FURNACE CONTROL USING INDUCED DRAFT BLOWER, EXHAUST GAS FLOW RATE SENSING AND DENSITY COMPENSATION

摘要

FURNACE CONTROL USING INDUCED DRAFT BLOWER, EXHAUST GAS FLOW RATE SENSING AND DENSITY COMPENSATION An induced draft combustion apparatus and its associated control system has a blower located in the exhaust stack or vent which is used to induce the movement of air and combustion products into, through and out of the combustion chamber. A flow-restricting orifice in the exhaust stack near the blower causes a region of higher pressure to exist upstream from the orifice with a region of lower pressure downstream from -the orifice. An exhaust gas pressure signal representative of the exhaust gas volume flow rate 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 exhaust gas volume flow rate. By controlling blower speeds and exhaust gas volume flow capacities as related to a selected orifice size, various firing rates for the furnace can be selected, from the design maximum down to various derated levels. Temperaturesensitive devices cooperating with the stack orifice or with the modulating gas valve are employed to compensate for changes in the density of the exhaust gas which accompany startup and changes in firing rate.