Abstract:
A heat recovery variable-frequency multi-split heat pump system and a control method thereof. The system comprises an outdoor unit and at least two indoor units. The system is a three-pipe heating recovery multi-split heat pump system designed on the basis of a four-way reversing valve, and one indoor unit thereof is provided with two electronic expansion valves and two heat exchangers so that any indoor unit in the system can operate independently under three working conditions of refrigeration, heating or heat recovery dehumidification. Under multi-split condition, the system can operate under six working conditions, namely, the full-refrigeration working condition, the full-heating working condition, the common-heat-recovery working condition, the common-heat-recovery-dehumidification working condition, the heat recovery dehumidification-refrigeration-combination working condition and the heat recovery dehumidification-heating-combination working condition. Under the heat recovery dehumidification condition, a lower outlet air temperature, during low-temperature dehumidification, is raised by means of heat removal of a condenser so as to achieve the purpose of dehumidification without temperature fall or temperature rise, so that the thermal comfort and efficiency of the system are improved, and the refrigerating capacity and heating capacity of the system are effectively improved.

Description:
[0001]    This application claims the benefit of Chinese Patent Application No. 201410039143.3, filed with the Chinese Patent Office on Jan. 27, 2014 and entitled “Heat recovery variable-frequency multi-connected heat pump system and control method thereof”, which is hereby incorporated by reference in its entirety. 
       FIELD 
       [0002]    The present invention relates to the field of variable-frequency multi-connected heat pumps, and particularly to a heat recovery variable-frequency multi-connected heat pump system, and a method for controlling the same. 
       BACKGROUND 
       [0003]    A variable-frequency multi-connected heat pump system is a complex refrigerating system with a complex structure, a large architecture, highly coupled internal parameters, and a variety of boundary conditions, which is characterized by a highly varying incumbent load demand, a large number of connected indoor units, a varying complex operating condition, etc., and is an important direction that air conditioners are evolving toward. As illustrated in  FIG. 1 , the multi-connected heat pump system generally includes one or more outdoor units  01 , one or more indoor units  02 , a central control network (CS-NET)  03 , a refrigerant pipe  04 , a manifold pipe OS, and a communication line  06 , where the outdoor units constitute a set of outdoor units controlled by the central control network through the communication line, and the outdoor units are connected with the indoor units through the refrigerant pipe and the manifold pipe, As compared with a number of air conditioners at home, the outdoor units of the multi-connected heat pump system are shared so that the cost of the devices can be lowered effectively, and the respective indoor units can he centrally managed, where one or more of the indoor units can he started separately or concurrently into operation, thus making it more flexible to control. 
         [0004]    At present, people are demanding high comfort of their living, in an engineering application, there are some of the indoor units in the multi-connected heat pump system being refrigerating while the other indoor units being heating. In order to make full use of advantageous energy-saving in the multi-connected heat pump system, heat in the system is recovered in the prior art to enable the operating indoor units to refrigerate and heat concurrently. However generally a number of four-way valves have to he arranged to allocate the refrigerant and to recover discharged heat, thus making it complex and costly to control the pipes of the system. Moreover if the prior art is applied in a rainy season and a moist region, then the system has to refrigerate for the purpose of temperature lowering and dehumidification, and if the temperature of the environment is low, then the effect of dehumidification may be poor, and the temperature of outgoing air may be low, thus resulting in degraded thermal comfort. 
       SUMMARY 
       [0005]    An object of the invention is to provide a heat recovery variable-frequency multi-connected heat pump system so as to address the technical problem in the prior art of being complex and costly to control the pipes of the system because the heat in the system has to be recovered using a number of four-way valves. 
