Patent Publication Number: US-11391496-B2

Title: Refrigerating cycle apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. national stage application of PCT/TH2017/000079 filed on Nov. 1, 2017, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a refrigerating cycle apparatus equipped with a plurality of outdoor units. Each outdoor unit has at least one high pressure type compressor. More specifically, the invention relates to improvement of oil balancing between the compressors in each outdoor unit. 
     BACKGROUND ART 
     In a refrigerating cycle apparatus, a plurality of high pressure type compressors are provided having a casing held with a lubricating oil in which the discharging tube and suction tube of these compressors are mutually connected. In the prior art, an imbalance occurs between an amount of lubricating oil discharged from each compressor in a state to be mixed in a refrigerant and an amount of lubricating oil returned back to the respective compressor and sometimes there will occur an oil shortage in the compressors. If such oil shortage occurs, the supply of the oil to sliding motion parts of the compressor is interrupted, thus exerting a bad effect on the service life of the compressor. 
     In order to solve such problem, a refrigerating cycle apparatus has been proposed. For example, in the Japanese patent Publication No. JPH08159580 A, an oil balancing tube is connected between a side of one compressor and a suction tube of an associated compressor to allow the exceeding oil from the one compressor to be returned back to the associated compressor. However, the oil balancing tubes may involve a complex connection. 
     In the refrigerating cycle apparatus where there are more than one outdoor unit, problems, such as oil leakage from the interconnected oil balancing pipes&#39; joints and fittings and pipe clog due to dirt and debris during unit installation, caused by unskillful workers, can easily occur and may cause severe damages to the compressor. 
     In EP 2 397 793B1, a refrigerating cycle apparatus with a plurality of outdoor units is disclosed. The oil amount of the compressor in the respective outdoor units is detected whether it is at a correct level by detecting temperature of the flow substance (refrigerant or oil) in the oil balancing tubes. However, this makes the apparatus complex and incurs high cost. Further, the apparatus still requires oil balancing tubes mutually connected between each outdoor unit. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, a refrigerating cycle apparatus is provided which can simply balance oil level of each compressor in each outdoor unit without installation of oil balancing pipes between each outdoor units. This can alleviate the above-mentioned problems, such as oil leakages from the interconnected oil balancing pipes&#39; joints or pipe clog due to installing the interconnected oil balancing pipes after installing multiple outdoor units. 
     In one embodiment, the refrigerating cycle apparatus is equipped with a plurality of outdoor units, each of which includes at least one high-pressure type compressor. Each one of the compressor has a casing hold therein an oil. A discharge pipe and a suction pipe are connected to the high-pressure side and the low-pressure side of the compressor, respectively. Each one of the discharge pipes of the respective compressor is connected to a main discharge pipe for jointly discharging refrigerant and oil to the indoor unit. Each one of the suction pipes of the respective compressors is connected for returning the refrigerant and the oil from the indoor unit to the compressor. Each one of the plurality of outdoor units comprise a first oil balancing circuit connected between a side surface of the casing of the respective compressor and the main discharge pipe for allowing a flowing-in of an excess amount of the oil in the casing of the respective compressor. 
     In another embodiment, the refrigerating cycle apparatus may further include a second oil balancing circuit connected between a side surface of the casing of the respective compressor and the main suction pipe. The first and second balancing circuits may be operated periodically and sequentially and controlled by electronic valves. 
     Therefore, oil balancing between the compressors of the plurality of outdoor units can be realized more efficiently. 
     In one aspect of the invention, each one of the compressor may further comprise an oil level sensor for detecting oil level of the oil in the casing of the respective compressor. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram of refrigerating cycle apparatus that include one compressor in each outdoor unit according to an embodiment 1 of the present invention; 
         FIG. 2  is a flowchart for oil balancing control of the refrigerating cycle apparatus shown in  FIG. 1 ; 
         FIG. 3  is a circuit diagram of refrigerating cycle that included a plurality of compressors in each outdoor unit according to an embodiment 2 of the present invention; 
         FIG. 4  is a flowchart for oil balancing control of the refrigerating cycle apparatus shown in  FIG. 3 ; 
         FIG. 5  is a circuit diagram of refrigerating cycle that included a plurality of compressors with an oil level sensor in each compressor according to an embodiment 3 of the present invention, and 
         FIG. 6  is a flowchart for oil balancing control of the refrigerating cycle apparatus shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiment 1 
     The embodiment 1 of the present invention will be described based on the  FIG. 1 . A refrigerating cycle apparatus  1  is comprised of an air conditioner comprising a plurality of outdoor units  2   a ,  2   b  and an indoor unit  3 . As for the indoor unit  3 , not only one but also a plurality of units may be connected. Each one of the outdoor units  2   a ,  2   b  include at least one high pressure type compressor  4 , an oil separator  6 , a discharge check valve  8 , a four-way valve  11 , an outdoor heat exchanger  12 , a receiver tank  13 , an accumulator  18 , and an outdoor controller  50 . The indoor unit  3  includes an expansion valve  15 , an indoor heat exchanger  16 , and an indoor controller (not shown). The outdoor units  2   a , 2   b  and the indoor unit  3  are connected by a liquid pipe  14  and a gas pipe  17 . 
