How coconut shell fired thermic fluid heaters work
Coconut shell fired thermic fluid heaters are used in desiccated coconut, coconut oil, coir and food processing plants and in textile units near coconut-growing areas. Shell is a dense, hard fuel with low moisture, typically 8 to 15%, a calorific value of about 4,000 to 4,500 kcal/kg and only 1 to 3% ash, but that ash is rich in potassium and chlorides. It burns hot and fast on a fixed or reciprocating grate, with high flame temperatures close to the coil. Flue gas leaves the coil at 330 to 380°C, passes an air preheater and cyclone, and reaches the ID fan and chimney. The FD fan supplies undergrate air; overfire air comes from a secondary fan or natural draft ports.
Where the FD fan sits
- Inlet screen
- FD fan
- Air preheater
- Undergrate plenum
- Shell grate
- Furnace
The fan pushes ambient air through the air preheater, which heats it to 150 to 200°C, and into the plenum under the grate. Shell forms a denser, deeper bed than husk or wood chips, so the grate resistance is higher, and the fan works at about 80 to 150 mmWC. Overfire air enters through ports above the grate from a secondary fan or by natural draft, and the FD fan is interlocked with the firing door on manually fed heaters.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the preheater | 150 to 200°C, downstream of the fan |
| Dust | Shell dust and fibre near the fuel store |
| Pressure at the fan outlet | 80 to 150 mmWC |
| Grate conditions | Dense, hot bed; ash fuses at low temperature |
| 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 coconut shell fired thermic fluid heaters
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on oil outlet temperature |
| Also specified | Inlet screen and firing door interlock |
What this FD fan has to cope with
Clinker on the grate
Potassium-rich shell ash melts at relatively low temperature. Too much hot undergrate air drives the bed temperature up and forms clinker that blocks the grate openings and starves parts of the bed. Controlling the air flow to the burning rate, and not over-preheating the air, keep the bed below its clinkering temperature.
Oil overheating
Dry shell burns fast after each charge and can overheat the oil film on the coil. FD fan control linked to oil outlet temperature cuts the undergrate air quickly to slow the fire, and an oil flow interlock stops the fan if circulation is lost.
Dense bed resistance
A deep bed of dense shell adds resistance, and it changes as the bed burns down between charges. The FD fan needs enough pressure to push air through the bed evenly at its deepest, and a steep fan curve keeps the flow steadier as the bed resistance falls.
Energy and control
A variable speed drive on the FD fan, controlled from oil outlet temperature, saves power and gives smoother control of a fast-burning fuel than on-off firing. The correct split between undergrate and overfire air improves burnout of the shell volatiles, keeps excess air low and reduces the fume reaching the ID fan.
ID fan and FD fan in coconut shell fired thermic fluid heaters
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone, before the chimney | Before the air preheater, under the grate |
| What it handles | Flue gas with fine potassium and chloride fume | Ambient air |
| Temperature at the fan | 200 to 230°C | Ambient |
| Main design concern | Sticky deposits and corrosion | Clinker and oil overheating |
| Control | Furnace draft | Oil outlet temperature |
See also the Induced Draft (ID) Fan for coconut shell fired thermic fluid heaters.
Questions buyers ask
Why does clinker form on a coconut shell heater grate?
Shell ash is rich in potassium, which lowers its melting point. Too much air under the grate, or air preheated too far, drives the bed hot enough to fuse the ash into clinker. Controlled undergrate air and a properly sized air preheater prevent this.
Does the FD fan on a coconut shell heater handle hot air?
No. It pushes ambient air into the air preheater, which heats it downstream on its way to the grate. The fan is a cold-air machine selected at ambient density in mild steel, with an inlet screen against shell dust.
What pressure does the FD fan need on a coconut shell fired thermic fluid heater?
Typically 80 to 150 mmWC, covering the air preheater, ducts, plenum and the dense shell bed. Deeper beds and grates with small air holes need the higher end. Send the grate data and we will calculate the duty for the coconut shell fired thermic fluid heater.
Information needed to quote an FD fan for coconut shell fired thermic fluid heaters
- 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.
- Grate. Grate type and area, and the depth of the shell bed.
- Air preheater. Air side resistance and hot air 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.