Patent Publication Number: US-11384995-B2

Title: Finless heat exchanger and refrigeration cycle apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a finless heat exchanger with no fins and a refrigeration cycle apparatus. 
     BACKGROUND ART 
     A finless heat exchanger, which has no fins, has been developed as a heat exchanger having heat exchange performance and compactness (refer to Patent Literature 1, for example). The finless heat exchanger disclosed in Patent Literature 1 includes two headers arranged apart from each other and a plurality of heat transfer tubes spaced apart and arranged side by side between the two headers, fitted at opposite ends in the two headers, and secured to the headers. The heat transfer tubes, which are flat tubes, are arranged parallel to each other such that the major axis of the cross-section of each flat tube extends in an air flow direction. 
     The finless heat exchanger disclosed in Patent Literature 1 is configured such that the flat tubes each having a short minor axis in cross-section are arranged at a narrow pitch. Such a configuration ensures the compactness and allows the heat exchanger to have higher heat exchange performance than a finned-tube heat exchanger. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2009-145010 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the finless heat exchanger disclosed in Patent Literature 1, the two headers each have a plurality of insertion holes equal in number to the heat transfer tubes. Increasing the number of heat transfer tubes to improve the heat exchange performance increases the number of insertion holes to be formed in each header. The insertion holes can be formed using any of various processing methods. If the insertion holes are formed by cutting or stamping, strain due to poor strength of portions between the insertion holes may remain in the headers, resulting in a reduction in ease of processing of the headers. If the insertion holes are formed by wire cutting or electrical discharge machining, the cost of processing may increase. 
     Other problems arising from an increase in the number of heat transfer tubes include the difficulty of handling the multiple heat transfer tubes during assembly. This difficulty results in a reduction in ease of assembly. 
     As described above, increasing the number of heat transfer tubes to improve the heat exchange performance reduces the ease of processing of the headers and the ease of overall assembling, leading to lower productivity. 
     The finless heat exchanger and the refrigeration cycle apparatus of the present disclosure has been made to overcome the above-described problems and aims to provide a finless heat exchanger and a refrigeration cycle apparatus in which, while heat exchange performance is maintained, a reduction in the number of heat transfer tubes and a reduction in the number of insertion holes are achieved to improve productivity. 
     Solution to Problem 
     A finless heat exchanger according to an embodiment of the present disclosure includes two headers; and a plurality of heat transfer tubes spaced apart from each other and arranged side by side, the two headers each having a plurality of insertion holes, to which both ends of the plurality of heat transfer tubes are fitted and connected, the plurality of heat transfer tubes each including straight portions extending in a direction orthogonal to an arrangement direction in which the plurality of heat transfer tubes are arranged and turning portions, the straight portions and the turning portions being alternately and continuously arranged. 
     Advantageous Effects of Invention 
     Each heat transfer tube in the embodiment of the present disclosure includes the straight portions extending in the direction orthogonal to the arrangement direction and the turning portions, and the straight portions and the turning portions are alternately and continuously arranged. In other words, the multiple straight portions arranged side by side are connected by the turning portions, thus forming a single heat transfer tube. Such a configuration achieves a reduction in the number of heat transfer tubes and a reduction in the number of insertion holes in the headers while maintaining heat exchange performance. This results in improved productivity. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram schematically illustrating the configuration of a refrigerant circuit of a refrigeration cycle apparatus according to Embodiment 1 of the present disclosure. 
         FIG. 2  includes diagrams schematically illustrating the structure of a finless heat exchanger according to Embodiment 1 of the present disclosure. 
         FIG. 3  is a diagram illustrating a finless heat exchanger according to Comparative Example. 
         FIG. 4  is a graph illustrating an example of the relationship between the heat exchange performance of the finless heat exchanger and the minor-axis dimension of each heat transfer tube under conditions where air flow resistance is constant. 
         FIG. 5  is a graph illustrating the relationship between the minor-axis dimension of the heat transfer tube and the range of tube pitches P in which the same air flow resistance is obtained. 
         FIG. 6  includes diagrams schematically illustrating the structure of a finless heat exchanger according to Embodiment 2 of the present disclosure, (a) is a front view of the heat exchanger, and (b) is a bottom view thereof. 
         FIG. 7  is an enlarged view illustrating turning portions of heat transfer tubes in contact with headers in  FIG. 6 . 
         FIG. 8  is a diagram illustrating a modification of the finless heat exchanger according to Embodiment 2 of the present disclosure. 
         FIG. 9  is a diagram illustrating a heat transfer tube included in a finless heat exchanger according to Embodiment 3 of the present disclosure. 
         FIG. 10  is an enlarged view of turning portions of the heat transfer tube of  FIG. 9 . 
         FIG. 11  is a diagram illustrating a heat transfer tube included in the finless heat exchanger according to Embodiment 1 as a comparative example. 
         FIG. 12  is an enlarged view of turning portions of the heat transfer tube of  FIG. 11 . 
         FIG. 13  is a diagram illustrating a modification of the heat transfer tube included in the finless heat exchanger according to Embodiment 3 of the present disclosure. 
