Patent Publication Number: US-10760807-B2

Title: Air conditioner and control method therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application which claims the benefit under 35 U.S.C. § 371 of International Patent Application No. PCT/KR2016/011631, filed on Oct. 17, 2016, which claims the priority benefit of Korean Patent Application No. 10-2015-0146020, filed on Oct. 20, 2015 in the Korean Patent and Trademark Office, the disclosures of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to an air conditioner capable of stably performing a cooling operation in an environment in which an outdoor temperature is lower than an indoor temperature. 
     BACKGROUND ART 
     Generally, an air conditioner is an apparatus which adjusts temperatures and humidity of indoor air using a refrigeration cycle and may cool a room by suctioning and heat-exchanging warm air with a low-temperature refrigerant and discharging the cooled air into the room, or on the other hand, may heat a room by suctioning and heat-exchanging an inside low-temperature air with a high-temperature refrigerant and discharging the heated air. 
     An air condition may include an outdoor unit installed in an outdoor space and an indoor unit installed in an indoor space. The outdoor unit may include a compressor for compressing a refrigerant, an outdoor heat exchanger for heat-exchanging outdoor air with a refrigerant, an air blowing fan, and a variety of pipes which connects the compressor to the indoor unit. The indoor unit may include an indoor heat exchanger for heat-exchanging indoor air with a refrigerant and an expansion device. 
     The air conditioner may cool or heat the room through a refrigerant cycle which circulates the compressor, the indoor heat exchanger (condenser), the expansion device, and the indoor heat exchanger (evaporator) in forward or reverse directions. 
     In detailed consideration of the refrigerant cycle, a gas refrigerant compressed by the compressor flows into the outdoor heat exchanger and phase-changes into a liquid refrigerant, heat is released outward while the refrigerant phase-changes at the outdoor heat exchanger, and then the refrigerant discharged from the outdoor heat exchanger expands while passing through the expansion device and flows into the indoor heat exchanger. 
     Afterward, the liquid refrigerant which flows into the indoor heat exchanger phase-changes into a gas refrigerant. Likewise, the refrigerant phase-changes at the indoor heat exchanger and absorbs outside heat. 
     As described above, the air conditioner adjust an indoor temperature by discharging air (cold air) heat-exchanged by characteristics in which ambient heat is absorbed when a liquid state refrigerant evaporates or the heat is discharged when a gas state refrigerant is liquefied. 
     Meanwhile, in a space in which a lot of large-sized servers and electronic equipment are installed, cooling is performed even in winter to stably operate the servers and electronic equipment. Particularly, when an outdoor temperature is low, a condensing temperature of a refrigerant which passes through an outdoor heat exchanger decreases and an evaporating temperature of a refrigerant which passes through an indoor heat exchanger decreases. 
     Also, a phenomenon in which a liquid refrigerant that flows into a compressor or an indoor heat exchanger freezes occurs and causes unstable operation of an air conditioner and an increase in power consumption from over-operating the compressor. 
     DISCLOSURE OF THE INVENTION 
     Technical Problem 
     One aspect of the present invention provides an air conditioner capable of stably performing a cooling operation in an environment in which an outdoor temperature is lower than an indoor temperature. 
     Also, one aspect of the present invention provides a method of controlling an air conditioner, capable of efficiently performing a cooling operation in an environment in which an outdoor temperature is lower than an indoor temperature without damaging the air conditioner. 
     Also, one aspect of the present invention provides an air conditioner configured to mount an additional outdoor unit, including a pump capable of low-temperature cooling, between an outdoor unit and indoor unit of an existing air conditioner. 
     Technical Solution 
     In accordance with an aspect of the present disclosure, an air conditioner may include an outdoor unit which comprises a first heat exchanger; an indoor unit which comprises a second heat exchanger; an accumulator configured to separate a refrigerant discharged from the first heat exchanger or the indoor unit into a liquid refrigerant and a gas refrigerant; a compressor configured to compress the gas refrigerant discharged from the accumulator and to supply the compressed gas refrigerant to the first heat exchanger; and a pump configured to pressurize the liquid refrigerant discharged from the accumulator and to supply the pressurized liquid refrigerant to the indoor unit. 
     In addition, the air conditioner may further include an expansion valve provided at a flow path which connects the first heat exchanger to the accumulator and configured to be adjusted an opening rate according to a supercooling degree of a refrigerant discharged from the first heat exchanger; and a control valve provided at a flow path which connects the indoor unit to the accumulator and configured to be opened when an outdoor temperature is lower, by a reference or more, than an indoor temperature. 
     In addition, the air conditioner may further include a reservoir provided at a flow path, which connects the first heat exchanger to the expansion valve, to store a refrigerant. 
     In addition, the air conditioner may further include a first check valve configured to allow a refrigerant flow from the compressor to the first heat exchanger; and a second check valve configured to allow a refrigerant flow from the pump to the outdoor unit. 
     In addition, the air conditioner may further include a bypass flow path which connects the first heat exchanger to the indoor unit to prevent a refrigerant from passing through the pump and at which a control valve configured to adjust a refrigerant flow is provided. 
     In addition, the air conditioner may further include a bypass flow path which connects the indoor unit to the first heat exchanger to prevent a refrigerant from passing through the compressor and at which a check valve configured to allow a refrigerant flow from the indoor unit to the first heat exchanger is provided. 
     In accordance with an aspect of the present disclosure, an air conditioner includes an outdoor unit which includes a first heat exchanger, a compressor, an accumulator, and a pump, an indoor unit which includes a second heat exchanger, a first flow path configured to connect the first heat exchanger to the indoor unit and at which the accumulator configured to divide a refrigerant discharged from the indoor unit into a liquid and a gas is provided and the pump configured to pressurize a liquid refrigerant discharged from the accumulator and supply the pressurized liquid refrigerant to the indoor unit, a second flow path configured to connect the indoor unit to the first heat exchanger and at which the accumulator configured to divide a refrigerant discharged from the first heat exchanger or the indoor unit into a liquid and a gas is provided and the compressor configured to compress a gas refrigerant discharged from the accumulator and supply the compressed gas refrigerant to the first heat exchanger is provided, a first bypass flow path configured to connect the first heat exchanger to the indoor unit not to allow a refrigerant to pass through the pump, a second bypass flow path configured to connect the indoor unit to the first heat exchanger not to allow a refrigerant to pass through the compressor, and a controller configured to allow a refrigerant to flow through one of the first flow path and the first bypass flow path and one of the second flow path and the second bypass flow path. 
     When an outdoor temperature is lower than an indoor temperature by a reference or less, the controller may allow a refrigerant to flow through the first flow path and the second flow path, may switch a refrigerant which is flowing through the first bypass flow path and the second flow path to flow through the first flow path and the second flow path, or may switch a refrigerant which is flowing through the first flow path and the second flow path to flow through the first flow and the second bypass flow path. 
     The air conditioner may include a first pressure sensor and a second pressure sensor at an outlet side and an inlet side of the pump provided at the first flow path. Here, when a difference between pressures detected by the first pressure sensor and the second pressure sensor is a lower limit or more of a reference range, the controller may flow a refrigerant to flow through the first flow path and the second flow path, may switch a refrigerant which is flowing through the first bypass flow path and the second flow path to flow through the first flow path and the second flow path, or may switch a refrigerant which is flowing through the first flow path and the second flow path to flow through the first flow and the second bypass flow path. 
     When the pressure detected by the first sensor is an allowable pressure or less of the pump, the controller may switch a refrigerant which is flowing through the first bypass flow path and the second flow path to flow through the first flow path and the second flow path. 
     The air conditioner may further include a temperature sensor provided at an outlet of the first heat exchanger. Here, when a supercooling temperature of a refrigerant at the outlet of the first heat exchanger exceeds an upper limit of a reference range, the controller may switch the refrigerant which is flowing through the first bypass flow path and the second flow path to flow through the first flow path and the second flow path. 
     The outdoor unit may further include an air blowing fan configured to suction air into the first heat exchanger and a sensor capable of measuring a rotational speed of the air blowing fan. Here, when the rotational speed of the air blowing fan is below a lower limit of a reference range, the controller may switch a refrigerant which is flowing through the first bypass flow path and the second flow path to flow through the first flow path and the second flow path. 
     In accordance with an aspect of the present disclosure, a method of controlling an air conditioner in a cooling operation of the air conditioner, which includes an outdoor unit having a first heat exchanger, a compressor and a pump and an indoor unit having a second heat exchanger, may include a first mode in which a refrigerant circulates through the first heat exchanger, the compressor, and the indoor unit; a second mode in which a refrigerant circulates through the first heat exchanger, the pump, and the indoor unit; and a third mode in which a refrigerant circulates through the first heat exchanger, the compressor, the pump, and the indoor unit. 
