Heat Exchanger

TECHNICAL FIELD

The invention relates to a heat exchanger heated by a combustion gas, which includes a rectangular cylindrical casing an inside of which the combustion gas flows in, on a premise that two orthogonal directions crossing a flow direction of the combustion gas in the casing are defined as an X-axis direction and a Y-axis direction, respectively, a plurality of heat absorbing fins stacked and arranged in the X-axis direction in the casing, a plurality of heat absorbing tubes piercing through the heat absorbing fins and side plates of both sides in the X-axis direction of the casing, and an inside of which a fluid to be heated flows in, and a connecting portion connecting the heat absorbing tubes in series at outsides of the side plates of both sides in the X-axis direction of the casing.

BACKGROUND ART

Conventionally, in such a heat exchanger of this kind, there has been known the heat exchanger in which the heat absorbing tubes are divided into two stages and arranged in the flow direction of the combustion gas, a meander passage connecting a plurality of the heat absorbing tubes in each stage from an outermost side of one side in the Y-axis direction to an outermost side of the other side in the Y-axis direction is constituted by a plurality of the heat absorbing tubes in each stage and the connecting portion for each stage, and one of the stages in which the meander passage is provided and which is positioned at an upstream side in the flow direction of the combustion gas is defined as a first stage and the other of the stages in which the meander passage is provided and which is positioned at a downstream side in the flow direction of the combustion gas is defined as a second stage (Patent document No.1, for example). In the heat exchanger, the heat absorbing tubes at upstream and downstream ends of the meander passage in the first stage are made to be positioned at the outermost side of the one side in the Y-axis direction and at the outermost side of the other side in the Y-axis direction, respectively. Then, the heat absorbing tube at the downstream end in the first stage and the heat absorbing tube at the upstream end, which is positioned at the outermost side of the other side in the Y-axis direction, of the meander passage in the second stage are connected to each other, allowing the fluid to be heated to flow from the meander passage in the first stage to the meander passage in the second passage.

REFERENCE

Patent document No.1: JP2014-137208 A

SUMMARY OF INVENTION
Technical Problem

Here, when the heat absorbing tubes thermally expand in the X-axis direction as a longitudinal direction thereof, the side plates of each side in the X-axis direction of the casing, through which the heat absorbing tubes pierce and on which the heat absorbing tubes are brazed, are pressed in the X-axis direction. In the above conventional heat exchanger, the heat absorbing tube (the heat absorbing tube in which the cold fluid to be heated flows) at the upstream end of the meander passage in the first stage and the heat absorbing tube (the heat absorbing tube from which the heated fluid is discharged) at the downstream end of the meander passage in the second stage are positioned at the outermost side of the one side in the Y-axis direction, respectively, and come close to each other. Temperature difference between these heat absorbing tubes is the largest among those between any one of the heat absorbing tubes of the meander passage in the first stage and any one of the heat absorbing tubes of the meander passage in the second stage. Therefore, due to the difference in the amount of thermal expansion in such two heat absorbing tubes, a large stress is applied to the side plates of each side in the X-axis direction of the casing. Then, cracks appear in the side plates due to repetition of the stress applied to the side plates.

In the light of the above problem, the invention provides a heat exchanger of which durability is improved by reducing the stress acting on the side plates of each side in the X-axis direction of the casing.

Solution to Problem

In order to solve the above problem, the invention presupposes a heat exchanger heated by a combustion gas, which includes a rectangular cylindrical casing an inside of which the combustion gas flows in; on a premise that two orthogonal directions crossing a flow direction of the combustion gas in the casing are defined as an X-axis direction and a Y-axis direction, respectively; a plurality of heat absorbing fins stacked and arranged in the

