TANK STRUCTURE OF HEAT EXCHANGER

Abstract

In a tank structure of a heat exchanger having a narrow width header plate, to reduce thermal stress generated in a tube on the outermost side of the header plate in a lengthwise direction. Insertion holes, except for end-portion tube insertion holes at least located at end portions of a header plate in the lengthwise direction, are linked with deformation prevention beads formed along the lengthwise direction of the header plate, and the end-portion tube insertion holes are not linked with the deformation prevention beads.

Description

TECHNICAL FIELD

The present invention relates to a tank structure of a heat exchanger that is most suitable for radiators for engine cooling water, oil coolers and other heat exchangers, and relates to one that improves heat durability thereof.

BACKGROUND ART

As a conventional tank of a heat exchanger, there has been found one in which the width of a header plate is narrowed for weight saving and space reduction.

In the tank of a heat exchanger in Patent Literature 1 below, a sealing surface 24 of a header plate 4a is formed in flat, as shown in FIG. 8B, to narrow the width of the header plate.

Further, between tube insertion holes formed in the header plate 4a, a deformation prevention beads 7, for supporting a tank main body 6 and preventing deformation of the tank main body 6 when the tank main body 6 is caulked, is provided.

CITATION LIST

Patent Literature

PTL 1: WO 2017/069280 A1

SUMMARY OF INVENTION

Technical Problem

In the header plate 4a described in the Patent Literature 1, a high burring 22 is formed on long side portions of respective tube insertion holes, and a low burring is formed on short side portions.

Then, a brazing joining line in the short side portion is located on a tank outer side as compared with that in the long side portion.

As the result of the examination of the present inventor, it has been found that concentration of thermal stress generated due to unevenness of temperature distribution in the heat exchanger on the tube short side portion became prominent in an instance where the brazing joining line in the short side portion was located on a tank outer side as compared with that in the long side portion as described above.

Meanwhile, the stress concentration is prominent in a tube at the end-portion in the lengthwise direction of the header plate 4a.

Moreover, in an instance where a tank is divided into a plurality of zones by partition portions and fluids having different temperatures each other flow into respective zones, the stress concentration is prominent in tubes near the partition portions.

Therefore, the present invention is directed to relax the concentration of thermal stress on a tube short side portion near a brazed portion between the tube and the header plate in a heat exchanger having a header plate with narrow width.

Solution to Problem

The present invention according to a first aspect thereof is a tank structure of a heat exchanger, including:

a header plate 4 having a plurality of flat tube insertion holes 3 constituted of a pair of facing short side portions 1 and a pair of long side portions 2 linking the both short side portions 1;

a plurality of flat tubes 5 whose end-portions have been inserted into the respective tube insertion holes 3, and the insertion portions have been brazed;

a tank main body 6 to which a rim of the header plate 4 is fixed by caulking, and

the short side portions 1 of the plurality of tube insertion holes 3 are located in a width direction of the header plate 4, and their tube insertion holes 3 are arranged separately each other in a lengthwise direction of the header plate 4, wherein

respective tube insertion holes 3, except for end-portion tube insertion holes 8 at least located at end-portions in the lengthwise direction, are linked by deformation prevention beads 7 formed along the lengthwise direction of the header plate 4; and

the end-portion tube insertion holes 8 are not linked by the deformation prevention beads 7.

The present invention according to a second aspect thereof is the tank structure of a heat exchanger according to the first aspect, wherein:

a low burring 9 having low height is formed on a hole brim of the short side portions 1 of the respective tube insertion holes 3, while a high burring 10 higher than the low burring 9 is formed on the long side portions 2 of the respective tube insertion holes 3 except for at least the end-portion tube insertion holes 8; and

the low burring 9 is formed on the hole brim of the end-portion tube insertion holes 8 throughout the entire circumference thereof.

The present invention according to the third aspect thereof is the tank structure of a heat exchanger according to the first aspect, wherein:

the low burring 9 having low height is formed on a hole brim of the short side portions 1 of the respective tube insertion holes 3, and the high burring 10 higher than the low burring 9 is formed on the long side portions 2 of the respective tube insertion holes 3 except for at least the end-portion tube insertion holes 8; and

the low burring 9 is formed on hole brims of both end-portions of the long side portions 2 of at least the end-portion tube insertion holes 8, and the high burring 10 is formed on a hole brim of the long side portion 2 except for both end-portions thereof.

