1 ). As the solenoid valve (SV- 1 ) is opened, the refrigeration oil separated by the oil separator ( 70 ) is returned to the main suction pipe ( 55 ). 
     The three oil equalization pipes ( 72 ,  73 ,  74 ) are a first oil equalization pipe ( 72 ), a second oil equalization pipe ( 73 ) and a third oil equalization pipe ( 74 ). They function as oil equalizers. One end of the first oil equalization pipe ( 72 ) is connected to part of a casing of the first compressor ( 11   a ) at a certain height and the other end thereof is connected to the connecting suction pipe ( 56 ) and has a solenoid valve (SV- 2 ). One of the second oil equalization pipe ( 73 ) is connected to part of a casing of the second compressor ( 11   b ) at a certain height and the other end thereof is connected to the third suction pipe branch ( 61   c ) by a third liquid injection pipe branch ( 86   c ) to be described later and has a solenoid valve (SV- 3 ). One end of the third oil equalization pipe ( 74 ) is connected to part of a casing of the third compressor ( 11   c ) at a certain height and the other end thereof is connected to the oil return pipe ( 71 ) and has a solenoid valve (SV- 4 ). The first oil equalization pipe ( 72 ) may be connected to the main suction pipe ( 55 ), the second oil equalization pipe ( 73 ) may be connected to the second suction pipe branch ( 61   b ) and the third oil equalization pipe ( 74 ) may directly be connected to the third suction pipe branch ( 61   c ). 
     The liquid injection pipes ( 86 ,  86   a ,  86   b ,  86   c ) include a main liquid injection pipe ( 86 ) and first to third liquid injection pipe branches ( 86   a ,  86   b ,  86   c ). One end of the main liquid injection pipe ( 86 ) is connected to part of the fourth liquid pipe ( 84 ) between the one end of the fourth liquid pipe ( 84 ) and the junction with the sixth liquid pipe ( 89 ). The other end of the main liquid injection pipe ( 86 ) is branched in two for connection with one end of the second liquid injection pipe branch ( 86   b ) and one end of the third liquid injection pipe branch ( 86   c ). The main liquid injection pipe ( 86 ) is provided with a third expansion valve ( 47 ). The third expansion valve ( 47 ) is an electronic expansion valve whose degree of opening is adjustable. One end of the first liquid injection pipe branch ( 86   a ) is connected to the barrel of the second liquid injection pipe branch ( 86   b ). The first to third liquid injection pipe branches ( 86   a ,  86   b ,  86   c ) have capillary tubes ( 87   a ,  87   b ,  87   c ), respectively, and the other ends of them are connected to the suction pipe branches ( 61   a ,  61   b ,  61   c ) of the first to third compressors ( 11   a ,  11   b ,  11   c ), respectively. Accordingly, a liquid refrigerant running through the third liquid pipe ( 83 ) flows into the liquid injection pipe branches ( 86   a ,  86   b ,  86   c ) via the fourth liquid pipe ( 84 ) and the main liquid injection pipe ( 86 ) and is supplied to the suction pipe branches ( 61   a ,  61   b ,  61   c ) of the compressors ( 11   a ,  11   b ,  11   c ). 
     The three oil collecting pipes ( 75 ,  76 ,  77 ) are a first oil collecting pipe ( 75 ), a second oil collecting pipe ( 76 ) and a third oil collecting pipe ( 77 ). One end of the first oil collecting pipe ( 75 ) is connected to part of the first suction pipe branch ( 61   a ) of the first compressor ( 11   a ) between the junction with the first liquid injection pipe branch ( 86   a ) and the other end of the first suction pipe branch ( 61   a ). One end of the second oil collecting pipe ( 76 ) is connected to part of the second suction pipe branch ( 61   b ) of the second compressor ( 11   b ) between the junction with the second liquid injection pipe branch ( 86   b ) and the other end of the second suction pipe branch ( 61   b ). One end of the third oil collecting pipe ( 77 ) is connected to part of the third suction pipe branch ( 61   c ) of the third compressor ( 11   c ) between the junction with the third liquid injection pipe branch ( 86   c ) and the other end of the third suction pipe branch ( 61   c ). The other ends of the oil collecting pipes ( 75 ,  76 ,  77 ) are connected to each other. 
     The outdoor circuit ( 20 ) further includes various sensors and pressure switches ( 95   a ,  95   b ,  95   c ,  95   d ). More specifically, a suction pressure sensor ( 25 ) and a suction temperature sensor ( 24 ) are assembled to the main suction pipe ( 55 ). A discharge pressure sensor ( 23 ) is assembled to the main discharge pipe ( 64 ). Discharge temperature sensors ( 19   a ,  19   b ,  19   c ) are assembled to the discharge pipe branches ( 64   a ,  64   b ,  64   c ), respectively. A temperature sensor ( 51 ) is assembled to part of the third liquid pipe ( 83 ) near the junction with the first flow path ( 50   a ) of the refrigerant heat exchanger ( 50 ). Pressure switches ( 95   a ,  95   b ,  95   c ,  95   d ) are assembled to a pipe between the gas-end stop valve ( 54 ) and the four-way switch valve ( 12 ) and the discharge pipe branches ( 64   a ,  64   b ,  64   c ), respectively. 
     The outdoor unit ( 2 ) further includes an outside temperature sensor ( 13   a ) and an outdoor fan ( 13   f ). Outdoor air is sent to the outdoor heat exchanger ( 13 ) by the outdoor fan ( 131 ). 
     (Refrigerant Pipes on the Suction Side of the Compressors) 
     As a feature of the present invention, the structure of the refrigerant pipes ( 60   a ,  60   b ,  61   a ,  61   b ,  61   c ) on the suction sides of the three compressors ( 11   a ,  11   b ,  11   c ) will be described in more detail with reference to  FIG. 2 . In  FIG. 2 , the third oil equalization pipe ( 74 ), the discharge pipe branches ( 64   a ,  64   b ,  64   c ) and parts of the first and second oil equalization pipes ( 72 ,  73 ) connected to the casings of the compressors ( 11   a ,  11   b ) are omitted. 
     The refrigerant pipes ( 60   a ,  60   b ,  61   a ,  61   b ,  61   c ) on the suction sides of the compressors ( 11   a ,  11   b ,  11   c ) are provided as described above. The main suction pipe ( 55 ) is branched by the primary branch element ( 102 ) into the first suction pipe branch ( 61   a ) and the connecting suction pipe ( 56 ). The connecting suction pipe ( 56 ) is branched by the secondary branch element ( 104 ) into the second suction pipe branch ( 61   b ) and the third suction pipe branch ( 61   c ). 
