The greatest heat loss of steam boiler is done by the hot flue gases in the stack. Recovery of several of heat losses can be performed by economizer which utilizes flue gas to heat boiler feedwater or utilize flue gases to heat combustion air via air heater. These methods of boiler heat recovery will certainly increase the overall thermal efficiency and efficiency of steam boiler.
Generally for every 7°C rise in temperature of feedwater, steam boiler can reduce fuel consumption about 1%. For every 20°C rise in temperature of combustion air through air heater, fuel consumption of boiler can be reduced by 1%.
|Figure 1: Boundary Layer of Gas Side|
(Source: The Boiler Operator Handbook - Graham & Trotmant)
An economizer is an extendable of steam boiler and it is operated at similar pressure, therefore is susceptible to similar protection needs as steam boiler. Moreover circulation of water temperature coming into the economizer should not be very low possibly inducing too much sulfur corrosion, or very high so vapor bubbles form in the tubes. As common guideline temperature of water must be over 55°C and not greater than 20°C below saturation temperature of steam at working pressure of steam boiler that could be attained through steam tables. So as to observe these temps, thermometers are needed at the inlet and outlet of the economizer.
Many heating tools could be developed or designed to utilize preheated combustion air. A tubular air heater in the flue gas flow recuperates heat. Metal air heaters are susceptible to sulfur corrosion if temperatures of flue gas decrease very low. In big installations, stainless steel can be installed to defeat corrosion.