Patent Publication Number: US-11022372-B2

Title: Air conditioner

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of PCT/JP2017/043016, filed on Nov. 30, 2017, which claims priority to Japanese Patent Application No. 2017-004542, filed on Jan. 13, 2017, the contents of which are hereby incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to an air conditioner having a heat exchanger. 
     2. Description of Related Art 
     Various proposals have been made for improving heat-exchange efficiency of heat exchangers of air conditioners. 
     For example, Japanese Patent Application Publication No. 2013-53812 presents proposals related to a heat exchanger in which a plurality of heat-transfer pipes extending in a horizontal direction are disposed at predetermined intervals in a vertical direction and header pipes extending in the vertical direction are provided at opposite ends of the plurality of heat transfer pipes. The interior of each header pipe is divided into a plurality of sections by partition plates. Refrigerant circulating in the heat exchanger flows downward while flowing through the heat-transfer pipes in both directions between the header tubes. Corrugated fins are interposed between the heat-transfer pipes. The refrigerant transfers heat to/from (exchanges heat with) an air flow passing the corrugated fins while the refrigerant passes through the heat-transfer pipes. 
     When the heat exchanger described above is used as a condenser, refrigerant in a gaseous state (gas refrigerant) gives off heat to an air flow (i.e., the refrigerant is cooled by the air flow) to condense into refrigerant in a liquid state (liquid refrigerant). 
     As the volume of the liquid refrigerant does not further diminish even when it is cooled, a liquid pool of the liquid refrigerant is formed in the heat-transfer pipes to narrow the region in which the gas refrigerant can give off heat to condense, resulting in a decrease in the heat-exchange efficiency. In view of the above, it is desirable to inhibit formation of the liquid pool of the liquid refrigerant. 
     As to the amount of refrigerant to be sealed, an insufficient amount of refrigerant cannot demonstrate desired heat exchange performance, whereas an excessive amount of refrigerant increases production costs. 
     Moreover, taking into account the Global Warming Potential (GWP) of the refrigerant to be used, it is desirable to avoid unnecessarily increasing the amount of refrigerant to be sealed. 
     The present invention has been made in view of the above circumstances and it is an object of the present invention to provide an air conditioner that can inhibit formation of a liquid pool in a heat exchanger to improve the heat-exchange efficiency and allow sealing an appropriate amount of refrigerant into the heat exchanger. 
     SUMMARY 
     To achieve the above-described object, an air conditioner according to the present invention includes a heat exchanger that includes: a plurality of heat-transfer pipes arranged to extend in a horizontal direction and to be spaced apart at predetermined intervals in a vertical direction and configured to allow a thermal medium to flow therein, wherein a part of the plurality of heat transfer pipes are used for at least one inflow path into which the thermal medium flows from an outside of the heat exchanger and the other part of the plurality of heat transfer pipes are used for at least one outflow path from which the thermal medium flows out to the outside of the heat exchanger; and at least one connection pipe through which an outlet side of one of the at least one inflow path communicates with an inlet side of one of the at least one outflow path, the at least one connection pipe having a hydraulic diameter of 4 mm or greater. A circulation flow rate Gr kg/s of the thermal medium and the number of the paths N satisfy 0.003≤Gr/N≤0.035. 
     Advantageous Effects of the Invention 
     The present invention provides an air conditioner that can inhibit formation of a liquid pool in a heat exchanger to allow for sealing an appropriate amount of refrigerant into the heat exchanger while improving the heat-exchange efficiency. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram representing the refrigeration cycle system of an air conditioner according to a present embodiment. 
         FIG. 2  is a perspective view showing a heat exchanger of the air conditioner according to the present embodiment. 
         FIG. 3  is an exploded perspective view illustrating the heat exchanger disassembled into a heat exchange section and headers. 
         FIG. 4  is a perspective view of a heat-transfer pipe of the heat exchanger. 
         FIG. 5  is a schematic view illustrating the configuration of the heat exchanger according to the present embodiment. 
         FIG. 6  is a cross-sectional view of a connection portion of the heat exchanger according to the present embodiment, which connection portion connects a fold back header of the heat exchanger to the heat exchange section of the heat exchanger. 
