Heat Exchanger

In a heat exchanger, a respective meander passage 31, 32 in each of two stages connecting heat absorbing tubes 31 divided into the two stages and arranged in a flow direction of a combustion gas in an inside of a casing 1 from one side to the other side in a Y-axis direction is constituted, and both a heat absorbing tube 31 of #11 at an upstream end of the meander passage 31 in a first stage and a heat absorbing tube 31 of #21 at a downstream end of the meander passage 32 in a second stage are made to be positioned at the one side in the Y-axis direction, notches are provided with each heat absorbing fin 2 at portions positioned at a more upstream side in the flow direction of the combustion gas than each heat absorbing tube 31 of the meander passage 32 in the second stage, and first notches 221 corresponding to the heat absorbing tube 31 at a downstream end of the meander passage 32 in the second passage are formed larger than second notches 222 corresponding to each heat absorbing tube 31 other than the heat absorbing tube 31 of #21 at the downstream end.

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 of 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

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 a first stage of an upstream side in the flow direction of the combustion gas and the meander passage is provided in a second stage of a downstream side in the flow direction of the combustion gas, the heat absorbing tube at an upstream end and the heat absorbing tube at a downstream end of the meander passage in the first stage are 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, and the heat absorbing tube at the downstream end of the meander passage 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 passage are connected, allowing the fluid to be heated to flow from the meander passage in the first stage to the meander passage in the second stage. In the heat exchanger, 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, which is positioned at the outermost side of the one side in the Y-axis direction of the meander passage in the second stage, are defined as first notches, 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 the invention, heat transfer to the heat absorbing tube at the downstream end of the meander passage in the second passage, of which temperature becomes the highest, through each heat absorbing fin can be reduced by the larger first notches. Therefore, 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, a stress acting on the slide plates of each side in the X-axis direction of the casing can be reduced, and durability can be improved.

In addition, in the invention, it is desirable that a width of a portion of each heat absorbing fin between the heat absorbing tube at the upstream end of the meander passage in the first stage and the first notch is wider than widths of portions of each heat absorbing fin between each second notch and such each heat absorbing tube other than the heat absorbing tube at the upstream end of the meander passage in the first stage as is adjacent to each second notch at the upstream side in the flow direction of the combustion gas. According to this, heat transfer to the heat absorbing tube at the upstream end of the meander passage in the first stage through each heat absorbing tube is promoted, and temperature of the combustion gas directed to the heat absorbing tube at the downstream end of the meander passage in the second stage, which is positioned at the downstream side in the flow direction of the combustion gas with respect to such heat absorbing tube of the meander passage in the first stage, is lowered. As a result, 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 more, and the stress acting on the side plates of each side in the X-axis direction of the casing can be reduced.

Further, in the invention, it is desirable that 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 first stage 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 second stage 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 first stage and such each heat absorbing tube other than the heat absorbing tube at the downstream end of the meander passage in the second stage 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 first stage. Thus, the stress acting on the side plates of each side in the X-axis direction of the casing can be reduced by lengthening the distance 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, between which the temperature difference becomes largest.

In addition, in the invention, 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 is difficult for the combustion gas to flow around the heat absorbing tube, of which temperature is 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 more, and the stress acting on the side plates of each side in the X-axis direction of the casing can be reduced.

Further, in the invention, in a case where 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 is 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 at the downstream end of the meander passage in the second stage is made to decrease more, and the stress acting on the side plates of each side in the X-axis direction of the casing can be reduced.

DESCRIPTION OF EMBODIMENTS

Referring toFIGS.1-4, a heat exchanger A of an embodiment of the invention includes a casing1having one end (an upper end inFIGS.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 casing1, 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 casing1, which is shown by arrows “a” inFIGS.3,4, are defined as an X-axis direction and a Y-axis direction, respectively, the casing1has a rectangular cylindrical shape with side plates11,12of both sides in the X-axis direction and side plates13,14of both sides in the Y-axis direction. The heat exchanger A also includes a plurality of heat absorbing fins2stacked and arranged in the X-axis direction in the casing1, a plurality of heat absorbing tubes31piercing through the heat absorbing fins2and the side plates11,12of both sides in the X-axis direction of the casing1, and into which a fluid to be heated such as water or the like flows, a connecting portion32connecting the heat absorbing tubes31in series at outsides of the side plates11,12of both sides in the X-axis direction of the casing1, and an exhaust gas gathering portion4covering an opening la at a downstream end (a lower end inFIGS.3,4) in the flow direction of the combustion gas of the casing1and having an exhaust port41opened for discharging the combustion gas. Meanwhile, the combustion gas discharged from the exhaust port41is introduced into a sub heat exchanger with latent heat recovery, which is not shown. In addition, each heat absorbing tube31is brazed on each heat absorbing fin2and each of the side plates11,12in a piercing state.

