How crude distillation unit heaters work
The crude distillation unit heater takes desalted crude from the preheat train at about 250 to 290°C and heats it to 350 to 375°C at the column flash zone, partly vaporising it. It is usually the largest heater in the refinery, built as a box or cabin with several cells and multiple parallel passes, each pass with its own flow control. Refinery fuel gas and fuel oil, sometimes heavy residue, are fired. The convection section preheats crude and often generates or superheats steam. On a balanced draft design, flue gas from one or more cells is collected, passed through a large air preheater, and drawn to a stack by ID fans, while FD fans supply combustion air through the preheater to many burners.
Where the ID fan sits
- Radiant cells
- Convection sections
- Common flue duct
- Air preheater, flue side
- ID fans in parallel
- Stack
Flue gas from each cell's convection section is collected in a common duct and taken to the air preheater, with the ID fans after it and before the stack. Large crude heaters often use two fans in parallel, each sized for a large share of the flow, so that losing one does not trip the crude unit. Suction is typically 150 to 250 mmWC, covering convection, ducts and preheater. Isolation dampers allow one fan to be maintained while the other runs.
Gas at the ID fan
| Gas temperature, normal | 160 to 200°C after the air preheater |
| Maximum or upset | 380 to 450°C with the preheater bypassed |
| Dust at the fan | Soot and oil ash when fuel oil is fired |
| Moisture and dew point | Acid dew point 120 to 150°C with sulphur-bearing fuel oil |
| Corrosive species | SO₃ and vanadium-bearing ash from residue firing |
| 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 crude distillation unit heaters
| Blade form | Backward curved or aerofoil, double inlet for large flows |
| Material of construction | Mild steel or boiler plate rated for the bypass temperature; wear protection at the blade inlet where oil ash is heavy |
| Drive and arrangement | Direct drive, double inlet arrangement 7 or 3, with large motors |
| Bearings and cooling | Oil-lubricated sleeve or rolling bearings with temperature and vibration monitoring |
| Flow control | Variable speed drive or inlet vanes, with parallel fan load sharing |
| Also specified | Isolation dampers for each fan, and API 673 or equivalent mechanical requirements |
What this ID fan has to cope with
Parallel fan instability
Two fans in parallel can hunt, with one taking more flow while the other approaches stall, especially at reduced crude throughput when both run back on their curves. Matched fans, a stable characteristic without a pronounced dip, and load sharing control from a common speed or vane signal keep both fans in their stable region.
Soot and oil ash deposits
When the heater fires fuel oil or residue, soot and ash build up on the impeller and in the preheater. Deposits unbalance the rotor and raise vibration. Soot blowing in the convection section, regular preheater cleaning, and an inspection door on the fan control the build-up.
A single fan trip stopping the crude unit
The crude heater feeds the whole refinery. Losing the only ID fan, with nothing but natural draft fallback, forces a large throughput cut that ripples through every downstream unit. Parallel fans, or a design that lets one fan carry most of the flow, together with automatic restart logic, avoid a refinery-wide upset from a single fan fault.
Acid attack at reduced throughput
At low crude rates, or when the preheater is clean and recovering well, the flue gas cools further and approaches the acid dew point at the cold end. Acid then condenses in the preheater baskets and carries to the fan inlet. Cold end temperature control, by air bypass or steam coils on the air side, protects both the preheater and the fan.
Energy and control
The crude heater runs continuously at a throughput set by refinery planning, and the ID fans are among the largest motors in the unit. Variable speed drives match flow to firing without damper loss and, with two fans, can share load efficiently. Low excess oxygen with tight draft control gives fuel savings that far exceed the fan power saving on a heater of this size.
ID fan and FD fan in crude distillation unit heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the common air preheater | Before the air preheater |
| What it handles | Large flue gas flow, soot with fuel oil | Ambient air |
| Temperature at the fan | 160 to 200°C | Ambient |
| Main design concern | Size, parallel operation and soot | Air distribution across cells |
| Control | Arch draft in each cell | Air flow with oxygen trim |
See also the Forced Draft (FD) Fan for crude distillation unit heaters.
Questions buyers ask
Why do crude distillation heaters have two ID fans?
The crude heater is critical to the whole refinery. Two ID fans in parallel let the heater continue at high load if one fan trips, and allow maintenance on one fan without a shutdown. Each is typically sized for a large share of the total flue gas flow.
What causes vibration on a crude heater ID fan?
Common causes are soot and oil ash deposits on the impeller when fuel oil or residue is fired, worn bearings, and unstable parallel operation at low throughput. Regular cleaning during preheater washes, continuous vibration monitoring, field balancing to ISO 21940-11 and correct parallel fan control usually bring a crude heater ID fan back to smooth running.
What temperature should a crude heater ID fan be designed for?
Normal operation after the air preheater is typically 160 to 200°C at the crude heater ID fan. The fan must also be checked at the preheater bypass condition, often 380 to 450°C, when the preheater is out for washing or repair, and the impeller and bearings must be fit for that temperature for as long as the bypass lasts.
Information needed to quote an ID fan for crude distillation unit 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.
- Heater configuration. Number of cells and passes, and whether flue gas from several heaters shares one preheater.
- Redundancy. Number of ID fans, the share of flow each must carry and the natural draft fallback capability.
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.