X-axis direction in the casing; a plurality of heat absorbing tubes piercing through the heat absorbing fins and side plates of both sides in the X-axis direction of the casing, and an inside of which a fluid to be heated flows in; and a connecting portion connecting the heat absorbing tubes in series at outsides of the side plates of both sides in the X-axis direction of the casing, wherein the heat absorbing tubes are divided into two stages and arranged in the flow direction of the combustion gas, a meander passage connecting a plurality of the heat absorbing tubes in each stage from an outermost side of one side in the Y-axis direction to an outermost side of the other side in the Y-axis direction is constituted by a plurality of the heat absorbing tubes in each stage and the connecting portion for each stage, the meander passage is provided in each of the two stages consisting of a first stage at the upstream side in the flow direction of the combustion gas and a second stage at the downstream side in the flow direction of the combustion gas, the heat absorbing tubes at the upstream and downstream ends of the meander passage in one of the first and second stages are positioned at the outermost sides of the one and the other sides in the Y-axis direction, respectively, and the heat absorbing tube at the downstream end in the one of the first and second stages and the heat absorbing tube at the upstream end, which is positioned at the outermost side of the other side in the Y-axis direction, of the meander passage in the other of the first and second stages are connected to each other, allowing the fluid to be heated to flow from the meander passage in the one of the first and second stages to the meander passage in the other of the first and second stages. In the heat exchanger, a distance between the heat absorbing tube at the upstream end positioned at the outermost side of the one side in the Y-axis direction of the meander passage in the one of the stages and the heat absorbing tube at the downstream end positioned at the outermost side of the one side in the Y-axis direction of the meander passage in the other of the stages is longer than a respective distance between each heat absorbing tube other than the heat absorbing tube at the upstream end of the meander passage in the one of the stages and such each heat absorbing tube other than the heat absorbing tube at the downstream end of the meander passage in the other of the stages as is the closest to each heat absorbing tube other than the heat absorbing tube at the upstream end of the meander passage in the one of the stages.

According to the invention, though the temperature difference between the heat absorbing tube at the upstream end of the meander passage in the one of the stages and the absorbing tube at the downstream end of the meander passage in the other of the stages is the largest, since the distance between such two heat absorbing tubes is made longer, a stress acting on the side plates of each side in the X-axis direction of the casing due to difference in the amount of the thermal expansion between such two heat absorbing tubes can be reduced. As a result, durability can be improved.

In addition, in the invention, it is desirable that a direction for lengthening the distance between the heat absorbing tube at the upstream end of the meander passage in the one of the stages and the heat absorbing tube at the downstream end of the meander passage in the other of the stages is set to the flow direction of the combustion gas. According to this, there is no need to increase a dimension in the Y-axis direction of the heat exchanger, and oversizing of the heat exchanger can be avoided.

Further, in the invention, in a case where the one of the stages is the first stage and the other of the stages is the second stage, it is desirable that notches are provided with each heat absorbing fin at portions positioned at a more upstream side in the flow direction of the combustion gas than each heat absorbing tube of the meander passage in the second stage, the notches corresponding to the heat absorbing tube at the downstream end of the meander passage in the second stage are defined as first notches and the notches corresponding to each heat absorbing tube other than the heat absorbing tube at the downstream end of the meander passage in the second stage are defined as second notches, and the first notches are formed larger than the second notches. According to this, heat transfer to the heat absorbing tube, of which temperature becomes the highest, at the downstream end of the meander passage in the second stage through each heat absorbing fin is reduced by the larger first notches. Therefore, the temperature difference between the heat absorbing tube at the upstream end of the meander in the first stage and the heat absorbing tube at the downstream end of the meander passage in the second stage is made to decrease, and the stress acting on the side plates of each side in the X-axis direction of the casing can be more reduced.

In addition, in the invention, in a case where the one of the stages is the first stage and the other of the stages is the second stage, it is desirable that a ventilation resistant portion which suppresses the combustion gas from being directed to the heat absorbing tube at the downstream end of the meander passage in the second stage is provided. According to this, it becomes difficult for the combustion gas to flow around the heat absorbing tube, of which temperature becomes the highest, at the downstream end of the meander passage in the second stage, and heat transfer from the combustion gas to such heat absorbing tube can be reduced. Therefore, the temperature difference between the heat absorbing tube at the upstream end of the meander passage in the first stage and the heat absorbing tube at the downstream end of the meander passage in the second stage is made to decrease, and the stress acting on the side plates of each side in the X-axis direction of the casing can be more reduced.