     Oil separator  6  is used for separating oil from the discharge gas and the separated oil then will be returned back to the compressor  4  through the suction line of the compressor  4 . 
     The discharge check valve  8  is used for preventing the refrigerant and oil from flowing back to the stopped compressor while other compressors are operating. 
     The four-way valve  11  is used for selecting one of the two operating modes of the apparatus, i.e. cooling mode and heating mode. 
     The receiver tank  13  is used for separating the liquid refrigerant from the mixed (gas and liquid) refrigerant before the refrigerant flows to the expansion device. 
     The accumulator  18  is used for separating gas refrigerant from mixed refrigerant before the refrigerant flows back to the suction line of the compressor  4 . 
     The outdoor controller  50  controls operation of the outdoor unit  2   a ,  2   b  and operation of the solenoid valve V 1 . Operation of the valve V 1  will be explained in details later. 
     The compressor  4  is a high pressure type compressor. After the refrigerant pass a compression chamber of the compressor, the high pressure and high temperature refrigerant will flow inside the casing of the compressor for cooling the motor (not shown) and thereafter flow to the discharge pipe  5 . During operation, interior of the compressor&#39;s casing is under high pressure. Lubrication oil L is stored inside the compressor&#39;s casing in the bottom. The discharge pipe  5  is connected between the discharge side (high-pressure side) of the compressor  4  and an inlet of the oil separator  6 . The suction pipe  20  is connected between a suction muffler  21  of the compressor  4  and an outlet of the accumulator  18 . The suction pipe  20  of the compressor  4  is connected for returning the refrigerant and the oil from the at least one indoor unit to the compressor  4 . 
     The oil separator  6  is connected between compressor  4  and a discharge check valve  8 . The inlet of the oil separator  6  is connected to the discharge pipe  5 . The outlet of the oil separator  6  is connected to the high-pressure outlet pipe  7 . A main discharge pipe  9  is connected between the discharge check valve  8  and the four-way valve  11 . The oil return pipe  31  is connected between the bottom of the oil separator  6  and a capillary tube (pressure reduction device)  32 . An oil return pipe  33  is connected between the capillary tube  32  and the suction pipe  20 . 
     According to the first embodiment of the invention, an oil balancing circuit  60 A is provided for each outdoor unit  2   a  and  2   b . And each oil balancing circuit  60 A includes an oil balancing pipe  22 , an oil check valve  23 , an oil balancing pipe  28 , an oil solenoid valve V 1 , and an oil balancing pipe  30 . 
     The oil balancing circuit is configured to allow a flowing-in of an excess amount of the oil L stored in the casing of the compressor  4 . 
     The oil balancing pipe  22  is formed as a first exceeding oil passage connected in fluid communication between the side surface of the casing of the compressor  4  and the main discharge pipe  9  via oil check valve  23  and the oil solenoid valve V 1 . In other word, the oil balancing pipe  28  is connected between the oil check valve  23  and the oil solenoid valve V 1 . The solenoid valve V 1  is interposed in the first exceeding oil passage for periodically controlling flow of the exceeding oil from the compressor  4  to the main discharge pipe  9 . Further, the oil balancing pipe  30  is connected between the oil solenoid valve V 1  and the main discharge pipe  9 . 
     It should be noted that in case where there is only one high-pressure type compressor  4  equipped in the outdoor unit  2   a ,  2   b , the main discharge pipe  9  becomes a main discharge pipe and functions for discharging refrigerant and oil to the at least one indoor unit  3 . Also, the suction pipe  20  becomes a main suction pipe and function for returning the refrigerant and the oil from the at least one indoor unit  3  to the respective compressor  4 . 
     (Explanation of Refrigerant Flow) 
     When the outdoor units  2   a ,  2   b  operate, the compressor  4  in each outdoor unit operates. The refrigerant and oil discharges from each compressor  4  flows through the discharge pipe  5  and flows into the oil separator  6 . The discharge refrigerant is high pressure in gas phase. The discharge refrigerant gas contains lubricating oil most of which will be separated by the oil separator  6  and return to the compressor  4 . The discharge refrigerant gas from which the lubricating oil is separated flows into the high-pressure outlet pipe  7  and flows pass through the discharge check valve  8  and to the main discharge pipe  9 . The discharge refrigerant gas then flows pass through the four-way valve  11 . 
     At the cooling mode operation, the discharge refrigerant gas flows to the outdoor heat exchanger  12 . At this time, the outdoor heat exchanger  12  functions as a condenser. After passing the outdoor heat exchanger  12 , the discharge refrigerant gas will be changed into a liquid phase and enters the receiver tank  13 . The liquid refrigerant then passes a service valve  34   a  and flow into the liquid pipe  14  and go into the indoor unit  3  via a service valve  34   c . The liquid refrigerant flows to the expansion valve  15  and enters the indoor heat exchanger  16  where it will be changed into a gas phase. The indoor heat exchanger  16  functions as an evaporator. The gas phase refrigerant will be suctioned pass another service valve  34   d  and flow into the gas pipe  17  and return to the outdoor unit  2   a , 2   b  through the service valve  34   b . It then flows into the accumulator  18  and subsequently to the suction pipe  20  and suction muffler  21  of each compressor  4 . 
