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 ID fan sits
- Hot gas generator
- Rotary dryer
- Discharge hood
- Cyclone
- Bag filter
- Exhaust (ID) fan
- Stack
The fan is placed after the bag filter. It works against the drum, the discharge hood, the cyclone and the filter, typically 180 to 320 mmWC, and keeps the drum ends under slight suction. The gas reaching it is cool and humid, and carries only what passes the filter: fine limestone, which is soft, and fine clay and laterite, which are more abrasive because of their quartz content.
Gas at the ID fan
| Exhaust temperature, normal | 90 to 120°C after the bag filter |
| Maximum or upset | Up to 200°C if feed is interrupted |
| Dust at the fan | Limestone and clay fines; quartz in laterite and clay raises the abrasion |
| Moisture and dew point | Dew point of about 45°C on dry limestone, 60 to 70°C on wet clay or laterite |
| Gas composition | Generator flue gas with tempering air; some CO₂ from partial calcination is negligible at these temperatures |
| Density at the design temperature | About 0.93 kg/m³ at 105°C, 78% 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. |
ID Fan selection for limestone and additive dryers
| Blade form | Backward curved, with wear liners when clay or laterite is dried |
| Material of construction | Mild steel; wear-resistant blade tips for siliceous additives |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on dryer suction, with stored set points for each material |
| Also specified | Insulated casing, drain, and an access door for clearing clay deposits |
What this ID fan has to cope with
Duty that changes with the material
Drying limestone at 4% moisture needs perhaps half the exhaust volume of drying laterite at 20%. A fan sized only for the wet material runs badly throttled most of the year, while a fan sized for limestone cannot hold draft on clay. A wide-range selection with a variable speed drive serves both without surge or overload.
Clay deposits on the impeller
Fine clay that passes the filter in a humid exhaust sticks to the blade pressure faces and builds into layers that break off unevenly. Keeping the exhaust well above its dew point, insulating the casing and washing the wheel during planned stops keep vibration under control.
Filter blinding and lost draft
Sticky clay fines and condensation at start-up can blind the bag filter. Its pressure drop rises, the fan moves back on its curve and the drum loses suction, so dust escapes at the seals. Preheating the filter before feed and a fan with pressure margin keep the dryer running.
Energy and control
Additive dryers often run in campaigns, switching between materials and sometimes standing idle. A variable speed drive on the exhaust fan, with set points for each material, cuts power when drying limestone or other low-moisture feed. Tight drum seals and a well-maintained discharge hood limit false air, which otherwise adds to fan volume and lowers the exhaust temperature towards its dew point.
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 Forced Draft (FD) Fan for limestone and additive dryers.
Questions buyers ask
How do I size the ID fan for a dryer that handles both limestone and clay?
Size it for the wettest material at the highest throughput, which sets the largest exhaust volume, and check that it runs stably at the smallest volume, usually dry limestone. A backward curved fan with a variable speed drive covers this range well without running near surge or overloading the motor.
Why does my limestone dryer exhaust fan vibrate after drying laterite?
Laterite and clay fines are sticky in humid exhaust and build up on the blades. When the build-up flakes off unevenly, the wheel goes out of balance. Washing the impeller after a clay campaign and keeping the exhaust above its dew point prevent the problem from recurring.
What temperature should a limestone dryer ID fan be rated for?
The normal exhaust temperature after the bag filter is usually 90 to 120°C. The fan is also checked for a short upset of about 200°C, which occurs if feed stops while the generator is still firing. A temperature trip on the filter inlet protects both the bags and the fan.
Information needed to quote an ID fan for limestone and additive dryers
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Materials dried. Each material dried, its moisture range and throughput, and the required product moisture.
- Campaign pattern. How often the dryer switches between materials and how long it runs on each.
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.