Patent Publication Number: US-2013227985-A1

Title: Air conditioner

Description:
TECHNICAL FIELD 
     The present disclosure relates to air conditioners which cool the air to be supplied to an indoor space through an air passage, such as a duct. 
     BACKGROUND ART 
     Air conditioners which cool the air to be supplied to an indoor space through a duct have been known. The air cooled by the air conditioners flows in the duct and is distributed into a plurality of rooms. 
     The air conditioners of this type are disclosed in Patent Document 1, for example. Patent Document 1 shows a marine air conditioner. In this air conditioner, the air that is cooled when it passes through an evaporator flows in a duct, and is supplied to a plurality of cabins. 
     CITATION LIST 
     Patent Document  
     Patent Document 1: Japanese Patent Publication No. 2008-008543 
     SUMMARY OF THE INVENTION 
     Technical Problem  
     The air conditioner disclosed in Patent Document 1 includes a plurality of compressors and one evaporator. Evaporators of the air conditioners of this type are designed to be capable of reliably evaporating a refrigerant while all the compressors are in operation. Further, in the air conditioner having a plurality of compressors, the number of compressors in operation needs to be changed according to an air conditioning load. Thus, in a situation where only some of the plurality of compressors are in operation, the capacity of the evaporator is relatively too much, and all the compressors may have to be stopped because of too much air-conditioning capability with respect to the air conditioning load, regardless of a reduction in the number of compressors in operation. 
     During operation of the compressors, moisture in the air is condensed into drain water in the evaporator. If all the compressors are stopped due to too much air-conditioning capability with respect to the air conditioning load, air is not cooled by the evaporator. In this situation, the drain water remaining around the evaporator is heated by the air passing through the evaporator, evaporated again, and supplied to an indoor space together with the air, which may increase the humidity of the indoor space, and reduce comfort of the space. 
     The present disclosure is thus intended to reduce the occurrence of situation where all compressors are stopped during operation of an air conditioner, and maintain high level of comfort of an indoor space. 
     Solution to the Problem  
     The first aspect of the present disclosure is directed to an air conditioner ( 10 ) including a refrigerant circuit ( 20 ) which performs a refrigeration cycle by circulating a refrigerant, for cooling air flowing in an air passage connected to a supply opening ( 102 ) of each of a plurality of rooms by the refrigerant. The refrigerant circuit ( 20 ) includes a compressor unit ( 30 ) having a plurality of compressors ( 31 ,  32 ,  33 ) connected to each other in parallel, an evaporator ( 50 ) provided at the air passage and having a plurality of heat exchanger sections ( 55 ,  60 ,  65 ) connected to each other in parallel to heat exchange the refrigerant with the air, and a flow control mechanism ( 17 ) configured to change the number of the heat exchanger sections ( 55 ,  60 ,  65 ) through which the refrigerant passes. 
     In the first aspect of the present disclosure, the refrigerant circuit ( 20 ) performs a refrigeration cycle. Air is cooled in the evaporator ( 50 ) of the refrigerant circuit ( 20 ). The air cooled in the evaporator ( 50 ) passes through the air passage and is distributed into a plurality of rooms. In the compressor unit ( 30 ), a plurality of compressors ( 31 ,  32 ,  33 ) are connected in parallel to each other. The operation capacity of the compressor unit ( 30 ) varies by changing the operation capacity of each of the compressors ( 31 ,  32 ,  33 ), or changing the number of compressors ( 31 ,  32 ,  33 ) in operation. The evaporator ( 50 ) includes a plurality of heat exchanger sections ( 55 ,  60 ,  65 ). In the evaporator ( 50 ), the plurality of heat exchanger sections ( 55 ,  60 ,  65 ) are connected in parallel to each other. For example, in the case where the refrigerant flows into all of the heat exchanger sections ( 55 ,  60 ,  65 ), the refrigerant sent to the evaporator ( 50 ) is distributed to the heat exchanger sections ( 55 ,  60 ,  65 ), takes heat from the air, and evaporates. The number of heat exchanger sections ( 55 ,  60 ,  65 ) to which the refrigerant flows is changed by the flow control mechanism ( 17 ). The capacity of the evaporator ( 50 ) is changed by changing the number of heat exchanger sections ( 55 ,  60 ,  65 ) to which the refrigerant flows. 
     The second aspect of the present disclosure is that in the first aspect of the present disclosure, the flow control mechanism ( 17 ) changes the number of the heat exchanger sections ( 55 ,  60 ,  65 ) through which the refrigerant passes, according to an operation capacity of the compressor unit ( 30 ). 
     In the second aspect of the present disclosure, the capacity of the evaporator ( 50 ) is changed according to the operation capacity of the compressor unit ( 30 ). If the operation capacity of the compressor unit ( 30 ) changes, the flow rate of the refrigerant which passes through the evaporator ( 50 ) also changes. Thus, it is possible to adjust the capacity of the evaporator ( 50 ) according to the flow rate of the refrigerant which passes through the evaporator ( 50 ) by changing the number of heat exchanger sections ( 55 ,  60 ,  65 ) through which the refrigerant flows according to the operation capacity of the compressor unit ( 30 ). 
     The third aspect of the present disclosure is that in the second aspect of the present disclosure, each of the compressors ( 31 ,  32 ,  33 ) in the compressor unit ( 30 ) has a fixed capacity, the compressor unit ( 30 ) is configured such that the operation capacity of the compressor unit ( 30 ) is adjusted by changing the number of the compressors ( 31 ,  32 ,  33 ) in operation, and the flow control mechanism ( 17 ) reduces the number of the heat exchanger sections ( 55 ,  60 ,  65 ) through which the refrigerant passes, when the number of the compressors ( 31 ,  32 ,  33 ) in operation is reduced. 
     In the third aspect of the present disclosure, the operation capacity of the compressor unit ( 30 ) is adjusted by changing the number of compressors ( 31 ,  32 ,  33 ) in operation. Thus, the operation capacity of the compressor unit ( 30 ) is changed in stages. If the number of compressors ( 31 ,  32 ,  33 ) in operation is reduced and the operation capacity of the compressor unit ( 30 ) is accordingly reduced, the capacity of the evaporator ( 50 ) is reduced by the flow control mechanism ( 17 ). That is, if the operation capacity of the compressor unit ( 30 ) is reduced and the flow rate of the refrigerant passing through the evaporator ( 50 ) is reduced, the capacity of the evaporator ( 50 ) is accordingly reduced. 
     The fourth aspect of the present disclosure is that in any one of the first to third aspects of the present disclosure, the refrigerant circuit ( 20 ) is provided with one expansion valve ( 40 ) which expands the refrigerant that is not yet branched for flowing into the heat exchanger sections ( 55 ,  60 ,  65 ) of the evaporator ( 50 ). 
     In the fourth aspect of the present disclosure, the refrigerant circuit ( 20 ) is provided with one expansion valve ( 40 ). The refrigerant which circulates in the refrigerant circuit ( 20 ) expands when it passes through the expansion valve ( 40 ), and thereafter the refrigerant is distributed into each of the heat exchanger sections ( 55 ,  60 ,  65 ) of the evaporator ( 50 ). 
     The fifth aspect of the present disclosure is that in any one of the first to third aspects of the present disclosure, the refrigerant circuit ( 20 ) is provided with a plurality of branch pipes ( 26 ,  27 ,  28 ) each of which is connected to a corresponding one of the heat exchanger sections ( 55 ,  60 ,  65 ) of the evaporator ( 50 ), and through which the refrigerant that is branched for flowing into the heat exchanger sections ( 55 ,  60 ,  65 ) flows, and each of the branch pipes ( 26 ,  27 ,  28 ) is provided with a corresponding one of expansion valves ( 41 ,  42 ,  43 ) which expand the refrigerant. 