     In the heating mode operation, the four-way valve  11  forces the refrigerant to flow in a reverse direction, comparing to that of the cooling mode operation. That is to say, the refrigerant will flow in the direction from the indoor heat exchanger  16  of the indoor unit  3  to the heat exchanger  12  of the outdoor unit  2   a ,  2   b . In this case, the indoor heat exchanger  16  functions as a condenser, and the outdoor heat exchanger  12  functions as an evaporator. The remaining operation in the heating mode will be omitted since it is the same as those already aforementioned above. 
     (Explanation of Oil Return from Oil Separator) 
     When the discharge refrigerant gas and oil from compressor  4  of each outdoor unit  2   a , 2   b  flows pass the oil separator  6 , the oil separated by the oil separator  6  will return to the compressor via the oil return pipe  31 , the capillary tube  32 , the oil return pipe  33  and the suction muffler  21 , respectively. 
     (Explanation of Oil Flow in Oil Balancing Circuit Between Outdoor Unit) 
     Even the oil separator  6  is used, unbalance of oil level in compressor  4  in each outdoor unit  2   a ,  2   b  can still occur because the oil separator  6  cannot completely separate the oil from the mixed refrigerant. Typically, the oil separator  6  has an oil recovery efficiency around 90 to 99%. As a result, some of compressor oil L still flows through the circuit and remains in some parts of the circuit, such as the indoor unit  3 , the accumulator  18 , and the piping. Since not all of the lubricating oil L can return to the compressor  4 , this may give rise to an unbalance oil level condition. Some compressors  4  may cause severe damages to the sliding motion parts of the compressor  4  without having sufficient lubricating oil L. 
     In case the oil L for lubrication in the compressor  4  of each outdoor unit  2   a ,  2   b  is high above the level of inlet of the oil balancing pipe  22  in the side surface of the casing of the compressor  4 , the exceeding oil will flow to the oil balancing pipe  22 . The exceeding oil flows through a first exceeding oil passage which is connected in fluid communication between the side surface of the casing of the compressor  4  and the main discharge pipe  9 . That is, the exceeding oil passes through the oil check valve  23  and flows into the oil balancing pipe  28 , the solenoid valve V 1 , and the oil balancing pipe  30 . The exceeding oil then flows to the main discharge pipe  9  and pass through the four-way valve  11 , the outdoor heat exchanger  12 , the receiver tank  13 , and the service valve  34   a , respectively, and flows into the liquid pipe  14 . From there, the exceeding oil passes through the service valve  34   c  of indoor unit  3 , and the expansion valve  15 , the indoor heat exchanger  16 , the service valve  34   d  of indoor unit  3  and flows in to the gas pipe  17 . Subsequently, the exceeding oil will return to each outdoor unit  2   a ,  2   b . The exceeding oil will be divided and pass through service valve  34   b  of each outdoor unit that is currently operating and passes through the four-way valve  11  to the accumulator  18 , and then flows into the suction pipe  20 . The exceeding oil will pass through the suction muffler  21  and fill into each compressor  4  of each outdoor unit  2   a ,  2   b . As a result, the oil level in each compressor  4  of each outdoor unit  2   a  and  2   b  becomes at the appropriate level. 
     It should be noted that the solenoid valve V 1  in the oil balancing circuit of each outdoor unit is preferably opened and closed periodically for a predetermined time by the outdoor controller  50 . For example, solenoid valve V 1  may be opened for 1-2 minutes for every two hours of the compressor operating time, when there are more than one operating outdoor units  2   a ,  2   b.    
     In the present invention, the oil balance circuit in each outdoor unit is assembled at the factory and is constructed as a built-in component of the outdoor unit. Therefore, there is no need to install the oil balancing pipe between each outdoor unit on site after installing the apparatus as it is in the conventional refrigerating cycle apparatus. This can prevent problems such as oil leakage, and clogging in the oil pipe. 
     (Explanation of Oil Balancing Logic) 
     Reference is now made to the flowchart shown in  FIG. 2 , the explanation will be made below about operation (oil balancing logic) of the outdoor controller  50 . 
     The outdoor controllers  50  are connected to each other by communication wires and cooperate to control the operation of this refrigerating cycle apparatus. 
     At step S 101 , if the refrigerating cycle apparatus is operating with only single outdoor unit  2   a  or  2   b , i.e. there is only one compressor  4  running, the oil balancing is not required. On the other hand, if the apparatus is operating with both outdoor unit  2   a  and  2   b  running (“Yes” at step S 101 ), i.e. there is more than one compressor  4  running, the oil balancing is required. Then, the outdoor controller  50  of each outdoor unit  2   a  and  2   b  will detect operating time of each compressor  4 . In the case where at least one compressor  4  among the plurality of compressors  4  is running continuously more than a second predetermined time T 2  (for example, two hours), the oil solenoid valve V 1  of each operating outdoor unit  2   a  and  2   b  will be opened for a first predetermined time T 1 , for example, for one minute to allow flowing of the exceeding oil from each compressor to the discharging pipe  9  and then be closed (Step S 103 ). In case the operating compressor  4  is stopped before the predetermined time (in this example two hours) passes away, the oil balancing between each outdoor unit  2   a  and  2   b  is not required. 
