Deaeration and Feedwater Temperature
Feedwater carries dissolved oxygen, the main cause of pitting in boilers and feed systems. Heating the water drives the oxygen out, which is what a deaerator does, and the same heat raises feedwater temperature and saves fuel. The limit is the feed pump: water close to boiling is hard to pump.
You'll learn
- Explain why oxygen must be removed and how heating removes it
- Describe a feed tank and a pressurized deaerator
- Connect feedwater temperature to fuel saving
- Understand NPSH and feed pump cavitation
Before this: Raw water quality, Saturation pressure and temperature
Why oxygen has to go
Water in contact with air dissolves oxygen — about 8 mg per liter near room temperature. In a boiler that oxygen attacks steel, forming small, deep pits that can perforate a tube long before general corrosion would. The feed tank, feed lines, economizer and the boiler itself are all exposed.
Hot water holds far less dissolved gas than cold water; at its boiling point it holds almost none. So the simplest way to remove oxygen is to heat the feedwater close to boiling and let the released gas escape through a vent. Any trace that remains is removed chemically with an oxygen scavenger.
Feed tanks and deaerators
- Hot well or feed tank: an atmospheric tank where returned condensate and make-up mix, often heated by direct steam injection to around 85–90 °C. It removes much of the oxygen.
- Pressurized deaerator: a vessel held slightly above atmospheric pressure, typically a fraction of a kg/cm², where incoming water is sprayed or cascaded over trays against steam and heated to saturation — 105.0 °C at 0.2 kg/cm²(g). The released gases are vented. It removes oxygen to a very low level.
Either way, the feedwater reaches the boiler hotter. Each 6 °C of feedwater temperature is worth about 1% of fuel (UNEP), so the heat spent warming the feedwater, when it comes from exhaust steam, flash steam or returned condensate, is recovered rather than added.
The feed pump limit: NPSH
Hot water is close to its boiling point at the pump suction. If the pressure at the pump inlet falls below the water’s vapor pressure, bubbles form in the pump and collapse violently as the pressure rises: cavitation, which erodes the impeller and starves the boiler of feed. Water at 90 °C has a vapor pressure of 0.70 bar absolute; at 100 °C, 1.01 bar. The hotter the feed, the less margin.
| Symbol | Meaning | Unit |
|---|---|---|
| Absolute pressure on the water surface | Pa | |
| Vapor pressure of the water at its temperature | Pa | |
| Height of water surface above the pump | m | |
| Friction loss in the suction pipe | m |
NPSH available must exceed the pump’s NPSH required, with a margin. For a deaerator at saturation, the pressure term is zero — the water is exactly at its vapor pressure — so all of the margin comes from height. That is why deaerators sit high above the feed pumps, and why suction pipes are kept short, generous in size and free of restrictions.
Key takeaways
- Dissolved oxygen causes pitting; hot water holds almost none, so heating removes it.
- A pressurized deaerator heats feed to saturation, 105 °C at 0.2 kg/cm²(g), and vents the gas.
- Hotter feedwater saves about 1% of fuel per 6 °C.
- Hot feed raises cavitation risk: NPSH comes from tank height when the water is at saturation.