JP2007248014A - Flat perforated pipe for heat exchanger - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a copper or copper alloy flat perforated pipe for a heat exchanger with improved pressure resistance, weight reduction, and bendability while maintaining advantages of copper or a copper alloy superior in corrosion resistance and thermal conductivity. <P>SOLUTION: A corrugated sheet 2 comprising copper or a copper alloy is arranged in a flat pipe 1 having a pair of opposing inner faces comprising copper or the copper alloy. The corrugated sheet 2 is bent so as to alternately contact with the pair of opposing faces of the flat pipe 1, and contact surfaces of the inner faces of the flat pipe 1 and the corrugate sheet 2 are joined by brazing 3. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a flat porous tube made of copper or copper alloy applied to a heat exchanger such as a room air conditioner, a packaged air conditioner, a refrigerator, an air conditioner for automobiles, and a refrigeration apparatus.

Conventionally, an aluminum flat porous tube having a plurality of refrigerant channels has been used as a heat transfer tube for a car air conditioner. Such a flat porous tube is manufactured by extrusion (Patent Document 1). However, the heat exchanger made of aluminum has a low pressure resistance, and it is necessary to increase the wall thickness in order to use it for a high-pressure refrigerant such as CO ₂ , so that there is a problem that the corrosion resistance is low. In particular, when a refrigerant having a high temperature is used, aluminum may be problematic in terms of material since heat resistance is low.

  In order to improve the disadvantages of such flat aluminum tubes, wrap a plurality of tubes with a brazing sheet and a brazing metal plate, braze the bent portion, and place a plurality of tubes in the flat tube. A copper flat porous tube having a structure in which tubes are juxtaposed has been proposed (Patent Document 2). This flat porous tube can greatly increase the amount of heat exchange compared to an aluminum flat tube, but the bent portion of the flat outer tube that wraps the inner tube is joined by brazing to form a flat tube Therefore, since there is a brazed portion on the outer surface, there is a problem in reliability with respect to pressure strength. In addition, the flat tube is formed into a flat tube by curving a plate material in a flat shape so as to wrap a plurality of juxtaposed circular tubes, bending both ends thereof and brazing the bent portions. This bent portion has the disadvantage that the mass of the entire flat porous tube becomes heavy.

Japanese Examined Patent Publication No. 7-65876 Japanese Utility Model Publication No.7-12771

  As described above, the conventional copper or copper alloy porous flat tube disclosed in Patent Document 2 has a problem that the pressure resistance is low and the mass is heavy although the corrosion resistance and the thermal conductivity are excellent. In addition, the contact portions between the plurality of tubes disposed inside and the metal sheet that wraps the tubes are brazed, but the contact portions between the plurality of tubes are difficult to rotate, so they are simply in contact with each other. ing. For this reason, when bending is performed on the copper flat porous tube produced in this way, the inner tube and the tube are displaced, and due to this displacement, the brazed portion between the tube and the metal sheet is peeled or cracked. This causes problems such as deterioration of heat transfer performance and fatigue failure due to long-term use.

  The present invention has been made in view of such problems, and improves the pressure strength, weight reduction, and bending workability while maintaining the superiority of copper or copper alloy having excellent corrosion resistance and thermal conductivity. An object of the present invention is to provide a flat porous tube for a heat exchanger made of copper or copper alloy.

  A flat porous tube for a heat exchanger according to the present invention is a flat tube made of copper or a copper alloy and having a pair of opposed inner surfaces, and is bent so as to be in contact with the pair of opposed surfaces disposed in the flat tube. A corrugated sheet made of copper or a copper alloy, and a contact surface between the inner surface of the flat tube and the corrugated sheet is joined by brazing.

  In the present invention, the space surrounded by the inner surface of the flat tube and the corrugated sheet is a flow path through which the medium flows. And since the flat porous tube of this invention is manufactured with copper or a copper alloy, pressure resistance strength, corrosion resistance, and thermal conductivity are excellent, and since there is no brazing part on the outer surface, a high-pressure refrigerant is used. Excellent pressure strength reliability. Further, since the flow path in the tube is formed by a corrugated sheet, the corrugated sheet does not need to bear strength, so that a thin one can be used, and thereby the mass of the flat porous tube can be reduced.