     The oil balancing pipe installation problem manifested in the EP 2 397 793B1 can be solved by this present invention. The oil balancing circuit between each outdoor unit is already built-in each outdoor unit at the time of factory shipment. According to the first embodiment of the invention, oil level in each compressor  4  of each outdoor unit  2   a  and  2   b  can be efficiently maintained at an appropriate level by the outdoor unit controller  50 . 
     Embodiment 2 
     An another embodiment of the present invention will be now described with respect to  FIG. 3 . 
     It should be noted that only the difference between the embodiment 1 and the embodiment 2 will be described below. The explanation of the common components will be omitted. The advantage of the second embodiment is explained as follows.
         1. Each outdoor unit can have more than one high pressure type compressors. This can help expanding indoor unit connections. Further, the plurality of compressors can provide better performance comparing to that of an apparatus having one big-sized compressor. For example, one of the advantages of the multiple compressors is the ability to control refrigeration capacity finely.   2. In the second embodiment, there is a second oil balancing circuit in each outdoor unit, in addition to the oil balancing circuit described in the first embodiment. The first balancing circuit balances oil between each compressor in difference outdoor unit. Whereas, the additional second Oil balancing circuit balances oil between each compressor in the respective outdoor unit. This can provide more efficiency in oil balancing to the refrigerating cycle apparatus.       

     In  FIG. 3 , the refrigerating cycle apparatus comprises a plurality of outdoor units  2   c , 2   d  and a plurality of indoor units  3   a - 3   e . The outdoor unit  2   c  have two compressors  4   a  and  4   b , two oil separators  6   a  and  6   b , two discharge check valves  8   a  and  8   b , two discharge pipes  9   a  and  9   b , a main discharge pipe  10 , a four-way valve  11 , an outdoor heat exchanger  12 , a receiver tank  13 , an accumulator  18  and an outdoor controller  50 . 
     In the embodiment, the outdoor unit  2   d  have three compressors  4   a , 4   b  and  4   c , three oil separators  6   a , 6   b  and  6   c , three discharge check valves  8   a , 8   b  and  8   c , three discharge pipes  9   a , 9   b  and  9   c , a main discharge pipe  10 , a four-way valve  11 , an outdoor heat exchanger  12 , a receiver tank  13 , an accumulator  18  and a outdoor controller  50 . Each of the indoor unit  3  includes an expansion valve  15 , an indoor heat exchanger  16  and an indoor controller (not shown). The Plurality of the outdoor unit  2   c , 2   d  and the plurality of indoor unit  3   a - 3   e  are connected by a liquid pipe  14  and a gas pipe  17 . 
     The outdoor controller  50  controls operation of the outdoor unit  2   c , 2   d  and operation of the oil solenoid valve V 1  for oil balancing between outdoor unit  2   c  and  2   d . Further, the outdoor controller  50  also controls the oil solenoid valve V 2  for oil balancing between compressor  4   a , 4   b , 4   c  inside each outdoor unit  2   c , 2   d.    
     The compressors  4   a , 4   b , 4   c  in each outdoor unit  2   c , 2   d  are high pressure type compressor. During operation, pressure inside compressor shell or casing is high. Lubrication oil L is contained inside the compressor casing at the bottom position. The discharge pipes  5   a , 5   b , 5   c  are connected between the discharge pipe of the compressor  4   a , 4   b , 4   c  and the inlet pipes of the oil separator  6   a , 6   b , 6   c . The suction pipes  20   a , 20   b , 20   c  are connected between respective suction muffler  21   a , 21   b , 21   c  of the compressor  4   a , 4   b , 4   c  and a main suction pipe  19 . The main suction pipe  19  is connected between the suction pipe  20   a , 20   b , 20   c  and the accumulator  18 . The oil balancing pipes  22   a , 22   b , 22   c  are connected to an oil storage position above the bottom position of side surfaces of the respective compressor  4   a , 4   b , 4   c.    
     The oil separators  6   a , 6   b , 6   c  in each outdoor unit  2   c , 2   d  are connected between the compressor  4   a , 4   b , 4   c  and the discharge check valve  8   a , 8   b , 8   c . The inlet pipes of the oil separators  6   a , 6   b , 6   c  are connected to the discharge pipes  5   a , 5   b , 5   c . The outlet pipes of oil separator  6   a , 6   b , 6   c  are connected to respective high pressure outlet pipe  7   a , 7   b , 7   c . The oil return pipes  31   a , 31   b , 31   c  are connected between the oil return pipe of the oil separators  6   a , 6   b , 6   c  and the capillary tubes  32   a , 32   b , 32   c . The oil retune pipes  33   a , 33   b , 33   c  are connected between the capillary tubes  32   a , 32   b , 32   c  and the suction pipes  20   a , 20   b , 20   c . The discharge check valves  8   a , 8   b , 8   c  are connected between high pressure the outlet pipes  7   a , 7   b , 7   c  and the discharge pipes  9   a , 9   b , 9   c.    
