What Is a Deaerator? Function, Types, Oxygen Targets, and Boiler Feedwater Role

A deaerator is a mechanical device that removes dissolved oxygen and carbon dioxide from boiler feedwater before it enters the boiler. It is a standard component of industrial steam systems, and its primary job is corrosion prevention—not heating or storage, though it performs both as secondary functions.

This article covers what a deaerator does, why dissolved gases damage boiler systems, how deaeration works physically, the main types used in industrial applications, and the parameters that matter for sizing and specification.

Why Dissolved Gases Damage Boiler Systems

Dissolved oxygen and carbon dioxide cause two distinct but compounding corrosion mechanisms in boiler and pipework systems.

Dissolved oxygen attaches to the internal walls of metal piping, heat exchanger surfaces, and the boiler drum, forming iron oxides. This pitting corrosion creates localized damage that progresses faster than general surface corrosion. It is the more aggressive of the two gases in most boiler water chemistry contexts.

Dissolved free carbon dioxide combines with water to form carbonic acid (H₂CO₃). This reduces feedwater pH and accelerates corrosion of carbon steel components throughout the feedwater and condensate return circuit. A deaerator removes dissolved oxygen and free carbon dioxide. Combined carbon dioxide—released from alkalinity decomposition in the boiler—may still enter the steam-condensate cycle downstream and requires separate condensate treatment, independent of deaerator performance.

The two gases also interact. Oxygen accelerates the corrosive effect of carbonic acid, so a system with both gases present suffers more damage than one with either gas in isolation.

Dissolved oxygen targets by system type Source: U.S. DOE Steam Tip Sheet #18, "Deaerators in Industrial Steam Systems." Additional chemistry targets should be confirmed against the applicable ASME/ABMA or boiler OEM guideline for the specific pressure class.

These targets apply to pressure deaerators for boiler feedwater conditioning. Vacuum deaerators—used in lower-duty water distribution or some process applications—typically achieve 330–650 ppb dissolved oxygen and are not suitable for medium- or high-pressure boiler feedwater.

Chemical oxygen scavengers such as sodium sulfite, DEHA, and carbohydrazide are used alongside mechanical deaeration to remove residual traces. They are supplements to mechanical removal, not substitutes for it.

How a Deaerator Works: The Physical Principles

Effective deaeration depends on four operating conditions: sufficient temperature, turbulence, thin-film water exposure, and adequate contact time between water and steam.

Two physical laws underpin the process.

Henry's Law states that gas solubility in a liquid decreases as the partial pressure of that gas above the liquid decreases. Reducing the partial pressure of oxygen and carbon dioxide in the vapor space above the feedwater drives those gases out of solution.

Dalton's Law of Partial Pressures supports this. When steam is introduced into the deaerator, it displaces oxygen and carbon dioxide in the vapor space. Their partial pressures drop, accelerating their release from the water.

In practice, the deaerator uses steam to simultaneously heat the feedwater to its saturation temperature and strip dissolved gases. Gas solubility drops sharply as water temperature approaches boiling point—cold water holds significantly more dissolved oxygen than water at saturation temperature. Heating to saturation is both a thermodynamic requirement and a performance condition.

The released gases exit through a vent at the top of the deaerator. A small fraction of the steam supplied to the deaerator—typically 5–14% of that steam—must be vented along with the stripped gases. This is not 5–14% of total boiler steam generation; it refers only to the steam entering the deaerator itself. Vent steam loss is a real operating cost and should be accounted for when sizing the steam supply.

The deaerated feedwater collects in the storage section at the bottom. A boiler feedwater pump draws from this storage section and must have sufficient net positive suction head (NPSH) available. Deaerator elevation and storage tank pressure should be checked against the feed pump NPSHr curve during system design.

Main Types of Industrial Deaerators

Two designs dominate industrial boiler feedwater applications.

Tray-type deaerator

The tray-type consists of a vertical deaeration column mounted above a horizontal storage vessel. Feedwater enters the top of the column and passes downward through perforated trays or baffles. Steam rises upward counter-current to the water flow. The trays break the water into thin films and droplets, maximizing steam-water contact surface area. This extended contact gives tray-type deaerators high deaeration efficiency and makes them the preferred choice for high-pressure systems and variable-load applications.