         FIG. 7  is a graph illustrating the relationship between the circulation flow rate of refrigerant per path and the pressure loss. 
         FIG. 8  is a graph illustrating the relationship between the circulation flow rate of refrigerant per path and the Froude number. 
         FIG. 9  is a graph illustrating the relationship between the hydraulic diameter and the pressure loss of a connection pipe. 
         FIG. 10  is a diagram illustrating the relationship between the hydraulic diameter of a connection pipe and the amount of refrigerant holding capacity per path. 
         FIG. 11  is a cross-sectional view of another configuration for the connection portion connecting the fold back header and the heat exchange section of the heat exchanger according to the present embodiment. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     Embodiments for carrying out the present invention will now be described in detail with reference to the drawings. In the description, the same symbols will be assigned to the respective same elements, and duplicative description will be omitted. 
     &lt;Configuration of Air Conditioner&gt; 
       FIG. 1  illustrates the refrigeration cycle of an air conditioner  1  in which the heat exchanger  101  according to the present invention is employed. 
     The air conditioner  1  has an outdoor unit  10  and an indoor unit  30 . 
     The outdoor unit  10  has a compressor  11 , a four-way valve  12 , an outdoor heat exchanger  13 , an outdoor blower  14 , an outdoor expansion valve  15 , and an accumulator  20 . 
     The indoor unit  30  has an indoor heat exchanger  31 , an indoor blower  32 , and an indoor expansion valve  33 . 
     The devices of the outdoor unit  10  and the devices of the indoor unit  30  are connected by a refrigerant piping  2  to form a refrigeration cycle. Refrigerant serving as a thermal medium is sealed in the refrigerant piping  2 . The refrigerant circulates between the outdoor unit  10  and the indoor unit  30  via the refrigerant piping  2 . 
     Next, a description will be given of the devices of the outdoor unit  10 . 
     The compressor  11  sucks and compresses refrigerant in a gaseous state (gas refrigerant) and discharges the compressed refrigerant. 
     The four-way valve  12  changes the direction of refrigerant flowing between the outdoor unit  10  and the indoor unit  30  while maintaining the direction of refrigerant flowing toward the compressor  11 . The four-way valve  12  switches between cooling and heating operations by changing the direction of the refrigerant. 
     The outdoor heat exchanger  13  has a heat exchanger  101  according to the present invention to exchange heat between the refrigerant and outdoor air. 
     The outdoor blower  14  supplies the outdoor air to the outdoor heat exchanger  13 . 
     The outdoor expansion valve  15  is a throttle valve for causing refrigerant in a liquid state (liquid refrigerant) to evaporate by adiabatic expansion. 
     The accumulator  20  is provided to accumulate liquid return in a transitional state. The accumulator  20  separates liquid refrigerant mixed in gas refrigerant to be supplied to the compressor  11  to maintain a moderate quality of the refrigerant. 
     Next, a description will be given of the devices of the indoor unit  30 . 
     The indoor heat exchanger  31  has a heat exchanger  101  according to the present invention to exchange heat between refrigerant and indoor air. 
     The indoor blower  32  supplies the indoor air to the indoor heat exchanger  31 . 
     The indoor expansion valve  33  is a throttle valve for causing refrigerant in a liquid state (liquid refrigerant) to evaporate by adiabatic expansion. The indoor expansion valve  33  is capable of changing the aperture size thereof to change the flow rate of refrigerant flowing in the indoor heat exchanger  31 . 
     &lt;Operation of Air Conditioner&gt; 
     Next, a description will be given of a cooling operation of the air conditioner  1  by which cool air is supplied into a room. 
     The solid arrows in  FIG. 1  represent the flow of refrigerant in the cooling operation. The four-way valve  12  controls the direction of the flow as indicated by the solid lines. 
     The gas refrigerant compressed to high-temperature and high-pressure by the compressor  11  flows into the outdoor heat exchanger  13  via the four-way valve  12 . 
     The gas refrigerant that has flowed into the outdoor heat exchanger  13  gives off heat to the outdoor air supplied by the outdoor blower  14 , to condense into a low-temperature, high-pressure liquid refrigerant. 
     That is, the outdoor heat exchanger  13  functions as a condenser in the cooling operation. 