         FIG. 14  is an enlarged view of turning portions of a heat transfer tube of  FIG. 13 . 
         FIG. 15  includes diagrams schematically illustrating the structure of a finless heat exchanger according to Embodiment 4 of the present disclosure, (a) is a front view of the heat exchanger, and (b) is a bottom view thereof. 
         FIG. 16  includes diagrams schematically illustrating the structure of a finless heat exchanger according to Embodiment 5 of the present disclosure, (a) is a front view of the heat exchanger, and (b) is a bottom view thereof. 
         FIG. 17  includes diagrams schematically illustrating the structure of a finless heat exchanger according to Embodiment 6 of the present disclosure, (a) is a front view of the heat exchanger, and (b) is a bottom view thereof. 
         FIG. 18  is a schematic front view of the structure of a finless heat exchanger according to Embodiment 7 of the present disclosure. 
         FIG. 19  is a perspective view of essential part of a heat transfer tube in  FIG. 18 . 
         FIG. 20  is a schematic front view of the structure of a finless heat exchanger according to Embodiment 8 of the present disclosure. 
         FIG. 21  includes schematic diagrams illustrating a finless heat exchanger according to Embodiment 9 of the present disclosure, (a) is a front view of the heat exchanger, (b) is a plan view thereof, and (c) is a side view thereof. 
         FIG. 22  is a schematic front view of a finless heat exchanger according to Embodiment 10 of the present disclosure. 
         FIG. 23  is a partial sectional view of a positioning part in  FIG. 22 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Heat exchangers according to embodiments of the present disclosure will be described in detail below with reference to the drawings. In the figures, the same elements or equivalents are designated by the same reference signs. The following embodiments should not be construed as limiting the present disclosure. Note that the relative sizes of components illustrated in the following figures may differ from those in actual apparatuses. 
     Embodiment 1 
       FIG. 1  is a diagram schematically illustrating the configuration of a refrigerant circuit of a refrigeration cycle apparatus according to Embodiment 1 of the present disclosure. An air-conditioning apparatus that conditions air in an indoor space, serving as an air-conditioned space, will be described as an example of the refrigeration cycle apparatus. 
     An air-conditioning apparatus  1  includes a heat source side unit  1 A and a use side unit  1 B. The heat source side unit  1 A and the use side unit  1 B constitute a refrigeration cycle through which refrigerant is circulated, and the heat source side unit  1 A discharges or supplies heat for air-conditioning. The heat source side unit  1 A is installed outside. The heat source side unit  1 A includes a compressor  110 , a flow switching device  160 , a heat source side heat exchanger  40 , an expansion device  150 , and an accumulator  170 . The heat source side unit  1 A further includes a fan  41  that sends air to the heat source side heat exchanger  4 , and the fan  41  faces the heat source side heat exchanger  4 . 
     The use side unit  1 B, which is installed in an indoor space, serving as an air-conditioned space, includes a use side heat exchanger  180  and a fan (not illustrated) that sends air to the use side heat exchanger  180 . The air-conditioning apparatus  1  includes the refrigeration cycle including the compressor  110 , the flow switching device  160 , the use side heat exchanger  180 , the heat source side heat exchanger  40 , and the expansion device  150 . 
     The compressor  110  compresses sucked refrigerant into a high temperature, high pressure state. The compressor  110  is configured as a scroll compressor or a reciprocating compressor. 
     The flow switching device  160  switches between a heating passage and a cooling passage in response to switching between an operation mode for a heating operation and an operation mode for a cooling operation. The flow switching device  160  is configured as a four-way valve. In the heating operation, the flow switching device  160  connects a discharge side of the compressor  110  and the use side heat exchanger  180  and connects the heat source side heat exchanger  40  and the accumulator  170 . In the cooling operation, the flow switching device  160  connects the discharge side of the compressor  110  and the heat source side heat exchanger  40  and connects the use side heat exchanger  180  and the accumulator  170 . Although  FIG. 1  illustrates a case where the four-way valve is used as the flow switching device  160 , the flow switching device may have any configuration. For example, a plurality of two-way valves may be combined into the flow switching device  160 . 
     The heat source side heat exchanger  40  is configured as a finless heat exchanger. The structure of the finless heat exchanger will now be described with reference to the figures. 
       FIG. 2  includes diagrams schematically illustrating the structure of the finless heat exchanger according to Embodiment 1 of the present disclosure, (a) is a front view of the heat exchanger, and (b) is a bottom view thereof. 
     The finless heat exchanger according to Embodiment 1 includes two headers  21  arranged apart from each other, a plurality of heat transfer tubes  22  connected at both ends to the two headers  21 , and a housing (not illustrated) containing the headers and the heat transfer tubes. The heat transfer tubes  22  are spaced apart from each other and arranged side by side. The two headers  21  are arranged apart from each other in a direction orthogonal to an arrangement direction in which the heat transfer tubes  22  are arranged side by side. The heat transfer tubes  22  are configured as flat tubes each having a flat cross-sectional shape with a major axis and a minor axis and each including a plurality of through-holes, serving as refrigerant passages. The heat transfer tubes  22  are made of aluminum-based material. The cross-sectional shape of each of the through-holes, serving as refrigerant passages, in the heat transfer tubes  22  is, for example, rectangular, square, trapezoidal, triangular, or circular. 
