Heat Exchanger

Abstract

A gas cooler includes paired header tanks, and a plurality of parallel flat tubes disposed between the header tanks. Each header tank is configured such that an outside plate, an inside plate, and an intermediate plate are brazed together in layers. When the height of each flat tube is represented by T (mm), the distance between each of the opposite longitudinal end surfaces of each flat tube and an outer surface of the corresponding intermediate plate is represented by L (mm), and the width of each communication hole of the intermediate plate is represented by W (mm), relations L.gtoreq.0.7 T and 1.1 T.ltoreq.W.ltoreq.2.5 T are satisfied. This gas cooler can minimize an increase in pressure loss when supercritical refrigerant flows from the flat tubes into first refrigerant flow sections of the outside plate and flows from the first refrigerant flow sections into the flat tubes.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a heat exchanger, and more particularly to a heat exchanger that can be favorably used as a gas cooler or an evaporator of a supercritical refrigeration cycle in which a CO.sub.2 (carbon dioxide) refrigerant or a like supercritical refrigerant is used.

[0002] Herein and in the appended claims, the term "supercritical refrigeration cycle" means a refrigeration cycle in which a refrigerant on the high-pressure side is in a supercritical state; i.e., assumes a pressure in excess of a critical pressure. The term "supercritical refrigerant" means a refrigerant used in a supercritical refrigeration cycle.

[0003] The applicant of the present application has proposed a heat exchanger for use in a supercritical refrigeration cycle (refer to Japanese Patent Application Laid-Open (kokai) No. 2005-300135). The proposed heat exchanger includes a pair of header tanks disposed apart from each other and each having at least one header section; and a plurality of flat tubes disposed in parallel between the two header tanks and having opposite end portions connected to the respective header tanks. Each of the two header tanks is configured such that an outside plate, an inside plate, and an intermediate plate intervening between the outer and inside plates are brazed together in layers. The outside plate has at least one outwardly bulging portion extending in the longitudinal direction of the header tank and having an opening closed by the intermediate plate. The interior of the outwardly bulging portion serves as a first refrigerant flow section. The inside plate has a plurality of tube insertion holes in the form of through-holes formed in a region corresponding to the outwardly bulging portion and spaced apart from one another along the longitudinal direction thereof. Opposite end portions of the flat tubes are inserted through the respective tube insertion holes of the inside plates of the two header tanks, and are brazed to the inside plates. The intermediate plate has communication holes in the form of through-holes formed for allowing the tube insertion holes of the inside plate to communicate with the first refrigerant flow section of the outside plate. The communication holes communicate with one another via communication portions, each of which is formed between adjacent communication holes of the intermediate plate. The communication portions and portions of the communication holes which correspond to the communication portions form a second refrigerant flow section in the intermediate plate. The second refrigerant flow section communicates with the first refrigerant flow section of the outside plate and allows refrigerant to flow therethrough in the longitudinal direction of the header tank. The header section is formed by portions of the three plates which constitute the corresponding header thank and which correspond to the outwardly bulging portion. The width of the second refrigerant flow section of the intermediate plate is smaller than that of the first refrigerant flow section of the outside plate. The longitudinal opposite ends of the flat tubes are positioned within the second refrigerant flow sections of the intermediate plates; i.e., at thicknesswise intermediate portions of the intermediate plates.

[0004] In the heat exchanger described in the above-mentioned publication, supercritical refrigerant flows from each flat tube into the first refrigerant flow section of the outside plate of each header tank as follows. That is, at a portion of each flat tube which faces the second refrigerant flow section of the intermediate plate, the supercritical refrigerant flows directly into the second refrigerant flow section, and then flows into the first refrigerant flow section via the second refrigerant flow section. Meanwhile, at portions of each flat tube which face the inner surface of the outside plate, the supercritical refrigerant first flows into the corresponding communication hole of the intermediate plate, flows within the communication hole in the longitudinal direction thereof to enter the second refrigerant flow section, and then flows into the first refrigerant flow section via the second refrigerant flow section.

[0005] Further, in the heat exchanger described in the above-mentioned publication, the supercritical refrigerant flows from the first refrigerant flow section of the outside plate of each header tank to each flat tube as follows. That is, at a portion of each flat tube which faces the second refrigerant flow section of the intermediate plate, the supercritical refrigerant flows from the first refrigerant flow section directly into the flat tube via the second refrigerant flow section. Meanwhile, at portions of each flat tube which face the inner surface of the outside plate, the supercritical refrigerant first flows from the first refrigerant flow section into the second refrigerant flow section, and then flows within the communication hole in the longitudinal direction thereof to enter the flat tube.

[0006] At the portions of each flat tube which face the inner surface of the outside plate, pressure loss unavoidably increases for both the case where the supercritical refrigerant flows from the flat tube into the first refrigerant flow section of the outside plate of the header tank and the case where the supercritical refrigerant flows from the first refrigerant flow section of the outside plate of the header tank into the flat tube. As a result, the performance of the heat exchanger deteriorates.

SUMMARY OF THE INVENTION

[0007] An object of the present invention is to solve the above problem and to provide a heat exchanger which can minimize an increase in pressure loss at the time when supercritical refrigerant flows from each flat tube into a first refrigerant flow section of an outside plate and when the supercritical refrigerant flows from the first refrigerant flow section of the outside plate into the flat tube.

[0008] To fulfill the above object, the present invention comprises the following modes.