     The liquid refrigerant that has condensed from the gas refrigerant is sent to the indoor unit  30  via the outdoor expansion valve  15 . As the outdoor expansion valve  15  does not function as an expansion valve in this process, the liquid refrigerant passes through the outdoor expansion valve  15  as is without adiabatic expansion. 
     The liquid refrigerant that has flowed into the indoor unit  30  adiabatically expands in the indoor expansion valve  33  and flows into the indoor heat exchanger  31 . 
     The liquid refrigerant takes latent heat of vaporization from the indoor air supplied by the indoor blower  32 , to evaporate into a low-temperature, low-pressure gas refrigerant. 
     That is, the indoor heat exchanger  31  functions as an evaporator in the cooling operation. 
     The indoor air is relatively cooled by being deprived of latent heat of vaporization, resulting in cool air blowing into the room. 
     The gas refrigerant that has evaporated from the liquid refrigerant is sent to the outdoor unit  10 . 
     The gas refrigerant that has returned to the outdoor unit  10  passes through the four-way valve  12  and flows into the accumulator  20 . 
     The liquid refrigerant mixed in the gas refrigerant having flowed into the accumulator  20  is separated in the accumulator  20 , adjusted to have a predetermined quality, and supplied to the compressor  11  to be compressed again. 
     In this way, the cooling operation for providing cool air indoors is achieved by circulating the refrigerant in the directions indicated by the solid arrows in the refrigeration cycle. 
     That is, in the cooling operation, the outdoor heat exchanger  13  functions as a condenser and the indoor heat exchanger  31  functions as an evaporator. 
     Next, a description will be given of a heating operation of the air conditioner  1  by which warm air is supplied into the room. 
     The dotted arrows in  FIG. 1  represent the flow of refrigerant in a heating operation. The four-way valve  12  controls the direction of the flow as indicated by the dotted lines. 
     The gas refrigerant that has been compressed to high-temperature and high-pressure by the compressor  11  flows into the indoor unit  30  via the four-way valve  12 . 
     The gas refrigerant that has flowed into the indoor heat exchanger  31  gives off heat to the indoor air supplied by the indoor blower  32  while passing through the indoor heat exchanger  31 , to condense into a low-temperature, high-pressure liquid refrigerant. 
     That is, the indoor heat exchanger  31  functions as a condenser in the heating operation. 
     The indoor air is relatively heated by receiving heat, resulting in warm air blowing into the room. 
     The liquid refrigerant that has condensed from the gas refrigerant passes the indoor expansion valve  33  to be sent to the outdoor unit  10 . As the indoor expansion valve  33  does not function as an expansion valve in this process, the liquid refrigerant passes through the indoor expansion valve  33  as is without adiabatic expansion. 
     The liquid refrigerant that has flowed into the outdoor unit  10  adiabatically expands in the outdoor expansion valve  15  and flows into the outdoor heat exchanger  13 . 
     The liquid refrigerant takes latent heat of vaporization from the outdoor air supplied by the outdoor blower  14 , to evaporate into a low-temperature, low-pressure gas refrigerant. 
     That is, the outdoor heat exchanger  13  functions as an evaporator in the heating operation. 
     The refrigerant that has flowed out of the outdoor heat exchanger  13  passes through the four-way valve  12  and flows into the accumulator  20 . 
     The liquid refrigerant mixed in the refrigerant having flowed into the accumulator  20  is separated in the accumulator  20 , adjusted to have a predetermined quality, and supplied to the compressor  11  to be compressed again. 
     In this way, a heating operation for providing warm air indoors is achieved by circulating the refrigerant in the directions indicated by the dotted arrows in the refrigeration cycle. 
     That is, in the heating operation, the indoor heat exchanger  31  functions as a condenser and the outdoor heat exchanger  13  functions as an evaporator. 
     Next, a description will be given of the heat exchanger  101  according to the present embodiment, which constitutes each of the above-described outdoor heat exchanger  13  and the indoor heat exchanger  31 . 
     The outdoor heat exchanger  13  and the indoor heat exchanger  31  in the above described air conditioner  1  are each constituted by the heat exchanger  101  of the present invention. It should be noted that the heat exchanger  101  exerts effects of the present invention even when only one of the outdoor heat exchanger  13  and the indoor heat exchanger  31  is constituted by the heat exchanger  101 . 