The present invention according to the fourth aspect thereof is a tank structure of a heat exchanger, including:

a header plate 4 having a plurality of flat tube insertion holes 3 constituted of a pair of facing short side portions 1 and a pair of long side portions 2 linking the both short side portions 1;

a plurality of flat tubes 5 whose end-portions have been inserted into the respective tube insertion holes 3, and the insertion portions have been brazed; and

a tank main body 6 to which a rim of the header plate 4 is fixed by caulking, in which

the short side portions 1 of the plurality of tube insertion holes 3 are located in a width direction of the header plate 4, and their tube insertion holes 3 are arranged separately each other in a lengthwise direction of the header plate 4,

wherein:

the tank main body 6 harbors a partition portion 19 that divides the same into a plurality of zones in a lengthwise direction;

a dummy tube insertion hole 11 is formed at the header plate 4 located near the partition portion 19, and the end-portion tube insertion hole 8 with a plurality of tube insertion holes 3 in parallel on both sides of the dummy tube insertion hole 11;

flat tubes 5 are inserted into respective tube insertion holes 3, 8, 11;

respective tube insertion holes 3, except for the dummy tube insertion hole 11 and an end-portion tube insertion hole 8 located at least to the first from the dummy tube insertion hole 11, are linked by the deformation prevention beads 7 formed along the lengthwise direction of the header plate 4; and

formed so that the deformation prevention beads 7 do not exist for the dummy tube insertion hole 11 and the end-portion tube insertion hole 8 located at least to the first from the dummy tube insertion hole 11.

The present invention according to the fifth aspect thereof is the tank structure of a heat exchanger according to the fourth aspect, wherein:

the low burring 9 is formed on short side portions 1 of the respective tube insertion holes 3;

the high burring 10 is provided for the long side portions 2 of the respective tube insertion holes 3 except for the dummy tube insertion hole 11 and the end-portion tube insertion hole 8 located at least to the first from the dummy tube insertion hole 11; and

the low burring 9 is provided for the end-portion tube insertion hole 8 located at least to the first from the dummy tube insertion hole 11 throughout the entire circumference thereof.

Advantageous Effects of Invention

The invention according to the first aspect is an invention, wherein: respective tube insertion holes 3, except for end-portion tube insertion holes 8 at least located at the end-portions of the header plate 4 in the lengthwise direction, are linked by the deformation prevention beads 7 formed along the lengthwise direction of the header plate 4, and the deformation prevention beads 7 are not linked to the end-portion tube insertion hole 8.

As a result of eliminating the deformation prevention beads 7 from the end-portion tube insertion hole 8 as described above, flexural rigidity of the part of the header plate 4 is reduced, and deformation of the part absorbs effectively thermal deformation due to thermal stress generated larger in the flat tube 5 that should be inserted into the end-portion. Moreover, when the rim of the header plate 4 is caulked and fixed to the tank main body 6, the caulking load is supported with deformation prevention beads 7 provided for other tube insertion holes 3. Therefore, the function of tank deformation prevention by the deformation prevention beads 7 is ensured and, furthermore, thermal stress generated in the flat tube 5 inserted into the end-portion tube insertion hole 8 is reduced.

The invention according to the second aspect is an invention, wherein: the low burring 9 having low height is formed on the hole brim of the short side portions 1 of respective tube insertion holes 3, while the high burring 10 higher than the low burring 9 is formed on the long side portions 2 of respective tube insertion holes 3 except for the end-portion tube insertion hole 8, and the low burring 9 is formed on the hole brim of the end-portion tube insertion holes 8 throughout the entire circumference of the hole brims.

Hereby, the brazing joining line of the end-portion tube insertion hole 8 with the flat tube 5 in the short side portion 1 becomes the same as that in the long side portion 2. Therefore, stress concentration on the flat tube 5 in the short side portion 1 is relaxed.

The invention according to the third aspect is an invention, wherein: the low burring 9 is formed on the hole brims of both end-portions of the long side portions 2 of the end-portion tube insertion holes 8, while the high burring 10 is formed on the hole brim of long side portions 2 except for the both end-portions.

As a result of forming the high burring 10 on the long side portions 2, except for both end-portions thereof, of the end-portion tube insertion holes 8 in the same way as in other tube insertion holes 3 as described above, expansion workability for the flat tube 5 to be inserted there can be made almost the same as that for flat tubes 5 in other sites.

