Patent Publication Number: US-7721559-B2

Title: Multi-type air conditioner and method for controlling the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of Korean Application No. P2004-93983, filed on Nov. 17, 2004, which is hereby incorporated by reference as if fully set forth herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to air conditioner, and more particularly, to a multi-type air conditioner and a method for controlling same, which can improve reliability of a compressor and system efficiency. 
   2. Discussion of the Related Art 
   In general, the air conditioner cools or/and heats rooms as the air conditioner performs steps of compression, condensing, expansion, and evaporation of refrigerant. In air conditioning systems, there are cooling air conditioner for supplying cold air to the room by operating a refrigerating cycle only in one direction, and cooling/heating air conditioner for supplying cold or warm air to the room by operating the refrigerating cycle in either direction, selectively. 
   Moreover, there are general air conditioners in which one indoor unit is connected to one outdoor unit, and multi-type air conditioners in which a plurality of indoor units are connected to one outdoor unit. In the meantime, the multi-type air conditioner may have one of more than one outdoor unit. 
   The multi-type air conditioner having one of more than one outdoor unit is provided with at least one compressor mounted on the outdoor unit. As the compressor, a single speed compressor of which operating frequency is constant, or a variable speed compressor of which operating frequency varies, is used. 
   However, the multi-type air conditioner having an outdoor unit with a plurality of compressors mounted thereon has the following problems. 
   That is, if the air conditioner is put into operation, there has been a problem of non-uniform distribution of oil among the plurality of compressors caused by differences of suction pressures of the compressors. Consequently, a compressor having a relative shortage of oil experiences drop of performance, to impair reliability. If the compressor is operated continuously in a state of oil shortage, the compressor is liable to burn. Moreover, the drop of performance of the compressor leads to drop an overall efficiency of the multi-type air conditioner, resulting to impair a cooling/heating performance. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed to a multi-type air conditioner and a method for controlling same that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
   An object of the present invention is to provide a multi-type air conditioner and a method for controlling same, in which an oil distribution among compressor is made uniform at an exact time the oil is distributed non-uniformly among the compressors, for improving the compressor performance, to improve a performance of the air conditioner. 
   Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
   To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a multi-type air conditioner includes a plurality of compressors, a plurality of temperature sensors mounted in the plurality of compressors for sensing temperatures in the compressors, respectively, and an equalizing pipe in communication with the plurality of compressors, for uniform distribution of oil among the plurality of compressors according to the temperatures sensed at the plurality of temperature sensors respectively. 
   Preferably, the temperature sensors are mounted adjacent tb ends of the equalizing pipe, respectively. 
   The ends of the equalizing pipe may be positioned at heights at least higher than a lowest oil level of the compressors, and the equalizing pipe may include branch pipes in communication with the plurality of compressors. 
   The multi-type air conditioner may further include a strainer in the equalizing pipe for separating foreign matters from flowing oil. 
   The plurality of compressors may include a variable capacity compressor of which capacity varies, and other one single speed compressor which is driven at a constant speed. 
   The multi-type air conditioner may further include a plurality of oil separators in communication with the plurality of compressors for separating oil from refrigerant from the compressors respectively, and a plurality of oil return pipes respectively connected between the oil separators and compressors for guiding oil separated at the oil separators to the compressors. The oil return pipes may be respectively connected to refrigerant suction pipes of the compressors into which refrigerant is drawn. 
   The multi-type air conditioner may further include strainers in the oil return pipes for separating foreign matters from flowing oil. 
   In the meantime, in another aspect of the present invention, a multi-type air conditioner includes at least one outdoor unit having a plurality of compressors for compressing refrigerant, at least one indoor unit connected to the outdoor unit with refrigerant pipes, a plurality of temperature sensors mounted in the plurality of compressors for sensing temperatures in the compressors, respectively, and an equalizing pipe in communication with the plurality of compressors, for uniform distribution of oil among the plurality of compressors according to the temperatures sensed at the plurality of temperature sensors respectively. 
   Preferably, the temperature sensors are mounted adjacent to the equalizing pipe in communication with insides of the compressors, respectively. 
   The ends of the equalizing pipe are positioned at heights at least higher than a lowest oil level of the compressors, and, if the multi-type air conditioner has one outdoor unit, the equalizing pipe may include branch pipes in communication with the plurality of compressors, and, if the multi-type air conditioner has a plurality of outdoor units, the equalizing pipe may include branch pipes in communication with the plurality of compressors in respective outdoor units, and connection pipes for making the plurality of the outdoor units in communication between the branch pipes. 