[0009] 1) A heat exchanger comprising a pair of header tanks disposed apart from each other and each having at least one header section; and a plurality of flat tubes disposed in parallel between the two header tanks and having opposite end portions connected to the respective header tanks, wherein each of the two header tanks is configured such that an outside plate, an inside plate, and an intermediate plate intervening between the outside and inside plates are brazed together in layers; the outside plate has at least one outwardly bulging portion extending in the longitudinal direction of the header tank and having an opening closed by the intermediate plate, the interior of the outwardly bulging portion serving as a first refrigerant flow section; the inside plate has a plurality of tube insertion holes in the form of through-holes formed in a region corresponding to the outwardly bulging portion and spaced apart from one another along the longitudinal direction of the inside plate; opposite end portions of the flat tubes are inserted through the respective tube insertion holes of the inside plates of the two header tanks, and are brazed to the inside plates; the intermediate plate has communication holes in the form of through-holes formed for allowing the tube insertion holes of the inside plate to communicate with the first refrigerant flow section of the outside plate, the communication holes communicating with one another via communication portions, each of which is formed between adjacent communication holes of the intermediate plate; the communication portions and portions of the communication holes which correspond to the communication portions form a second refrigerant flow section in the intermediate plate, the second refrigerant flow section communicating with the first refrigerant flow section of the outside plate and allowing refrigerant to flow therethrough in the longitudinal direction of the header tank; the header section is formed by portions of the three plates which constitute the corresponding header thank and which correspond to the outwardly bulging portion; and the longitudinal opposite ends of the flat tubes are positioned within the second refrigerant flow sections of the intermediate plates of the two header tanks, wherein when the height of each flat tube is represented by T (mm), the distance between each of the opposite longitudinal end surfaces of each flat tube and an outer surface of the corresponding intermediate plate is represented by L (mm), and the width of each communication hole of the intermediate plate is represented by W (mm), relations L.gtoreq.0.7 T and 1.1 T.ltoreq.W.ltoreq.2.5 T are satisfied.

[0010] 2) A heat exchanger according to par. 1), wherein a relation L.ltoreq.2.5 T is satisfied.

[0011] 3) A heat exchanger according to par. 1 or 2), wherein the first header tank includes a plurality of header sections arranged in the longitudinal direction of the header tank; the second header tank includes a header section(s), the number of which is one fewer than the number of header sections of the first header tank and which face adjacent two header sections of the first header tank; a header section at one end portion of the first header thank is an inlet header section having a refrigerant inlet, and a header section at the other end portion of the first header thank is an outlet header section having a refrigerant outlet.

[0012] 4) A heat exchanger according to par. 3), wherein the first header tank includes two header sections, and the second header tank includes a single header section.

[0013] According to the heat exchanger of par. 1), when the height of each flat tube is represented by T (mm), the distance between each of the opposite longitudinal end surfaces of each flat tube and an outer surface of the corresponding intermediate plate is represented by L (mm), and the width of each communication hole of the intermediate plate is represented by W (mm), the relations L.gtoreq.0.7 T and 1.1 T.ltoreq.W.ltoreq.2.5 T are satisfied. Therefore, it is possible to minimize an increase in pressure loss at the time when supercritical refrigerant first flows from portions of each flat tube which face the inner surface of the outside plate first into the corresponding communication hole of the intermediate plate, flows within the communication hole in the longitudinal direction thereof to enter the second refrigerant flow section, and then flows into the first refrigerant flow section via the second refrigerant flow section. Further, it is possible to minimize an increase in pressure loss at the time when the supercritical refrigerant first flows from the first refrigerant flow section into the second refrigerant flow section, flows within the communication hole in the longitudinal direction thereof, and then flow into portions of each flat tube which face the inner surface of the outside plate. Accordingly, deterioration in the performance of the heat exchanger can be suppressed. In addition, since a decrease in the joint area between the outside plate and the intermediate plate can be minimized, a drop in the withstanding pressure of the header tanks can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a perspective view showing the overall construction of a gas cooler to which the heat exchanger according to the present invention is applied;

[0015] FIG. 2 is a fragmentary view in vertical section showing the gas cooler of FIG. 1 as it is seen frontward from rear;

[0016] FIG. 3 is a perspective view showing a first header tank of the gas cooler of FIG. 1;

[0017] FIG. 4 is an enlarged view in section taken along line A-A of FIG. 2;

[0018] FIG. 5 is an enlarged view in section taken along line B-B of FIG. 2;

[0019] FIG. 6 is an enlarged view in section taken along line C-C of FIG. 5;

[0020] FIG. 7 is an exploded perspective view showing the first header tank of the gas cooler of FIG. 1;

[0021] FIG. 8 is an exploded perspective view showing a second header tank of the gas cooler of FIG. 1;

[0022] FIG. 9 is a cross-sectional view showing a flat tube of the gas cooler of FIG. 1;

[0023] FIG. 10 is a fragmentary enlarged view of FIG. 9;

[0024] FIG. 11 is a view showing a method of manufacturing the flat tube shown in FIG. 9;

[0025] FIG. 12 is a diagram showing the flow of a refrigerant through the gas cooler of FIG. 1;

[0026] FIG. 13 is a graph sowing results of Test Example 1; and

[0027] FIG. 14 is a graph sowing results of Test Example 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0028] An embodiment of the present invention will next be described in detail with reference to the drawings. This embodiment is implemented by applying a heat exchanger according to the present invention to a gas cooler of a supercritical refrigeration cycle.

[0029] In the following description, the upper, lower, left-hand, and right-hand sides of FIGS. 1 and 2 will be referred to as "upper," "lower," "left," and "right," respectively. Also, the downstream side of flow (represented by arrow X in FIG. 1) of air through air-passing clearances between adjacent flat tubes will be referred to as the "front," and the opposite side as the "rear."

[0030] Further, in the following description, the term "aluminum" encompasses aluminum alloys in addition to pure aluminum.

[0031] FIGS. 1 and 2 show the overall construction of a gas cooler to which the heat exchanger according to the present invention is applied. FIGS. 3 to 8 show the configuration of essential portions of the gas cooler. FIGS. 9 and 10 show a flat tube. FIG. 11 shows a method of manufacturing the flat tube. FIG. 12 shows the flow of a refrigerant through the gas cooler of FIG. 1.