     In the fifth aspect of the present disclosure, the refrigerant circuit ( 20 ) is provided with the same number of expansion valves ( 41 ,  42 ,  43 ) as the number of heat exchanger sections ( 55 ,  60 ,  65 ) of the evaporator ( 50 ). The refrigerant which circulates in the refrigerant circuit ( 20 ) is branched for flowing into the heat exchanger section ( 55 ,  60 ,  65 ) of the evaporator ( 50 ), then passes through the expansion valve ( 41 ,  42 ,  43 ) and is expanded, and thereafter flows into the heat exchanger section ( 55 ,  60 ,  65 ) corresponding to the expansion valve ( 41 ,  42 ,  43 ) through which the refrigerant passes. 
     Advantages of the Invention  
     In the present disclosure, the capacity of the evaporator ( 50 ) is changed by changing the number of heat exchanger sections ( 55 ,  60 ,  65 ) to which the refrigerant flows, using the flow control mechanism ( 17 ). Thus, if the operation capacity of the compressor unit ( 30 ) is reduced to make the air-conditioning capability of the air conditioner ( 10 ) accord with the air conditioning load, the air-conditioning capability of the air conditioner ( 10 ) can be reliably reduced by reducing the number of heat exchanger sections ( 55 ,  60 ,  65 ) to which the refrigerant flows and thereby reducing the capacity of the evaporator ( 50 ). As a result, a lower limit of a range of adjustment of the air-conditioning capability of the air conditioner ( 10 ) can be reduced to a point lower than before, and it is possible to reduce the frequency of occurrence of the situation where all the compressors ( 31 ,  32 ,  33 ) are stopped during operation of the air conditioner ( 10 ). That is, according to the present disclosure, it is possible to reduce the occurrence of a phenomenon in which drain water evaporates again in the state where all the compressors ( 31 ,  32 ,  33 ) are stopped, and is delivered into an indoor space, and maintain high level of comfort of the indoor space. 
     In the second aspect of the present disclosure, the number of heat exchanger sections ( 55 ,  60 ,  65 ) through which the refrigerant passes is changed according to the operation capacity of the compressor unit ( 30 ). Thus, the capacity of the evaporator ( 50 ) can be adjusted according to the flow rate of the refrigerant which passes through the evaporator ( 50 ). According to the present disclosure, the capacity of the evaporator ( 50 ) can be set appropriately, and the air-conditioning capability of the air conditioner ( 10 ) can be adjusted more appropriately. 
     In the third aspect of the present disclosure, if the number of compressors ( 31 ,  32 ,  33 ) in operation is increased/reduced, the number of heat exchanger sections ( 55 ,  60 ,  65 ) through which the refrigerant passes is accordingly increased/reduced. Thus, according to the present disclosure, the capacity of the evaporator ( 50 ) can be appropriately changed according to the operation capacity of the compressor unit ( 30 ) which is changed in stages, thereby making it possible to adjust the air-conditioning capability of the air conditioner ( 10 ) more appropriately. 
     In the fourth aspect of the present disclosure, the refrigerant flowing into all the heat exchanger sections ( 55 ,  60 ,  65 ) can be expanded using one expansion valve ( 40 ). Thus, according to the present disclosure, an increase in the number of components of the air conditioner ( 10 ) can be prevented. 
     In the fifth aspect of the present disclosure, the flow rate of the refrigerant flowing into the heat exchanger sections ( 55 ,  60 ,  65 ) can be individually controlled by adjusting the openings of the expansion valves ( 41 ,  42 ,  43 ) which respectively correspond to the heat exchanger sections ( 55 ,  60 ,  65 ). Thus, according to the present disclosure, the flow rate of the refrigerant flowing through the heat exchanger sections ( 55 ,  60 ,  65 ) of the evaporator ( 50 ) can be appropriately adjusted, and the air-conditioning capability of the air conditioner ( 10 ) can be maximized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration of a marine air-conditioning system. 
         FIG. 2  is a schematic configuration of an air conditioner. 
         FIG. 3  is a configuration of a main part of a refrigerant circuit according to the first variation of the first embodiment. 
         FIG. 4  is a configuration of a main part of a refrigerant circuit according to the first variation of the first embodiment. 
         FIG. 5  is a configuration of a main part of a refrigerant circuit according to the second variation of the first embodiment. 
         FIG. 6  is a configuration of a main part of a refrigerant circuit according to the second embodiment which corresponds to the refrigerant circuit of  FIG. 2 . 
         FIG. 7  is a configuration of a main part of a refrigerant circuit according to the second embodiment which corresponds to the refrigerant circuit of  FIG. 3 . 
         FIG. 8  is a configuration of a main part of a refrigerant circuit according to the second embodiment which corresponds to the refrigerant circuit of  FIG. 4 . 
         FIG. 9  is a configuration of a main part of a refrigerant circuit according to the second embodiment which corresponds to the refrigerant circuit of  FIG. 5 . 
         FIG. 10  is a configuration of a main part of a refrigerant circuit according to the second embodiment which corresponds to the refrigerant circuit of  FIG. 6 . 
         FIG. 11  is a configuration of a main part of a refrigerant circuit according to the first variation of another embodiment which corresponds to the refrigerant circuit of  FIG. 2 . 
         FIG. 12  is a configuration of a main part of a refrigerant circuit according to the first variation of another embodiment which corresponds to the refrigerant circuit of  FIG. 6 . 
         FIG. 13  is a schematic configuration of a main part of an evaporator according to the second variation of another embodiment which corresponds to the evaporator of  FIG. 2 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present disclosure will be described in detail below based on the drawings. 
     First Embodiment of Invention 
     The first embodiment of the present disclosure will be described. The air conditioner ( 10 ) of the present embodiment is provided at a marine air-conditioning system to supply conditioned air to cabins ( 103 ), i.e., rooms. 
     As shown in  FIG. 1 , an intake duct ( 100 ) and an air supply duct ( 101 ) are connected to a casing ( 11 ) of the air conditioner ( 10 ). The intake duct ( 100 ), the air supply duct ( 101 ), and a space which is formed in the casing ( 11 ) and communicates with the intake duct ( 100 ) and the air supply duct ( 101 ), form an air passage in which air flows. The indoor air in the cabins ( 103 ) and the outdoor air are taken into the intake duct ( 100 ). The indoor air and the outdoor air are mixed, and the mixed air is sent to the air conditioner ( 10 ) through the intake duct ( 100 ). The air supply duct ( 101 ) is connected to a supply opening ( 102 ) provided at each of the cabins ( 103 ). The air blown out from the air conditioner ( 10 ) passes through the air supply duct ( 101 ) and is distributed into the plurality of cabins ( 103 ). 
     As shown in  FIG. 2 , the air conditioner ( 10 ) of the present embodiment includes a refrigerant circuit ( 20 ), a blower ( 15 ), and a controller ( 16 ). The refrigerant circuit ( 20 ), the blower ( 15 ), and the controller ( 16 ) are housed in the casing ( 11 ). In the casing ( 11 ), the blower ( 15 ) and an evaporator ( 50 ), described later, of the refrigerant circuit ( 20 ) are located in the space which communicates with the intake duct ( 100 ) and the air supply duct ( 101 ). 