   The multi-type air conditioner may further include a strainer in the equalizing pipe for separating foreign matters from flowing oil. 
   If the multi-type air conditioner has a plurality of outdoor units, one of the outdoor units includes the plurality of compressors having a variable capacity compressor of which capacity varies, and other one single speed compressor which is driven at a constant speed, and other one of the outdoor units includes the plurality of compressors having the single speed compressors. 
   The multi-type air conditioner may further include a plurality of oil separators in communication with the plurality of compressors for separating oil from refrigerant from the compressors respectively, and a plurality of oil return pipes respectively connected between the oil separators and compressors for guiding oil separated at the oil separators to the compressors. The oil return pipes are respectively connected to refrigerant suction pipes of the compressors into which refrigerant are drawn. 
   The multi-type air conditioner may further include strainers in the oil return pipes for separating foreign matters from flowing oil. 
   Preferably, the refrigerant pipes connected between the outdoor unit and the indoor unit are parallel. 
   In another aspect of the present invention, a method for controlling a multi-type air conditioner having at least one outdoor unit having a plurality of compressors for compressing refrigerant, at least one indoor unit connected to the outdoor unit with refrigerant pipes, a plurality of temperature sensors mounted in the plurality of compressors for sensing temperatures in the compressors respectively, and an equalizing pipe in communication with the plurality of compressors, for uniform distribution of oil among the plurality of compressors according to the temperatures sensed at the plurality of temperature sensors respectively, includes a sensing step of sensing temperatures of insides of the plurality of compressors with the plurality of temperature sensors respectively, a comparing step of comparing the temperatures of the insides of the compressors sensed thus to preset critical temperatures of the compressors respectively, and an operation step of distributing oil uniformly among the plurality of compressors according to a result of the comparison. 
   The operation step is performed if there is at least one temperature below the critical temperature in the temperatures sensed with the temperature sensors, and the operation step includes the step of operating the plurality of compressors alternately for a preset time period. 
   The critical temperature is a temperature inside of the compressor when an amount of oil stored in the compressor is a minimum amount the compressor requires. 
   It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings; 
       FIG. 1  illustrates a perspective view of a multi-type air conditioner in accordance with a first preferred embodiment of the present invention, with partial cut away views; 
       FIG. 2  illustrates a diagram of the multi-type air conditioner in accordance with a first preferred embodiment of the present invention during cooling operation, schematically; 
       FIG. 3  illustrates key parts of an equalizing pipe arrangement between compressors in  FIG. 2 , schematically; 
       FIG. 4  illustrates a flow chart showing the steps of a method for controlling a multi-type air conditioner in accordance with a first preferred embodiment of the present invention; 
       FIG. 5  illustrates a graph showing a critical temperature Tc of a compressor, and a correlation between a compressor internal temperature T and time, of a multi-type air conditioner of the present invention; 
       FIG. 6  illustrates a perspective view of a multi-type air conditioner in accordance with a second preferred embodiment of the present invention, with partial cut away views; 
       FIG. 7  illustrates a diagram of the multi-type air conditioner in accordance with a second preferred embodiment of the present invention during cooling operation, schematically; 
       FIG. 8  illustrates a flow chart showing the steps of a method for controlling a multi-type air conditioner in accordance with a second preferred embodiment of the present invention; 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   A multi-type air conditioner in accordance with a first preferred embodiment of the present invention will be described, with reference to  FIGS. 1 to 5 . 
   Referring to  FIG. 1 , the multi-type air conditioner includes one outdoor unit  80 , a plurality of indoor units  51 ,  52 ,  53 , and  54 , and refrigerant pipelines  100  connected between the outdoor unit  80  and the indoor units  51 ,  52 ,  53 , and  54 . 
   The plurality of indoor units  51 ,  52 ,  53 , and  54  are connected in parallel to the outdoor unit  80  in parallel with the refrigerant pipelines  100 . That is, refrigerant flow between the outdoor unit  80  and the indoor units  51 ,  52 ,  53 , and  54  is made through the refrigerant pipelines  100 . 
   Referring to  FIG. 2 , the indoor unit  51 ,  52 ,  53 , or  54  includes an indoor fan  74  for drawing air from a room and discharging the air to the room again, and an indoor heat exchanger  72  for making the air drawn into the indoor unit  51 ,  52 ,  53 , or  54  to heat exchange with refrigerant, to cool or heat the air. 
