How tea waste fired boilers work
Two kinds of tea waste are fired in boilers. Spent tea leaves from instant tea and tea extract plants leave the extractors at 70 to 80% moisture and are pressed to 55 to 65% before firing. Factory tea waste, the fibre, stalk and fluff removed during sorting, is dry at about 10% moisture. Both carry 4 to 8% ash and a dry calorific value of about 4,000 to 4,500 kcal/kg, but pressed spent tea yields only about a third of that as fired. Spent tea is burnt on a reciprocating or travelling grate with hot undergrate air, often blended with wood, factory waste or coal. The flue gas passes the boiler, an air heater and economiser, and a multicyclone or bag filter before the ID fan.
Where the FD fan sits
- Inlet screen
- FD fan
- Air heater
- Undergrate plenum
- Reciprocating grate
The FD fan pushes ambient air through a tubular air heater, where it is heated to about 200 to 250°C, into the plenum under the reciprocating or travelling grate. The front of the grate is a drying zone in which hot air drives water out of the fuel before ignition. The fan pressure covers the air heater, ducts, zone dampers and a thick, wet fuel bed, typically 120 to 220 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 15 to 35°C in tea-growing hills, higher in the plains |
| Air after the air heater | 200 to 250°C, downstream of the fan |
| Dust | Clean air; tea fluff near the fuel store |
| Pressure at the fan outlet | Typically 120 to 220 mmWC |
| Altitude | Hill plants at 1,000 to 2,000 m have air 10 to 20% lighter than at sea level |
| Density at the design temperature | About 1.15 kg/m³ at 35°C, 96% 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 tea waste fired boilers
| Blade form | Backward curved, selected at site air density |
| Material of construction | Mild steel |
| Drive and arrangement | Direct coupled or overhung |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive with oxygen trim |
| Also specified | Inlet screen, zone dampers, and motor sized at site density |
What this FD fan has to cope with
Fan rated at sea level, installed in the hills
At 1,500 m the air is about 15% lighter than at sea level. A fan selected on standard density develops less pressure and moves less mass than its catalogue curve shows, so the grate runs short of air. Selecting the FD fan at site density, with the speed or diameter to suit, gives the air the grate actually needs.
Wet bed resistance
A thick bed of wet spent tea resists air more than a dry fuel bed, and resistance rises further when the fuel arrives wetter than usual. The fan needs pressure margin to push air through the bed at the highest moisture, or drying stops, the fire dies back toward the rear of the grate and output falls.
Air heater bypass at low load
At low load the air heater can cool the flue gas toward its dew point. Bypassing part of the air around the air heater reduces heat pickup and protects the gas side, but it also changes the resistance the FD fan works against and the air temperature at the grate. The fan is checked at both positions.
Energy and control
The FD fan on a spent tea boiler runs continuously through the processing season. Variable speed with oxygen trim keeps excess air down, which matters here because each extra cubic metre of air adds to an already large wet gas volume at the ID fan. Better pressing of the spent tea reduces the hot air needed for drying, and so the load on both fans.
ID fan and FD fan in tea waste fired boilers
| ID fan | FD fan | |
|---|---|---|
| Position | After the multicyclone or bag filter | Before the air heater, feeding the grate |
| What it handles | Wet flue gas with fine ash | Ambient air |
| Temperature at the fan | 150 to 180°C, up to 230°C | Ambient, 15 to 35°C |
| Main design concern | Wet gas volume and condensation | Hot drying air and site altitude |
| Control | Furnace draft | Air flow and excess oxygen |
See also the Induced Draft (ID) Fan for tea waste fired boilers.
Questions buyers ask
Why is hot air needed for a spent tea fired boiler?
Spent tea is pressed to 55 to 65% moisture and will not burn until most of that water is driven off. Hot undergrate air from the air heater dries the fuel on the front of the grate. The FD fan pushes ambient air through the air heater to supply it at about 200 to 250°C.
Does altitude affect the FD fan in a tea factory boiler?
Yes. Many tea plants are at 1,000 to 2,000 m, where the air is 10 to 20% less dense. A forced draft fan selected at sea-level density will deliver less mass flow and pressure than expected. It must be selected for site density, and the motor checked at the same condition.
What pressure does the FD fan need on a tea waste fired boiler?
Typically 120 to 220 mmWC, covering the air heater, ducts, zone dampers and the wet fuel bed. Wetter fuel and deeper beds need more. We size it from the boiler data, the fuel moisture range and the site altitude, then confirm against site readings where the boiler is running.
Information needed to quote an FD fan for tea waste fired boilers
- 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.
- Site altitude. Height of the plant above sea level and the ambient temperature range.
- Air heater. Hot air temperature and the air-side pressure drop of the air heater.
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.