What is an example of an open feed water heater?

An open feed water heater is specifically designed to preheat water before it enters a steam boiler, and the correct answer, which is a deaerator, serves this exact purpose. A deaerator reduces the levels of dissolved gases, like oxygen and carbon dioxide, from boiler feed water, which can cause corrosion and other issues within the system. By performing this function, a deaerator improves the overall efficiency and longevity of the boiler operations.

In addition to facilitating the preheating of feed water, deaerators also help in maintaining the water quality by removing harmful gases. This not only aids in the optimization of boiler performance but also enhances system reliability.

The other options, while related to the context of heat transfer or boiler operation, do not serve the same function as an open feed water heater. For instance, a shell and tube heat exchanger and a heat recovery steam generator are primarily designed for heat transfer between fluids, rather than specifically focusing on preheating feed water and removing gases. A boiler drum, on the other hand, is part of the boiler system itself and is primarily used to hold steam and water, rather than to preheat water or reduce gas content.