     Here, the first mode may include closing an expansion valve provided at a first flow path, at which the pump is provided, to prevent a refrigerant discharged from the first heat exchanger from flowing through the first flow path; opening a first control valve provided at a first bypass flow path connected to the indoor unit to allow a refrigerant discharged from the first heat exchanger to flow through the first bypass flow path; and opening a second control valve provided at a second flow path, at which the compressor is provided, to allow a refrigerant discharged from the indoor unit to flow through the second flow path instead of flowing through a second bypass flow path which directly connects the indoor unit to the first heat exchanger. 
     In addition, the second mode may include opening an expansion valve provided at a first flow path, at which the pump is provided, to allow a refrigerant discharged from the first heat exchanger to flow through the first flow path; closing a first control valve provided at a first bypass flow path connected to the indoor unit, to prevent a refrigerant discharged from the first heat exchanger from flowing through the first bypass flow path; and closing a second control valve provided at a second flow path, at which the compressor is provided, to allow a refrigerant discharged from the indoor unit to flow through a second bypass flow path which directly connects the indoor unit to the first heat exchanger instead of flowing through the second flow path. 
     In addition, the third mode may include opening an expansion valve provided at a first flow path, at which the pump is provided, to allow a refrigerant discharged from the first heat exchanger to flow through the first flow path; closing a first control valve provided at a first bypass flow path connected to the indoor unit, to prevent a refrigerant discharged from the first heat exchanger from flowing through the first bypass flow path; and opening a second control valve provided at a second flow path, at which the compressor is provided, to allow a refrigerant discharged from the indoor unit to flow through the second flow path instead of flowing through a second bypass flow path which directly connects the indoor unit to the first heat exchanger. 
     The method may further include determining whether an outdoor temperature is lower, by a reference or more, than an indoor temperature; and measuring a pressure at an outlet of the pump and a pressure at an inlet of the pump after performing test operation of the pump for a certain period of time or more. When the outdoor temperature is lower, by the reference or more, than the indoor temperature and a difference between the pressure at the outlet of the pump and the pressure at the inlet thereof is equal to or above a lower limit of a reference range, the method is able to operate the air conditioner in the second mode when the air conditioner is in a stopped state or to switch to the third mode when the air conditioner is operating in the first mode. 
     In addition, when the air conditioner operates in the first mode, the method may further include measuring a temperature of a refrigerant at an outlet of the first heat exchanger and measuring a pressure at the inlet of the pump and a pressure at the outlet of the pump. When a supercooling degree of the refrigerant at the outlet of the first heat exchanger is above an upper limit of a reference range, the pressure at the outlet of the pump is equal to and lower than an allowable pressure of the pump, and the difference between the pressures at the inlet and outlet of the pump is equal to and lower than an allowable differential pressure of the pump, the method is able to switch to the third mode. 
     In addition, when the air conditioner operates in the first mode, the method may further include measuring a rotational speed of an air blowing fan which allows air to flow into the first heat exchanger. When the rotational speed of the air blowing fan is below a lower limit of a reference range, the method is able to switch to the third mode. 
     In addition, the air conditioner may further include an accumulator configured to divide a refrigerant discharged from the first heat exchanger and a refrigerant discharged from the indoor unit into a liquid and a gas and supply the liquid and the gas to the pump and the compressor. Here, the method may include calculating dryness of a refrigerant which flows into the accumulator and dryness of a refrigerant which is discharged from the first heat exchanger and passes through the expansion valve when the air conditioner operates in the third mode, increasing an opening rate of the expansion valve provided at a flow path configured to connect the first heat exchanger to the accumulator when a difference between the dryness of the refrigerant which flows into the accumulator and the dryness of the refrigerant which is discharged from the first heat exchanger and passes through the expansion valve exceeds an upper limit of a reference range, reducing the opening rate of the expansion valve when the difference between the dryness of the refrigerant which flows into the accumulator and the dryness of the refrigerant which is discharged from the first heat exchanger and passes through the expansion valve is below a lower limit of the reference range. 
     In addition, the method may include increasing a rotational speed of the pump when the rotational speed of the pump is lower than a rotational speed limit of the pump and a greater load is put on the air conditioner while the air conditioner operates in the third mode. 
     In addition, the method may include calculating the dryness of the refrigerant which flows into the accumulator and the dryness of the refrigerant which is discharged from the first heat exchanger and passes through the expansion valve when the pump rotates at the rotational speed limit, increasing a speed of the compressor when the difference between the dryness of the refrigerant which flows into the accumulator and the dryness of the refrigerant which is discharged from the first heat exchanger and passes through the expansion valve exceeds the upper limit of the reference range, and reducing the speed of the compressor when the difference between the dryness of the refrigerant which flows into the accumulator and the dryness of the refrigerant which is discharged from the first heat exchanger and passes through the expansion valve is below the lower limit of the reference range. 
     In addition, when the air conditioner operates in the second mode, the method may further include measuring a temperature of a refrigerant at an outlet of the first heat exchanger. When a supercooling degree of the refrigerant at the outlet of the first heat exchanger is below a lower limit of a reference range, the method is able to increase a rotational speed of an air blowing fan which allows air to flow into the first heat exchanger. When the supercooling degree of the refrigerant at the outlet of the first heat exchanger is above an upper limit of the reference range, the method is able to reduce the rotational speed of the air blowing fan. 
     In addition, the method may include determining whether a compression ratio of the compressor exceeds a minimum compression ratio when the air conditioner operates in the third mode, increasing the rotational speed of the air blowing fan when the compression ratio of the compressor exceeds the minimum compression ratio, and reducing the rotational speed of the air blowing fan when the compression ratio of the compressor is below the minimum compression ratio. 
     In addition, when the air conditioner operates in the second mode or the third mode, the method is able to switch to the first mode when a difference between a set temperature of the indoor unit and a saturation temperature of the outlet of the pump is below a lower limit of a reference range. 
     In accordance with another aspect of the present disclosure, an air conditioner includes a first outdoor unit which includes a first heat exchanger and a compressor, an indoor unit which includes a second heat exchanger, an accumulator configured to divide a refrigerant discharged from the first outdoor unit or the indoor unit into a liquid and a gas, and a second outdoor unit which includes a pump configured to pressurize a liquid refrigerant discharged from the accumulator and supply the pressurized liquid refrigerant to the indoor unit. Here, a gas refrigerant discharged from the accumulator may be supplied to the first outdoor unit. 
     Here, the second outdoor unit may further include a third heat exchanger configured to heat-exchange a refrigerant discharged from the indoor unit and a bypass flow path configured to connect the indoor unit to the third heat exchanger not to allow the refrigerant to pass through the compressor of the first outdoor unit and at which a control valve capable of adjusting a flow of the refrigerant which moves from the indoor unit toward the first heat exchanger is provided. 
     In addition, the second outdoor unit may include a bypass flow path configured to connect the first outdoor unit to the indoor unit not to allow a refrigerant to pass through the pump and at which a control valve configured to adjust a refrigerant flow. 
     In accordance with still another aspect of the present disclosure, an air conditioner includes a first outdoor unit which includes a first heat exchanger and a compressor, an indoor unit which includes a second heat exchanger, and a second outdoor unit disposed between the first outdoor unit and the indoor unit to receive a refrigerant from the first outdoor unit and supply the refrigerant to the indoor unit or to receive a refrigerant from the indoor unit and supply the refrigerant to the first outdoor unit. Here, the second outdoor unit may include a third heat exchanger configured to heat-exchange the refrigerant discharged from the indoor unit, an accumulator configured to divide a refrigerant discharged from the third heat exchanger into a liquid or a gas, and a pump configured to pressurize a liquid refrigerant discharged from the accumulator and supply the pressurized liquid refrigerant to the indoor unit. 
     Here, the second outdoor unit may further include a first transfer flow path configured to connect the first outdoor unit to the indoor unit to receive a refrigerant from the first outdoor unit and supply the refrigerant to the indoor unit and a second transfer flow path configured to connect the indoor unit to the first outdoor unit not to allow a refrigerant discharged from the indoor unit to pass through the third heat exchanger, the accumulator, and the pump. 
     Advantageous Effects 
     Since an air conditioner according to the concept of the present invention includes both a compressor capable of compressing and circulating a gas state refrigerant, and a pump capable of pressurizing and circulating a liquid state refrigerant, the air conditioner may stably perform a cooling operation even in an environment in which an outdoor temperature is lower than an indoor temperature. 
