Economizer is a heat exchanger through which the feedwater is pumped. Another flue gas heat recovery method is through the use on an economizer. The economizer heats the feedwater to improve steam boiler efficiency and reduce heat loss to the stack. The increased heat in the feedwater reduces the boiler’s requirement for fuel and combustion air. In the economizer arrangement, the fluea gas leaves the steam boiler and enters the economizer where it make contact with the heat transfer surface, in the form of water tubes, through which the steam boiler feedwater flows. Since the flue gas is at higher temperature than the water, the flue gas is cooled and the water temperature is increased. Cooling the flue gas reduces its heat loss in amount equal to the increased heat in the feedwater to the steam boiler.
The feedwater thus arrive the steam boiler at higher temperature than would be the case if no economizer was fitted. Less energy is then required to raise the steam. Alternatively if the same quantity of energy is supplied, then more steam is raised. This results in a higher efficiency. In board terms a 10o C in feedwater temperature will give efficiency improvement of 2%.