     As shown in  FIGS. 2 and 3 , the heat exchanger  101  according to the present embodiment is a fin-tube type heat exchanger and has a heat exchange section  110  and headers  130 . 
     The heat exchange section  110  is a part to exchange heat between refrigerant and air. The heat exchange section  110  has a plurality of heat-transfer fins  111  and a plurality of heat-transfer pipes  112  (see  FIG. 3 ) 
     The plurality of heat-transfer fins  111  are each constituted by a rectangular, plate-shaped member. The plurality of heat-transfer fins  111  are arranged in a stacked manner such that the rectangular plate-shaped members have their length directions in the vertical direction and are spaced apart at predetermined intervals, with adjacent rectangular plate-shaped members facing with each other. The outdoor air or indoor air passes through gaps between the stacked heat-transfer fins  111 . 
     As shown in  FIG. 4 , each heat-transfer pipe  112  is constituted by a flat tubular member with a cross section having a substantially oval shape. The interior of the flat tubular member is divided by partition walls  113  into a plurality of flow channels  114  extending in the length direction of the flat tubular member. The heat-transfer pipes  112  have upper and lower portions that correspond to flat portions of the oval shape and extend in the horizontal direction, and are spaced apart at predetermined intervals in the vertical direction. The heat-transfer pipes  112  penetrate the stacked heat-transfer fins  111  and are joined thereto. 
     The heat-transfer pipes  112  each have opposite ends that communicate with respective headers  130 . 
     In the use of the heat exchanger  101  as a condenser, the plurality of heat-transfer pipes  112  provide inflow paths  121  into which the refrigerant (gas refrigerant) flows from the outside and outflow paths  122  from which the refrigerant (liquid refrigerant) flows out to the outside. 
     As shown in  FIG. 5 , in the heat exchanger  101  according to the present embodiment, the inflow paths  121  and the outflow paths  122  are alternately arranged in the vertical direction. The inflow paths  121  and the outflow paths  122  are not necessarily alternately arranged in the vertical direction if they are arranged such that they are not likely to be influenced by the gravity. 
     In the condenser, the ratio of gas refrigerant to the whole refrigerant is high upstream of the heat exchange section  110 , whereas the ratio of liquid refrigerant to the whole refrigerant increases as the refrigerant flows downstream. That means that the volume of the refrigerant in each outflow path  122  is smaller than that in the corresponding inflow path  121 . In  FIG. 6 , for simplicity of drawing, each inflow path  121  and each outflow path  122  have the same number of heat-transfer pipes  112 . However, it is desirable to select the number of heat-transfer pipes for each path so that refrigerant flows at a necessary speed in accordance with whether the refrigerant flowing through the path is in a condensed state or a vapor state. 
     The refrigerant that has flowed out of inflow paths is in a gas-liquid two-phase state, in which the refrigerant has not completely condensed. By making the refrigerant that has flowed out of the inflow paths flow into connection pipes  151  and flow downward or upward in the connection pipes  151 , influences of gravity on the refrigerant between the paths can be reduced and formation of a liquid pool at lower paths can be inhibited. 
     As shown in  FIGS. 5 and 6 , the headers  130  are constituted by a distribution/collection header  131  and a fold back header  132  that bundle the heat-transfer pipes  112  at opposite ends thereof. The distribution/collection header  131  distributes/collects refrigerant to/from the heat-transfer pipes  112 . 
     The distribution/collection header  131  includes a part called distribution section  133  that distributes refrigerant flowing from the outside into the distribution/collection header  131  to the inflow paths  121  when the heat exchanger  101  is used as a condenser. The distribution/collection header  131  further includes a part called collection section  134  that collects the refrigerant flowing out of the outflow paths  122  and discharges the refrigerant to the outside when the heat exchanger  101  is used as a condenser. 
     As shown in  FIG. 6 , the interior of the fold back header  132  is divided by partition plates  135  into compartments each of which is assigned to respective one of the inflow paths  121  and the outflow paths  122 . The fold back header  132  is provided with the connection pipes  151 . The interior of the distribution section  133  is divided by the partition plates  135  into compartments each of which is assigned to respective one of the inflow paths  121  in a similar manner to the fold back header  132 . The interior of the collection section  134  is divided by the partition plates  135  into compartments each of which is assigned to respective one of the outflow paths  122  in a similar manner to the fold back header  132 . 