The heat absorbing tubes31, as specified inFIG.3, are divided into two stages and arranged in the flow direction of the combustion gas. In particular, six heat absorbing tubes31, i.e., #11-#16, from the heat absorbing tube31positioned at an outermost side of one side in the Y-axis direction to the heat absorbing tube31positioned 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 tubes31, i.e., #21-#25from the heat absorbing tube31positioned at the outermost side of the one side in the Y-axis direction to the heat absorbing tube31positioned 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 tube31in the second stage is arranged with a positional relationship such that a Y-axis direction center of such each heat absorbing tube31coincides with a Y-axis direction center between any two of the heat absorbing tubes31,31adjacent to each other in the first stage. Meanwhile, each heat absorbing fin2is provided with each swelling portion21at a Y-axis direction position coincident with each heat absorbing tube31in the first stage, which swells to the upstream side in the flow direction of the combustion gas. Then, each heat absorbing tube31in the first stage pierces through each swelling portion21.

In addition, a meander passage31in the first stage sequentially connecting the heat absorbing tubes31in the first stage from the heat absorbing tube31of #11positioned at the outermost side of the one side in the Y-axis direction to the heat absorbing tube31of #16positioned at the outermost side of the other side in the Y-axis direction is constituted by the six heat absorbing tubes31in the first stage and a connecting portion321for the first stage sequentially connecting the heat absorbing tubes31in the first stage. A meander passage32in the second stage sequentially connecting the heat absorbing tubes31from the heat absorbing tube31of #21positioned at the outermost side of the one side in the

Y-axis direction to the heat absorbing tube of #25positioned at the outermost side of the other side in the Y-axis direction is constituted by the five heat absorbing tubes31in the second stage and a connecting portion322for the second stage sequentially connecting the heat absorbing tubes31in the second stage.

In this connection, the connecting portion321for the first stage is constituted by three U-shaped tubes321a and two U-shaped tubes321b. The three U-shaped tubes321a connect the heat absorbing tubes31,31of #11and #12, the heat absorbing tubes31,31of #13and #14, and the heat absorbing tubes31,31of #15and #16, respectively, which are disposed at an outside of the side plate11of the one side in the X-axis direction of the casing1. The two U-shaped tubes321b connect the heat absorbing tubes31,31of #12and #13, and the heat absorbing tubes31,31of #14and #15, respectively, which are disposed at an outside of the side plate12of the other side in the X-axis direction of the casing1. In addition, the connecting portion322for the second stage is constituted by two U-shaped tubes322a and two U-shaped tubes322b. The two U-shaped tubes322a connect the heat absorbing tubes31,31of #22and #23, and the heat absorbing tubes31,31of #24and #25, respectively, which are disposed at the outside of the side plate11of the one side in X-axis direction of the casing1. The two U-shaped tubes322b connect the heat absorbing tubes31,31of #21and #22, and the heat absorbing tubes31,31of #23and #24, respectively, which are disposed at the outside of the side plate12of the other side in the X-axis direction of the casing1.

An inflow tube33is connected to the heat absorbing tube31of #11at the upstream end positioned at the outermost side of the one side in the Y-axis direction of the meander passage31in the first stage at the outside of the side plate12of the other side in the X-axis direction of the casing1. In addition, the heat absorbing tube31of #16at the downstream end positioned at the outermost side of the other side in the Y-axis direction of the meander passage31in the first stage is connected to the heat absorbing tube31of #25at the upstream end positioned at the outermost side of the other side in the Y-axis direction of the meander passage32in the second passage at the outside of the side plate12of the other side in the X-axis direction of the casing1through an intermediate connecting portion3212consisting of a U-shaped tube, allowing the fluid to be heated to flow from the meander passage31in the first stage to the meander passage32in the second stage. Further, an outflow side joint34having an outflow port34a at an end portion thereof is connected to the heat absorbing tube31of #21at the downstream end positioned at the outermost side of the one side in the Y-axis direction of the meander passage32in the second stage at the outside of the side plate11of the one side in the X-axis direction of the casing1. Thus, the connecting portion32which sequentially connects all of the heat absorbing tubes31in series is made up of the connecting portion321for the first stage, the connecting portion322for the second stage, and the intermediate connecting portion3212.

