What is primarily preheated by an economizer?

The correct answer is water. An economizer is a component typically found in steam boilers and other heat recovery systems that is designed to improve efficiency by preheating the feedwater before it enters the boiler. By utilizing the waste heat from the flue gases, the economizer raises the temperature of the incoming water, which reduces the amount of energy needed to convert that water into steam. This process conserves fuel, enhances the overall thermal efficiency of the system, and reduces emissions by enabling the boiler to operate more effectively.

Combustion air, while also important in the context of improving combustion efficiency, is not the primary focus of an economizer. Fuel oil is generally preheated separately to ensure good combustion and prevent issues such as clogging. Steam is typically produced as a result of boiling water, but it is not preheated by the economizer; rather, it is the outcome of the heating process applied to the water in the boiler.