Deaerators Explained
Dissolved oxygen is the leading cause of boiler corrosion. Deaerators strip dissolved gases from feedwater before it enters the boiler. Learn how deaerators work, different types, and maintenance requirements.
What is a Deaerator?
A deaerator is a device that removes dissolved oxygen and other gases (primarily CO2) from boiler feedwater. Dissolved oxygen is extremely corrosive to boiler steel, causing pitting corrosion that can penetrate tube walls in months. Even trace amounts (above 0.007 mg/L) cause measurable corrosion.
Deaerators work on the principle that the solubility of gases in water decreases as temperature increases. By heating feedwater to the saturation temperature at the operating pressure, dissolved gases are released and vented away.
How It Works
The most common type is the spray-type deaerator. Cold feedwater enters the top and is sprayed into a steam-filled chamber. As the fine droplets contact steam, they are heated to saturation temperature almost instantly, releasing dissolved gases.
The water then falls onto a series of trays or fill plates where it forms thin films, providing additional surface area for any remaining gases to be stripped by the steam. The deaerated water collects in a storage tank below, while gases are vented from the top of the vessel.
Types of Deaerators
- Spray-type: Most common. Water is sprayed into steam, heated, and degassed. Good for most industrial applications.
- Tray-type: Water flows over trays in contact with steam. Very effective at removing trace gases. Common in large power boilers.
- Vacuum deaerator: Operates under vacuum, reducing the boiling point. Used when steam is not available for heating. Less common in boiler applications.
Operating Parameters
| Parameter | Target | Notes |
|---|---|---|
| Dissolved O2 in outlet | Below 0.007 mg/L (7 ppb) | Measured with a colourimetric or electrochemical test |
| Operating pressure | 0.2 to 0.3 bar (3 to 5 psi) | Slight positive pressure prevents air ingress |
| Water temperature | Saturation temp (105 to 108 degrees C at 0.2 bar) | Must be at saturation for effective degassing |
| Vent rate | Per manufacturer spec | Too much vent wastes steam. Too little leaves gases in the water. |
Chemical scavenger is a backup, not a replacement
Sulphite or hydrazine dosing is a backup to the deaerator, not a replacement for it. If the deaerator fails, the chemical scavenger cannot keep up with the oxygen load. Always repair deaerator problems immediately.
Tip: Test dissolved oxygen in the deaerator outlet weekly. If O2 is above 0.007 mg/L, check: steam pressure and flow, spray nozzle condition, vent valve position, and water inlet temperature. A cold water inlet can overwhelm the deaerator.