     Part of the main suction pipe ( 55 ) downstream of the junction with the oil return pipe ( 71 ) extends horizontally and has a primary flow biasing element ( 110 ). The primary flow biasing element ( 1110 ) includes a primary curved portion ( 101 ) and the primary branch element ( 102 ). 
     The primary curved portion ( 101 ) is an elbow joint that connects pipes on the upstream and downstream sides of the primary curved portion ( 101 ) at an angle of 90°. Accordingly, a refrigerant running through the main suction pipe ( 55 ) in the direction from the right back to the front in  FIG. 2  turns at a substantially right angle at the primary curved portion ( 101 ) and flows to the left side. 
     The primary branch element ( 102 ) is a branch joint that divides a refrigerant flow into two flows and has a first branch port ( 102   a ) and a second branch port ( 102   b ). In the primary branch element ( 102 ), the first branch port ( 102   a ) is positioned obliquely below the second branch port ( 102   b ) at an angle of 45° and positioned outside the second branch port ( 102   b ) in the direction of a radius of curvature of the primary curved portion ( 101 ). The first suction pipe branch ( 61   a ) of the first compressor ( 11   a ) is connected to the first branch port ( 102   a ) and the connecting suction pipe ( 56 ) is connected to the second branch port ( 102   b ). Accordingly, in the primary branch element ( 102 ), the first suction pipe branch ( 61   a ) is at the bottommost position and the outermost position in the direction of a radius of curvature of the primary curved portion ( 101 ). 
     One end of the first suction pipe branch ( 61   a ) is connected to the first branch port ( 102   a ) of the primary branch element ( 102 ) and the other end thereof is connected to the first compressor ( 11   a ). More specifically, the first suction pipe branch ( 61   a ) includes a horizontally extending linear oil storing portion ( 58 ) connected to the first branch port ( 102   a ) of the primary branch element ( 102 ) at one end, an oblique portion ( 59 ) connected to the other end of the oil storing portion ( 58 ) at one end and extends obliquely upward in the downstream direction and a vertical portion ( 60 ) extending vertically downward from the top of the oblique portion ( 59 ) and connected to the first compressor ( 11   a ). Further, the first liquid injection pipe branch ( 86   a ) is connected to the top wall of the oil storing portion ( 58 ) of the first suction pipe branch ( 61   a ), and the first oil collecting pipe ( 75 ) is connected to the bottom wall of the oil storing portion ( 58 ) to be located downstream of the first liquid injection pipe branch ( 86   a ). 
     The connecting suction pipe ( 56 ) extends horizontally and has a secondary flow biasing element ( 120 ). The secondary flow biasing element ( 120 ) includes a secondary curved portion ( 103 ) and the secondary branch element ( 104 ). The first oil equalization pipe ( 72 ) is connected to part of the connecting suction pipe ( 56 ) downstream of the secondary curved portion ( 103 ). 
     The secondary curved portion ( 103 ) is an elbow joint that connects pipes on the upstream and downstream sides of the secondary curved portion ( 103 ) at an angle of 90°. Accordingly, a refrigerant flowing from one end of the connecting suction pipe ( 56 ) to the left side in  FIG. 2  turns at a substantially right angle at the secondary curved portion ( 103 ) and flows to the back. 
     The secondary branch element ( 104 ) is a branch joint that divides a refrigerant flow into two flows and has a first branch port ( 104   a ) and a second branch port ( 104   b ). In the secondary branch element ( 104 ), the first branch port ( 104   a ) is positioned obliquely below the second branch port ( 104   b ) at an angle of 45° and positioned outside the second branch port ( 104   b ) in the direction of a radius of curvature of the secondary curved portion ( 103 ). The second suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is connected to the first branch port ( 104   a ) and the third suction pipe branch ( 61   c ) of the third compressor ( 11   c ) is connected to the second branch port ( 1044   b ). 
     One end of the second suction pipe branch ( 61   b ) is connected to the first branch port ( 104   a ) of the secondary branch element ( 104 ) and the other end thereof is connected to the suction side of the second compressor ( 11   b ). More specifically, the second suction pipe branch ( 61   b ) includes a horizontally extending linear oil storing portion ( 58 ) connected to the first branch port ( 104   a ) of the secondary branch element ( 104 ) at one end, an oblique portion ( 59 ) connected to the other end of the oil storing portion ( 58 ) at one end and extends obliquely upward in the downstream direction and a vertical portion ( 60 ) extending vertically downward from the top of the oblique portion ( 59 ) and connected to the second compressor ( 12   a ). Further, the second liquid injection pipe branch ( 86   b ) and the second oil collecting pipe ( 76 ) are connected to the top and bottom walls of the oil storing portion ( 58 ), respectively. 
     One end of the third suction pipe branch ( 61   c ) is connected to the second branch port ( 104   b ) of the secondary branch element ( 104 ) and the other end thereof is connected to the suction side of the third compressor ( 11   c ). The third suction pipe branch ( 61   c ) does not have the oil storing portion ( 58 ) and the oblique portion ( 59 ). The third suction pipe branch ( 61   c ) extends horizontally in the direction from the one end to the other end and is bent vertically downward at the other end. The third liquid injection pipe branch ( 86   c ) and the second oil equalization pipe ( 73 ) combined with each other are connected to the horizontal top wall of the third suction pipe branch ( 61   c ), and the third oil collecting pipe ( 77 ) is connected to the bottom wall of the third liquid injection pipe branch ( 86   c ) to be located downstream of the junction with the third liquid injection pipe branch ( 86   c ) and the second oil equalization pipe ( 73 ). 
     The other ends of the first to third oil collecting pipes ( 75 ,  76 ,  77 ) are connected with each other immediately below the junction of the second suction pipe branch ( 61   b ) with the second oil collecting pipe ( 76 ). 
     (Cold Storage Unit) 
     As shown in  FIG. 1 , a cold-storage circuit ( 30 ) of the cold storage unit ( 3 ) includes two cold-storage heat exchangers ( 16 ,  17 ), two drain pan heaters ( 26 ,  27 ) and two cold-storage expansion valves ( 15   a ,  15   b ). 