         [0006]    In order to address the technical problem above, the invention is embodied in the following technical solutions: 
         [0007]    A heat recovery variable-frequency multi-connected heat pump system includes an outdoor unit and at least two indoor units, wherein the outdoor unit and the indoor units are connected through a high-pressure liquid pipe, a high-pressure gas pipe, and a low-pressure gas return pipe, wherein: 
         [0008]    the outdoor unit includes a four-way valve ( 4 ), an outdoor heat exchanger ( 5 ), an outdoor electronic expansion valve ( 6 ), a second electromagnetic valve ( 10 ), a third electromagnetic valve ( 11 ), and at least one compressor ( 1 ); 
         [0009]    an exhaust end of the compressor ( 1 ) is connected with an A end of the four-way valve ( 4 ); a B end of the four-way valve ( 4 ) is connected with the outdoor heat exchanger ( 5 ), and an other end of the outdoor heat exchanger ( 5 ) is connected with the high-pressure liquid pipe through the outdoor electronic expansion valve ( 6 ); a C end of the four-way valve ( 4 ) is connected with a suction end of the compressor ( 1 ); a D end of the four-way valve ( 4 ) is connected with the low-pressure gas return pipe; the third electromagnetic valve ( 11 ) is connected between the high-pressure gas pipe and the low-pressure gas return pipe; and one end of the second electromagnetic valve ( 10 ) is connected with the A end of the four-way valve ( 4 ), and the other end thereof is connected between the third electromagnetic valve ( 11 ) and the high-pressure gas pipe; 
         [0010]    each of the indoor units includes a first indoor heat exchanger ( 16 ), a first electronic expansion valve ( 15 ), a second indoor heat exchanger ( 18 ), and a second electronic expansion valve ( 17 ); and 
         [0011]    one end of the first indoor heat exchanger ( 16 ) is connected with the high-pressure gas pipe, and an other end of the first indoor heat exchanger ( 16 ) is connected with the high-pressure liquid pipe through the first electronic expansion valve ( 15 ); and one end of the second indoor heat exchangers ( 18 ) is connected with the low-pressure gas return pipe, and an other end of the second indoor heat exchanger ( 18 ) is connected with the high-pressure liquid pipe through the second electronic expansion valve ( 17 ). 
         [0012]    Optionally an oil separator ( 2 ) is connected between the exhaust end of the compressor ( 1 ), and the A end of the four-way valve ( 4 ). 
         [0013]    Optionally the oil separator ( 2 ) is connected with the suction end of the compressor ( 1 ) through a first electromagnetic valve ( 8 ) and a capillary ( 9 ). 
         [0014]    Optionally a check valve ( 3 ) is connected between the oil separator ( 2 ) and the A end of the four-way valve ( 4 ). 
         [0015]    Optionally a vapor-liquid separator ( 7 ) is connected between the C end of the four-way valve ( 4 ), and the suction end of the compressor ( 1 ). 
         [0016]    Optionally a liquid-side stop valve ( 12 ) is arranged on the high-pressure liquid pipe; a gas-side stop valve ( 13 ) is arranged on the low-pressure gas return pipe; and a discharge line valve ( 14 ) is arranged on the high-pressure gas pipe. 
         [0017]    An embodiment of the invention further provides a first method for controlling the heat recovery variable-frequency multi-connected heat pump system, the method controlling the heat pump system under a fully-refrigerating condition to perform operations of: connecting the A end and the B end of the four-way valve ( 4 ), and connecting the C end and the D end thereof; closing the second electromagnetic valve ( 10 ), and opening the third electromagnetic valve ( 11 ); fully-opening the outdoor electronic expansion valve ( 6 ); and enabling the first electronic expansion valves ( 15 ) and the second electronic expansion valves ( 17 ) of the respective indoor units to throttle for lower pressure; so that exhaust refrigerant from the exhaust end of the compressor ( 1 ) is condensed by the outdoor heat exchanger ( 5 ), wherein one branch of the refrigerant flows through the first electronic expansion valves ( 15 ) and the first indoor heat exchangers ( 16 ) of the respective indoor units, and the third electromagnetic valve ( 11 ) to the D end of the four-way valve ( 4 ), and an other branch of the refrigerant flows through the second electronic expansion valves ( 17 ) and the second indoor heat exchangers ( 18 ) of the respective indoor units to the D end of the four-way valve ( 4 ); and thereafter the refrigerant enters the suction end of the compressor ( 1 ) through the C end of the four-way valve ( 4 ). 
         [0018]    An embodiment of the invention further provides a second method for controlling the heat recovery variable-frequency multi-connected heat pump system, the method controlling the heat pump system under a fully-heating condition to perform operations of: connecting the A end and the D end of the four-way valve ( 4 ), and connecting the B end and the C end thereof; opening the second electromagnetic valve ( 10 ), and closing the third electromagnetic valve ( 11 ); enabling the outdoor electronic expansion valve ( 6 ) to throttle for lower pressure; and enabling both the first electronic expansion valves ( 15 ) and the second electronic expansion valves ( 17 ) of the respective indoor units to throttle for lower pressure; so that one branch of exhaust refrigerant from the exhaust end of the compressor ( 1 ) flows out of the A and D ends of the four-way valve ( 4 ) to the second indoor heat exchangers ( 18 ) and the second electronic expansion valves ( 17 ) of the respective indoor units, and an other branch of the refrigerant flows out of the second electromagnetic valve ( 10 ) to the first indoor heat exchangers ( 16 ) and the first electronic expansion valves ( 15 ) of the respective indoor units; and the refrigerant from the first electronic expansion valve ( 15 ), and the refrigerant from the second electronic expansion valve ( 17 ) meet, and then flow through the outdoor electronic expansion valve ( 6 ), the outdoor heat exchanger ( 5 ), and the B and C ends of the four-way valve ( 4 ), and enter the suction end of the compressor ( 1 ). 