  In this flat porous tube for a heat exchanger, it is preferable that T ≧ t, where T is the thickness of the flat tube and t is the thickness of the corrugated sheet.

  The thickness of the flat tube is determined according to the pressure of the refrigerant, but the corrugated sheet constituting the refrigerant flow path is loaded with pressure from both flow paths on both sides of the sheet partitioned by the corrugated sheet. For this reason, the corrugated sheet is not required to be so strong. For this reason, the thickness of the corrugated sheet can be reduced in order to reduce the weight of the flat porous tube. However, in order to ensure the reliability of the brazed portion, the corrugated sheet needs to have a certain thickness and needs to have an appropriate thickness. Thus, in order to ensure the reliability of the brazed portion, the thickness T (mm) of the flat tube satisfies T ≧ t with respect to the thickness t (mm) of the corrugated sheet. Thereby, while being able to hold | maintain sufficient pressure strength, the weight reduction of the heat exchanger using a flat porous tube can be achieved. The thickness t of the corrugated sheet is preferably as thin as possible to reduce the weight of the flat porous tube. However, depending on the thickness of the brazing material and the brazing conditions (heating temperature, heating time), the corrugated sheet is eaten by the brazing, Since the thickness may be somewhat reduced, the wall thickness t is preferably 0.1 mm or more.

  Moreover, it is preferable that the curvature radius of the bent part of the corrugated sheet is 0.01 to 1 mm.

  The pressure resistance of the flat porous tube depends on the brazing strength of the inner surface of the flat tube and the corrugated sheet as well as the thickness of the flat tube. In the corrugated sheet, when the flat tube is placed horizontally, the upper portion (contacting the lower inner surface of the flat tube) and the lower portion (contacting the upper inner surface of the flat tube) are alternately connected. However, if the bending radius of the bent portion is large, the contact surface between the corrugated sheet and the inner surface of the flat tube becomes small, and this contact surface becomes the joint surface in brazing, so the joint area is small and the joint strength is small. End up. For this reason, pressure resistance performance will fall. Therefore, it is desirable that the radius of curvature of the bent portion of the corrugated sheet is 0.01 to 1 mm.

Furthermore, when the tensile strength when the flat tube and the copper or copper alloy material of the corrugated sheet are heated to 800 to 850 ° C. for 1 to 30 minutes is σ _B and σ _B2 , respectively, σ _B ≧ σ _B2 Preferably there is.

The flat tube and the corrugated sheet are heated and brazed at 800 to 850 ° C. for 1 to 30 minutes to produce a flat porous tube. Copper or copper alloy is softened by brazing. When a heat-resistant copper-based alloy having higher heat resistance than the flat tube is used for the corrugated sheet, the corrugated sheet may be distorted when the flat porous tube is bent. For this reason, the pressure resistance performance may deteriorate. Therefore, it is desirable to use a copper alloy in which σ _B ≧ σ _B2, where σ _B and σ _B2 are the tensile strengths of the flat tube and the corrugated sheet when heated to 800 to 850 ° C. for 1 to 30 minutes, respectively. By not making the heat resistance strength (high temperature tensile strength) of the flat tube smaller than the tensile strength of the corrugated sheet, when the flat porous tube is bent, the corrugated sheet is distorted and peels off from the flat tube. The situation can be prevented.

  Furthermore, it is preferable that the average crystal grain size is 200 μm or less in the cross section along the longitudinal direction of the flat tube.

  When the crystal grains of the flat tube become coarse due to brazing of the flat tube and the corrugated sheet, the pressure resistance performance decreases due to strength reduction and the surface cracks during bending work tend to occur. The average grain size is desirably 200 μm or less. The crystal grain size is more preferably 100 μm or less, and further preferably 50 μm or less.

  According to the present invention, the flat porous tube has excellent corrosion resistance and thermal conductivity, and the flat porous tube has improved pressure resistance, light weight, and bending workability, and improved performance of a heat exchanger using the same, Significant contribution to reliability improvement and miniaturization.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the accompanying drawings. FIG. 1 is a sectional view (fluid passage sectional view) showing a flat porous tube according to an embodiment of the present invention. As shown in FIG. 2, the flat porous tube of this embodiment has a horizontal section 2a, a vertical segment 2b, and a horizontal segment 2c, as shown in FIG. The corrugated sheet 2 is bent substantially vertically. The flat tube 1 and the corrugated sheet 2 have the same cross-sectional shape in the longitudinal direction.

