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 ID fan sits
- Furnace or hot air generator
- Mixing chamber
- Rotary drum or flash duct
- Product cyclones
- ID fan
- Stack or wet scrubber
The fan sits after the product cyclones, since the cyclones are part of the process and must run under suction to discharge pith cleanly through their rotary valves. Upstream are the furnace, the mixing chamber, the drum or flash column and the cyclone pair. The fan inlet suction is typically 150 to 300 mmWC on a flash dryer and somewhat less on a drum, where the drum resistance is low but the cyclones and ducting still take most of the head.
Gas at the ID fan
| Gas temperature, normal | 70 to 110°C after the cyclones |
| Maximum or upset | 150 to 200°C if the feed stops while the furnace is still firing |
| Dust at the fan | Pith fines, short fibre strands and fine sand; grams per cubic metre with cyclones alone |
| Moisture and dew point | High water vapour load; dew point typically 55 to 70°C |
| Corrosive species | Chlorides from unwashed or poorly washed pith in any condensate |
| Density at the design temperature | About 0.95 kg/m³ at 100°C, 79% 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 coir pith dryers
| Blade form | Radial tipped or open paddle, to shed fibre and tolerate fines |
| Material of construction | Mild steel with wear-resistant plate on the blade leading edges and casing scroll; SS 304 or coated wetted parts where salty condensate is a problem |
| Drive and arrangement | Belt driven, arrangement 3 or 9, to choose the speed and allow later re-rating |
| Bearings and cooling | Plummer block bearings outside the gas stream; no cooling needed at this temperature |
| Flow control | Variable speed drive on dryer outlet temperature or inlet suction |
| Also specified | Large inspection door, casing drain and a water wash connection |
What this ID fan has to cope with
Fibre wrapping and build-up
Short coir strands that pass the cyclones catch on blade roots, the hub and the inlet cone, then hold damp fines until the wheel is visibly out of balance. An open radial wheel with no ledges, a smooth inlet and a door that allows a quick clean at each shift change keep vibration under control.
Sand erosion
Pith that is not thoroughly washed carries fine silica sand, which erodes the blade tips and the scroll cut-off. Wear plate on these zones, a moderate tip speed and an efficient high-efficiency cyclone in front of the fan extend the life of the rotor.
Condensation on start and stop
At start-up the cold fan casing condenses moisture from the exhaust, and pith fines turn to a paste that sets as the fan warms. Warming the system with the furnace before feed, insulating the casing and fitting a drain reduce the deposits.
Fire in the exhaust path
Dry pith fines are combustible and sparks from a coconut shell furnace can be carried through. A spark arrestor or settling chamber after the furnace and a temperature trip on the fan inlet protect the fan and the cyclones.
Energy and control
Coir pith units often run one or two shifts and follow the supply of wet pith, so the dryer is started and stopped frequently and runs at part load for long periods. A variable speed drive on the ID fan, set to hold outlet temperature or suction, saves more than throttling a damper and keeps the pith moisture steadier. Pressing the pith harder before the dryer cuts the evaporation load and the gas volume.
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 Forced Draft (FD) Fan for coir pith dryers.
Questions buyers ask
What type of ID fan is used on a coir pith dryer?
A radial tipped or open paddle wheel is usual on a coir pith dryer, because the exhaust after the cyclones still carries fibre strands, fines and some sand. Backward curved wheels are more efficient but collect fibre and wear faster on this duty unless a bag filter is fitted ahead of the fan.
Why does the coir pith dryer fan vibrate after a few days?
Damp pith and fibre build up unevenly on the blades and hub, usually after stops when the casing has cooled and condensate has formed. Regular cleaning through an inspection door, casing insulation and an open wheel design reduce the problem; balancing alone will not cure it.
Does salt in coir pith affect the dryer fan?
Unwashed pith contains sodium and potassium salts. When the exhaust condenses in the fan or ducting, chlorides concentrate in the condensate and pit mild steel. Washing the pith, keeping the fan above the dew point and using stainless or coated wetted parts address this.
Information needed to quote an ID fan for coir pith 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.
- Dryer type. Rotary drum or flash dryer, evaporation rate in kg/h, and the inlet and outlet gas temperatures.
- Pith quality. Whether the pith is washed or buffered and how much sand it typically carries.
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.