PURPOSE:To improve thermal efficiency by providing multiple flame blow-out steps and making the flame in a lower step contact with the flame in the next upper step. CONSTITUTION:Multiple flame blow-out steps 1 and 2 are provided, and the lower flame 4 from the lower flame blow-out step 2 is made to contact with the upper flame 5 of the upper flame blow-out step 1. With this arrangement, the thermal efficiency can be raised. In the figure, there are two flame blow-out steps, but with three or four flame blow-out steps the thermal efficiency can be raised as well.
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