  Then, with the flat tube 1 placed horizontally, the corrugated sheet 2 is inserted horizontally into the flat tube 1, the upper horizontal segment 2c of the corrugated sheet 2 is overlaid on the upper inner surface of the flat tube 1, and the lower side of the corrugated sheet 2 The horizontal segment 2a is overlaid on the lower inner surface of the flat tube 1, the brazing material 3 is disposed between the segment 2a and the lower inner surface of the flat tube 1, and between the segment 2c and the upper inner surface of the flat tube 1, and these are heated. Then, the segment 2a and the lower inner surface of the flat tube 1 and the segment 2c and the upper inner surface of the flat tube 1 are brazed.

  The flat perforated pipe thus manufactured is composed of a space surrounded by the upper segment 2c, the two vertical segments 2b, and the inner surface of the flat tube 1, the lower segment 2a, the two vertical segments 2b, and the inner surface of the flat tube 1. The space surrounded by the pipe becomes the flow path 6 through which the refrigerant flows.

  An aluminum flat porous tube is manufactured relatively easily by extrusion, but it is technically difficult to produce a similar one by extruding copper or a copper alloy. As shown in FIG. 1, by flattening and integrating a flat tube 1 made of copper or copper alloy and a corrugated sheet 2, a flat porous plate made of copper or copper alloy having the same shape as a flat porous tube made of aluminum or aluminum alloy. Tubes can be manufactured. By adopting a seamless flat tube, pressure resistance is improved. Moreover, by employ | adopting the corrugated sheet | seat 2 for the partition in the flat tube 1, a partition plate can be thinned and a weight can be lightened. By reducing the pitch between the vertical segments 2b of the corrugated sheet 2, the cross-sectional area of the refrigerant flow path can be reduced, and the pressure resistance performance and the heat exchanger performance can be improved.

  In this invention, what is necessary is just to select a suitable thing from copper or various copper alloys according to the heat conductivity, pressure strength, corrosion resistance, and bending workability required as a heat exchanger. If pressure strength is not so much required, OFC (oxygen-free copper) or phosphorous deoxidized copper (C1220 of JISH3300) may be used. If pressure strength for carbon dioxide refrigerant or the like is important, a flat tube is used. For example, as described in JP-A-10-130754, a copper alloy having heat resistance and excellent thermal conductivity may be used. Also, precipitation-strengthening alloys such as Cu-Ni-Si and Cu-Cr are used, and additional elements or intermetallic compounds are precipitated during cooling after brazing, or the compounds are precipitated after brazing. If a method such as this is used, a flat porous tube with higher strength can be produced.

  As shown in FIG. 2, the cross-sectional shape of the flat tube 1 is not limited to a rectangular shape, and a long side constituting a pair of opposing surfaces and a short side connecting the long sides are straight. For example, as in the flat tube 4 shown in FIG. 3, the short side may have a semicircular shape having a radius of curvature that is ½ of the interval between a pair of long sides (opposing surfaces). Such a flat tube is manufactured by drawing a copper or copper alloy tube having an appropriate outer diameter, thickness, and material using a die having a rectangular cross section and a plug having a rectangular cross section. Can do.

  As described above, the material of the flat tubes 1 and 4 may be selected from various copper or copper alloys, but the tensile strength after heating at 800 to 850 ° C. for 1 to 30 minutes is the tensile strength of the corrugated sheet. It is desirable to be more than strength. When the crystal grains of the flat tube become coarse due to heating during brazing, the strength is lowered and sufficient pressure strength cannot be obtained. Further, if the flat perforated pipe is bent, the surface may be roughened and wrinkles or cracks may occur during the bending of the heat exchanger. Therefore, it is important in terms of pressure resistance that the average crystal grain size in the cross section in the longitudinal direction of the flat tube is 200 μm or less. The crystal grain size is more desirably 100 μm or less, and further desirably 50 μm or less.

  Since the dimensions of the flat tubes 1 and 4 are determined according to the dimensions of the heat exchanger, the dimensions are not particularly limited. However, there is a problem if the cross-sectional width is 20 to 300 mm and the height is about 5 to 10 mm. Can be produced without. The thickness is determined by the required pressure resistance and the tensile strength after brazing.