         [0019]    An embodiment of the invention further provides a third method for controlling the heat recovery variable-frequency multi-connected heat pump system, the method controlling the heat pump system under a normally-heat-recovery condition to perform operations of: connecting the A end and the B end of the four-way valve ( 4 ), and connecting the C end and the D end thereof; opening the second electromagnetic valve ( 10 ), and closing the third electromagnetic valve ( 11 ); fully-opening the outdoor electronic expansion valve ( 6 ); and fully-opening the first electronic expansion valve ( 15 ) and fully-closing the second electronic expansion valve ( 17 ) in each of a part of the indoor units, and fully-closing the first electronic expansion valve ( 15 ) and enabling the second electronic expansion valve ( 17 ) to throttle for lower pressure in each of other indoor units; so that one branch of exhaust refrigerant from the exhaust end of the compressor ( 1 ) flows through the second electromagnetic valve ( 10 ), the first indoor heat exchanger ( 16 ), and the first electronic expansion valve ( 15 ), and an other branch of the refrigerant flows out of the A and B ends of the four-way valve ( 4 ) to the outdoor heat exchanger ( 5 ) for being condensation, flows through the outdoor electronic expansion valve( 6 ), then meets the refrigerant flowing out of the first electronic expansion valve ( 15 ), further flows through the second electronic expansion valve ( 17 ) and the indoor heat exchanger ( 18 ) of each of the other indoor units sequentially, and thereafter enters the suction end of the compressor ( 1 ) through the D and C ends of the four-way valve ( 4 ). 
         [0020]    An embodiment of the invention further provides a fourth method for controlling the heat recovery variable-frequency multi-connected heat pump system, the method controlling the heat pump system under a normally-heat-recovery and dehumidifying condition to perform operations of connecting the A end and the B end of the four-way valve ( 4 ), and connecting the C end and the D end thereof; opening the second electromagnetic valve ( 10 ), and closing the third electromagnetic valve ( 11 ); fully-opening the outdoor electronic expansion valve ( 6 ); and fully-opening the first electronic expansion valve ( 15 ) and enabling the second electronic expansion valve ( 17 ) to throttle for lower pressure in each of indoor units; so that one branch of exhaust refrigerant from the exhaust end of the compressor ( 1 ) flows through the outdoor heat exchanger ( 5 ), and an other branch of the refrigerant flows through the second electromagnetic valve ( 10 ), and the first indoor heat exchanger ( 16 ), and  - the refrigerant flowing out of the first indoor heat exchanger ( 16 ), and the refrigerant flowing out of the outdoor heat exchange ( 5 ) meet at the second electronic expansion valve ( 17 ) and the second indoor heat exchanger ( 18 ), and thereafter enter the suction end of the compressor ( 1 ) through the D and C ends of the four-way valve ( 4 ). 
         [0021]    An embodiment of the invention further provides a fifth method for controlling the heat recovery variable-frequency multi-connected heat pump system, the method controlling the heat pump system under a heat-recovery and dehumidifying plus refrigerating hybrid condition to perform operations of: connecting the A end and the B end of the four-way valve ( 4 ), and connecting the C end and the D end thereof; opening the second electromagnetic valve ( 10 ), and closing the third electromagnetic valve ( 11 ); fully-opening the outdoor electronic expansion valve ( 6 ); and fully-opening the first electronic expansion valve ( 15 ) and enabling the second electronic expansion valve ( 17 ) to throttle for lower pressure in each of a part of the indoor units, and fully-closing the first electronic expansion valve ( 15 ) and enabling the second electronic expansion valve ( 17 ) to throttle for lower pressure in each of other indoor units; so that one branch of exhaust refrigerant from the exhaust end of the compressor ( 1 ) flows through the outdoor heat exchanger ( 5 ), and the second electronic expansion valve ( 17 ) and the second indoor heat exchanger ( 18 ) of each of the other indoor units, and an other branch of the refrigerant flows through the second electromagnetic valve ( 10 ), and the first indoor heat exchanger ( 16 ) of each of the part of the indoor units; and the refrigerant flowing out of the first indoor heat exchangers ( 16 ), and the refrigerant flowing out of the outdoor heat exchanger ( 5 ) meet at the second electronic expansion valve ( 17 ) and the second indoor heat exchanger ( 18 ), and the refrigerant flowing through the second indoor heat exchanger ( 18 ) of each of the part of the indoor units, and the refrigerant flowing through the second indoor heat exchanger ( 18 ) of each of the other indoor units meet, and thereafter enter the suction end of the compressor ( 1 ) through the D and C ends of the four-way valve ( 4 ). 