Spray-type deaerator

The spray-type introduces feedwater through spray nozzles that atomize the water into fine droplets, creating high surface area for steam contact. Deaerated water collects in the storage vessel below. Spray-type units are more compact, have fewer internal components, and suit lower-pressure systems or installations where height is constrained.

Both tray-type and properly designed spray deaerators can achieve the 7 ppb dissolved oxygen target. Selection depends on load turndown requirements, available installation height, condensate return variability, and maintenance preference.

Secondary Functions: Storage and Feedwater Preheating

Feedwater storage. The storage vessel provides a buffer of deaerated, heated feedwater for the boiler feed pump. This buffer absorbs short-term fluctuations in condensate return or makeup water supply without causing the feed pump to run dry or the boiler water level to drop suddenly. Storage volume is commonly specified at 10–20 minutes of feedwater demand at maximum boiler output. Ten minutes is often used as a minimum baseline; higher storage is selected for unstable condensate return or critical boiler loads.

Feedwater preheating. The deaerator heats feedwater to saturation temperature at its operating pressure. As a widely referenced rule of thumb, every 6°C increase in feedwater temperature can reduce fuel consumption by approximately 1%. The actual saving depends on boiler pressure, fuel type, excess air level, condensate return rate, and whether an economizer is also installed.

In our experience specifying deaerators for industrial boiler installations in food processing, textile, and chemical applications, facilities that retrofit a deaerator onto systems previously relying on chemical scavenging alone consistently report reductions in both fuel consumption and maintenance frequency. This is especially clear in systems with low condensate return, where the cold makeup water proportion is highest.

Key Installation and Sizing Variables

A correctly sized and installed deaerator delivers its rated dissolved oxygen performance. An incorrectly sized or improperly installed unit may deliver 40–50 ppb even when mechanically functional.

Operating pressure and temperature. Pressure deaerators are operated at a pressure selected to maintain feedwater near the corresponding saturation temperature. Common design examples range from approximately 0.02 MPa(g) to 0.4 MPa(g), depending on required outlet temperature and site steam conditions. Operating pressure and saturation temperature should always be specified together—one determines the other.

Pressure regulation. A dedicated pressure-regulating valve on the steam supply is required to maintain constant deaerator operating pressure. Sudden increases in flash steam entering the deaerator can spike internal pressure and temporarily re-oxygenate the feedwater.

Vent sizing. The vent must carry away stripped gases continuously without over-venting steam. Under-venting raises dissolved gas levels. Over-venting wastes steam and increases operating cost.

Insulation. The deaeration column, storage vessel, and all connecting pipework must be insulated. Heat loss causes feedwater to cool below saturation temperature, which increases gas solubility and reduces deaeration effectiveness.

Condensate return rate. High condensate return reduces the proportion of cold makeup water and lowers the steam demand for heating. Systems with condensate return above 80% require significantly less steam for deaeration than low-return systems. This affects both steam supply sizing and vent steam loss.

Design code. The deaerator vessel is a pressure vessel. In the US market, it is typically designed and manufactured to ASME Section VIII Division 1. Confirm the applicable code with the local authority having jurisdiction before specifying.

We size deaerators against confirmed peak feedwater demand, condensate return rate, steam supply conditions, dissolved oxygen target, storage time requirement, and applicable code—not from catalog tables alone.

Chemical Oxygen Scavengers: Complement, Not Substitute

Even a well-operated mechanical deaerator may leave residual dissolved oxygen above the target during startup, load changes, or periods of high makeup water flow. Chemical oxygen scavengers are added to the deaerated feedwater to remove these residual traces.

Common scavengers used in industrial boiler systems:

· Sodium sulfite (Na₂SO₃): Reacts with oxygen to form sodium sulfate, which is non-scaling. Not recommended above 60 bar, as it decomposes to sulfur dioxide at high temperature and pressure.

