What Is a Boiler Deaerator? A Simple Guide for Boiler Operators

A boiler deaerator is a tank-like vessel in the feedwater system whose whole job is to drive dissolved gases out of the water before that water ever reaches the boiler. Ask "what is a boiler deaerator" and the short answer is: it is the boiler's protection against a silent killer — oxygen in the feedwater. New boiler operators sometimes walk right past the deaerator on rounds because it just sits there hissing quietly, but understanding it is one of the basics of power plant operations.

What Is a Boiler Deaerator and Why Feedwater Needs One?

Ordinary makeup water and even returned condensate carry small amounts of dissolved air — mostly oxygen and carbon dioxide. Inside a boiler, dissolved oxygen attacks steel and causes tiny pits in the metal (pitting corrosion), while carbon dioxide travels with the steam and forms carbonic acid in the condensate piping, eating away at the returns. Both shorten equipment life and both are hard to fix after the fact. The deaerator's job is to remove these gases from the feedwater upstream of the boiler feed pump, where the fix is cheap and easy.

How a Deaerator Works

The deaerator works on a simple idea: hot water holds far less dissolved gas than cold water. Low-pressure steam heats the incoming feedwater close to its boiling point, and the dissolved oxygen and carbon dioxide bubble out. These gases, along with a little steam, escape through a vent at the top of the vessel — a thin wisp of vent plume is the sign it is doing its job. The hot, de-gassed water collects in a storage section below, which gives the boiler feed pump a steady, hot supply and helps prevent pump cavitation.

What the Operator Watches

On rounds, a stationary engineer checks the deaerator for a few simple things:

• The vent: no venting means no deaerating — a plugged or closed vent defeats the whole purpose of the vessel.
• Temperature: the water should be very hot, close to the temperature of the heating steam.
• Steam supply: the heating steam must be flowing; without it, the deaerator is just a warm tank.
• Water level: keep the storage section at its normal level so the boiler feed pump never starves.

A well-run deaerator, combined with proper feedwater chemical treatment, is what keeps boiler steel and condensate piping alive for decades. It is a simple vessel doing a vital job — and now you know what to look for.

Want to learn more about feedwater systems and the rest of the steam cycle? Fundamentals of Power Plant Operations and Stationary Engineering explains every piece of the boiler room in plain, beginner-friendly language.