     In this embodiment, a first oil balancing circuit  60 B in each outdoor unit  2   c , 2   d  includes an oil balancing pipe  22   a , 22   b , 22   c , an oil check valve  23   a , 23   b , 23   c , an oil balancing pipe  28   a , 28   b , 28   c , an oil balancing pipe  29 , an oil solenoid valve V 1  and an oil balancing pipe  30 . The oil balancing pipe  22   a , 22   b , 22   c  is respectively connected between side surface of compressor  4   a , 4   b , 4   c  and the oil balancing pipe  28   a , 28   b   28   c . The oil check valve  23   a , 23   b , 23   c  is installed on the other end of the oil balancing pipe  22   a , 22   b , 22   c . The Oil balancing pipe  28   a , 28   b , 28   c  each is branched out from the corresponding oil check valve  23   a , 23   b , 23   c , to an oil balancing pipe  29 . The oil solenoid valve V 1  is connected between the oil balancing pipe  29  and the oil balancing pipe  30  which is connected to the main discharge pipe  10 . 
     The oil balancing circuit  60 B of the second embodiment further comprises a second exceeding oil passage which is connected in fluid communication between the side surface of the respective compressor  4   a ,  4   b ,  4   c  and the main suction pipe  19 . A second solenoid valve V 2  is interposed in the second exceeding oil passage for controlling flow of the exceeding oil from each compressor  4   a ,  4   b ,  4   c  to the main suction pipe  19 . Reference  60 B is indicated by example in outdoor unit  2   c  of  FIG. 3  only to avoid obscuring the figures. 
     The second exceeding oil passage balances oil between the compressors  4   a , 4   b , 4   c  in the same outdoor unit  2   c , 2   d . Each of the second exceeding oil passage includes the oil balancing pipe  22   a , 22   b , 22   c , the oil check valve  23   a , 23   b , 23   c , a capillary tube  24   a , 24   b , 24   c  for reduction of oil pressure, an oil balancing pipe  25   a , 25   b , 25   c , an oil balancing pipe  26 , an oil solenoid valve V 2  and an oil balancing pipe  27 . The oil balancing pipes  22   a , 22   b , 22   c  are connected between the side surface of compressor  4   a , 4   b , 4   c  and the oil check valve  23   a , 23   b , 23   c . The capillary tubes  24   a , 24   b , 24   c  are connected to the respective oil check valves  23   a , 23   b , 23   c . The oil balancing pipes  25   a , 25   b , 25   c  are then jointly connected between the capillary tubes  24   a , 24   b , 24   c  and the oil balancing pipe  26 . The above oil balancing pipes  28   a ,  28   b ,  28   c  respectively branch off from the position between the oil check valves  23   a ,  23   b ,  23   c  and the capillary tubes  24   a ,  24   b ,  24   c . The oil balancing pipe  26  is connected to the oil solenoid valve V 2 . The oil solenoid valve V 2  is connected to the main suction pipe  19  via the oil balancing pipe  27 . 
     (Explanation of Refrigerant Flow) 
     Refrigerant flow of the refrigerating cycle apparatus  1   b  in the second embodiment is the same as refrigerant flow of the refrigerating cycle apparatus  1   a  explained in the first embodiment above. 
     The differences between the embodiment 1 and embodiment 2 are explained below. 
     1. In embodiment 1, the refrigerant flow in one outdoor unit is from one high pressure type compressor, while in the embodiment 2 the refrigerant flow in one outdoor unit is from more than one compressors. 
     2. In this embodiment, discharged gas being discharged from two or three compressors will flow from the discharge pipes  9   a , 9   b , 9   c  and be mixed to the main discharge pipe  10 . 
     3. In this embodiment, suction gas flow being suctioned from the main suction pipe  19  will be separated to each compressor by the suction pipe  20   a , 20   b , 20   c.    
     (Explanation of Oil Return from Oil Separator) 
     In this embodiment, the oil return from the oil separators  6   a , 6   b , 6   c  in the outdoor unit  2   c  and  2   d  are the same as oil return from the oil separator  6  in outdoor unit  2   a  and  2   b  in the embodiment 1. Therefore, the explanation is omitted. 
     (Explanation of Oil Flow in Oil Balancing Circuit Between the Outdoor Units) 
     In case the amount of oil L in some compressors  4   a , 4   b , 4   c  in each outdoor unit  2   c  and  2   d  is high above the connection position on a side surface of the respective oil balancing pipe  22   a , 22   b , 22   c . The exceeding oil L flows into the oil balancing pipe  22   a , 22   b , 22   c  from inside of the compressor casing and flow pass the oil check valve  23   a , 23   b , 23   c  into either one of the following passages depending on conditions of operation: 
     1) the oil balancing pipe  28   a , 28   b , 28   c , the oil balancing pipe  29 , the oil solenoid valve V 1 , the oil balancing pipe  30 , and the main discharge pipe  10 . and 
     2) the capillary tubes  24   a , 24   b , 24   c , the oil balancing pipe  25   a , 25   b , 25   c , the oil balancing pipe  26 , the oil solenoid valve V 2 , the oil balancing pipe  27  and the main suction pipe  19 . 
     The above 1) is the oil balancing circuit between the outdoor units and 2) is the oil balancing circuit between compressors inside outdoor unit. 
     The following is an explanation about the flow of the exceeding oil in the oil balancing circuit between the outdoor units shown in 1). 