· Hydrazine alternatives (DEHA, carbohydrazide): Used in high-pressure systems above the temperature limit for sodium sulfite. Also provide some passivation of metal surfaces.

· Erythorbic acid, tannins: Used in specific lower-pressure applications.

Scavenger dosage should be controlled by residual sulfite, ORP, or dissolved oxygen testing—not by fixed dosing alone. Using chemical scavengers as the primary oxygen removal method without a deaerator results in high chemical consumption, inconsistent protection, and increased total dissolved solids from reaction products accumulating in the boiler water.

Deaerator Specification Parameters

When requesting a deaerator quotation or specification, provide these parameters to allow accurate sizing:

FAQ

What does a deaerator remove from feedwater?

A deaerator removes dissolved oxygen and free carbon dioxide—the gases responsible for corrosion in boiler systems. Combined carbon dioxide from alkalinity decomposition in the boiler is not removed by the deaerator and requires separate condensate treatment. The deaerator also does not remove entrained air from loose pipe connections or pump cavitation—those are separate system issues.

What dissolved oxygen level does a pressure deaerator achieve?

Most pressure deaerators for boiler feedwater are designed to reduce dissolved oxygen to 7 ppb by weight—commonly expressed as 0.005 cm³/L under HEI performance terminology. High-pressure systems above 200 psig typically target 5 ppb. Vacuum deaerators used in lower-duty applications achieve 330–650 ppb and are not suitable for medium- or high-pressure boiler feedwater.

Is a deaerator required for all industrial boilers?

For continuous-duty medium- and high-pressure steam systems, a deaerator is generally treated as a standard feedwater component. Low-pressure or intermittent-duty systems may use chemical oxygen scavenging alone, but this should be evaluated against the dissolved oxygen target, chemical cost, condensate return rate, and expected equipment service life.

Where is a deaerator positioned in the steam system?

The deaerator sits between the condensate return and makeup water system on the inlet side and the boiler feedwater pump on the outlet side. It receives a mixture of returned condensate and fresh makeup water, deaerates and preheats the combined flow, and stores it as ready-to-pump feedwater. Deaerator elevation and storage tank pressure must be confirmed against the boiler feed pump NPSHr curve.

How much steam does a deaerator consume?

Steam consumption depends on the proportion of cold makeup water and the target outlet temperature. Systems with high condensate return use less steam. Typically, 5–14% of the steam supplied to the deaerator must be vented to carry away stripped gases. This percentage applies to the steam entering the deaerator—not to total boiler steam generation.

What standards apply to deaerator pressure vessel design?

In the US, deaerators are typically designed and manufactured to ASME Section VIII Division 1. The applicable standard depends on the installation country and authority having jurisdiction. Confirm the required code and any third-party inspection requirements before procurement.

What maintenance does a deaerator require?

Regular inspection of the vent, spray nozzles or tray assemblies, and pressure controls. Tray assemblies should be checked periodically for fouling, corrosion, or mechanical damage that reduces water-steam contact efficiency. The pressure regulating valve should be calibrated and tested. Insulation should be checked annually. Internal surfaces should be inspected for corrosion during scheduled shutdowns.

Conclusion

A deaerator is a necessary component of continuous-duty industrial steam systems where dissolved oxygen and carbon dioxide corrosion is a concern. It works by heating feedwater to saturation temperature and stripping dissolved gases through steam-water contact, governed by Henry's Law and Dalton's Law of Partial Pressures. Pressure deaerators in tray-type or spray-type configurations are the standard for boiler feedwater applications, achieving dissolved oxygen levels of 7 ppb or below.

Beyond gas removal, the deaerator contributes to boiler efficiency through feedwater preheating and protects the boiler feed pump through pressurized storage. These secondary functions make it a system-level investment, not just a corrosion control measure.

If you are specifying a deaerator for a new or existing industrial boiler installation, the variables that matter most are peak feedwater demand, condensate return rate, steam supply pressure and quality, dissolved oxygen target, required storage time, and applicable pressure vessel code. Share those parameters with our team and we will confirm the right deaerator type, sizing, and integration with your boiler system.