   In the meantime, referring to  FIG. 2 , the outdoor unit  80  includes an outdoor fan  81  (see  FIG. 1 ) for drawing outdoor air therein and discharging the outdoor air again, an outdoor heat exchanger  82  for making the air drawn by the outdoor fan  81  to heat exchange with refrigerant, a plurality of compressors  83 , and  84  for compressing refrigerant, and a plurality of oil separators  85 , and  86  connected to refrigerant discharge pipes  83   a , and  84   a  of the compressors  83 , and  84  for separating oil from refrigerant from the compressor  83 , and  84 , respectively. 
   Though not shown, the compressor  83 , or  84  includes a compression unit having a compression chamber for compressing refrigerant, a motor unit for compressing the compression chamber, and an oil pump for pumping oil for lubrication of the motor unit or the compression unit. When the compressor discharges high temperature, and high pressure gaseous refrigerant, the oil is discharged together with the refrigerant, most of which is separated at the oil separator  85 , or  86 , and returns to a refrigerant suction pipe  83   b , or  84   b  of the compressor  83 , or  84 , such that some of the oil circulates the refrigerating cycle together with refrigerant. 
   It is preferable that the compressors  83 , and  84  include a variable capacity compressor  83  having a variable capacity, and a single speed compressor  84  which is driven at a constant speed, so that, if a load on the indoor units  51 ,  52 ,  53 , and  54  is low, for an example, in a case one or two of the plurality of indoor units  51 ,  52 ,  53 , and  54  is in operation, only the variable capacity compressor  83  may be operated in correspondence to the load on the indoor unit  51 ,  52 ,  53 , and  54 , and, if the load on the indoor units  51 ,  52 ,  53 , and  54  is relatively high, for an example, in a case three or four of the plurality of indoor units  51 ,  52 ,  53 , and  54  are in operation, the single speed compressor  84  is operated together with the variable capacity compressor  83 . 
   An unexplained reference numeral  87   a  denotes a common accumulator in the outdoor unit  80  connected to the refrigerant suction pipes  83   b  and  84   b  of the variable capacity compressor  83  and the signal speed compressor  84 , for accumulating liquid refrigerant so that only gaseous refrigerant is introduced into the variable capacity compressor  83  and the single speed compressor  84 . An unexplained reference numeral  87   b  denotes a 4-way valve in the outdoor unit  80  for changing over a flow path such that refrigerant from the oil separator flows either to the indoor heat exchanger or the outdoor heat exchanger  82  so that the plurality of indoor units  51 ,  52 ,  53 , and  54  can be used as coolers or heaters. An unexplained reference numeral  87   c  denotes a receiver in the outdoor unit  80  for storing surplus refrigerant, and making only liquid refrigerant to circulate toward an indoor unit side in cooling operation. An unexplained reference numeral  88   a  denotes an expansion device, such as an orifice or an electronic expansion valve, or so on, on the refrigerant pipeline  100  between the outdoor heat exchanger  82  and the indoor heat exchangers  72  for expanding refrigerant passed through the outdoor heat exchanger  82  or the indoor heat exchanger  72  to low temperature, and low pressure refrigerant. An unexplained reference numeral  88   b  denotes an electronic expansion valve on the outdoor unit for controlling a flow passage of the refrigerant for controlling a flow rate of the refrigerant circulating through the refrigerating cycle. An unexplained reference numeral  89  denotes a check valve on each of the refrigerant discharge pipes  83   a , and  84   a  of the variable capacity compressor  83  and the single speed compressor  84  for preventing reverse flow of the refrigerant or the oil. 
   In the meantime, referring to  FIG. 3 , the multi-type air conditioner further includes temperature sensors  163 , and  164  in the compressors respectively, for sensing inside temperatures of the compressors  83 , and  84  respectively, and an equalizing pipe  150  in communication with the compressors  83 , and  84  for making uniform distribution of oil between the compressors  83 , and  84  according to the temperatures sensed at the temperature sensors  163 , and  164 . 
   The temperature sensors  163 , and  164  are mounted adjacent to ends of the equalizing pipe  150 , respectively. That is, it is preferable that the temperature sensors  163 , and  164  are mounted at opposite ends of the equalizing pipe  150 , respectively. 
