How ethylene cracking furnaces work
An ethylene cracking furnace heats a hydrocarbon feed, ethane, propane, naphtha or gas oil, mixed with dilution steam in radiant coils to about 800 to 860°C for a fraction of a second, cracking it to ethylene and propylene. The cracked gas is quenched immediately in transfer line exchangers that raise high-pressure steam. Firing is with floor and wall burners burning methane-rich fuel gas from the plant's own cold box. Flue gas leaves the radiant box at about 1,050 to 1,150°C and passes a tall convection section with feed preheat, dilution steam superheat, high-pressure steam superheat and boiler feed water coils, leaving at only 100 to 140°C. Most cracking furnaces have natural draft burners and an ID fan; air preheat with an FD fan is less common.
Where the FD fan sits
- Inlet filter
- FD fan
- Air preheater
- Air ducts
- Floor burner plenum
- Wall burners
Where fitted, the fan is at grade beside the furnace. It delivers ambient air to an air preheater, often a steam coil heater or a flue gas preheater in the convection section, and then to the floor burner plenum and wall burner manifolds. Pressure needed is typically 100 to 250 mmWC. On furnaces without air preheat, there is no FD fan and the burners draw air directly on firebox draft.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 100 to 300°C, depending on the preheat source |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | Typically 100 to 250 mmWC |
| Burners | Floor and wall burners with different air needs |
| 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 ethylene cracking furnaces
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on air flow with oxygen trim |
| Also specified | Inlet filter and silencer |
What this FD fan has to cope with
Floor and wall burner split
Floor burners take most of the heat and wall burners shape the heat flux along the coil. Air must be split correctly between them, or coil outlet temperatures vary and coking increases. Separate manifolds with balancing dampers, and enough fan pressure to control both, give the right split.
Converting natural draft burners
Adding air preheat to an existing cracking furnace means changing the burners or adding plenums, ducts and an FD fan around a crowded furnace floor. The new arrangement must still hold the right draft in the firebox, keep the floor and wall heat release unchanged, and allow the furnace to keep running at reduced firing if the new fan trips.
Fan trip and natural draft fallback
A furnace converted to forced draft burners must be able to fall back to natural draft at reduced firing, or trip safely, when the FD fan stops. Natural draft doors on the floor plenum, a burner design that works in both modes, and an ID fan able to raise the firebox draft quickly let the furnace stay on feed through a fan fault.
Energy and control
Combustion air preheat on a cracking furnace saves firebox fuel, and the FD fan's power is small by comparison. Speed control with oxygen trim keeps excess air low through the run and the decoke, when firing falls sharply. With many furnaces in a plant, efficient fans and drives add up to a meaningful saving, and low excess air also reduces the ID fan load on each furnace.
ID fan and FD fan in ethylene cracking furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | At the convection section outlet | Before the air preheater, where fitted |
| What it handles | Clean flue gas after deep heat recovery | Ambient air |
| Temperature at the fan | 100 to 140°C | Ambient |
| Main design concern | Steady draft for natural draft burners | Floor and wall burner split |
| Control | Firebox draft | Air flow with oxygen trim |
See also the Induced Draft (ID) Fan for ethylene cracking furnaces.
Questions buyers ask
Do ethylene furnaces have combustion air fans?
Most ethylene cracking furnaces do not have combustion air fans; they use natural draft burners, and the ID fan provides the firebox draft that pulls air in. Some furnaces have combustion air preheat, and those need an FD fan to push the air through the preheater and the burner plenums, because natural draft cannot overcome that resistance.
Why would a cracker add combustion air preheat?
Preheating the combustion air reduces fuel use in the firebox for the same cracking duty, and the reduction in fuel also cuts CO₂ emissions. Sources include low-grade steam, gas turbine exhaust or flue gas from the convection section. An FD fan is then needed to push the air through the preheater and the burner plenums.
What pressure does an ethylene furnace FD fan need?
Typically 100 to 250 mmWC, enough for the air preheater, the ducts and the burner registers on the floor and wall burners. Wall burners usually need a different air pressure from floor burners, so the fan pressure is set by the more demanding of the two, with dampers to trim the other manifold.
Information needed to quote an FD fan for ethylene cracking furnaces
- 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.
- Preheat source. Steam coil, gas turbine exhaust or flue gas preheater, with the air temperature.
- Burner split. Number of floor and wall burners and the air split between them.
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.