How delayed coker heaters work
The delayed coker heater takes vacuum residue and heats it rapidly to about 490 to 505°C, just short of the point where it cokes in the tubes, and sends it to coke drums where the cracking and coking take place. To keep coking in the tubes as slow as possible, the heater is designed for high velocity, short residence and very uniform heat flux, usually with double-fired tubes, burners firing both sides of a single tube row. Velocity steam is injected into the coil. Coke still builds inside the tubes over months, and the heater is decoked by online spalling, steam-air decoking or pigging, pass by pass. Refinery fuel gas, which may contain coker gas, is the usual fuel. Balanced draft with air preheat is common.
Where the FD fan sits
- Inlet filter
- FD fan
- Air preheater, air side
- Hot air ducts
- Burner plenums on both sides
- Burners
The fan is at grade beside the preheater. It delivers cold air to the preheater and then, through split hot air ducts, to plenums on both sides of each radiant cell. Pressure needed is typically 120 to 250 mmWC. The plenums are long and serve many burners on each side, so the fan pressure must leave enough at the registers for the burners to be balanced individually.
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 120 to 250 mmWC |
| Burners | Double-fired, both sides of each tube row |
| 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 delayed coker heaters
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct drive |
| Bearings and cooling | Standard or oil-lubricated bearings |
| Flow control | Variable speed drive on air flow with oxygen trim |
| Also specified | Inlet filter and silencer, API 673 or equivalent |
What this FD fan has to cope with
Uneven air between the two sides
If one side of a double-fired cell gets more air than the other, its flames differ and the tubes see uneven flux around their circumference. Coke builds faster on the hotter side, and the run length shortens. Balanced plenums on both sides, with enough fan pressure that the registers control the flow, correct the imbalance.
Burner air changes during spalling
During online spalling the firing on one pass is changed, often reduced, while the others carry on. The combustion air to the burners on that pass must follow, or excess air rises in that section and cools the cell unevenly. Burner registers that can be adjusted quickly, and a fan control that holds header pressure, make this manageable.
Fan trip and natural draft
As on other refinery heaters, the coker heater must fall back to natural draft at reduced firing or trip safely when the FD fan trips. For a coker this matters more than usual, because a heater trip risks coking the coil solid. Natural draft doors on both plenums and burners proven in natural draft mode reduce that risk.
Energy and control
The FD fan runs continuously with the coker. Speed control with oxygen trim keeps excess air low as firing changes with drum switches and spalling, without throttling losses at a damper. Low excess air saves fuel, reduces the ID fan's load and limits the oxygen available for tube scaling, while even air distribution protects run length.
ID fan and FD fan in delayed coker heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the air preheater | Before the air preheater |
| What it handles | Flue gas, sour where fuel gas has H₂S | Ambient air |
| Temperature at the fan | 160 to 200°C | Ambient |
| Main design concern | Draft through spalling and acid dew point | Even air to double-fired burners |
| Control | Cell draft | Air flow with oxygen trim |
See also the Induced Draft (ID) Fan for delayed coker heaters.
Questions buyers ask
What is a double-fired coker heater?
A double-fired heater has a single row of tubes in the middle of the firebox with burners on both sides, so each tube receives heat from both directions. This gives a much more uniform heat flux around the tube circumference than single firing against a wall, and it is common on delayed coker heaters to slow coking.
What does the FD fan do on a coker heater?
On a delayed coker heater the FD fan is the combustion air fan. It supplies air through the air preheater to burners on both sides of each tube row. Its main task, beyond flow and pressure, is to give the plenums enough pressure for the burners on each side to be set evenly, which supports uniform heat flux.
How much pressure does a coker heater FD fan need?
Typically 120 to 250 mmWC, covering the air preheater, the split hot air ducts and the burner registers on both sides of the cells. The burner supplier's required pressure at full firing sets the figure, with a margin for preheater fouling and for the longer plenums that double-fired cells need.
Information needed to quote an FD fan for delayed coker heaters
- 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.
- Burner layout. Number of burners on each side of each cell.
- Preheater. Air side pressure drop and air temperature.
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.