PROBLEM TO BE SOLVED: To provide an observation hole for a combustion device allowing visual observation of a combustion state of an interior without causing deterioration of combustion performance.;SOLUTION: The observation hole 25 for visual observation of the combustion state in a steam boiler (the combustion device) from outside the device can be inserted and fixed in a cylinder 26 pierced through an outer wall (a body side wall) of a premixed gas passage 6 of the steam boiler, and is composed by including an observation hole tube 27 attached with glasses 36, 37 on longitudinal both end openings, and air filling means (a blower 5 and an air supply tube 28 branched off from a discharge side of the blower 5 and connected to the observation hole tube 27) for filling air into an interior of the observation hole tube 27 and maintaining its internal pressure almost constant. A flange 32 is attached to a longitudinal middle outer circumference of the observation hole tube 27, and by securing the flange 32 to the cylinder 26, the observation hole tube 27 can be attached to the cylinder 26 such that it can be inserted and detached.;COPYRIGHT: (C)2011,JPO&INPIT
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