   The opposite ends of the equalizing pipe  150  are connected to position at a height at least higher than a lowest limit H 0  of an oil level of the compressors  83 , and  84 . It is preferable that the opposite ends of the equalizing pipe  150  are positioned at a height higher than the lowest limit H 0  of the oil level of the compressors  83 , and  84 . If there are three of more than three compressors in the outdoor unit, it is preferable that the equalizing pipe  150  is designed to be in communication with the compressors, to serve as a distribution pipe which prevents the oil from concentrating on one of the compressors, but makes the oil distributed among all the compressors. It is more preferable that a strainer  150   a  is further provided in the equalizing pipe  150  for separating foreign matters from flowing oil. 
   In the meantime, the oil separated at the oil separators  85 , and  86  returns to the compressors  83 , and  84  through oil return pipes  156 , and  157  connected between the oil separators  85 , and  86 , and the refrigerant suction pipes  83   b , and  84   b  of the compressors  83 , and  84 , respectively. 
   It is preferable that strainers  156   a , and  157   a  are mounted on the oil return pipes  156 , and  157  for separating foreign matters from the oil. 
   A method for controlling the foregoing multi-type air conditioner in accordance with a first preferred embodiment of the present invention will be described. For reference, if refrigerant is made to circulate in a direction of the outdoor heat exchanger  82 , the expansion device  88   a , and the indoor heat exchanger  72  starting from the compressors  83 , and  84  by means of the 4-way valve  87   b , the multi-type air conditioner forms a cooling cycle such that the indoor units  51 ,  52 ,  53 , and  54  cool rooms. If the refrigerant flow is changed over by means of the 4-way valve  87   b  such that the refrigerant flows in a reverse direction of the cooling, a heating cycle is formed such that the indoor units  51 ,  52 ,  53 , and  54  heat rooms. Accordingly, in the following description, only a case will be described, in which the multi-type air conditioner forms the cooling cycle. 
   If the multi-type air conditioner is operated such that some of the plurality of indoor units  51 ,  52 ,  53 , and  54  are in cooling operation, for an example, one or two of the indoor units  51 ,  52 ,  53 , and  54  is in cooling operation, the multi-type air conditioner operates the variable capacity compressor  83  only in the outdoor unit  80 , leaving the single speed compressor  83  in the outdoor unit  80  stationary. 
   Accordingly, the variable capacity compressor  83  compresses the refrigerant to a high temperature, high pressure refrigerant and discharges to the refrigerant discharge pipe  83   a  together with the oil, and most of the oil is separated from the refrigerant as the discharged refrigerant and oil passes through the oil separator  85 , and a portion of the oil circulates the refrigerating cycle together with the refrigerant. 
   That is, the refrigerant passed through the oil separator  85  passes through the 4-way valve  87   b , the outdoor heat exchanger  82 , the expansion device  88   a  in succession, and is introduced into the indoor heat exchanger of the indoor unit in cooling operation, vaporizes while cooling air around the indoor unit  72  such that the indoor unit serves as a cooler, and returns to the variable capacity compressor  83 . 
   The oil separated at the oil separator  85  returns to the refrigerant suction pipe  83   b  of the variable capacity compressor  83  through the oil return pipe  156 , and, therefrom to the variable capacity compressor  83  together with the refrigerant returning to the variable capacity compressor  83 . 
   In the meantime, the multi-type air conditioner causes a non-uniform distribution of oil between the compressors  83 , and  84  as oil in the refrigerating system concentrates on the variable capacity compressor  83  if operation of the variable capacity compressor  83  is continued for a long time period. 
   In this instance, the multi-type air conditioner of the present invention performs an operation in which the oil is distributed uniformly between the compressors  83 , and  84  at an exact time point through the equalizing pipe  150  and the temperature sensors  163 , and  164  at the opposite ends of the equalizing pipe  150 , for preventing wear and noise from the compressors  83 , and  84 , extending lifetimes of the compressor  83 , and  84 , and improving a system efficiency. 
   That is, referring to  FIG. 4 , upon putting the multi-type air conditioner in accordance with a first preferred embodiment of the present invention into operation, the temperature sensors  163 , and  164  at the opposite ends of the equalizing pipe  150  sense temperatures inside of the compressors  83 , and  84 , respectively. 
