Introduction
Boiler feedwater carries dissolved oxygen and carbon dioxide into equipment that runs hot and pressurized around the clock. Left untreated, those gases eat through economizer tubes, feedwater piping, and boiler steel from the inside out. A deaerator exists to strip those gases out before the water ever reaches the boiler.
Many plant operators struggle with inconsistent steam supply, poor venting, or control-valve sizing that undermines deaerator performance even when the vessel itself is sound. Dissolved oxygen above target levels can accelerate pitting at the economizer inlet, along downcomer tubes, and at feedwater-distribution holes, according to Veolia's water handbook.
This guide covers what a deaerator does, how steam contact removes gases, where that steam comes from, how to estimate steam consumption, and how to spot supply or venting problems before they cause damage.
Key Takeaways
- Deaerators strip dissolved O₂, CO₂, and other gases from feedwater before the boiler
- Near-saturation temperature, steam contact, residence time, and venting make deaeration work
- Meter supply steam from extraction, auxiliary, cold-reheat, or another plant source
- Feedwater flow, inlet temperature, condensate return, and pressure set steam consumption
What Is a Deaerator?
A deaerator is an open feedwater-heating vessel that uses direct steam contact and a controlled vent to remove dissolved gases from water before it enters the boiler. It is an active process vessel, not a passive holding tank, where heat and turbulence strip the gases.
Why it matters: dissolved oxygen and carbon dioxide are the two gases most responsible for preboiler and boiler corrosion. Oxygen pitting can be severe at the economizer inlet, and deep pits lead to rapid failures in feedwater lines, economizers, boiler tubes, and condensate piping.
Beyond gas removal, deaerators handle a few supporting jobs:
- Preheating feedwater closer to boiler saturation temperature, reducing thermal shock
- Providing a buffered volume of treated water ahead of the boiler feed pumps
- Smoothing short-term swings in condensate return or makeup water demand
What a Deaerator Is Not
It helps to separate a deaerator from equipment it sometimes gets confused with:
- Chemical dosing system - injects oxygen scavengers or amines; doesn't physically strip gases
- Condensate receiver or surge tank - collects water but provides no heating or gas stripping
- Desuperheater or attemperator - conditions steam temperature downstream, not water chemistry
Common Configurations
| Design | How it works | Typical fit |
|---|---|---|
| Spray | Nozzles atomize incoming water into droplets for steam contact | Smaller or simpler systems |
| Tray | Water cascades over perforated trays while steam rises through it | Larger or more variable loads |
| Spray-tray combination | First-stage spray followed by second-stage tray contact | Systems needing fast preheat plus thorough stripping |
Internal geometry, operating pressure, load swing, and condensate-return conditions all factor into which configuration a given plant specifies.
How Does a Deaerator Work and How Is Steam Supplied?
The deaerating process follows a sequence: feedwater enters, steam heats and contacts it, released gases exit through a vent, and treated water collects for delivery to the boiler. Each stage depends on the one before it.
Initiation
Condensate return, makeup water, and other feedwater streams enter through controlled inlet connections. Flow varies constantly with boiler demand and system losses, so the deaerator has to absorb those swings without losing control of pressure or level.
Operation is normally continuous and automated. Startup looks different: before extraction or process steam is available, many plants rely on auxiliary steam to bring the unit up to temperature.
Watch these dependencies closely:
- Inlet-water temperature and flow rate
- Tank level and steam pressure
- Control-valve capacity
- Strainer and isolation-valve condition
Steam Supply and Heating
Heating steam does two things at once: it raises water temperature and lowers the solubility of dissolved gases, letting oxygen leave the liquid phase. Steam sources vary by plant design:
- Extraction steam - pulled from a turbine stage
- Cold-reheat steam - taken from HP-turbine exhaust, a common extraction point in power-cycle service
- Auxiliary or startup steam - used when extraction isn't yet available
- Other plant-specific sources - determined by cycle design and available pressure
A pegging-steam or deaerator control valve regulates pressure and flow to keep conditions within the range the specific plant and manufacturer call for. There's no single universal setpoint. Check your plant standard or OEM documentation rather than borrow a number from another system.
Large pressure drops across that control valve can generate noise, vibration, or downstream erosion. Mitigation typically involves valve trim selected for the service, staged pressure reduction across multiple steps, or a downstream diffuser where engineering analysis supports it.
Core Operation
In spray-type units, nozzles divide incoming water into fine droplets, multiplying the surface area in contact with steam before the water drops into storage.
Tray-type units work differently. Water flows over perforated trays or packing while steam rises through it, creating repeated contact at each level. This design tends to suit larger systems or ones with more variable loads, since staged contact gives gases more chances to escape.
