Which type of feedwater heater is commonly used to transfer heat from the exhaust steam?

A closed feedwater heater is designed to enhance the efficiency of a steam system by transferring heat from exhaust steam to the feedwater without permitting direct contact between the two. In this arrangement, the exhaust steam flows through a network of tubes, which are surrounded by the feedwater. The heat from the exhaust steam is transferred through the walls of the tubes to the feedwater, raising its temperature before it enters the boiler.

This method effectively utilizes heat that would otherwise be wasted, increasing the overall thermal efficiency of the system. The closed design also helps prevent contamination of the feedwater since the steam and water do not mix. This characteristic is particularly important for maintaining the quality and purity of the feedwater, ensuring optimal boiler operation and longevity.

In contrast, options such as an open feedwater heater allow for direct contact between the steam and feedwater, which can lead to issues with contamination and requires careful monitoring of water quality. A flash drum serves a different purpose related to vaporizing liquid in a pressure control context, while a desuperheater is utilized specifically for cooling superheated steam before it enters a turbine or a process, rather than for heating feedwater.