   Then, the control unit (not shown) compares the temperatures T 3 , and T 4  of the compressors  83 , and  84  sensed with the temperature sensors  163 , and  164  to a preset critical temperature Tc of the compressors  83 , and  84 . The critical temperature Tc is an inside temperature of the compressor  83 , or  84  when an amount of oil stored in the compressor  83 , or  84  is a minimum amount the compressor  83 , or  84  requires. 
   Accordingly, if one of the temperatures T 3 , and T 4  of the compressors  83 , or  84  drops below the critical temperature Tc of the compressors  83 , and  84 , an operation is performed for a predetermined time period to make oil distribution between the compressor  83 , and  84  uniform. 
   For an example, by operating the compressors  83 , and  84  alternately for the predetermined time period, the oil is transferred from the compressor  83  on which the oil is concentrated to the compressor  84  which has shortage of oil. 
   Thus, by performing the operation for uniform distribution of oil between the compressors  83 , and  84  at the exact time point at which the oil is distributed between the compressors  83 , and  84  non-uniformly, not only the performance of the compressor  83 , and  84  can be improved, but also system efficiency of the air conditioner having the compressors  83 , and  84  can be improved. 
   For reference, a principle of a method for equalizing oil between the compressors of the present invention will be described, with reference to  FIG. 5 . 
   During operation of the compressors, mixture of gaseous refrigerant, and oil is held in the compressor, and an amount of oil in the compressor which is relatively stationary is reduced gradually, leading to drop a concentration of oil in the compressor. 
   Once the oil concentration drops, a pressure in the compressor drops, leading to drop the inside temperature of the compressor, too. 
   Accordingly, by defining a temperature in the compressor in a case a minimum required quantity of oil is held in the compressor as a critical temperature Tc of the compressor, and a time point when the temperature T of the inside of the compressor is below the critical temperature Tc as a critical time point, the operation for equalizing oil between the compressors is performed, if the compressor at the critical time point is at least one. 
   In the meantime, the multi-type air conditioner operates both the variable capacity compressor  83  and the single speed compressor  84  in correspondence to the load on the indoor units  51 ,  52 ,  53 , and  54 , if three or four of the indoor units  51 ,  52 ,  53 , and  54  are operated. 
   In this case, by the method for equalizing oil between the compressors  83 , and  84 , an operation for distributing oil between the compressors  83 , and  84  uniformly is performed at an exact time point the non-uniform oil distribution occurs between the compressors  83 , and  84 , thereby preventing wear down and drop of performance of the compressors, and improving system efficiency of the air conditioner having the compressors. 
   Next, a multi-type air conditioner in accordance with a second preferred embodiment of the present invention will be described with reference to  FIGS. 6 to 8 . 
   Referring to  FIG. 6 , the multi-type air conditioner includes a plurality of outdoor units  80 , and  90 , a plurality of indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64 , and refrigerant pipelines  100  connected between the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64 , and the outdoor units  80 , and  90 . 
   Referring to  FIG. 7 , the refrigerant pipelines  100  include a plurality of indoor unit connection pipes  101 ,  102 ,  103 ,  104 ,  111 ,  112 ,  113 , and  114  respectively connected to the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64 , a plurality of outdoor unit connection pipes  120 , and  130  respectively connected to the outdoor units  80 , and  90 , and indoor/outdoor connection pipes  140  respectively connected between the indoor unit connection pipes  101 ,  102 ,  103 ,  104 ,  111 ,  112 ,  113 , and  114  and the connection pipes  120 , and  130 , to connect the plurality of indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  to the plurality of outdoor units  80 , and  90  in parallel. 
   The refrigerant passed through the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  joins at the indoor/outdoor connection pipes  140 , and distributed to the outdoor units  80 , and  90 , and the refrigerant passed through the outdoor units  80 , and  90  joins at the indoor/outdoor connection pipes  140 , and distributed to the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64 . 
   Moreover, each of the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  has an indoor fan  72  for drawing air from a room and discharging the air to the room again, and an indoor heat exchanger  74  for making the air drawn into the indoor unit to heat exchange with the refrigerant, to cool or heat the room. 
   In the meantime, though the present invention is not limited to numbers of the outdoor units  80 , and  90 , and the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64 , for convenience sake, the description of the multi-type air conditioner in accordance with a second preferred embodiment of the present invention will be proceeded with two outdoor units  80 , and  90 , and eight indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64 , taken as an example. 
