Patent ID: 10330312

Abstract:
A premixing apparatus in which a downstream end of a gas supply passage having interposed therein a flow control valve is connected to a gas suction section disposed in an air supply passage on an upstream side of a fan. The premixing apparatus has: a butterfly valve as an air resistance changeover device for changing over, between high and low, a ventilation resistance in that section of the air supply passage which is on the upstream side of the gas suction section; and a gas resistance changeover device for changing over, between high and low, a ventilation resistance in that section of the gas supply passage which is on the downstream side of the flow control valve. Wind noises at the time of closing the butterfly valve can be prevented. For that purpose, in that section of the air supply passage which is on the upstream side of the gas suction section, an inner tube containing therein the butterfly valve is disposed while leaving a clearance to an inner circumferential wall surface of the air supply passage. A subsidiary passage which is parallel with a main passage inside the inner tube is constituted by the clearance between the inner circumferential wall surface of the air supply passage and an outer peripheral surface of the inner tube.