     The refrigerant circuit ( 20 ) includes a compressor unit ( 30 ), a condenser ( 35 ), an expansion valve ( 40 ), and an evaporator ( 50 ). The refrigerant circuit ( 20 ) is filled with a refrigerant. The refrigerant circuit ( 20 ) is a closed circuit formed by sequentially connecting the compressor unit ( 30 ), the condenser ( 35 ), the expansion valve ( 40 ), and the evaporator ( 50 ) with pipes. 
     The compressor unit ( 30 ) includes three compressors ( 31 ,  32 ,  33 ). The number of compressors ( 31 ,  32 ,  33 ) provided in the compressor unit ( 30 ) is merely an example. The compressors ( 31 ,  32 ,  33 ) are hermetic scroll compressors ( 31 ,  32 ,  33 ). Each of the compressors ( 31 ,  32 ,  33 ) has a fixed capacity, that is, the rotational speed cannot be changed. 
     In the compressor unit ( 30 ), the three compressors ( 31 ,  32 ,  33 ) are connected to each other in parallel. Specifically, suction pipes ( 31   a ,  32   a ,  33   a ) of the compressors ( 31 ,  32 ,  33 ) are connected to an outlet pipe ( 52 ) of the evaporator ( 50 ), described later. Further, discharge pipes ( 31   b ,  32   b ,  33   b ) of the compressors ( 31 ,  32 ,  33 ) are connected to a refrigerant inlet of the condenser ( 35 ). The compressors ( 31 ,  32 ,  33 ) compress the refrigerant suctioned through the suction pipes ( 31   a ,  32   a ,  33   a ) and discharge the compressed refrigerant through the discharge pipes ( 31   b ,  32   b ,  33   b ). 
     The operation capacity of the compressor unit ( 30 ) is adjusted by changing the number of compressors ( 31 ,  32 ,  33 ) in operation. In general, the operation capacity of the compressor unit ( 30 ) can be adjusted by changing the rotational speed of each compressor ( 31 ,  32 ,  33 ) using an inverter. However, if an inverter is used, electromagnetic noise is generated, and this may adversely affect radio communication such as rescue communication. Further, a negative phase sequence current generated in the inverter may reduce the capability of an electric generator. Thus, if an inverter is used to adjust the operation capacity of the compressor unit ( 30 ), the marine air conditioner ( 10 ) requires measures for preventing the above adverse effects and this may increase the fabrication costs. For this reason, the compressor unit ( 30 ) of the present embodiment is configured such that the operation capacity of the compressor unit ( 30 ) is adjusted by changing the number of compressors ( 31 ,  32 ,  33 ) in operation. 
     The condenser ( 35 ) is a so-called a shell and tube heat exchanger, in which a refrigerant is heat exchanged with cooling water (e.g., seawater or water taken from a river etc.). The refrigerant outlet of the condenser ( 35 ) is connected to the evaporator ( 50 ) with a pipe ( 25 ). The pipe ( 25 ) is provided with the expansion valve ( 40 ). 
     The expansion valve ( 40 ) is a so-called thermostatic automatic expansion valve. A feeler bulb ( 40   a ) of the expansion valve ( 40 ) is attached to the outlet pipe ( 52 ) of the evaporator ( 50 ), and is in contact with the surface of the outlet pipe ( 52 ). 
     The pipe ( 25 ) is branched into two pipes at a downstream side of the expansion valve ( 40 ). A first branch pipe ( 26 ) is connected to one end of a first flow pass ( 56 ) of the evaporator ( 50 ), and a second branch pipe ( 27 ) is connected to one end of a second flow pass ( 61 ) of the evaporator ( 50 ). The second branch pipe ( 27 ) of the pipe ( 25 ) is provided with a solenoid valve ( 70 ) which comprises a flow control mechanism ( 17 ). 
     The evaporator ( 50 ) is a so-called cross-fin type fin-and-tube heat exchanger, and includes a heat-transfer tube made of copper and a fin ( 51 ) made of aluminum. The evaporator ( 50 ) exchanges heat between the refrigerant and air. 
     The evaporator ( 50 ) has a first heat exchanger section ( 55 ) and a second heat exchanger section ( 60 ). The heat exchanger section ( 55 ,  60 ) is comprised of the flow pass ( 56 ,  61 ) made of a heat-transfer tube, and fins ( 51 ) attached to the heat-transfer tube comprising the flow pass ( 56 ,  61 ). In the evaporator ( 50 ), the fins ( 51 ) which comprise the heat exchanger sections ( 55 ,  60 ) are integrally formed. 
     As described above, in the evaporator ( 50 ), one end of the first flow pass ( 56 ) is connected to the expansion valve ( 40 ) via the first branch pipe ( 26 ), and one end of the second flow pass ( 61 ) is connected to the expansion valve ( 40 ) via the second branch pipe ( 27 ). In the evaporator ( 50 ), the other end of each of the flow passes ( 56 ,  61 ) is connected to the outlet pipe ( 52 ). 
     The air conditioner ( 10 ) is provided with a supply air temperature sensor ( 81 ) and an evaporation temperature sensor ( 82 ). The supply air temperature sensor ( 81 ) is located at a downstream side of the evaporator ( 50 ) along an airflow passage. The supply air temperature sensor ( 81 ) measures a temperature of the air delivered to the air supply duct ( 101 ) through the evaporator ( 50 ). The evaporation temperature sensor ( 82 ) is attached to the heat-transfer tube comprising the first flow pass ( 56 ) of the evaporator ( 50 ), and is in contact with the surface of the heat-transfer tube. The evaporation temperature sensor ( 82 ) measures a. temperature of the surface of the heat-transfer tube as a temperature at which the refrigerant evaporates in the evaporator ( 50 ). 
     The controller ( 16 ) performs operation of adjusting the operation capacity of the compressor unit ( 30 ), and operation of controlling the solenoid valve ( 70 ). Specifically, a value measured by the supply air temperature sensor ( 81 ), and a value measured by the evaporation temperature sensor ( 82 ) are input to the controller ( 16 ). The controller ( 16 ) adjusts the operation capacity of the compressor unit ( 30 ) based on the value measured by the supply air temperature sensor ( 81 ), and opens/closes the solenoid valve ( 70 ) based on the value measured by the evaporation temperature sensor ( 82 ). 
     —Operation Mechanism— 
     An operation mechanism of the air conditioner ( 10 ) will be described. 
     First, an operation of the refrigerant circuit ( 20 ) will be described with reference to  FIG. 2 . In this example, a state in which the operation capacity of the compressor unit ( 30 ) is largest and the solenoid valve ( 70 ) is open will be described. 
     When the operation capacity of the compressor unit ( 30 ) is largest, all the compressors ( 31 ,  32 ,  33 ) are operated. The refrigerant discharged from each of the compressors ( 31 ,  32 ,  33 ) is merged together, flows into the condenser ( 35 ), dissipates heat into cooling water, and is condensed. The refrigerant condensed in the condenser ( 35 ) is depressurized when passing through the expansion valve ( 40 ), and changed to a gas-liquid two-phase state. 
     The refrigerant having passed through the expansion valve ( 40 ) flows into the evaporator ( 50 ). Specifically, part of the refrigerant having passed through the expansion valve ( 40 ) goes through the first branch pipe ( 26 ) to flow into the first flow pass ( 56 ) of the first heat exchanger section ( 55 ), and the other part of the refrigerant goes through the second branch pipe ( 27 ) to flow into the second flow pass ( 61 ) of the second heat exchanger section ( 60 ). The refrigerant flowing in the flow pass ( 56 ,  61 ) absorbs heat from the air passing between the fins ( 51 ) and evaporates, and usually becomes superheated vapors and flows into the outlet pipe ( 52 ). 
