Estimation of Fuel Savings by Recuperation of Furnace Exhausts to Preheat Combustion Air

Abstract

The recovery of waste energy in furnace exhaust gases is gaining in importance as fuel costs continue to escalate. Installation of a recuperator in the furnace exhaust stream to preheat the combustion air can result in considerable savings in fuel usage. These savings are primarily the result of the sensible heat increase of the combustion air and, to some extent, improved combustion efficiency. The amount of fuel saved will depend on the exhaust gas temperature, amount of excess air used, the type of burner and the furnace control system. These fuel savings may be accurately measured by metering the energy consumption per unit of production before and after installation of the recuperator.

In the design of a waste heat recuperation system, it is necessary to be able to estimate the fuel saved by use of such a system. Standard industrial practice refers to the method described in the North American Combustion Handbook with its curves and tables that directly predict the percentage fuel savings. This paper analyzes the standard estimation technique and suggests a more realistic approach to calculation of percent fuel savings. Mass and enthalpy balances are provided for both methods and a typical furnace recuperation example is detailed to illustrate the differences in the two methods of calculating the percent energy saved.