     The cold-storage heat exchangers ( 16 ,  17 ) are cross-fin type fin-and-tube heat exchangers, each of which performs heat exchange between the refrigerant and air in the cooling chamber. Each of the cold-storage heat exchangers ( 16 ,  17 ) is connected to one end of the corresponding drain pan heater ( 26 ,  27 ) by the corresponding cold-storage expansion valve ( 15   a ,  15   b ) at one end, and is connected to one end of a corresponding gas-end pipe branch ( 22   a ,  22   b ) at the other end. The other ends of the gas-end pipe branches ( 22   a ,  22   b ) are united with each other and connected to the other end of the gas-end connecting pipe ( 22 ). 
     The cold-storage expansion valves ( 15   a ,  15   b ) are electronic expansion valves whose degree of opening is adjustable. The cold-storage heat exchangers ( 16 ,  17 ) have first refrigerant temperature sensors ( 16   b ,  17   b ) for measuring the temperature at which the refrigerant evaporates, respectively. The cold-storage heat exchangers ( 16 ,  17 ) further have second refrigerant temperature sensors ( 18   a ,  18   b ) assembled to the other ends thereof, respectively. The degree of opening of the cold-storage expansion valves ( 15   a ,  15   b ) is adjustable so that the temperature measured by the second refrigerant temperature sensors ( 18   a ,  18   b ) is higher than the evaporation temperature of the refrigerant measured by the first refrigerant temperature sensors ( 16   b ,  17   b ) by a predetermined temperature value (e.g., 5° C.). 
     The drain pan heaters ( 26 ,  27 ) are assembled to drain pans (not shown) of the cold-storage heat exchangers ( 16 ,  17 ) and heat the drain pans as a high-temperature and high-pressure refrigerant flows through the drain pan heaters so that frost and ice are not formed on the drain pans. The other ends of the drain pan heaters ( 26 ,  27 ) are connected to the one ends of liquid-end pipe branches ( 21   a ,  21   b ), respectively. The other ends of the liquid-end pipe branches ( 21   a ,  21   b ) are combined with each other and connected to the other end of the liquid-end connecting pipe ( 21 ). 
     The cold storage unit ( 3 ) further includes cooling chamber temperature sensors ( 16   a ,  17   a ) and cooling chamber fans ( 16   f ,  17   f ). The air in the cooling chamber is sent to the cold-storage heat exchangers ( 16 ,  17 ) by the cooling chamber fans ( 16   f ,  17   f ). 
     (Controller) 
     The controller ( 100 ) switches the valves (SV- 1 , SV- 2 , SV- 3 , SV- 4 ,  12 ,  46 ,  47 ,  48 ,  15   a ,  15   b ) provided in the refrigerant circuit ( 10 ) and adjusts the degree of opening of the valves. The controller ( 100 ) also drives the compressors ( 11   a ,  11   b ,  11   c ) and the fans ( 13   f ,  16   f ,  17   f ) to control the operation of the refrigeration system ( 1 ). 
     —Operation— 
     Now, the operation of the refrigeration system ( 1 ) according to the present embodiment is explained. 
     The refrigeration system ( 1 ) performs a cooling operation for cooling the air in the cooling chamber to the chosen temperature of 5° C., for example, and temporarily stops the cooling operation to perform a defrosting operation. 
     (Cooling Operation) 
     For the cooling operation, as shown in  FIG. 3 , the controller ( 100 ) puts the four-way switch valve ( 12 ) of the outdoor circuit ( 20 ) into the first state and the first expansion valve ( 45 ) is fully opened. In this state, the first to third compressors ( 11   a ,  11   b ,  11   c ) are driven, and the cold-storage expansion valves ( 15   a ,  15   b ), the second expansion valve ( 46 ) and the third expansion valve ( 47 ) are opened to a suitable degree of opening to circulate the refrigerant in the direction of solid arrows indicated in  FIG. 3 . At the same time, the outdoor fan ( 13   f ) and the cold-storage fans ( 16   f ,  17   f ) are driven. The controller ( 100 ) allows the solenoid valve (SV- 1 ) of the oil return pipe ( 71 ) to open or close as required and controls the solenoid valves of the oil equalization pipes ( 72 ,  73 ,  74 ) so that, for example, the solenoid valve (SV- 2 ) of the first oil equalization pipe ( 72 ), the solenoid valve (SV- 3 ) of the second oil equalization pipe ( 73 ) and the solenoid valve (SV- 4 ) of the third oil equalization pipe ( 74 ) are sequentially opened in this order. 
     The flows of the refrigerant discharged out of the first to third compressors ( 11   a ,  11   b ,  11   c ) run through the discharge pipe branches ( 64   a ,  64   b ,  64   c ), respectively, to flow into the main discharge pipe ( 64 ), and then sent to the outdoor heat exchanger ( 13 ) through the four-way switch valve ( 12 ). In the outdoor heat exchanger ( 13 ), the refrigerant is condensed and liquefied as it dissipates heat to the outdoor air. The liquefied refrigerant enters the first liquid pipe ( 81 ), passes through the receiver ( 14 ) and the second liquid pipe ( 82 ), and then flows into the first flow path ( 50   a ) of the refrigerant heat exchanger ( 50 ). The liquefied refrigerant runs through the first flow path ( 50   a ) and the third liquid pipe ( 83 ), while a portion of which flows into the fourth liquid pipe ( 84 ) as indicated in  FIG. 3  by broken line arrows (a, b). 
     A portion of the refrigerant flowed into the fourth liquid pipe ( 84 ) as indicated by the broken line arrow (a) is reduced in pressure as it passes through the second expansion valve ( 46 ). Then, it flows into the second flow path ( 50   b ) of the refrigerant heat exchanger ( 50 ) and exchanges heat with the liquid refrigerant in the first flow path ( 50   a ) to evaporate, thereby cooling the liquid refrigerant in the first flow path ( 50   a ) to a predetermined low temperature. After being cooled by the branched flow of the refrigerant in the second flow path ( 50   b ) to 15° C., for example, the liquid refrigerant in the first flow path ( 50   a ) passes through the third liquid pipe ( 83 ), the liquid-end stop valve ( 53 ) and the liquid-end connecting pipe ( 21 ) and flows into the cold-storage circuit ( 30 ). The branched flow of the liquid refrigerant in the second flow path ( 50   b ) evaporates and is injected into the main suction pipe ( 55 ) through the gas injection pipe ( 85 ). 