Meanwhile, into an opening end-portion of flat tube 5 to be inserted into each of insertion holes 3, 8, an expansion jig is inserted and the outer circumference thereof is enlarged, to thereby bring the outer circumference of the flat tube 5 into close contact with each of insertion holes 3, 8. In this condition, the flat tube 5 and the header plate 4 are brazed.

Moreover, the low burring 9 has been formed on hole brims at both end-portions of the long side portions 2 of the end-portion tube insertion hole 8, and therefore stress concentration on the flat tube 5 at a joining portion with the short side portion 1 thereof is relaxed.

The invention according to the fourth aspect is an invention, wherein: the tank main body 6 harbors a partition portion 19 that divides the same into a plurality of zones in a lengthwise direction; a dummy tube insertion hole 11 is formed at the header plate 4 located near the partition portion 19, with the end-portion tube insertion hole 8 and a plurality of tube insertion holes 3 in parallel on both sides of the dummy tube insertion hole 11; respective tube insertion holes 3, except for the dummy tube insertion hole 11 and an end-portion tube insertion hole 8 located at least to the first from the dummy tube insertion hole 11, are linked by the deformation prevention beads 7 formed along the lengthwise direction of the header plate 4; and formed such that the deformation prevention beads 7 do not exist for the dummy tube insertion hole 11 and the end-portion tube insertion hole 8 located at least to the first from the dummy tube insertion hole 11.

Hereby, the flexural rigidity of the header plate 4 corresponding to the part from which the deformation prevention beads 7 have been eliminated is reduced and the part deforms, to thereby absorb deformation due to thermal stress generated in the flat tube 5 inserted there.

Moreover, when the rim of the header plate 4 is caulked and fixed to the tank main body 6, the load to be added is supported with deformation prevention beads 7 formed at other tube insertion holes 3. Therefore, the function of tank deformation prevention is ensured and, furthermore, thermal stress generated in the flat tube 5 inserted into the end-portion tube insertion hole 8 is reduced.

The invention according to the fifth aspect is an invention, wherein: the high burring 10 has been provided for long side portions 2 of respective tube insertion holes 3, except for the dummy tube insertion hole 11 and the end-portion tube insertion hole 8, and the low burring 9 has been formed to the end-portion tube insertion hole 8 throughout the entire circumference thereof.

Hereby, the brazing joining line of the end-portion tube insertion hole 8 with the flat tube 5 in the short side portion 1 becomes similar to that in the long side portion 2. Therefore, stress concentration on the flat tube 5 in the short side portion 1 is relaxed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A illustrates an exploded perspective view of a tank structure of a heat exchanger in a first Example of the present invention, and FIG. 1B illustrates a main part perspective view of a header plate 4 thereof.

FIG. 2A illustrates a plan view of the header plate 4 in the first Example, FIG. 2B is a view seen along a B-B arrow in FIG. 2A, FIG. 2C is a view seen along a C-C arrow in FIG. 2A, FIG. 2D is a view seen along a D-D arrow in FIG. 2A, FIG. 2E is a perspective view of an end-portion tube insertion hole 8 seen from the underside of FIG. 2D, FIG. 2F is a view seen along an F-F arrow in FIG. 2A, and FIG. 2G is a perspective view seen from the underside of FIG. 2F.

FIG. 3 illustrates a perspective view of a header plate 4 of a tank structure of a heat exchanger in a second Example of the present invention.

FIG. 4A illustrates a plan view of the header plate 4 in the second Example, FIG. 4B—is a view seen along a B-B arrow in FIG. 4A, FIG. 4C is a view seen along a C-C arrow in FIG. 4A, FIG. 4D is a view seen along a D-D arrow in FIG. 4A, FIG. 4E is a perspective view of an end-portion tube insertion hole 8 seen from the underside of FIG. 4D, FIG. 4F is a view seen along an F-F arrow in FIG. 4A, and FIG. 4G is a perspective view seen from the underside of FIG. 4F.

FIG. 5 illustrates a front view of a tank structure of a heat exchanger in a third Example of the present invention.

FIG. 6A illustrates a main part plan view seen along a VI-VI arrow in FIG. 5, FIG. 6B is a cross-sectional view seen along a B-B arrow in FIG. 6A, FIG. 6C is a view seen along a C-C arrow in FIG. 6A, and FIG. 6D is a view seen along a D-D arrow in FIG. 6A.

