Deaerators: Removing Oxygen from Boiler Feedwater is the mechanical process that strips dissolved oxygen and carbon dioxide from feedwater before it enters a steam boiler, protecting the boiler, economizer, piping and condensate system from corrosion. Nearly every industrial boiler above the smallest package units relies on some form of deaeration, and getting it wrong quickly turns the system into a corrosion problem.
Makeup water and returning condensate both carry dissolved atmospheric gases, mainly oxygen and carbon dioxide. Dissolved oxygen is aggressively corrosive to carbon steel at boiler temperatures, producing localized pitting on tube walls, drums and feedwater piping; a small amount can cause a perforation long before uniform wall thinning would be noticed. Carbon dioxide forms carbonic acid, lowering pH and driving corrosion in condensate lines, traps and heat exchangers, far downstream of the boiler. Removing both gases before they reach hot metal is far cheaper than living with the consequences.
Gas solubility in water falls sharply as temperature rises, reaching a practical minimum near the water's boiling point at the operating pressure. A deaerator heats incoming feedwater with steam to bring it to, or very close to, saturation temperature, so dissolved gases come out of solution as free gas. Heating alone is not enough: the released gas must be carried away before it redissolves, so deaerators pair heating with mechanical scrubbing, breaking water into fine droplets against counterflowing steam and venting the liberated gas before it cools and redissolves.
Selection comes down to boiler size, load variability and the residual oxygen target required downstream.
A properly sized, properly operated deaerator with adequate steam supply and correct venting routinely reduces dissolved oxygen to the low parts-per-billion range, commonly cited as 7 ppb (0.005 cm3/L) or better in HEI and ASME guidance, with carbon dioxide cut to a negligible level too. Holding that range takes sufficient steam flow to maintain saturation temperature at all loads, correct venting, and adequate residence time in the scrubber.
| Parameter | Poorly operated deaerator | Well-run deaerator |
|---|---|---|
| Residual dissolved oxygen | 50 to 100+ ppb | 7 ppb (0.005 cm3/L) or less |
| Operating temperature vs saturation | Several degrees C subcooled | Within about 1C of saturation |
| Vent rate | Under-vented or over-vented | Set per manufacturer curve for load |
| Storage level control | Erratic, prone to flashing | Stable, sized for pump NPSH margin |
Below the deaerating section, every deaerator has a storage tank holding deaerated water at saturation temperature, ready for the feed pumps. The tank buffers demand swings and, just as important, provides the elevation and static head feed pumps need for their NPSH requirement. Because storage water sits near its boiling point, the margin between available and required NPSH is thin by design, which is why storage tanks are almost always mounted on elevated steel structures, sometimes called deaerator towers, so the extra head prevents cavitation at the pump inlet. Level and pressure control here are treated as protective functions, not just inventory management.
Mechanical deaeration removes the great majority of dissolved oxygen but cannot reach zero. Chemical scavengers, such as sodium sulfite for lower and medium pressure boilers or agents like hydrazine substitutes and carbohydrazide derivatives for higher pressure units, are dosed downstream of the deaerator to react with that residual. Scavengers are a backup and polishing measure, not a substitute for mechanical deaeration.
Performance also depends on how much condensate a plant returns and in what condition. Condensate arrives hot and largely gas-free if the condensate return system is tight and well vented, easing the deaerator's load, while air in-leakage at pumps or damaged traps reintroduces oxygen already stripped out once. Deaerator performance, blowdown rates and condensate integrity are best reviewed together; see our overview of boiler blowdown practice for how dissolved solids concentrate and purge from the boiler.
Routine maintenance covers spray valve and tray inspection for scale or erosion, vent rate checks against load, storage level and pressure instrumentation, and periodic dissolved oxygen testing at the outlet. These tasks are easy to defer, and the consequences show up months later as pitted tubes, so many teams track inspections and dosing logs in a CMMS platform like Fabrico, keeping the work on a fixed interval so drift gets flagged early. Book a Fabrico demo to see how deaerator PM schedules fit into a broader reliability program.
A plain feedwater tank just holds water for the boiler feed pumps. A deaerator actively heats and mechanically scrubs the water with steam to strip out dissolved oxygen and carbon dioxide, then vents the liberated gases away.
The storage section holds water near its boiling point, so the margin against pump cavitation is thin. Elevating the tank adds static head at the pump suction, satisfying the NPSH requirement and preventing flashing at the pump inlet.
Some very small or low-pressure systems use chemical treatment alone, but for most industrial boilers this is impractical at scale, since dosing has to track a constantly moving oxygen load without mechanical removal. Mechanical deaeration plus a scavenger as a polishing step is the standard approach.
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