     The other portion of the refrigerant flowed into the fourth liquid pipe ( 84 ) runs through the main liquid injection pipe ( 86 ) as indicated by the broken line arrow (b) and passes through the third expansion valve ( 47 ) opened to a suitable degree. Then, it is distributed into the liquid injection pipe branches ( 86   a ,  86   b ,  86   c ) and supplied the suction pipe branches ( 61   a ,  61   b ,  61   c ) of the compressors ( 11   a ,  11   b ,  11   c ). 
     In the cold-storage circuit ( 30 ), a liquid refrigerant of 15° C. is distributed into the liquid-end pipe branches ( 21   a ,  21   b ). The distributed flows of the refrigerant run through the drain pan heaters ( 26 ,  27 ) to prevent frosting on the drain pans and surely melt frost fell on the drain pans from the cold-storage heat exchangers ( 16 ,  17 ). The liquid refrigerant flows discharged out of the drain pan heaters ( 26 ,  27 ) are reduced in pressure and expanded as they pass through the cold-storage expansion valves ( 15   a ,  15   b ) and enter the cold-storage heat exchangers ( 16 ,  17 ). In each of the cold-storage heat exchangers ( 16 ,  17 ), the refrigerant absorbs heat of the air in the cooling chamber and evaporates at an evaporation temperature of, for example, about −5° C. In this way, in the cold storage unit ( 3 ), the air cooled by the cold-storage heat exchangers ( 16 ,  17 ) is supplied to the cooling chamber to keep the temperature in the cooling chamber to the chosen temperature of 5° C. 
     The flows of refrigerant gas resulting from the evaporation in the cold-storage heat exchangers ( 16 ,  17 ) run through the gas-end pipe branches ( 22   a ,  22   b ) and are united in the gas-end connecting pipe ( 22 ). Then, the united flow of the refrigerant gas runs through the gas-end connecting pipe ( 22 ) and the four-way switch valve ( 12 ) to enter the main suction pipe ( 55 ). The refrigerant flowed through the main suction pipe ( 55 ) is distributed into the first suction pipe branch ( 61   a ) and the connecting suction pipe ( 56 ). The refrigerant flowed into the first suction pipe branch ( 61   a ) is sucked into the first compressor ( 11   a ) and compressed. The refrigerant flowed into the connecting suction pipe ( 56 ) is distributed into the second suction pipe branch ( 61   b ) and the third suction pipe branch ( 61   c ). The refrigerant entered the second suction pipe branch ( 61   b ) is sucked into the second compressor ( 11   b ) and compressed. The refrigerant entered the third suction pipe branch ( 61   c ) is sucked into the third compressor ( 11   c ) and compressed. 
     (Refrigeration Oil Returning Operation) 
     When all the three compressors ( 11   a ,  11   b ,  11   c ) are working for the cooling operation, the refrigeration oil is returned from the main suction pipe ( 55 ) to the first, second and third compressors so that the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors, respectively. The refrigeration oil is supplied from the compressors ( 11   a ,  11   b ) which receive a larger amount of the returned refrigeration oil to the compressors ( 11   b ,  11   c ) which receive a smaller amount of the returned refrigeration oil through the oil equalization pipes ( 72 ,  73 ,  74 ) so as to equalize the amounts of the refrigeration oil in the compressors ( 11   a ,  11   b ,  11   c ). 
     More specifically, as shown in  FIG. 2 , the refrigeration oil separated from the discharged refrigerant by the oil separator ( 70 ) is supplied to the main suction pipe ( 55 ) through the oil return pipe ( 71 ). In part of the main suction pipe ( 55 ) downstream of the oil return pipe ( 71 ), the refrigerant and the refrigeration oil flow in a mixed state. In this state, gravity is exerted on the refrigerant and the refrigeration oil as they flow in the main suction pipe ( 55 ) and centrifugal force is exerted on the refrigerant and the refrigeration oil as they flow in the primary curved portion ( 101 ). Accordingly, in the downstream part of the primary curved portion ( 101 ), the refrigerant flows in an upper inside region in the primary curved portion ( 101 ) in the direction of a radius of curvature of the primary curved portion ( 101 ), and the refrigeration oil flows in a lower outside region in the primary curved portion ( 101 ) in the direction of a radius of curvature of the primary curved portion ( 101 ). Since the first suction pipe branch ( 61   a ) of the first compressor ( 11   a ) is at the bottommost and outermost position in the primary branch element ( 102 ) in the direction of a radius of curvature of the primary curved portion ( 101 ), most of the refrigeration oil in the main suction pipe ( 55 ) flows into the first suction pipe branch ( 61   a ). Further, as the first compressor ( 11   a ) is a capacity invariable compressor, the refrigeration oil flowed into the first suction pipe branch ( 61   a ) is surely sucked into and accumulated in the first compressor ( 11   a ). 
     The refrigerant flowed into the connecting suction pipe ( 56 ) contains a small amount of refrigeration oil. The refrigerant and the refrigeration oil experience gravity as they flow in the connecting suction pipe ( 56 ) and centrifugal force as they flow in the secondary curved portion ( 103 ). Accordingly, in the downstream part of the secondary curved portion ( 103 ), the refrigerant flows in an upper inside region in the secondary curved portion ( 103 ) in the direction of a radius of curvature of the secondary curved portion ( 103 ), and the refrigeration oil flows in a lower outside region in the secondary curved portion ( 103 ) in the direction of a radius of curvature of the secondary curved portion ( 103 ). In the secondary branch element ( 104 ), the second suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is positioned lower than and outside the third suction pipe branch ( 61   c ) of the third compressor ( 11   c ) in the direction of a radius of curvature of the secondary curved portion ( 103 ). Therefore, most of the refrigeration oil in the connecting suction pipe ( 56 ) flows into the second suction pipe branch ( 61   b ). Further, as the second compressor ( 11   b ) is a capacity invariable compressor, the refrigeration oil flowed into the second suction pipe branch ( 61   b ) is surely sucked into and accumulated in the second compressor ( 11   b ). 
     The refrigerant and the rest of the refrigeration oil flow into the third suction pipe branch ( 61   c ) of the third compressor ( 11   c ) and sucked into the third compressor ( 11   c ). In this way, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors, respectively. 