         [0022]    An embodiment of the invention further provides a sixth method for controlling the heat recovery variable-frequency multi-connected heat pump system, the method controlling the heat pump system under a normally-heat-recovery and dehumidifying plus heating hybrid condition to perform operations of connecting the A end and the B end of the four-way valve ( 4 ), and connecting the C end and the D end thereof; opening the second electromagnetic valve ( 10 ), and closing the third electromagnetic valve ( 11 ); fully-closing the outdoor electronic expansion valve ( 6 ); fully-opening the first electronic expansion valves ( 15 ) of the respective indoor units; and enabling the second electronic expansion valves ( 17 ) of a part of the indoor units to throttle for lower pressure, and fully-closing the second electronic expansion valves ( 17 ) of the other indoor units; so that exhaust refrigerant from the exhaust end of the compressor ( 1 ) flows through the second electromagnetic valve ( 10 ), wherein one branch of the refrigerant flows through the first indoor heat exchanger ( 16 ) and the first electronic expansion valves ( 15 ) of each of the part of the indoor units, and an other branch of the refrigerant flows through the first indoor heat exchanger ( 16 ) and the first electronic expansion valve ( 15 ) of each of the other indoor units; and the refrigerant flowing out of the first electronic expansion valves ( 15 ) meets, and then flows through the second electronic expansion valve ( 17 ) and the second indoor heat exchanger ( 18 ), and the D and C ends of the four-way valve ( 4 ) sequentially, and thereafter enters the suction end of the compressor ( 1 ). 
         [0023]    As compared with the prior art, advantages and positive effects of the invention lie in that the invention is a heat recovery variable-frequency multi-connected heat pump system in which three pipes are designed using a four-way valve, and the indoor units include two sets of electronic expansion valves, and two sets of heat exchangers, so that any one of the indoor units in the system can operate separately to refrigerate, to heat, or to recover heat and dehumidify; and there are six operating conditions in which the multi-connected system may function, i.e., a fully-refrigerating condition, a fully-heating condition, a normally-heat-recovery condition, a normally-heat-recovery and dehumidifying condition, a heat-recovery and dehumidifying plus refrigerating hybrid condition, and a heat-recovery and dehumidifying plus heating hybrid condition. Particularly the principle of heat-recovery and dehumidifying lies in the use of the condenser to exhaust heat to thereby improve the low temperature of outgoing air while dehumidifying at low temperature for the purpose of dehumidifying without lowering the temperature, or of humidifying while raising the temperature, thus improving the thermal comfort and the efficiency of the system, and also improve in effect the capacity of refrigerating and heating by the system. The application scope of the invention can be extended in effect by selecting one of the six operating conditions in which the system functions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    In order to make the technical solutions according to the embodiments of the invention more apparent, the drawings to which reference is made in the description of the embodiments will be described below briefly, and apparently the drawings described below are merely illustrative of some of the embodiments of the invention, and those ordinarily skilled in the art can further obtain other drawings from these drawings without any inventive effort. in the drawings: 
           [0025]      FIG. 1  is a schematic diagram of the connection of the multi-connected heat pump system in the prior art; 
           [0026]      FIG. 2  is a schematic diagram of a system according to an embodiment of the invention; 
           [0027]      FIG. 3  is a schematic diagram of the system operating under a fully-refrigerating condition according to an embodiment of the invention; 
           [0028]      FIG. 4  is a schematic diagram of the system operating under a fully-heating condition according to an embodiment of the invention; 
           [0029]      FIG. 5  is a schematic diagram of the system operating under a normally-heat-recovery condition according to an embodiment of the invention; 
           [0030]      FIG. 6  is a schematic diagram of system operating under a heat-recovery and dehumidifying condition according to an embodiment of the invention; 
           [0031]      FIG. 7  is a schematic diagram of the system operating under a heat-recycling and dehumidifying plus refrigerating hybrid condition according to an embodiment of the invention; 
           [0032]      FIG. 8  is a schematic diagram of the system operating under a heat-recycling and dehumidifying plus heating hybrid condition according to an embodiment of the invention; and 
           [0033]      FIG. 9  is a schematic diagram of a system according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0034]    In order to make the objects, technical solutions, and advantages of the invention more apparent, the invention will be described below in further details with reference to the drawings, and apparently the embodiments described below are only a part but not all of the embodiments of the invention. Based upon the embodiments here of the invention, all the other embodiments which can occur to those skilled in the art without any inventive effort shall fall into the scope of the invention. 