     As shown in  FIGS. 5 and 6 , the connection pipes  151  are constituted by down-flow pipes  152  and up-flow pipes  153 . The down-flow pipes  152  and the up-flow pipes  153  have the same cross section. In  FIGS. 2 and 3 , illustration of the connection pipes  151  is omitted for convenience of drawing. 
     Each down-flow pipe  152  allows, in the fold back header  132 , the compartment on the outlet side of a corresponding inflow path  121  (outlet-side compartment AR 1  of the corresponding inflow path  121 ) to communicate with the compartment on the inlet side of a corresponding outflow path  122  (inlet-side compartment AR 2  of the corresponding outflow path  122 ) located below the corresponding inflow path  121 , via the down-flow pipe  152 . 
     Each up-flow pipe  153  allows the outlet-side compartment AR 1  of a corresponding inflow path  121  to communicate with the inlet-side compartment AR 2  of a corresponding outflow path  122  located above the corresponding inflow path  121 , via the up-flow pipe  153 . 
     In the present embodiment, the uppermost inflow path  121  communicates with the lowermost outflow path  122  via one of the down-flow pipes  152 . The lowermost inflow path  121  communicates with the uppermost outflow path  122  via one of the up-flow pipes  153 . 
     The second uppermost inflow path  121  communicates with the second lowermost outflow path  122  via one of the down-flow pipes  152 . The second lowermost inflow path  121  communicates with the second uppermost outflow path  122  via one of the up-flow pipes  153 . 
     When the heat exchanger  101  is used as a condenser, the high-temperature, high-pressure gas refrigerant introduced into the distribution section  133  of the distribution/collection header  131  condenses into gas-liquid two-phase refrigerant, which is a mixture of gas refrigerant and liquid refrigerant, by exchanging heat with air while passing through the inflow paths  121 . The gas-liquid two-phase refrigerant is introduced from the outlet-side compartments AR 1  of the inflow paths  121  in the fold back header  132  into the inlet-side compartments AR 2  of the outflow paths  122  in the fold back header  132 , via the down-flow pipes  152  or the up-flow pipes  153 . The gas-liquid two-phase refrigerant in the inlet-side compartments AR 2  of the outflow paths  122  condenses further into gas-liquid two-phase refrigerant in which liquid refrigerant is dominant, by exchanging heat with air when passing through the outflow paths  122 . 
     The pressure of refrigerant flowing downward in the down-flow pipes  152  increases as the refrigerant moves from the outlet-side compartments AR 1  of the inflow paths  121  to the inlet-side compartments AR 2  of the outflow paths  122 . This partially cancels a decrease in the pressure of refrigerant flowing upward in the up-flow pipes  153 , resulting in a decrease in the pressure difference Δp due to influences of gravity. 
     As a result, the pressure difference Δp in the vertical direction in the heat exchange section  110  is decreased, inhibiting formation of a liquid pool of refrigerant in lower heat-transfer pipes  112 . This allows for exchanging heat with high-efficiency. 
     Next, a description will be given of the flow rate of refrigerant circulating in the air conditioner  1 . 
     Hereinafter, the amount of refrigerant circulating per second when the air conditioner  1  is in operation at a rated cooling capacity of the air conditioner  1  is referred to as refrigerant circulation flow rate Gr [kg/s], and the number of inflow paths  121  to which the distribution/collection header  131  distributes the refrigerant, i.e., the number of branches of the distribution section  133 , is referred to as the number of paths N. The number of paths N is equal to the number of outflow paths  122  and the number of connection pipes  151 . The rated cooling capacity of the air conditioner  1  refers to an output of the air conditioner  1  when room air is cooled to a temperature of 27° C., under the condition where a temperature of outdoor air is 35° C. and a relative humidity of the room air is 45%. 
       FIG. 7  is a graph illustrating the relationship between the refrigerant circulation flow rate per path (flow channel) Gr/N [kg/s] and the pressure loss ΔP [kPa] in the connection pipes  151 . 
