How fired heaters with air preheat systems work
An air preheat system recovers heat from the flue gas of one or more fired heaters by warming the combustion air, raising fired efficiency from about 80 to 85% on a natural draft heater to 90% or more. The system comprises an FD fan, an air preheater, an ID fan, hot air and flue gas ducts, dampers, and the controls that let the heaters fall back to natural draft. Preheaters may be rotary regenerative, cast iron or glass tube recuperative, plate type, or heat pipe; some systems use an indirect circulating fluid. Flue gas enters the preheater at 300 to 450°C and leaves at 150 to 200°C, while air is heated from ambient to 200 to 300°C. A common system often serves several heaters through shared ducts.
Where the FD fan sits
- Inlet filter and silencer
- FD fan
- Steam coil air heater
- Air preheater, air side
- Hot air header
- Heater burner plenums
The fan is at the start of the air path, usually ahead of a steam coil air heater used for cold end protection, and then the main preheater. The hot air goes through an insulated header to the burner plenum of each heater, with an isolation damper on each branch. Pressure needed is typically 150 to 300 mmWC for the steam coil, preheater, header and burner registers.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 200 to 300°C |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | Typically 150 to 300 mmWC |
| Leakage | Fan supplies air lost to the flue side through preheater seals |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
FD Fan selection for fired heaters with air preheat systems
| Blade form | Backward curved or aerofoil |
| Material of construction | Mild steel |
| Drive and arrangement | Direct drive |
| Bearings and cooling | Oil-lubricated on large fans |
| Flow control | Variable speed drive on air header pressure |
| Also specified | Inlet filter and silencer, API 673 or equivalent |
What this FD fan has to cope with
Leakage means the fan supplies more than burners need
Preheater leakage takes air from the FD fan straight to the flue side without reaching any burner. The fan must therefore supply the burner air plus the leakage. If seals wear and leakage grows, the burners run short of air at full firing while the fan appears to be at full flow. Fan margin for worn seals and routine seal maintenance keep supply adequate.
Cold end control with air bypass
Bypassing part of the air around the preheater raises the cold end metal temperature in cold weather or at low load, protecting it from acid, but it reduces heat recovery. The bypass ducts and dampers add flow paths with their own resistance, and the FD fan must hold header pressure whether the bypass is closed or fully open.
Natural draft fallback
When the air preheat system trips, every connected heater must fall back to natural draft at reduced firing or trip safely. Fast-opening natural draft doors on each burner plenum, stack dampers that open on the trip signal, and logic tested at each turnaround are needed so that a single fan fault does not take several heaters down.
Energy and control
The FD fan supplies combustion air for all the connected heaters, so it runs whenever any heater is on preheat. Speed control on air header pressure saves power at part load and when a heater is offline. Steam coil preheating for cold end protection uses steam, so its control should keep the cold end just safe, not far above the dew point, and switch off in hot weather.
ID fan and FD fan in fired heaters with air preheat systems
| ID fan | FD fan | |
|---|---|---|
| Position | After the preheater, before the stack | Before the steam coil and preheater |
| What it handles | Flue gas plus leakage air | Ambient air, including leakage |
| Temperature at the fan | 150 to 200°C | Ambient |
| Main design concern | Leakage and cold end corrosion | Supplying burner air plus leakage |
| Control | Suction header pressure | Air header pressure |
See also the Induced Draft (ID) Fan for fired heaters with air preheat systems.
Questions buyers ask
What is a steam coil air heater?
A steam coil air heater is a finned coil heated by low-pressure steam, placed between the FD fan and the main preheater. It warms the combustion air before the preheater, raising the cold end metal temperature to protect it from acid corrosion, especially at low firing, in cold weather or with high-sulphur fuel.
Why does the FD fan supply more air than the burners use?
Some air leaks across the preheater, through the seals of a rotary preheater or through damaged tubes or plates, from the higher-pressure air side to the flue side. This air never reaches the burners. The FD fan in an air preheat system must therefore supply the burner air plus this leakage, and the leakage grows with seal wear.
What happens on an air preheat system trip?
On a trip of either fan, the connected heaters fall back to natural draft: natural draft doors open on the burner plenums, the preheater is isolated and the stack dampers open. Firing is reduced to what natural draft can support. If a heater cannot run on natural draft, its burner management system trips it safely instead.
Information needed to quote an FD fan for fired heaters with air preheat systems
- Duty point. Combustion air quantity in m³/h or kg/h and the static pressure at the fan outlet, with the margins you want.
- Air condition. Site ambient temperature range and altitude, and whether the fan takes ambient air or air already heated.
- System. Boiler or furnace capacity and fuel, resistance of the air heater, windbox, grate or burner, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), noise limit, rotation and discharge orientation.
- Cold end protection. Steam coil or air bypass, and the design cold end temperature.
- System layout. Number of heaters, header arrangement and the burner air pressure needed.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.