  Next, the corrugated sheet 2 will be described. As shown in FIG. 4, the shape of the corrugated sheet 2 has a large number of “U” -shaped cross sections that open to the upper and lower parts when the corrugated sheet 2 is placed horizontally. The “U” -shaped portion is formed adjacent to each other so as to open upward and open downward. Such a corrugated sheet is formed by pressing one or several pieces of “U” shape with an upper die and a lower die made of copper or a copper alloy thin plate, or “U” shape. The corrugated sheet shape 2 can be produced by rolling one to several “U” shapes with the upper roll and the lower roll on which the film is formed. By heating the corrugated sheet 2 together with the brazing material into the flat tube, the horizontal segments 2a and 2c of the corrugated sheet 2 are joined to the inner surface of the flat tube 1, and the vertical segment 2b of the corrugated sheet 2 is adjacent to the refrigerant flow. It becomes the road partition. The corrugated sheet 2 in FIG. 4 has the horizontal segment 2a at the end, but the end may be the vertical segment 2b as in the corrugated sheet 5 shown in FIG.

  The dimensions of the corrugated sheets 2 and 5 may be determined so that they can be inserted into the flat tube 1 without difficulty. In particular, the height of the vertical segment 2b is important in performing brazing soundly, and it is desirable that the height of the vertical segment 2b be 0.005 to 0.30 mm smaller than the interval between the inner surfaces of the flat tubes 1. As described above, by properly maintaining the clearance, it is possible to perform brazing in which a sound fillet is formed without any defect, and good thermal conductivity and pressure resistance can be obtained. The lateral length of the corrugated sheet 2 (the total length of the horizontal segments 2a and 2c) is less than the length in the width direction of the inner surface of the flat tube 1 (the distance between the inner surfaces of the short sides of the flat tubes 1 and 4). What is necessary is just to make it small about 01 thru | or 0.3 mm. When the corrugated sheet 5 having the shape shown in FIG. 5 is used, the vertical segment 2b at the end and the end surface of the flat tube 1 are brazed, so that the width of the inner surface of the flat tube 1 is 0.005 to 0.3 mm. What is necessary is just to make it small.

  The vertical segments 2b of the corrugated sheets 2 and 5 serve as partition walls of the refrigerant flow path, and the same pressure acts from the refrigerant flow paths on both sides, so that the strength is not so necessary. Therefore, in order to reduce the mass of the flat porous tube, the corrugated sheet 2 should be thin, and an appropriate value may be determined so that the correct shape can be maintained after brazing without being deformed when inserted into the flat tube 1. The thickness of the corrugated sheet 2 is, for example, about 0.1 to 1 mm. In order to secure a sufficient pressure resistance and to reduce the weight of the flat porous tube, it is desirable to set the ratio of (thickness of flat tubes 1, 4 / thickness of corrugated sheets 2, 5) to 1 to 5. .

  In order to improve the heat transfer performance of the flat porous tube and increase the pressure resistance, it is advantageous that the cross-sectional area of the refrigerant channel is small, and the interval between adjacent vertical segments 2b is suitably about 0.1 to 3 mm. . However, the smaller the interval, the larger the mass of the flat porous tube.

  If the radius of curvature of the corner portions of the corrugated sheets 2 and 5 is large, the joint surface with the inner surface of the flat tube 1 becomes small, the joint strength becomes small, and the pressure resistance performance is lowered. In order to ensure a sufficient bonding area between the corrugated sheet 2 and the inner surface of the flat tube 1, the radius of curvature is preferably 0.01 to 1 mm.

  The corrugated sheets 2 and 5 are preferably made of a copper alloy having a strength after heating equal to or smaller than that of the flat tube 1. If the corrugated sheet 2 is made of a copper alloy having a strength higher than that of the flat tube 1 after heating, the corrugated sheet 2 may be distorted when the flat porous tube is bent. Therefore, it is desirable that the corrugated sheets 2 and 5 have a tensile strength after heating at 800 to 850 ° C. for 1 to 30 minutes not more than the tensile strength of the flat tube.