         [0035]    First Embodiment: 
         [0036]    This embodiment relates to an embodiment of the invention including two indoor units by way of an example. 
         [0037]    As illustrated in  FIG. 2 , a multi-connected heat pump system according to this embodiment includes an outdoor unit  01 , and two indoor units  02  and  02 ′. The outdoor unit  01  includes a compressor  1 , an oil separator  2 , a check valve  3 , a four-way valve  4 , an outdoor heat exchanger  5 , an outdoor electronic expansion valve  6 , a vapor-liquid separator  7 , a first electromagnetic valve  8 , a capillary  9 , a second electromagnetic valve  10 , a third electromagnetic valve  11 , a liquid-side stop valve  12 , a gas-side stop valve  13 , and a discharge line valve  14 . 
         [0038]    The first indoor unit  02  includes a first indoor electronic expansion valve  15 , a first indoor heat exchanger  16 , a second indoor electronic expansion valve  17 , a second indoor heat exchanger  18 , and a fourth manifold pipe  05 . 
         [0039]    The second indoor unit  02 ′ includes the same components as the first indoor unit  02 , and it includes a first indoor electronic expansion valve  15 , a first indoor heat exchanger  16 , a second electronic expansion valve  17 , a second indoor heat exchanger  18 , and a fourth manifold pipe  05 . 
         [0040]    The compressor  01  is a variable-frequency compressor which can include one or more compressors connected in parallel. The outdoor heat exchanger  5 , the first indoor heat exchanger  16 , and the second indoor heat exchanger  18  are air-cooling fin-tube heat exchanger, or aluminum micro-channel heat exchanger. 
         [0041]    In a real application, a fan of the outdoor unit blows the air to flow through the outdoor heat exchanger  5  for heat exchange between the air and a refrigerant. A fan of the indoor unit blows the return air to flow through the second indoor heat exchanger  18  and the first indoor heat exchanger  16  sequentially, that is, the air entering an air return port of the indoor unit exchanges heat with firstly the second indoor heat exchanger  18  and then the first indoor heat exchanger  16 , and finally flows out of the indoor unit. 
         [0042]    The respective components in the outdoor unit  01  are connected in such a way that a exhaust end of the compressor  1  is connected with an inlet end of the oil separator  2 , an air outlet end of the oil separator  2  is connected with one end of the check valve  3 , the other end of the check valve  3  is connected with an A-end of the four-way valve  4 , a B end of the four-way valve  4  is connected with one end of the outdoor heat exchanger  5 , the other end of the outdoor heat exchanger  5  is connected with one end of the outdoor electronic expansion valve  6 , the other end of the outdoor electronic expansion valve  6  is connected with one end of the liquid-side stop valve  12 , a C end of the four-way valve  4  is connected with one end of the vapor-liquid separator  7 , and the other end of the vapor-liquid separator  7  is connected with a suction end of the compressor  1 ; a D end of the four-way valve  4  is connected with one end of the third electromagnetic valve  11 , and one end of the gas-side stop valve  13 , both the other end of the third electromagnetic valve  11 , and one end of the discharge line valve  14  are connected with one end of the second electromagnetic valve  10 , and the other end of the second electromagnetic valve  10  is connected with the A-end of the four-way valve  4 ; and an oil outlet end of the oil separator  2  is connected with one end of the first electromagnetic valve  8 , the other end of the first electromagnetic valve  8  is connected with one end of the capillary  9 , and the other end of the capillary  9  is connected with a suction end of the compressor  1 . 
         [0043]    The outdoor unit  01  is connected with the indoor units  02  and  02 ′ in such a way that the other end of the liquid-side stop valve  12  is connected with one end of the first indoor electronic expansion valve  15 , and one end of the second electronic expansion valve  17  through a first manifold pipe  051  and the fourth manifold pipe  05 , the other end of the first indoor electronic expansion valve  15  is connected with one end of the first indoor heat exchanger  16 , and the other end of the first indoor heat exchanger  16  is connected with the other end of the discharge line valve  14  through a third manifold pipe  053 ; and the other end of the second electronic expansion valve  17  is connected with one end of the second indoor heat exchanger  18 , and the other end of the second indoor heat exchanger  18  is connected with the other end of the gas-side stop valve  13  through a second manifold pipe  052 . 