     The exceeding oil flows to the main discharge pipe  10  and pass the four-way valve  11 , the outdoor heat exchanger  12 , the receiver tank  13 , the service valve  34   a , and then flows into the liquid pipe  14 . The exceeding oil then flows pass the service valve  34   c  of respective indoor unit  3   a - 3   e , pass the expansion valve  15 , the indoor heat exchanger  16 , and pass the service valve  34   d  of each indoor unit  3   a - 3   e  and flows into the gas pipe  17 . From there, the exceeding oil then flows back to each outdoor unit  2   c  and  2   d  and flows pass through service valve  34   b  and passes the four-way valve  11 , the accumulator  18  and then flow into the main suction pipe  19 . The exceeding oil is then suctioned into the suction pipe  20   a , 20   b , 20   c  and fill in each compressor  4   a , 4   b , 4   c  in each outdoor unit  2   c  and  2   d  via the suction muffler  21   a , 21   b , 21   c . Thereby, the oil level in each compressor  4   a , 4   b , 4   c  of each outdoor unit  2   c  and  2   d  will be filled up to an appropriate level. 
     (Explanation of Oil Flow in Oil Balancing Circuit Between Compressors Inside Outdoor Unit) 
     Next, the flow of the exceeding oil in the oil balancing circuit between compressors inside outdoor unit shown in 2) will be explained. 
     The exceeding oil flows into the respective oil balancing pipe  22   a , 22   b , 22   c  and passes through the oil check valve  23   a , 23   b , 23   c  and the capillary tube  24   a , 24   b , 24   c  and then flows into the oil balancing pipe  25   a , 25   b , 25   c  and jointly flows into the oil balancing pipe  26 . The exceeding oil flows through the oil solenoid valve V 2  in controllable manner and flows to the main suction pipe  19  via the oil balancing pipe  27 . Consequently, the exceeding oil will be suctioned into the suction pipe  20   a , 20   b , 20   c  and fill in each compressor  4   a , 4   b , 4   c  in each outdoor unit  2   c  and  2   d  via the suction muffler  21   a , 21   b , 21   c . Thereby, the oil level in each compressor  4   a , 4   b , 4   c  of each outdoor unit  2   c  and  2   d  can be efficiently filled up to an appropriate level. 
     (Explanation of Oil Balancing Logic) 
     The oil balancing logic performed by each outdoor controller  50  will be explained with reference to  FIG. 4 . 
     At step S 201 , if the apparatus is operating with only single outdoor unit  2   c  or  2   d  and there is only one compressor  4   a , 4   b  or  4   c  being operated (“No” at step S 201 ), the oil balancing is not required. On the other hand, if the apparatus is operating using multiple compressors  4   a , 4   b , 4   c  (“Yes” at step S 201 ) and if the outdoor unit is operated using only single outdoor unit  2   c  or  2   d  (“No” at step S 202 ), the oil balancing between compressors inside the same outdoor unit  2   c  or  2   d  is required but oil balancing among the outdoor units  2   c  and  2   d  is not required, only when the outdoor controller  50  detects that at least one compressor  4   a , 4   b  or  4   c  is running continuously longer than a fourth predetermined time T 4  (“Yes” at step S 207 ). The fourth predetermined time T 4  is, for example, two hours. Thereby, the solenoid valve V 2  of the operating outdoor unit  2   c  or  2   d  will be opened for a third predetermined time T 3  to allow the exceeding oil from the respective compressor to flow from the operating compressors to the main suction pipe  19  and then the solenoid valve V 2  closed (Step S 208 ). The third predetermined time T 3  is, for example, one minute. Otherwise (“No” at step S 207 ), the oil balancing is not required. 
     If the apparatus is operated by multiple compressor  4   a , 4   b , 4   c  (“Yes” at step S 201 ) and operated by multiple outdoor units  2   c  and  2   d  (“Yes” at step S 202 ), in this case, both of oil balancing between compressors inside the respective outdoor unit  2   c  or  2   d  and oil balancing among outdoor units  2   c  and  2   d  are required, only when the outdoor controller  50  detects that at least one compressor  4   a , 4   b  or  4   c  is running continuously longer than a second predetermined time T 2  (“Yes” at step S 203 ). The second predetermined time T 2  is, for example, two hours. Consequently, the oil solenoid valve V 2  of the respective operating outdoor unit  2   c  or  2   d  will be opened for the third predetermined time T 3  and then closed (Step S 204 ). After that, the oil solenoid valve V 1  of the operating outdoor unit  2   c  or  2   d  will be opened for the first predetermined T 1  and then closed (Step S 205 ). The first predetermined time T 1  is, for example, one minute. Later, the oil solenoid valve V 2  of the operating outdoor unit  2   c  or  2   d  will be opened again for one min and then closed (Step S 206 ). In other words, the first solenoid valve V 1  and the second solenoid valve V 2  of the operating outdoor unit are alternatively opened and closed in sequence one after another. In this embodiment, oil balance between the compressors  4   a ,  4   b ,  4   c  in each outdoor unit  2   c ,  2   d  is implemented in the steps S 204 , S 206 , and/or S 208  and oil balance between compressors in different outdoor units is implemented in the step S 205 . 
     When oil level unbalance situation occurs in some of the plurality of compressors in any outdoor unit  2   c ,  2   d , the compressors  4   a ,  4   b ,  4   c  having insufficient oil can get lubrication oil from other compressors  4   a ,  4   b ,  4   c  in same outdoor unit much more quickly by performing the step S 204 . Next, the compressors  4   a ,  4   b ,  4   c  can get further oil from other compressors in the difference outdoor units during the step S 205 . However, after performing the step S 205 , oil level in each one of the compressors in the same outdoor unit may be unbalanced. The step S 06  is performed again so as to balance the oil level of the compressors  4   a ,  4   b ,  4   c  in the same outdoor unit  2   c ,  2   d . As a result, by performing oil balancing procedure periodically and in sequence as mentioned above, the oil level in each compressor  4   a ,  4   b ,  4   c  of each outdoor unit  2   c ,  2   d  can be maintained in an appropriate level. 
