How ceramic body spray dryers work
Ceramic tile and sanitaryware plants spray-dry body slip to make free-flowing granules for pressing. The slip, milled in ball mills to about 62 to 68% solids, is pumped to nozzles in the lower part of a wide chamber that spray upward in a fountain, so drying is mixed-flow. Hot gas at typically 500 to 600°C enters at the top from a natural gas or LPG burner, a coal or biomass hot air generator, or the exhaust of a gas turbine in cogeneration plants. Granules at 5 to 7% moisture leave the cone. The exhaust, at about 90 to 110°C, carries fine clay, feldspar and quartz dust and is cleaned in cyclones and a wet scrubber or bag filter. The ID fan draws it to the stack.
Where the FD fan sits
- Air intake
- FD (combustion air) fan
- Burner or hot air generator
- Dilution
- Hot gas duct
- Spray dryer roof inlet
The fan sits at the burner or hot air generator and takes ambient air. With a gas burner, combustion and dilution air are often supplied by one fan into the burner chamber; with a coal or biomass hot air generator, the FD fan supplies undergrate air and a separate fan supplies dilution air. The pressure covers the burner or grate, the duct to the dryer roof and the inlet distributor.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Drying gas | 500 to 600°C at the dryer inlet, downstream of the fan |
| Dust | Ambient air in a dusty body preparation area; intake filtered |
| Pressure | Moderate; burner or grate and hot gas duct |
| Heat source | Gas burner, hot air generator, or gas turbine exhaust |
| 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 ceramic body spray dryers
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct driven with a variable speed drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive linked to firing rate |
| Also specified | Intake filter, silencer and combustion air flow measurement |
What this FD fan has to cope with
Dusty intake
Body preparation halls are dusty, and clay and quartz dust drawn into the fan wears the wheel and fouls the burner. The intake is placed outside the hall or fitted with a filter.
Inlet temperature control
Granule moisture depends on a steady inlet gas temperature. Combustion and dilution air must be measured and controlled; a fan that drifts with filter loading changes granule quality.
Hot air generator variants
Coal or biomass hot air generators need undergrate air at higher pressure and separate dilution air, and carry fly ash into the dryer. Their fan arrangement differs from a clean gas burner.
Energy and control
The combustion air fan runs with the dryer. Speed control matched to firing rate keeps excess air low and saves fuel. Where a plant moves from a burner to gas turbine exhaust, the FD fan becomes a standby for when the turbine is out, and its duty should be reviewed.
ID fan and FD fan in ceramic body spray dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After cyclones and scrubber or bag filter | At the burner or hot air generator |
| What it handles | Humid exhaust with quartz and clay fines | Ambient combustion air |
| Temperature at the fan | 90 to 110°C, or 50 to 60°C after a scrubber | Ambient |
| Main design concern | Abrasion and slurry deposits | Steady drying gas temperature |
| Control | Chamber pressure | Firing rate |
See also the Induced Draft (ID) Fan for ceramic body spray dryers.
Questions buyers ask
What is the FD fan on a ceramic spray dryer?
It is the combustion air fan of the burner or hot air generator. It supplies air for combustion and dilution so the drying gas reaches the chamber at 500 to 600°C. It does not handle dryer exhaust.
Does a coal-fired hot air generator need a different FD fan?
Yes. It needs undergrate air at a higher pressure to pass the grate and fuel bed, plus dilution air to bring the gas to the drying temperature. Fly ash also enters the dryer, which the ID fan must allow for.
Should the combustion air fan be on a VFD?
Yes for most dryers. The firing rate follows press demand and slip density, and speed control keeps the air to fuel ratio right without damper losses. It also lets the plant reduce air at low output instead of throttling.
Information needed to quote an FD fan for ceramic body spray 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.
- Burner or hot air generator. Type, fuel and the combustion and dilution air flows required.
- Cogeneration. Whether a gas turbine supplies drying gas and the FD fan acts as backup.
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.