         [0044]    Rules of closing and opening the respective components of, and flowing of the refrigerant throughout, the multi-connected heat pump system according to this embodiment under respective operating conditions will be described below: 
         [0045]    1) Fully-Refrigerating Condition 
         [0046]    If all the indoor units in the system are refrigerating, then the system will be operating under a fully-refrigerating condition as illustrated in  FIG. 3 . Under this operating condition, the A end and the B end of the four-way valve  4  of the outdoor unit are connected, and the C end and the D end thereof are connected, the second electromagnetic valve  10  is closed, the third electromagnetic valve  11  is opened, and the outdoor electronic expansion valve  6  is fully-opened. In the indoor units  02  and  02 ′, both the first electronic expansion valves  15  and the second electronic expansion valves  17  are throttling for lower pressure. The outdoor heat exchangers  5  are condensers, and both the first indoor heat exchangers  16  and the second indoor heat exchangers  18  are evaporators. 
         [0047]    The exhaust high-pressure gaseous refrigerant from an exhaust end of the compressor  1  passes the oil separator  2 , the check valve  3 , and the A and B ends of the four-way valve  4 , and is condensed by the outdoor heat exchanger  5  into the high-pressure liquid refrigerant, and the high-pressure liquid refrigerant passing through the liquid-side stop valve  12 , the first manifold pipe  051 , and the fourth manifold pipe  05  is divided into two streams of refrigerant, where one stream of refrigerant flows through the first electronic expansion valves  15  and the first indoor heat exchangers  16  of the indoor units  02  and  02 ′, the third manifold pipe  053 , the discharge line valve  14 , and the third electromagnetic valve  11  sequentially; and the other stream of refrigerant flows through the second electronic expansion valves  17  and the second indoor heat exchangers  18  of the indoor units  02  and  02 ′, the second manifold pipe  052 , and the gas-side stop valve  13  sequentially, and then meets the first stream of refrigerant. The confluent refrigerant enters the vapor-liquid separator  7  through the D and C ends of the four-way valve  4 , and is separated into gas and liquid entering the suction end of the compressor  1 . 
         [0048]    2) Fully-Heating Condition 
         [0049]    If all the indoor units in the system are heating, then the system will be operating under a fully-heating condition as illustrated in  FIG. 4 . Under this operating condition, the A end and the D end of the four-way valve  4  of the outdoor unit are connected, and the B end and the C end thereof are connected, the second electromagnetic valve  10  is opened, the third electromagnetic valve  11  is closed, and the outdoor electronic expansion valve  6  is throttling for lower pressure. In the indoor units  02  and  02 ′, the first electronic expansion valves  15  and the second electronic expansion valves  17  are throttling. The outdoor heat exchanger  5  is evaporator, and both the first indoor heat exchangers  16  and the second indoor heat exchangers  18  are condensers. 
         [0050]    The exhaust high-pressure gaseous refrigerant from an exhaust end of the compressor  1  passes the oil separator  2  to the check valve  3 , and the high-pressure gaseous refrigerant flowing out of the check valve  3  is divided into two streams, where one stream of the refirgerant flows out of the A and D ends of the four-way valve  4 , and flows through the gas-side stop valve  13 , the second manifold  052 , and the second indoor heat exchangers  18  and the second indoor electronic expansion valves  17  of the indoor units  02  and  02 ′ sequentially; and the other stream of refrigerant flows out of the second electromagnetic valve  10 , and thereafter flows through the discharge line valve  14 , the third manifold pipe  053 , and the first indoor heat exchangers  16  and the first electronic expansion valves  15  of the indoor units  02  and  02 ′ sequentially. The two streams of refrigerant meet at the fourth manifold pipe  05 , and then flows through the first manifold pipe  051 , the liquid-side stop valve  12 , the outdoor electronic expansion valve  6 , the outdoor heat exchanger  5 , the B and C ends of the four-way valve  4 , and the vapor-liquid separator  7  sequentially, and thereafter enters suction end of the compressor  1 . 
         [0051]    3) Normally-Heat-Recovery Condition 
         [0052]    If a part of the indoor units are heating, and the other indoor units are refrigerating, then the system will be operating under a normally-heat recovery condition which will he exemplified in an embodiment as illustrated in  FIG. 5  where the first indoor unit  02  is heating, and the second indoor unit  02 ′ is refrigerating. At this time, the A end and the B end of the four-way valve  4  of the outdoor unit are connected, and the C end and the D end thereof are connected, the second electromagnetic valve  10  is opened, the third electromagnetic valve  11  is closed, and the outdoor electronic expansion valve  6  is fully-opened. In the indoor unit  02 , the first electronic expansion valve  15  is fully-opened, and the second electronic expansion valve  17  is fully-closed; and in the indoor unit  02 ′, the second indoor electronic expansion valve  17  is throttling, and the first indoor electronic expansion valve  15  is fully-closed. Both the outdoor heat exchanger  5 , and the first indoor heat exchanger  16  of the indoor unit  02  are condensers, the second indoor heat exchanger  18  of the indoor unit  02 ′ is an evaporator, neither the second indoor heat exchanger  18  of the indoor unit  02  nor the first indoor heat exchanger  16  of the indoor unit  02 ′ exchanges heat. 
