How rotary dryers work
Rotary dryers dry granular and lumpy solids in a slightly inclined rotating drum fitted with lifters that shower the material through the gas. Direct co-current dryers, the most common type, bring hot gas from a burner or a coal, biomass or oil-fired hot air generator in at the feed end, often at 400 to 900°C, and exhaust it at 80 to 130°C. Counter-current dryers send the gas against the solids, giving a hotter product and exhaust. Indirect dryers heat the solids through steam tubes or an external jacket, with only a small sweep of air to carry off vapour. Rotary dryers handle sand, minerals, coal, fertiliser, salt, sludge, biomass and distillery residues. The exhaust passes cyclones and a bag filter or wet scrubber, and the ID fan draws it to the stack.
Where the FD fan sits
- Air intake
- FD (combustion air) fan
- Burner or grate
- Hot air generator
- Dilution air
- Rotary drum inlet
The fan sits at the hot air generator. With oil or gas burners, the burner fan supplies combustion air and a dilution fan or damper controls the drum inlet temperature. With coal or biomass, the FD fan supplies undergrate air at higher pressure, and dilution air is added at the mixing chamber. Indirect dryers have no FD fan in this sense: steam heats the tubes.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Gas to the drum | 400 to 900°C, depending on product, downstream of the fan |
| Dust | Ambient air, often in a dusty yard; intake filtered |
| Pressure | Moderate for burners; higher for grate-fired hot air generators |
| Heat source | Oil, gas, coal, biomass or waste heat |
| 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 rotary dryers
| Blade form | Backward curved for clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct driven, variable speed |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive linked to firing rate |
| Also specified | Intake filter and separate dilution air fan where needed |
What this FD fan has to cope with
Dusty plant air
Rotary dryer yards are dusty, and dust drawn into the combustion air fan wears the wheel and fouls the burner. The intake is filtered or placed away from handling.
Inlet temperature control
Too hot an inlet scorches or ignites organic products, too cool reduces output. Combustion and dilution air are measured and controlled together, so the drum inlet temperature holds steady as the firing rate changes.
Fuel changes
Switching from oil to biomass or coal changes the combustion air pressure and the fan arrangement, adding undergrate and dilution fans. The FD fan is chosen for the fuel the plant will actually burn over the next years, not the one on the original drawing.
Energy and control
Firing rate follows feed moisture and throughput, both of which vary with season and source. Speed control on the combustion air fan holds the air to fuel ratio and saves power compared with dampers. Recirculating exhaust instead of fresh dilution air saves fuel where the product allows, and lowers oxygen in the drum for products that can burn.
ID fan and FD fan in rotary dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After cyclones and bag filter or scrubber | At the burner or hot air generator |
| What it handles | Moist drum exhaust with product dust | Ambient combustion and dilution air |
| Temperature at the fan | 80 to 200°C by dryer type | Ambient |
| Main design concern | Dust, moisture and seal leakage | Inlet temperature and fuel |
| Control | Drum hood suction | Firing rate |
See also the Induced Draft (ID) Fan for rotary dryers.
Questions buyers ask
What is the FD fan on a rotary dryer?
It is the combustion air fan of the burner or hot air generator. It may also supply dilution air, or a separate fan does. It handles ambient air, not the drum exhaust.
Does a biomass-fired rotary dryer need a different FD fan?
Yes. A biomass or coal furnace needs undergrate air at higher pressure and often secondary and dilution air. The fan arrangement is more complex than for a gas burner.
Do steam tube dryers have an FD fan?
Not for combustion. Steam heats the tubes, and only a small sweep air flow enters the drum, drawn in by the ID fan to carry the evaporated water away. Some steam tube dryers warm the sweep air in a small coil, but no forced draft fan is needed.
Information needed to quote an FD fan for rotary dryers
- 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.
- Heat source. Fuel, burner or furnace type and the drum inlet temperature.
- Air streams. Combustion, dilution and any recirculation air flows.
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.