What does an economizer use to heat feedwater?

An economizer is a device designed to improve the efficiency of a steam boiler system by capturing waste heat from the flue gases produced during the combustion of fuel. In this process, it utilizes the heat energy from the combustion gases—these are the hot gases that exit the boiler after burning fuel to produce steam. By transferring this residual heat to the feedwater that is introduced into the boiler, the economizer preheats the water before it enters the boiler itself. This preheating reduces the amount of energy required to reach the necessary steam generation temperature, thereby improving overall efficiency and reducing fuel consumption.

The other options, while related to heating or thermodynamic processes, do not accurately describe how an economizer functions:

• Electric heaters rely on electrical energy for heating, which is not the primary mode of operation for economizers.

• Radiant heat refers to heat transferred via radiation, which is not specific to the economizer’s operation as it directly utilizes heat from combustion gases.

• Steam vapor would be used in other contexts but isn't the primary source of heat for an economizer, as it specifically depends on the heat carried by the flue gases.