Further, in the invention, in a case where the one of the stages is the first stage and the other of the stages is the second stage, an exhaust gas gathering portion covering an opening at the downstream end in the flow direction of the combustion gas of the casing is provided, and an exhaust port for discharging the combustion gas is opened at the exhaust gas gathering portion, it is desirable that the exhaust port is provided at a deviated portion of the other side in the Y-axis direction of the exhaust gas gathering portion. According to this, the combustion gas does not gather in the vicinity of the heat absorbing tube, of which temperature becomes the highest, at the downstream end of the meander passage in the second stage, and the heat transfer from the combustion gas to such heat absorbing tube can be reduced. Therefore, the temperature difference between the heat absorbing tube at the upstream end of the meander passage in the first stage and the heat absorbing tube of the meander passage at the downstream end of the meander passage in the second stage is made to decrease, and the stress acting on the side plates of each side in the X-axis direction of the casing can be more reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a heat exchanger of an embodiment of the invention.

FIG. 2 is a perspective view of the heat exchanger of the embodiment, viewed from an opposite direction to FIG. 1.

FIG. 3 is a section cut along a III-III line of FIG. 2.

FIG. 4 is a cutaway side view cut along a IV-IV line.

FIG. 5 is an enlarged view of one portion of FIG. 3.

FIG. 6 is an enlarged perspective view of a major portion of each heat absorbing fin provided with the heat exchanger of the embodiment.

DESCRIPTION OF EMBODIMENTS

Referring to FIGS. 1-4, a heat exchanger A of an embodiment of the invention includes a casing 1 having one end (an upper end in FIGS. 3, 4) on which a burner, not shown, is mounted. A combustion gas generated by combustion of an air-fuel mixture discharged from the burner flows in the casing 1, and the heat exchanger A is heated by the combustion gas. On a premise that two orthogonal directions crossing a flow direction of the combustion gas in the casing 1, which is shown by arrows “a” in FIGS. 3, 4, are defined as an X-axis direction and a Y-axis direction, respectively, the casing 1 has a rectangular cylindrical shape with side plates 11, 12 of both sides in the X-axis direction and side plates 13, 14 of both sides in the Y-axis direction. The heat exchanger A also includes a plurality of heat absorbing fins 2 stacked and arranged in the X-axis direction in the casing 1, a plurality of heat absorbing tubes 31 piercing through the heat absorbing fins 2 and the side plates 11, 12 of both sides in the X-axis direction of the casing 1, and into which a fluid to be heated such as water or the like flows, a connecting portion 32 connecting the heat absorbing tubes 31 in series at outsides of the side plates 11, 12 of both sides in the X-axis direction of the casing 1, and an exhaust gas gathering portion 4 covering an opening 1a at a downstream end (a lower end in FIGS. 3, 4) in the flow direction of the combustion gas of the casing 1 and having an exhaust port 41 opened for discharging the combustion gas. Meanwhile, the combustion gas discharged from the exhaust port 41 is introduced into a sub heat exchanger with latent heat recovery, which is not shown. In addition, each heat absorbing tube 31 is brazed on each heat absorbing fin 2 and each of the side plates 11, 12 in a piercing state.

The heat absorbing tubes 31, as specified in FIG. 3, are divided into two stages and arranged in the flow direction of the combustion gas. In particular, six heat absorbing tubes 31, i.e., #11-#16, from the heat absorbing tube 31 positioned at an outermost side of one side in the Y-axis direction to the heat absorbing tube 31 positioned at an outermost side of the other side in the Y-axis direction are arranged in a first stage which is a stage at an upstream side in the flow direction of the combustion gas, and five heat absorbing tubes 31, i.e., #21-#25 from the heat absorbing tube 31 positioned at the outermost side of the one side in the Y-axis direction to the heat absorbing tube 31 positioned at the outermost side of the other side in the Y-axis direction are arranged in a second stage which is a stage at a downstream side in the flow direction of the combustion gas. Each heat absorbing tube 31 in the second stage is arranged with a positional relationship such that a Y-axis direction center of such each heat absorbing tube 31 coincides with a Y-axis direction center between any two of the heat absorbing tubes 31, 31 adjacent to each other in the first stage. Meanwhile, each heat absorbing fin 2 is provided with each swelling portion 21 at a Y-axis direction position coincident with each heat absorbing tube 31 in the first stage, which swells to the upstream side in the flow direction of the combustion gas. Then, each heat absorbing tube 31 in the first stage pierces through each swelling portion 21.

