Protecting Your Precious Recuperators in High Temperature Processes

Abstract

Recuperators are very useful heat exchangers that recover waste heat from products of combustion (poc) in a furnace stack and give them back to the heating operation in the form of preheated combustion air for the burners. Since part of the chemical energy in our purchased fuel must first be used to raise the air and fuel to flame temperature, the use of preheated air leaves more heat for transfer to the furnace load, or permits reduction of overall fuel consumption. Also, this heat-recycling affords a good relationship, time-wise, between the need for input and the availability of hot flue gases for air preheating. Unlike the heat exchange surface of waste heat boilers, however, recuperators re gas-to-gas heat exchangers that can overheat and develop hot spots because the only coolant to protect the heat exchanger material is the air being heated. Air is a good insulator and therefore a poor coolant; whereas the heat exchange surface of a waste heat boiler is backed by a good coolant-water-with a high latent heat, making it very forgiving. The flow of air coolant through a recuperator diminishes as the burner input is turned down to lower firing rates. But, the furnace temperature, and therefore the flue gas temperature, stays at about the same level. Although the flow of hot poc is reduced, the net effect is that heat exchange surface temperature rises, often above the limit of its materials. This is only one of several ways in which over-enthusiastic engineers have been 'burned' by recuperator failures.