       FIG. 7  shows that as the refrigerant circulation flow rate per path Gr/N [kg/s] increases, the pressure loss ΔP [kPa] increases. 
     The pressure loss ΔP [kPa] of the heat exchanger  101  is derived from the pressure loss in the heat-transfer pipes  112  and the pressure loss in the connection pipes  151 . 
     It is required that the pressure loss in the connection pipes  151  be inhibited to such a degree that the power consumption of the air conditioner  1  is not increased. This is because the connection pipes  151  are not portions for exchanging heat between the refrigerant and air positively. 
     From calculations, it is derived that the refrigerant circulation flow rate per path Gr/N [kg/s] is preferably less than or equal to 0.035. 
     In other words, influences of pressure loss by the connection pipes  151  can be inhibited by setting the refrigerant circulation flow rate Gr of the air conditioner and the number of paths N so as to satisfy Inequality 1.
 
 N≥Gr/ 0.035  Inequality 1
 
     As described above, the connection pipes  151  are constituted by the up-flow pipes  153  and down-flow pipes  152 . The refrigerant flowing through the connection pipes  151  is being condensed and thus is in the form of gas-liquid two-phase refrigerant, which is a mixture of gas refrigerant and liquid refrigerant. A certain flow rate is necessary for the gas-liquid two-phase refrigerant including liquid refrigerant mixed therein to flow upward in the up-flow pipes  153 , to move into the inlet-side compartments AR 2  of the outflow paths  122  located on the upper side. Thus, the flow rate of the refrigerant will be discussed next. 
     The Froude number Fr is known as an index for estimating a rising limit of a liquid. The Froude number Fr is calculated by Equation 2:
 
 Fr =(ρ G·uG 2+ρ L·uG 2)/(ρ L·g·d )  Equation 2
 
where ρL is the density of the liquid refrigerant, ρG is the density of the gas refrigerant, uG is the flow rate of the gas refrigerant, g is the gravitational acceleration, and d is the inner diameter of the pipe.
 
     By setting the flow rate of gas-liquid two-phase refrigerant such that the Froude number Fr takes a value greater than or equal to a predetermined value (=1), the gas-liquid two-phase refrigerant including liquid refrigerant mixed therein is able to flow upward in the up-flow pipes  153 . 
     When the Froude number Fr is less than the predetermined value (=1), the mixed liquid refrigerant adheres to the wall surfaces of the up-flow pipes  153  and is unable to flow upward further. As a result, liquid pools are formed in the outlet-side compartments AR 1  of the inflow paths  121  located on the lower side. 
     To obtain a Froude number Fr of a predetermined value (=1) or greater, it is necessary that the refrigerant circulation flow rate per path Gr/N [Kg/s] be greater than or equal to 0.003 [kg/s] (see  FIG. 8 ). 
     Therefore, in combination with the conditions described above, it is required to determine the number of paths N with respect to the refrigerant circulation flow rate Gr such that the refrigerant circulation flow rate per path Gr/N [Kg/s] satisfies Inequality 3. 
     This inhibits the pressure loss ΔP [kPa] due to the arrangement of connection pipes  151  and inhibits formation of liquid pools in the connection pipes  151 .
 
0.003≤ Gr/N≤ 0.035 [kg/s]  Inequality 3
 
     Next, a description will be given of the configuration of the connection pipes  151 . 
     The connection pipes  151  are not limited as to their cross sectional shape, but are configured as having their hydraulic diameter D in the range given by Inequality 4.
 
4≤ D≤ 11 [mm]  Inequality 4
 
     The range of hydraulic diameter D represented by Inequality 4 is derived from  FIGS. 9  and  10 . 
       FIG. 9  shows the relationship between the hydraulic diameter D [mm] of the connection pipes  151  and the pressure loss ΔP [kPa] in the connection pipes  151 , in three conditions that satisfy Inequality 3. 
     From  FIG. 9 , it is obvious that, in a region where the hydraulic diameter D is less than a certain value, as the refrigerant circulation flow rate Gr increases, the pressure loss ΔP [kPa] increases. From  FIG. 9 , to reduce the influence of the pressure loss ΔP [kPa] for any refrigerant circulation flow rate Gr and the number of paths N, it is preferable that the hydraulic diameter D of the connection pipes  151  be 4 mm or greater. 
