Deaerator Parameter Encyclopedia: Complete Guide to Performance, Selection, and Maintenance

Overview of Deaerator

A deaerator is a mechanical device used in thermal power plants, boiler systems, and industrial steam generation to remove dissolved gases—primarily oxygen and carbon dioxide—from feedwater. By reducing oxygen content to below 0.005 mg/L (5 ppb) and CO₂ to near zero, deaerators prevent corrosion in boilers, pipelines, and heat exchangers. Typical operating pressures range from 0.02 MPa to 0.5 MPa, with water flow capacities from 10 t/h to 2000 t/h. Deaerators are classified as either thermal (spray or tray type) or vacuum type, with thermal deaerators being the most common in high-demand industrial settings.

Working Principle of Deaerator

The deaerator operates on Henry’s Law and gas solubility principles. Heating feedwater to saturation temperature at a given pressure reduces the solubility of oxygen and CO₂. Steam is introduced through spray nozzles or tray stacks, stripping dissolved gases from the water. The freed gases are vented to the atmosphere via a vent condenser, while deaerated water collects in the storage tank. In a tray-type deaerator, water cascades over multiple perforated trays while steam flows upward, maximizing surface area and mass transfer. A properly tuned deaerator achieves dissolved oxygen levels ≤ 0.007 mg/L (7 ppb) under standard conditions.

Definition of Deaerator

A deaerator is a pressure vessel system that combines mechanical scrubbing, thermal heating, and venting to reduce dissolved oxygen and non-condensable gases in boiler feedwater. It typically consists of a deaeration head (where gas removal occurs) and a storage tank (holding deaerated water). The device is essential for extending boiler life and maintaining water quality per ASME and EN standards. In industry, deaeration efficiency is defined as the percentage of dissolved oxygen removed relative to inlet concentration—commonly exceeding 99.5%.

Application Scenarios of Deaerator

Deaerators are deployed in:
- Utility and industrial steam boilers (coal, gas, biomass, waste-to-energy)
- Combined cycle power plants (HRSG systems)
- Chemical and petrochemical plants (process steam)
- Food and beverage processing (sterilization steam)
- District heating networks (hot water boilers)
- Paper mills, refineries, and steel mills
Typical feedwater temperature at deaerator inlet: 60–90 °C; outlet temperature: 104 °C (at 0.12 MPa) to 150 °C (at 0.5 MPa). System pressure is closely matched to downstream boiler requirements.

Classification of Deaerator

Spray-type deaerators are simpler and cost-effective for low-to-medium capacity. Tray-type units offer higher efficiency and are preferred for large utility boilers. Vacuum deaerators operate below atmospheric pressure and suit applications requiring low temperature deaeration (50–80 °C).

Performance Indicators of Deaerator

Key performance metrics include:
- Dissolved Oxygen Removal Efficiency: ≥ 99.5% (typical outlet ≤ 0.007 mg/L)
- Carbon Dioxide Removal: ≤ 1 mg/L (often undetectable)
- Vent Rate: 0.1–0.5% of total steam flow (optimized to minimize steam loss)
- Storage Tank Capacity: 5–60 minutes of full-load water demand
- Temperature Rise: 30–60 °C from inlet to deaeration temperature
- Pressure Drop Across Internal Stacks: 0.01–0.05 MPa
- Feedwater Purity: pH 8.5–9.5 (ammonia/amine treated) after deaeration

Key Parameters of Deaerator

Industry Standards for Deaerator

Deaerators must comply with:
- ASME Boiler and Pressure Vessel Code (Section VIII, Div. 1)
- HEI (Heat Exchange Institute) Standards for Deaerators (latest edition)
- EN 12953 (European shell boilers) / EN 13445 (unfired pressure vessels)
- GB/T 1576 (China industrial boiler water quality)
- ISO 9001 / ISO 3834-2 for welding quality
HEI standard mandates that thermal deaerators achieve dissolved oxygen ≤ 0.007 mg/L at rated capacity. Vent rates must be between 0.1% and 0.5% of steam flow. Storage tank retention time should be ≥ 10 minutes at peak demand.

