Premixing apparatus

Abstract

In a premixing apparatus for mixing a fuel gas with air and supplying an air-fuel mixture to a burner through a fan, a downstream end of a gas supply passage is connected to a gas suction portion that is provided in an air supply passage which is located on an upstream side of the fan, and a butterfly valve is provided in a portion of the passage, which is located on an upstream side from the gas suction portion, and is rotatable to at least an open attitude and a closed attitude. At least one ventilation opening through which air flows from an upstream side to a downstream side of the valve at the closed attitude is formed in the valve, and a size of the valve is set so that, at the closed attitude, a clearance between an outer peripheral edge of the valve and a peripheral wall surface of the passage is substantially closed.

1 . A premixing apparatus for mixing a fuel gas with air and supplying an air-fuel mixture to a burner through a fan,
 wherein a downstream end of a gas supply passage is connected to a gas suction portion that is provided in an air supply passage which is located on an upstream side of the fan, and a butterfly valve is provided in a portion of the air supply passage, which is located on an upstream side from the gas suction portion, and   wherein, provided that a rotational axis line along a predetermined radial direction of the butterfly valve serves as a center, the butterfly valve is configured to be rotatable to at least two attitudes: an open attitude parallel to a longitudinal direction of the air supply passage and a closed attitude orthogonal to the longitudinal direction of the air supply passage,   wherein:   at least one ventilation opening through which air flows from an upstream side to a downstream side of the butterfly valve at the closed attitude is formed in the butterfly valve, and a size of the butterfly valve is set so that, at the closed attitude, a clearance between an outer peripheral edge of the butterfly valve and a peripheral wall surface of the air supply passage is substantially closed, and air flows only through the ventilation opening.   
     
     
         2 . The premixing apparatus as claimed in  claim 1 , wherein multiple ventilation openings are formed, spaced in a circumferential direction of the butterfly valve. 
     
     
         3 . The premixing apparatus as claimed in  claim 2 , wherein, in the air supply passage, a venturi portion, which is located on a downstream side from the portion in which the butterfly valve is disposed and has a smaller diameter than the portion in which the butterfly valve is disposed, is provided, and the gas suction portion is provided to surround the venturi portion,
 wherein:   each of the ventilation openings is configured so that a part of each of the ventilation openings expands outward in the radial direction from a plane to which the venturi portion is projected upstream when the butterfly valve is rotated to the closed attitude.   
     
     
         4 . The premixing apparatus as claimed in  claim 1 , wherein each of the ventilation openings is circular in shape.