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 FD fan sits
- Inlet filter and silencer
- FD fans in parallel
- Air preheater, air side
- Hot air ducts to each cell
- Burner plenums
- Burners
The fans sit at grade beside the air preheater. They deliver cold air to the preheater, where it is heated to 220 to 300°C, and then through long insulated ducts to the burner plenums of each cell. Pressure needed is typically 120 to 250 mmWC, covering the preheater air side, the hot air ducts, cell dampers and burner registers. As on the flue side, two FD fans in parallel are common.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 220 to 300°C |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | Typically 120 to 250 mmWC |
| Turndown | Following crude throughput, 60 to 100% in normal operation |
| 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 crude distillation unit heaters
| Blade form | Backward curved or aerofoil, double inlet for large flows |
| Material of construction | Mild steel |
| Drive and arrangement | Direct drive, double inlet |
| Bearings and cooling | Oil-lubricated bearings on large fans |
| Flow control | Variable speed drive on total air flow with oxygen trim; cell dampers for distribution |
| Also specified | Inlet silencer and filter, and API 673 or equivalent requirements |
What this FD fan has to cope with
Uneven air between cells
Long hot air ducts to several cells give different pressure losses to each, so cells far from the fan run short of air while near cells run with excess. The heater then fires on the worst cell's oxygen. Duct design checked by flow modelling, cell dampers and an oxygen analyser on each cell keep firing even across the heater.
Pass temperature imbalance
Uneven air within a cell gives uneven flames and heat flux across the passes, so some passes run hot while others run cool, and the pass flow controllers fight to balance outlet temperatures. Enough fan pressure that the burner registers, not the ducts, are the main resistance, together with careful register setting, gives uniform heat flux.
Loss of an FD fan
With parallel fans, one fan trip must not cause a heater trip. Fast-closing outlet dampers on the tripped fan prevent air flowing backwards through it, and the running fan increases speed to carry the load. The running fan's motor and curve must cover this single-fan case, which is often the true sizing condition.
Energy and control
The crude heater's FD fans run all year and move a large air flow. Speed control on air flow with oxygen trim keeps excess air low across throughput changes. Improving air distribution between cells allows lower overall excess air, which is often worth more in fuel than any change to the fan itself.
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 Induced Draft (ID) Fan for crude distillation unit heaters.
Questions buyers ask
How is combustion air distributed in a multi-cell crude heater?
On a multi-cell crude heater the FD fans feed a common air preheater and then hot air ducts to each cell's burner plenum. Cell dampers and the individual burner registers set the distribution, guided by oxygen readings in each cell. Good duct design matters, because a combustion air fan can only supply pressure, not even distribution.
Why are two FD fans used on crude heaters?
Two forced draft fans in parallel provide redundancy, so the crude heater can continue at high load if one fan trips, and they allow maintenance on one fan without shutting down the crude unit. Each fan is sized for a large share of the total combustion air, often enough to hold most of the design throughput alone.
What happens if a crude heater FD fan fails?
With parallel fans, the other fan picks up the load, its speed rises and the heater may reduce firing slightly. With a single fan, the heater falls back to natural draft at reduced firing or trips. In either case the burner management system closes the tripped fan's damper to stop backflow of air through it.
Information needed to quote an FD fan for crude distillation unit 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 count and layout. Number of burners per cell and the plenum arrangement.
- Parallel operation. Number of FD fans and the share of flow each must carry.
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.