     The refrigerant having flowed into the outlet pipe ( 52 ) from the flow passes ( 56 ,  61 ) flows out from the evaporator ( 50 ), and is separately sucked into the three compressors ( 31 ,  32 ,  33 ) thereafter. The refrigerant sucked into the compressors ( 31 ,  32 ,  33 ) is compressed and thereafter discharged from the compressors ( 31 ,  32 ,  33 ). 
     As described above, the feeler bulb ( 40   a ) of the expansion valve ( 40 ) is attached to the outlet pipe ( 52 ) of the evaporator ( 50 ). Thus, the opening of the expansion valve ( 40 ) is adjusted such that a degree of superheat of the refrigerant flowing in the outlet pipe ( 52 ) will be a target degree of superheat. That is, when the degree of superheat of the refrigerant flowing in the outlet pipe ( 52 ) is too high, the opening of the expansion valve ( 40 ) is increased to lower the degree of superheat. On the other hand, when the degree of superheat of the refrigerant flowing in the outlet pipe ( 52 ) is too low, the opening of the expansion valve ( 40 ) is reduced to increase the degree of superheat. 
     Now, flow of the air will be described with reference to  FIG. 1 . The blower ( 15 ) is driven during operation of the air conditioner ( 10 ). The blower ( 15 ) takes air through the air supply duct ( 101 ). Thus, the air in the cabins ( 103 ) and the air outside the boat are taken in the air conditioner ( 10 ) through the air supply duct ( 101 ). 
     The air taken in the air conditioner ( 10 ) is cooled by the refrigerant when passing through the evaporator ( 50 ). In general, the temperature of the air having passed through the evaporator ( 50 ) is lower than the dew-point temperature of the air to be delivered to the evaporator ( 50 ). Thus, in the evaporator ( 50 ), water vapors contained in the air are condensed into drain water. In other words, the air is cooled and dehumidified in the evaporator ( 50 ). The cooled and dehumidified air is delivered into the air supply duct ( 101 ) from the air conditioner ( 10 ). The air flowing in the air supply duct ( 101 ) is distributed into the supply opening ( 102 ) provided at each cabin ( 103 ), and blown into the cabin ( 103 ) from the supply opening ( 102 ). 
     —Operation of Controller— 
     Now, an operation of the controller ( 16 ) will be described. 
     First, an operation for adjusting the operation capacity of the compressor unit ( 30 ) will be described. The controller ( 16 ) adjusts the operation capacity of the compressor unit ( 30 ) such that a temperature measured by the supply air temperature sensor ( 81 ) will be a predetermined temperature. 
     Specifically, if the temperature measured by the supply air temperature sensor ( 81 ) is lower than the predetermined temperature, the controller ( 16 ) reduces, one by one, the number of compressors ( 31 ,  32 ,  33 ) in operation in the compressor unit ( 30 ) to increase the value measured by the supply air temperature sensor ( 81 ). That is, in this case, the controller ( 16 ) reduces the operation capacity of the compressor unit ( 30 ) in stages. Further, if the temperature measured by the supply air temperature sensor ( 81 ) is lower than the predetermined temperature even in a situation where only one of the compressors ( 31 ,  32 ,  33 ) is operated, the controller ( 16 ) stops all of the compressors ( 31 ,  32 ,  33 ). 
     On the other hand, if the temperature measured by the supply air temperature sensor ( 81 ) is higher than the predetermined temperature, the controller ( 16 ) increases, one by one, the number of compressors ( 31 ,  32 ,  33 ) in operation in the compressor unit ( 30 ) to reduce the value measured by the supply air temperature sensor ( 81 ). That is, in this case, the controller ( 16 ) increases the operation capacity of the compressor unit ( 30 ) in stages. 
     Next, an operation for controlling the solenoid valve ( 70 ) will be described. The controller ( 16 ) opens/closes the solenoid valve ( 70 ) so that the value measured by the evaporation temperature sensor ( 82 ) is maintained in a predetermined reference range. 
     Specifically, when the value measured by the evaporation temperature sensor ( 82 ) exceeds an upper limit of the reference range in a state where the solenoid valve ( 70 ) is open, the controller ( 16 ) closes the solenoid valve ( 70 ). In the evaporator ( 50 ), if the solenoid valve ( 70 ) is closed, the refrigerant does not flow in the second flow pass ( 61 ) of the second heat exchanger section ( 60 ), but flows only in the first flow pass ( 56 ) of the first heat exchanger section ( 55 ). 
     If the solenoid valve ( 70 ) is open in a state where the operation capacity of the compressor unit ( 30 ) is small, the capacity of the evaporator ( 50 ) is too much with respect to the flow rate of the refrigerant which circulates in the refrigerant circuit ( 20 ), and it is highly likely that the temperature at which the refrigerant evaporates at the evaporator ( 50 ) will increase. In such a case, the controller ( 16 ) closes the solenoid valve ( 70 ) to reduce the capacity of the evaporator ( 50 ). If the solenoid valve ( 70 ) is closed, the refrigerant flows only to the first flow pass ( 56 ), and the capacity of the evaporator ( 50 ) is accordingly reduced. Consequently, the evaporation temperature of the refrigerant at the evaporator ( 50 ) is reduced. 
     On the other hand, if the value measured by the evaporation temperature sensor ( 82 ) is smaller than a lower limit of the reference range in a state where the solenoid valve ( 70 ) is closed, the controller ( 16 ) opens the solenoid valve ( 70 ). In the evaporator ( 50 ), if the solenoid valve ( 70 ) is open, the refrigerant flows into both of the first flow pass ( 56 ) of the first heat exchanger section ( 55 ) and the second heat exchanger section ( 60 ) of the first flow pass ( 56 ). 
     If the solenoid valve ( 70 ) is closed in a state where the operation capacity of the compressor unit ( 30 ) is large, the capacity of the evaporator ( 50 ) is too small with respect to the flow rate of the refrigerant which circulates in the refrigerant circuit ( 20 ), and it is highly likely that the temperature at which the refrigerant evaporates at the evaporator ( 50 ) will decrease. In such a case, the controller ( 16 ) opens the solenoid valve ( 70 ) to increase the capacity of the evaporator ( 50 ). If the solenoid valve ( 70 ) is opened, the refrigerant flows to both of the first flow pass ( 56 ) and the second flow pass ( 61 ), and the capacity of the evaporator ( 50 ) is accordingly increased. Consequently, the evaporation temperature of the refrigerant in the evaporator ( 50 ) is increased. 
     Advantages of First Embodiment 
     As described above, the controller ( 16 ) adjusts the operation capacity of the compressor unit ( 30 ) during the operation of the air conditioner ( 10 ). If the cooling load of the cabins ( 103 ) is very small, all the compressors ( 31 ,  32 ,  33 ) of the compressor unit ( 30 ) may be stopped even during the operation of the air conditioner ( 10 ). The air conditioner ( 10 ) takes air in which indoor air and outdoor air are mixed, and supplies the mixed air to the cabins ( 103 ). In other words, the air conditioner ( 10 ) performs not only cooling, but also ventilation of the cabins ( 103 ). The cabins ( 103 ) need to be ventilated all the time, irrespective of the cooling load of the cabins ( 103 ). Therefore, during the operation of the air conditioner ( 10 ), the blower ( 15 ) is kept driven even in the state where all the compressors ( 31 ,  32 ,  33 ) of the compressor unit ( 30 ) are stopped. 
