BACKGROUND OF INVENTION

Field of the Invention

The present invention relates to a heat exchanger that causes heat exchange between a high-temperature gas and a liquid having a lower temperature than the high-temperature gas to heat the liquid.

Background Art

Heat exchangers that cause heat exchange between a high-temperature gas and a liquid having a lower temperature than the high-temperature gas to heat the liquid are incorporated and used in various pieces of equipment. For example, water heaters are widely used to burn fuel gas to generate hot water. Such a water heater burns fuel gas to produce high-temperature combustion exhaust gas and causes an internal heat exchanger to generate hot water through heat exchange between the combustion exhaust gas and water.

The heat exchanger includes a frame defining a part of the gas passage through which a high-temperature gas flows, multiple heat transfer plates (typically referred to as heat exchanger fins) arranged at regular intervals within the frame, and a hollow pipe extending through the multiple heat exchanger fins. The hollow pipe extends through the heat exchanger fins, and is bent in a U-shape to extend again through the heat exchanger fins in the opposite direction. This is repeated to allow the pipe to repeatedly extend through the multiple heat exchanger fins. The hollow pipe and the heat exchanger fins are formed from copper or other metal with high thermal conductivity. The hollow pipe extending through each heat exchanger fin is joined to each fin by, for example, brazing.

In the heat exchanger with this structure, a low-temperature liquid (e.g., water) is fed through one end of the hollow pipe while a high-temperature gas (e.g., combustion exhaust gas) is being fed to the gas passage, and thus the high-temperature gas flowing through the spaces between the multiple heat exchanger fins exchanges heat with the liquid flowing through the hollow pipe. The liquid heated through the heat exchange (e.g., hot water) flows out through the other end of the hollow pipe. The high-temperature gas is cooled through the heat exchange during the passage through the spaces between the multiple heat exchanger fins.

Although the heat exchanger fins in the heat exchanger come in contact with the high-temperature gas and become hot, the hollow pipe is cooled by the liquid flowing through it and remains at a lower temperature than the heat exchanger fins. In this state, the hollow pipe with the lower temperature restricts the hot heat exchanger fins that tend to expand, causing large thermal stress on the heat exchanger fins. The heat exchanger used under such a thermally heavy load for a long time may cause cracking in the heat exchanger fins due to repeated applications of thermal stress. In particular, upstream parts of the heat exchanger fins receiving a high-temperature gas flow reach a higher temperature and are likely to crack under large thermal stress.

A technique has been developed for cutting slits in the heat exchanger fins from the upstream edges receiving high-temperature gas into the heat exchanger fins to positions between adjacent hollow pipes (Patent Literature 1). Although heat exchanger fins reach a higher temperature, the slits cut in the heat exchanger fins with this technique absorb the thermal expansion of the heat exchanger fins, reducing thermal stress and preventing cracking.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application Publication No. 11-108456

SUMMARY OF INVENTION

However, with the above technique, the heat exchanger fins have slits between all adjacent hollow pipes and thus have smaller surface areas. This lowers the performance of the heat exchanger (heat-exchange efficiency).

In response to the above issue, one or more aspects of the present invention are directed to a heat exchanger that prevents cracking in heat exchanger fins due to thermal stress while avoiding lowering heat-exchange efficiency.

In response to the above issue, a heat exchanger according to an aspect of the present invention has the structure below. The heat exchanger causes heat exchange between a high-temperature gas and a liquid having a lower temperature than the high-temperature gas to heat the liquid. The heat exchanger includes a frame defining a part of a gas passage for the high-temperature gas, a plurality of heat transfer plates arranged within the frame with a space left between the plurality of heat transfer plates for the high-temperature gas to flow, and a hollow pipe extending through the plurality of heat transfer plates. The hollow pipe receives the liquid to exchange heat with the high-temperature gas flowing through the space between the plurality of heat transfer plates. The hollow pipe extends through the plurality of heat transfer plates at predetermined N positions aligned in a direction crossing a direction in which the high-temperature gas flows through the space between the plurality of heat transfer plates, where N is an integer of at least three. The plurality of heat transfer plates each include N−1 inter-pipe portions between adjacent parts of the hollow pipe extending through the plurality of heat transfer plates at the N positions, and the N−1 inter-pipe portions of each heat transfer plate include a selected inter-pipe portion including a thermal expansion absorber between an edge of the heat transfer plate receiving inflow of the high-temperature gas and the selected inter-pipe portion to absorb thermal expansion of the heat transfer plate.