In addition, a meander passage 3₁in the first stage sequentially connecting the heat absorbing tubes 31 in the first stage from the heat absorbing tube 31 of #11 positioned at the outermost side of the one side in the Y-axis direction to the heat absorbing tube 31 of #16 positioned at the outermost side of the other side in the Y-axis direction is constituted by the six heat absorbing tubes 31 in the first stage and a connecting portion 32₁for the first stage sequentially connecting the heat absorbing tubes 31 in the first stage. A meander passage 3₂in the second stage sequentially connecting the heat absorbing tubes 31 from the heat absorbing tube 31 of #21 positioned at the outermost side of the one side in the Y-axis direction to the heat absorbing tube 31 of #25 positioned at the outermost side of the other side in the Y-axis direction is constituted by the five heat absorbing tubes 31 in the second stage and a connecting portion 32₂for the second stage sequentially connecting the heat absorbing tubes 31 in the second stage.

In this connection, the connecting portion 32₁for the first stage is constituted by three U-shaped tubes 32₁a and two U-shaped tubes 32₁b. The three U-shaped tubes 32₁a connect the heat absorbing tubes 31, 31 of #11 and #12, the heat absorbing tubes 31, 31 of #13 and #14, and the heat absorbing tubes 31, 31 of #15 and #16, respectively, which are disposed at an outside of the side plate 11 of the one side in the X-axis direction of the casing 1. The two U-shaped tubes 32₁b connect the heat absorbing tubes 31, 31 of #12 and #13, and the heat absorbing tubes 31, 31 of #14 and #15, respectively, which are disposed at an outside of the side plate 12 of the other side in the X-axis direction of the casing 1. In addition, the connecting portion 32₂for the second stage is constituted by two U-shaped tubes 32₂a and two U-shaped tubes 32₂b. The two U-shaped tubes 32₂a connect the heat absorbing tubes 31, 31 of #22 and #23, and the heat absorbing tubes 31, 31 of #24 and #25, respectively, which are disposed at the outside of the side plate 11 of the one side in X-axis direction of the casing 1. The two U-shaped tubes 32₂b connect the heat absorbing tubes 31, 31 of #21 and #22, and the heat absorbing tubes 31, 31 of #23 and #24, respectively, which are disposed at the outside of the side plate 12 of the other side in the X-axis direction of the casing 1.

An inflow tube 33 is connected to the heat absorbing tube 31 of #11 at the upstream end positioned at the outermost side of the one side in the Y-axis direction of the meander passage 3₁in the first stage at the outside of the side plate 12 of the other side in the X-axis direction of the casing 1. In addition, the heat absorbing tube 31 of #16 at the downstream end positioned at the outermost side of the other side in the Y-axis direction of the meander passage 3₁in the first stage is connected to the heat absorbing tube 31 of #25 at the upstream end positioned at the outermost side of the other side in the Y-axis direction of the meander passage 3₂in the second passage at the outside of the side plate 12 of the other side in the X-axis direction of the casing 1 through an intermediate connecting portion 32₁₂consisting of a U-shaped tube, allowing the fluid to be heated to flow from the meander passage 3₁in the first stage to the meander passage 3₂in the second stage. Further, an outflow side joint 34 having an outflow port 34a at an end portion thereof is connected to the heat absorbing tube 31 of #21 at the downstream end positioned at the outermost side of the one side in the Y-axis direction of the meander passage 3₂in the second stage at the outside of the side plate 11 of the one side in the X-axis direction of the casing 1. Thus, the connecting portion 32 which sequentially connects all of the heat absorbing tubes 31 in series is made up of the connecting portion 32₁for the first stage, the connecting portion 32₂for the second stage, and the intermediate connecting portion 32₁₂.

