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 ID fan sits
- Heater convection outlets
- Flue gas collection duct
- Air preheater, flue side
- ID fan
- Stack
The fan is after the preheater, before a common stack or the heater's own stack. It handles the flue gas from all connected heaters plus air leaking from the air side. Suction is typically 120 to 250 mmWC, covering the collection ducts and the preheater. Each heater has an isolation damper, and a bypass allows the preheater to be isolated.
Gas at the ID fan
| Gas temperature, normal | 150 to 200°C after the preheater |
| Maximum or upset | 300 to 450°C with the preheater bypassed |
| Dust at the fan | Negligible on gas firing; soot and ash with oil |
| Moisture and dew point | Acid dew point 110 to 150°C with sulphur-bearing fuel |
| Leakage air | Rotary preheaters add typically several percent of air flow to the flue gas |
| Density at the design temperature | About 0.78 kg/m³ at 180°C, 65% 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. |
ID Fan selection for fired heaters with air preheat systems
| Blade form | Backward curved or aerofoil |
| Material of construction | Mild steel or boiler plate rated for the bypass temperature |
| Drive and arrangement | Direct drive, single or double inlet |
| Bearings and cooling | Cooling disc; oil-lubricated bearings on large fans |
| Flow control | Variable speed drive on suction header pressure |
| Also specified | API 673 or equivalent, and isolation dampers |
What this ID fan has to cope with
Preheater leakage raises the fan load
Rotary preheaters leak air through seals into the flue gas, increasing the ID fan flow and diluting the gas. As seals wear, leakage grows, and the fan can run out of capacity. Seal maintenance and fan margin for worn seals keep the system at full duty.
Cold end corrosion
If the flue gas leaving the preheater falls below the acid dew point, sulphuric acid condenses on the cold end elements and in the fan inlet, and deposits stick to the wet surfaces. Cold end temperature control, by bypassing part of the air around the preheater or warming it in a steam coil, keeps the cold end metal above the dew point.
Shared system with several heaters
When one ID fan serves several heaters, a change on one heater, a burner trip or a damper movement, shifts the draft on the others. Individual heater isolation dampers, draft measurement on each heater and a common suction header pressure control let the fan follow the total load while each heater keeps its own stable draft.
Energy and control
An air preheat system exists to save fuel, and its fans are its main running cost. Variable speed drives on both fans, controlled from header pressure and air flow, keep power low at part load and when one heater on a shared system is down. Keeping preheater leakage low reduces both fan loads, so seal maintenance is energy work as well as reliability work.
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 Forced Draft (FD) Fan for fired heaters with air preheat systems.
Questions buyers ask
What is an air preheat system on a fired heater?
It is a system of an FD fan, an air preheater, an ID fan, ducts and dampers that uses flue gas heat to warm the combustion air for one or more fired heaters. It raises fired heater efficiency from about 80 to 85% on natural draft to 90% or more, and it includes the controls that let the heaters fall back to natural draft.
How does air preheater leakage affect the ID fan?
Air leaking from the air side to the flue side of the preheater adds to the flue gas flow and lowers its temperature, so the ID fan must handle more volume than the heaters produce. Rotary regenerative preheaters leak the most, and leakage grows as seals wear, so the fan is sized with an allowance for worn seals.
Can one air preheat system serve several heaters?
Yes. A common air preheat system with shared ducts, fans and preheater is common in refineries, especially for groups of smaller heaters. Each heater has isolation dampers and its own draft control, so it can be taken out of service or run on natural draft while the system continues to serve the others.
Information needed to quote an ID fan for fired heaters with air preheat systems
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Preheater type and leakage. Preheater type, design leakage and its pressure drop.
- Connected heaters. Number of heaters on the system and the flue gas flow from each.
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.