   Referring to  FIG. 7 , the outdoor unit  80  includes an outdoor fan  81  (see  FIG. 6 ) for drawing outdoor air therein and discharging the outdoor air again, an outdoor heat exchanger  82  for making the air drawn by the outdoor fan  81  to heat exchange with refrigerant, a plurality of compressors  83 , and  84  for compressing refrigerant, and a plurality of oil separators  85 , and  86  connected to refrigerant discharge pipes  83   a , and  84   a  of the compressors  83 , and  84  for separating oil from refrigerant from the compressor  83 , and  84 , respectively. 
   In the meantime, though not shown, the compressor  83 , or  84  includes a compression unit having a compression chamber for compressing refrigerant, a motor unit for compressing the compression chamber, and an oil pump for pumping oil for lubrication of the motor unit or the compression unit. When the compressor discharges high temperature, and high pressure gaseous refrigerant, the oil is discharged together with the refrigerant, most of which is separated at the oil separator  85 , or  86 , and returns to a refrigerant suction pipe  83   b , or  84   b  of the compressor  83 , or  84 , such that some of the oil circulates the refrigerating cycle together with refrigerant. 
   It is preferable that the compressors  83 , and  84  include a variable capacity compressor  83  having a variable capacity, and a first single speed compressor  84  which is driven at a constant speed, so that, if a load on the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  is low, for an example, in a case one or two of the plurality of indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  is in operation, only the variable capacity compressor  83  may be operated in correspondence to the load on the indoor unit  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64 , and, if the load on the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  is relatively high, for an example, in a case three or four of the plurality of indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  are in operation, the first single speed compressor  84  is operated together with the variable capacity compressor  83 . 
   An unexplained reference numeral  87   a  denotes a common accumulator in the first outdoor unit  80  connected to the refrigerant suction pipes  83   b  and  84   b  of the variable capacity compressor  83  and the first signal speed compressor  84 , for accumulating liquid refrigerant so that only gaseous refrigerant is introduced into the variable capacity compressor  83  and the first single speed compressor  84 . An unexplained reference numeral  87   b  denotes a 4-way valve in the first outdoor unit  80  for changing over a flow path such that refrigerant from the oil separators  85 , and  86  flows either to the indoor heat exchanger  74  or the outdoor heat exchanger  82  so that the plurality of indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  can be used as coolers or heaters. An unexplained reference numeral  87   c  denotes a receiver in the first outdoor unit  80  for storing surplus refrigerant, and making only liquid refrigerant to circulate toward an indoor unit side in cooling operation. An unexplained reference numeral  88   a  denotes an expansion device, such as an orifice or an electronic expansion valve, or so on, on the refrigerant pipeline  100  between the outdoor heat exchanger  82  and the indoor heat exchangers  72  for expanding refrigerant passed through the outdoor heat exchanger  82  or the indoor heat exchanger  72  to low temperature, and low pressure refrigerant. An unexplained reference numeral  88   b  denotes an electronic expansion valve on the first outdoor unit for controlling a flow passage of the refrigerant for controlling a flow rate of the refrigerant circulating through the refrigerating cycle. An unexplained reference numeral  89  denotes a check valve on each of the refrigerant discharge pipes  83   a , and  84   a  of the variable capacity compressor  83  and the first single speed compressor  84  for preventing reverse flow of the refrigerant or the oil. 
   Referring to  FIG. 7 , system and operation of an outdoor fan  91  (see  FIG. 6 ), indoor an outdoor heat exchanger  92 , oil separators  95 ,  96 , a common accumulator  97   a , a 4-way valve  97   b , a receiver  97   c , an electronic expansion valve, and a check valve of the second outdoor unit  90  are the same with the first outdoor unit  80 , except the compressors  93 , and  94  for compressing refrigerant, detailed description thereof will be omitted. 
   It is preferable that the compressor of the second outdoor unit  90  includes second, and third single speed compressors  93 , and  94 , operated selectively depending on a load on the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64 . 
   That is, if the load on the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  is high, for an example, five or six of the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  are in operation, the second single speed compressor  93  may be operated together with the variable capacity compressor  83 , and the first single speed compressor  84 , and if the load on the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  is maximum, for an example, seven or eight of the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  are in operation, the third single speed compressor  94  may be operated, together with the variable speed compressor  83 , the first single speed compressor  84 , and the second single speed compressor  93 . 
