Furnace control using induced draft blower and exhaust stack flow rate sensing

摘要

Apparatus is provided for constructing an induced draft furnace and its control system, to produce an induced draft furnace having increased efficiency. A blower located in the furnace exhaust stack is used to induce movement of air, fuel and combustion products into, through and out of the combustion chamber. A flow-limiting orifice in the exhaust stack in proximity to the blower causes a region of higher pressure to exist upstream from the orifice, with a region of lower pressure downstream from the orifice. A pressure signal representative of the flow rate of exhaust stack gases is sensed on one side of the orifice and is fed back to a modulating gas valve which controls the outlet gas flow from the valve to be proportional to the magnitude of the pressure signal representing exhaust stack flow rate. By selecting blower speeds and flow capacities, various firing rates for the furnace can be selected, from the design maximum of the furnace down to various derated levels.