     Then, a liquid refrigerant is injected to the suction pipe branches ( 61   a ,  61   b ,  61   c ) through the liquid injection pipe branches ( 86   a ,  86   b ,  86   c ), respectively. If the liquid refrigerant is injected into the main suction pipe ( 55 ) or the connecting suction pipe ( 56 ), the liquid refrigerant is dissolved in the refrigeration oil and the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors, respectively. However, as long as the refrigerant is injected to the suction pipe branches ( 61   a ,  61   b ,  61   c ), respectively, the flows of the refrigerant discharged from the compressors ( 11   a ,  11   b ,  11   c ) are surely reduced in temperature, thereby preventing excessive temperature rise in the compressors ( 11   a ,  11   b ,  11   c ). 
     As described above, the controller ( 100 ) controls the solenoid valves of the oil equalization pipes ( 72 ,  73 ,  74 ) to open, for example, in the order of the solenoid valve (SV- 2 ) of the first oil equalization pipe ( 72 ), the solenoid valve (SV- 3 ) of the second oil equalization pipe ( 73 ), and the solenoid valve (SV- 4 ) of the third oil equalization pipe ( 74 ). 
     First, the solenoid valve (SV- 2 ) of the first oil equalization pipe ( 72 ) is opened to supply the refrigeration oil accumulated in the casing of the first compressor ( 11   a ) to the connecting suction pipe ( 56 ) through the first oil equalization pipe ( 72 ). The refrigeration oil supplied to the connecting suction pipe ( 56 ) runs through a lower region in the connecting suction pipe ( 56 ) due to the difference in weight between the refrigerant and the refrigeration oil. As a result, most of the refrigeration oil flows into the second suction pipe branch ( 61   b ). In this way, the refrigeration oil is supplied from the first compressor ( 11   a ) to the second compressor ( 11   b ) through the first oil equalization pipe ( 72 ) and surely accumulated in the second compressor ( 11   b ). 
     The first oil equalization pipe ( 72 ) may be connected to part of the connecting suction pipe ( 56 ) upstream of the secondary curved portion ( 103 ). In this case, most of the refrigeration oil supplied to the connecting suction pipe ( 56 ) through the first oil equalization pipe ( 72 ) flows into the second suction pipe branch ( 61   b ) due to the gravity and the centrifugal force exerted thereon in the secondary curved portion ( 103 ). Alternatively, the first oil equalization pipe ( 72 ) may be connected not to the connecting suction pipe ( 56 ) but to the second suction pipe branch ( 61   b ). 
     With a large amount of the refrigeration oil accumulated in the casing of the second compressor ( 11   b ), the solenoid valve (SV- 3 ) of the second oil equalization pipe ( 73 ) is opened. As a result, the refrigeration oil accumulated in the casing of the second compressor ( 11   b ) is supplied to the third suction pipe branch ( 61   c ) through the second oil equalization pipe ( 73 ) and flows into the third compressor ( 11   c ). In this way, the refrigeration oil is surely accumulated in the third compressor ( 11   c ). 
     With a large amount of the refrigeration oil accumulated in the casing of the third compressor ( 11   c ), the solenoid valve (SV- 4 ) of the third oil equalization pipe ( 74 ) is opened. As a result, a surplus of the refrigeration oil in the third compressor ( 11   c ) is supplied to the oil return pipe ( 71 ) through the third oil equalization pipe ( 74 ) and returned to the first compressor ( 11   a ) through the main suction pipe ( 55 ). 
     The operation of the first compressor ( 11   a ) may be stopped depending on the operating state of a utilization side (cooling load). In this case, the refrigeration oil and the liquid refrigerant injected through the liquid injection pipes ( 86 ,  86   a ) remain in the oil storing portion ( 58 ) of the suction pipe branch ( 61   a ) of the first compressor ( 11   a ). As the second and third compressors ( 11   b ,  11   c ) are working, the refrigeration oil and the liquid refrigerant remaining in the oil storing portion ( 58 ) of the first suction pipe branch ( 61   a ) are introduced to the suction pipe branches ( 61   b ,  61   c ) of the second and third compressors ( 11   b ,  11   c ) through the oil collecting pipes ( 75 ,  76 ,  77 ) and sucked into the second and third compressors ( 11   b ,  11   c ). Therefore, the stopped first compressor ( 11   a ) does not suck a large amount of the refrigeration oil in the liquid state upon restart. That is, there is no possibility that the compressor ( 11   a ) performs liquid compression immediately upon restart. 
     Even when the second compressor ( 11   b ) is stopped with the first compressor ( 11   a ) being stopped, the refrigeration oil and the liquid refrigerant remaining in the oil storing portions ( 58 ) of the first and second suction pipes ( 61   a ,  61   b ) is sucked into the working third compressor ( 11   c ) through the oil collecting pipes ( 75 ,  76 ,  77 ). 
     (Defrosting Operation) 
     For a defrosting operation, though not shown, the four-way switch valve ( 12 ) is set to the second state and the cold-storage expansion valves ( 15   a ,  15   b ) and the second expansion valve ( 46 ) are fully opened. The first and second expansion valves ( 45 ,  46 ) are opened to a suitable degree of opening. In this state, reverse-cycle defrosting is performed by circulating the refrigerant in the direction opposite to the circulating direction during the cooling operation. 
     More specifically, the flows of the refrigerant discharged from the three compressors ( 11   a ,  11   b ,  11   c ) run through the cold-storage heat exchangers ( 16 ,  17 ) and the drain pan heaters ( 26 ,  27 ) and are condensed and liquefied as they dissipate heat to the frost on the cold-storage heat exchangers ( 16 ,  17 ) and the drain pans. The liquefied refrigerant flows into the outdoor circuit ( 20 ) through the liquid-end connecting pipe ( 21 ). Then, it passes through the fifth liquid pipe ( 88 ), the receiver ( 14 ) and the first flow path ( 50   a ) of the refrigerant heat exchanger ( 50 ). Then, the refrigerant is expanded by the first expansion valve ( 45 ) as it flows in the sixth liquid pipe ( 89 ), condensed in the outdoor heat exchanger ( 13 ) and flows into the main suction pipe ( 55 ) through the four-way switch valve ( 12 ). Then, the refrigerant is distributed into the suction pipe branches ( 61   a ,  61   b ,  61   c ) and sucked into the compressors ( 11   a ,  11   b ,  11   c ). 
