What is a Deaerating Heater and How Does It Work?

A deaerating heater, or deaerator, heats boiler feedwater and brings it into direct contact with steam so dissolved oxygen, carbon dioxide and other non-condensable gases can leave through a controlled vent. Lower dissolved oxygen reduces one cause of corrosion in pre-boiler and boiler equipment. Deaeration works with condensate control, makeup-water treatment and cycle chemistry; it does not replace them.

Common direct-contact configurations use trays, sprays or a combination of spray and packed or tray contact sections. Steam heats incoming water to near the saturation temperature at deaerator pressure and strips released gases towards the vent. Steam is not a source of hydrazine, sodium sulfite or any other oxygen scavenger. If chemistry requires a scavenger, trained water-treatment staff select and dose it separately.

Tray Type Deaerating Heater

In a tray section, incoming water is sprayed or distributed over perforated trays. Water forms droplets and films while steam passes through the contact stages. The repeated surface renewal improves heat and mass transfer. Deaerated water drains to the storage section, while non-condensable gases and a controlled amount of steam move to the vent or vent condenser.

The advantages of a tray-type deaerating heater are:

• • A well-designed tray bank can provide several contact stages and stable performance across its specified operating range.

• • It can meet a low residual-oxygen guarantee when pressure, temperature, flow, venting and inlet conditions stay within the tested specification.

• • A separate storage section can buffer feedwater demand and provide the required suction conditions for downstream boiler-feed pumps.

The disadvantages of a tray-type deaerating heater are:

• • Trays add pressure drop and can be damaged by hydraulic or mechanical upset. Steam use comes mainly from the heat balance and vent rate, not from the tray label alone.

• • Internal trays, supports, spray valves and vent components require access, inspection and maintenance.

• • Deposits, debris or poor water distribution can block openings and reduce contact. Upstream treatment and inspection intervals must match the water source.

Spray Type Deaerating Heater

A spray-type deaerator uses spring-loaded or controlled spray valves to divide incoming water into droplets and expose a large surface to steam. Vessel orientation and the direction of steam and water flow vary by design. Some units add a scrubber or second contact stage. Water enters storage after contact, and gases leave through the controlled vent path.

The advantages of a spray-type deaerating heater are:

• • A spray arrangement can be compact and has fewer tray components, depending on the required capacity and storage volume.

• • Spray valves provide rapid surface renewal when their pressure differential and flow remain within the specified range.

• • Accessible spray devices can simplify inspection in some products, but fouling resistance depends on nozzle design and water quality.

The disadvantages of a spray-type deaerating heater are:

• • Droplet formation and contact can degrade outside the rated turndown, inlet-temperature and spray-valve differential-pressure range.

• • Residual oxygen cannot be inferred from the word spray. Use the supplier’s guarantee and an ASME PTC 12.3 test or other specified acceptance method.

• • Storage capacity is a separate design choice. An undersized tank can cause rapid level changes or inadequate pump suction margin regardless of contact-section type.

Factors Affecting Deaeration Efficiency

Residual dissolved oxygen and terminal temperature difference depend on these operating conditions:

• • Temperature and pressure: gas solubility generally falls as water temperature rises and rises with gas partial pressure. The deaerator brings water close to the saturation temperature for its controlled steam-space pressure. Compare outlet temperature with that saturation temperature as terminal temperature difference; do not assume a universal 5 °C target or 0.2 bar pressure.

• • Steam and venting: the heat balance must cover the coldest expected makeup and condensate mix. Pressure control maintains saturation conditions, while the vent must remove incoming gas. Too little venting retains gas; excessive venting wastes steam and treated water.

• • Contact and operation: spray pattern, tray condition, surface renewal, residence time and load range affect transfer. Monitor outlet dissolved oxygen, pressure, temperature, level and vent behaviour. A vent condenser can recover heat while allowing non-condensable gases to escape.

Benefits of Deaerating Heaters

When performance is verified and the full chemistry programme is maintained, a deaerator can:

• • Reduce oxygen-driven corrosion and pitting risk in feedwater heaters, economisers, boilers and connected piping

• • Lower the residual-oxygen load that any separately dosed scavenger must handle

• • Recover heat from extraction steam, flash steam or other suitable sources while heating feedwater; the net cycle benefit depends on the complete heat balance and vent loss

• • Provide hot feedwater storage and steady suction conditions for boiler-feed pumps; deaeration does not remove hardness or prevent scale

Conclusion

A deaerator heats feedwater to near saturation, renews its surface in contact with steam and vents released non-condensable gases. Tray, spray and combined internals are design choices, not fixed performance rankings. Verify residual oxygen and terminal temperature difference under specified inlet and load conditions. Trend pressure, temperature, level, venting and dissolved oxygen, then investigate a change before increasing chemicals or steam. Boiler-water specialists must set any oxygen-scavenger programme separately.