     Also, in a method of controlling an air conditioner according to the concept of the present invention, when operation efficiency of a compressor decreases in an environment in which an outdoor temperature is lower than an indoor temperature, a pump is operated simultaneously or only the pump is separately operated such that the air conditioner may efficiently perform a cooling operation without discontinuities of a cooling function, and the compressor and the pump may be prevented from being damaged by controlling a refrigerant flow. 
     Also, in the air conditioner according to the concept of the present invention, since an outdoor unit which includes a pump may be mounted on an existing outdoor unit for low-temperature cooling, a low-temperature cooling system may be embodied utilizing the existing outdoor unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view illustrating a state in which a compressor and a pump of an air conditioner according to one embodiment of the present invention are driven simultaneously 
         FIG. 2  is a view illustrating a state in which only the compressor of the air conditioner shown in  FIG. 1  is driven. 
         FIG. 3  is a view illustrating a state in which only the pump of the air conditioner shown in  FIG. 1  is driven. 
         FIG. 4  is a control block diagram of the air conditioner shown in  FIG. 1 . 
         FIGS. 5 a  to 5 c    are flowcharts illustrating methods of operating the air conditioner shown in  FIG. 1  in the first mode, the second mode, and third mode. 
         FIG. 6  is a flowchart illustrating a method of controlling the expansion valve while the air conditioner shown in  FIG. 1  operates in the third mode. 
         FIG. 7  is a flowchart illustrating a method of controlling the compressor or the pump while the air conditioner shown in  FIG. 1  operates in the third mode. 
         FIG. 8  is a flowchart illustrating a method of controlling the air blowing fan while the air conditioner shown in  FIG. 1  operates in the second mode. 
         FIG. 9  is a flowchart illustrating a method of controlling the air blowing fan while the air conditioner shown in  FIG. 1  operates in the third mode. 
         FIG. 10  is a flowchart illustrating a method of controlling the air conditioner shown in  FIG. 1  such that the air conditioner, which operates in the second mode or the third mode, is switched to the first mode. 
         FIG. 11  is a view illustrating a state in which a compressor and a pump of the air conditioner according to another embodiment of the present invention are driven simultaneously. 
         FIG. 12  is a view illustrating a state in which only a compressor of the air conditioner shown in  FIG. 11  is driven. 
         FIG. 13  is a view illustrating a state in which only a pump of the air conditioner shown in  FIG. 11  is driven. 
         FIG. 14  is a view illustrating a state in which only a compressor of the air conditioner according to still another embodiment of the present invention is driven. 
         FIG. 15  is a view illustrating a state in which only a pump of the air conditioner shown in  FIG. 14  is driven. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiments described herein and configurations shown in the drawings are merely exemplary examples. Also, various modified examples with which these embodiments and the drawings could be replaced may be present at the time of filing of the present application. 
     Also, throughout the drawings of the specification, the same reference numerals or symbols refer to components or elements which perform substantially same functions. 
     Also, the terms used herein explain the embodiments but are not intended to restrict and/or limit the present disclosure. Singular expressions, unless clearly defined otherwise in context, include plural expressions. Throughout the specification, the terms “comprise,” “include,” “have”, and the like are used herein to specify the presence of stated features, numbers, steps, operations, elements, components or combinations thereof but do not preclude the presence or addition of one or more other features, numbers, steps, operations, elements, components, or combinations thereof. 
     Also, even though the terms including ordinals such as “first”, “second”, and the like may be used to describe various components, the components are not be limited by the terms and the terms are used only for distinguishing one element from others. For example, without departing from the scope of the present disclosure, a first component may be referred to as a second component, and similarly, a second component may be referred to as a first component. The term “and/or” includes any and all combinations of one or a plurality of associated listed items. 
     Hereinafter, an air conditioner and a method of controlling the same according to one embodiment of the present invention will be described with reference to the attached drawings. 
       FIG. 1  is a view illustrating a state in which a compressor and a pump of an air conditioner according to one embodiment of the present invention are driven simultaneously,  FIG. 2  is a view illustrating a state in which only the compressor of the air conditioner shown in  FIG. 1  is driven, and  FIG. 3  is a view illustrating a state in which only the pump of the air conditioner shown in  FIG. 1  is driven. Also,  FIG. 4  is a control block diagram of the air conditioner shown in  FIG. 1 . 
     Referring to  FIGS. 1 to 3 , an air conditioner  1  according to one embodiment of the present invention includes an outdoor unit  10  including a first heat exchanger  100  and an indoor unit  20  including a second heat exchanger  21 . Generally, in a cooling operation, the first heat exchanger  100  included in the outdoor unit  10  is used as a condenser and the second heat exchanger  21  included in the indoor unit  20  is used as an evaporator. 
     The air conditioner  1  may include a compressor  150  and an expansion device  22 , which form a refrigeration cycle. The compressor  150  may be included in the outdoor unit  10  and the expansion device  22  may be included in the indoor unit  20 . 
     Also, the air conditioner  1  may further include a pump  140  for efficiently operating the air conditioner  1  when an outdoor temperature of a place where the outdoor unit  10  is installed, is lower, by a certain degree or more, than an indoor temperature of a place where the indoor unit  20  is installed. 
     Also, the air conditioner  1  may include an accumulator  130  capable of separating a refrigerant discharged from the first heat exchanger  100  of the outdoor unit  10  or the second heat exchanger  21  of the indoor unit  20  into a liquid and a gas and the supplying the liquid and gas to the compressor  150  and the pump  140 . 
     A gaseous refrigerant collected at the accumulator  130  is supplied to the compressor  150  through a flow path  66  which connects an outlet provided at a top of the accumulator  130  to the compressor  150 , and a liquid refrigerant collected at the accumulator  130  is supplied to the pump  140  through a flow path  63  which connects an outlet provided at a bottom of the accumulator  130  to the pump  140 . 
     The compressor  150  may compress the gaseous refrigerant discharged from the accumulator  130  and supply the compressed gaseous refrigerant to the first heat exchanger  100  of the outdoor unit  10 , and the pump  140  may pressurize the liquid refrigerant discharged from the accumulator  130  and supply the pressurized liquid refrigerant to the indoor unit  20 . 
     An expansion valve  120  which adjusts an opening rate according to a supercooling degree of a refrigerant discharged from the first heat exchanger  100  may be provided at flow paths  61  and  62  which connect the first heat exchanger  100  to the accumulator  130 , and a control valve  170  which is opened when an outdoor temperature is lower, by a reference value or more, than an indoor temperature and it is necessary to drive the compressor  150  and the pump  140  simultaneously may be provided at a flow path  65  which connects the indoor unit  20  to the accumulator  130 , that is, the flow path  65  which connects an inlet valve  12  of the outdoor unit  10 , through which the refrigerant flows from the indoor unit  20  to the outdoor unit  10 , to the accumulator  130 . 
     Also, a reservoir  110  capable of storing a liquid refrigerant which will be discharged from the first heat exchanger  100  and pressurized by the pump  140  may be provided at the flow path  61  which connects the first heat exchanger  100  to the expansion valve  120 , and a liquid level sensor (not shown) capable of checking an amount of the liquid refrigerant stored at the reservoir  110  may be provided at the reservoir  110 . 
     Also, a first check valve  14  which allows a refrigerant to flow from the compressor  150  to the first heat exchanger  100 , may be provided at a flow path  67  which connects the compressor  150  to the first heat exchanger  100 , and a second check valve  15  which allows a refrigerant to flow from the pump  140  to the indoor unit  20  may be provided at a flow path  64  which connects the pump  140  to the outdoor unit  10 , that is, the flow path  64  which connects an outlet valve  11  of the outdoor unit  10 , through which a refrigerant flows from the outdoor unit  10  to the indoor unit  20 , to the pump  140 . 
     The air conditioner  1  may further include a first bypass flow path  68  so as to perform a cooling operation using only the compressor  150  without using the pump  140  when a normal cooling operation is necessary rather than a low-temperature cooling case in which an outdoor temperature is lower than an indoor temperature. The first bypass flow path  68  connects the first heat exchanger  100  to the indoor unit  20  or the outlet valve  11  of the outdoor unit  10  to prevent a refrigerant from passing through the pump  140 , and a control valve  160  capable of adjusting a refrigerant flow may be provided at the first bypass flow path  68 . 