     Also in the defrosting operation, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors, respectively. Among the compressors ( 11   a ,  11   b ,  11   c ), the amount of refrigeration oil is equalized suitably by sequentially supplying the refrigeration oil from the compressors ( 11   a ,  11   b ) which receive a larger amount of the returned refrigeration oil to the compressors ( 11   b ,  11   c ) which receive a smaller amount of the returned refrigeration oil through the oil equalization pipes ( 72 ,  73 ,  74 ). 
     Effect of Embodiment 1 
     In the refrigeration system ( 1 ), the largest amount of the refrigeration oil is returned to the first compressor ( 11   a ) by making use of the difference in gravity and centrifugal force exerted on the refrigerant and the refrigeration oil as they pass through the main suction pipe ( 55 ). Therefore, the refrigeration oil is surely accumulated in the casing of the first compressor ( 11   a ). Further, with the help of the difference in gravity and centrifugal force exerted on the refrigerant and the refrigeration oil as they pass through the connecting suction pipe ( 56 ), a larger amount of the refrigeration oil is returned to the second compressor ( 11   b ) than to the third compressor ( 11   c ). Thus, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors, respectively. 
     Through the first oil equalization pipe ( 72 ), the refrigeration oil accumulated in the casing of the first compressor ( 11   a ) containing the largest amount of the refrigeration oil is supplied to the second compressor ( 11   b ) so that the refrigeration oil is also accumulated in the second compressor ( 11   b ). Then, the refrigeration oil accumulated in the casing of the second compressor ( 11   b ) is supplied to the third compressor ( 11   c ) through the second oil equalization pipe ( 73 ) so that the refrigeration oil is also accumulated in the third compressor ( 11   c ). Further, through the third oil equalization pipe ( 74 ), a surplus of the refrigeration oil in the third compressor ( 11   c ) is returned to the first compressor ( 11   a ). In this way, the refrigeration oil is sequentially supplied from the compressors ( 11   a ,  11   b ) which receive a larger amount of the returned refrigeration oil to the compressors ( 11   b ,  11   c ) which receive a smaller amount of the returned refrigeration oil and a surplus of the refrigeration oil in each casing is circulated among the compressors ( 11   a ,  11   b ,  11   c ) so as to suitably equalize the amounts of the refrigeration oil. 
     When only the first compressor ( 11   a ) is stopped or the first and second compressors ( 11   a ,  11   b ) are stopped, the refrigeration oil and the liquid refrigerant remaining in the oil storing portion ( 58 ) of the suction pipe branch ( 61   a ,  61   b ) of the stopped compressor ( 11   a ,  11   b ) is sucked into the other working compressors ( 11   b ,  11   c ) through the oil collecting pipes ( 75 ,  76 ,  77 ). Therefore, the stopped compressor ( 11   a ) does not suck a large amount of the refrigeration oil and the refrigerant in the liquid state upon restart. This allows preventing the stopped compressor ( 11   a ) from performing liquid compression immediately upon restart. In particular, according to the present embodiment, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second, third suction pipe branches ( 61   a ,  61   b ,  61   c ) of the first, second and third compressors, respectively. Then, as the cooling load is reduced during the operation, the first, second and third compressors are stopped in this order. Thus, the effect of the oil collecting pipes ( 75 ,  76 ,  77 ) is significantly exerted. 
     As described above, the refrigeration system ( 1 ) is able to prevent lack of the refrigeration oil in the compressors ( 11   a ,  11   b ,  11   c ). Even when the first and second compressors ( 11   a ,  11   b ) are stopped when the refrigeration system ( 1 ) is working, the stopped compressors are prevented from performing liquid compression upon restart. That is, according to the refrigeration system ( 1 ), the oil amounts in the compressors ( 11   a ,  11   b ,  11   c ) are checked with accuracy, thereby improving the reliability of the compressors ( 11   a ,  11   b ,  11   c ). 
     Embodiment 2 
     In Embodiment 1, the primary flow biasing element ( 110 ) includes the primary curved portion ( 101 ) and the primary branch element ( 102 ) and the secondary flow biasing element ( 120 ) includes the secondary curved portion ( 103 ) and the secondary branch element ( 104 ). According to the present embodiment shown in  FIG. 4 , different from Embodiment 1, the primary flow biasing element ( 110 ) is formed of the primary branch element ( 102 ) only and the secondary flow biasing element ( 120 ) is formed of the secondary branch element ( 104 ) only. More specifically, according to the present embodiment, centrifugal force is not exerted on the refrigerant and the refrigeration oil as they pass through the curved portions ( 101 ,  103 ) of the pipes ( 55 ,  56 ) on the suction side. Only by making use of the difference gravity exerted on the refrigerant and the refrigeration oil, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors, respectively. In  FIG. 4 , the liquid injection pipe branches ( 86   a ,  86   b ,  86   c ) are omitted. 
     More specifically, as shown in  FIG. 4 , the main suction pipe ( 55 ) extends horizontally in part thereof downstream of the junction with the oil return pipe ( 71 ) and is connected to a primary branch element ( 102 ) serving as the primary flow biasing element ( 110 ) at the most downstream end thereof. 
     The primary branch element ( 102 ) includes a first branch port ( 102   a ) and a second branch port ( 102   b ). The first branch port ( 102   a ) is positioned obliquely below the second branch port ( 102   b ) at an angle of 45°. The first suction pipe branch ( 61   a ) of the first compressor ( 11   a ) is connected to the first branch port ( 102   a ) and the connecting suction pipe ( 56 ) is connected to the second branch port ( 102   b ). That is, the first suction pipe branch ( 61   a ) is at the bottommost position in the primary branch element ( 102 ). 
     One end of the first suction pipe branch ( 61   a ) is connected to the first branch port ( 102   a ) of the primary branch element ( 102 ) and the other end thereof is connected to the suction side of the first compressor ( 11   a ). More specifically, the first suction pipe branch ( 61   a ) includes a descending portion ( 63 ) connected to the first branch port ( 102   a ) of the primary branch element ( 102 ) at one end and extends obliquely downward to be away from the connecting suction pipe ( 56 ), a horizontally extending linear oil storing portion ( 58 ) connected to the other end of the descending portion ( 63 ) at one end, an oblique portion ( 59 ) connected to the other end of the oil storing portion ( 58 ) at one end and extends obliquely upward in the downstream direction and a vertical portion ( 60 ) extending vertically downward from the top of the oblique portion ( 59 ) and connected to the first compressor ( 11   a ). One end of a first oil collecting pipe ( 75 ) is connected to the bottom wall of the oil storing portion ( 58 ) at the most downstream end of the oil storing portion ( 58 ) of the first suction pipe branch ( 61   a ). 
