How tea dryers work
After withering, rolling or CTC cutting and fermentation, tea leaf is dried to about 3% moisture to stop fermentation and fix the quality. CTC factories mostly use vibro fluid bed dryers; orthodox factories use endless chain pressure (ECP) dryers with trays moving through a hot air chamber. Hot air at typically 100 to 140°C at the inlet is produced by an air heater fired with coal, firewood, gas or oil, or by a thermic fluid heater. The main dryer fan pushes air from the heater into the dryer. Exhaust air leaves at about 45 to 60°C carrying tea fluff and fine particles, through a cyclone or a fibre separator. The air heater furnace has its own flue gas path, often with an ID fan.
Where the FD fan sits
- Fresh air inlet
- Air heater
- FD (main hot air) fan
- Hot air duct
- Plenum
- Fluid bed or ECP dryer
In the common arrangement the main fan sits after the air heater, drawing air through the heater tubes and pushing it into the dryer. It therefore handles air at the dryer inlet temperature, typically 100 to 140°C. Some newer installations place the fan before the heater so that it handles ambient air, but the heater then runs under pressure and leaks push air outward.
Air at the FD fan
| Air temperature at the fan | 100 to 140°C where the fan follows the heater; ambient where it precedes it |
| Air after the fan | Same as the fan inlet |
| Dust | Clean hot air; heater tube leaks can add smoke |
| Humidity | Ambient humidity in the monsoon flush season |
| Pressure at the fan outlet | Typically 100 to 250 mmWC for VFBD, lower for ECP |
| Density at the design temperature | About 0.88 kg/m³ at 130°C, 73% 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 dryers
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Belt driven, motor outside the hot air |
| Bearings and cooling | Bearings with cooling disc for hot air |
| Flow control | Variable speed drive or damper on dryer inlet temperature |
| Also specified | Insulated casing and inspection door |
What this FD fan has to cope with
Bearing overheating
Air at 100 to 140°C heats the shaft, and heat conducts to the bearings. Grease breaks down and bearings fail in mid-season. A cooling disc on the shaft, bearings mounted well clear of the casing and a high-temperature grease keep bearing temperatures in range. Vibration checks catch early wear.
Smoke in the tea
A leaking air heater tube lets flue gas into the drying air. Where the main fan draws through the heater, the heater runs under suction and leaks pull smoke straight into the dryer, tainting the tea. Inspecting and pressure-testing the tubes before each season is essential, and a strong furnace ID fan keeps the flue side at lower pressure.
Air flow during peak flush
In peak flush the dryer runs at full capacity for long hours. A fan selected without margin, or one with a worn impeller, limits throughput just when the factory needs it most. Selection with margin for peak flush, and checking fan performance in the lean season, avoid this.
Energy and control
The main hot air fan runs throughout the manufacturing season and is one of the larger motors in the factory. A variable speed drive controlled on dryer inlet temperature or air flow saves power in the lean months, when leaf intake is low. Keeping the air heater tubes clean lowers both the air side resistance and the fuel used.
ID fan and FD fan in tea dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the fluff cyclone | After the air heater, before the dryer |
| What it handles | Exhaust air with tea fluff | Hot drying air |
| Temperature at the fan | 45 to 60°C | 100 to 140°C |
| Main design concern | Fluff build-up and fire | Hot bearings and steady flow |
| Control | Hood pressure | Dryer inlet temperature |
See also the Induced Draft (ID) Fan for tea dryers.
Questions buyers ask
Is the tea dryer main fan before or after the air heater?
In most tea factories it is after the heater, drawing air through the heater tubes and handling hot air at 100 to 140°C. Some newer layouts place it before the heater so that it handles ambient air, but then any tube leak pushes air into the flue gas rather than smoke into the air.
Why do tea dryer fan bearings fail?
Heat from the hot air conducts along the shaft into the bearings, and ordinary grease breaks down. A cooling disc, high-temperature grease, correct bearing selection and regular greasing during the season prevent most failures. Belt tension that is too high also shortens bearing life. Checking bearing temperature weekly in the season helps.
What causes smoky tea from the dryer?
Usually a leaking air heater tube. Where the main fan draws air through the heater, any leak pulls flue gas into the drying air. Pressure-testing the heater, repairing tubes and keeping the furnace side under firm suction stops it. A rise in carbon monoxide or a smoky smell at the dryer inlet is the early sign, and the tea taster often notices it first.
Information needed to quote an FD fan for tea 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.
- Fan position. Before or after the air heater.
- Dryer air flow. Air flow and pressure at the dryer inlet.
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.