[0032] With reference to FIG. 1, a gas cooler 1 of a supercritical refrigeration cycle wherein a supercritical refrigerant, such as CO.sub.2, is used includes two header tanks 2 and 3 extending vertically and spaced apart from each other in the left-right direction; a plurality of flat tubes 4 arranged in parallel between the two header tanks 2 and 3 and spaced apart from one another in the vertical direction in such a manner that the width direction of the flat tubes 4 coincides with the front-rear direction; corrugated fins 5 arranged in respective air-passing clearances between adjacent flat tubes 4 and at the outside of the upper-end and lower-end flat tubes 4 and each brazed to the adjacent flat tubes 4 or to the upper-end or lower-end flat tube 4; and side plates 6 of aluminum arranged externally of and brazed to the respective upper-end and lower-end corrugated fins 5. In the case of this embodiment, the header tank 2 at the right will be referred to as the "first header tank," and the header tank 3 at the left as the "second header tank."

[0033] As shown in FIGS. 2 to 7, the first header tank 2 is configured such that an outside plate 7, an inside plate 8, and an intermediate plate 9 intervening between the outside plate 7 and the inside plate 8 are brazed together in layers. The outside plate 7 and the inside plate 8 are each formed from a brazing sheet having a brazing material layer on each of opposite sides; herein, an aluminum brazing sheet. The intermediate plate 9 is formed from a bare metal material; herein, a bare aluminum material. The first header tank 2 is configured such that an inlet header section 10A and an outlet header section 10B are arranged in the vertical direction.

[0034] The outside plate 7 has a plurality of; herein, two, dome-like outwardly bulging portions 11A and 11B spaced apart from each other in the vertical direction. The outwardly bulging portions 11A and 11B have the same bulging height, length, and width. In the outside plate 7, a peripheral portion around a leftward-facing opening of each of the outwardly bulging portions 11A and 11B is brazed to the intermediate plate 9, whereby the intermediate plate 9 covers the leftward-facing openings of the outwardly bulging portions 11A and 11B. As a result, the interiors of the outwardly bulging portions 11A and 11B serve as first refrigerant flow sections 11a and 11b whose upper and lower ends are closed and through which refrigerant flows in the vertical direction.

[0035] A refrigerant inlet 12 is formed in a crest portion of the upper outwardly bulging portion 11A of the outside plate 7. An inlet member 13 of a metal; herein, a bare aluminum material, having a refrigerant inflow channel 14 in communication with the refrigerant inlet 12 is brazed to the outer surface of the outwardly bulging portion 11A by use of the brazing material on the outer surface of the outside plate 7. A refrigerant outlet 15 is formed in a crest portion of the lower outwardly bulging portion 11B. An outlet member 16 of a metal; herein, a bare aluminum material, having a refrigerant outflow channel 17 in communication with the refrigerant outlet 15 is brazed to the outer surface of the outwardly bulging portion 11B by use of the brazing material on the outer surface of the outside plate 7. The outside plate 7 is formed, by press work, from an aluminum brazing sheet having a brazing material layer on each of opposite sides.

[0036] A plurality of tube insertion holes 18 elongated in the front-rear direction are formed through the inside plate 8 and are vertically spaced apart from one another. An upper-half group of tube insertion holes 18 are formed within a vertical range corresponding to the upper outwardly bulging portion 11A of the outside plate 7. Similarly, a lower-half group of tube insertion holes 18 are formed within a vertical range corresponding to the lower outwardly bulging portion 11B. The tube insertion holes 18 have a front-to-rear length slightly longer than the front-to-rear width of the outwardly bulging portions 11A and 11B such that front and rear end portions thereof project outward beyond the front and rear ends, respectively, of the outwardly bulging portions 11A and 11B. Front and rear edge portions of the inside plate 8 have integrally formed respective cover walls 19. The cover walls 19 project rightward such that their ends reach the outer surface of the outside plate 7, and cover respective boundary portions between the outside plate 7 and the intermediate plate 9 along the overall length of the boundary portions. The cover walls 19 are brazed to the front and rear side surfaces, respectively, of the outside plate 7 and the intermediate plate 9. The projecting end of each of the cover walls 19 has a plurality of integrally formed engaging portions 21 which are vertically spaced apart from one another and which are engaged with the outer surface of the outside plate 7. The engaging portions 21 of the cover walls 19 are engaged with and brazed to the outer surface of the outside plate 7. Notably, before the three plates 7, 8, and 9 are layered, as indicated by a chain line in FIG. 7, the engaging portions 21 are not bent and extend straight from the cover walls 19. The straight engaging portions before being bent are denoted by 21A.

[0037] A pair of projecting portions 26 are integrally formed on the inside plate 8 at positions located at opposite sides of each tube insertion hole 18 with respect to the tube-width direction (vertical direction). The projecting portions 26 project outward with respect to the left-right direction; i.e., toward the intermediate plate 9, via inclined portions which are slightly inclined toward the tube-insertion-hole-18 side and toward the intermediate plate 9 (outward with respect to the left-right direction). The projecting portions 26 are formed by bending portions of the inside plate 8 corresponding to the opposite edges of each tube insertion hole 18 outward with respect to the left-right direction. The vertically outer surfaces of the projecting portions 26 are inclined outward with respect to the left-right direction and toward the tube-insertion-hole-18 side. The inside plate 8 is formed, by press work, from an aluminum brazing sheet having a brazing material layer on each of opposite sides.