     Each heat transfer tube  22  includes straight portions  23  and turning portions  24  arranged alternately and continuously, and the straight portions  23  are substantially parallel to each other. The heat transfer tube  22  is a single-piece component formed by bending a tubular material. The heat transfer tube  22  is connected at both ends, or two positions, to the two headers  21 . In  FIG. 2 , the air flows in a direction perpendicular to the drawing sheet of  FIG. 2 . The heat transfer tube  22  is placed such that the major axis in the cross-section of the heat transfer tube  22  is parallel to the air flow direction. 
     Each header  21  is, for example, a cylindrical pipe. The header  21  has a structure in which a first end of the cylindrical pipe is completely closed and a second end thereof except a refrigerant inlet-outlet  26  is closed. The header  21  has insertion holes  25 , to which the ends of the heat transfer tubes  22  are fitted. The heat transfer tubes  22  are joined to the header  21 . Portions of the heat transfer tubes  22  in contact with the insertion holes  25  of the header  21  are joined to the header  21  by brazing, for example. 
     Advantageous effects of the finless heat exchanger configured as described above will be described. To more clearly describe the advantageous effects of the finless heat exchanger according to Embodiment 1, a finless heat exchanger including heat transfer tubes including only straight portions will be described as Comparative Example, which is illustrated in  FIG. 3 . The finless heat exchanger according to Embodiment 1 will be described in comparison with the finless heat exchanger according to Comparative Example.  FIG. 3  is a diagram illustrating the finless heat exchanger according to Comparative Example. 
     The finless heat exchanger,  400 , according to Comparative Example has the same size and heat exchange performance as those of the finless heat exchanger according to Embodiment 1. Heat transfer tubes  220  each include only a straight portion. The straight portion  23  is connected at opposite ends to headers  210 . The heat transfer tubes  220  in Comparative Example have the same major-axis and minor-axis dimensions as those of the heat transfer tubes  22  in Embodiment 1. Furthermore, the heat transfer tubes are arranged at a tube pitch P1, which is equal to a tube pitch P in  FIG. 2 . The tube pitch P is the interval between the adjacent straight portions  23 . 
     The comparison between the finless heat exchanger  400  according to Comparative Example and the finless heat exchanger according to Embodiment 1 reveals that each heat transfer tube  22  of the finless heat exchanger according to Embodiment 1 can be formed by connecting the heat transfer tubes  220  in Comparative Example with the turning portions  24 . In the finless heat exchanger according to Embodiment 1, therefore, a reduction in the number of heat transfer tubes  22  is achieved while the same heat exchange performance as that in Comparative Example is maintained. The larger the number of turning portions  24 , the smaller the number of heat transfer tubes  22 . 
     As described above, while the heat exchange performance is maintained, a reduction in the number of heat transfer tubes  22  is achieved in the finless heat exchanger according to Embodiment 1. This results in a reduction in the number of ends of the heat transfer tubes  22  fitted in the headers  21  and a reduction in the number of insertion holes  25  of the headers  21 . Consequently, the insertion holes  25  can be arranged at relatively long intervals in the headers  21 . This ensures that portions between the insertion holes of the headers have a width sufficient for reducing the likelihood of a processing failure, such as deformation upon processing. This leads to improved ease of processing of the headers. Thus, the headers  21  can be relatively easily produced at low cost. 
     A reduction in the number of heat transfer tubes  22  facilitates handling the heat transfer tubes  22  during assembly of the heat exchanger, significantly improving the ease of assembly. 
     Furthermore, a reduction in the number of ends of the heat transfer tubes  22  fitted in the headers  21  can provide distribution closer to ideal distribution by an amount corresponding to a reduction in the number of heat transfer tubes  22  when the refrigerant is distributed from the headers  21  to the individual heat transfer tubes  22 . This leads to improved performance of refrigerant distribution to the individual heat transfer tubes  22  in the headers  21 , thus enhancing the heat exchange performance. This can relatively easily provide a high-performance finless heat exchanger. In addition, the enhancement of the heat exchange performance allows a finless heat exchanger to be compact in size while the heat exchange performance is maintained. 
     A reduction in the number of heat transfer tubes  22  results in a reduction in the number of joints between the headers  21  and the heat transfer tubes  22 , reducing the likelihood of poor joints. This improves the reliability of the finless heat exchanger. 
     Furthermore, since the finless heat exchanger does not include fins, the cost of material, the cost of processing, and the cost of die can be reduced, resulting in a significant reduction in cost of the heat exchanger. 
     As described above, according to Embodiment 1, each heat transfer tube  22  includes the straight portions  23  extending in the direction orthogonal to the arrangement direction and the turning portions  24  such that the straight portions  23  and the turning portion  24  are alternately and continuously arranged. In other words, the multiple straight portions  23  arranged side by side are connected by the turning portions  24 , thus forming a single heat transfer tube. Such a configuration achieves a reduction in the number of heat transfer tubes of the entire finless heat exchanger while maintaining the heat exchange performance equivalent to that of the heat exchanger of  FIG. 3 . This results in a reduction in the number of insertion holes  25  of the headers  21 , improving the ease of processing of the headers  21  and the ease of overall assembly. This leads to improved productivity. The improved productivity enables lower cost production. 
