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 ID fan sits
- Double-fired radiant cells
- Convection section
- Air preheater, flue side
- ID fan
- Stack
The fan is after the air preheater, at grade. A coker heater usually has two or more double-fired radiant cells sharing a convection section and preheater, each cell with its own outlet damper. Suction is typically 120 to 220 mmWC, covering the convection coils, flue ducts and preheater. As on other refinery heaters, a bypass and stack damper allow natural draft operation at reduced firing if the fan trips.
Gas at the ID fan
| Gas temperature, normal | 160 to 200°C after the air preheater |
| Maximum or upset | 350 to 420°C with the preheater bypassed |
| Dust at the fan | Negligible on fuel gas firing |
| Moisture and dew point | Acid dew point 110 to 140°C where fuel gas contains H₂S |
| Corrosive species | SO₂ and SO₃ from sour fuel 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 delayed coker heaters
| Blade form | Backward curved or aerofoil, for efficiency on clean gas |
| Material of construction | Mild steel or boiler plate rated for the bypass temperature |
| Drive and arrangement | Direct drive, arrangement 8 |
| Bearings and cooling | Cooling disc; oil-lubricated bearings with monitoring |
| Flow control | Variable speed drive on arch draft, with a wide stable range |
| Also specified | Insulated casing and inlet box, drain and API 673 or equivalent |
What this ID fan has to cope with
Firing changes during online spalling
When one pass is spalled online, its firing is changed, steam rates rise and the cell's heat release shifts. The ID fan must hold draft in each cell through these changes. A wide stable operating range and a fast control loop keep the cells at the right draft.
Sour fuel gas and acid dew point
Coker gas and other refinery gases can carry H₂S if amine treating is upset. The SO₃ raises the acid dew point and the preheater cold end corrodes. Cold end temperature control, insulation of the fan and inlet ducts, and a corrosion allowance protect the fan.
Draft imbalance between cells
Two or more cells sharing one ID fan can end up at different draft if their outlet dampers, burner settings or convection fouling differ. One cell goes positive and pushes hot gas through peepholes, while the other pulls in tramp air. Draft measurement in each cell and individual cell dampers, with the fan holding a common suction, balance them.
Energy and control
A delayed coker heater runs continuously with a firing rate that changes as drums switch and passes are decoked. Variable speed control on arch draft keeps the fan efficient across these changes. Stable draft and good air control let the heater run at low excess oxygen, which saves fuel and lowers the flue gas volume.
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 Forced Draft (FD) Fan for delayed coker heaters.
Questions buyers ask
What is online spalling on a coker heater?
Online spalling removes coke from one pass at a time while the heater stays in service. The pass is put on steam, and its temperature is changed so the coke layer cracks and spalls off into the flow. Firing on that part of the heater changes, and the coker heater ID fan must hold the cell draft steady through those changes.
Why is uniform heat flux important on a coker heater?
Vacuum residue cokes quickly wherever heat flux or film temperature is high. Uniform flux, from double firing and even air to every burner, slows coke build-up in the tubes and extends the run between decoking. Steady draft from the ID fan keeps flame shapes constant, which is part of keeping the heat flux uniform along each pass.
Does the coker heater ID fan need to be corrosion resistant?
If the refinery fuel gas contains H₂S, for example when the amine unit is upset or coker gas is fired, the flue gas carries SO₃ and its acid dew point rises. Keeping the gas above the dew point with insulation and cold end control is the main protection; corrosion-resistant materials are added only where cold spots cannot be avoided.
Information needed to quote an ID fan for delayed coker heaters
- 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.
- Cell arrangement. Number of cells, whether they share one ID fan, and cell draft control.
- Fuel gas. H₂S content of the fuel gas, normal and upset.
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.