How gas fired hot air generators work
Gas fired hot air generators burn natural gas, LPG or biogas, and because the combustion products are clean they are usually direct fired, even for food. A line burner, also called a duct burner, sits inside the process air duct and fires directly into the moving air stream; or a packaged burner fires into a small combustion chamber and the flame gas mixes with process air. Typical uses are grain, spice, pasta, snack, tea and milk powder dryers, and paint and coating ovens. The air leaves at 80 to 250°C with a little added water vapour and CO₂. The dryer exhaust fan is part of the dryer rather than the generator, so this page covers the FD fan that supplies the process and combustion air.
Where the FD fan sits
- Inlet filter
- FD (process air) fan
- Line burner profile
- Mixing section
- Dryer
The fan sits upstream of the burner, on cold filtered air, and pushes it through the burner profile plate and the dryer. Placing it before the burner keeps it at ambient temperature, small and simple. The profile plate around the line burner is sized for a set air velocity past the flame, typically 10 to 15 m/s, and the fan must hold that velocity as the dryer resistance and filters change.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the burner | 80 to 250°C, downstream of the fan |
| Dust | Filtered air; the product sees whatever the fan delivers |
| Pressure at the fan outlet | Set by the filters, burner profile and dryer, often 100 to 250 mmWC |
| Burner condition | Air velocity across the profile held within the burner maker's range |
| 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 gas fired hot air generators
| Blade form | Backward curved or aerofoil, for efficiency on clean air |
| Material of construction | Mild steel with food-grade coating, or stainless steel for food dryers |
| Drive and arrangement | Direct coupled or belt driven |
| Bearings and cooling | Standard bearings outside the air stream; no cooling needed |
| Flow control | Variable speed drive holding air flow or profile pressure drop |
| Also specified | Air flow proving switch for the burner safety system, inlet filter and flexible connections |
What this FD fan has to cope with
Profile velocity out of range
A line burner is designed for a narrow band of air velocity. Too little air and the flame lengthens and overheats the burner; too much and it lifts off or produces CO. As filters load and dryer resistance changes, a fixed speed fan drifts out of the band. Speed control holding profile pressure drop keeps the burner in its window.
Purge and air proving
Gas safety codes require proven air flow and a pre-purge before ignition. The fan must deliver the purge volume through the whole dryer, and an air flow or pressure switch must prove it. A fan selected without considering purge can leave the burner unable to light reliably.
Combustion products in food
Direct gas firing adds water vapour, CO₂ and small amounts of NOx to the drying air. Most food dryers accept this with clean, well-adjusted low NOx burners. Keeping the fan delivering the design air flow, so the burner never runs rich, keeps CO and NOx low in the product air.
Energy and control
The process air fan moves all the drying air, so it is the main electrical load of a gas fired generator and runs for the whole production period. A high efficiency backward curved or aerofoil wheel and a variable speed drive pay back quickly. Recirculating part of the dryer exhaust to the fan inlet, where humidity allows, raises the inlet temperature and reduces gas consumption; the fan is then selected at the mixed temperature.
Questions buyers ask
Can a gas fired hot air generator be used directly for food drying?
Yes, in most cases. Natural gas and LPG burn cleanly, and direct gas fired dryers are common for grain, spices, pasta, snacks and tea. The burner must be well adjusted and the process air fan must hold the design air flow so the flame stays clean.
What air velocity does a line burner need from the process air fan?
Typically 10 to 15 m/s across the burner profile, though each burner maker gives its own range. The fan is selected to hold that velocity at all dryer loads, usually with a speed drive controlled on the pressure drop across the profile plate, so the flame stays stable as filters load and dryer resistance changes.
Does a gas fired hot air generator need an ID fan?
Not as part of the generator. The combustion products go into the dryer with the process air, and the dryer exhaust fan handles them with the moisture from the product. That exhaust fan is selected as a dryer fan, on the exhaust temperature, humidity and product dust.
Information needed to quote an FD fan for gas fired hot air generators
- 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. Line burner or packaged burner, firing range and the required profile velocity or wind box pressure.
- Dryer. Product, dryer type and the air temperature and volume required.
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.