[0038] The intermediate plate 9 has communication holes 22 in the form of through-holes for allowing the tube insertion holes 18 of the inside plate 8 to communicate with the interiors of the outwardly bulging portions 11A and 11B of the outside plate 7 and in a number equal to the number of the tube insertion holes 18. The communication holes 22 positionally coincide with the respective tube insertion holes 18 of the inside plate 8. The width of the communication holes 22 of the intermediate plate 9 with respect to the vertical direction is greater than that of the tube insertion holes 18 of the inside plate 8, except for the front and rear end portions of the communication holes. The projecting portions 26 on the upper and lower sides of each tube insertion hole 18 of the inside palate 8 enter the corresponding communication hole 22. Notably, the width of the front and rear end portions of each communication hole 22 with respect to the vertical direction is generally equal to that of each tube insertion hole 18. Stepped portions 25 are formed on the peripheral wall surface of each communication hole 22 of the intermediate plate 9 at opposite end portions thereof with respect to the hole-length direction (front and rear end portions). The stepped portions 25 are located at an intermediate position with respect to the plate-thickness direction of the intermediate plate 9, and project inward with respect to the hole-length direction of the communication hole 22. An end surface of the flat tube 4 abuts the stepped portions 25. The projecting height of the stepped portion 25 of the intermediate plate 9 from the peripheral wall surface of the communication hole 22 is determined so as not to cover a refrigerant channel 4a, which will be described later, of the flat tube 4. An upper-half group of tube insertion holes 18 of the inside plate 8 communicate with a first refrigerant flow section 11a within the upper outwardly bulging portion 11A of the outside plate 7 via an upper-half group of respective communication holes 22 of the intermediate plate 9. Similarly, a lower-half group of tube insertion holes 18 communicate with a first refrigerant flow section 11b within the lower outwardly bulging portion 11B of the outside plate 7 via a lower-half group of respective communication holes 22 of the intermediate plate 9. All of the communication holes 22 in communication with the upper-side first refrigerant flow section 11a of the outside plate 7 communicate with one another via communication portions 23, and all of the communication holes 22 in communication with the lower-side first refrigerant flow section 11b of the outside plate 7 communicate with one another via the communication portions 23. The communication portions 23 are formed by cutting off central portions (with respect to the front-rear direction) of portions of the intermediate plate 9 between the adjacent communication holes 22. As a result, the communication portions 23 which connect the connection holes 22 facing the first refrigerant flow section 11a of the outside plate 7 and the central portions (with respect to the front-rear direction) of the communication holes 22 (portions of the communication holes 22 corresponding to the communication portions 23) form in the intermediate plate 9 a second refrigerant flow section 9a which communicates with the first refrigerant flow section 11a of the outside plate 7 and through which refrigerant flows in the vertical direction. Similarly, the communication portions 23 which connect the connection holes 22 facing the first refrigerant flow section 11b of the outside plate 7 and the central portions (with respect to the front-rear direction) of the communication holes 22 (portions of the communication holes 22 corresponding to the communication portions 23) form in the intermediate plate 9 a second refrigerant flow section 9b which communicates with the first refrigerant flow section 11b of the outside plate 7 and through which refrigerant flows in the vertical direction. The right ends of the flat tubes 4 are positioned within the second refrigerant flow sections 9a and 9b of the intermediate plate 9; i.e., at a thicknesswise intermediate portion of the intermediate plate 9. The intermediate plate 9 is formed, by press work, from a bare aluminum material.

[0039] Respective portions of the three plates 7, 8, and 9 of the first header tank 2 which correspond to the outwardly bulging portions 11A and 11B form the inlet header section 10A and the outlet header section 10B. The two first refrigerant flow sections 11a and 11b of the outside plate 7 and the two second refrigerant flow sections 9a and 9b of the intermediate plate 9 form the internal refrigerant flow spaces of the inlet header section 10A and the outlet header section 10B.

[0040] The second header tank 3 has substantially the same construction as the first header tank 2, and like members and portions are designated by like reference numerals (see FIG. 2). The two header tanks 2 and 3 are disposed such that the respective inside plates 8 face each other.

[0041] As shown in FIGS. 1, 2, and 8, the outside plate 7 of the second header tank 3 has dome-like outwardly bulging portions provided in a number one fewer than the outwardly bulging portions 11A and 11B of the first header tank 2; herein, a single dome-like outwardly bulging portion 24, which extends from an upper end portion to a lower end portion of the outside plate 7 and is opposed to the outwardly bulging portions 11A and 11B of the first header tank 2. In the outside plate 7, a peripheral portion around a rightward-facing opening of the outwardly bulging portion 24 is brazed to the intermediate plate 9, whereby the intermediate plate 9 covers the rightward-facing opening of the outwardly bulging portion 24. As a result, the interior of the outwardly bulging portion 24 serves as a first refrigerant flow section 24a whose upper and lower ends are closed and through which refrigerant flows in the vertical direction. The outwardly bulging portion 24 has neither a refrigerant inlet nor a refrigerant outlet.

[0042] All of the tube insertion holes 18 of the inside plate 8 of the second header tank 3 are formed within a vertical range corresponding to the outwardly bulging portion 24 of the outside plate 7. All of the tube insertion holes 18 of the inside plate 8 communicate with the first refrigerant flow section 24a within the outwardly bulging portion 24 of the outside plate 7 via all of the communication holes 22 of the intermediate plate 9. All of the communication holes 22 of the intermediate plate 9 communicate with one another via communication portions 23, which are formed by cutting off central portions (with respect to the front-rear direction) of portions of the intermediate plate 9 between the adjacent communication holes 22. As a result, the communication portions 23 and the central portions (with respect to the front-rear direction) of the communication holes 22 (portions of the communication holes 22 corresponding to the communication portions 23) form in the intermediate plate 9 a second refrigerant flow section 9c which communicates with the refrigerant flow section 24a of the outside plate 7 and through which refrigerant flows in the vertical direction.

[0043] Respective portions of the three plates 7, 8, and 9 of the second header tank 3 which correspond to the outwardly bulging portion 24 form intermediate header sections 20, the number of which is one fewer than the two header sections 10A and 10B of the first header tank 2; here, a single intermediate header section 20, in such a manner that the intermediate header section 20 faces the two header sections 10A and 10B of the first header tank 2. The refrigerant flow section 24a of the outside plate 7 and the second refrigerant flow section 9c of the intermediate plate 9 form the internal refrigerant flow space of the intermediate header section 20.

[0044] The remaining portion of the second header tank 3 has the same structure as the first header tank 2, and like members and portions are denoted by the same reference numerals.