     Since the number of insertion holes  25  of the headers  21  can be reduced as described above, a low-cost, high-performance, high-quality, and compact finless heat exchanger can be provided. 
     Although Embodiment 1 has been described with respect to a case where the flat tube is used as an example of the heat transfer tube  22 , the heat transfer tube  22  is not limited to the flat tube. The heat transfer tube  22  may be a cylindrical tube. If the heat transfer tubes  22  are cylindrical tubes, the same advantageous effects can be obtained. Note that the heat transfer tubes  22  are not limited to flat tubes. The same applies to the following embodiments unless otherwise stated. For the material for the heat transfer tubes  22 , the aluminum-based material has been described as an example. If the heat transfer tubes  22  are made of copper-based material or iron-based material, the same advantageous effects can be obtained. The same applies to the following embodiments. 
     Specific dimensions of the finless heat exchanger including the flat tubes as the heat transfer tubes  22  will now be discussed. 
       FIG. 4  is a graph illustrating an example of the relationship between the heat exchange performance of the finless heat exchanger and the minor-axis dimension of each heat transfer tube under conditions where air flow resistance is constant.  FIG. 5  is a graph illustrating the relationship between the minor-axis dimension of the heat transfer tube and the range of tube pitches P in which the same air flow resistance is obtained. As described above, the tube pitch P is the interval between the adjacent straight portions  23 . In  FIG. 5 , a hatched portion represents a range in which the same air flow resistance is obtained. 
       FIG. 4  demonstrates that the minor-axis dimension of the heat transfer tubes  22  has only to be reduced to provide higher heat exchange performance under conditions where the air flow resistance is constant. Furthermore,  FIG. 5  demonstrates that, to obtain the same air flow resistance with different minor-axis dimensions, the smaller the minor-axis dimension of the heat transfer tube  22  is, the more the tube pitch has to be reduced. In other words, it is clear that the minor-axis dimension of the heat transfer tube  22  and the tube pitch have to be reduced to improve the heat exchange performance under conditions where the air flow resistance is constant. 
       FIGS. 4 and 5  demonstrate that the minor-axis dimension of the heat transfer tube  22  may be set to 1.5 mm and the tube pitch may be set in the range of 2.1 mm to 3.3 mm so that the finless heat exchanger exhibits heat exchange performance equivalent to target heat exchange performance X1. The term “target heat exchange performance X1” as used herein refers to heat exchange performance of a finned-tube heat exchanger including a plurality of fins. It is therefore clear that the minor-axis dimension of the heat transfer tube  22  may be set to 1.5 mm and the tube pitch may be set in the range of 2.1 mm to 3.3 mm so that the finless heat exchanger exhibits heat exchange performance equivalent to that of the finned-tube heat exchanger under conditions where the air flow resistance in the finless heat exchanger is the same as that in the finned-tube heat exchanger. 
     Furthermore, the minor-axis dimension of the heat transfer tube  22  may be further reduced to 0.6 mm and the tube pitch may be set in a lower range, or the range of 1.2 mm to 2.4 mm, so that the finless heat exchanger exhibits heat exchange performance X2 that is higher than the heat exchange performance X1. 
     As can be seen based on the area of the hatched portion in  FIG. 5 , the minor-axis dimension of the heat transfer tube  22  may be less than or equal to 1.5 mm and greater than 0 to allow the finless heat exchanger to exhibit heat exchange performance equivalent to the target heat exchange performance X1 under conditions where the air flow resistance is constant. In addition, a value obtained by subtracting the minor-axis dimension from the tube pitch may range from 0.6 [mm] to 1.8 [mm]. The lower limit “0.6” of this range is a value obtained by subtracting 1.5 from 2.1. The upper limit “1.8” is a value obtained by subtracting 1.5 from 3.3. Considering the performance of the air-conditioning apparatus, the air flow resistance does not necessarily have to be equal to that in the finned-tube heat exchanger. The finless heat exchanger has only to be designed so that the sum of the work of the compressor and the work of the indoor-unit fan or the outdoor-unit fan decreases. 
     As described above, when the minor-axis dimension of the heat transfer tube  22  is reduced under conditions where the air flow resistance is constant, the tube pitch has to be reduced. In other words, the number of heat transfer tubes  22  can be increased. Therefore, setting the minor-axis dimension of the heat transfer tube  22  to a small value prevents degradation of the ease of processing of the headers  21  and improves the heat exchange performance of the finless heat exchanger. 
     Embodiment 2 
     Embodiment 2 relates to a technique for eliminating the inconvenience of variations in the intervals between the straight portions  23  of the heat transfer tubes  22  during production. The following description will focus on components different from those in Embodiment 1. Components that are not described in Embodiment 2 are the same as those in Embodiment 1. 