     In the state where all the compressors ( 31 ,  32 ,  33 ) are stopped, the refrigerant is not supplied to the evaporator ( 50 ), and cooling of the air does not occur in the evaporator ( 50 ). On the surface of the evaporator ( 50 ) or around the evaporator ( 50 ), there remains drain water generated during the operation of the compressors ( 31 ,  32 ,  33 ). If air passes through the evaporator ( 50 ) in the state where all the compressors ( 31 ,  32 ,  33 ) are stopped, the drain water on the surface of the evaporator ( 50 ) and around the evaporator ( 50 ) is heated by the air, evaporates again, and is delivered to the cabins ( 103 ) together with the air. Therefore, if all the compressors ( 31 ,  32 ,  33 ) of the compressor unit ( 30 ) are stopped during the operation of the air conditioner ( 10 ), the humidity of the air to be supplied to the cabins ( 103 ) increases, which may reduce comfort of the interior of the cabins ( 103 ). 
     In the marine air conditioner ( 10 ), in particular, it is difficult to use an inverter in order to adjust the operation capacity of the compressor unit ( 30 ) in terms of cost. Therefore, in general, the operation capacity of the compressor unit ( 30 ) is adjusted by changing the number of compressors ( 31 ,  32 ,  33 ) in operation. It is thus difficult to adjust the operation capacity of the compressor unit ( 30 ) in detail, and it frequently happens that all the compressors ( 31 ,  32 ,  33 ) of the compressor unit ( 30 ) are stopped. 
     Further, in the air conditioner ( 10 ) of the present embodiment, a thermostatic automatic expansion valve is used as the expansion valve ( 40 ), and the feeler bulb ( 40   a ) of the expansion valve ( 40 ) is attached to the outlet pipe ( 52 ) of the evaporator ( 50 ). A degree of superheat of the refrigerant flowing in the outlet pipe ( 52 ) is increased when the capacity of the evaporator ( 50 ) is too much with respect to the flow rate of the refrigerant which circulates in the refrigerant circuit ( 20 ), and therefore, the opening of the expansion valve ( 40 ) is increased to reduce the degree of superheat of the refrigerant. However, in the state where the opening of the expansion valve ( 40 ) is large, it is difficult to sufficiently reduce the flow rate of the refrigerant passing through the evaporator ( 50 ) by reducing the number of compressors ( 31 ,  32 ,  33 ) in operation. It is thus difficult to sufficiently reduce the lower limit of a range of adjustment of the cooling capability of the air conditioner ( 10 ), and this is also a cause of frequent occurrence of the situation in which all the compressors ( 31 ,  32 ,  33 ) of the compressor unit ( 30 ) are stopped. 
     In the air conditioner ( 10 ) of the present embodiment, the controller ( 16 ) controls the solenoid valve ( 70 ) based on a value measured by the evaporation temperature sensor ( 82 ), thereby changing the number of heat exchanger sections ( 55 ,  60 ) in the evaporator ( 50 ) through which the refrigerant flows, such that the value measured by the evaporation temperature sensor ( 82 ) is maintained in a reference range. Thus, for example, if only one compressor ( 31 ,  32 ,  33 ) of the compressor unit ( 30 ) is in operation and the evaporation temperature of the refrigerant at the evaporator ( 50 ) increases and exceeds the upper limit of the reference range, the controller ( 16 ) closes the solenoid valve ( 70 ), and the refrigerant flows only to the first flow pass ( 56 ) of the first heat exchanger section ( 55 ). 
     In the air conditioner ( 10 ) of the present embodiment, as described above, if the flow rate of the refrigerant passing through the evaporator ( 50 ) is reduced due to a reduction in the operation capacity of the compressor unit ( 30 ), the number of heat exchanger sections ( 55 ,  60 ) in the evaporator ( 50 ) through which the refrigerant flows is reduced, thereby reducing the capacity of the evaporator ( 50 ). Thus, in the present embodiment, the capacity of the evaporator ( 50 ) can be reduced according to the operation capacity of the compressor unit ( 30 ), and it is possible to reduce the lower limit of a range of adjustment of the cooling capability. As a result, it is possible to reduce the frequency of the occurrence of the situation where all the compressors ( 31 ,  32 ,  33 ) of the compressor unit ( 30 ) are stopped, and possibility that comfort of the indoor space is reduced due to the reevaporation of drain water. 
     Further, in the case where the flow rate of the refrigerant passing through the evaporator ( 50 ), an excessive increase in the degree of superheat of the refrigerant flowing in the outlet pipe ( 52 ) of the evaporator ( 50 ) is prevented by reducing the number of heat exchanger sections ( 55 ,  60 ) in the evaporator ( 50 ) through which the refrigerant flows. Thus, the opening of the expansion valve ( 40 ) can be smaller than a certain degree, and it is possible to reliably reduce the flow rate of the refrigerant passing through the evaporator ( 50 ). 
     First Variation of First Embodiment 
     In the evaporator ( 50 ) of the first embodiment, one or both of the first flow pass ( 56 ) of the first heat exchanger section ( 55 ) and the second flow pass ( 61 ) of the second heat exchanger section ( 60 ) may have a plurality of paths ( 56   a,    56   b,    61   a ,  61   b ). 
     In an example shown in  FIG. 3 , each of the first flow pass ( 56 ) and the second flow pass ( 61 ) includes a first path ( 56   a,    61   a ), a second path ( 56   b,    61   b ), a distributer ( 57 ,  62 ), and a junction pipe ( 58 ,  63 ). In the first flow pass ( 56 ) shown in  FIG. 3 , one end of each of the first path ( 56   a ) and the second path ( 56   b ) is connected to an outlet side of the distributer ( 57 ), and the other end of each of the first path ( 56   a ) and the second path ( 56   b ) is connected to the outlet pipe ( 52 ) via the junction pipe ( 58 ). The first branch pipe ( 26 ) of the pipe ( 25 ) is connected to an intake side of the distributer ( 57 ). In the second flow pass ( 61 ) shown in  FIG. 3 , one end of each of the first path ( 61   a ) and the second path ( 61   b ) is connected to an outlet side of the distributer ( 62 ), and the other end of each of the first path ( 61   a ) and the second path ( 61   b ) is connected to the outlet pipe ( 52 ) via the junction pipe ( 63 ). The second branch pipe ( 27 ) of the pipe ( 25 ) is connected to an intake side of the distributer ( 62 ). 
     In an example shown in  FIG. 4 , only the second flow pass ( 61 ) includes the first path ( 61   a ), the second path ( 61   b ), the distributer ( 62 ), and the junction pipe ( 63 ). In the second flow pass ( 61 ) shown in  FIG. 4 , one end of each of the first path ( 61   a ) and the second path ( 61   b ) is connected to the outlet side of the distributer ( 62 ), and the other end of each of the first path ( 61   a ) and the second path ( 61   b ) is connected to the outlet pipe ( 52 ) via the junction pipe ( 63 ). The second branch pipe ( 27 ) of the pipe ( 25 ) is connected to the intake side of the distributer ( 62 ). 
     Second Variation of First Embodiment 
     The evaporator ( 50 ) of the first embodiment may include three or more heat exchanger sections ( 55 ,  60 ,  65 ). In this example, a refrigerant circuit ( 20 ) provided with an evaporator ( 50 ) which includes three heat exchanger sections ( 55 ,  60 ,  65 ) will be described with reference to  FIG. 5 . 