[0045] Here, the tube height, which is the thickness of each flat tube 4 as measured in the vertical direction, is represented by T (mm); the distance between each of the opposite longitudinal end surfaces of each flat tube 4 and the outer surface of the corresponding intermediate plate 9 is represented by L (mm); and the width of each communication hole 22 of the intermediate plate 9, except for the narrow portions at the front and rear ends thereof, is represented by W (mm). The gas cooler 1 must satisfy the relations L.gtoreq.0.7 T and 1.1 T.ltoreq.W.ltoreq.2.5 T (see FIG. 6).

[0046] When L<0.7 T, the clearance between each of the opposite ends of each flat tube 4 and the corresponding outside plate 7 becomes too small, so that the pressure loss increases at the inlet header section 10A of the first header tank 2 and a lower half portion of the intermediate header section 20 of the second header tank 3. That is, at the inlet header section 10A of the first header tank 2, the pressure loss increases when the supercritical refrigerant flows from the first refrigerant flow section 11a into the second refrigerant flow section 9a, flows within the communication holes 22 in the longitudinal direction thereof, and flows into the flat tubes 4 from portions of the first ends of the flat tubes 4 facing the inner surface of the corresponding outside plate 7. Similarly, at the lower half portion of the intermediate header section 20 of the second header tank 3, the pressure loss increases when the supercritical refrigerant flows from the first refrigerant flow section 24a into the second refrigerant flow section 9c, flows within the communication holes 22 in the longitudinal direction thereof, and flows into the flat tubes 4 from portions of the second ends of the flat tubes 4 facing the inner surface of the corresponding outside plate 7. When the pressure loss increases, the performance of the gas cooler 1 deteriorates. Further, the pressure loss increases at the outlet header section 10B of the first header tank 2 and an upper half portion of the intermediate header section 20 of the second header tank 3. That is, at the outlet header section 10B of the first header tank 2, the pressure loss increases when the supercritical refrigerant flows from the portions of the first ends of the flat tubes 4 facing the inner surface of the corresponding outside plate 7 into the communication holes 22 of the intermediate plate 9, flows within the communication holes 22 in the longitudinal direction thereof to enter the second refrigerant flow section 9b, and then flows into the first refrigerant flow section 11b via the second refrigerant flow section 9b. Similarly, at the upper half portion of the intermediate header section 20 of the second header tank 3, the pressure loss increases when the supercritical refrigerant flows from the portions of the second ends of the flat tubes 4 facing the inner surface of the corresponding outside plate 7 into the communication holes 22 of the intermediate plate 9, flows within the communication holes 22 in the longitudinal direction thereof to enter the second refrigerant flow section 9c, and then flows into the first refrigerant flow section 24a via the second refrigerant flow section 9c. When the pressure loss increases, the performance of the gas cooler 1 deteriorates. Preferably, the upper limit of L is 2.5 T. Even when L>2.5 T, the above-described effect of suppressing an increase in pressure loss is almost the same, and in addition, it is necessary to decrease the tube height T of the flat tubes 4 and/or increase the thickness of the intermediate plate 9. When the tube height T of the flat tubes 4 is decreased, an increase in the pressure loss of the flat tubes 4 themselves may possibly become greater than the degree of suppression of an increase in the pressure loss at the time when the supercritical refrigerant flows from the first refrigerant flow sections 11a and 24a into the flat tubes 4 and when the supercritical refrigerant flows from the flat tubes 4 into the first refrigerant flow sections 11b and 24a. When the thickness of the intermediate plate 9 is increased, the sizes and weights of the header tanks 2 and 3 increase.

[0047] W is preferably set to a large value, because when the value of W is small, pressure loss changes sharply with a change in L and when the value of W is large, pressure loss changes mildly with a change in L. However, when the value of W is excessively large, the joint area between the outer surface of the intermediate plate 9 and the inner surface of the outside plate 7 decreases, and the withstanding pressure of the header tanks 2 and 3 decreases. Further, when the value of W is excessively small, inserting the flat tubes 4 into the communication holes 22 becomes difficult. Accordingly, the width W of the communication holes 22 must be chosen within the range of 1.1 T to 2.5 T.

[0048] The first and second header tanks 2 and 3 are manufactured as follows. That is, after the three plates 7, 8, and 9 are stacked in layers, the straight engagement portions 21A are bent to form the engagement portions 21, which are engaged with the outside plate 7 so as to form a provisionally fixed assembly. After that, the provisionally fixed assembly is heated to a predetermined temperature so as to braze the three plates 7, 8, and 9 together by use of the brazing material layers of the outside plate 7 and the inside plate 8, braze the cover walls 19 to the front and rear end surfaces of the intermediate plate 9 and the outside plate 7, and further braze the engagement portions 21 to the outside plate 7. Thus are manufactured the two header tanks 2 and 3.

[0049] As shown in FIGS. 9 and 10, each flat tube 4 includes mutually opposed flat upper and lower walls 31 and 32 (a pair of flat walls); front and rear side walls 33 and 34 which extend over front and rear side ends, respectively, of the upper and lower walls 31 and 32; and a plurality of reinforcement walls 35 which are provided at predetermined intervals between the front and rear side walls 33 and 34 and extend longitudinally and between the upper and lower walls 31 and 32. By virtue of this structure, the flat tube 4 internally has a plurality of refrigerant channels 4a arranged in the width direction thereof.

[0050] The front side wall 33 has a dual structure and includes an outer side-wall-forming elongated projection 36 which is integrally formed with the front side end of the upper wall 31 in a downward raised condition and extends along the entire height of the flat tube 4; an inner side-wall-forming elongated projection 37 which is located inside the outer side-wall-forming elongated projection 36 and is integrally formed with the upper wall 31 in a downward raised condition; and an inner side-wall-forming elongated projection 38 which is integrally formed with the front side end of the lower wall 32 in an upward raised condition. The outer side-wall-forming elongated projection 36 is brazed to the two inner side-wall-forming elongated projections 37 and 38 and the lower wall 32 while a lower end portion thereof is engaged with a front side edge portion of the lower surface of the lower wall 32. The two inner side-wall-forming elongated projections 37 and 38 are brazed together while butting against each other. The rear side wall 34 is integrally formed with the upper and lower walls 31 and 32. A projection 38a is integrally formed on the tip end face of the inner side-wall-forming projection 38 of the lower wall 32 and extends in the longitudinal direction of the inner side-wall-forming projection 38 along the entire length thereof. A groove 37a is formed on the tip end face of the inner side-wall-forming elongated projection 37 of the upper wall 31 and extends in the longitudinal direction of the inner side-wall-forming elongated projection 37 along the entire length thereof. The projection 38a is press-fitted into the groove 37a.