       FIG. 6  includes diagrams schematically illustrating the structure of a finless heat exchanger according to Embodiment 2 of the present disclosure, (a) is a front view of the heat exchanger, and (b) is a bottom view thereof.  FIG. 7  is an enlarged view of turning portions of heat transfer tubes in contact with headers in  FIG. 6 . 
     The finless heat exchanger according to Embodiment 2 differs from that according to Embodiment 1 in the configuration of each header  21 . In Embodiment 2, each header  21 A has recesses  30  located to face the turning portions  24  of the heat transfer tubes  22  and to support the turning portions  24 . The recesses  30 , each of which is shaped to fit the outer shape of the turning portion  24 , are used as a positioning structure that supports the turning portions  24  to maintain the intervals between the straight portions  23  during production. Although  FIG. 6  illustrates an example in which the recesses  30  are grooves arranged in components, serving as the headers  21 A, the recesses  30  may be formed by curving the components, serving as the header  21 A. Furthermore, although  FIG. 6  illustrates the configuration in which the two headers each have the recesses  30 , either one of the headers may have the recesses. 
     If the minor-axis dimension of each heat transfer tube  22  is reduced so that the heat transfer tubes  22  are closely arranged to improve the heat exchange performance, the rigidity of the heat transfer tube  22  will decrease. As a result, when both the ends of the heat transfer tubes  22  are joined to the headers  21 A by brazing, residual thermal stress can be generated, deforming the heat transfer tubes  22 . The deformation of the heat transfer tubes  22  can cause variations in the intervals between the adjacent turning portions  24 . 
     For this reason, when both the ends of the heat transfer tubes  22  are fitted into the insertion holes  25  of the headers  21 A, the turning portions  24  of the heat transfer tubes  22  are placed in the recesses  30 , so that the turning portion  24  are positioned. In such a state, both the ends of the heat transfer tubes  22  are brazed to the headers  21 A. This can prevent variations in the intervals between the adjacent turning portions  24  during production. Consequently, the turning portions  24  can be stably positioned, thus maintaining a uniform pitch between the adjacent straight portions  23 . This reduces or eliminates a reduction in heat exchange performance caused by variations in the pitch of the straight portions  23 . 
     As described above, since the header  21 A have the recesses  30  to support the turning portions  24  of the heat transfer tubes  22 , Embodiment 2 offers the following advantageous effects as well as the same advantageous effects as those of Embodiment 1. Specifically, the pitch between the adjacent straight portions  23  can be maintained uniform, reducing or eliminating a reduction in heat exchange performance caused by variations in the pitch. 
     The finless heat exchanger according to Embodiment 2 may be modified as follows. Such a modification also offers the same advantageous effects. 
       FIG. 8  is a diagram illustrating a modification of the finless heat exchanger according to Embodiment 2 of the present disclosure. 
     Although  FIG. 7  described above illustrates the structure in which the turning portions  24  of the heat transfer tubes  22  are directly supported by the recesses  30  of the headers  21 A, a structure in which, as illustrated in  FIG. 8 , heat insulating material  31  is interposed between the turning portions  24  of the heat transfer tubes  22  and the recesses  30  to support the turning portions  24  may be used. The heat insulating material  31  placed in the above-described manner can reduce or eliminate the transfer of heat from the turning portions  24  of the heat transfer tubes  22  to the headers  21 A. This can prevent loss of heat exchange, leading to higher heat exchange performance than in a case without the heat insulating material  31 . 
     Embodiment 3 
     The turning portions  24  of each heat transfer tube  22  are formed by bending the tubular material. It is easier to process the turning portions  24  as the bend radius of each turning portion  24  is larger. Embodiment 3 relates to the shape of the heat transfer tube based on the ease of processing of the turning portions  24 . The following description will focus on components different from those in Embodiment 1. Components that are not described in Embodiment 3 are the same as those in Embodiment 1. 
     A heat transfer tube  22 A in Embodiment 3 will be described below in comparison with the heat transfer tube  22  in Embodiment 1.  FIG. 9  is a diagram illustrating the heat transfer tube of a finless heat exchanger according to Embodiment 3 of the present disclosure.  FIG. 10  is an enlarged view of turning portions of the heat transfer tube of  FIG. 9 .  FIG. 11  is a diagram illustrating the heat transfer tube of the finless heat exchanger according to Embodiment 1 as a comparative example.  FIG. 12  is an enlarged view of the turning portions of the heat transfer tube of  FIG. 11 . 
     As illustrated in  FIG. 10 , each turning portion  24  of the heat transfer tube  22 A in Embodiment 3 includes a first part  24   a , which is curved, and a pair of second parts  24   b  extending from both ends of the first part  24   a  toward each other. The straight portions  23  extend from ends of the second parts  24   b.    