     One end of the connecting suction pipe ( 56 ) is connected to the second branch port ( 102   h ) of the primary branch element ( 102 ) and the other end thereof is connected to a secondary branch element ( 104 ) serving as the secondary flow biasing element ( 120 ). The connecting suction pipe ( 56 ) extends horizontally in the direction from one end to the other end thereof with a first oil equalization pipe ( 72 ) connected to the barrel thereof. 
     The secondary branch element ( 104 ) includes a first branch port ( 104   a ) and a second branch port ( 104   b ). The first branch port ( 104   a ) is positioned obliquely below the second branch port ( 104   b ) at an angle of 45°. One end of the second suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is connected to the first branch port ( 104   a ) and the third suction pipe branch ( 61   c ) of the third compressor ( 11   c ) is connected to the second branch port ( 104   b ). 
     One end of the second suction pipe branch ( 61   b ) is connected to the first branch port ( 104   a ) of the secondary branch element ( 104 ) and the other end thereof is connected to the suction side of the second compressor ( 11   b ). More specifically, the second suction pipe branch ( 61   b ) includes a horizontally extending linear oil storing portion ( 58 ) connected to the first branch port ( 104   a ) of the secondary branch element ( 104 ) at one end, an oblique portion ( 59 ) connected to the other end of the oil storing portion ( 58 ) at one end and extends obliquely upward in the downstream direction, a horizontally extending horizontal portion ( 62 ) connected to the top of the oblique portion ( 59 ) at one end and a vertical portion ( 60 ) connected to the other end of the horizontal portion ( 62 ) at one end and extends vertically downward and connected to the second compressor ( 12   a ). One end of a second oil collecting pipe ( 76 ) is connected to the bottom wall of the oil storing portion ( 58 ) at the most downward end of the oil storing portion ( 58 ) of the second suction pipe branch ( 61   b ). 
     One end of the third suction pipe branch ( 61   c ) is connected to the second branch port ( 104   b ) of the secondary branch element ( 104 ) and the other end thereof is connected to the suction side of the third compressor ( 11   c ). The third suction pipe branch ( 61   c ) does not have the oil storing portion ( 58 ) and the oblique portion ( 59 ), but extends horizontally in the direction from the one end to the other end and is bent vertically downward at the other end. A second oil equalization pipe ( 73 ) is connected to the top wall of the horizontal part of the third suction pipe branch ( 61   c ) and one end of a third oil collecting pipe ( 77 ) is connected to the bottom wall of the horizontal part to be located downstream of the second oil equalization pipe ( 73 ). 
     According to the present embodiment, the refrigerant flows in an upper region and the refrigeration oil flows in a lower region in the main suction pipe ( 55 ) due to the difference in gravity exerted on the refrigerant and the refrigeration oil. Since the suction pipe branch ( 61   a ) of the first compressor ( 11   a ) is at the bottommost position in the primary branch element ( 102 ), most of the refrigeration oil flowing in the lower region in the main suction pipe ( 55 ) flows into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ). 
     The refrigerant flowed into the connecting suction pipe ( 56 ) contains a small amount of refrigeration oil. The refrigerant flows in an upper region and the refrigeration oil flows in a lower region in connecting suction pipe ( 56 ) due to the difference in gravity exerted on the refrigerant and the refrigeration oil. Since the second suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is positioned lower than the third suction pipe branch ( 61   c ) of the third compressor ( 11   c ) in the secondary branch element ( 104 ), most of the refrigeration oil running through the connecting suction pipe ( 56 ) flows into the second suction pipe branch ( 61   b ). 
     The refrigerant and the rest of the refrigeration oil flow into the third suction pipe branch ( 61   c ) of the third compressor ( 11   c ) and sucked into the third compressor ( 11   c ). In this way, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors, respectively. 
     The refrigeration oil in the casing of the first compressor ( 11   a ) is supplied to the connecting suction pipe ( 56 ) through the first oil equalization pipe ( 72 ). The supplied refrigeration oil flows in the lower region in the connecting suction pipe ( 56 ) due to the difference in gravity exerted on the refrigerant and the refrigeration oil. As a result, most of the refrigeration oil flows into the second suction pipe branch ( 61   b ). That is, the refrigeration oil is supplied from the first compressor ( 11   a ) to the second compressor ( 11   b ) through the first oil equalization pipe ( 72 ) and the refrigeration oil is surely accumulated in the second compressor ( 11   b ). 
     The refrigeration oil in the casing of the second compressor ( 11   b ) is supplied to the third suction pipe branch ( 61   c ) through the second oil equalization pipe ( 73 ) and sucked into the third compressor ( 11   c ). In this manner, the refrigeration oil is supplied from the second compressor ( 11   b ) to the third compressor ( 11   c ) and the refrigeration oil is surely accumulated in the third compressor ( 11   c ). A surplus of the refrigeration oil in the third compressor ( 11   c ) is supplied to the oil return pipe ( 71 ) through the third oil equalization pipe (not shown) and returned to the first compressor ( 11   a ) through the main suction pipe ( 55 ). In this way, the amounts of the refrigeration oil in the compressors ( 11   a ,  11   b ,  11   c ) are suitably equalized. 
     When only the first compressor ( 11   a ) is stopped or the first and second compressors ( 11   a ,  11   b ) are stopped, the refrigeration oil remaining in the oil storing portions ( 58 ) of the suction pipe branches ( 61   a ,  61   b ) of the stopped compressors ( 11   a ,  11   b ) is sucked into the other working compressors ( 11   b ,  11   c ) through the oil collecting pipes ( 75 ,  76 ,  77 ). This prevents the stopped compressors ( 11   a ,  11   b ) from sucking a large amount of the refrigeration oil in the liquid state and performing liquid compression upon restart. 
     Except the above, the structure and the effect of Embodiment 2 are the same as those of Embodiment 1. 