     In the refrigerant circuit ( 20 ) of the present variation, the pipe ( 25 ) connecting the condenser ( 35 ) and the evaporator ( 50 ) is divided into three branch pipes ( 26 ,  27 ,  28 ) at a portion on the downstream side of the expansion valve ( 40 ). The first branch pipe ( 26 ) is connected to one end of the first flow pass ( 56 ) of the first heat exchanger section ( 55 ). The second branch pipe ( 27 ) is connected to one end of the second flow pass ( 61 ) of the second heat exchanger section ( 60 ). The third branch pipe ( 28 ) is connected to one end of the third flow pass ( 66 ) of the third heat exchanger section ( 65 ). The other end of each of the flow passes ( 56 ,  61 ,  66 ) is connected to the outlet pipe ( 52 ). In the refrigerant circuit ( 20 ) of the present variation, the second branch pipe ( 27 ) of the pipe ( 25 ) is provided with a first solenoid valve ( 71 ), and the third branch pipe ( 28 ) is provided with a second solenoid valve ( 72 ). In the evaporator ( 50 ) of the present variation, the number of heat exchanger sections ( 55 ,  60 ,  65 ) through which the refrigerant flows is any number from one to three. 
     Second Embodiment of Invention 
     The second embodiment of the present disclosure will be described. A refrigerant circuit ( 20 ) of the present embodiment includes the same number of expansion valves ( 41 ,  42 ) as the number of heat exchanger sections ( 55 ,  60 ) of the evaporator ( 50 ). 
     The refrigerant circuit ( 20 ) shown in  FIG. 6  is obtained by applying the present embodiment to the refrigerant circuit ( 20 ) shown in  FIG. 2 . In the refrigerant circuit ( 20 ) shown in  FIG. 6 , branch pipes ( 26 ,  27 ) of the pipe ( 25 ) are provided with expansion valves ( 41 ,  42 ), respectively. The second expansion valve ( 42 ) is provided on the second branch pipe ( 27 ) of the pipe ( 25 ) at a location on the upstream side of the solenoid valve ( 70 ). 
     Each of the expansion valves ( 41 ,  42 ) of the refrigerant circuit ( 20 ) shown in  FIG. 6  is a so-called thermostatic automatic expansion valve. A feeler bulb ( 41   a ) of the first expansion valve ( 41 ) provided at the first branch pipe ( 26 ) is attached to a pipe which comprises an outlet side end of the first flow pass ( 56 ), and is in contact with a surface of this pipe. The opening of the first expansion valve ( 41 ) is adjusted such that a degree of superheat of the refrigerant flowing out of the first heat exchanger section ( 55 ) will be a target degree of superheat. A feeler bulb ( 42   a ) of the second expansion valve ( 42 ) provided at the second branch pipe ( 27 ) is attached to a pipe which comprises an outlet side end of the second flow pass ( 61 ), and is in contact with a surface of this pipe. The opening of the second expansion valve ( 42 ) is adjusted such that a degree of superheat of the refrigerant flowing out of the second heat exchanger section ( 60 ) will be a target degree of superheat. 
     The refrigerant circuit ( 20 ) shown in  FIG. 7  is obtained by applying the present embodiment to the refrigerant circuit ( 20 ) shown in  FIG. 3 . In the refrigerant circuit ( 20 ) shown in  FIG. 7 , branch pipes ( 26 ,  27 ) of the pipe ( 25 ) are provided with expansion valves ( 41 ,  42 ), respectively. The second expansion valve ( 42 ) is provided on the second branch pipe ( 27 ) of the pipe ( 25 ) at a location on the upstream side of the solenoid valve ( 70 ). 
     Each of the expansion valves ( 41 ,  42 ) of the refrigerant circuit ( 20 ) shown in  FIG. 7  is a so-called thermostatic automatic expansion valve. A feeler bulb ( 41   a ) of the first expansion valve ( 41 ) provided at the first branch pipe ( 26 ) is attached to a junction pipe ( 58 ) of the first flow pass ( 56 ), and is in contact with a surface of the junction pipe ( 58 ). The opening of the first expansion valve ( 41 ) is adjusted such that a degree of superheat of the refrigerant flowing out of the paths ( 56   a,    56   b ) of the first heat exchanger section ( 55 ) will be a target degree of superheat. A feeler bulb ( 42   a ) of the second expansion valve ( 42 ) provided at the second branch pipe ( 27 ) is attached to a junction pipe ( 63 ) of the second flow pass ( 61 ), and is in contact with a surface of the junction pipe ( 63 ). The opening of the second expansion valve ( 42 ) is adjusted such that a degree of superheat of the refrigerant flowing out of the paths ( 61   a ,  61   b ) of the second heat exchanger section ( 60 ) will be a target degree of superheat. 
     The refrigerant circuit ( 20 ) shown in  FIG. 8  is obtained by applying the present embodiment to the refrigerant circuit ( 20 ) shown in  FIG. 4 . In the refrigerant circuit ( 20 ) shown in  FIG. 8 , branch pipes ( 26 ,  27 ) of the pipe ( 25 ) are provided with expansion valves ( 41 ,  42 ), respectively. The second expansion valve ( 42 ) is provided on the second branch pipe ( 27 ) of the pipe ( 25 ) at a location on the upstream side of the solenoid valve ( 70 ). 
     Each of the expansion valves ( 41 ,  42 ) of the refrigerant circuit ( 20 ) shown in  FIG. 8  is a so-called thermostatic automatic expansion valve. A feeler bulb ( 41   a ) of the first expansion valve ( 41 ) provided at the first branch pipe ( 26 ) is attached to a pipe which comprises an outlet side end of the first flow pass ( 56 ), and is in contact with a surface of this pipe. The opening of the first expansion valve ( 41 ) is adjusted such that a degree of superheat of the refrigerant flowing out of the first heat exchanger section ( 55 ) will be a target degree of superheat. A feeler bulb ( 42   a ) of the second expansion valve ( 42 ) provided at the second branch pipe ( 27 ) is attached to a junction pipe ( 63 ) of the second flow pass ( 61 ), and is in contact with a surface of the junction pipe ( 63 ). The opening of the second expansion valve ( 42 ) is adjusted such that a degree of superheat of the refrigerant flowing out of the paths ( 61   a ,  61   b ) of the second heat exchanger section ( 60 ) will be a target degree of superheat. 
     The refrigerant circuit ( 20 ) shown in  FIG. 9  is obtained by applying the present embodiment to the refrigerant circuit ( 20 ) shown in  FIG. 5 . In the refrigerant circuit ( 20 ) shown in  FIG. 9 , branch pipes ( 26 ,  27 ,  28 ) of the pipe ( 25 ) are provided with expansion valves ( 41 ,  42 ,  43 ), respectively. The second expansion valve ( 42 ) is provided on the second branch pipe ( 27 ) of the pipe ( 25 ) at a location on the upstream side of the first solenoid valve ( 71 ). The third expansion valve ( 43 ) is provided on the third branch pipe ( 28 ) of the pipe ( 25 ) at a location on the upstream side of the second solenoid valve ( 72 ). 
     Each of the expansion valves ( 41 ,  42 ,  43 ) of the refrigerant circuit ( 20 ) shown in  FIG. 9  is a so-called thermostatic automatic expansion valve. A feeler bulb ( 41   a ) of the first expansion valve ( 41 ) provided at the first branch pipe ( 26 ) is attached to a pipe which comprises an outlet side end of the first flow pass ( 56 ), and is in contact with a surface of this pipe. The opening of the first expansion valve ( 41 ) is adjusted such that a degree of superheat of the refrigerant flowing out of the first heat exchanger section ( 55 ) will be a target degree of superheat. A feeler bulb ( 42   a ) of the second expansion valve ( 42 ) provided at the second branch pipe ( 27 ) is attached to a pipe which comprises an outlet side end of the second flow pass ( 61 ), and is in contact with a surface of this pipe. The opening of the second expansion valve ( 42 ) is adjusted such that a degree of superheat of the refrigerant flowing out of the second heat exchanger section ( 60 ) will be a target degree of superheat. A feeler bulb ( 43   a ) of the third expansion valve ( 43 ) provided at the third branch pipe ( 28 ) is attached to a pipe which comprises an outlet side end of the third flow pass ( 66 ), and is in contact with a surface of this pipe. The opening of the third expansion valve ( 43 ) is adjusted such that a degree of superheat of the refrigerant flowing out of the third heat exchanger section ( 65 ) will be a target degree of superheat. 