     Assuming that the tube pitch P, serving as the interval between the adjacent straight portions  23 , in the heat transfer tube  22 A in Embodiment 3 of  FIG. 10  is the same as that in the heat transfer tube  22  in Embodiment 1 of  FIG. 12 , the bend radius of each turning portion  24  in Embodiment 3 will be compared with that in Embodiment 1. The bend radius, R, of the turning portion  24  in Embodiment 1 of  FIG. 12  is a dimension of (tube pitch P−minor-axis dimension L)/2. In contrast, the bend radius R of the first part  24   a  of each turning portion  24  in Embodiment 3 of  FIG. 10  can be increased up to a dimension close to (tube pitch P−minor-axis dimension L)/2×2 if the bend radius is permitted to increase so that the adjacent turning portions  24  come into contact with each other. 
     As described above, since each turning portion  24  of the heat transfer tube  22 A is shaped to include the first part  24   a  that is curved and the pair of second parts  24   b  extending from both the ends of the first part  24   a  toward each other, Embodiment 3 offers the following advantageous effects as well as the same advantageous effects as those of Embodiment 1. Specifically, the bend radius R of the turning portion  24  can be increased without increasing the tube pitch P. This improves the ease of processing of the heat transfer tube  22 A and thus improves the productivity of the finless heat exchanger. This provides a high-quality heat transfer tube with improved ease of processing of the turning portion  24 . 
     To reduce or eliminate a reduction in heat exchange performance, the heat transfer tubes  22 A are preferably not in contact with each other. If the heat transfer tubes  22 A are in contact with each other such that only the first parts  24   a  of the turning portions  24  are in contact with each other, the heat exchange performance will not decrease markedly because the area of contact is small. 
     An increase in bend radius R of the turning portion  24  results in a reduction in residual strain caused by bending the heat transfer tube  22 A, thus reducing or eliminating a reduction in strength of the heat transfer tube  22 A. This can reduce or eliminate a reduction in factor of safety for internal pressure and a reduction in quality of the heat transfer tube  22 A. 
     An increase in bend radius R of the turning portion  24  also results in a reduction in distance between the turning portions  24  of the adjacent heat transfer tubes  22 A or contact of these turning portions. The heat transfer tubes  22  may be vibrated or deformed depending on operation conditions of the air-conditioning apparatus  1 , so that the heat transfer tubes  22 A may come into contact with each other and thus may be damaged or experience accumulation of fatigue. Unfortunately, the heat transfer tubes  22 A may be broken. To prevent such breakage, portions of the adjacent heat transfer tubes  22 A that are close to or in contact with each other are preferably joined together. This enhances the quality of the heat transfer tubes  22 A and allows the heat transfer tubes  22 A to be stably positioned, resulting in a uniform pitch of the heat transfer tubes  22 A. This leads to improved heat exchange performance. 
     The heat transfer tube  22 A, which has a configuration in  FIGS. 9 and 10 , of the finless heat exchanger according to Embodiment 3 may be modified as follows. Such a modification also offers the same advantageous effects. 
       FIG. 13  is a diagram illustrating a modification of the heat transfer tube of the finless heat exchanger according to Embodiment 3 of the present disclosure.  FIG. 14  is an enlarged view of turning portions of the heat transfer tube of  FIG. 13 . 
     In this modification, the adjacent turning portions  24  are staggered in the arrangement direction of the heat transfer tubes  22 A. Such a configuration allows the bend radius R of each turning portion  24  to increase up to approximately (tube pitch P−minor-axis dimension L)/2×3. 
     For the range of bend radii R of the turning portions  24  of the heat transfer tubes  22  and  22 A illustrated in  FIGS. 9 to 14 , each bend radius R satisfies r&lt;R≤3r, where r=(tube pitch P−minor-axis dimension L)/2. This range of bend radii applies to a case where the heat transfer tube is a flat tube. The present disclosure includes a configuration in which the bend radius R of at least one turning portion  24  of the heat transfer tube satisfies the above-described expression. 
     Embodiment 4 
     Embodiment 4 relates to miniaturization of the headers  21 . The following description will focus on components different from those in Embodiment 1. Components that are not described in Embodiment 4 are the same as those in Embodiment 1. 
       FIG. 15  includes diagrams schematically illustrating the structure of a finless heat exchanger according to Embodiment 4 of the present disclosure, (a) is a front view of the heat exchanger, and (b) is a bottom view thereof. 
     The finless heat exchanger according to Embodiment 4 includes headers  21 B instead of the headers  21  in Embodiment 1. The headers  21 B are headers miniaturized by making intervals L1 between the insertion holes  25  of the headers  21  to be smaller than arrangement intervals P2 between the adjacent heat transfer tubes  22  to such an extent as not to significantly reduce the ease of processing. Specifically, the length, L2, of each header  21 B in the arrangement direction of the heat transfer tubes  22  is shorter than the overall length, L3, of an arrangement region where the multiple heat transfer tubes are arranged. The finless heat exchanger according to Embodiment 4 is configured such that the ends of the heat transfer tubes  22  are guided to the headers  21 B, which are miniaturized in the above-described manner, via bends  32  as appropriate and are joined to the insertion holes  25 . 
     Embodiment 4 offers the same advantageous effects as those in Embodiment 1. Furthermore, since the heat exchanger includes the miniaturized headers  21 B, a reduction in internal volume of each header  21 B is achieved. This results in a reduction in amount of refrigerant. 
