How limestone and additive dryers work
Limestone, dolomite, laterite, clay, iron ore fines and similar additives are dried before grinding or blending in cement plants, steel plant flux preparation and chemical plants. Moisture ranges from 3 to 6% for most limestone to 15 to 25% for clay and laterite in the wet season. A co-current rotary dryer is the most common type, with hot gas at 500 to 800°C from a coal, biomass, oil or gas-fired hot gas generator or from kiln and cooler waste gas. Clay-rich additives become sticky as they dry and form balls and coatings. The exhaust passes through a cyclone and a bag filter to the ID fan and stack, and the FD fan supplies the generator combustion air.
Where the FD fan sits
- Inlet screen
- FD fan
- Hot gas generator
- Tempering chamber
- Rotary dryer
The fan sits at the hot gas generator and pushes ambient air to the grate, stoker or burner. Tempering air, added to bring the gas to the dryer inlet temperature, may come from a branch of the same fan or be drawn in by the ID fan through a damper. The fan works against the generator and its air distribution, usually 100 to 250 mmWC. When the dryer uses kiln or cooler waste gas, the generator and its fan stand by or run only for top-up heat.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Limestone and coal dust in the plant; inlet screening needed |
| Pressure at the fan outlet | Typically 100 to 250 mmWC |
| Turndown | Large, from wet clay at full firing to dry limestone at low firing |
| 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 limestone and additive dryers
| Blade form | Backward curved, on clean ambient air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard pedestal bearings |
| Flow control | Variable speed drive linked to the firing control |
| Also specified | Inlet screen and a damper on any tempering air branch |
What this FD fan has to cope with
Poor turndown on dry material
When the dryer moves from wet clay to dry limestone, firing falls to a fraction of design. A damper-controlled fan near the bottom of its range gives unstable air pressure and poor combustion at the generator. A variable speed drive keeps the fan in a stable region of its curve across the full range of firing.
Air without fuel control
If fuel feed and combustion air are adjusted separately, the generator runs with either smoke or very high excess air whenever the operator changes material. Linking the fan speed to the fuel feed, with an oxygen trim, makes material changes smoother and saves fuel. It also keeps the gas volume at the exhaust fan predictable.
Inlet in a dusty area
Generators in cement and steel plants often sit near crushers and conveyors. Limestone and coal dust at the inlet build up on the impeller and wear bearing seals. An inlet duct from a cleaner area, with a screen and filter, avoids this at little cost and keeps the fan in balance.
Energy and control
Firing on an additive dryer changes with every switch of material and with the season, so a fixed-speed combustion air fan spends long periods throttled. A variable speed drive linked to the firing control saves most of that energy. Holding excess air low at the generator saves fuel and also reduces the gas volume handled by the exhaust fan, which uses more power.
ID fan and FD fan in limestone and additive dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone and bag filter | At the hot gas generator |
| What it handles | Humid exhaust with limestone and clay fines | Clean ambient air |
| Temperature at the fan | 90 to 120°C | Ambient |
| Main design concern | Wide duty swing and clay build-up | Turndown between materials |
| Control | Dryer suction per material | Generator firing rate |
See also the Induced Draft (ID) Fan for limestone and additive dryers.
Questions buyers ask
What is the FD fan on a limestone dryer?
It is the combustion air fan of the hot gas generator that heats the dryer. It handles clean ambient air and supplies the grate or burner, and sometimes the tempering air. The dusty, humid exhaust from the drum is handled by the separate ID or exhaust fan after the bag filter.
Does a limestone dryer using kiln waste gas need a combustion air fan?
Only if it has a hot gas generator for start-up or top-up heat. When kiln or cooler gas provides all the heat, the dryer typically needs only an exhaust fan and possibly a booster fan on the waste gas duct. The generator and its fan then run only when the kiln is down.
Can a single combustion air fan supply both the grate and tempering air?
Yes, with separate dampers on each branch. The fan is then sized for the sum of both flows at the higher pressure, which is usually the grate. On larger generators separate fans give better control, because tempering air changes with dryer inlet temperature while grate air follows fuel.
Information needed to quote an FD fan for limestone and additive 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.
- Generator. Fuel, firing capacity and whether the generator also supplies tempering air.
- Heat sources. Whether kiln or cooler waste gas is also used and in what share.
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.