Referring to FIG. 3, three cooling passages 35₄, 35₈, 35₁₂of a fourth, an eighth, and a twelfth, consisting of a tube, from the upstream side of the flow direction of the combustion gas in sequence are disposed at an inner side of a portion of the side plate 13 of the one side in the Y-axis direction of the casing 1, which is positioned at a more upstream side of the flow direction of the combustion gas than each heat absorbing fin 2, so as to come into contact with the side plate 13. Three cooling passages 35₂, 35₆, 35₁₀of a second, a sixth, and a tenth, consisting of a tube, from the upstream side of the flow direction of the combustion gas in sequence are disposed at an inside of a portion of the side plate 14 of the other side in the Y-axis direction of the casing 1, which is positioned at the more upstream side of the flow direction of the combustion gas than each heat absorbing fin 2, so as to come into contact with the side plate 14. In addition, as shown in FIGS. 1, 4, a third cooling passage 35₃connecting the second cooling passage 35₂and the fourth cooling passage 35₄, a seventh cooling passage 35₇connecting the sixth cooling passage 35₆and the eighth cooling passage 35₈, and an eleventh cooling passage 35₁₁connecting the tenth cooling passage 35₁₀and the twelfth cooling passage 35₁₂are provided at a portion of the side plate 11 of the one side in the X-axis direction of the casing 1, which is positioned at the more upstream side in the flow direction of the combustion gas than each heat absorbing fin 2. Further, as shown in FIGS. 2, 4, a first cooling passage 35₁which is connected to the second cooling passage 35₂and at an end portion of which an inflow port 35₁a in which the fluid to be heated flows from the sub heat exchanger is provided, a fifth cooling passage 35₅connecting the fourth cooling passage 35₄and the sixth cooling passage 35₆, and a ninth cooling passage 35₉connecting the eighth cooling passage 35₈and the tenth cooling passage 35₁₀are provided at a portion of the side plate 12 of the other side in the X-axis direction of the casing 1, which is positioned at the more upstream side in the flow direction of the combustion gas than each heat absorbing fin 2. The heat absorbing tube 31 of #11 at the upstream side of the meander passage 3₁in the first stage is connected to the twelfth cooling passage 35₁₂through the inflow tube 33. Therefore, the fluid to be heated flows in the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage from the inflow port 35₁a through the first to the twelfth passages 35₁-35₁₂. Thus, each of the side plates 11-14 of the casing 1 is allowed to be cooled by the fluid to be heated flowing in the first to the twelfth cooling passages 35₁-35₁₂.

Meanwhile, each of the third, seventh, and eleventh cooling passages 35₃, 35₇, 35₁₁, and each of the first, fifth, and ninth cooling passages 35₁, 35₅, 35₉are constituted by recesses extending inward in the X-axis direction and formed at each of the side plates 11, 12, and covers 351 mounted on an outer surface of each of the side plates 11, 12 to cover the recesses, respectively. In addition, a plurality of holes 141 into which ignition electrodes, flame rods, or the like are inserted are formed in the side plate 14 of the other side in the Y-axis direction of the casing 1.

Incidentally, when the heat absorbing tubes 31 thermally expand in the X-axis direction as the longitudinal direction thereof, the side plates 11, 12 of each side in the X-axis direction of the casing 1, through which the heat absorbing tubes 31 pierce and on which the heat absorbing tubes 31 are brazed, are pressed in the X-axis direction. In the heat exchanger A of the embodiment, temperature difference between the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage and the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage becomes the largest, and the two heat absorbing tubes 31, 31 of #11 and #21 are positioned at the outermost side of the one side in the Y-axis direction, respectively. Therefore, when a distance between the two heat absorbing tubes 31, 31 of #11 and #21 is short, a large stress acts on the side plates 11, 12 of each side in the X-axis direction of the casing 1 due to difference in the amount of heat expansion between these heat absorbing tubes 31, 31 of #11 and #21. As a result, cracks appear in the side plates 11, 12 due to repeated effects of the stress.

Then, in the embodiment, as specified in FIG. 5, a distance La between the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage and the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage is made longer than a respective distance Lb between each heat absorbing tube 31 other than the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage and such each heat absorbing tube 31 other than the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage as is the closest to each heat absorbing tube 31 other than the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage. In particular, a position of the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage is shifted to a more downstream side of the flow direction of the combustion gas than each heat absorbing tube 31 other than the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage, and thereby allowing the distance La between the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage and the heat absorbing tube 31 of #21 at the downstream end of the second meander passage 3₂in the second stage to be larger.

