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 FD fan sits
- Inlet screen
- FD fan
- Air preheater (if fitted)
- Burner or under-grate plenum
- Furnace
- Thermic fluid coil
On oil- and gas-fired heaters the fan is usually part of the burner, mounted on the heater front. On wood and briquette heaters a separate fan at ground level pushes air through the air preheater, if fitted, into the plenum under the grate, with a branch to the overfire nozzles. Either way the fan handles ambient air and works against the burner head or grate, the fuel bed and the air preheater, typically 80 to 200 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the air preheater | 150 to 200°C, downstream of the fan |
| Dust | Wood chips, briquette dust and plant debris near the heater |
| Pressure at the fan outlet | Typically 80 to 200 mmWC |
| Turndown | Frequent load changes with fryer start, stop and product change |
| 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 snack fryer thermal oil heaters
| Blade form | Backward curved, for a stable curve and good efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct mounted on the motor shaft, or direct coupled on larger heaters |
| Bearings and cooling | Motor or pedestal bearings; no cooling needed |
| Flow control | Variable speed drive linked to thermic fluid outlet temperature |
| Also specified | Inlet screen, air pressure switch interlocked to the fuel supply, and enough flow for pre-purge |
What this FD fan has to cope with
Slow response to fryer start-up
When the fryer restarts after a stop, cold product loads the frying oil and thermic fluid temperature drops quickly. A combustion air fan with a slow damper or fixed speed limits how fast the heater can recover, and the oil temperature sags. A variable speed drive linked to thermic fluid temperature responds faster and shortens the dip.
Pre-purge and safety interlocks
Oil- and gas-fired heaters must purge the furnace with air before every ignition. The fan must deliver the purge flow reliably and its air pressure switch must not chatter, or the heater locks out and the fryer line waits. A fan with margin over the burner requirement avoids these stoppages.
Grate starvation on briquettes
Briquettes swell as they burn and form a dense bed that raises resistance at the grate. A fan without pressure margin delivers less air as the bed builds, leading to smoke and slow heating. Sizing on the dense bed, and a drive that holds plenum pressure, keep combustion right.
Energy and control
Snack lines change products and stop for cleaning many times a week, and the heater spends long periods at low fire. A combustion air fan on variable speed follows the burner or grate firing instead of throttling, and supplies only the air needed, which cuts flue losses. Where an air preheater is fitted, the air the fan delivers is warmed by the flue gas before the furnace, saving fuel at every load.
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 Induced Draft (ID) Fan for snack fryer thermal oil heaters.
Questions buyers ask
What does the FD fan do on a thermic fluid heater for a fryer?
It supplies combustion air to the burner, or to the grate of a solid-fuel heater. The heater warms thermic fluid, which then heats the frying oil in a heat exchanger. The fan handles ambient air, and its job is a steady, controllable air supply so the heater can follow the load of the fryer line.
Why does my fryer oil temperature drop when the line starts?
Starting a fryer line loads the oil with cold product, and the heater must raise its firing quickly. If the combustion air fan responds slowly or is undersized for full fire, the heater lags and oil temperature sags. A variable speed drive on the forced draft fan helps the heater recover faster.
Can a wood-fired thermic fluid heater use the same FD fan as an oil-fired one?
Not usually. An oil burner fan delivers a smaller flow at higher pressure through a burner head, while a wood or briquette heater needs a larger flow at lower pressure through a grate and fuel bed, split between undergrate and overfire air. A change of fuel generally means a new fan selection.
Information needed to quote an FD fan for snack fryer 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.
- Firing system. Burner make and rating, or grate area and fuel for solid-fuel heaters.
- Air preheater. Whether one is fitted, and its resistance on the air side.
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.