[0051] The reinforcement walls 35 are formed such that reinforcement-wall-forming elongated projections 40 and 41, which are integrally formed with the upper wall 31 in a downward raised condition, and reinforcement-wall-forming elongated projections 42 and 43, which are integrally formed with the lower wall 32 in an upward raised condition, are brazed together while the reinforcement-wall-forming elongate projections 40 and 41 butt against the reinforcement-wall-forming elongated projections 43 and 42, respectively. The upper wall 31 has the reinforcement-wall-forming elongated projections 40 and 41, which are of different projecting heights and are arranged alternately in the front-rear direction. The lower wall 32 has the reinforcement-wall-forming elongated projections 42 and 43, which are of different projecting heights and are arranged alternately in the front-rear direction. The reinforcement-wall-forming elongated projections 40 of a long projecting height of the upper wall 31 and the respective reinforcement-wall-forming elongated projections 43 of a short projecting height of the lower wall 32 are brazed together. The reinforcement-wall-forming elongated projections 41 of a short projecting height of the upper wall 31 and the respective reinforcement-wall-forming elongated projections 42 of a long projecting height of the lower wall 32 are brazed together. Hereinafter, the reinforcement-wall-forming elongated projections 40 and 42 of a long projecting height of the upper and lower walls 31 and 32 are called the first reinforcement-wall-forming elongated projections. Similarly, the reinforcement-wall-forming elongated projections 41 and 43 of a short projecting height of the upper and lower walls 31 and 32 are called the second reinforcement-wall-forming elongated projections. A groove 44 (45) is formed on the tip end face of the second reinforcement-wall-forming elongated projection 41 (43) of the upper wall 31 (lower wall 32) and extends in the longitudinal direction of the second reinforcement-wall-forming elongated projection 41 (43) along the entire length thereof. A tip end portion of the first reinforcement-wall-forming elongated projection 42 (40) of the lower wall 32 (upper wall 31) is fitted into the groove 44 (45) of the second reinforcement-wall-forming elongated projection 41 (43) of the upper wall 31 (lower wall 32). While tip end portions of the first reinforcement-wall-forming elongated projections 40 and 42 of the upper and lower walls 31 and 32, respectively, are fitted into the respective grooves 45 and 44, the reinforcement-wall-forming elongated projections 40 and 43 are brazed together, and the reinforcement-wall-forming elongated projections 41 and 42 are brazed together.

[0052] The flat tube 4 is manufactured by use of a tube-forming metal sheet 50 as shown in FIG. 11(a). The tube-forming metal sheet 50 is formed, by rolling, from an aluminum brazing sheet having a brazing material layer on each of opposite sides. The tube-forming metal sheet 50 includes a flat upper-wall-forming portion 51 (flat-wall-forming portion); a flat lower-wall-forming portion 52 (flat-wall-forming portion); a connection portion 53 connecting the upper-wall-forming portion 51 and the lower-wall-forming portion 52 and adapted to form the rear side wall 34; the inner side-wall-forming elongated projections 37 and 38, which are integrally formed with the side ends of the upper-wall-forming and lower-wall-forming portions 51 and 52 opposite the connection portion 53 in an upward raised condition and which are adapted to form an inner portion of the front side wall 33; an outer side-wall-forming-elongated-projection forming portion 54, which extends outward from the side end of the upper-wall-forming portion 51 opposite the connection portion 53; and a plurality of reinforcement-wall-forming elongated projections 40, 41, 42, and 43, which are integrally formed with the upper-wall-forming and lower-wall-forming portions 51 and 52 in an upward raised condition and which are arranged at predetermined intervals in the width direction of the tube-forming metal sheet 50. The first reinforcement-wall-forming elongated projections 40 of the upper-wall-forming portion 51 and the second reinforcement-wall-forming elongated projections 43 of the lower-wall-forming portion 52 are located symmetrically with respect to the centerline of the width direction of the connection portion 53. Similarly, the second reinforcement-wall-forming elongated projections 41 of the upper-wall-forming portion 51 and the first reinforcement-wall-forming elongated projections 42 of the lower-wall-forming portion 52 are located symmetrically with respect to the centerline of the width direction of the connection portion 53. The projection 38a is formed on the tip end face of the inner side-wall-forming elongated projection 38 of the lower-wall-forming portion 52, and the groove 37a is formed on the tip end face of the inner side-wall-forming elongated projection 37 of the upper-wall-forming portion 51. The groove 44 (45), into which a tip end portion of the first reinforcement-wall-forming elongated projection 42 (40) of the lower-wall-forming portion 52 (upper-wall-forming portion 51) is fitted, is formed on the tip end face of the second reinforcement-wall-forming elongated projection 41 (43) of the upper-wall-forming portion 51 (lower-wall-forming portion 52).

[0053] The inner side-wall-forming elongated projections 37 and 38 and the reinforcement-wall-forming elongated projections 40, 41, 42, and 43 are integrally formed, by rolling, on one side of the aluminum brazing sheet whose opposite sides are clad with a brazing material, whereby a brazing material layer (not shown) is formed on the opposite side surfaces and tip end faces of the inner side-wall-forming elongated projections 37 and 38 and the reinforcement-wall-forming elongated projections 40, 41, 42, and 43; on the peripheral surfaces of the grooves 44 and 45 of the second reinforcement-wall-forming elongated projections 41 and 43; and on the vertically opposite surfaces of the upper-wall-forming and lower-wall-forming portions 51 and 52 and the outer side-wall-forming-elongated-projection forming portion 54.

