How coir pith dryers work
Coir pith, the spongy dust left after fibre is extracted from coconut husk, is washed or buffered to lower its salt content, pressed, and then dried from about 65 to 80% moisture down to 15 to 20% before it is compressed into blocks, discs or grow bags. Most mechanical dryers are rotary drums or pneumatic flash dryers fed with hot gas from a furnace or hot air generator burning coconut shell, husk chips or firewood. The gas is tempered with ambient air to about 250 to 450°C at the dryer inlet and leaves at 70 to 110°C, laden with water vapour and fine pith. A cyclone or a pair of cyclones recovers the product, and the exhaust fan after them discharges to a stack or a wet scrubber.
Where the FD fan sits
- Air inlet
- FD fan
- Undergrate air box
- Grate and fuel bed
- Mixing chamber
- Dryer
The fan sits beside the furnace and blows ambient air into the air box beneath the grate, sometimes with a branch to overfire nozzles above the bed. Tempering air that cools the furnace gas to the dryer inlet temperature usually enters the mixing chamber by the suction of the ID fan, through a damper, rather than from the FD fan. The FD fan pressure covers the grate, the fuel bed and the air box, typically 50 to 120 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 40°C |
| Dust | Clean air, but the fan stands in a dusty yard with pith and husk fines |
| Pressure at the fan outlet | Typically 50 to 120 mmWC, depending on grate and bed depth |
| Fuel | Coconut shell, husk chips or firewood, with moisture varying by season |
| Turndown | Wide; firing follows the wet pith feed and stops with it |
| 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 coir pith dryers
| Blade form | Backward curved, for a stable curve on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled, or belt driven on small units |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive or inlet damper on dryer inlet temperature |
| Also specified | Inlet mesh guard and a filter or hood where the yard is dusty |
What this FD fan has to cope with
Temperature swings with batch firing
Manual charging of shell or husk causes a burst of volatiles and then a dying fire, so the dryer inlet temperature rises and falls. Linking the FD fan speed to the inlet temperature smooths the firing and keeps the pith from scorching or leaving wet.
Dusty intake
A fan drawing from a pith yard ingests fibre and dust that clog the inlet guard and coat the blades. Locating the intake away from the pith heap or fitting a simple screen and hood keeps the air supply clean.
Grate overheating at low load
When the dryer slows down, cutting the fan too far starves the grate of cooling air and burns the bars. A minimum fan speed and a separate idle firing setting protect the grate.
Energy and control
The FD fan on a coir pith furnace is small next to the ID fan, but it decides how well the fuel is burnt. Matching air to fuel with a speed-controlled fan avoids the excess air that otherwise cools the gas, raises the tempering load and enlarges the ID fan duty. Running the fan only as hard as the dryer needs also cuts fuel consumption.
ID fan and FD fan in coir pith dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the product cyclones | Beside the furnace, under the grate |
| What it handles | Humid gas with pith fines, fibre and sand | Ambient air |
| Temperature at the fan | 70 to 110°C | Ambient |
| Main design concern | Fibre build-up, erosion and condensation | Steady combustion with batch firing |
| Control | Dryer suction or outlet temperature | Dryer inlet temperature |
See also the Induced Draft (ID) Fan for coir pith dryers.
Questions buyers ask
Is the FD fan on a coir pith dryer the same as the combustion air blower?
Yes. On a coir pith dryer the forced draft fan supplies the combustion air to the furnace grate, and is often just called the blower. The tempering air that cools the gas is normally drawn in by the ID fan through a damper.
How much pressure does a coir pith furnace FD fan need?
Usually 50 to 120 mmWC at the fan outlet, enough to push air through the grate, the fuel bed and the air box. Deeper beds of husk chips or wet fuel need the upper end of the range.
Can the dryer run without an FD fan?
Small natural draft furnaces work, but the firing rate then depends on the ID fan suction and the chimney, and the inlet temperature is harder to hold. A forced draft fan gives better control of combustion and the dryer inlet temperature.
Information needed to quote an FD fan for coir pith 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.
- Furnace details. Grate area, fuel type and the firing rate in kg/h at full dryer load.
- Air distribution. Whether overfire air is taken from the same fan and the pressure needed at the nozzles.
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.