         [0053]    The high-pressure gaseous refrigerant flowing out of the check valve  3  is divided into two streams, where one stream of refrigerant flows through the second electromagnetic valve  10  and the discharge line valve  14  sequentially, thereafter enters the first indoor unit  02 , further flows through the first indoor heat exchanger  16 , the first indoor electronic expansion valve  15 , and the fourth manifold pipe  05  sequentially, and thereafter enters the first manifold pipe  051 ; and the other stream of refrigerant flows out of the A and B ends of the four-way valve  4 , and is condensed by the condenser  5  into the high-pressure liquid refrigerant, and the high-pressure liquid refrigerant flows through the outdoor electronic expansion valve  6  and the liquid-side stop valve  12 , and then also enters the first manifold pipe  051 . The two streams of refrigerant meet at the first manifold pipe  051 , thereafter enter the refrigerating second indoor unit  02 ′, further flow through the fifth manifold pipe  05 , the second electronic expansion valve  17 , and the second indoor heat exchanger  18  sequentially, then flow out of the second indoor unit  02 ′, further flow into the vapor-liquid separator  7  through the second manifold pipe  052 , the gas-side stop valve  13 , and the D and C ends of the four-way valve  4 , and thereafter enter the suction end of the compressor  1 . 
         [0054]    4) Normally-Heat-Recovery and Dehumidifying Condition 
         [0055]    If the system is operating in a rainy season or a low-temperature and high-humidity environment, then the system will dehumidify indoors and also improve the temperature of outgoing air so as to prevent the temperature of outgoing air from becoming too low, which would otherwise result in degraded thermal comfort. If all the indoor units are operating under the heat-recovery and dehumidifying condition, then the refrigerant will flow as schematically illustrated in  FIG. 6 . Under the normally-heat-recovery and dehumidifying condition, the respective components in the outdoor unit  01  will be connected, the valves will be opened and closed, and the heat exchangers will function, in substantially the same way as they are under the normally-heat-recovery condition, so a repeated description thereof will be omitted here. 
         [0056]    In the indoor units  02  and  02 ′, the first electronic expansion valves  15  are fully-opened, and the second indoor electronic expansion valves  17  are throttling for lower pressure. The first indoor heat exchangers  16  are condensers which discharge heat to heat the outgoing air through the refrigerant so as to improve the temperature of outgoing air. The second indoor heat exchangers  18  are evaporators which lower the temperature of and dehumidify the air. 
         [0057]    The refrigerant after the condensation and heat exchange by the outdoor heat exchanger  5 , and the high-pressure liquid refrigerant after condensation and heat exchange by the first indoor heat exchangers  16  meet at the fourth manifold pipe  05 , then flow through the second indoor electronic expansion valves  17 , the second indoor heat exchangers  18 , the second manifold pipe  052 , and the D and C ends of four-way branch  4  sequentially, and thereafter enter the vapor-liquid separator  7 , thus completing the refrigeration loop of the refrigerant 
         [0058]    5) Heat-Recovery and Dehumidifying plus Refrigerating Hybrid Condition 
         [0059]    As illustrated in  FIG. 7 , there is a schematic diagram where a part of the indoor units are operating wider heat-recovery and dehumidifying condition and the other indoor units are under refrigerating condition. The only difference of this hybrid operating condition from the normally-heat recovery and dehumidifying condition lies in that the first indoor electronic expansion valve  15  of the refrigerating indoor unit  02 ′ is fully-closed so that the second indoor heat exchanger  16  does not exchange heat. Except for this, the respective components in the outdoor units and the indoor units will be connected, the valves will be opened and closed, and the refrigerant will flow, in substantially the same way as they are under the normally-heat-recovery and dehumidifying condition, so a repeated description thereof will be omitted here. 
         [0060]    6) Heat-Recovery and Dehumidifying Plus Heating Hybrid Condition 
         [0061]    As illustrated in  FIG. 8 , there is a schematic diagram where a part of the indoor units are under heat-recovery and dehumidifying condition and the other indoor units are under heating condition. Under this hybrid operating condition, the outdoor electronic expansion valve  6  is fully-closed and the outdoor heat exchanger  5  does not exchange heat. The first electronic expansion valve  15  of the indoor unit  02  is fully-opened, and the second electronic expansion valve  17  thereof is throttling for lower pressure; and the first electronic expansion valve  15  of the indoor unit  02 ′ is fully-opened, and the second indoor electronic expansion valve  17  thereof is fully-closed. The first indoor heat exchanger  16  of the indoor unit  02  is a condenser, and the second indoor heat exchanger  18  thereof is an evaporator; and the first indoor heat exchanger  16  of the indoor unit  02 ′ is a condenser, and the second indoor heat exchanger  18  does not exchange heat. 
