How mineral rotary dryers work
Mineral rotary dryers dry silica sand, quartz, feldspar, limestone, dolomite, bauxite, kaolin, gypsum and heavy mineral sands such as ilmenite, rutile, zircon and garnet. Feed moisture is usually 5 to 15%, and the product leaves at below 0.5% for glass, foundry and abrasive grades. Most are co-current drums fired by oil, gas or a coal hot air generator, with inlet gas at 600 to 900°C because minerals tolerate high temperatures. Some sand dryers are counter-current to discharge a hot, dry product for screening. The exhaust, at about 90 to 150°C, carries hard, fine mineral dust through cyclones and a pulse-jet bag filter. The collected fines are often valuable and are returned to the product.
Where the FD fan sits
- Air intake
- FD (combustion air) fan
- Burner or hot air generator
- Dilution
- Rotary drum
The fan sits at the burner or hot air generator and draws ambient air. Because minerals tolerate a high drum inlet temperature, the gas needs less dilution air than in food or organic dryers. With an oil or gas burner, one fan usually supplies combustion and dilution air; with a coal-fired hot air generator, the FD fan supplies undergrate air at higher pressure and a separate fan or damper adds dilution air.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Gas to the drum | 600 to 900°C, downstream of the fan |
| Dust | Ambient air in a dusty plant; filtered intake |
| Pressure | Burner or grate and furnace |
| Fuel | Oil, gas or coal |
| 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 mineral rotary dryers
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct driven, variable speed |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive with firing rate |
| Also specified | Intake filter and silencer |
What this FD fan has to cope with
Silica dust in the intake
Mineral plants have airborne silica and feldspar dust around crushers, screens and stockpiles. Drawn into the combustion air fan it wears the wheel and fouls the burner. The intake is placed away from handling areas, upwind where possible, and fitted with a filter.
Burner turndown with wet feed swings
Feed moisture can double after rain, then fall back within a day. The burner and its fan must turn down stably across this range without flame instability. A fan selected for the full range and controlled by speed handles it better than a damper.
Ash from coal-fired hot air generators
Coal-fired hot air generators carry fly ash into the drum, which can discolour white minerals such as kaolin, feldspar or glass sand. For these grades the plant fires oil or gas, and the FD fan arrangement follows the cleaner fuel.
Energy and control
The combustion air fan follows the firing rate, which follows feed moisture. Speed control holds the air to fuel ratio without damper losses and keeps excess air low. Running the highest inlet temperature the mineral and drum allow reduces the gas volume per kg of water evaporated, which shrinks both the combustion air and the ID fan duty.
ID fan and FD fan in mineral rotary dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter | At the burner or hot air generator |
| What it handles | Exhaust with hard mineral fines | Ambient combustion air |
| Temperature at the fan | 90 to 150°C | Ambient |
| Main design concern | Abrasion during filter bypass | Burner turndown |
| Control | Drum suction | Firing rate |
See also the Induced Draft (ID) Fan for mineral rotary dryers.
Questions buyers ask
What is the FD fan on a mineral rotary dryer?
It is the combustion air fan of the burner or hot air generator. It takes ambient air and supplies combustion and, often, dilution air so that hot gas at 600 to 900°C enters the drum. It does not handle drum exhaust or mineral dust under normal conditions.
Why do mineral dryers use high inlet gas temperatures?
Most minerals are not damaged by heat, so a high inlet temperature evaporates more water per cubic metre of gas. That reduces the total gas volume, the size of the ID fan and bag filter, and the fuel lost with the exhaust.
Does coal firing affect mineral product quality?
It can. Ash and unburnt carbon from a coal-fired hot air generator may discolour white minerals such as kaolin or glass sand. Plants producing these grades usually fire oil or gas, while coloured minerals and construction sand tolerate coal firing.
Information needed to quote an FD fan for mineral 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.
- Fuel and burner. Fuel, burner type and inlet gas temperature.
- Dilution. Dilution air flow.
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.