According to this, the stress acting on the side plates 11, 12 of each side in the X-axis direction of the casing 1 due to the difference in the amount of the heat expansion between the two heat absorbing tubes 31, 31 of #11 and #21, between which the temperature difference is the largest, can be reduced by the longer distance La between the two heat absorbing tubes 31, 31 of #11 and #21, and durability can be improved. In addition, in the embodiment, the distance La between the two heat absorbing tubes 31, 31 of #11 and #21 is made longer by shifting the position of the heat absorbing tube 31 of #21 to the downstream side in the flow direction of the combustion gas. Namely, a direction making the distance La between the two heat absorbing tubes 31, 31 of #11 and #21 longer is made equal to the flow direction of the combustion gas. Therefore, there is no need to increase the dimension in the Y-axis direction of the heat exchanger A, and oversizing of the heat exchanger A can be avoided. Meanwhile, it is also possible to make the distance La longer by shifting the position of the heat absorbing tube 31 of #21 not only to the downstream side of the flow direction of the combustion gas, but also to the other side in the Y-axis direction. In this case, when the position of the heat absorbing tube 31 of #21 is shifted so that a distance between centers of the two heat absorbing tubes 31, 31 of #21 and #22, becomes equal to that between the two heat absorbing tubes 31, 31 of #23 and #24, the two heat absorbing tubes 31, 31 of #21 and #22, can be connected not with a dedicated U-shaped tube but with the U-shaped tube having the same shape and the same dimension as the U-shaped tube 32₂b for connecting the two heat absorbing tubes 31, 31 of #23 and #24 and this makes it advantageous.

In addition, in the embodiment, notches are provided with each heat absorbing fin 2 at portions positioned at the more upstream side in the flow direction of the combustion gas than each heat absorbing tube 31 of the meander passage 3₂in the second stage. In particular, the notches are provided at each heat absorbing fin 2 by making the notches positioned at base side edge portions of each swelling portion 21 positioned at the more upstream side in the flow direction of the combustion gas than each heat absorbing tube 31 of the meander passage 3₂in the second stage. Then, the notches corresponding to the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage are defined as first notches 22₁, the notches corresponding to each heat absorbing tube 31 other than the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage are defined as second notches 22₂, and the first notches 22₁are formed larger than the second notches 22₂.

According to this, the heat transfer to the heat absorbing tube 31 of #21, of which temperature becomes the highest, at the downstream end of the meander passage 3₂in the second stage through each heat absorbing fin 2 can be made to reduce by the larger first notches 22₁. Therefore, the temperature difference between the two heat absorbing tubes 31, 31 of #11 and #21 is made to decrease, and the stress acting on the side plates 11, 12 of each side in the X-axis direction of the casing 1 can be made to more reduce.

In addition, in the embodiment, a width Wa of a portion of each heat absorbing fin 2 between the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage and the first notch 22₁formed at a side edge portion of the other side in the Y-axis direction of a base of the swelling portion 21 through which such heat absorbing tube 31 of #11 pierces is made wider than widths Wb of portions of each heat absorbing fin 2 between each second notch 22₂and such each heat absorbing tube 31 other than the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage as is adjacent to each second notch 22₂at the upstream side in the flow direction of the combustion gas. According to this, the heat transfer to the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage through each heat absorbing fin 2 is promoted, and temperature of the combustion gas directed to the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage, which is positioned at the downstream side in the flow direction of the combustion gas with respect to the heat absorbing tube 31 of #11, is lowered. Therefore, the heat transfer from the combustion gas to the heat absorbing tube 31 of #21 can be reduced. As a result, the temperature difference between the two heat absorbing tubes 31, 31 of #11 and #21 is made to decrease more, and the stress acting on the side plates 11, 12 of each side in the X-axis direction of the casing 1 can be reduced.

Meanwhile, the first notch 22₁is formed not only at the side edge portion of the other side in the Y-axis direction of the base of the swelling portion 21 through which the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage pierces but also at the side edge portion of the one side in the Y-axis direction of the base of the swelling portion 21 through which the heat absorbing tube 31 of #12 adjacent to the heat absorbing tube 31 of #11 pierces. Then, the portion of each heat absorbing fin 2 between the heat absorbing tube 31 of #12 and the first notch 22₁is also made the same width Wa as the portion of each heat absorbing fin 2 between the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage and the first notch 22₁.

Further, in the embodiment, as shown in FIG. 3, a ventilation resistant portion 23 is provided to suppress the combustion gas from being directed to the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage. According to this, it is difficult for the combustion gas to flow around the heat absorbing tube 31 of #21 of which the temperature becomes the highest, and the heat transfer from the combustion gas to such heat absorbing tube 31 of #21 can be made to be reduced. Therefore, the temperature difference between the two heat absorbing tubes 31, 31 of #11 and #21 is made to decrease, and the stress acting on the side plates 11, 12 of each side in the X-axis direction of the casing 1 can be more reduced.