   In the meantime, referring to  FIG. 3 , the multi-type air conditioner further includes temperature sensors  163 ,  164 ,  173 , and  174  in the compressors  83 ,  84 ,  93 , and  94  respectively, for sensing inside temperatures of the compressors  83 ,  84 ,  93 , and  94  respectively, and an equalizing pipe  150  in communication with the compressors  83 ,  84 ,  93 , and  94  for making uniform distribution of oil among the compressors  83 ,  84 ,  93 , and  94  according to the temperatures sensed at the temperature sensors  163 ,  164 ,  173 , and  174 . 
   The equalizing pipe  150  is in communication with the variable capacity compressor  83 , the first single speed compressor  84 , the second single speed compressor, and the third single speed compressor, to prevent the oil from concentrating on one of the compressors  83 ,  84 ,  93 , and  94 , and to make the oil distributed among all the compressors  83 ,  84 ,  93 , and  94 . The equalizing pipe  150  includes a plurality of branch pipes  151 ,  152 ,  153 , and  154 , and a connection pipe  155  between the outdoor units  80 , and  90  to make the branch pipes  151 , and  152 , and the branch pipes  153 , and  154  in communication. Ends of the branch pipes  151 ,  152 ,  153 , and  154  are connected to position at a height at least higher than a lowest limit of an oil level of the compressors  83 ,  84 ,  93 , and  94 . It is preferable that the ends of the branch pipes  151 ,  152 ,  153 , and  154  are positioned at a height higher than the lowest limit of the oil level of the compressors  83 ,  84 ,  93 , and  94 . It is more preferable that strainers  151   a ,  152   a ,  153   a , and  154   a  are further provided in the branch pipes  151 ,  152 ,  153 , and  154  respectively, for separating foreign matters from flowing oil. 
   The temperature sensors  163 ,  164 ,  173 , and  174  are respectively mounted in the compressors  83 ,  84 ,  93 , and  94  adjacent to the ends of the branch pipes  151 ,  152 ,  153 , and  154  in communication with insides of the compressors  83 ,  84 ,  93 , and  94 . 
   In the meantime, the oil separated at the oil separators  85 ,  86 ,  95 , and  96  returns to the compressors  83 ,  84 ,  93 , and  94  through oil return pipes  156 ,  157 ,  158 , and  159  connected between the oil separators  85 ,  86 ,  95 , and  96  and the refrigerant suction pipes  83   b ,  84   b ,  93   b , and  94   b  of the compressors  83 ,  84 ,  93 , and  94 , respectively. 
   It is preferable that strainers  156   a ,  157   a ,  158   a , and  159   a  are mounted in the oil return pipes  156 ,  157 ,  158 , and  159  for separating foreign matters from the oil. 
   A method for controlling the foregoing multi-type air conditioner in accordance with a second preferred embodiment of the present invention will be described. For reference, if refrigerant is made to circulate in a direction of the outdoor heat exchanger  82 , and  92 , the expansion device  88   a , and the indoor heat exchanger  72  starting from the compressors  83 ,  84 ,  94 , and  95  by means of the 4-way valves  87   b , and  97   b , the multi-type air conditioner forms a cooling cycle such that the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  cool rooms. If the refrigerant flow is changed over by means of the 4-way valves  87   b , and  97   b  such that the refrigerant flows in a reverse direction of the cooling, a heating cycle is formed such that the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  heat rooms. Accordingly, in the following description, only a case will be described, in which the multi-type air conditioner forms the cooling cycle. 
   If the multi-type air conditioner is operated such that some of the plurality of indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  are in cooling operation, for an example, one of the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  is in cooling operation, the multi-type air conditioner operates the variable capacity compressor  83  only in the outdoor unit  80 , leaving the first single speed compressor  83  in the first outdoor unit  80 , the second, and third single speed compressors  93 , and  94  in the second outdoor unit  90  stationary. 
   The variable capacity compressor  83  compresses the refrigerant to a high temperature, high pressure refrigerant and discharges to the refrigerant discharge pipe  83   a  together with the oil, and most of the oil is separated from the refrigerant as the discharged refrigerant and oil passes through the oil separator  85 , and a portion of the oil circulates the refrigerating cycle together with the refrigerant. 
   That is, the refrigerant passed through the oil separator passes through the 4-way valve  87   b , the outdoor heat exchanger  82 , the expansion device  88   a  in succession, and is introduced into the indoor heat exchanger of the indoor unit in cooling operation, vaporizes while cooling air around the indoor unit  72  such that the indoor unit serves as a cooler, and returns to the variable capacity compressor  83 . 
