How snack fryer thermal oil heaters work
Continuous fryers for potato chips, banana chips and namkeen hold cooking oil at about 160 to 190°C. The oil is heated outside the fryer, in a shell and tube heat exchanger, by thermic fluid at roughly 230 to 280°C from a thermic fluid heater, which avoids hot spots that break down the frying oil. Snack plants fire the heater on diesel, LDO or natural gas, or on wood and briquettes over a grate. Flue gas leaves the coil bank at about 300 to 380°C; larger heaters add an air preheater or economiser coil that brings it down to 180 to 230°C. A combustion air fan supplies the burner or grate, and an ID fan, after a multicyclone on solid fuel, sends the flue gas to the chimney.
Where the ID fan sits
- Furnace
- Thermic fluid coil bank
- Air preheater (if fitted)
- Multicyclone
- ID fan
- Chimney
The fan sits after the multicyclone on wood or briquette heaters, so it handles the fine fly ash the cyclone misses; on oil- or gas-fired heaters it follows the coil or air preheater directly and sees clean gas. Because the thermic fluid must leave the heater hotter than the frying oil, the flue gas cannot be cooled as far as in a boiler, and without heat recovery the fan runs at 300°C or more. Suction at the fan inlet is typically 80 to 180 mmWC.
Gas at the ID fan
| Gas temperature, normal | 280 to 340°C without heat recovery; 180 to 230°C after an air preheater |
| Maximum or upset | About 400 to 420°C with a sooted coil or when the fryer stops suddenly |
| Dust at the fan | Fine wood or briquette fly ash with some unburnt char; negligible on oil or gas |
| Moisture and dew point | Wood at 20 to 30% moisture raises the water dew point, but the gas stays well above it in normal running |
| Corrosive species | SO₂ from diesel or LDO firing; acid condensation only at a cold air preheater end |
| Density at the design temperature | About 0.62 kg/m³ at 300°C, 52% 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 snack fryer thermal oil heaters
| Blade form | Backward curved on clean gas; radial tipped behind a multicyclone on solid fuel |
| Material of construction | Mild steel casing; impeller in boiler-quality plate, or heat-resisting steel where upsets exceed about 400°C |
| Drive and arrangement | Direct coupled arrangement 8, with the casing on sliding feet |
| Bearings and cooling | Shaft cooling disc, heat shield, and oil-lubricated bearings above about 300°C |
| Flow control | Variable speed drive on furnace draft, linked to heater firing |
| Also specified | Insulated casing, inlet damper for isolation and purge, and inspection door |
What this ID fan has to cope with
High exit temperature with no heat recovery
Small heaters often have no air preheater, so the fan runs at 300°C or more continuously. Mild steel is losing strength at that temperature, and an excursion to 400°C is common when the fryer stops. The impeller is stressed for the upset temperature, and the bearings are protected by a cooling disc, a heat shield and oil lubrication.
Heat surge when the fryer stops
When the fryer line stops for a product change, oil demand drops sharply, the thermic fluid returns hot and the flue temperature climbs before the burner or grate responds. A fan rated for the surge, and draft control fast enough to follow the firing cut-back, protect both the coil and the fan.
Coil leak and furnace fire
A thermic fluid leak inside the furnace burns fiercely and fills the coil bank with oily soot. The ID fan must keep running to hold suction while the fire is brought under control, so its motor and starter are on a protected supply, and the casing has a drain and an inspection door for clearing soot afterwards.
Fly ash wear on solid fuel
Wood and briquette ash carries sand, which wears the blade leading edges even after a multicyclone. Lower tip speed, a radial tipped wheel and bolted liners on the leading edges extend impeller life on solid-fuel heaters, and allow repairs at site during a planned stop.
Energy and control
Snack lines run in shifts with frequent product changes and cleaning stops, so the heater spends many hours at part load or idling hot. An ID fan on variable speed holding furnace draft saves power during these periods, and stops a fixed fan drawing excess air through an idling furnace, which wastes fuel. Adding an air preheater lowers flue temperature, so the fan moves a smaller gas volume and its bearings run cooler.
ID fan and FD fan in snack fryer thermal oil heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the coil and multicyclone | On the burner or below the grate |
| What it handles | Flue gas, with fly ash on solid fuel | Clean ambient air |
| Temperature at the fan | 280 to 340°C, up to about 420°C upset | Ambient, 25 to 45°C |
| Main design concern | High temperature and heat surges | Fast response to load swings |
| Control | Furnace draft | Thermic fluid temperature |
See also the Forced Draft (FD) Fan for snack fryer thermal oil heaters.
Questions buyers ask
What ID fan temperature should I specify for a thermic fluid heater?
Specify the measured flue temperature at the fan inlet and the highest temperature it reaches during upsets. Without an air preheater, a thermic fluid heater serving a fryer typically runs at 280 to 340°C and can exceed 400°C when the coil is sooted or the load drops suddenly. The fan is designed for the upset figure.
Why does the ID fan on my snack fryer heater trip on vibration?
Common causes are uneven soot or ash deposits on the blades, a shaft bowed by stopping the fan while hot, or bearing damage from heat. Cleaning the wheel, checking shaft run-out, and fitting a cooling disc or heat shield usually solve it. Rebalancing after cleaning restores smooth running.
Is a separate fan needed for fryer fume extraction?
Yes. The fryer hood extraction removes oil mist and steam from above the frying oil and is a separate exhaust system, usually with an oil mist separator. The induced draft fan of the thermic fluid heater handles only flue gas from the heater and should never be connected to the fryer hood.
Information needed to quote an ID fan for snack fryer thermal oil 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 rating and fuel. Heater output in kcal per hour, fuel fired, and whether an air preheater or economiser is fitted.
- Upset history. Highest flue temperature recorded at the fan and how the heater behaves when the fryer line stops.
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.