FIG. 7 illustrates a plan view of a tank structure of a heat exchanger in a fourth Example.

FIG. 8A illustrates a main part plan view of a tank structure of a conventional heat exchanger, FIG. 8B is a B-B cross-sectional view of FIG. 8A, and FIG. 8C is a main part perspective view of the tank structure.

DESCRIPTION OF EMBODIMENTS

Next, embodiments of the present invention will be explained on the basis of the drawings, with Examples.

Heat exchangers in each of Examples explained below can be utilized as a radiator for cooling engine cooling water, a sub-radiator for cooling auxiliary machinery cooling water, or the like, as an example.

Example 1

FIG. 1 and FIG. 2 show the tank structure of the heat exchanger in the first Example of the present invention, in which FIG. 1A illustrates an exploded perspective view thereof, and FIG. 1B illustrates a perspective view of the header plate 4 for use in it. FIG. 2A illustrates a main part plan view of the header plate 4, and FIG. 2B to FIG. 2G illustrate cross-sectional views and perspective views, respectively, in FIG. 2A.

This heat exchanger is a heat exchanger, in which a flat tube 5 and a corrugated fin 12 are arranged alternately in parallel to form a core, end-portions of the flat tubes 5 are inserted into tube insertion holes 3 and end-portion tube insertion holes 8, respectively, provided for a bottom face 41 of the header plate 4, and their insertion portions are brazed and fixed.

Meanwhile, on both sides in the width direction of the core, a pair of side plates 15 (that on the left side is omitted) are arranged.

Inner circumferences of respective tube insertion holes 3 and two end-portion tube insertion holes 8 are formed slightly larger than an outer circumference of the flat tube 5 to be inserted there. When the flat tube 5 is inserted there later, an expansion jig is inserted into the tip portion thereof to expand it due to plastic deformation, and each hole brim of the tube insertion holes 3 and end-portion tube insertion holes 8 is brought into close contact with the end-portion of the flat tube 5.

Subsequently, the flat tube 5 and each of tube insertion holes 3 and end-portion tube insertion holes 8 are integrally brazed and fixed.

Next, a tank main body 6 is constituted of a synthetic resin material in this Example, in which a small flange 6a is formed on the outer circumference while tooth portions 6b are integrally protruded at regular intervals on the inner circumference. Between each of the tooth portions 6b, a tube escape portion 6c is formed, and marginal portions on a long axis side of tip portions of respective flat tubes 5 intrude into the tube escape portion. Further, a seal ring 13 is arranged between the tooth portion 6b and the inner surface of the tank main body 6.

Further, nail portions for caulking 16 provided at regular intervals for the header plate 4 are caulked to the outer circumference of the small flange 6a of the tank main body 6, to thereby form the tank structure of the heat exchanger. Meanwhile, in FIG. 1, an underside tank structure is omitted, but the underside tank is formed in the same way.

The header plate 4 of this Example has been formed into a planar rectangle. In the bottom face 41 of the header plate 4, a plurality of flat tube insertion holes 3 constituted of a pair of facing short side portions 1 and a pair of long side portions 2 linking both short side portions 1 have been drilled at regular intervals. At both end-portions in the lengthwise direction of the header plate 4 (in FIG. 1, FIG. 2A, the left side is omitted), in this Example, two end-portion tube insertion holes 8 constituted of a pair of short side portions 1 and a pair of long side portions 2 have been drilled in the same way as the tube insertion hole 3.

The inner circumferences of respective tube insertion holes 3 and end-portion tube insertion holes 8 are identical.

As shown in FIG. 1 and FIG. 2A, FIG. 2F, FIG. 2G, respective tube insertion holes 3 are connected with a pair of parallel deformation prevention beads 7 protruding toward the inner face side of the tank main body 6. However, in this Example, the deformation prevention beads 7 are not linked to two end-portion tube insertion holes 8.

Hereby, the surrounding of two end-portion tube insertion holes 8 is formed with the rigidity weaker than that in the surrounding of other tube insertion holes 3.

Moreover, in this Example, a burring protruding toward the inner face side of the tank main body 6 has been formed on hole brims of respective tube insertion holes 3 and end-portion tube insertion holes 8.