   The oil separated from refrigerant at the oil separator  85  returns to the refrigerant suction pipe  83   b  of the variable capacity compressor  83  through the oil return pipe  156 , and, therefrom to the variable capacity compressor  83  together with the refrigerant returning to the variable capacity compressor  83 . 
   In the meantime, the multi-type air conditioner causes a non-uniform distribution of oil among the compressors  83 ,  84 ,  94 , and  95  as oil in the refrigerating system concentrates on the variable capacity compressor  83  if operation of the variable capacity compressor  83  is continued for a long time period. 
   In this instance, the multi-type air conditioner of in accordance with a second preferred embodiment of the present invention performs an operation in which the oil is distributed uniformly among the compressors  83 ,  84 ,  93 , and  94  at an exact time point through the equalizing pipe  150  having the branch pipes  151 ,  152 ,  153 , and  154 , and the connection pipe  155 , and the temperature sensors  163 ,  164 ,  173 , and  174  at the ends of the branch pipes  151 ,  152 ,  153 , and  154 , for preventing wear and noise from the compressors  83 ,  84 ,  93 , and  94 , extending lifetimes of the compressor  83 ,  84 ,  93 , and  94 , and improving a system efficiency. 
   That is, referring to  FIG. 8 , upon putting the multi-type air conditioner in accordance with a second preferred embodiment of the present invention into operation, the temperature sensors  163 ,  164 ,  173 , and  174  at the ends of the branch pipes  151 ,  152 ,  153 , and  154  sense temperatures inside of the compressors  83 ,  84 ,  93 , and  94 , respectively. 
   Then, the control unit (not shown) compares the temperatures T 13 , T 14 , T 23 , and T 24  of the compressors  83 ,  84 ,  93 , and  94  sensed with the temperature sensors  163 ,  164 ,  173 , and  174  to a preset critical temperature Tc of the compressors  83 ,  84 ,  93 , and  94 . The critical temperature Tc is an inside temperature of the compressor when an amount of oil stored in the compressor is a minimum amount the compressor requires. 
   Accordingly, if one of the temperatures T 13 , T 14 , T 23 , and T 24  of the compressors  83 ,  84 ,  93 , and  94  drops below the critical temperature Tc of the compressors  83 ,  84 ,  93 , and  94 , an operation is performed for a predetermined time period to make oil distribution among the compressor  83 ,  84 ,  93 , and  94  uniform. 
   For an example, by operating the compressors  83 ,  84 ,  93 , and  94  alternately for the predetermined time period, the oil is transferred from the compressor  83  on which the oil is concentrated to the compressors  84 ,  93 , and  94  which have shortage of oil. 
   Thus, by performing the operation for uniform distribution of oil among the compressors  83 ,  84 ,  93 , and  94  at the exact time point at which the oil is distributed among the compressors  83 ,  84 ,  93 , and  94  non-uniformly, not only the performance of the compressor  83 ,  84 ,  93 , and  94  can be improved, but also system efficiency of the air conditioner having the compressors  83 ,  84 ,  93 , and  94  can be improved. 
   In the meantime, if two, or more than two of the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64  are in operation, the first, second, and third single speed compressors  84 ,  93 , and  94  are operated in correspondence to the load on the indoor units  51 ,  52 ,  53 ,  54 ,  61 ,  62 ,  63 , and  64 . 
   In this case too, the performance of the operation for uniform distribution of oil among the compressors  83 ,  84 ,  93 , and  94  at the exact time point at which the oil is distributed among the compressors  83 ,  84 ,  93 , and  94  non-uniformly, not only permits improvement of the performance of the compressors  83 ,  84 ,  93 , and  94 , but also permits improvement of system efficiency of the air conditioner having the compressors  83 ,  84 ,  93 , and  94 . 
   The multi-type air conditioner of the present invention has the following advantages. 
   First, the uniform distribution of oil among the compressors at an exact time point at which the oil is distributed among the compressors non-uniformly, not only permits improvement of the performance of the compressors, but also permits improvement of system efficiency of the air conditioner having the compressors. 
   Second, the performance of the operation for uniform distribution of oil among the compressors only at a time point at which the oil is distributed among the compressors non-uniformly permits to minimize power consumption, to reduce an energy consumption effectively. 
   The improvement of performance and efficiency of the multi-type air conditioner coming from improvement of the compressors permits to improve the cooling/heating performance of the multi-type air conditioner. 
   It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.