Precise Selection Points and Matching Principles of Deaerator

Selecting the correct deaerator requires matching:
1. Capacity: Deaerator max flow = (boiler maximum evaporation + blowdown + auxiliary steam) × 1.1 safety margin.
2. Pressure: Deaerator operating pressure must be 0.02–0.1 MPa higher than boiler feed pump NPSH requirement. Typical values: 0.12 MPa for 104 °C, 0.35 MPa for 140 °C.
3. Oxygen removal target: For high-pressure boilers (≥ 6.4 MPa), require ≤ 0.003 mg/L; for low-pressure (≤ 2.5 MPa), ≤ 0.007 mg/L is sufficient.
4. Steam supply: Ensure heating steam pressure is at least 0.2 MPa above deaerator operating pressure. Flow: 5–15 kg steam per 100 kg of water (varies with inlet temperature).
5. Storage tank volume: Match to boiler startup and transient demand (typical 15–30 minutes of full load).
6. Material: For high-purity water (pH > 9.5), use stainless steel internals; standard units use carbon steel with corrosion allowance.

Procurement Pitfalls of Deaerator

Common pitfalls to avoid:
- Underrated vent condenser: Causes steam loss > 1% and poor gas removal. Specify vent rate ≤ 0.3% at design conditions.
- Incorrect tray spacing: Production units often use 150–200 mm tray spacing; too small leads to flooding, too large reduces efficiency. Verify manufacturer’s tray loading (kg/m²·h) matches flow.
- Ignoring water level control: Poor level control causes carryover into steam lines. Demand two independent level transmitters plus redundancy.
- Neglecting NPSH for downstream pump: Deaerator height must provide ≥ 3 meters static head above pump centerline to avoid cavitation.
- Cheap materials: Chinese-made deaerators often use Q245R instead of ASME SA-516 Gr.70; request material certificates and impact test results for temperatures below -20 °C if outdoor installation.

Usage and Maintenance Guide of Deaerator

Operation and maintenance best practices:
- Startup: Fill with preheated water (≥ 60 °C) to avoid thermal shock. Gradually open steam valve to raise temperature at ≤ 10 °C/min.
- Daily checks: Monitor dissolved oxygen (online analyzer), vent flow, pressure, temperature, and storage tank water level. Typical deviation limits: oxygen < 0.010 mg/L, pressure ± 0.005 MPa.
- Monthly: Inspect spray nozzles for blockage; clean or replace if flow deviation > 10%. Check tray flatness and hole fouling.
- Annual overhaul: Hydraulic test at 1.5× design pressure (per ASME). Replace gaskets at flange joints. Recalibrate all instruments.
- Corrosion prevention: Maintain water pH 8.5–9.5 using amine dosing. Inspect storage tank bottom for sludge every 6 months.
- Winter operation: Insulate all exposed piping and add trace heating to vent line to prevent freezing. For outdoor units, minimum ambient temperature during operation: -10 °C (with proper insulation).

Common Misconceptions of Deaerator

Misconception 1: Higher pressure always gives better deaeration.
Fact: Deaeration efficiency depends on reaching saturation temperature at corresponding pressure. Excess pressure increases steam consumption and may cause flashing. Optimal pressure matches boiler feed requirements.

Misconception 2: A deaerator can handle any flow above design capacity.
Fact: Overloading beyond 110% rated capacity leads to poor oxygen removal (outlet > 0.020 mg/L) and water carryover. Always size with 10–15% margin.

Misconception 3: Vent closing saves energy.
Fact: Reducing vent rate below 0.1% of steam flow severely degrades oxygen removal. Minimal vent rate should be 0.15% for spray type and 0.2% for tray type (per HEI).

Misconception 4: Stainless steel internals are always necessary.
Fact: For feedwater pH < 9.2, carbon steel is adequate with proper corrosion allowance. Use stainless only when pH > 9.5 or when water contains chlorides > 50 mg/L.