As shown in FIG. 2B, the high burring 10 having high height from the bottom face 41 of the header plate 4 to the tip of the burring has been formed on hole brims of a pair of long side portions 2 of respective tube insertion holes 3. Then, as shown in FIG. 2C, the low burring 9 having lower height than the high burring 10 has been formed on hole brims of a pair of short side portions 1 of respective tube insertion holes 3.

Further, the low burring 9 and the high burring 10 are connected smoothly.

Moreover, as shown in FIG. 2B and FIG. 2C, the low burring 9 having lower height than the high burring 10 has been formed on the hole brim of the end-portion tube insertion hole 8, and the height of the low burring 9 is kept at an approximately the same throughout the entire circumference of the end-portion tube insertion hole 8.

Hereby, the brazing joining line of the end-portion tube insertion hole 8 with the flat tube 5 in the short side portion 1 becomes similar to that in the long side portion 2, and therefore stress concentration on the flat tube 5 in this short side portion 1 is relaxed.

Example 2

Next, FIG. 3 and FIG. 4 show a second Example of the present invention. This Example is different from the first Example in the shape of a burring at the hole brim portions of two end-portion tube insertion holes 8 provided on both end-portions in the lengthwise direction of the header plate 4.

In this Example, as shown in FIG. 3 and FIG. 4C, the low burring 9 has been formed on hole brims of both end-portions in the long side portion 2 of the end-portion tube insertion hole 8, and, as shown in FIG. 3 and FIG. 4B, the high burring 10 has been formed on the hole brim of the long side portion 2 except for the both end-portions.

As a result of setting the same high burring 10 as that of other tube insertion holes 3 on the long side portion 2 of the end-portion tube insertion hole 8 except for both end-portions thereof as described above, the expansion workability of the flat tube 5 to be inserted there can be made almost the same as that of other flat tubes 5.

Moreover, on hole brims of both end-portions of the long side portion 2 of the end-portion tube insertion hole 8, the low burring 9 has been formed, and therefore stress concentration on the flat tube 5 at the joining portion with the short side portion 1 thereof is relaxed.

Example 3

Next, FIG. 5 and FIG. 6 show a third Example of the present invention, in which FIG. 5 is a front view thereof, FIG. 6A is a view seen along a VI-VI arrow in FIG. 5, FIG. 6B is a view seen along a B-B arrow in FIG. 6A, FIG. 6C is a view seen along a C-C arrow in FIG. 6A, and FIG. 6D is a view seen along a D-D arrow in FIG. 6A.

In this tank structure of a heat exchanger, in FIG. 5, upper and lower tanks constituted of a tank main body 6 and the header plate 4 have, via valley portions provided at a middle in the lengthwise direction, a first tank portion 17 and a second tank portion 18 on both sides thereof.

In the inside of the tank main body 6, as shown in FIG. 6B, a pair of partition portions 19 dividing the inside of the tank in the lengthwise direction into a plurality of zones are arranged facing each other, and end-portions thereof are joined with the inner surface of the header plate 4 via annular sealing rings 13 respectively. As shown in FIG. 6A, FIG. 6B, to this header plate 4, the dummy tube insertion hole 11 has been formed at a position corresponding to the vicinity of the partition portion 19, and, while sandwiching the dummy tube insertion hole 11, end-portion tube insertion holes 8 and tube insertion holes 3 are arranged one by one in a line. Further, the end-portion of the flat tube 5 has been inserted into each of insertion holes 3, 8, 11, and the flat tube 5 and each of insertion portions are brazed and fixed.

In this way, on both sides in the lengthwise direction of the dummy tube insertion hole 11, a first flow path 20 and a second flow path 21 each constituted of an assembly of a plurality of the flat tubes 5 are arranged, and different heat media circulate through these flow paths. As an example, engine cooling water may be made to circulate through the first flow path 20, and auxiliary machinery cooling water may be made to circulate through the second flow path 21.

Here, the dummy tube insertion hole 11, and the end-portion tube insertion hole 8 located to the first from the dummy tube insertion hole 11 are formed so that a deformation prevention beads 7 do not exist, as shown in FIG. 6A.

Further, on the hole brim portion of each tube insertion hole 3, the high burring 10 has been formed on the long side portion 2 thereof, and the low burring 9 (not shown) has been formed on the short side portion 1. Moreover, on hole brim portions of the end-portion tube insertion holes 8, the low burring 9 has been formed, respectively.