     Also, the air conditioner  2  may further include a second bypass flow path  69  so as to perform a cooling operation using only the pump  140  without using the compressor  150  when an outdoor temperature is lower than an indoor temperature and a low-temperature cooling operation is performed. The second bypass flow path  69  connects the indoor unit  20  or the inlet valve  12  of the outdoor unit  10  to the first heat exchanger  100 , and a check valve  13  which allows a refrigerant which flows from the indoor unit  20  to the first heat exchanger  100  to flow may be provided at the second bypass flow path  69 . 
     Also, the outdoor unit  10  may include an air blowing fan  180  which is provided at the first heat exchanger  100  and helps heat exchange at the first heat exchanger  100  by allowing air to flow into the first heat exchanger  100 . 
     Hereinafter, the air conditioner  1  according to one embodiment of the present invention will be described according to a refrigerant flow. 
     Referring to  FIGS. 1 to 3 , the air conditioner  1  may include first flow paths  61 ,  62 ,  63 , and  64  which connect the first heat exchanger  100  to the indoor unit  20  and at which the accumulator  130  and the pump  140  are provided, and second flow paths  65 ,  66 , and  67  which connect the indoor unit  20  to the first heat exchanger  100  and at which the accumulator  130  and the compressor  150  are provided. 
     At the accumulator  130  at which the first flow paths intersect with the second flow paths, a gaseous refrigerant of a refrigerant which flows from the first heat exchanger  100  to the accumulator  130  may mixedly flow into the second flow paths, and a liquid refrigerant of a refrigerant which flows from the indoor unit  20  to the accumulator  130  may mixedly flow into the first flow paths. 
     Also, the air conditioner  1  may include the first bypass flow path  68  which diverges from the flow path  61 , which connects the first heat exchanger  100  to the expansion valve  120  to prevent the refrigerant discharged from the first heat exchanger  100  from passing through the pump  140 , and directly connects the first heat exchanger  100  to the indoor unit  20  and may include the second bypass flow path  69  which diverges from the flow path  65 , which connects the indoor unit  20  to the accumulator  130  to prevent the refrigerant discharged from the indoor unit  20  from passing through the compressor  150 , and directly connects the indoor unit  20  to the first heat exchanger  100 . 
     Also, the air conditioner  1  may include a controller  600  capable of allowing a refrigerant to flow through one of the first flow paths  61 ,  62 ,  63 , and  64  which pass the pump  140  and the first bypass flow path  68  which does not pass the pump  140  and capable of allowing a refrigerant to flow through one of the second flow paths  65 ,  66 , and  67  which pass the compressor  150  and the second bypass flow path  69  which does not pass the compressor  150 . 
     Referring to  FIGS. 1 to 4 , the air conditioner  1  may include a sensor  250  for measuring an outdoor temperature Tout and a sensor  260  for measuring an indoor temperature Tin. When the indoor temperature Tin is lower, by a reference value or more, than the outdoor temperature Tout, the controller  600  may move a refrigerant to the first flow paths  61 ,  62 ,  63 , and  64  and the second flow paths  65 ,  66 , and  67 , may switch the refrigerant which is flowing through the first bypass flow path  68  and the second flow paths  65 ,  66 , and  67  to flow through the first flow paths  61 ,  62 ,  63 , and  64  and the second flow paths  65 ,  66 , and  67 , or may switch the refrigerant which is flowing through the first flow paths  61 ,  62 ,  63 , and  64  and the second flow paths  65 ,  66 , and  67  to flow through the first flow paths  61 ,  62 ,  63 , and  64  and the second bypass flow path  69 . 
     Also, the air conditioner  1  may include a first pressure sensor  240  and a second pressure sensor  220  provided at the first flow path  64  connected to an outlet side of the pump  140  and provided at the first flow path  63  connected to an inlet side of the pump  140 , respectively. When a difference between an outlet pressure Pout of the pump  140  detected by the first pressure sensor  240  and an inlet pressure Pin of the pump  140  detected by the second pressure sensor  220  is above a lower limit of a reference range, the controller  600  may move a refrigerant to the first flow paths  61 ,  62 ,  63 , and  64  and the second flow paths  65 ,  66 , and  67 , may switch the refrigerant which is flowing through the first bypass flow path  68  and the second flow paths  65 ,  66 , and  67  to flow through the first flow paths  61 ,  62 ,  63 , and  64  and the second flow paths  65 ,  66 , and  67 , or may switch the refrigerant which is flowing through the first flow paths  61 ,  62 ,  63 , and  64  and the second flow paths  65 ,  66 , and  67  to flow through the first flow paths  61 ,  62 ,  63 , and  64  and the second bypass flow path  69 . 
     Here, when a refrigerant of the air conditioner  1  is already flowing through the first bypass flow path  68  and the second flow paths  65 ,  66 , and  67 , so as to switch a flow of the refrigerant to the first flow paths  61 ,  62 ,  63 , and  64  and the second flow paths  65 ,  66 , and  67 , it is necessary that a pressure of the refrigerant which is flowing through the flow path  68  is equal to or lower than an allowable pressure of the pump  140 . Since the flow path  64  connected to the outlet side of the pump  140  is attached to the flow path  68 , which directly connects the first heat exchanger to the indoor unit  20 , and is connected to the outlet valve  11  of the outdoor unit  10 , the first pressure sensor  240  may measure a pressure of a refrigerant which flows to the flow path  68  and the pressure may become the outlet pressure Pout of the pump  140 . Accordingly, when the outlet pressure Pout of the pump  140  detected by the first pressure sensor  240  is equal to or lower than the allowable pressure of the pump  140 , the pump  140  may be driven without damage and the controller  600  may switch a refrigerant which is flowing through the first bypass flow path  68  and the second flow paths  65 ,  66 , and  67  to flow through the first flow paths  61 ,  62 ,  63 , and  64  and the second flow paths  65 ,  66 , and  67 . 
     Also, the air conditioner  1  may include a temperature sensor  210  provided at the flow path connected to an outlet side of the first heat exchanger  100 . Since a supercooling degree of a refrigerant at the outlet of the first heat exchanger  100  is an index which indicates how much a liquid refrigerant amount capable of being supplied to the pump  140  is included in the refrigerant at the outlet of the first heat exchanger  100 , when the supercooling degree of the refrigerant at the outlet of the first heat exchanger  100  is above an upper limit of the reference range on the basis of a temperature Tc detected by the temperature sensor  210  and the refrigerant amount of a reference value or more is secured, the controller may shift a refrigerant which is flowing through the first bypass flow path  68  and the second flow paths  65 ,  66 , and  67  to flow through the first flow paths  61 ,  62 ,  63 , and  64  and the second flow paths  65 ,  66 , and  67 . 
     Also, the air conditioner  1  may further include a sensor  270  capable of measuring a rotational speed Vf of the air blowing fan  180  provided at the first heat exchanger  100  side. When an outdoor temperature Tout at a place where the outdoor unit  10  is installed, a condensing pressure of a refrigerant at the first heat exchanger  100  is decreased. When the condensing pressure at the first heat exchanger  100  is decreased, an air volume of the air blowing fan  180  is reduced so as to secure a compression ratio at the compressor  150 . When the rotational speed Vf of the air blowing fan  180  is decreased to be equal to or below a lower limit of a reference range, it is impossible to perform a cooling operation using only the compressor  150 . Accordingly, when the rotational speed Vf of the air blowing fan  180  is decreased to be equal to or below the lower limit of the reference range, the controller  600  may shift a refrigerant which is flowing through the first bypass flow path  68  and the second flow paths  65 ,  66 , and  67  to flow through the first flow paths  61 ,  62 ,  63 , and  64  and the second flow paths  65 ,  66 , and  67 . 
     The sensor  270  which measures the rotational speed Vf of the air blowing fan  180  may replace measurement of the rotational speed Vf with measurement of power consumption of the air blowing fan  180 . 
     Hereinafter, a method of controlling the air conditioner according to one embodiment of the present invention will be described with reference to  FIGS. 1 to 10 . 
     As shown in  FIG. 4 , the air conditioner  1  may include an input portion  200  which runs a start of a cooling operation or a heating operation from a user. The user may input performing of the cooling operation through the input portion  200  to as well as may input a set temperature Ts which is desired by the user. The input portion  200  may be provided at the indoor unit  20 . 
     When the performing of the cooling operation is received through the input portion  200 , the controller  600  may control the expansion valve  120 , the first control valve  160  provided at the first bypass flow path  68 , the second control valve  170  provided at the second flow paths  65 ,  66 , and  67 , the compressor  150 , the pump  140 , the air blowing fan  180  provided at the first heat exchanger  100 , and the like on the basis of data detected by a variety of sensors so as to allow the air conditioner  1  to efficiently operate. 
