Heat exchanger

A heat exchanger heated by a combustion gas, comprising,

• a box-shaped 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, 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,
• wherein:
• notches are provided over each heat absorbing tube of the meander passage in the second stage, and projecting in the upstream direction of the combustion gas flow between each heat absorbing tube of the meander passage in the first stage wherein each notch confined within a diameter of the heat absorbing tubes, 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 of the second passage are defined as second notches, and the first notches are formed larger than the second notches, and
• wherein a closest width of a portion of each heat absorbing fin in a radial direction of the heat absorbing tube between the heat absorbing tube at the upstream end of the meander passage in the first stage and the first notch is wider than a closest width of each portion of the heat absorbing fin in the radial direction of the heat absorbing tube between each second notch and the closest heat absorbing tube other than the heat absorbing tube at the upstream end of the meander passage in the first stage that is adjacent to the corresponding second notch at the upstream side in the flow direction of the combustion gas.