The recirculation valve of economizer is equipped in order to protect the economizer when the feedwater is interrupted if fire is made in the  boiler or the boiler is shutdown. Because during the period of making a fire and shutdown of the boiler, when the water is interrupted, the water in the economizer does not flow, the high temperature flue gas will burn out the economizer tube. Open the recirculation valve of the economizer and use the density difference of working medium of the steam drum and economizer to produce natural cycle, so that the economizer tube can be cooled.