Temperature Compensated Air/Fuel Ratio Control on a Recuperated Furnace

Abstract

When recuperation is added to a furnace, air/ fuel ratio control seemingly becomes more complicated. Two methods normally used are mass flow control where the fuel pressure or flow is proportional to the mass flow of air or cross-connected control where fuel pressure is proportional to air pressure. Both methods can control fuel input from upstream (cold side) or downstream (hot side) of the recuperator.

With either system, cold side control may not compensate for changes in recuperator leakage and hot side control suffers from preheated air temperature variations. Variation in recuperator pressure drop with temperature also complicates a cold side cross-connected system.

This paper discusses advantages and disadvantages of various ratio control schemes when recuperation is used. Temperature compensated hot side cross-connection is described in detail. With this method, variations in preheated air temperature are sensed with a thermostatic metal. This varies the free flow area of a bleed orifice and compensates the pressure signal which controls the fuel flow.