What Is a Deaerator? Boiler Feedwater
A deaerator is a mechanical device that removes dissolved gases—primarily oxygen and carbon dioxide—from boiler feedwater. Dissolved oxygen is the primary cause of pitting corrosion in boilers, steam piping, and condensate systems. Even small concentrations (as low as 7 ppb) can cause significant damage over time. Deaerators reduce dissolved oxygen to less than 7 ppb (0.005 cm3/L), providing effective corrosion protection for the entire steam and condensate system.
Deaerators operate on Henry’s Law: the solubility of a gas in a liquid is proportional to the partial pressure of that gas above the liquid. By heating the feedwater to its saturation temperature with steam and reducing the partial pressure of oxygen above the water surface, dissolved gases are released and vented to atmosphere.
Deaerator Types and Specifications
| Feature | Spray-Type Deaerator | Tray-Type Deaerator |
|---|---|---|
| Construction | Water sprayed into steam atmosphere in a horizontal vessel | Water cascades over stacked trays in a vertical section above the storage tank |
| Oxygen removal | Less than 7 ppb | Less than 7 ppb |
| Water heating | Water heated to saturation temperature by direct steam contact | Water heated progressively as it flows over trays |
| Response to load changes | Fast—spraying provides rapid mixing | Moderate—tray contact requires more time |
| Maintenance | Low—fewer internal components | Higher—tray maintenance required |
| Best for | Variable load, smaller capacity | Steady load, larger capacity |
Key Design Parameters
| Parameter | Typical Specification |
|---|---|
| Design standard | ASME Section VIII (storage tank), HEI Standards (performance) |
| Operating pressure | 0.2-0.7 barg (low-pressure steam) |
| Operating temperature | 105-165 degrees C (at corresponding saturation pressure) |
| Oxygen outlet | Less than 7 ppb (guaranteed performance) |
| CO2 outlet | Essentially zero |
| Steam source | Low-pressure extraction steam or pressure-reduced main steam |
| Vent rate | 0.5-1.0% of steam flow (must be maintained for gas removal) |
| Storage capacity | 10-20 minutes of boiler feedwater demand |
| Materials | Carbon steel shell with stainless steel internals (trays, spray nozzles) |
Deaerator in the Steam System
The deaerator sits between the condensate system and the boiler feed pumps. It receives:
- Condensate returns from process heat exchangers and steam traps
- Makeup water from the water treatment plant
- Low-pressure steam for heating and gas stripping
| Component | Function |
|---|---|
| Deaerating section | Heats water and strips dissolved gases using steam |
| Storage tank | Provides feedwater reserve for boiler feed pumps |
| Vent condenser | Recovers heat from vented steam before atmospheric discharge |
| Level control | Maintains storage tank level via makeup water valve |
| Pressure control | Regulates steam pressure to maintain saturation conditions |
Deaerators are standard equipment in power plants, refineries, and any facility with a high-pressure steam system. They connect to the steam and condensate piping system through flanged nozzles and require pressure relief protection per ASME code requirements.
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