     In controlling of the air conditioner according to the present invention, a reference value range of control may be set in consideration of hysteresis and the controller may control the air conditioner with an upper limit and a lower limit of the reference range as critical points. 
     The method of controlling according to one embodiment of the present invention may include a first mode  700 , a second mode  800 , or a third mode  900 , in which the compressor  150  and/or the pump  140  are driven according to internal and external operation environments of the air conditioner  1 . 
     The first mode  700  is an operation mode in which a refrigerant circulates through the first heat exchanger  100 , the compressor  150 , and the indoor unit  20  such that only the compressor is separately driven. The second mode  800  is an operation mode in which a refrigerant circulates through the first heat exchanger  100 , the pump  140 , and the indoor unit  20  such that only the pump  140  is separately driven. The third mode  900  is an operation mode in which a refrigerator circulates through the first heat exchanger  100 , the compressor  150 , the pump  140 , and the indoor unit  20  such that both the compressor  150  and the pump  140  are driven simultaneously. 
     Each of the operation modes will be described with reference to  FIGS. 1 to 3 and 5   a  to  5   c.    
       FIG. 2  illustrates circulation of a refrigerant in the first mode  700 . In the first mode  700 , the expansion valve  120  provided at the first flow paths is closed such that a refrigerant discharged from the first heat exchanger  100  may not flow through the first flow paths  61 ,  62 ,  63 , and  64  at which the pump  140  is provided ( 710 ), the first control valve  160  provided at the first bypass flow path  68  is opened such that the refrigerant discharged from the first heat exchanger  100  may flow through the first bypass flow path  68  ( 720 ), and the second control valve  170  provided at the second control flow path  65 ,  66 , and  67  is opened such that the refrigerant discharged from the indoor unit  20  may not flow to the second bypass flow path  69  which directly connects the indoor unit  20  to the first heat exchanger  100  and may flow through the second flow paths  65 ,  66 , and  67  at which the compressor  150  is provided ( 730 ), and the only the compressor  150  may be separately driven ( 740 ). 
       FIG. 3  illustrates circulation of a refrigerant in the second mode  800 . In the second mode  800 , the expansion valve  120  provided at the first flow paths  61 ,  62 ,  63 , and  64  is opened such that a refrigerant discharged from the first heat exchanger  100  may flow through the first flow paths  61 ,  62 ,  63 , and  64  at which the pump  140  is provided ( 810 ), the first control valve  160  provided at the first bypass flow path  68  is closed such that the refrigerant discharged from the first heat exchanger  100  may not flow through the first bypass flow path  68  ( 820 ), and the second control valve  170  provided at the second control flow path  65 ,  66 , and  67  is closed such that the refrigerant discharged from the indoor unit  20  may not flow through the second flow paths  65 ,  66 , and  67  at which the compressor  150  is provided and may flow through the second bypass flow path  69  which directly connects the indoor unit  20  to the first heat exchanger  100  ( 830 ), and the only the pump  140  may be separately driven ( 840 ). 
       FIG. 1  illustrates circulation of a refrigerant in the third mode  900 . In the third mode  900 , the expansion valve  120  provided at the first flow paths  61 ,  62 ,  63 , and  64  is opened such that a refrigerant discharged from the first heat exchanger  100  may flow through the first flow paths  61 ,  62 ,  63 , and  64  at which the pump  140  is provided ( 910 ), the first control valve  160  provided at the first bypass flow path  68  is closed such that the refrigerant discharged from the first heat exchanger  100  may not flow through the first bypass flow path  68  ( 920 ), and the second control valve  170  provided at the second control flow path  65 ,  66 , and  67  is opened such that the refrigerant discharged from the indoor unit  20  may not flow through the second bypass flow path  69  which directly connects the indoor unit  20  to the first heat exchanger  100  and may flow through the second flow paths  65 ,  66 , and  67  at which the compressor  150  is provided ( 930 ), and both the compressor  150  and the pump  140  may be driven simultaneously ( 940 ). 
     Hereinafter, the method of controlling the air conditioner  1 , which includes the first mode  700 , the second mode  800 , and the third mode  900 , will be described. 
       FIGS. 5 a  to 5 c    are flowcharts illustrating methods of operating the air conditioner shown in  FIG. 1  in the first mode, the second mode, and third mode. 
     When a cooling operation is input to the input portion  200  by a user ( 1000 ), an outdoor temperature Tout and an indoor temperature Tin are measured by the sensor  250  for measuring the outdoor temperature Tout and the sensor  260  for measuring the indoor temperature Tin ( 1010 ). It is determined whether the outdoor temperature Tout is lower, by a reference value α or more, than the indoor temperature Tin ( 1020 ). When the outdoor temperature Tout is not lower, by the reference value α or more, than the indoor temperature Tin, since it is not a low-temperature cooling environment, the air conditioner  1  performs a normal cooling operation in the first mode  700 . 
     When the outdoor temperature Tout is lower, by the reference value α or more, than the indoor temperature Tin, a test operation of the pump  140  is performed for more than a certain period of time η to check whether a liquid refrigerant amount capable of driving the pump  140  is prepared ( 1030 ), and an inlet pressure Pin of the pump  140  and an outlet pressure Pout of the pump  140  are measured by the pressure sensor  220  provided at the inlet of the pump  140  and the pressure sensor  240  provided at the outlet of the pump  140  ( 1040 ). 
     It is determined whether the outlet pressure Pout of the pump  140  and the inlet pressure Pin of the pump  140  are higher than a lower limit βmin of a reference range ( 1050 ). When the outlet pressure Pout of the pump  140  and the inlet pressure Pin of the pump  140  are not higher than a lower limit βmin of the reference range, since a liquid refrigerant amount is inadequate, it is impossible to operate the pump  140 , and the air conditioner  1  performs operation in the first mode  700 . 
     When the outlet pressure Pout of the pump  140  and the inlet pressure Pin of the pump  140  are higher than the lower limit βmin of the reference range, it is determined whether the air conditioner  1  is in a stopped state ( 1060 ). When the air conditioner  1  is in the stopped state in which the air conditioner  1  does not start operating, the air conditioner  1  performs operation in the second mode  800 . 
     When the air conditioner  1  is not in the stopped state and operates in a random operation mode, it is determined whether the air conditioner  1  operates in the first mode  700  ( 1070 ). When the air conditioner  1  does not operate in the first mode  700 , starting operations of the flowcharts shown in  FIGS. 5 a  to 5 c    are performed again and an operation environment of the air conditioner  1  is determined again. 
     When the air conditioner  1  operates in the first mode  700 , a temperature Tc of a refrigerant at the outlet of the first heat exchanger  100  is measured by the temperature sensor  210  provided at the outlet of the first heat exchanger  100  ( 1080 ). When a supercooling degree K of the refrigerant is above an upper limit Kmax of a reference range on the basis of the temperature Tc of the refrigerant at the outlet of the first heat exchanger  100 , since a ratio of a liquid refrigerant to the refrigerant discharged from the first heat exchanger  100  is high, cooling efficiency is decreased in the first mode  700  in which only the compressor  150  is separately driven. 
     Accordingly, it is determined whether the supercooling degree K of the refrigerant at the outlet of the first heat exchanger  100  is above the upper limit Kmax of the reference range ( 1090 ). When the supercooling degree K of the refrigerant at the outlet of the first heat exchanger  100  is not above the upper limit Kmax of the reference range, the air conditioner continuously operates in the first mode  700  and the starting operation of the flowchart is performed again such that the operation environment of the air conditioner  1  is determined again. When the supercooling degree K of the refrigerant at the outlet of the first heat exchanger  100  is above the upper limit Kmax of the reference range, it is determined whether the outlet pressure Pout of the pump  140  is less than an allowable pressure θ so as to check whether the pump  140  is driven without damage ( 1100 ). 
     Referring to  FIGS. 1 to 3 , the first bypass flow path  68  and the flow path  64  connected to the outlet side of the pump  140  are attached, pass the outlet valve  11  of the outdoor unit  10 , and are connected to the indoor unit  20 . Accordingly, since a refrigerant is flowing through the first bypass flow path  68  when the air conditioner  1  is operating in the first mode  700  as shown in  FIG. 2 , a pressure of the refrigerant at the first bypass flow path  68  becomes the outlet pressure Pout of the pump  140 . It is necessary that the outlet pressure Pout of the pump  140  is lower than the allowable pressure θ such that the pump  140  may be driven without damage. 
     Since it is impossible to drive the pump  140  when the outlet pressure Pout of the pump  140  is not less than the allowable pressure θ, the starting operation of the flowchart is performed again and the operation environment of the air conditioner  1  is determined again while the air conditioner  1  continuously operates in the first mode  700 . 
