What does an economizer do in a boiler?

A boiler economizer (also known as an economizer) are mechanical devices intended to reduce energy consumption or perform a useful function such as preheating a fluid. A boiler economizer is essentially a heat exchanger that makes a system more energy efficient by taking enthalpy in fluid streams that are hot, but not hot enough to be used in a boiler – hence recovering more useful enthalpy and improving the steam boiler’s efficiency.

After producing steam, flue gas comes out from the system (flue gas is the gas exiting to the atmosphere via a flue). Some heat energy remains in the flue gas when it goes out. If we cannot utilize this heat energy, it gets lost. Boiler economizer is a device that uses a portion this remained energy of the flue gas to heat up inlet water (feed water) to the boiler. Since the heat energy is given to the water before it gets fed to the boiler, the requirement of fuel for producing steam is optimized. Because of that, we call this device as an economizer.