     It should be understood by skilled people in the art that the sequence above is only an exemplary operation and number of times and sequence for performing oil balance between the compressors in the same outdoor unit and oil balancing between the outdoor units can be varied or modified dependent on other factors and operating conditions designed for specific apparatus, such as size of the high pressure compressors, number of the indoor units and outdoor units, installations of the, and the like. For example, the apparatus may be configured to omit the step S 204  and perform only the steps S 205  and S 206 . The oil level in each compressors still become in an appropriate level. However, it is preferable to perform the step S 204 , since the oil in each compressor will be at appropriate level more quickly. After opening and closing the first solenoid valve V 1  of the operating outdoor unit  2   c ,  2   d , it is preferable to perform the step of opening and closing the second solenoid valve (V 2 ). 
     Embodiment 3 
     The embodiment 3 of the present invention will be described with reference to  FIG. 5  As illustrated in  FIG. 5 , the embodiment 3 is almost the same as that of the embodiment 2, except that every compressors in the embodiment 3 is equipped with an oil level sensor  35   a ,  35   b , and  35   c . The oil level sensor  35   a ,  35   b ,  35   c  is a sensor for detecting oil level of the lubrication oil L in the casing of the respective compressor. Output of the oil level sensor  35   a ,  35   b ,  35   c  will be used in oil balancing for the respective compressor  40   a ,  40   b ,  40   c  in the outdoor unit  2   e ,  2   f . An oil level sensor may be a capacitive type oil level sensor, or the like. The oil level sensor is, for example, disclosed in EP 2 772 731 A1 publication, entitled “The electrostatic capaciative liquid surface sensor”. 
     In this embodiment, a refrigerating cycle apparatus  1   c  is configured between a plurality of outdoor units  2   e ,  2   f  and a plurality of indoor units  3   a - 3   e . All components, as well as piping and connections inside the outdoor unit  2   e  and  2   f  are the same as those of the outdoor unit  2   c  and  2   d  in the embodiment 2, respectively. However, this embodiment is different from the embodiment 2 in that the compressors  40   a  and  40   b  are equipped with the oil level sensors  35   a , and  35   b , respectively. Therefore, explanation for those similar components as those of the above described first and second embodiments will be omitted. 
     The outdoor controller  50  controls operation of the outdoor unit  2   e , 2   f  and operation of the oil solenoid valve V 1  for oil balancing between different outdoor units  2   e  and  2   f  and control the oil solenoid valve V 2  for oil balancing between the compressors  40   a , 40   b , 40   c  in the same outdoor unit. The outdoor controller  50  will perform oil balancing based on the detection results from the oil level sensor  35   a ,  35   b ,  35   c  of each compressor. Further details of operation will be described below. 
     (Explanation of Refrigerant Flow) 
     Refrigerant flow in the refrigerating cycle apparatus  1   c  is the same as the refrigerant flow in the refrigerating cycle apparatus  1   b  explained in the embodiment 2. 
     (Explanation of Oil Return from Oil Separator) 
     Oil return from the oil separator  6   a , 6   b ,  6   c  in the outdoor unit  2   e  and  2   f  is the same as the oil return from the oil separator  6   a , 6   b , 6   c  in the outdoor unit  2   c  and  2   d  as explained in the embodiment 2. 
     (Explanation of Oil Flow in Oil Balancing Circuit Between Outdoor Units) 
     Oil flow in each one of the oil balancing circuit between the outdoor units  2   e  and  2   f  is the same as oil flow in each one of the oil balancing circuit between outdoor units  2   c  and  2   d  as explained in the embodiment 2. 
     (Explanation of Oil Flow in Oil Balancing Circuit Between the Compressors Inside Outdoor Unit) 
     Oil flow in the oil balancing circuit between compressor  40   a , 40   b , 40   c  of the outdoor unit  2   e  and  2   f  is the same as oil flow in the oil balancing circuit between the compressors  4   a , 4   b , 4   c  of the outdoor units  2   c  and  2   d  as explained in the embodiment 2. 
     (Explanation of the Oil Balancing Logic) 
     Oil balancing logic in the outdoor controller  50  is explained by referring to  FIG. 6 . If the apparatus is operating using only one outdoor unit  2   e  or  2   f  and utilizes one compressor  40   a ,  40   b  or  40   c , the oil balancing is not required (“No” at step S 301 ). If the apparatus is operating by multiple compressors  40   a , 40   b , 40   c  (“Yes” at step S 301 ) and utilize only one outdoor unit  2   e  or  2   f  (“No” at step S 302 ), the oil balancing between compressors in the same outdoor unit  2   e  or  2   f  is required, but oil balancing between the outdoor units  2   e  and  2   f  is not required. 