Regardless of design, effective deaeration needs four things at once:
- Temperature near the saturation point for the operating pressure
- Enough turbulence or surface area
- Sufficient residence time
- A stable flow pattern
Skimp on any one and dissolved oxygen readings climb.
Regulation and Control
Pressure, level, steam flow, water flow, and venting all get monitored to keep the unit inside its intended operating window. The vent plays a dual role: letting stripped gases and a controlled amount of carrier steam escape without wasting excess steam.
DOE's steam tip sheet notes that typically 5% to 14% of deaerator steam is vented for stripping, with the rate climbing when makeup water runs cold and oxygen-rich. The same source points out that a sudden surge in flash steam can spike deaerator pressure and reoxygenate feedwater- a strong argument for a dedicated pressure-regulating valve.
Estimating steam demand comes down to a mass-and-energy balance: feedwater flow, inlet enthalpy or temperature, steam enthalpy, condensate-return conditions, and the target deaerator condition all factor in. Spirax Sarco's engineering reference walks through this calculation in detail. Plant data and a qualified engineer should confirm the numbers for your specific system.
Common symptoms of poor control, per HEI's troubleshooting guide, include:
| Symptom | Possible cause |
|---|---|
| High dissolved oxygen | Air in-leakage, improperly installed trays, or inlet conditions outside design |
| Excessive pressure fluctuation | Undersized or miscalibrated steam PRV, or inlet-temperature variation |
| Water discharging from the vent | Cracked vent welds, improper vent piping, or carryover |
Output and Result
Deaerated, heated water collects in storage and is pumped toward the boiler or steam-generation equipment. Consistent feedwater condition supports lower corrosion risk, less thermal shock, and more stable boiler operation.
A deaerator alone doesn't replace water chemistry monitoring, vent inspection, oxygen-scavenger control where specified, or routine valve maintenance.
Where Deaerators Are Used
In a typical power or process cycle, condensate and makeup water enter the deaerator, get heated and stripped of gases, then sit in storage until boiler feed pumps send them toward the boiler or heat-recovery steam generator.
You'll find deaerators across a wide range of U.S. facilities:
- Fossil-fuel and HRSG power plants
- Industrial boiler rooms and cogeneration facilities
- Refineries and chemical plants
- Pulp-and-paper operations
- Waste-to-energy systems
- Campus utilities and district-heating plants
System conditions shape design choices:
- High makeup-water rates increase gas-removal duty
- High condensate-return volumes shift the heat balance
- Fluctuating loads put more weight on storage capacity, control range, and dependable steam supply
Deaerators sometimes operate alongside other steam-conditioning equipment in the same plant. B-Tech Valve's LPD large-pipe and SPAD small-pipe desuperheaters, for instance, control steam temperature downstream. They're not a substitute for a deaerator, but they can matter when the broader steam system also needs application-engineered attemperation.
LPD units are sized for steam pipes 6 inches and larger; SPAD units handle 4-inch and smaller lines, drawing water from a separate temperature-control valve.
Conclusion
A deaerator's logic is straightforward: controlled steam heating and steam-water contact release dissolved gases, a vent carries those gases out, treated water collects in storage, and the boiler gets warmer, less corrosive feedwater.
Understanding steam supply (where it comes from, how the control valve is sized, and how the vent is designed) feeds directly into better decisions on startup operation, troubleshooting, and maintenance. For plant-specific steam-conditioning or desuperheater applications, reach out to B-Tech Valve LLC for application engineering guidance at Sales@B-TechValves.com or 609-321-2205.
Frequently Asked Questions
What is the purpose of a deaerator?
A deaerator removes dissolved oxygen and carbon dioxide from boiler feedwater while preheating it, which protects against corrosion. It also provides a buffered storage volume ahead of the boiler feed pumps.
How does a deaerator remove oxygen?
Steam heats the water near saturation temperature, which lowers gas solubility and drives dissolved oxygen out of solution. A controlled vent then carries the released gases, along with a small amount of carrier steam, out of the system.
What are the three main types of deaerators?
Spray-type units atomize water for steam contact, tray-type units cascade water over perforated trays, and combination designs use both stages. Exact terminology and configurations vary by manufacturer.
What does deaeration mean?
Deaeration is the removal of dissolved gases, mainly oxygen and carbon dioxide, from water. It's typically accomplished through heat, direct steam contact, and controlled venting.
How do you calculate deaerator steam consumption?
Steam consumption comes from a mass-and-energy balance using feedwater flow, inlet and outlet temperatures, condensate-return conditions, steam properties, and vent losses. Plant data and qualified engineering guidance should confirm the final figures.
Which chemical is used in deaerators?
Deaerators remove gases mechanically and thermally rather than chemically. Many facilities still add oxygen scavengers, such as sulfite- or amine-based treatments, afterward depending on water chemistry and plant specifications.