     Although  FIG. 15  illustrates the configuration in which each of the two headers  21  is miniaturized, at least one of the headers  21  may be miniaturized. 
     Embodiment 5 
     Embodiment 5 relates to the configuration of a finless heat exchanger including the miniaturized headers  21  described in Embodiment 4 and this configuration is intended to reduce the size of the entire finless heat exchanger. The following description will focus on components different from those in Embodiment 4. Components that are not described in Embodiment 5 are the same as those in Embodiment 4. 
       FIG. 16  includes diagrams schematically illustrating the structure of the finless heat exchanger according to Embodiment 5 of the present disclosure, (a) is a front view of the heat exchanger, and (b) is a bottom view thereof. 
     Although the two headers  21 B are arranged on the opposite ends of the heat transfer tubes  22  in Embodiment 4, Embodiment 5 relates to a configuration in which the two headers  21 B are arranged on one side where both ends of the heat transfer tubes  22  are arranged. Although the two headers  21 B are arranged on a lower side where both the ends of the heat transfer tubes are arranged in the illustrated configuration, the headers may be arranged on an upper side where both the ends of the heat transfer tubes are arranged. 
     Since the two miniaturized headers  21 B are arranged together on one side where both the ends of the heat transfer tubes  22  are arranged, Embodiment 5 offers the following advantageous effects as well as the same advantageous effects as those of Embodiment 4. Specifically, the arrangement region, in which the multiple heat transfer tubes  22  are arranged, in the housing is allowed to have a larger size than in the case where the two headers  21 B are separately arranged on opposite sides where the opposite ends of the heat transfer tubes  22  are arranged. This results in an increase in area of a front surface of the finless heat exchanger. This leads to an increase in area of heat transfer, improving the heat exchange performance. 
     Embodiment 6 
     Embodiment 6 relates to a combined structure of the two headers  21 B in Embodiment 5. The following description will focus on components different from those in Embodiment 5. Components that are not described in Embodiment 6 are the same as those in Embodiment 5. 
       FIG. 17  includes diagrams schematically illustrating the structure of a finless heat exchanger according to Embodiment 6 of the present disclosure, (a) is a front view of the heat exchanger, and (b) is a bottom view thereof. 
     Instead of the two headers  21 B arranged on one side where both the ends of the heat transfer tubes  22  are arranged in Embodiment 5, the finless heat exchanger according to Embodiment 6 includes a header  21 C formed by combining the two headers  21 B. In the header  21 C, a space connected to first ends of the heat transfer tubes  22  is separated from a space connected to second ends of the heat transfer tubes  22  by a partition plate  42 . 
     Embodiment 6 offers the same advantageous effects as those of Embodiment 5. Furthermore, since the header  21 C has a configuration formed by combining two headers, the header  21 C exhibits enhanced rigidity, leading to improved rigidity of the finless heat exchanger. Thus, the heat transfer tubes  22  are stably positioned and the tube pitch P of the straight portions  23  is kept at a predetermined pitch, leading to improved heat exchange performance. 
     Embodiment 7 
     Although each heat transfer tube  22  in Embodiment 1 described above is a single-piece component formed by bending the tubular material, each heat transfer tube  22  in Embodiment 7 is formed by joining multiple tubular materials. The following description will focus on components different from those in Embodiment 1. Components that are not described in Embodiment 7 are the same as those in Embodiment 1. 
       FIG. 18  is a schematic front view of the structure of a finless heat exchanger according to Embodiment 7 of the present disclosure.  FIG. 19  is a perspective view of essential part of the heat transfer tube in  FIG. 18 . 
     Each heat transfer tube  22 B in Embodiment 7 includes straight and turning portions  23  and  24 , which are formed as separate parts, joined by brazing, for example. Specifically, the turning portions  24  are configured as U-bent tubes. 
     Embodiment 7 offers the same advantageous effects as those of Embodiment 1. 
     Embodiment 8 
     Embodiment 8 differs from Embodiment 1 in the arrangement direction of the components of the finless heat exchanger. The following description will focus on components different from those in Embodiment 1. Components that are not described in Embodiment 8 are the same as those in Embodiment 1. 
       FIG. 20  is a schematic front view of the structure of a finless heat exchanger according to Embodiment 8 of the present disclosure. 
     In the finless heat exchanger according to Embodiment 1 described above, the heat transfer tubes  22  are arranged side by side in a horizontal direction. As illustrated in  FIG. 20 , in the finless heat exchanger according to Embodiment 8, the heat transfer tubes  22  are arranged side by side in a vertical direction. 
     Embodiment 8 offers the same advantageous effects as those of Embodiment 1. 
     Embodiment 9 
     Although the finless heat exchanger according to Embodiment 1 described above has a flat overall form, a finless heat exchanger according to Embodiment 9 has an L-shaped overall form. The following description will focus on components different from those in Embodiment 1. Components that are not described in Embodiment 9 are the same as those in Embodiment 1. 
       FIG. 21  includes schematic diagrams illustrating the finless heat exchanger according to Embodiment 9 of the present disclosure, (a) is a front view of the heat exchanger, (b) is a plan view thereof, and (c) is a side view thereof. 
