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 ID fan sits
- Radiant section
- Convection section
- Air preheater, flue side
- ID fan
- Stack
The fan is after the preheater and before the stack, usually at grade with ducts from the convection outlet. With high-sulphur fuel, the preheater outlet is held well above the acid dew point, so gas reaches the fan at 170 to 220°C rather than the lower figures possible on clean gas firing. Suction is typically 100 to 200 mmWC. A bypass and stack damper allow natural draft operation at reduced firing if the fan trips.
Gas at the ID fan
| Gas temperature, normal | 170 to 220°C after the preheater |
| Maximum or upset | 350 to 420°C with the preheater bypassed |
| Dust at the fan | Oil ash and soot with heavy fuel oil firing |
| Moisture and dew point | Acid dew point 130 to 150°C with high-sulphur fuel |
| Corrosive species | SO₃, vanadium and sodium compounds in oil ash |
| Density at the design temperature | About 0.75 kg/m³ at 200°C, 63% 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 vacuum distillation unit heaters
| Blade form | Backward curved or backward inclined, with blade shapes that resist ash build-up |
| Material of construction | Mild steel or boiler plate; corrosion-resistant lining at the inlet box where cold spots occur |
| 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 |
| Also specified | Insulated inlet box and casing, drain, inspection doors and API 673 or equivalent requirements |
What this ID fan has to cope with
Acid condensation in a cold spot
Even with the main gas stream above the dew point, uninsulated inlet boxes, damper shafts, access doors and casing corners can fall below it, especially in the monsoon or at low firing. Acid condenses there and eats through the plate from the inside. Full insulation, cladding over stiffeners and doors, and duct layouts without dead pockets avoid these cold spots.
Oil ash deposits
Heavy fuel oil ash containing vanadium and sodium builds up on preheater surfaces and on the fan impeller. It is sticky at higher temperatures and sets hard at lower ones, so deposits grow unevenly and unbalance the rotor. Regular cleaning, a blade form that does not trap deposits, and vibration monitoring keep the fan running between turnarounds.
Draft swings and coil coking
The vacuum heater coil is sensitive to changes in heat flux, and any pass that runs hot begins to coke, raising pressure drop and shortening the run. Draft swings change flame shape and the distribution of heat. Fine, steady draft control from a speed controlled ID fan helps keep flames stable and heat flux uniform across the passes.
Energy and control
The vacuum heater runs at a throughput set by the crude unit. Variable speed control on arch draft keeps the fan efficient and the draft stable. Holding preheater outlet temperature just above the dew point, not far above it, recovers heat safely; the ID fan should be designed for this operating window.
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 Forced Draft (FD) Fan for vacuum distillation unit heaters.
Questions buyers ask
What is the acid dew point on a vacuum heater burning fuel oil?
With high-sulphur fuel oil, the sulphuric acid dew point of the flue gas from a vacuum heater is typically 130 to 150°C, depending on the sulphur content and the SO₃ conversion. The preheater outlet and the ID fan inlet are kept above this, often at 170 to 220°C, to prevent corrosion at the cold end and in the fan.
Why is steady draft important on a vacuum heater?
The residue in a vacuum heater coil cokes if heat flux rises in any pass. Draft swings change flame length and the way heat is distributed to the tubes, and a hot pass begins to coke. Steady draft from a well-controlled ID fan keeps the flames and heat flux stable, which extends the run between decoking or pigging.
What maintenance does a vacuum heater ID fan need?
Regular inspection and cleaning of oil ash on the impeller, checks for acid corrosion at the inlet box, casing corners and drains, and bearing and vibration monitoring. At each turnaround the impeller should be checked for thinning and cracks, and rebalanced to ISO 21940-11 if deposits or repairs have changed its balance.
Information needed to quote an ID fan for vacuum 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.
- Fuel sulphur and ash. Fuel oil sulphur and ash content, and the preheater outlet temperature set point.
- Natural draft operation. Whether the heater must run on natural draft if the fan trips.
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.