  In order to manufacture the flat porous tube of the present invention, it is preferable to use a brazing material such as phosphor copper brazing. A sheet brazing material having a thickness of several tens of μm to 0.3 mm is placed between the corrugated sheet 2 and the flat tube 1 and heated to a temperature equal to or higher than the melting point of the brazing material in an inert, reducing or vacuum atmosphere. By this, the corrugated sheet 2 is joined to the flat tube 1, and a flat porous tube can be manufactured. The width and length of the brazing material may be matched to the width and length of the corrugated sheet. In order to braze using phosphor copper brazing, it is necessary to heat to 800 ° C. or higher. By heating, the brazing material in contact with the horizontal segment of the corrugated sheet is melted and brazed between the inner surface of the flat tube, and the brazing material not in contact with the horizontal segment is sucked into the horizontal segment by capillary action. As a result, fillets are formed at the corners of the horizontal segments, and good brazing strength can be ensured. When it is desired to reduce the strength decrease due to heating, for example, a corrugated sheet 2 with Sn plated to an appropriate thickness is inserted into the flat tube 1 and heated to about 250 to 400 ° C. It is possible to produce.

  In addition, if it brazes in the state which applied the pressure to the wide surface of the flat tube 1, the joining strength of a brazing part will become high and the reliability as a heat exchanger will improve more.

  Hereinafter, examples and comparative examples of the present invention will be specifically described, and effects of the present invention will be described. A flat tube and a corrugated sheet were produced by the method described above. The flat tube, corrugated sheet, brazing material, and brazing method used for the production of the flat porous tube are shown below.

  As shown in Table 2 below, the material of the flat tube is JISH3300-C1220, Cu-0.65% Sn-0.03% P, or Al-0.05% Mn-0.05% Cu-0.03. % Ti. The flat tube has a width of 45.0 mm, a height of 4.5 mm, a length of 420 mm, and a wall thickness of 0.8 mm.

  The material of the corrugated sheet is C1220 (phosphorus deoxidized copper) or Al-0.8% Mn-0.2% Zr as shown in Table 1 below. As shown in FIG. 6, the corrugated sheet has a width of 43.2 mm, a height of 2.67 mm, a lateral length of 400 mm, and a wall thickness of 0.5 to 1.0 mm. The radius of curvature of the bent portion of the corrugated sheet is 0.05 to 0.5 mm, and the length of the horizontal segment is 3 mm.

また、ろう材は、表１に示すように、厚さが０．１ｍｍ、幅４３．２ｍｍ、長さ４００ｍｍの板状ＢＣＵＰ２又はＡｌ−１０．２％Ｓｉ−０．６％Ｍｇである。ろう付け条件は銅系素材については、８３０℃のＲＸ炉（Ｎ_２：４９％、ＣＯ：２１％、Ｈ_２：２９％）中で８分加熱した。アルミニウム系素材については、真空炉中で６００℃、５分加熱した
上述の扁平管、コルゲートシート、及びろう材を脱脂し、コルゲートシートの水平セグメント上下にろう材を配置した状態で、ろう材とコルゲートシートを扁平管に挿入後、厚さが５ｍｍ、幅が６０ｍｍ、長さが４５０ｍｍの鋼板で挟み、長手方向の６箇所を万力で加圧した。この状態のまま加熱してろう付けにより一体化し、実施例及び比較例の扁平多孔管を作製した。このようにして作製した扁平多孔管について、以下の方法でコルゲートシートの接合状況、耐圧試験、及び曲げ加工試験を行った。その結果を下記表２に示す。

Further, as shown in Table 1, the brazing material is plate-like BCUP2 having a thickness of 0.1 mm, a width of 43.2 mm, and a length of 400 mm, or Al-10.2% Si-0.6% Mg. As for the brazing condition, the copper-based material was heated in an RX furnace (N ₂ : 49%, CO: 21%, H ₂ : 29%) at 830 ° C. for 8 minutes. For aluminum-based materials, the above-mentioned flat tube, corrugated sheet, and brazing material heated at 600 ° C. for 5 minutes in a vacuum furnace are degreased, and brazing material is placed above and below the horizontal segment of the corrugated sheet. After the corrugated sheet was inserted into the flat tube, it was sandwiched between steel plates having a thickness of 5 mm, a width of 60 mm, and a length of 450 mm, and six locations in the longitudinal direction were pressurized with a vise. The flat porous tube of the Example and the comparative example was produced by heating in this state and integrating by brazing. The flat perforated pipe thus produced was subjected to a corrugated sheet joining condition, a pressure resistance test, and a bending work test by the following methods. The results are shown in Table 2 below.