     Embodiment 3 
     In Embodiment 1, the secondary flow biasing element ( 120 ) includes the secondary curved portion ( 103 ) and the secondary branch element ( 104 ) assembled to the connecting suction pipe ( 56 ). According to the present embodiment as shown in  FIG. 5 , different from Embodiment 1, the secondary flow biasing element ( 120 ) is formed of the primary curved portion ( 101 ) assembled to the main suction pipe ( 55 ) and the secondary branch element ( 104 ) assembled to the connecting suction pipe ( 56 ). More specifically, according to the present embodiment, centrifugal force caused in the primary curved portion ( 101 ) of the main suction pipe ( 55 ) is used to bias the flow of the refrigeration oil running through the connecting suction pipe ( 56 ). In  FIG. 5 , the liquid injection pipe branches ( 86   a ,  86   b ,  86   c ) are omitted. 
     More specifically, as shown in  FIG. 5 , in part of the main suction pipe ( 55 ) downstream of the junction with the oil return pipe ( 71 ), a primary curved portion ( 101 ) and a primary branch element ( 102 ) are provided as the primary flow biasing element ( 110 ). The piping downstream of the primary branch element ( 102 ) is the same as that of Embodiment 2 shown in  FIG. 4 . 
     According to this structure, in the refrigeration system ( 1 ), the refrigerant flows in an upper inside region and the refrigeration oil flows in a lower outside region in the main suction pipe ( 55 ) in the direction of a radius of curvature of the primary curved portion ( 101 ) due to the difference in gravity exerted on the refrigerant and the refrigeration oil and the difference in centrifugal force exerted on the refrigerant and the refrigeration oil as they pass through the primary curved portion ( 101 ). The suction pipe branch ( 61   a ) of the first compressor ( 11   a ) is at the bottommost and outermost position in the primary branch element ( 102 ) in the direction of a radius of curvature of the primary curved portion ( 101 ). Therefore, most of the refrigeration oil in the main suction pipe ( 55 ) flows into the suction pipe branch ( 61   a ) of the first compressor ( 11   a ). 
     The refrigerant flowed into the connecting suction pipe ( 56 ) contains a small amount of refrigeration oil. Due to the difference in gravity exerted on the refrigerant and the refrigeration oil and the difference centrifugal force exerted on the refrigerant and the refrigeration oil as they pass through the primary curved portion ( 101 ) of the main suction pipe ( 55 ), the refrigerant flows in an upper inside region and the refrigeration oil flows in a lower outside region in the connecting suction pipe ( 56 ) in the direction of a radius of curvature of the primary curved portion ( 101 ). Since the suction pipe branch ( 61   b ) of the second compressor ( 11   b ) is positioned lower than and outside the suction pipe branch ( 61   c ) of the third compressor ( 11   c ) in the direction of a arduous of curvature of the primary curved portion ( 101 ) of the main suction pipe ( 55 ), more refrigeration oil in the connecting suction pipe ( 56 ) flows into the suction pipe branch ( 61   c ) of the second compressor ( 11   b ) than into the suction pipe branch ( 61   c ) of the third compressor ( 11   c ). 
     The refrigerant and the rest of the refrigeration oil flow into the third suction pipe branch ( 61   c ) of the third compressor ( 11   c ). In this way, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors, respectively. 
     Except the above, the structure and the effect of Embodiment 3 are the same as those of Embodiment 1. 
     Other Embodiments 
     The above-described embodiments may be modified in the following manner. 
     According to Embodiments 1 to 3, the first branch ports ( 102   a ,  104   a ) are positioned lower than the second branch ports ( 102   b ,  104   b ) in the primary branch element ( 102 ) and the secondary branch element ( 104 ). However, the first branch ports ( 102   a ,  104   a ) and the second branch ports ( 102   b ,  104   b ) may be arranged horizontally on the same level. Even in this case, the first branch port ( 102   a ) of the primary branch element ( 102 ) is positioned outside the second branch port ( 102   b ) in the direction of a radius of curvature of the primary curved portion ( 101 ) and the first branch port ( 104   a ) of the secondary branch element ( 104 ) is positioned outside the second branch port ( 104   b ) in the direction of a radius of curvature of the secondary curved portion ( 103 ) or the primary curved portion ( 101 ). Therefore, only by making use of the centrifugal force caused in the curved portions ( 101 ,  103 ), the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors, respectively. 
     In each of the embodiments described above, the refrigeration system ( 1 ) includes three compressors. However, the number of the compressors is not limited to three. For example, two compressors are connected in parallel and more refrigeration oil may be returned to one of the compressors. 
     The structure of the flow biasing elements ( 110 ,  120 ) is not limited to those described in the embodiments. The structure of the main suction pipe ( 55 ) branched into the suction pipe branches ( 61   a ,  61   b ,  61   c ) is not limitative. For example, the suction pipe branch ( 61   c ) of the third compressor ( 11   c ) may be branched off the upstream part of the main suction pipe ( 55 ), and then the suction pipe branches ( 61   a ,  61   b ) of the first compressor ( 11   a ) and the second compressor ( 11   b ) may be branched off the downstream part of the main suction pipe ( 55 ). In this case, the largest amount of the refrigeration oil is supplied from the main suction pipe ( 55 ) to suction pipe branch ( 61   a ) to the first compressor ( 11   a ). Further in this case, the largest, the second largest and the smallest amounts of the refrigeration oil are returned to the first, second and third compressors, respectively. 
     The refrigeration system ( 1 ) of Embodiment 1 includes the refrigerant circuit ( 10 ) which performs one-stage refrigerant compression by vapor compression refrigerating cycles. However, the refrigeration system ( 1 ) may include a refrigerant circuit which performs two-stage refrigerant compression. In this case, a plurality of compressors (first to third compressors) may be connected in parallel in each of a low-stage compressor mechanism and a high-stage compressor mechanism for the two-stage compression. In each of the compressor mechanisms the largest, the second largest and the smallest amounts of the refrigeration oil may be returned to the first, second and third compressors, respectively. Further, in each of the compressor mechanisms, the refrigeration oil may be supplied from the compressor which receives a larger amount of the refrigeration oil to the compressor which receives a smaller amount of the refrigeration oil through the oil equalization pipe. In the low-stage compressor mechanism, the refrigeration oil in the casing of the third compressor which receives the smallest amount of the refrigeration oil may be supplied to the suction side of the high-stage compressor mechanism through the oil equalization pipe. 
     The above embodiments are merely preferred embodiments in nature and are not intended to limit the scope, applications and use of the invention. 
     INDUSTRIAL APPLICABILITY 
     As described above, the present invention is useful for refrigeration systems including a plurality of compressors connected in parallel.