     Then, the outdoor controller  50  is detecting an amount of the oil L in the compressor  40   a , 40   b , 40   c  using the oil level sensor  35   a , 35   b , 35   c , respectively during operation. In case the amount of the oil L in some compressors  40   a ,  40   b  or  40   c  is below a predetermined level (i.e. too low) (“Yes” at step S 307 ), the controller  50  will then start to perform the oil balancing between the compressors in the same outdoor unit to allow flowing of the exceeding oil from each compressor to the main suction pipe  19  and return to each compressors  40   a ,  40   b ,  40   c . The oil solenoid valve V 2  of the operating outdoor unit  2   e  or  2   f  will be opened for the third predetermined time T 3  (for example, one minute), and then closed (Step S 308 ). 
     In case the apparatus is operating by multiple compressors  40   a , 40   b , 40   c  (“Yes” at step S 301 ) and utilize multiple outdoor units  2   e  and  2   f  (“Yes” at step S 302 ), both oil balancing between the compressors  40   a ,  40   b ,  40   c  in the same outdoor units and oil balancing between the outdoor units  2   e  and  2   f  are required. The outdoor controller  50  of each outdoor units  2   e  and  2   f  detect the amount of the oil L in each compressors  40   a ,  40   b ,  40   c  using the oil level sensor  35   a , 35   b , 35   c , respective. If the oil amount in some compressors  40   a ,  40   b  or  40   c  is below the predetermined level or “too low” (“Yes” at step S 303 ), the oil solenoid valve V 2  in all operating outdoor unit  2   e  and  2   f  will be opened for the third predetermined time T 3  (for example, one minute), and then closed (Step S 304 ) in order to allow the exceeding oil flow to the main suction pipe  19 , thereby balancing the oil level in the compressors  40   a ,  40   b ,  40   c  of the same outdoor unit. Then, the oil solenoid valve V 1  in all operating outdoor unit  2   e  and  2   f  will be opened for the first predetermined time T 1  (for example, one minute), and then closed (Step S 305 ) in order to allow the exceeding oil flow to the main discharge pipe  10 , thereby balancing the oil level in between the operating outdoor units  2   e ,  2   f . Later, in the Step S 306 , the oil solenoid valve V 2  of all operating outdoor unit  2   e  and  2   f  will be opened for the third predetermined time T 3  (for example, one minute), and then closed in order to allow the exceeding oil flow to the main suction pipe  19 , thereby balancing the oil level in the compressors  40   a ,  40   b ,  40   c  of the same outdoor unit again. In case the amount of oil L in compressor  40   a , 40   b  and  40   c  of each outdoor unit  2   e  and  2   f  is in an appropriate level (“No” at step S 303 ), the oil balancing is not required. 
     In the step S 305 , if it is found out that the oil level of all the compressors is at the appropriate level by the output of the oil level sensor  35   a ,  35   b ,  35   c  before the first predetermined time T 1  elapses, the first solenoid valve V 1  of each operating outdoor unit may be closed, and step S 306  may be omitted. 
     In this embodiment, oil balancing is controlled by the virtue of sensing oil level by the oil level sensors, instead of sensing the compressor&#39;s running time. It is considered to have more advantages than other embodiments because oil balancing will be performed only when it is actually required, that is, only when the oil level in each compressor is not in an appropriate level. Therefore, the embodiment 3 of the present invention has a better performance. 
     REFERENCE SIGN LIST 
     
         
           1   a ,  1   b ,  1   c : Refrigerant cycle apparatus. 
           2   a , 2   b , 2   c , 2   d , 2   e , 2   f : Outdoor unit. 
           3 , 3   a , 3   b , 3   c , 3   d , 3   e : Indoor unit. 
           4 , 4   a , 4   b , 4   c , 40   a , 40   b , 40   c : High pressure type compressor. 
           5 , 5   a , 5   b , 5   c : Discharge pipe. 
           6 , 6   a , 6   b , 6   c : Oil separator. 
           7 , 7   a , 7   b , 7   c : High pressure outlet pipe. 
           8 , 8   a , 8   b , 8   c : Discharge check valve. 
           9 : Main discharge pipe. 
           9   a , 9   b , 9   c : Discharge pipe. 
           10 : Main discharge pipe. 
           11 :  4  way valve. 
           12 : Outdoor heat exchanger. 
           13 : Receiver tank. 
           14 : Liquid pipe. 
           15 : Expansion valve. 
           16 : Indoor heat exchanger. 
           17 : Gas pipe. 
           18 : Accumulator. 
           19 : Main suction pipe. 
           20 , 20   a , 20   b , 20   c : Suction pipe. 
           21 , 21   a , 21   b , 21   c : Suction muffler. 
           22 , 22   a , 22   b , 22   c , 28 , 28   a , 28   b , 28   c , 29 , 30 , 25   a , 25   b , 25   c , 26 , 27 : Oil balancing pipe. 
           23 , 23   a , 23   b , 23   c : Oil check valve. 
           24   a , 24   b , 24   c , 32 , 32   a , 32   b , 32   c : Capillary tube. 
           31 , 31   a , 31   b , 31   c , 33 , 33   a , 33   b , 33   c : Oil return pipe. 
           34   a , 34   b , 34   c , 34   d : Service valve. 
           35 , 35   a , 35   b , 35   c : Oil level sensor. 
           50 : Outdoor controller 
         V 1 ,V 2 : Solenoid valve