Next, with reference also to FIG. 6, the ventilation resistant portion 23 will be particularly explained. In FIG. 6, a symbol “2a” represents a burring process hole through which each heat absorbing tube 31 pierces. A first protruding piece portion 231, upper and lower second protruding piece portions 232₁, 232₂, and a third protruding piece portion 233 are provided at each heat absorbing fin 2 as constituent elements of the ventilation resistant portion 23. The first protruding piece portion 231 is made to be positioned at the more downstream side in the flow direction of the combustion gas than the meander passage 3₁in the first stage and between the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage and the heat absorbing tube 31 of #22 of the meander passage 3₂in the second stage adjacent to the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage, and protrudes in the X-axis direction. The upper and lower second protruding piece portions 232₁, 232₂protruding in the X-axis direction are made to be positioned at the more downstream side in the flow direction of the combustion gas than the meander passage 3₁in the first stage and between the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage and the side plate 13 of the one side in the Y-axis direction of the casing 1. The third protruding piece portion 233 protruding in the X-axis direction is made to be positioned between any two of the heat absorbing tubes 31, 31 which are adjacent to each other in the Y-axis direction and are other than the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage.

Here, a width W1a of a minimum portion of a clearance between the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage and the first protruding piece portion 231 is made narrower than a width W1b of a minimum portion of a clearance between the heat absorbing tube 31 of #22 of the meander passage 3₂in the second stage and the first protruding piece portion 231. According to this, the combustion gas flows in a larger amount in the clearance between the heat absorbing tube 31 of #22 of the meander passage 3₂in the second stage and the first protruding piece portion 231, which is wider than that between the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage and the first protruding piece portion 231. Therefore, the combustion gas can be suppressed from being directed to the heat absorbing tube 31 of #21.

In addition, widths W2₁a, W2₂a of minimum portions of clearances between the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage and the upper and lower second protruding piece portions 232₁, 232₂are made narrower than widths W2₁b, W2₂b of minimum portions of clearances between the side plate 13 of the one side in the Y-axis direction of the casing 1 and the upper and lower second protruding piece portions 232₁, 232₂, respectively. According to this, the combustion gas flows in the large amount to the clearances between the side plate 13 of the one side in the Y-axis direction of the casing 1 and each of the second protruding piece portions 232₁, 232₂, which are wider than those between the heat absorbing tube 31 of #21 of the meander passage 3₂in the second stage and the second protruding piece portions 232₁, 232₂. Therefore, the combustion gas can be suppressed from being directed to the heat absorbing tube 31 of #21.

Meanwhile, the upper second protruding piece portion 232₁has a cylindrical shape. A width W2₁c of a minimum portion of a clearance between the upper second protruding piece portion 232₁and the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage is made wider than the width W2₁a of the minimum portion of the clearance between the upper second protruding piece portion 232₁and the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage so that the combustion gas flows in the larger amount in the clearance between the upper second protruding piece portion 232₁and the heat absorbing tube 31 of #11.

In addition, a Y-axis direction width W1 of the first protruding piece portion 231 is made wider than a Y-axis direction width W3 of the third protruding piece portion 233 so that the ventilation resistance given by the first protruding piece portion 231 is made larger than that given by the third protruding piece portion 233. According to this, the combustion gas flows in the larger amount between any two of the heat absorbing tubes 31, 31 adjacent to each other in the Y-axis direction, except the heat absorbing tube 31 at the downstream end of the meander passage 3₂in the second stage, between which the third protruding piece portion 233 by which the given ventilation resistance is small exists. Therefore, the combustion gas can be suppressed from being directed to the heat absorbing tube 31 of #21 of the meander passage 3₂in the second stage. In this connection, a protruding length in the X-axis direction of the first protruding piece portion 231 is made longer than that of the third protruding piece portion 233, whereby it is also possible to make the ventilation resistance given by the first protruding piece portion 231 larger than that given by the third protruding piece portion 233.