     As illustrated in  FIG. 21 , the finless heat exchanger according to Embodiment 9 includes a plurality of heat transfer tubes  22  having bends  60  in middle portions thereof in the longitudinal direction of the heat transfer tubes  22 . The finless heat exchanger has an L-shaped overall form. Specifically, the heat transfer tubes  22  have the bends at identical positions in the longitudinal direction. The finless heat exchanger according to Embodiment 9 is intended to be used as a heat exchanger for an indoor unit. 
     Embodiment 9 offers the same advantageous effects as those of Embodiment 1. Furthermore, since the finless heat exchanger according to Embodiment 9 has an L-shaped overall form, the heat exchanger can be effectively used, as an indoor-unit heat exchanger, in an indoor unit because it is difficult to allow the indoor unit to have a large front surface. 
     Embodiment 10 
     Embodiment 10 relates to a configuration in which the straight portions  23  of the heat transfer tubes  22  are arranged at a constant tube pitch P, or regular intervals, if the heat transfer tubes  22  are vibrated during operation of the air-conditioning apparatus  1 . The following description will focus on components different from those in Embodiment 1. Components that are not described in Embodiment 10 are the same as those in Embodiment 1. 
       FIG. 22  is a schematic front view of the structure of a finless heat exchanger according to Embodiment 10 of the present disclosure.  FIG. 23  is a sectional view illustrating part of a positioning part in  FIG. 22 . 
     The finless heat exchanger according to Embodiment 10 includes positioning parts  70 , which are included in a positioning structure maintaining the tube pitch P of the straight portions  23  of the heat transfer tubes  22  constant. In such an example, two positioning parts  70  are arranged apart in the longitudinal direction of the heat transfer tubes  22 . Each positioning part  70  is a rod-shaped component and has a plurality of indented insertion slots  71 , to which the straight portions  23  of the heat transfer tubes  22  are fitted, arranged in the longitudinal direction of the positioning part  70 . The insertion slots  71  are arranged at regular intervals corresponding to the intervals between the adjacent straight portions  23 . The straight portions  23  are fitted in the insertion slots  71  of the positioning parts  70  so that the tube pitch P of the straight portions  23  can be maintained constant if the heat transfer tubes  22  are vibrated during operation of the air-conditioning apparatus  1 . The positioning parts  70  are preferably made of resin having low thermal conductivity or heat insulating material. 
     Embodiment 10 offers the same advantageous effects as those of Embodiment 1. Furthermore, the heat transfer tubes  22  are positioned by the positioning parts  70 , so that the tube pitch P is maintained constant. This leads to improved heat exchange performance. 
     A finless heat exchanger is reduced in diameter of heat transfer tubes to obtain heat exchange performance equivalent to that of a finned-tube heat exchanger, and such heat transfer tubes tend to have lower rigidity. However, since the positioning parts  70  are arranged, the straight portions  23  of the heat transfer tubes  22  are fitted in and supported by the insertion slots  71  of the positioning parts  70 . This eliminates or reduces a reduction in rigidity of the heat transfer tubes  22 , leading to improved rigidity of the heat exchanger. 
     The form of each positioning part  70 , the number of positioning parts  70 , and the positions of the positioning parts  70  do not necessarily have to be limited to those in  FIGS. 22 and 23  and can be changed as appropriate without departing from the scope of operation of the positioning parts  70 . For example, the number of positioning parts  70  is not limited to two, and may be one or three or more. 
     The present disclosure is not limited to Embodiments 1 to 10 described above, and can be variously modified within the scope of the present disclosure. Specifically, the configurations according to Embodiments described above may be appropriately modified and at least one element of the configurations may be substituted for another element. Furthermore, a component whose location is not particularly limited does not necessarily have to be disposed at the location described in Embodiments, and may be disposed at any location that enables the component to achieve its function. 
     Although Embodiments 1 to 10 have been described as different embodiments, the features of Embodiments 1 to 10 may be appropriately combined into a finless heat exchanger. For example, Embodiment 2 and Embodiment 4 may be combined, and the headers  21 B in  FIG. 15  may have the recesses  30  in Embodiment 2. For the modifications of the components in Embodiments 1 to 10, similar components in the embodiments other than the embodiment in which the modification has been described may be similarly modified. 
     Although the case where the finless heat exchanger according to the present disclosure is used as a heat source side heat exchanger has been described as an example, the finless heat exchanger according to the present disclosure may be used as a use side heat exchanger. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  air-conditioning apparatus  1 A heat source side unit  1 B use side unit  4  heat source side heat exchanger  21  header  21 A header  21 B header  21 C header  22  heat transfer tube  22 A heat transfer tube  22 B heat transfer tube  23  straight portion  24  turning portion  24   a  first part  24   b  second part  25  insertion hole  26  refrigerant inlet-outlet  30  recess  31  heat insulating material  32  bend  40  heat source side heat exchanger  41  fan  42  partition plate  60  bend  70  positioning part  71  insertion slot  110  compressor  150  expansion device  160  flow switching device  170  accumulator  180  use side heat exchanger  210  header  220  heat transfer tube  400  finless heat exchanger