     When the outlet pressure Pout of the pump  140  is less than the allowable pressure θ, it is necessary to determine whether a difference between the outlet pressure Pout of the pump  140  and the inlet pressure Pin of the pump  140  is less than an allowable differential pressure γ of the pump  140  ( 1110 ). Although the outlet pressure Pout of the pump  140  is less than the allowable pressure θ, the pump  140  may be damaged when a differential pressure between the inlet and outlet of the pump  140  is not less than the allowable differential pressure γ. 
     Accordingly, since it is impossible to drive the pump  140  when the difference between the outlet pressure Pout of the pump  140  and the inlet pressure Pin of the pump  140  is not less than the allowable differential pressure γ of the pump  140 , the starting operation of the flowchart is performed again and the operation environment of the air conditioner  1  is determined again while the air conditioner  1  continuously operates in the first mode  700 . 
     When the difference between the outlet pressure Pout of the pump  140  and the inlet pressure Pin of the pump  140  is less than the allowable differential pressure γ of the pump  140 , it may be determined that an environment capable of starting operation of the pump  140  is provided. As a next stage, it is determined whether switching the air conditioner  1  which is operating in the first mode  700  to operate in the third mode  900  is operating with high efficiency. 
     It may be determined, by measuring the rotational speed Vf of the air blowing fan  180  using the sensor  270  provided at the air blowing fan  180  to measure the rotational speed, whether switching the air conditioner  1  which is operating in the first mode  700  to operate in the third mode  900  is operating with high efficiency. 
     When the rotational speed Vf of the air blowing fan  180  decreases below a lower limit min of a reference range, it may be determined that heat exchange efficiency of the first heat exchanger  100  is decreased by supplying a refrigerant using only the compressor  150  ( 1130 ). Accordingly, when the rotational speed Vf of the air blowing fan  180  is below the lower limit min of the reference range, the air conditioner  1  which is operating in the first mode  700  is switched to operate in the third mode  900 . When the rotational speed Vf of the air blowing fan  180  is not below the lower limit min of the reference range, the starting operation of the flowchart is performed again and the operation environment of the air conditioner  1  is determined again while the air conditioner  1  operates in the first mode  700 . 
       FIG. 6  is a flowchart illustrating a method of controlling the expansion valve while the air conditioner shown in  FIG. 1  operates in the third mode. 
     In the air conditioner  1  which is operating in the third mode  900  according to the flowcharts shown in  FIGS. 5 a  to 5 c    ( 1200 ), a refrigerant discharged from the first heat exchanger  100  passes through the flow paths  61  and  62  at which the expansion valve  120  is provided and is supplied to the accumulator  130 , and a refrigerant discharged from the indoor unit  20  passes through the flow path  65  at which the second control valve  170  is provided and is supplied to the accumulator  130 . 
     To efficiently operate the compressor  150  and the pump  140  of the air conditioner  1 , it is necessary to adjust amounts of a liquid refrigerant and a gaseous refrigerant supplied by the accumulator  130 . An opening rate of the expansion valve  120  may be controlled so as to adjust the amounts of the liquid refrigerant and gaseous refrigerant. 
     A dryness D of a refrigerant which flows into the accumulator  130  and a dryness E of a refrigerant which is discharged from the first heat exchanger  100  and passes the expansion valve  120  are measured ( 1210 ). When the dryness D of the refrigerant which flows into the accumulator  130  is above an upper limit δmax of a reference range than the dryness E of the refrigerant which is discharged from the first heat exchanger  100  and passes the expansion valve  120  ( 1220 ), it means deficiency in a liquid refrigerant amount. Accordingly, the opening rate of the expansion valve  120  is increased so as to secure the liquid refrigerant amount ( 1230 ). 
     Also, when the dryness D of the refrigerant which flows into the accumulator  130  is below a lower limit δmin of the reference range than the dryness E of the refrigerant which is discharged from the first heat exchanger  100  and passes the expansion valve  120  ( 1240 ), it means deficiency in a gaseous refrigerant amount. Accordingly, the opening rate of the expansion valve  120  is reduced so as to secure the gaseous refrigerant amount ( 1250 ). 
     In detail, the dryness D of the refrigerant which flows into the accumulator  130  may be calculated using a mean enthalpy value hm of a refrigerant which passes the pump  140  and a refrigerant which passes the compressor  150  under evaporating pressure. 
     The mean enthalpy value hm of the refrigerant which passes the pump  140  and the refrigerant which passes the compressor  150  is obtained by a following equation.
 
Mean Enthalpy hm=[(Pump Flow Rate*Outlet Enthalpy of Indoor Unit)+(Compressor Flow Rate*Outlet Enthalpy of First Heat Exchanger)]/[Pump Flow Rate+Compressor Flow Rate]
 
     Also, the dryness E of the refrigerant which passes the expansion valve  120  may be calculated using an enthalpy value of a refrigerant at the outlet of the first heat exchanger  100  under evaporating pressure. 
       FIG. 7  is a flowchart illustrating a method of controlling the compressor or the pump while the air conditioner shown in  FIG. 1  operates in the third mode. 
     In the air conditioner  1  which is operating in the third mode  900  according to the flowcharts shown in  FIGS. 5 a  to 5 c    ( 1300 ), rotational speeds of the compressor  150  and the pump  140  may be adjusted for efficient operation. 
     A rotational speed Vp of the pump  140  is measured ( 1310 ). When the rotational speed Vp of the pump  140  is less than a rotational speed limit v ( 1320 ), the rotational speed Vp of the pump  140  is increased ( 1330 ). When the pump  140  rotates at the rotational speed limit v ( 1340 ), the dryness D of the refrigerant which flows into the accumulator  130  and the dryness E of the refrigerant which is discharged from the first heat exchanger  100  and passes the expansion valve  120  are measured ( 1350 ). 
     When the dryness D of the refrigerant which flows into the accumulator  130  is above the upper limit δmax of the reference range than the dryness E of the refrigerant which is discharged from the first heat exchanger  100  and passes the expansion valve  120  ( 1360 ), it indicates a sufficient amount of gaseous refrigerant. Accordingly, a speed Vc of the compressor  150  is increased ( 1370 ). 
     Also, when the dryness D of the refrigerant which flows into the accumulator  130  is below the lower limit δmin of the reference range than the dryness E of the refrigerant which is discharged from the first heat exchanger  100  and passes the expansion valve  120  ( 1380 ), it indicates a deficient amount of a gaseous refrigerant. Accordingly, the speed Vc of the compressor  150  is reduced ( 1390 ). 
       FIG. 8  is a flowchart illustrating a method of controlling the air blowing fan while the air conditioner shown in  FIG. 1  operates in the second mode. 
     In the air conditioner  1  which is operating in the second mode  800  according to the flowcharts shown in  FIGS. 5 a  to 5 c    ( 1400 ), the rotational speed Vf of the air blowing fan  180  may be adjusted for efficient operation. 
     A temperature Tc of a refrigerant at the outlet of the first heat exchanger  100  is measured by the temperature sensor  210  provided at the outlet of the first heat exchanger  100  ( 1410 ). When the supercooling degree K of the refrigerant is below the lower limit Kmin of the reference range on the basis of the temperature Tc of the refrigerant at the outlet of the first heat exchanger  100  ( 1420 ), the rotational speed Vf of the air blowing fan  180  is increased so as to increase heat exchange efficiency of the first heat exchanger  100  ( 1430 ). 
     Also, when the supercooling degree K of the refrigerant is above the upper limit Kmax of the reference range on the basis of the temperature Tc of the refrigerant at the outlet of the first heat exchanger  100  ( 1440 ), since the supercooling degree K of the refrigerant at the outlet of the first heat exchanger  100  is unnecessarily high, the rotational speed Vf of the air blowing fan  180  is reduced ( 1450 ). 
       FIG. 9  is a flowchart illustrating a method of controlling the air blowing fan while the air conditioner shown in  FIG. 1  operates in the third mode. 
     In the air conditioner  1  which is operating in the third mode  900  according to the flowcharts shown in  FIGS. 5 a  to 5 c    ( 1500 ), the rotational speed Vf of the air blowing fan  180  may be adjusted for efficient operation. 
     When a compression ratio R which is a ratio between an inlet pressure and an outlet pressure of the compressor  150  is equal to or lower than a minimum compression ratio Rmin, the compressor  150  can not perform a function of the compressor  150 . 