In the heat exchanger according to the aspect of the present invention, the plurality of heat transfer plates are arranged within the frame with the space left between the heat transfer plates. When the high-temperature gas flows through the space, the high-temperature gas exchanges heat with the liquid in the hollow pipe. During the heat exchange, the heat transfer plates heated by the high-temperature gas tend to expand. The heat transfer plates, through which multiple parts of the hollow pipe extend, are elongated in the direction in which the parts of the hollow pipe are aligned, and can thus have large thermal expansion accumulating in the elongation direction. When the large thermal expansion is restricted by the hollow pipe, the heat transfer plates can have large thermal stress and can crack. However, the heat transfer plates in the heat exchanger according to the aspect of the present invention each include the thermal expansion absorber between the edge of the heat transfer plate receiving the inflow of the high-temperature gas and the selected inter-pipe portion to absorb thermal expansion of the heat transfer plate. In this structure, the thermal expansion absorber absorbs thermal expansion before accumulating, reducing thermal stress on the heat transfer plate and reducing cracking. Although each heat transfer plate receiving N parts of the hollow pipe includes N−1 inter-pipe portions, a selected one or more of the N−1 inter-pipe portions, rather than all of the N−1 inter-pipe portions, may include the thermal expansion absorber to prevent large thermal expansion from accumulating, thus reducing thermal stress on the heat transfer plate and reducing cracking. Such selected one or more of the N−1 inter-pipe portions including the thermal expansion absorber can also avoid lowering the heat-exchange efficiency as compared with the structure with all the inter-pipe portions including thermal expansion absorbers.

In the heat exchanger according to the above aspect of the present invention, the thermal expansion absorber may be a cut extending from the edge of the heat transfer plate receiving the inflow of the high-temperature gas to the selected inter-pipe portion.

This thermal expansion absorber can be easily formed at multiple positions of the heat transfer plates.

In the heat exchanger according to the above aspect of the present invention, the inter-pipe portions positioned described below may be selected, from the inter-pipe portions at the multiple (N−1) positions of each heat transfer plate, as selected inter-pipe portions each including the thermal expansion absorber. As described above, the N parts of the hollow pipe extend through the heat transfer plate, and the N−1 inter-pipe portions are formed between the N parts of the hollow pipe. Thus, selecting one or more (or K, smaller than N−1) inter-pipe portions from the N−1 inter-pipe portions may mean that the K inter-pipe portions (selected inter-pipe portions) are selected to divide the heat transfer plate into multiple (K+1) sub-areas through which the hollow pipe extends. When the K inter-pipe portions are selected from different positions to divide the heat transfer plate into K+1 sub-areas, each sub-area receives a different number of parts of the hollow pipe. The selected inter-pipe portions may be positioned to allow each of the sub-areas to receive not more than three parts of the hollow pipe extending through the sub-areas.

For example, with three parts of the hollow pipe extending through each sub-area, thermal expansion of the heat transfer plate surrounding the central hollow pipe part pushes the right hollow pipe part rightward and the left hollow pipe part leftward. Then, the heat transfer plate surrounding the left hollow pipe part pushed leftward and the right hollow pipe part pushed rightward expands thermally, causing the leftward and rightward thermal expansion to accumulate. However, the accumulating thermal expansion is absorbed by the thermal expansion absorber formed on the left of the left hollow pipe part and the thermal expansion absorber formed on the right of the right hollow pipe part, causing substantially no accumulation of thermal expansion. Thus, with the inter-pipe portions selected from positions that allow not more than three parts of the hollow pipe to extend through each sub-area, the sub-area avoids accumulation of thermal expansion and also prevents cracking.

In the heat exchanger according to the above aspect of the present invention, the inter-pipe portions positioned described below may be selected from multiple inter-pipe portions as selected inter-pipe portions. More specifically, the selected inter-pipe portions may be positioned to allow the sub-areas to receive the same number of parts of the hollow pipe or numbers of parts of the hollow pipe different from one another by not more than one.

When the heat transfer plate is heated, thermal expansion of each sub-area accumulates in the elongation direction. However, the selected inter-pipe portions each including the thermal expansion absorber do not accumulate thermal expansion across the selected inter-pipe portions. The thermal expansion accumulates within each sub-area, greater in a long sub-area (receiving more parts of the hollow pipe) than in a short sub-area (receiving fewer parts of the hollow pipe). Thus, for a heat transfer plate having a long sub-area and a short sub-area, the long sub-area is likely to crack. However, with the selected inter-pipe portions positioned to allow multiple sub-areas to receive the same number of parts of the hollow pipe or numbers of parts of the hollow pipe different from one another by not more than one, each sub-area is not more likely to crack than the other sub-areas, thus reducing cracking in the heat transfer plates.

In the heat exchanger according to the above aspect of the present invention, the selected inter-pipe portion may be at least one of inter-pipe portions symmetric to each other among the N−1 inter-pipe portions.

This structure prevents the heat transfer plates from having thermal stress biased in the elongation direction, and thus avoids warping of the heat transfer plates.

In the heat exchanger according to the above aspect of the present invention, each heat transfer plate may receive the hollow pipe extending through the heat transfer plate at the N positions, and receive a plurality of parts of the hollow pipe extending through the heat transfer plate at positions downstream in a direction in which the high-temperature gas flows. In this structure, the selected inter-pipe portion may be selected from the N−1 inter-pipe portions between upstream parts of the hollow pipe extending through the heat transfer plate at the N positions.

The high-temperature gas is cooled through heat exchange during the passage through the space between the heat transfer plates. As the high-temperature gas has a lower temperature downstream, the heat transfer plates also have a lower temperature downstream than upstream. With multiple upstream and downstream parts of the hollow pipe extending through the heat transfer plates in the high-temperature gas flow, inter-pipe portions each including the thermal expansion absorber may be selected from inter-pipe portions between the upstream parts of the hollow pipe to prevent cracking in the heat transfer plates.