         [0062]    All the exhaust high-pressure refrigerant gas from the discharge end of the compressor  1  flow through the second electromagnetic valve  10 , and the discharge line valve  14 , and further enters the third manifold pipe  053  and is divided into two streams, where one stream of refrigerant flows through the first indoor heat exchanger  16  and the first electronic expansion valve  15  of the indoor unit  02  sequentially, and thereafter enters the fourth manifold pipe  05 ; and the other stream of refrigerant flows through the first indoor heat exchanger  16  and the first electronic expansion valve  15  of the indoor unit  02 ′, and the fifth manifold pipe  05  sequentially, and further passes the first manifold pipe  051  and thereafter enters the fourth manifold pipe  05  of the indoor unit  02 . The two streams of refrigerant meet at the fourth manifold pipe  05  of the indoor unit  02 , then flow through the second electronic expansion valve  17 , the second indoor heat exchanger  18 , the second manifold pipe  052 , the gas-side stop valve  13 , and the D and C ends of the four-way valve  4 , and thereafter enter the vapor-liquid separator  7 , thus completing the loop of the refrigerant. 
         [0063]    Second Embodiment: 
         [0064]    A multi-connected heat pump system according to this embodiment includes an outdoor unit  01 , and three indoor units  02 ,  02 ′ and  02 ″, where the outdoor unit includes two variable-frequency compressors connected in parallel. Pipes of the indoor unit  02 ″ meet pipes of the indoor unit  02 ′ through manifold pipes  051 ′,  052 ′, and  053 ′ respectively. Structures and operating conditions of the other components will be the same as in the first embodiment, so a repeated description thereof will be omitted here. 
         [0065]    It shall be noted that the methods according to the invention can be applied to a system including an outdoor unit and at least two indoor units, on the first embodiment of the invention will encompass the embodiment of the invention including two indoor units as exemplified in details without any limitation thereto, and alike the second embodiment of the invention will encompass the embodiment of the invention including three indoor units as exemplified in details without any limitation thereto. 
         [0066]    Those skilled in the art shall appreciate that the embodiments of the invention can he embodied as a method, a system or a computer program product. Therefore the invention can be embodied in the form of a fully-hardware embodiment, a fully-software embodiment or an embodiment of software and hardware in combination. Furthermore the invention can he embodied in the form of a computer program product embodied in one or more computer useable storage mediums (including but not limited to a disk memory, a CD-ROM, an optical memory, etc.) in which computer useable program codes are contained. 
         [0067]    The invention has been described in a flow chart and/or a block diagram of the method, the device (system) and the computer program product according to the embodiments of the invention. It shall be appreciated that respective flows and/or blocks in the flow chart and/or the block diagram and combinations of the flows and/or the blocks in the flow chart and/or the block diagram can be embodied in computer program instructions. These computer program instructions can be loaded onto a general-purpose computer, a specific-purpose computer, an embedded processor or a processor of another programmable data processing device to produce a machine so that the instructions executed on the computer or the processor of the other programmable data processing device create means for performing the functions specified in the flow(s) of the flow chart and/or the block(s) of the block diagram. 
         [0068]    These computer program instructions can also be stored into a computer readable memory capable of directing the computer or the other programmable data processing device to operate in a specific manner so that the instructions stored in the computer readable memory create an article of manufacture including instruction means which perform the functions specified in the flow(s) of the flow chart and/or the block(s) of the block diagram, 
         [0069]    These computer program instructions can also be loaded onto the computer or the other programmable data processing device so that a series of operational steps are performed on the computer or the other programmable data processing device to create a computer implemented process so that the instructions executed on the computer or the other programmable device provide steps for performing the functions specified in the flow(s) of the flow chart and/or the block(s) of the block diagram. 
         [0070]    Although the preferred embodiments of the invention have been described, those skilled in the art benefiting from the underlying inventive concept can make additional modifications and variations to these embodiments. Therefore the appended claims are intended to be construed as encompassing the preferred embodiments and all the modifications and variations coming into the scope of the invention. 
         [0071]    Evidently those skilled in the art can make various modifications and variations to the invention without departing from the spirit and scope of the invention. Thus the invention is also intended to encompass these modifications and variations thereto so long as the modifications and variations come into the scope of the claims appended to the invention and their equivalents.