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 ID fan sits
- Shell grate
- Radiant and convective coils
- Air preheater
- Cyclone
- ID fan
- Chimney
The fan sits after the cyclone at the chimney base, handling gas at about 200 to 230°C after the air preheater. The cyclone catches coarse ash and char, but the potassium and chloride fume is too fine for it and passes to the fan. Suction is typically 80 to 160 mmWC on heaters of 1 to 4 million kcal/h. Shell burns quickly, so heat release and gas volume rise sharply after each charge, and the fan must follow without letting the furnace go positive.
Gas at the ID fan
| Gas temperature, normal | 200 to 230°C after the air preheater |
| Maximum or upset | Up to 350°C on hard firing or with fouled coils |
| Dust at the fan | Low quantity; fine potassium and chloride fume with some char |
| Moisture and dew point | Low fuel moisture; water dew point well below fan temperature |
| Corrosive species | Chlorides in the fume; corrosive when deposits absorb moisture at shutdown |
| Density at the design temperature | About 0.73 kg/m³ at 215°C, 61% 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 coconut shell fired thermic fluid heaters
| Blade form | Backward curved with smooth blades, or radial tipped where deposits are heavy |
| Material of construction | Mild steel casing; corrosion-resistant steel impeller where chloride deposits stay wet between runs |
| Drive and arrangement | Direct coupled or belt driven |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on furnace draft with fast response |
| Also specified | Inspection door, casing drain, and backup power with the oil pump |
What this ID fan has to cope with
Sticky alkali deposits
Potassium and chloride fume from shell ash is fine enough to pass the cyclone and condenses as a sticky layer on blades and the inlet cone. Build-up is uneven and causes vibration. Smooth blades, an inspection door and regular cleaning keep it under control.
Corrosion during idle periods
Many coconut processors run one or two shifts, and the heater stands idle overnight. When it stops, chloride deposits on the fan absorb moisture from the air and pit the steel within weeks. Cleaning before long stops, casing insulation and a corrosion-resistant impeller reduce the damage.
Fast firing and draft swings
Dry shell flares up quickly after each charge, and the gas volume rises sharply within a minute. If the fan cannot follow, the furnace goes positive and the oil overheats. A fan with reserve pressure and a fast-acting variable speed drive keep the furnace under suction through each charge.
Energy and control
Shell heaters often run in one or two shifts and at varying load. A variable speed drive on furnace draft saves power and handles the fast swings of shell combustion better than a damper. Even, frequent charging of smaller quantities of shell smooths combustion, improves heater efficiency and lowers fan power at the same time.
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 Forced Draft (FD) Fan for coconut shell fired thermic fluid heaters.
Questions buyers ask
Why do deposits form on the ID fan of a coconut shell heater?
Coconut shell ash is rich in potassium and chlorides, which form a very fine fume in the hot furnace. It passes the cyclone and sticks to the blades and inlet cone as the gas cools. Regular cleaning, smooth blades and an inspection door control the build-up and the vibration it causes.
Does coconut shell cause corrosion in the ID fan?
It can. Chloride deposits absorb moisture when the heater is idle overnight or over weekends and pit the steel. Cleaning the fan before long stops, insulating the casing, and using a corrosion-resistant impeller on plants with long idle periods all help.
How hot is the gas at the ID fan of a coconut shell fired thermic fluid heater?
Typically 200 to 230°C with the air preheater working, and up to about 350°C on hard firing or with fouled coils. The fan is sized on the measured gas temperature and volume, with shaft cooling and bearings selected for the hot case.
Information needed to quote an ID fan for coconut shell fired thermic fluid heaters
- 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.
- Firing pattern. Manual or automatic feeding, shell firing rate, and shifts per day.
- Deposit history. How often the present fan is cleaned and photographs of the deposits.
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.