How vacuum distillation unit heaters work
The vacuum unit heater takes atmospheric residue from the bottom of the crude column at about 340 to 360°C and heats it to 390 to 420°C before it enters the vacuum column. Residue cracks and cokes easily, so the heater is designed for low and even heat flux, high mass velocity and short residence, with coil steam injected to increase velocity and reduce coking. Tubes are enlarged towards the outlet as vaporisation increases. The heater is usually smaller than the crude heater, often a cabin or vertical cylindrical design. Many refineries fire it on fuel gas and fuel oil, sometimes high-sulphur residue. With an air preheat system, an FD fan and an ID fan form a balanced draft arrangement similar to the crude heater, but at smaller flows.
Where the FD fan sits
- Inlet filter
- FD fan
- Air preheater, air side
- Hot air duct
- Burner plenum
- Burners
The fan is at grade beside the preheater. It delivers cold air to the preheater, where it is heated to 200 to 280°C, and then hot air through an insulated duct to the burner plenum below the heater floor. Pressure required is typically 100 to 200 mmWC. The plenum must distribute air evenly to burners placed around the coil, so the fan pressure is chosen with the plenum design.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 200 to 280°C |
| Dust | Clean, filtered air |
| Pressure at the fan outlet | Typically 100 to 200 mmWC |
| Turndown | Follows crude unit throughput |
| 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 vacuum distillation unit heaters
| Blade form | Backward curved, for stable pressure |
| 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 silencer and API 673 or equivalent requirements |
What this FD fan has to cope with
Long flames from short air
Burners that are short of air produce long, lazy flames that can reach the tubes and create hot spots, which on a residue coil is a direct cause of coking and tube damage. Even air distribution in the plenum, and enough fan pressure that each burner register can be set accurately, keep flames compact and clear of the coil.
Heavy oil atomisation
Fuel oil burners need good air mixing to burn the oil spray completely. Too little air pressure at the register gives soot, sparklers and longer flames, and adds to the ash load on the convection section and the ID fan. The fan pressure is selected for the burner's needs at full firing with the heaviest fuel oil expected.
Fallback to natural draft
On a fan trip the heater must continue at reduced firing on natural draft or trip safely. The burner design, the plenum doors and the preheater bypass must all work with the much lower air pressure available from the stack. The FD fan's own resistance when stopped and its outlet damper position are part of this fallback check.
Energy and control
The vacuum heater runs continuously, and the FD fan runs with it at a load that follows crude throughput. Speed control, with oxygen trim, holds low excess air without throttling losses. The fan's main contribution to fuel saving is enabling low excess air without long flames, which also protects the residue coil from coking and extends the run length between decokes.
ID fan and FD fan in vacuum distillation unit heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the air preheater | Before the air preheater |
| What it handles | Flue gas with oil ash and SO₃ | Ambient air |
| Temperature at the fan | 170 to 220°C | Ambient |
| Main design concern | Acid dew point and ash deposits | Even air for flame shape |
| Control | Arch draft | Air flow with oxygen trim |
See also the Induced Draft (ID) Fan for vacuum distillation unit heaters.
Questions buyers ask
Why does a vacuum heater need even air distribution?
The residue in a vacuum heater coil cokes easily. Uneven air gives uneven flames, which raises heat flux on some tubes and starts coking there, shortening the run. Even air from the FD fan and a well-designed burner plenum keeps heat flux uniform around the coil, which is why air distribution gets more attention here than on most heaters.
What air pressure do vacuum heater burners need?
Typically 100 to 200 mmWC at the fan outlet, enough for the preheater air side, the hot air duct and the burner registers. Fuel oil burners need enough air pressure at the register for good mixing with the oil spray. The burner supplier's required pressure at full firing should set the fan pressure, with a margin for preheater fouling.
Is the FD fan on a vacuum heater the combustion air fan?
Yes. On a vacuum distillation unit heater with air preheat, the FD fan is the combustion air fan. It takes ambient air and pushes it through the air preheater to the burner plenum. The air is heated after the fan, so the fan itself handles only cool, clean air and can be built in mild steel.
Information needed to quote an FD fan for vacuum 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 type. Gas, oil or combination burners and the air pressure required.
- Preheater. Type and air side pressure drop.
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.