     Variation of Second Embodiment 
     In the refrigerant circuit ( 20 ) of the present embodiment, the expansion valve ( 42 ,  43 ) and the solenoid valve ( 71 ,  72 ) may change places with each other at the branch pipe ( 27 ,  28 ) of the pipe ( 25 ). 
     The refrigerant circuit ( 20 ) shown in  FIG. 10  is obtained by applying the present variation to the refrigerant circuit ( 20 ) shown in  FIG. 6 . A second expansion valve ( 42 ) is provided on the second branch pipe ( 27 ) of the refrigerant circuit ( 20 ) shown in  FIG. 10  at a location on the downstream side of the solenoid valve ( 70 ). 
     Other Embodiments 
     —First Variation— 
     The refrigerant circuits ( 20 ) shown in  FIG. 2  to  FIG. 10  may include a so-called electronic expansion valve as the expansion valve ( 40 ,  41 ,  42 ). 
     The refrigerant circuit ( 20 ) shown in  FIG. 11  is obtained by applying the present variation to the refrigerant circuit ( 20 ) shown in  FIG. 2 . 
     In the refrigerant circuit ( 20 ) shown in  FIG. 11 , a refrigerant temperature sensor ( 85 ) is attached to the outlet pipe ( 52 ) of the evaporator ( 50 ). The refrigerant temperature sensor ( 85 ) is in contact with the outlet pipe ( 52 ) and measures a temperature of a surface of the outlet pipe ( 52 ) as a temperature of the refrigerant flowing in the outlet pipe ( 52 ). A degree of superheat of the refrigerant flowing in the outlet pipe ( 52 ) can be calculated by subtracting a value measured by the evaporation temperature sensor ( 82 ) from a value measured by the refrigerant temperature sensor ( 85 ). The controller ( 16 ) of the present variation controls the opening of the expansion valve ( 40 ) in the refrigerant circuit ( 20 ) shown in  FIG. 11  such that the value obtained by subtracting the value measured by the evaporation temperature sensor ( 82 ) from the value measured by the refrigerant temperature sensor ( 85 ) will be a target degree of superheat. 
     The refrigerant circuit ( 20 ) shown in  FIG. 12  is obtained by applying the present variation to the refrigerant circuit ( 20 ) shown in  FIG. 6 . 
     In the refrigerant circuit ( 20 ) shown in  FIG. 12 , a first refrigerant temperature sensor ( 86 ) is attached to a pipe which comprises an outlet side end of the first flow pass ( 56 ). The first refrigerant temperature sensor ( 86 ) is in contact with the pipe and measures a temperature of a surface of the pipe as a temperature of the refrigerant flowing out from the first flow pass ( 56 ). A degree of superheat of the refrigerant flowing out of the first flow pass ( 56 ) can be calculated by subtracting a value measured by the evaporation temperature sensor ( 82 ) from a value measured by the first refrigerant temperature sensor ( 86 ). The controller ( 16 ) of the present variation adjusts the opening of the first expansion valve ( 41 ) in the refrigerant circuit ( 20 ) shown in  FIG. 12  such that the value obtained by subtracting the value measured by the evaporation temperature sensor ( 82 ) from the value measured by the first refrigerant temperature sensor ( 86 ) will be a target degree of superheat. 
     Further, in the refrigerant circuit ( 20 ) shown in  FIG. 12 , a second refrigerant temperature sensor ( 87 ) is attached to a pipe which comprises an outlet side end of the second flow pass ( 61 ). The second refrigerant temperature sensor ( 87 ) is in contact with the pipe and measures a temperature of a surface of the pipe as a temperature of the refrigerant flowing out of the second flow pass ( 61 ). A degree of superheat of the refrigerant flowing out of the second flow pass ( 61 ) can be calculated by subtracting a value measured by the evaporation temperature sensor ( 82 ) from a value measured by the second refrigerant temperature sensor ( 87 ). The controller ( 16 ) of the present variation adjusts the opening of the second expansion valve ( 42 ) in the refrigerant circuit ( 20 ) shown in  FIG. 12  such that the value obtained by subtracting the value measured by the evaporation temperature sensor ( 82 ) from the value measured by the second refrigerant temperature sensor ( 87 ) will be a target degree of superheat. 
     A solenoid valve ( 70 ) is omitted in the refrigerant circuit ( 20 ) shown in  FIG. 12 . That is, only the second expansion valve ( 42 ) is provided on the second branch pipe ( 27 ) of the pipe ( 25 ). In this refrigerant circuit ( 20 ), the second expansion valve ( 42 ) also serves as a flow control mechanism ( 17 ). That is, the opening of the second expansion valve ( 42 ), which is an electronic expansion valve, can be freely determined by a control signal from the controller ( 16 ). Thus, in the case where the refrigerant is intended to flow only in the first flow pass ( 56 ), the controller ( 16 ) closes the second expansion valve ( 42 ) completely. 
     —Second Variation— 
     In the evaporator ( 50 ) shown in  FIG. 2  to  FIG. 10 , heat-transfer tubes comprising the flow passes ( 56 ,  61 ,  66 ) may be alternately arranged. 
     The evaporator ( 50 ) shown in  FIG. 13  is obtained by applying the present variation to the evaporator ( 50 ) shown in  FIG. 2 . In the evaporator ( 50 ) shown in  FIG. 13 , a heat-transfer tube comprising the first flow pass ( 56 ) and a heat-transfer tube comprising the second flow pass ( 61 ) are alternately arranged in a longitudinal direction of the fin ( 51 ). The evaporator ( 50 ) of the present variation allows the air passing through the evaporator ( 50 ) to have a uniform temperature even in the state where the refrigerant flows only in the first flow pass ( 56 ). 
     —Third Variation— 
     The controller ( 16 ) of the above embodiments may be configured to change the number of heat exchanger sections ( 55 ,  60 ,  65 ) through which the refrigerant flows in the evaporator ( 50 ), based on an evaporation pressure of the refrigerant in the evaporator ( 50 ). In this example, the present variation is applied to the air conditioner ( 10 ) of the first embodiment shown in  FIG. 2 . 
     The controller ( 16 ) of the present variation opens/closes the solenoid valve ( 70 ) such that an evaporation pressure of the refrigerant in the evaporator ( 50 ) (i.e., a low pressure of refrigeration cycle) is maintained in a reference range. 
     Specifically, when the evaporation pressure of the refrigerant exceeds an upper limit of the reference range in the state where the solenoid valve ( 70 ) is open, the controller ( 16 ) closes the solenoid valve ( 70 ). In the state where the solenoid valve ( 70 ) is closed, the refrigerant does not flow into the second flow pass ( 61 ) of the second heat exchanger section ( 60 ) in the evaporator ( 50 ), but flows only into the first flow pass ( 56 ) of the first heat exchanger section ( 55 ). 
