The economizer is a heat exchanger using the waste heat of exhaust fume from boiler to heat the water supply. The economizer absorbs the waste heat of the exhaust fume, reduces the temperature of exhaust fume and improve the boiler efficiency In addition, because the water temperature of feedwater entering the drum is increased after it passes through the economizer, so the thermal stress of steam drum caused by temperature difference is reduced, thus improving the working conditions of steam drum and prolonging the service life of the steam drum.