How granulated slag dryers for cement work
Granulated blast furnace slag arrives from the steel plant with typically 8 to 15% moisture after water granulation. Where it is ground in a ball mill for slag cement or GGBS, it is first dried, usually in a co-current rotary dryer or a flash dryer, to about 1% moisture. Hot gas comes from a coal, biomass or oil-fired hot gas generator, or from cooler exhaust where the dryer sits in a cement plant, at an inlet temperature of 500 to 750°C. Slag is a glassy, angular calcium alumino-silicate, harder than most minerals dried in cement plants. The exhaust passes a cyclone and bag filter to the ID fan. Plants grinding slag in a vertical roller mill dry it inside the mill, where the mill fan does this job instead.
Where the ID fan sits
- Hot gas generator
- Rotary dryer
- Drop-out box
- Cyclone
- Bag filter
- Exhaust (ID) fan
- Stack
The fan is installed after the bag filter. Upstream, the drum, the drop-out box at the discharge end, the cyclone and the filter together take 200 to 350 mmWC at full evaporation. The fan holds the dryer feed end and discharge hood at a small suction so that hot gas and dust do not escape around the drum seals. Slag carry-over from the drum is coarse and sharp, so most of it must be removed before the fan.
Gas at the ID fan
| Exhaust temperature, normal | 100 to 130°C after the bag filter |
| Maximum or upset | Around 200°C when slag feed is interrupted |
| Dust at the fan | Angular glassy slag fines; low after a bag filter, highly erosive if a bag fails or the filter is bypassed |
| Moisture and dew point | Evaporation of 8 to 15% feed moisture gives a dew point of about 55 to 65°C |
| Other | Fine slag is latent hydraulic and hardens if it is wetted in the casing |
| Density at the design temperature | About 0.91 kg/m³ at 115°C, 76% 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 granulated slag dryers for cement
| Blade form | Backward curved with wear liners; radial in wear plate where only a cyclone precedes the fan |
| Material of construction | Mild steel casing with a wear liner at the cut-off; blades in wear-resistant plate or with hard-faced leading edges |
| Drive and arrangement | Direct coupled, arrangement 8, with tip speed kept moderate |
| Bearings and cooling | Pedestal bearings with a cooling disc |
| Flow control | Variable speed drive on dryer suction and outlet temperature |
| Also specified | Insulated casing, drain, and an inspection door positioned to view the blade tips |
What this ID fan has to cope with
Erosion of blades and scroll
Slag particles are glassy and sharp, and the fines that pass the filter, or the heavy load when a bag fails, cut the blade leading edges and the scroll near the cut-off. Wear plate on the blades, a bolted liner at the cut-off and tip speed held well below that of a clean-air fan extend the interval between repairs.
Slag setting in the fan casing
Granulated slag reacts with water slowly but surely. Fines that settle in a cold, wet casing after a stop form a hard crust that is difficult to remove. Insulation, a warm purge before shutdown and a drain kept open stop this from building up between maintenance periods.
Change in slag moisture
Slag moisture depends on how long it has drained in the stockpile, and the evaporation load can double after rain. A fan sized only for average moisture cannot hold dryer draft on wet days. Sizing for the wettest expected slag, with a variable speed drive, keeps draft and output on target.
Energy and control
Slag dryers usually run continuously alongside the mill, but the evaporation load follows stockpile moisture and the season. A variable speed drive on the exhaust fan, controlled by dryer suction and outlet temperature, cuts power when slag is drier. Sealing the drum ends and the discharge hood reduces false air, which otherwise adds volume and cools the exhaust towards its dew point.
ID fan and FD fan in granulated slag dryers for cement
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone and bag filter | At the hot gas generator |
| What it handles | Humid exhaust with glassy slag fines | Clean ambient air |
| Temperature at the fan | 100 to 130°C | Ambient |
| Main design concern | Erosion and slag setting in the casing | Steady generator outlet temperature |
| Control | Dryer suction and outlet temperature | Generator firing rate |
See also the Forced Draft (FD) Fan for granulated slag dryers for cement.
Questions buyers ask
What ID fan is suitable for a granulated slag dryer?
A backward curved fan with wear liners works well after a bag filter, giving good efficiency with protection against the occasional slag fines. If the fan must follow a cyclone alone, a radial wheel in wear-resistant plate is preferred, because the sharp, glassy slag erodes thin blades rapidly.
Do I need a separate dryer for slag if I grind it in a VRM?
Usually not. A vertical roller mill dries the slag inside the mill with hot gas, and the mill fan does the job of the dryer exhaust fan. A separate slag dryer is used ahead of ball mills, which tolerate only a small amount of moisture in the feed.
Why does the slag dryer exhaust fan lose draft after rain?
Wet slag from the stockpile raises the water evaporated in the dryer, so the exhaust volume increases. A fan sized for normal moisture reaches the end of its capacity and draft at the drum falls. A larger margin on volume and a variable speed drive handle the wet season.
Information needed to quote an ID fan for granulated slag dryers for cement
- 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.
- Slag moisture. Normal and wet-season moisture of the slag feed, and the required product moisture.
- Heat source. Whether the dryer uses a hot gas generator or waste gas from the plant, and its temperature.
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.