What is the function of an in a Economizer in a boiler ?

The Economizer in a boiler is used to preheat the water which is fed into the boiler by using the exhaust gases of the boiler.  In this way, it is able to "economize" or save energy.  The heat of the exhaust gases will be in the range of 380 to 550 degrees Celsius.  By utilizing this heat energy, the economizer increases the overall efficiency of the boiler. 

The Economizer is in the form of vertical tubes in which the water flows.  The gases on the way to the exhaust stack transfer their heat to the economizer.  The temperature of the inlet water to the economizer should not be too low as that can result in fouling and corrosion. 

The outlet temperature of the economizer is also below the boiling point of the water.