As such, the rim of the hole brim portion of the end-portion tube insertion hole 8 and the rim portion of the dummy tube insertion hole 11 are formed with the rigidity weaker than the rigidity of others, to thereby absorb stress generated in the flat tube 5 inserted into the dummy tube insertion hole 11 and the end-portion tube insertion hole 8.

Furthermore, the brazing joining line of the end-portion tube insertion hole 8 with the flat tube 5 in the short side portion 1 becomes similar to that in the long side portion 2, and therefore stress concentration on the flat tube 5 in the short side portion 1 is relaxed.

Example 4

Next, FIG. 7 is a plan view of a header plate 4 in a fourth Example of the present invention. This invention is different from the invention in third Example in that three end-portion tube insertion holes 8 have been formed, respectively, while neighboring both sides of the dummy tube insertion hole 11.

Thereby, stress concentration on the flat tube 5 in the short side portion 1 of the end-portion tube insertion hole 8 is even more relaxed.

REFERENCE SIGNS LIST

• 1: short side portion
• 2: long side portion
• 3: tube insertion hole
• 4: header plate
• 4 a: header plate
• 5: flat tube
• 6: tank main body
• 6 a: small flange
• 6 b: tooth portion
• 6 c: tube escape portion
• 7: deformation prevention bead
• 8: end-portion tube insertion hole
• 9: low burring
• 10: high burring
• 11: dummy tube insertion hole
• 12: corrugated fin
• 13: seal ring
• 14: tooth portion
• 15: side plate
• 16: nail portion for caulking
• 17: first tank portion
• 18: second tank portion
• 19: partition portion
• 20: first flow path
• 21: second flow path
• 22: burring
• 22 a: R portion
• 24: sealing surface
• 41: bottom face

Claims

1. A tank structure of a heat exchanger, comprising: a header plate having a plurality of flat tube insertion holes constituted of a pair of facing short side portions and a pair of long side portions linking the both short side portions;a plurality of flat tubes whose end-portions have been inserted into the respective tube insertion holes, and the insertion portions have been brazed;a tank main body to which a rim of the header plate is fixed by caulking, andthe short side portions of the plurality of tube insertion holes are located in a width direction of the header plate, and their tube insertion holes are arranged separately each other in a lengthwise direction of the header plate, wherein:respective tube insertion holes, except for end-portion tube insertion holes at least located at end-portions in the lengthwise direction, are linked by deformation prevention beads formed along the lengthwise direction of the header plate; andthe end-portion tube insertion holes are not linked by the deformation prevention beads.

2. The tank structure of a heat exchanger according to claim 1, wherein: a low burring having low height is formed on a hole brim of the short side portions of the respective tube insertion holes, while a high burring higher than the low burring is formed on the long side portions of the respective tube insertion holes except for at least the end-portion tube insertion holes; andthe low burring is formed on the hole brim of the end-portion tube insertion holes throughout the entire circumference thereof.

3. The tank structure of a heat exchanger according to claim 1, wherein: the low burring having low height is formed on a hole brim of the short side portions of the respective tube insertion holes, and the high burring higher than the low burring is formed on the long side portions of the respective tube insertion holes except for at least the end-portion tube insertion holes; andthe low burring is formed on hole brims of both end-portions of the long side portions of at least the end-portion tube insertion holes, and the high burring is formed on a hole brim of the long side portion except for both end-portions thereof.

4. A tank structure of a heat exchanger, including: a header plate having a plurality of flat tube insertion holes constituted of a pair of facing short side portions and a pair of long side portions linking the both short side portions;a plurality of flat tubes whose end-portions have been inserted into the respective tube insertion holes, and the insertion portions have been brazed; anda tank main body to which a rim of the header plate is fixed by caulking, in whichthe short side portions of the plurality of tube insertion holes are located in a width direction of the header plate, and their tube insertion holes are arranged separately each other in a lengthwise direction of the header plate,

5. The tank structure of a heat exchanger according to claim 4, wherein: the low burring is formed on short side portions of the respective tube insertion holes;the high burring is provided for the long side portions of the respective tube insertion holes except for the dummy tube insertion hole and the end-portion tube insertion hole located at least to the first from the dummy tube insertion hole; andthe low burring is provided for the end-portion tube insertion hole located at least to the first from the dummy tube insertion hole throughout the entire circumference thereof.