Referring toFIG.3, three cooling passages354,358,3512of 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 plate13of the one side in the Y-axis direction of the casing1, which is positioned at a more upstream side of the flow direction of the combustion gas than each heat absorbing fin2, so as to come into contact with the side plate13. Three cooling passages352,356,3510of 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 plate14of the other side in the Y-axis direction of the casing1, which is positioned at the more upstream side of the flow direction of the combustion gas than each heat absorbing fin2, so as to come into contact with the side plate14. In addition, as shown inFIGS.1,4, a third cooling passage353connecting the second cooling passage352and the fourth cooling passage354, a seventh cooling passage357connecting the sixth cooling passage356and the eighth cooling passage358, and an eleventh cooling passage3511connecting the tenth cooling passage3510and the twelfth cooling passage3512are provided at a portion of the side plate11of the one side in the X-axis direction of the casing1, which is positioned at the more upstream side in the flow direction of the combustion gas than each heat absorbing fin2. Further, as shown inFIGS.2,4, a first cooling passage351which is connected to the second cooling passage352and at an end portion of which an inflow port351ain which the fluid to be heated flows from the sub heat exchanger is provided, a fifth cooling passage355connecting the fourth cooling passage354and the sixth cooling passage356, and a ninth cooling passage359connecting the eighth cooling passage358and the tenth cooling passage3510are provided at a portion of the side plate12of the other side in the X-axis direction of the casing1, which is positioned at the more upstream side in the flow direction of the combustion gas than each heat absorbing fin2. The heat absorbing tube31of #11at the upstream side of the meander passage31in the first stage is connected to the twelfth cooling passage3512through the inflow tube33. Therefore, the fluid to be heated flows in the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage from the inflow port351a through the first to the twelfth passages351-3512. Thus, each of the side plates11-14of the casing1is allowed to be cooled by the fluid to be heated flowing in the first to the twelfth cooling passages351-3512.

Meanwhile, each of the third, seventh, and eleventh cooling passages353,357,3511, and each of the first, fifth, and ninth cooling passages351,355,359are constituted by recesses extending inward in the X-axis direction and formed at each of the side plates11,12, and covers351mounted on an outer surface of each of the side plates11,12to cover the recesses, respectively. In addition, a plurality of holes141into which ignition electrodes, flame rods, or the like are inserted are formed in the side plate14of the other side in the Y-axis direction of the casing1.

Incidentally, when the heat absorbing tubes31thermally expand in the X-axis direction as the longitudinal direction thereof, the side plates11,12of each side in the X-axis direction of the casing1, through which the heat absorbing tubes31pierce and on which the heat absorbing tubes31are brazed, are pressed in the X-axis direction. In the heat exchanger A of the embodiment, temperature difference between the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage and the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage becomes the largest, and the two heat absorbing tubes31,31of #11and #21are positioned at the outermost side of the one side in the Y-axis direction, respectively. Therefore, when a distance between the two heat absorbing tubes31,31of #11and #21is short, a large stress acts on the side plates11,12of each side in the X-axis direction of the casing1due to difference in the amount of heat expansion between these heat absorbing tubes31,31of #11and #21. As a result, cracks appear in the side plates11,12due to repeated effects of the stress.

Then, in the embodiment, as specified inFIGS.5,6, notches are provided with each heat absorbing fin2at portions positioned at the more upstream side in the flow direction of the combustion gas than each heat absorbing tube31of the meander passage32in the second stage. In particular, the notches are provided at each heat absorbing fin2by making the notches positioned at base side edge portions of each swelling portion21positioned at the more upstream side in the flow direction of the combustion gas than each heat absorbing tube31of the meander passage32in the second. Then, the notches corresponding to the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage are defined as first notches221, the notches corresponding to each heat absorbing tube31other than the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage are defined as second notches222, and the first notches221are formed larger than the second notches222.

According to this, the heat transfer to the heat absorbing tube31of #21, of which the temperature becomes the highest, at the downstream end of the meander passage32in the second stage through each heat absorbing fin2can be made to reduce by the larger first notches221. Therefore, the temperature difference between the two heat absorbing tubes31,31of #11and #21is made to decrease, the stress acting on the side plates11,12of each side in the X-axis direction of the casing1can be reduced, and durability can be improved.