     Incidentally, when the connection pipes  151  have a larger hydraulic diameter D, radius for bending the connection pipes  151  needs to be increased. As a result, a larger space is required for installing the heat exchanger  101 . However, the space for installing the heat exchanger  101  is limited. Thus, it is desirable that the heat exchanger  101  be as small as possible. 
     In addition, from  FIG. 10 , it is obvious that as the hydraulic diameter D of the connection pipes  151  increases, the amount of refrigerant held per connection pipe increases. An increase in the amount of refrigerant held increases production cost of the air conditioner  1  as a whole. For this reason, it is desirable not to hold more than necessary refrigerant. 
     For this reason, taking into account the installation of heat exchanger  101  in a machine casing (not shown) or the like of the outdoor unit  10 , it is preferable to select pipes having a hydraulic diameter D of 11 mm or less as the connection pipes  151 . 
     In view of the foregoing, the connection pipes  151  are configured such that the hydraulic diameter D thereof falls within the range given by Inequality 4. 
     Next, a description will be given of the effects of the heat exchanger  101  according to the present embodiment. 
     In the heat exchanger  101  according to the present embodiment, the inflow paths  121  and the outflow paths  122  are connected via the connection pipes  151  such that at least one of the inflow paths  121  communicates with one of the outflow paths  122  located below the at least one of the inflow paths  121 , and at least another one of the inflow paths  121  communicates with another one of the outflow paths  122  located above the at least another one of the inflow paths  121 . 
     With this configuration, an increase in the pressure of refrigerant flowing downward in the down-flow pipes  152  cancels at least some of the decrease in the pressure of refrigerant flowing upward in the up-flow pipes  153 , resulting in a decrease in the pressure difference Δp due to influences of gravity. 
     As a result, the pressure difference Δp in the vertical direction in the heat exchange section  110  is decreased, inhibiting formation of liquid pools of refrigerant in the heat-transfer pipes  112  located on the lower side. This allows for exchanging heat with high-efficiency. 
     In the heat exchanger  101  according to the present embodiment, the refrigerant circulation flow rate per path Gr/N [Kg/s] is adjusted so as to fall within the range given by Inequality 3. 
     This inhibits formation of liquid pools in the heat-transfer pipes  112  and allows for exchanging heat (condensation of thermal medium) with high-efficiency. 
     In the heat exchanger  101  according to the present embodiment, the connection pipes  151  are configured to have a hydraulic diameter D falling within the range given by Inequality 4. 
     Selecting a hydraulic diameter D of 4 mm or greater reduces influence of the pressure loss of the refrigerant flowing through the connection pipes  151 . 
     Selecting a hydraulic diameter D of 11 mm or smaller contributes to space saving of the device as a whole. Further, configuring the connection pipes  151  to have a hydraulic diameter D of 11 mm or smaller inhibits the amount of thermal medium held in the connection pipes  151 , leading to cost reduction of the device as a whole. 
     In the heat exchanger  101  according to the present embodiment, each heat-transfer pipe  112  is constituted by a flat tubular member with a cross section having a substantially oval shape. 
     With this structure, each heat-transfer pipe  112  can have a smaller cross-sectional area than a circular cylindrical pipe having the same surface area, and thus can reduce the amount of the thermal medium to be held, even with the same surface area (heat exchange area) as that of the circular cylindrical pipe. 
     In addition, the interior of each heat-transfer pipe  112  is divided into the plurality of flow channels  114  by the partition walls  113  to increase the area where the thermal medium and the heat-transfer pipe  112  are in contact with each other. 
     This increases the amount of heat to be exchanged without increasing the amount of the thermal medium to be held. 
     In the heat exchanger  101  according to the present embodiment, it is preferable to use at least one of the refrigerants: R410A, R404A, R32, R1234yf, R1234ze(E), and HFO1123 as the thermal medium. 
     These refrigerants have an ozone depletion potential of zero. Selecting at least one of those refrigerants on the basis of the necessary refrigeration capacity and operation temperature allows for ensuring refrigeration capacity at any evaporation pressure. As a result, the embodiment allows for reducing the amount of the refrigerant to be held compared to that in conventional heat exchangers. 
