How edible oil refinery thermal oil heaters work
Edible oil refineries and vanaspati plants need heat at temperatures steam cannot easily reach: deodorisers run at 240 to 270°C under vacuum, and physical refining of palm and rice bran oil needs the upper end. Most plants use a thermal oil heater supplying fluid at 280 to 320°C, alongside a steam boiler for the lower temperature duties. The heater is a coil-type unit fired by furnace oil or gas through a forced draft burner, or by coal, wood or briquettes on a fixed or reciprocating grate. Because the fluid returns hot, the flue gas leaves the coil at 350 to 400°C and passes an air preheater or economiser to about 180 to 230°C, then a multicyclone and bag filter on solid fuel, before the ID fan and chimney.
Where the FD fan sits
- Inlet
- FD fan
- Air preheater
- Undergrate plenum or burner
- Furnace
The fan stands beside the heater and delivers ambient air into the air preheater, where it is heated to about 150 to 250°C, then to the undergrate plenum or burner windbox. On grate-fired heaters a separate overfire air fan may be fitted. Outlet pressure is typically 100 to 250 mmWC for grate firing; burner fans on oil or gas heaters need more, depending on the burner.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 150 to 250°C, downstream of the fan |
| Dust | Clean ambient air; fuel yard dust on solid fuel plants |
| Pressure at the fan outlet | Typically 100 to 250 mmWC for grate firing |
| Turndown | Firing swings with deodoriser cycles and idles at low fire |
| 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 edible oil refinery thermal oil heaters
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on firing rate, linked to the ID fan |
| Also specified | Inlet screen and silencer where the heater is near work areas |
What this FD fan has to cope with
Poor combustion at low fire
Refinery heaters spend long periods at low fire. A fan throttled hard gives uneven air under the grate and smoky combustion. Speed control and a fan sized close to the real duty keep combustion clean at low load.
Air preheater leakage
Leaks in the air preheater let air pass to the flue side without reaching the grate. The FD fan works harder and the ID fan carries extra air. Checking oxygen before and after the preheater shows the leak.
Coordination with the ID fan
If the FD fan responds faster than the ID fan, the furnace goes positive during load changes. Linking the two in the control system keeps the furnace under suction.
Energy and control
The combustion air fan runs whenever the heater fires, which in a refinery is long hours at a varying load. Speed control linked to firing rate, with oxygen trim, saves fan power and, more importantly, keeps excess air low, since the hot flue gas from a thermal oil heater carries a large share of the fuel heat out of the chimney.
ID fan and FD fan in edible oil refinery thermal oil heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter, before the chimney | Before the air preheater |
| What it handles | Flue gas at moderate temperature | Ambient air |
| Temperature at the fan | 180 to 230°C | Ambient |
| Main design concern | Stable draft through load swings | Clean combustion at low fire |
| Control | Furnace draft | Firing rate |
See also the Induced Draft (ID) Fan for edible oil refinery thermal oil heaters.
Questions buyers ask
Is the FD fan on a thermic fluid heater a combustion air fan?
Yes. It supplies combustion air to the grate or burner, usually through an air preheater. It handles ambient air only. On oil or gas firing it is the burner fan, and its pressure is set by the burner maker.
Why is an air preheater common on edible oil refinery heaters?
The thermal oil returns hot, so the flue gas leaves the coil at 350 to 400°C. An air preheater recovers much of this heat into the combustion air, saving fuel and lowering the ID fan temperature.
Should the FD and ID fans be linked on a refinery heater?
Yes. Refinery heaters change firing quickly. Linking the fans, with the ID fan on furnace draft and the FD fan on firing rate, keeps the furnace under suction through load swings.
Information needed to quote an FD fan for edible oil refinery thermal oil 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.
- Fuel and firing. Fuel type, grate or burner, and air pressure required at full firing.
- Air preheater. Air side pressure drop and hot air temperature.
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.