     Accordingly, the compression ratio R of the compressor  150  is measured by a sensor  280  for measuring the compression ratio R of the compressor  150  ( 1510 ). When the compression ratio R is more than a minimum compression ratio Rmin ( 1520 ), since the compressor  150  normally operates, the rotational speed Vf of the air blowing fan  180  is increased ( 1530 ). When the compression ratio R is less than the minimum compression ratio Rmin ( 1540 ), the rotational speed Vf of the air blowing fan  180  is reduced ( 1550 ). 
       FIG. 10  is a flowchart illustrating a method of controlling the air conditioner shown in  FIG. 1  such that the air conditioner, which operates in the second mode or the third mode, is switched to the first mode. 
     In the air conditioner  1  which is operating in the second mode  800  or the third mode  900  according to the flowcharts shown in  FIGS. 5 a  to 5 c   , when it is impossible to achieve a target cooling effect through refrigerant circulation by the pump  140 , although cooling by the compressor  150  is inefficient, it is possible to switch operation to be in the first mode  700  so as to achieve the target cooling effect. 
     A saturation temperature Tp of a refrigerant at the outlet of the pump  140  is measured by a temperature sensor  230  provided at the flow path  64  ( 1610 ). When the saturation temperature Tp of the refrigerant at the outlet of the pump  140  is below a lower limit w of a reference range set at the indoor unit  20  by the input portion  200  ( 1620 ), since it is impossible to cool to a setting temperature through the refrigerant circulation by the pump  140 , operation is switched to the first mode  700  in which the compressor  150  is separately driven. 
     Also, while the air conditioner  1  is operating in the second mode  800  or the third mode  900 , when power consumption of the pump  140  is reduced to be equal to or below a reference, a differential pressure of the pump  140  is reduced to be equal to or below a reference, a difference between the outdoor temperature Tout and the indoor temperature Tin becomes smaller to be equal to or lower than a reference a, and a liquid level in the reservoir  110  becomes lower to be equal to or below a reference, the pump  140  is determined to be incapable of normally circulating a refrigerant and switched to the first mode  700  such that only the compressor  150  is separately driven. 
     Hereinafter, an air conditioner  2  according to another embodiment of the present invention will be described with reference to  FIGS. 11 to 13 . 
       FIG. 11  is a view illustrating a state in which a compressor and a pump of the air conditioner according to another embodiment of the present invention are driven simultaneously,  FIG. 12  is a view illustrating a state in which only a compressor of the air conditioner shown in  FIG. 11  is driven, and  FIG. 13  is a view illustrating a state in which only a pump of the air conditioner shown in  FIG. 11  is driven. 
     Referring to  FIGS. 11 to 13 , in the air conditioner  2  according to another embodiment of the present invention, it is possible to dispose a second outdoor unit  40  configured to circulate a refrigerant through a pump  440  between a first outdoor unit  30  and the indoor unit  20 , which are already installed. 
     The air conditioner  2  includes the first outdoor unit  30  including a first heat exchanger  300  and the indoor unit  20  including the second heat exchanger  21 . Generally, in a cooling operation, the first heat exchanger  300  included in the first outdoor unit  30  is used as a condenser and the second heat exchanger  21  included in the indoor unit  20  is used as an evaporator. 
     The air conditioner  2  may include a compressor  350  and the expansion device  22 , which form a refrigeration cycle. The compressor  350  may be included in the first outdoor unit  30 , and the expansion device  22  may be included in the indoor unit  20 . 
     Also, when an outdoor temperature is lower, by more than a certain degree, than an indoor temperature, the air conditioner  2  includes the second outdoor unit  40  which includes the pump  440  for efficiently operating the air conditioner  2 . 
     Also, the second outdoor unit  40  may include a first accumulator  430  capable of separating a refrigerant discharged from the first heat exchanger  300  of the first outdoor unit  30  or the second heat exchanger  21  of the indoor unit  20  into a liquid and a gas and the supplying the liquid and gas to the pump  440  and the compressor  350  of the first outdoor unit  30 . 
     A gaseous refrigerant collected at the first accumulator  430  is discharged from the second outdoor unit  40  and supplied to the first outdoor unit  30  through a flow path  86  which connects an outlet provided at a top of the first accumulator  430  to a first outlet valve  41  of the second outdoor unit  40 . The gaseous refrigerant which flows into an inlet valve  32  of the first outdoor unit  30  approaches a four-way valve  390  at which a flow path is switched according to a cooling operation and a heating operation, through a flow path  72  connected to the inlet valve  32  and flows through a flow path  73  connected to a second accumulator  310 . Leaving a liquid refrigerant, condensed while the refrigerant flows, at the second accumulator  310  to prevent the compressor  350  from being damaged, only the gaseous refrigerant is supplied again to the compressor  350  through a flow path  74  which connects an outlet provided at a top of the second accumulator  310  to the compressor  350 . 
     The compressor  350  may compress the gaseous refrigerant discharged from the second accumulator  310  and may supply the gaseous refrigerant to the first heat exchanger  300  of the first outdoor unit  30  through the four-way valve  390 . A check valve  33  is provided at a flow path  75  which connects the compressor  350  to the four-way valve  390  such that the gaseous refrigerant flows to only the four-way valve  390  side, and the gaseous refrigerant which flows into the four-way valve  390  is supplied to the first heat exchanger  300  through a flow path  76  which connects the four-way valve  390  to the first heat exchanger  300 . 
     A condensed refrigerant discharged from the first heat exchanger  300  may be supplied to the second outdoor unit  40  through the first heat exchanger  300  and an outlet valve  31  of the first outdoor unit. An expansion valve  320  may be provided at a flow path  71  which connects the first heat exchanger  300  to the outlet valve  31  of the first outdoor unit  30 , and a bypass flow path at which a check valve  34  is provided may be provided in parallel with the expansion valve  320  to allow a refrigerant to reversely flow during a heating operation. 
     A refrigerant which is discharged from the first outdoor unit  30  and flows into a first inlet valve  42  of the second outdoor unit  40  may be supplied to the first accumulator  430  through flow paths  87  and  82  connected to the first accumulator  430 . An expansion valve  420  with an opening rate adjusted according to a supercooling degree of a refrigerant discharged from the first outdoor unit  30  may be provided at the flow paths  87  and  82  which connect the first inlet valve to the first accumulator  430 . A reservoir  410  for storing a liquid refrigerant to be pressurized at the pump  440  may be provided at the flow path  87  which connects the first inlet valve  42  of the second outdoor unit  40  to the expansion valve  420 . A liquid level sensor (not shown) capable of checking an amount of a stored liquid refrigerant may be provided at the reservoir  410 . 
     The flow path  82  which connects the expansion valve  420  to the first accumulator  430  is attached to a flow path  85  which connects the indoor unit  20  to the first accumulator  430 , in detail, the flow path  85  which connects a second inlet valve  44 , through which a refrigerant flows from the indoor unit  20  into the second outdoor unit  40 , to the first accumulator  430 . A control valve  470  provided at the flow path which connects the second inlet valve  44  to the first accumulator  430  may be opened when it is necessary to drive the compressor  350  and the pump  440  simultaneously due to an outdoor temperature lower, by a reference or more, than an indoor temperature. 
     A liquid refrigerant collected at the first accumulator  430  is supplied to the pump  440  through a flow path  83  which connects an outlet provided at a bottom of the first accumulator  430  to the pump  440 . 
     The pump  440  may pressurize the liquid refrigerant discharged from the first accumulator  430  and may supply the liquid refrigerant to the indoor unit  20  through a second outlet valve  43  of the second outdoor unit  40 . A check valve  46  is provided at a flow path  84  which connects the pump  440  to the second outlet valve  43  so as to allow the liquid refrigerant to flow through only the second outlet valve  43 , and the refrigerant discharged from the second outdoor unit  40  through the second outlet valve  43  is supplied to the indoor unit  20 . 
     The air conditioner  2  may further include a first bypass flow path  88  which diverges from the flow path  87 , which connects the first inlet valve  42  of the second outdoor unit  40  to the expansion valve  420 , so as to perform a cooling operation using only the compressor  150  provided at the first outdoor unit  30  without using the pump  440  provided at the second outdoor unit  40  when a normal cooling operation, which is not low-temperature cooling in which an outdoor temperature is lower than an indoor temperature, is necessary. The first bypass flow path  88  connects the first outdoor unit  30  to the indoor unit  20  to prevent a refrigerant from passing through the pump  440 , and a control valve  460  capable of adjusting a refrigerant flow may be provided at the first bypass flow path  88 . 