表１に示すコルゲートシートの接合状況は、製作した扁平多孔管の両端より長さ１０ｍｍの観察用試料を切断し、その断面を研磨して目視及び実体顕微鏡（倍率２０倍）によりコルゲートシートと扁平管の接合状況を観察した。

The bonding state of the corrugated sheet shown in Table 1 is that the observation sample having a length of 10 mm is cut from both ends of the manufactured flat perforated tube, the cross section is polished, and the corrugated sheet and the flat are visually and stereomicroscopically (magnification 20 times). The joining condition of the tube was observed.

    In the pressure test shown in Table 2, headers were brazed to both sides of a flat porous tube, the opening of one header was sealed, and water pressure of a predetermined pressure was applied from one end of the remaining header (10 to 50 MPa). It was held for 5 minutes in a state where a predetermined pressure was applied, and the presence or absence of swelling and destruction of the flat porous tube was investigated. If there is a part that is not completely brazed, water enters the gap between the horizontal segment of the corrugated sheet and the flat tube, and the part is swollen (swelled). In addition, if the location of defective brazing is large, a crack may occur in the swollen portion, and pressure may not be applied (breakage).

  As a bending test, the flat porous tube was bent with a cylindrical mold having a predetermined radius, and the flat porous tube was warped and roughened. In the bent portion, the cross-sectional shape parallel to the bending axis of the flat porous tube may change from a rectangular shape to an arc shape. The warpage was measured by measuring the size of the curve on the surface in contact with the bending mold, and the warpage was determined to be 1 mm or more.

  As shown in Tables 1 and 2, the samples of Examples 1 to 3 were all joined together by brazing, and no blistering or breaking occurred as a result of the pressure resistance test. In addition, as a result of the bending process, the samples of Examples 1 and 3 having small crystal grains do not generate warpage or rough skin regardless of the bending radius, and the bending radius is 40 mm in other samples of the present invention. Neither warpage nor rough skin occurred.

  On the other hand, since the sample of Comparative Example 6 was manufactured by brazing a chile member, the strength was weak, breakage occurred in the pressure test, and bending could not be performed. Moreover, the sample of the comparative example 6 had a location where the brazing was poor at the end portion of the corrugated sheet, and bulging and breaking occurred at this portion by the pressure resistance test.

It is a cross-sectional view showing a flat porous tube according to an embodiment of the present invention. It is a cross-sectional view showing a flat tube. It is a cross-sectional view showing another flat tube. It is a cross-sectional view showing a corrugated sheet. It is a cross-sectional view showing another corrugated sheet. It is a cross-sectional view which shows the dimension of the corrugated sheet | seat of an Example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 4: Flat tube 2, 5: Corrugated sheet 2a, 2c: Horizontal segment 2b: Vertical segment 3: Brazing material 6: Flow path

Claims (5)

A flat tube made of copper or a copper alloy and having a pair of opposed inner surfaces; and a corrugated sheet made of copper or a copper alloy that is arranged in the flat tube and bent so as to alternately contact the pair of opposed surfaces. A flat porous tube for a heat exchanger, wherein an inner surface of the flat tube and a contact surface of the corrugated sheet are joined by brazing. 2. The flat porous tube for a heat exchanger according to claim 1, wherein T ≧ t, where T is a thickness of the flat tube and t is a thickness of the corrugated sheet. The flat porous tube for a heat exchanger according to claim 1 or 2, wherein a radius of curvature of a bent portion of the corrugated sheet is 0.01 to 1 mm. When the tensile strength when the flat tube and the copper or copper alloy material of the corrugated sheet are heated to 800 to 850 ° C. for 1 to 30 minutes is σ _B and σ _B2 , respectively, σ _B ≧ σ _B2 The flat porous tube for heat exchangers according to any one of claims 1 to 3, wherein: The flat porous tube for a heat exchanger according to any one of claims 1 to 4, wherein an average crystal grain size is 200 µm or less in a cross section along the longitudinal direction of the flat tube.

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