Further, in the embodiment, in order for each heat absorbing fin 2 to efficiently absorb heat from the combustion gas, bridge-shaped convex portions 24 being longitudinal the Y-axis direction are provided at each heat absorbing fin 2. Each bridge-shaped convex portion 24 is made to be positioned at the more downstream side of the flow direction of the combustion gas than the meander passage 3₁in the first stage and between any two of the heat absorbing tubes 31, 31 adjacent to each other in the Y-axis direction of the meander passage 3₂in the second stage, and protrudes in an X-axis opposite direction to each of the above-mentioned protruding piece portions 231-233 so as to form tunnel-like passages through which the combustion gas passes. Here, a Y-axis direction center of the bridge-shaped convex portion 24 of #1 provided between the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage and the heat absorbing tube 31 of #22 at the downstream end of the meander passage 3₂in the second stage adjacent to the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage at the other side in the Y-axis direction is more one-sided to the other side of the Y-axis direction than a Y-axis direction center between the two heat absorbing tubes 31, 31 of #21 and #22 of the meander passage 3₂in the second stage. According to this, the bridge-shaped convex portion 24 of #1 is made away from the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage. Therefore, the heat transfer to the heat absorbing tube 31 of #21 through the bridge-shaped convex portion 24 of #1 is reduced. As a result, the temperature difference between the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage and the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage can be made to decrease, which contributes to reducing the stress acting on the side plates 11, 12 of each side in the X-axis direction of the casing 1.

In addition, in the embodiment, as shown in FIG. 3, the exhaust port 41 of the exhaust gas gathering portion 4 is provided at a deviated portion of the other side in the Y-axis direction of the exhaust gas gathering portion 4. According to this, the combustion gas which flows toward the exhaust port 41 does not gather in the vicinity of the heat absorbing tube 31 of #21 of which the temperature becomes the highest at the downstream end of the meander passage 3₂in the second stage, which is positioned at the outermost side of the one side in the Y-axis direction, and the heat transfer from the combustion gas to the heat absorbing tube 31 of #21 can be reduced. Therefore, the temperature difference between the two heat absorbing tubes 31, 31 of #11 and #21 is made to decrease more, and the stress acting on the side plates 11, 12 of each side in the X-axis direction can be more reduced.

Though the embodiment of the invention is explained with reference to the drawings, the invention is not limited to the embodiment. For example, though the distance Lb between each heat absorbing tube 31 other than the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage and such each heat absorbing tube 31 other than the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage as is the closest to each heat absorbing tube 31 other than the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage is made constant, and only the distance La between the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage and the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage is made longer than the distance Lb, it may be possible that the distance Lb between each heat absorbing tube 31 other than the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage and such each heat absorbing tube 31 other than the heat absorbing tube 31 of #21 at the downstream end of the meander passage 3₂in the second stage as is the closest to each heat absorbing tube 31 other than the heat absorbing tube 31 of #11 at the upstream end of the meander passage 3₁in the first stage is made to gradually increase from the other side in the Y-axis direction to the one side in the Y-axis direction. In addition, in the embodiment, though the fluid to be heated is caused to flow from the meander passage 3₁in the first stage to the meander passage 3₂in the second stage, it may be possible to cause the fluid to be heated to flow from the meander passage 3₂in the second stage to the meander passage 3₁in the first stage.

Explanation of symbols

- A Heat exchanger
- 1 Casing
- 11, 12 Side plates of each side in X-axis direction of casing
- 2 Heat absorbing fin
- 22
  ₁First notches
- 22
  ₂Second notches
- 23 Ventilation resistant portion
- 3
  ₁Meander passage in first stage (Meander passage in one of stages)
- 3
  ₂Meander passage in second passage (Meander passage in the other of stages)
- 31 Heat absorbing tube
- 32 Connecting portion
- 32
  ₁Connecting portion for first stage
- 32
  ₂Connecting portion for second stage
- 4 Exhaust gas gathering portion
- 41 Exhaust port
- La Distance between heat absorbing tube at upstream end of meander passage in first stage and heat absorbing tube at downstream end of meander passage in second stage
- Lb Respective distance between each heat absorbing tube other than heat absorbing tube at upstream end of meander passage in first stage and such each heat absorbing tube other than heat absorbing tube at downstream end of meander passage in second stage as is the closest to each heat absorbing tube other than heat absorbing tube at upstream end of meander passage in first stage

Heat Exchanger

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CPC

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Priority Claims (1)