In addition, in the embodiment, a width Wa of a portion of each heat absorbing fin2between the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage and the first notch221formed at a side edge portion of the other side in the Y-axis direction of a base of the swelling portion21through which such heat absorbing tube31of #11pierces is made wider than widths Wb of portions of each heat absorbing fin2between each second notch222and such each heat absorbing tube31other than the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage as is adjacent to each second notch222at the upstream side in the flow direction of the combustion gas. According to this, the heat transfer to the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage through each heat absorbing fin2is promoted, and temperature of the combustion gas directed to the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage, which is positioned at the downstream side in the flow direction of the combustion gas with respect to the heat absorbing tube31of #11, is lowered. Therefore, the heat transfer from the combustion gas to the heat absorbing tube31of #21can be reduced. As a result, the temperature difference between the two heat absorbing tubes31,31of #11and #21is made to decrease more, and the stress acting on the side plates11,12of each side in the X-axis direction of the casing1can be reduced.

Meanwhile, the first notch221is formed not only at a side edge portion of the other side in the Y-axis direction of the base of the swelling portion21through which the heat absorbing tube31of #11at the upstream end of the meander passage31in 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 portion21through which the heat absorbing tube31of #12adjacent to the heat absorbing tube31of #11pierces. Then, the portion of each heat absorbing fin2between the heat absorbing tube31of #12and the first notch221is also made the same width Wa as the portion of each heat absorbing fin2between the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage and the first notch221.

Further, in the embodiment, a distance La between the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage and the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage is made longer than a respective distance Lb between each heat absorbing tube31other than the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage and such each heat absorbing tube31other than the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage as is the closest to each heat absorbing tube31other than the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage. According to this, the stress acting on the side plates11,12of each side in the X-axis direction of the casing1due to the difference in the amount of the heat expansion between the two heat absorbing tubes31,31of #11and #21, between which the temperature difference is the largest, can be reduced by the longer distance La between the two heat absorbing tubes31,31of #11and #21.

In addition, in the embodiment, as shown inFIG.3, a ventilation resistant portion23is provided to suppress the combustion gas from being directed to the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage. According to this, it is difficult for the combustion gas to flow around the heat absorbing tube31of #21of which the temperature becomes the highest, and the heat transfer from the combustion gas to such heat absorbing tube31of #21can be made to be reduced. Therefore, the temperature difference between the two heat absorbing tubes31,31of #11and #21is made to decrease more, and the stress acting on the side plates11,12of each side in the X-axis direction of the casing1can be reduced.

Next, referring toFIGS.5,6, the ventilation resistant portion23will be particularly explained. In FIG.6, a symbol “2a” represents a burring process hole through which each heat absorbing tube31pierces. A first protruding piece portion231, upper and lower second protruding piece portions2321,2322, and a third protruding piece portion233are provided at each heat absorbing fin2as constituent elements of the ventilation resistant portion23. The first protruding piece portion231is made to be positioned at the more downstream side in the flow direction of the combustion gas than the meander passage31in the first stage and between the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage and the heat absorbing tube31of #22of the meander passage32in the second stage adjacent to the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage, and protrudes in the X-axis direction. The upper and lower second protruding piece portions2321,2322protruding 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 passage31in the first stage and between the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage and the side plate13of the one side in the Y-axis direction of the casing1. The third protruding piece portion233protruding in the X-axis direction is made to be positioned between any two of the heat absorbing tubes31,31which are adjacent to each other in the Y-axis direction and are other than the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage.

Here, a width W1a of a minimum portion of a clearance between the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage and the first protruding piece portion231is made narrower than a width W1b of a minimum portion of a clearance between the heat absorbing tube31of #22of the meander passage32in the second stage and the first protruding piece portion231. According to this, the combustion gas flows in a larger amount in the clearance between the heat absorbing tube31of #22of the meander passage32in the second stage and the first protruding piece portion231, which is wider than that between the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage and the first protruding piece portion231. Therefore, the combustion gas can be suppressed from being directed to the heat absorbing tube31of #21.

In addition, widths W21a, W22a of minimum portions of clearances between the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage and the upper and lower second protruding piece portions2321,2322are made narrower than widths W21b, W22b of minimum portions of clearances between the side plate13of the one side in the Y-axis direction of the casing1and the upper and lower second protruding piece portions2321,2322, respectively. According to this, the combustion gas flows in the large amount to the clearances between the side plate13of the one side in the Y-axis direction of the casing1and each of the second protruding piece portions2321,2322, which are wider than those between the heat absorbing tube31of #21of the meander passage32in the second stage and the second protruding piece portions2321,2322. Therefore, the combustion gas can be suppressed from being directed to the heat absorbing tube31of #21.

