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 FD fan sits
- Inlet screen
- FD fan
- Grate or burner
- Hot gas generator
- Mixing chamber
- Dryer drum
The fan stands at the hot gas generator and pushes ambient air under the grate of a coal or biomass generator, or to the burner on oil or gas. Tempering air enters the mixing chamber either from a branch of this fan or by suction from the ID fan through a damper. The fan works against the fuel bed or burner and the generator, typically 100 to 250 mmWC. Where the dryer uses waste gas from a clinker cooler, there may be no FD fan at all.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Slag and coal dust in the yard; inlet screening needed |
| Pressure at the fan outlet | Typically 100 to 250 mmWC |
| Load variation | Firing follows slag moisture, from near full load in the monsoon to half load in dry months |
| 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 granulated slag dryers for cement
| 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 tied to the firing rate |
| Also specified | Inlet screen, silencer where needed, and a separate damper for the tempering air branch |
What this FD fan has to cope with
Unsteady generator outlet temperature
When combustion air does not follow the fuel feed, the generator outlet temperature swings by 100°C or more, and dryer outlet temperature and slag moisture follow it. A combustion air fan with a stable rising curve, controlled together with the fuel, holds the generator at its set point and gives the mill a consistent feed.
Grit and coal fines at the inlet
Slag yards and coal handling areas are gritty, and the fan inlet often faces both. Grit wears the impeller and fines coat it, causing unbalance over time. Taking the inlet from a clean side of the building, with a screen and a simple filter, protects the fan at very low cost.
Too much air in dry weather
A generator sized for the wet season runs with a lot of excess air in dry months if the fan is not turned down. That excess air passes on to the ID fan and raises its power. A variable speed drive and an oxygen check at the generator outlet keep combustion air close to what the fuel needs.
Energy and control
The combustion air fan follows seasonal changes in slag moisture, so its duty falls well below design for much of the year. A variable speed drive tied to the firing control saves most of the energy that a throttling damper wastes. Lower excess air also reduces fuel and cuts the gas volume at the exhaust fan, which is the larger consumer.
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 Induced Draft (ID) Fan for granulated slag dryers for cement.
Questions buyers ask
What does the FD fan do on a slag dryer?
It supplies combustion air to the hot gas generator that heats the dryer, and sometimes the tempering air that brings the hot gas down to the dryer inlet temperature. It handles ambient air only and is a much simpler, smaller fan than the dryer exhaust fan that handles the dusty gas.
Is an FD fan needed if the slag dryer uses kiln or cooler waste gas?
Not always. If the dryer takes hot gas from a clinker cooler or kiln exhaust, the heat source already has its own fans, and the dryer may need only an exhaust fan and a booster. A small hot gas generator with its own combustion air fan is sometimes kept as a standby heat source.
How much pressure does the slag dryer combustion air fan need?
Usually 100 to 250 mmWC. A coal or biomass grate generator needs more because air passes through the fuel bed, while an oil or gas burner may need less, depending on its design. The fan is selected on the generator maker's data plus duct and damper losses.
Information needed to quote an FD fan for granulated slag dryers for cement
- 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. Fuel fired in the hot gas generator and its firing capacity.
- Tempering air. How tempering air is supplied and its share of the total gas to the dryer.
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.