[0054] The tube-forming metal sheet 50 is gradually folded at opposite side edges of the connection portion 53 by a roll forming process (see FIG. 11(b)) until a hairpin form is assumed. The inner side-wall-forming elongated projections 37 and 38 are caused to butt against each other; tip end portions of the first reinforcement-wall-forming elongated projections 40 and 42 are fitted into the respective grooves 45 and 44 of the second reinforcement-wall-forming elongated projections 43 and 41; and the projection 38a is press-fitted into the groove 37a.

[0055] Next, the outer side-wall-forming-elongated-projection forming portion 54 is folded along the outer surfaces of the inner side-wall-forming elongated projections 37 and 38, and a tip end portion thereof is deformed so as to be engaged with the lower-wall-forming portion 52, thereby yielding a folded member 55 (see FIG. 11(c)).

[0056] Subsequently, the folded member 55 is heated at a predetermined temperature so as to braze together tip end portions of the inner side-wall-forming elongated projections 37 and 38; to braze together tip end portions of the first and second reinforcement-wall-forming elongated projections 40 and 43; to braze together tip end portions of the first and second reinforcement-wall-forming elongated projections 42 and 41; and to braze the outer side-wall-forming-elongated-projection forming portion 54 to the inner side-wall-forming elongated projections 37 and 38 and to the lower-wall-forming portion 52. Thus is manufactured the flat tube 4.

[0057] While opposite end portions of the flat tubes 4 are inserted through the respective tube insertion holes 18 of the inside plates 8 and into the respective communication holes 22 of the intermediate plates 9 of the header tanks 2 and 3, and end surfaces of the opposite end portions abut the respective stepped portions 25 of the intermediate plates 9, the opposite end portions of the flat tubes 4 are brazed to the respective peripheral wall surfaces of the tube insertion holes 18 of the inside plates 8 and to the respective peripheral wall surfaces of the front and rear narrow end portions of the communication holes 22 of the intermediate plates 9 by utilization of the brazing material layers of the inside plates 8 and the brazing material layers of the tube-forming metal sheets 50.

[0058] Accordingly, right end portions of an upper-half group of flat tubes 4 are connected to the first header tank 2 so as to communicate with the interior of the upper outwardly bulging portion 11A, and left end portions are connected to the second header tank 3 so as to communicate with the interior of the outwardly bulging portion 24. Also, right end portions of a lower-half group of flat tubes 4 are connected to the first header tank 2 so as to communicate with the interior of the lower outwardly bulging portion 11B, and left end portions are connected to the second header tank 3 so as to communicate with the interior of the outwardly bulging portion 24.

[0059] Each of the corrugated fins 5 is made in a wavy form from a brazing sheet; herein, an aluminum brazing sheet, having a brazing material layer on each of opposite sides.

[0060] The gas cooler 1 is manufactured by the steps of: preparing the aforementioned two provisionally fixed assemblies to be manufactured into the header tanks 2 and 3, a plurality of the aforementioned folded members 55, and a plurality of corrugated fins 5; arranging the two provisionally fixed assemblies in such a manner as to be spaced apart from each other with the inside plates 8 facing each other; arranging alternately the folded members 55 and the corrugated fins 5; inserting opposite end portions of the folded members 55 through the respective tube insertion holes 18 of the inside plates 8 and into the respective communication holes 22 of the intermediate plates 9 of the two provisionally fixed assemblies, and causing the end surfaces of the opposite end portions to abut the respective stepped portions 25 of the intermediate plate 9; arranging the side plates 6 externally of the respective opposite-end corrugated fins 5; arranging the inlet member 13 and the outlet member 16 on the outwardly bulging portions 11A and 11B, respectively, of the outside plate 7 used to form the first header tank 2; and brazing necessary portions of the provisionally fixed assemblies as mentioned above to thereby form the header tanks 2 and 3, brazing necessary portions of the folded members 55 as mentioned above to thereby form the flat tubes 4, brazing the flat tubes 4 to the header tanks 2 and 3, brazing the corrugated fins 5 to the flat tubes 4, brazing the side plates 6 to the respective corrugated fins 5, and brazing the inlet member 13 and the outlet member 16 to the outwardly bulging portions 11A and 11B, respectively.

[0061] The gas cooler 1, together with a compressor, an evaporator, a pressure-reducing device, and an intermediate heat exchanger for performing heat exchange between refrigerant from the gas cooler and refrigerant from the evaporator, constitutes a supercritical refrigeration cycle. The refrigeration cycle is installed in a vehicle, for example, in an automobile, as a car air conditioner.

[0062] As shown in FIG. 12, in the gas cooler 1 described above, CO.sub.2 from a compressor flows through the refrigerant inflow channel 14 of the inlet member 13 and enters the first refrigerant flow section 11a of the upper outwardly bulging portion 11A of the first header tank 2 through the refrigerant inlet 12. Then, the CO.sub.2 dividedly flows into the refrigerant channels 4a of all the flat tubes 4 in communication with the upper outwardly bulging portion 11A via the upper-side second refrigerant flow section 9a and the communication holes 22 of the intermediate plate 9. The CO.sub.2 in the refrigerant channels 4a flows leftward through the refrigerant channels 4a and enters the first refrigerant flow section 24a of the outwardly bulging portion 24 of the second header tank 3. The CO.sub.2 in the first refrigerant flow section 24a of the outwardly bulging portion 24 flows downward through the first refrigerant flow section 24a and through the second refrigerant flow section 9c of the intermediate plate 9; dividedly flows into the refrigerant channels 4a of all the flat tubes 4 in communication with the lower outwardly bulging portion 11B via the second refrigerant flow section 9c and the communication holes 22; flows rightward through the refrigerant channels 4a; and enters the first refrigerant flow section 11b of the lower outwardly bulging portion 11B via the lower-side second refrigerant flow section 9b and the communication holes 22 of the intermediate plate 9 of the first header tank 2. Subsequently, the CO.sub.2 flows out of the gas cooler 1 via the refrigerant outlet 15 and the refrigerant outflow channel 17 of the outlet member 16. While flowing through the refrigerant channels 4a of the flat tubes 4, the CO.sub.2 is subjected to heat exchange with the air flowing through the air-passing clearances in the direction of arrow X shown in FIGS. 1 and 12, thereby being cooled.

