Premixing Apparatus and Method Thereof

A method of premixing apparatus for mixing fuel gas with air to supply thus obtained air-fuel mixture, through a fan, to a burner,

• the premixing apparatus being so arranged that a downstream end of a gas supply passage having interposed therein a flow control valve for supplying fuel gas is connected to a gas suction section disposed in an air supply passage on an upstream side of the fan,
• the premixing apparatus comprising:
• an air resistance changeover means for changing over, between high and low, a first ventilation resistance in a first section of the air supply passage wherein the first section is on an upstream side of the gas suction section;
• and
• a gas resistance changeover means for changing over, between high and low, a second ventilation resistance in a second section of the gas supply passage wherein the second section is on the downstream side of the flow control valve,
• the air resistance changeover means being constituted by a butterfly valve disposed in the first section of the air supply passage so as to be rotatable into an opened posture parallel with a longitudinal direction of the air supply passage or a closed posture at right angles to the longitudinal direction of the air supply passage, and
• the gas resistance changeover means being constituted by a changeover valve disposed in the gas supply passage in an openable and closable manner, and has an interlocking mechanism to open or close the changeover valve in interlocking relationship with a rotation of the butterfly valve into the opened posture or into the closed posture,
• wherein
• the interlocking mechanism being arranged to maintain the changeover valve in a fully-closed state when the butterfly valve lies in a predetermined first rotation angle range inclusive of the closed posture, and maintains the changeover valve in a fully-opened state when the butterfly valve lies in a predetermined second rotation angle range inclusive of the opened posture;
• wherein the method of controlling the premixing apparatus comprises:
• a step of setting the rotation angle of the butterfly valve at which the excess air ratio of the air-fuel mixture becomes a predetermined value within the first rotation angle range is set to be a closing-side stop angle of the butterfly valve;
• a step of setting the rotation angle of the butterfly valve at which the excess air ratio of the air-fuel mixture becomes a predetermined value within the second rotation angle range is set to be an open-side stop angle of the butterfly valve;
• at a time of changing over to a small-capacity state in which the first ventilation resistance in the air supply passage is increased and also in which the second ventilation resistance in the gas supply passage is increased, a step of rotating the butterfly valve down to the closing side stop angle;
• and
• at a time of changing over to a large-capacity state in which the first ventilation resistance in the air supply passage is decreased and also in which the second ventilation resistance in the gas supply passage is decreased, a step of rotating the butterfly valve up to the open-side stop angle.