     Also, the air conditioner  2  may further include a third heat exchanger  400  and a second bypass flow path  89  to perform a cooling operation using only the pump  440  of the second outdoor unit  40  without using the compressor  350  of the first outdoor unit  30  when a low-temperature cooling operation is performed due to an outdoor temperature lower, by a certain degree or more, than an indoor temperature. The third heat exchanger  400  heat-exchanges a refrigerant discharged from the indoor unit  20 , and the second bypass flow path  89  connects the indoor unit  20  or the second inlet valve  44  of the second outdoor unit  40  to the third heat exchanger  400  to prevent the refrigerant from passing through the compressor  350  of the first outdoor unit  30 . A control valve  471  capable of adjusting a refrigerant flow through supply a refrigerant discharged from the indoor unit  20  only when the third heat exchanger  400  is used may be provided at the second bypass flow path  89 . 
     A flow path  81  provided at an outlet side of the third heat exchanger  400  so as to supply a refrigerant discharged from the third heat exchanger  400  to the first accumulator  430  may be attached to the flow paths  87  and  82  which connect the first inlet valve  42  of the second outdoor unit  40  to the first accumulator  430 . A check valve  45  which allows only a flow of a refrigerant discharged from the third heat exchanger  400  may be provided at the flow path  81  provided at the outlet side of the third heat exchanger  400  to prevent a refrigerant which flows into the first inlet valve  42  of the second outdoor unit  40  from flowing into the third heat exchanger  400   
     Also, the first outdoor unit  30  may include an air blowing fan  380  which is provided at the first heat exchanger  300  side and helps heat exchange at the first heat exchanger  300  by allowing air to flow into the first heat exchanger  300 , and the second outdoor unit  40  may include an air blowing fan  480  which is provided at the third heat exchanger  400  and helps heat exchange at the third heat exchanger  400  by allowing air to flow into the third heat exchanger  400 . 
     Also, the air conditioner  2  may include a variety of sensors which provide operation environment information of the air conditioner to operate by driving both the compressor  350  and the pump  440  simultaneously as shown in  FIG. 11 , driving only the compressor  350  as shown in  FIG. 12 , or driving only the pump  440  as shown in  FIG. 13 . 
     Particularly, the air conditioner  2  may include the temperature sensor  210  provided at the flow path  81  connected to an outlet side of the third heat exchanger  400  of the second outdoor unit  40  and may include the first pressure sensor  240  and the second pressure sensor  220  provided at the flow path  84  connected to an outlet side of the pump  440  and the flow path  83  connected to an inlet side thereof, respectively. Also, the temperature sensor  230  provided at the flow path  84  connected to the outlet of the pump  440  may be included. 
     The air conditioner  2  may perform all the same functions as those of the air conditioner  1  according to one embodiment of the present invention, which has been described above with reference to  FIGS. 1 to 10 , by additionally installing the second outdoor unit  40  in addition to the first outdoor unit  30  and the indoor unit  20  which are already installed. 
     Hereinafter, an air conditioner  3  according to still another embodiment of the present invention will be described with reference to  FIGS. 14 to 15 . 
       FIG. 14  is a view illustrating a state in which only a compressor of the air conditioner according to still another embodiment of the present invention is driven, and  FIG. 15  is a view illustrating a state in which only a pump of the air conditioner shown in  FIG. 14  is driven. 
     Referring to  FIGS. 14 to 15 , in a air conditioner  3  according to still another embodiment of the present invention, it is possible to dispose a second outdoor unit  50  configured to circulate a refrigerant through a pump  540  between the first outdoor unit  30  and the indoor unit  20 , which are already installed. 
     The first outdoor unit  30  and the indoor unit  20  of the air conditioner  3  have the same components as those of the first outdoor unit  30  and the indoor unit  20  of the air conditioner  2  according to the embodiment shown in  FIGS. 11 to 13 . 
     Accordingly, in a cooling operation, a refrigerant, which flows into the inlet valve  32  of the first outdoor unit  30 , passes through the compressor  350  and the first heat exchanger  300  of the first outdoor unit  30  and flows out through the outlet valve  31  like the method shown in  FIGS. 11 to 13 . 
     A refrigerant discharged through the outlet valve  31  of the first outdoor unit  30  flows into the second outdoor unit  50  through a first inlet valve  52  of the second outdoor unit  50 . The second outdoor unit  50  may receive a refrigerant from the first outdoor unit  30  and supply the refrigerant to the indoor unit  20  through a first transfer flow path  95  which connects the first inlet valve  52  of the second outdoor unit  50  to a first outlet valve  53  or may receive a refrigerant from the first outdoor unit  30  and supply the refrigerant to the first outdoor unit  30  through a second transfer flow path  96  which connects a second inlet valve  54  of the second outdoor unit  50  to a second outlet valve  51 . 
     Accordingly, when a normal cooling operation, which is not low-temperature cooling in which an outdoor temperature is lower than an indoor temperature, is necessary, the second outdoor unit  50  may perform a function of only transferring a refrigerant without passing through internal components of the second outdoor unit  50  by opening a first valve  58  provided at the first transfer flow path  95  and a second valve  57  provided at the second transfer flow path  96 . 
     Meanwhile, in the case of low-temperature cooling in which an outdoor temperature is lower, by a certain degree or more, than an indoor temperature, the air conditioner  3  may collect a refrigerant from the first outdoor unit  30  and perform a cooling operation using the second outdoor unit  50  including the pump  540 . 
     The second outdoor unit  50  may include a third heat exchanger  500  which heat-exchanges a refrigerant discharged from the indoor unit  20 , an accumulator  510  which separates a refrigerant discharged from the third heat exchanger  500  into a liquid and a gas, and the pump  540  which pressurizes a liquid refrigerant discharged from the accumulator  510  and supplies the pressurized liquid refrigerant to the indoor unit  20 . 
     A refrigerant, which flows from the indoor unit  20  into the second outdoor unit  50  through the second inlet valve  54 , may be supplied to the third heat exchanger  500  through a flow path  94  which diverges from the second transfer flow path  96  and connects the second inlet valve  54  to the third heat exchanger  500 . A third valve  55  may be provided at the flow path  94  which connects the second inlet valve  54  to the third heat exchanger  500 . When a low-temperature cooling operation is performed using the second outdoor unit  50 , the second valve  57  is closed and the third valve  55  is opened. 
     A refrigerant, which flows into the third heat exchanger  500 , passes through a flow path  91 , which connects an outlet of the third heat exchanger  500  to the accumulator  510 , and flows into the accumulator  510 . A liquid refrigerant separated at the accumulator  510  passes through a flow path  92  which connects an outlet of the accumulator  510  to the pump  540  and flows into the pump  540 . 
     A refrigerant pressurized at the pump  540  may pass through a flow path  93  connected to an outlet of the pump  540  and attached to the first transfer flow path  95  and be supplied to the indoor unit  20  through the first outlet valve  53  of the second outdoor unit  50 . The check valve  46  which allows only a flow of a refrigerant toward the indoor unit  20  may be provided at the flow path  93  connected to the outlet of the pump  540 . When a low-temperature cooling operation is performed using the second outdoor unit  50 , the first valve  58  is closed. 
     Also, the second outdoor unit  50  may include an air blowing fan  580  which is provided at the third heat exchanger  500  and helps heat exchange at the third heat exchanger  500  by allowing air to flow into the third heat exchanger  500 . 
     The air conditioner  3  may include a variety of sensors which provide operation environment information of the air conditioner to operate by driving only the compressor  350  as shown in  FIG. 14  or driving only the pump  440  as shown in  FIG. 15 . 
     Particularly, the air conditioner  3  may include the temperature sensor  210  provided at the flow path  91  connected to the outlet side of the third heat exchanger  500  of the second outdoor unit  50  and may include the first pressure sensor  240  and the second pressure sensor  220  provided at the flow path  93  connected to the outlet side of the pump  540  and the flow path  92  connected to an inlet side thereof, respectively. Also, the temperature sensor  230  provided at the flow path  93  connected to the outlet of the pump  540  may be included. 
     The second outdoor unit  50  of the air conditioner  3  has a structure simpler than that of the second outdoor unit  40  of the air conditioner  2  according to another embodiment of the present invention described above with reference to  FIGS. 11 to 13 . 
     Accordingly, a user may configure the air conditioner  3  capable of performing pump circulation in a low-temperature cooling environment at a low cost by additionally installing the second outdoor unit  50  in addition to the first outdoor unit  30  and the indoor unit  20  which are already installed. 
     The scope of the present invention is not limited the above-described particular embodiments. It should be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the disclosure, the scope of which is defined in the claims and their equivalents.