[0063] Next, there will be described text examples performed by use of the above-described gas cooler 1.

TEST EXAMPLE 1

[0064] Three types of gas coolers 1 which differ in the width W of the communication holes 22 of the intermediate plate 9 were prepared, and, while the distance L between each of the opposite longitudinal end surfaces of each flat tube 4 and the inner surface of the corresponding outside plate 7 was changed to various values, the relation between the above-described distance L and pressure loss was investigated for the gas cooler 1 of each type. Notably, the gas coolers 1 are identical in terms of the dimensions of the components and portions, other than the distance L. FIG. 13 shows the results of the test.

[0065] The results shown in FIG. 13 demonstrate that in each gas cooler 1, an increase in pressure loss can be suppressed when the relation L.ltoreq.0.7 T is satisfied, where T represents the tube height in mm of the flat tubes 4. Further, the results demonstrate that the smaller the width W of the communication holes 22 of the intermediate plate 9, the greater the degree to which a change in pressure loss with a change in L becomes sharp, and the greater the width W, the greater the degree to which a change in pressure loss with a change in L becomes mild.

TEST EXAMPLE 2

[0066] Gas coolers 1 were fabricated, while the width W of the communication holes 22 of the intermediate plate 9 was changed to various values, and the relation between the width W and the withstanding pressure of the header tanks 2 and 3 was investigated. The gas coolers 1 are identical in terms of the dimensions of the components and portions, other than the width W. FIG. 14 shows the results of the test.

[0067] The results shown in FIG. 14 demonstrate that the withstanding pressure of the header tanks 2 and 3 increases when W.ltoreq.2.5 T.

[0068] In the above-described embodiment, the heat exchanger of the present invention is applied to a gas cooler of a supercritical refrigeration cycle. However, the heat exchanger of the present invention may be applied to an evaporator of the above-mentioned supercritical refrigeration cycle. This evaporator, together with a compressor, a gas cooler, a pressure-reducing device, and an intermediate heat exchanger for performing heat exchange between refrigerant from the gas cooler and refrigerant from the evaporator, constitutes a supercritical refrigeration cycle which uses a supercritical refrigerant such as CO.sub.2. This refrigeration cycle is installed in a vehicle, for example, in an automobile, as a car air conditioner.

[0069] In the above-described embodiment, each of the header tanks 2 and 3 is formed by stacking three types of plates; i.e., the outside plate 7, the inside plate 8, and the intermediate plate 9, one sheet each. However, the present invention is not limited thereto, and two or more intermediate plates 9 may be stacked.

[0070] Although CO.sub.2 is used as a supercritical refrigerant of a supercritical refrigeration cycle in the above-described embodiments, the refrigerant is not limited thereto, but ethylene, ethane, nitrogen oxide, or the like may be alternatively used.

[0071] The above-described embodiment uses, for forming the flat tube 4, a folded member 55 which is formed by bending a tube-forming metal sheet in the form of an aluminum brazing sheet having a brazing material layer on each of opposite sides. However, the present invention is not limited thereto. For example, an aluminum extrudate having a brazing material layer on its outer surface may be used to form the flat tube 4.

Claims

1. A heat exchanger comprising a pair of header tanks disposed apart from each other and each having at least one header section; and a plurality of flat tubes disposed in parallel between the two header tanks and having opposite end portions connected to the respective header tanks, wherein each of the two header tanks is configured such that an outside plate, an inside plate, and an intermediate plate intervening between the outside and inside plates are brazed together in layers; the outside plate has at least one outwardly bulging portion extending in the longitudinal direction of the header tank and having an opening closed by the intermediate plate, the interior of the outwardly bulging portion serving as a first refrigerant flow section; the inside plate has a plurality of tube insertion holes in the form of through-holes formed in a region corresponding to the outwardly bulging portion and spaced apart from one another along the longitudinal direction of the inside plate; opposite end portions of the flat tubes are inserted through the respective tube insertion holes of the inside plates of the two header tanks, and are brazed to the inside plates; the intermediate plate has communication holes in the form of through-holes formed for allowing the tube insertion holes of the inside plate to communicate with the first refrigerant flow section of the outside plate, the communication holes communicating with one another via communication portions, each of which is formed between adjacent communication holes of the intermediate plate; the communication portions and portions of the communication holes which correspond to the communication portions form a second refrigerant flow section in the intermediate plate, the second refrigerant flow section communicating with the first refrigerant flow section of the outside plate and allowing refrigerant to flow therethrough in the longitudinal direction of the header tank; the header section is formed by portions of the three plates which constitute the corresponding header thank and which correspond to the outwardly bulging portion; and the longitudinal opposite ends of the flat tubes are positioned within the second refrigerant flow sections of the intermediate plates of the two header tanks, wherein when the height of each flat tube is represented by T (mm), the distance between each of the opposite longitudinal end surfaces of each flat tube and an outer surface of the corresponding intermediate plate is represented by L (mm), and the width of each communication hole of the intermediate plate is represented by W (mm), relations L.gtoreq.0.7 T and 1.1 T.ltoreq.W.ltoreq.2.5 T are satisfied.

2. A heat exchanger according to claim 1, wherein a relation L.ltoreq.2.5 T is satisfied.

3. A heat exchanger according to claim 1 or 2, wherein the first header tank includes a plurality of header sections arranged in the longitudinal direction of the header tank; the second header tank includes a header section(s), the number of which is one fewer than the number of the header sections of the first header tank and which faces adjacent two header sections of the first header tank; a header section at one end portion of the first header thank is an inlet header section having a refrigerant inlet, and a header section at the other end portion of the first header thank is an outlet header section having a refrigerant outlet.

4. A heat exchanger according to claim 3, wherein the first header tank includes two header sections, and the second header tank includes a single header section.