     If the solenoid valve ( 70 ) is open in the state where the operation capacity of the compressor unit ( 30 ) is small, the capacity of the evaporator ( 50 ) is too much with respect to the flow rate of the refrigerant which circulates in the refrigerant circuit ( 20 ), and it is highly likely that the temperature at which the refrigerant evaporates at the evaporator ( 50 ) will increase. In such a case, the controller ( 16 ) closes the solenoid valve ( 70 ) to reduce the capacity of the evaporator ( 50 ). When the solenoid valve ( 70 ) is closed, the refrigerant flows only to the first flow pass ( 56 ), and the capacity of the evaporator ( 50 ) is accordingly reduced. Consequently, the evaporation temperature of the refrigerant at the evaporator ( 50 ) is reduced. 
     On the other hand, if the evaporation pressure of the refrigerant in the evaporator ( 50 ) is lower than a lower limit of the reference range in the state where the solenoid valve ( 70 ) is closed, the controller ( 16 ) opens the solenoid valve ( 70 ). In the state where the solenoid valve ( 70 ) is open, the refrigerant flows into both of the first flow pass ( 56 ) of the first heat exchanger section ( 55 ) and the second flow pass ( 61 ) of the second heat exchanger section ( 60 ) in the evaporator ( 50 ). 
     If the solenoid valve ( 70 ) is closed in the state where the operation capacity of the compressor unit ( 30 ) is large, the capacity of the evaporator ( 50 ) is too small with respect to the flow rate of the refrigerant which circulates in the refrigerant circuit ( 20 ), and it is highly likely that the evaporation pressure of the refrigerant in the evaporator ( 50 ) decreases. In such a case, the controller ( 16 ) opens the solenoid valve ( 70 ) to increase the capacity of the evaporator ( 50 ). When the solenoid valve ( 70 ) is open, the refrigerant flows into both of the first flow pass ( 56 ) and the second flow pass ( 61 ), and the capacity of the evaporator ( 50 ) is accordingly increased. Consequently, the evaporation pressure of the refrigerant in the evaporator ( 50 ) is increased. 
     —Fourth Variation— 
     The controller ( 16 ) of the above embodiments may be configured to change the number of heat exchanger sections ( 55 ,  60 ,  65 ) through which the refrigerant flows in the evaporator ( 50 ), based on a value measured by the supply air temperature sensor ( 81 ). In this example, the present variation is applied to the air conditioner ( 10 ) of the first embodiment shown in  FIG. 2 . 
     The controller ( 16 ) of the present variation adjusts the operation capacity of the compressor unit ( 30 ) and controls the solenoid valve ( 70 ) such that a value measured by the supply air temperature sensor ( 81 ) will be a predetermined temperature. 
     Specifically, if the temperature measured by the supply air temperature sensor ( 81 ) is lower than the predetermined temperature in the state where the solenoid valve ( 70 ) is open, the controller ( 16 ) reduces, one by one, the number of compressors ( 31 ,  32 ,  33 ) in operation in the compressor unit ( 30 ) to increase the value measured by the supply air temperature sensor ( 81 ). Further, if the temperature measured by the supply air temperature sensor ( 81 ) is lower than the predetermined temperature even in a state where only one of the compressors ( 31 ,  32 ,  33 ) is operated in the compressor unit ( 30 ), the controller ( 16 ) closes the solenoid valve ( 70 ). In the state where the solenoid valve ( 70 ) is closed, the refrigerant does not flow into the second flow pass ( 61 ) of the second heat exchanger section ( 60 ) in the evaporator ( 50 ), but flows only to the first flow pass ( 56 ) of the first heat exchanger section ( 55 ). 
     If the solenoid valve ( 70 ) is open in the state where only one of the compressors ( 31 ,  32 ,  33 ) is in operation, the capacity of the evaporator ( 50 ) is too much, and therefore it is highly likely that the temperature of the air having passed through the evaporator ( 50 ) still remains lower than the predetermined temperature. In such a case, the controller ( 16 ) closes the solenoid valve ( 70 ) to reduce the capacity of the evaporator ( 50 ). If the solenoid valve ( 70 ) is closed, the refrigerant flows only to the first flow pass ( 56 ), and the capacity of the evaporator ( 50 ) is accordingly reduced. Consequently, the temperature of the air having passed through the evaporator ( 50 ) is increased. 
     On the other hand, if the temperature measured by the supply air temperature sensor ( 81 ) is higher than the predetermined temperature in the state where the solenoid valve ( 70 ) is closed, the controller ( 16 ) increases, one by one, the number of compressors ( 31 ,  32 ,  33 ) in operation in the compressor unit ( 30 ) to reduce the value measured by the supply air temperature sensor ( 81 ). Further, if the temperature measured by the supply air temperature sensor ( 81 ) is still higher than the predetermined temperature even in a situation where only two of the compressors ( 31 ,  32 ,  33 ) are operated in the compressor unit ( 30 ), the controller ( 16 ) opens the solenoid valve ( 70 ). In the state where the solenoid valve ( 70 ) is open, the refrigerant flows into both of the first flow pass ( 56 ) of the first heat exchanger section ( 55 ) and the second flow pass ( 61 ) of the second heat exchanger section ( 60 ) in the evaporator ( 50 ). 
     If the solenoid valve ( 70 ) is closed in the state where two of the compressors ( 31 ,  32 ,  33 ) are operated, the capacity of the evaporator ( 50 ) is too small with respect to the flow rate of the refrigerant which circulates in the refrigerant circuit ( 20 ), and it is highly likely that the temperature of the air having passed through the evaporator ( 50 ) still remains higher than the predetermined temperature. In such a case, the controller ( 16 ) opens the solenoid valve ( 70 ) to increase the capacity of the evaporator ( 50 ). If the solenoid valve ( 70 ) is open, the refrigerant flows into both of the first flow pass ( 56 ) and the second flow pass ( 61 ), and the capacity of the evaporator ( 50 ) is accordingly increased. Consequently, the temperature of the air having passed through the evaporator ( 50 ) is reduced. 
     —Fifth Variation— 
     The controller ( 16 ) of the above embodiments may be configured to change the number of compressors ( 31 ,  32 ,  33 ) in operation in the compressor unit ( 30 ), and also change the number of heat exchanger sections ( 55 ,  60 ,  65 ) through which the refrigerant flows in the evaporator ( 50 ). In this example, the present variation is applied to the air conditioner ( 10 ) of the first embodiment shown in  FIG. 2 . 
     As described above, the controller ( 16 ) of the first embodiment adjusts the operation capacity of the compressor unit ( 30 ) such that the value measured by the supply air temperature sensor ( 81 ) will be a predetermined temperature. When the number of compressors ( 31 ,  32 ,  33 ) operating in the compressor unit ( 30 ) is reduced to two to one, the controller ( 16 ) closes the solenoid valve ( 70 ) simultaneously. When the number of compressors ( 31 ,  32 ,  33 ) operating in the compressor unit ( 30 ) is increased from one to two, the controller ( 16 ) opens the solenoid valve ( 70 ) simultaneously. 
     The foregoing embodiments are merely preferred examples in nature, and are not intended to limit the scope, applications, and use of the present disclosure. 
     INDUSTRIAL APPLICABILITY 
     As described above, the present disclosure is useful as an air conditioner which cools air to be supplied to an indoor space through a duct. 
     Description of Reference Characters  
       10  air conditioner 
       17  flow control mechanism 
       20  refrigerant circuit 
       26  first branch pipe 
       27  second branch pipe 
       28  third branch pipe 
       30  compressor unit 
       31  first compressor 
       32  second compressor 
       33  third compressor 
       35  condenser 
       40  expansion valve 
       41  first expansion valve 
       42  second expansion valve 
       43  third expansion valve 
       50  evaporator 
       55  first heat exchanger section 
       56  first flow pass 
       60  second heat exchanger section 
       61  second flow pass 
       65  third heat exchanger section 
       66  third flow pass