     It should be noted that, in the present embodiment, although the configuration of the invention of the present application is applied to a fin-tube type heat exchanger, the invention of the present application is not limited thereto. The invention of the present application is applicable to any heat exchanger in which a plurality of heat-transfer pipes extending in the horizontal direction and spaced apart at predetermined intervals in the vertical direction are arranged and the plurality of heat-transfer pipes are used (assigned) as a plurality of paths via headers. Examples of such a heat exchanger include corrugated fin type heat exchangers. The invention of the present application applied to such a heat exchanger is able to achieve the same effects. 
     Although, in the present embodiment, the connection pipes  151  are arranged such as to be exposed outside the fold back header  132 , the present application is not limited thereto. 
     For example, as shown in  FIG. 11 , connection pipes  151 A can be arranged inside the fold back header  132 . 
     With this configuration, as the fold back header  132  has no irregularity on the external side, the heat exchangers  101  are easily arranged in casings of the outdoor unit  10  and the indoor unit  30 . 
     In the present embodiment, the number of heat-transfer pipes  112  constituting each inflow path  121  is the same as the number of heat-transfer pipes  112  constituting each outflow path  122 . However, the present invention is not limited thereto. It is possible to assign different number of heat-transfer pipes  112  to them. 
     For example, as described above, in a condenser, the ratio of gas refrigerant to the whole refrigerant is high upstream of the heat exchange section  110 , whereas the ratio of liquid refrigerant to the whole refrigerant increases as the refrigerant flows downstream. Thus, the volume of the refrigerant in each outflow path  122  is smaller than that in the corresponding inflow path  121 . 
     Taking this into account, each inflow path  121  may be constituted by a larger number of heat-transfer pipes  112  than those constituting each outflow path  122 . 
     With this configuration, when the heat exchanger  101  is used as a condenser, the area where gas refrigerant gives off heat is large, improving the heat-exchange efficiency. 
     That is, in the inflow paths and the outflow paths, it is desirable to select the number of heat-transfer pipes used in each outflow path and the number of folding back and the like, in accordance with the distribution of warm air speed and expected heat exchange state of refrigerant. Those numbers may not be necessarily the same between the inflow paths and the outflow paths. 
     Next, a description will be given of another embodiment of a method of evaluating the flow rate of the refrigerant circulating in the heat exchanger  101 . 
     The heat exchanger  101  has the same configuration as that of the above-described embodiment. That is, the connection pipes  151  are configured to have a hydraulic diameter D [mm] falling within the range given by Inequality 4. 
     The present embodiment differs from the above-described embodiment in that the former defines a condition for gas-liquid two-phase refrigerant including mixed liquid refrigerant to flow upward in the connection pipes  151  in terms of a rated cooling capacity Q rather than a refrigerant circulation flow rate Gr in relation with the Froude number Fr. 
     The rated cooling capacity Q refers to an output of the air conditioner  1  when room air is cooled to a temperature of 27° C., under the condition where a temperature of outdoor air is 35° C. and a relative humidity of the room air is 45%. 
     As physical properties used for calculating the Froude number Fr vary per refrigerant to be used, the obtainable enthalpy difference and density change. For this reason, depending on the type of the refrigerant, gas-liquid two-phase refrigerant may possibly not flow upward in the connection pipe  151  even when the refrigerant circulation flow rate Gr derived from Froude number Fr falls within the range given by Inequality 3. 
     In view of this, the present evaluation method uses the rated cooling capacity Q [kW] as an index that substitutes for the refrigerant circulation flow rate Gr [kg/s]. 
     Inequality 5 expresses a range corresponding to the range given by Inequality 3.
 
0.75≤ Q/N≤ 3.5 [kW]  Inequality 5
 
     Controlling the rated cooling capacity per path Q/N to fall within the range given by Inequality 5 achieves the same effects as those intended by Inequality 3, even with refrigerant having different physical properties. 
     That is, gas-liquid two-phase refrigerant is able to flow upward in the connection pipes  151  and formation of liquid pools in the connection pipes  151  can be inhibited. 
     Therefore, formation of liquid pools in the heat exchanger  101  can be inhibited and an appropriate amount of refrigerant can be sealed while improving the heat-exchange efficiency. 
     Reference Signs List