Meanwhile, the upper second protruding piece portion2321has a cylindrical shape. A width W21c of a minimum portion of a clearance between the upper second protruding piece portion2321and the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage is made wider than the width W21a of the minimum portion of the clearance between the upper second protruding piece portion2321and the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage so that the combustion gas flows in the larger amount in the clearance between the upper second protruding piece portion2321and the heat absorbing tube31of #11.

In addition, a Y-axis direction width W1of the first protruding piece portion231is made wider than a Y-axis direction width W3of the third protruding piece portion233so that the ventilation resistance given by the first protruding piece portion231is made larger than that given by the third protruding piece portion233. According to this, the combustion gas flows in the larger amount between any two of the heat absorbing tubes31,31adjacent to each other in the Y-axis direction, except the heat absorbing tube31at the downstream end of the meander passage32in the second stage, between which the third protruding piece portion233by which the given ventilation resistance is small exists. Therefore, the combustion gas can be suppressed from being directed to the heat absorbing tube31of #21of the meander passage32in the second stage. In this connection, a protruding length in the X-axis direction of the first protruding piece portion231is made longer than that of the third protruding piece portion233, whereby it is also possible to make the ventilation resistance given by the first protruding piece portion231larger than that given by the third protruding piece portion233.

Further, in the embodiment, in order for each heat absorbing fin2to efficiently absorb heat from the combustion gas, bridge-shaped convex portions being longitudinal in the Y-axis direction24are provided at each heat absorbing fin2. Each bridge-shaped convex portion24is made to be positioned at the more downstream side of the flow direction of the combustion gas than the meander passage31in the first stage and between any two of the heat absorbing tubes31,31adjacent to each other in the Y-axis direction of the meander passage32in the second stage, and protrudes in an X-axis opposite direction to each of the above-mentioned protruding piece portions231-233so as to form tunnel-like passages through which the combustion gas passes. Here, a Y-axis direction center of the bridge-shaped convex portion24of #1provided between the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage and the heat absorbing tube31of #22adjacent to the heat absorbing tube31of #21at the downstream end of the meander passage32in 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 tubes31,31of #21and #22of the meander passage32in the second stage. According to this, the bridge-shaped convex portion24of #1is made away from the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage. Therefore, the heat transfer to the heat absorbing tube31of #21through the bridge-shaped convex portion24of #1is reduced. As a result, the temperature difference between the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage and the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage can be made to decrease, which contributes to reducing the stress acting on the side plates11,12of each side in the X-axis direction of the casing1.

In addition, in the embodiment, as shown inFIG.3, the exhaust port41of the exhaust gas gathering portion4is provided at a deviated portion of the other side in the Y-axis direction of the exhaust gas gathering portion4. According to this, the combustion gas which flows toward the exhaust port41does not gather in the vicinity of the heat absorbing tube31of #21of which the temperature becomes the highest at the downstream end of the meander passage32in 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 tube31of #21can be reduced. Therefore, the temperature difference between the two heat absorbing tubes31,31of #11and #21is made to decrease more, and the stress acting on the side plates11,12of 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 tube31other than the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage and such each heat absorbing tube31other than the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage as is the closest to each heat absorbing tube31other than the heat absorbing tube31at the upstream end of the meander passage31in the first stage is made constant, and only the distance La between the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage and the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage is made longer than the distance Lb, it may be possible that the distance Lb between each heat absorbing tube31other than the heat absorbing tube31of #11at the upstream end of the meander passage31in the first stage and such each heat absorbing tube31other than the heat absorbing tube31of #21at the downstream end of the meander passage32in the second stage as is the closest to each heat absorbing tube31other than the heat absorbing tube31of #11at the upstream end of the meander passage31in 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.

Explanation of Symbols

A Heat exchanger1Casing11,12Side plates of each side in X-axis direction of casing2Heat absorbing fin221First notches222Second notches23Ventilation resistant portion31Meander passage in first stage32Meander passage in second passage31Heat absorbing tube32Connecting portion321Connecting portion for first stage322Connecting portion for second stage4Exhaust gas gathering portion41Exhaust portWa Width of portion of each heat absorbing fin between heat absorbing tube at upstream end of meander passage in first stage and first notchWb Widths of portions of each heat absorbing fin between each second notch and such heat absorbing tube other than heat absorbing tube at upstream end of meander passage in first stage as is adjacent to each second notch at upstream side of flow direction of combustion gasLa 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 stageLb 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