Fuel/air Ratio Control Recuperative Gas Fired Furnaces

Abstract

The application of recuperation by combustion air preheat to industrial gas fired furnaces can result in an appreciable saving in fuel. The application of such recuperation has, however, been limited,since changes in the combustion air temperature can cause the fuel/air ratio to vary widely from the optimum value. Such variations can nullify the potential fuel saving offered by the recuperator and, in extreme cases, cause burner instability. In examining this problem the value of recuperation by combustion air preheat is assessed and data is presented giving the fuel saving that can be achieved with recuperators of various efficiencies operating over a range of furnace gas temperatures. The general need to control fuel/air ratio to the stoichiometric value is illustrated and the cost, in terms of fuel wastage, of not doing so is calculated. The effect of changes in combustion air temperature on the performance of air blast burner equipment is examined in detail and possible techniques for producing an air blast injector, that can maintain a constant air/gas ratio regardless of changes in throughput and combustion air preheat, have been studied. A pneumatic bridge flow ratio controller has been developed which, applied to an air blast tunnel burner, is shown to maintain the air/gas ratio to within = 5% of the stoichiometric value over a range of throughputs and combustion air temperatures. Incidental information presented includes fundamental data on the performance of gas governors and a throughput control technique designed to have a linear effective characteristic.