How herbal extract spray dryers work
Herbal and nutraceutical extracts such as ashwagandha, turmeric, moringa, green tea, senna and many others are extracted with water or water-alcohol, filtered, concentrated in an evaporator to about 30 to 45% solids and spray dried, often with a carrier such as maltodextrin. Dryers are usually small to medium, 25 to 500 kg/h of water evaporation, co-current with a rotary atomiser. Inlet air is about 160 to 200°C, heated by steam, a thermic fluid heater or electric heaters, and the outlet about 80 to 95°C. Exhaust air carries fine, hygroscopic and often bitter or potent extract dust through cyclones and a bag filter or wet scrubber. Multi-product plants change extracts often, and residual alcohol in the concentrate is a fire concern.
Where the FD fan sits
- Air intake
- Filter
- FD (supply) fan
- Steam, thermic fluid or electric heater
- Air disperser
- Drying chamber
The fan sits after the intake filter and before the heater, so it handles cool ambient air. In small dryers the fan, filter and heater are often a packaged unit on the dryer platform, with short ducts to the air disperser on the chamber roof. The fan pressure covers the filter, the heater coil or element bank and the disperser, usually modest compared with the exhaust side.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the heater | 160 to 200°C, downstream of the fan |
| Dust | Filtered ambient air |
| Pressure | Filter, heater and air disperser |
| Heat source | Steam coil, thermic fluid coil or electric heater |
| 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 herbal extract spray dryers
| Blade form | Backward curved |
| Material of construction | Stainless steel or coated mild steel |
| Drive and arrangement | Direct driven, compact, variable speed |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on flow |
| Also specified | Intake filter and inspection door |
What this FD fan has to cope with
Dusty intake air
Herbal plants grind and sieve dry raw herbs, and the dust from these rooms is fine and organic. If it enters the supply air it ends up in the product and on the heater. The intake is placed away from grinding and handling areas, and a filter is fitted ahead of the fan.
Inlet temperature stability
Small dryers with electric heaters respond quickly, so a small change in air flow shifts the inlet temperature and then the powder moisture and colour. A supply fan with flow measurement and speed control keeps the air steady, letting the heater control hold the inlet temperature.
Turndown
Different extracts need different air flows and outlet temperatures, and one dryer may run a dozen products a month. The fan is selected to run stably across the full flow range, not just at its rated point, so low-flow products do not push it into unstable operation.
Energy and control
The supply fan is small and runs in campaigns of hours to days. Speed control matches the air flow to each extract and its outlet temperature. The heater uses far more energy than the fan, so where a plant runs most days, exhaust heat recovery to the inlet air is the larger saving; on small, infrequently used dryers it rarely pays back.
ID fan and FD fan in herbal extract spray dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the bag filter or scrubber | After the intake filter, before the heater |
| What it handles | Humid exhaust with hygroscopic extract fines | Filtered ambient air |
| Temperature at the fan | 80 to 95°C | Ambient |
| Main design concern | Changeover cleaning and residual solvent | Stable inlet temperature |
| Control | Chamber pressure | Drying air flow |
See also the Induced Draft (ID) Fan for herbal extract spray dryers.
Questions buyers ask
Does a herbal spray dryer FD fan need to be stainless?
Many plants use stainless steel for the whole air path, especially for nutraceuticals and export products. A coated mild steel fan before the heater is also used where the standard allows.
Which heater suits a herbal spray dryer?
Steam where it is available, thermic fluid where the plant already has a heater, and electric for small dryers. All allow the 160 to 200°C inlet temperature typical of herbal extracts.
Why use a VFD on a small supply fan?
Different extracts need different air flows and outlet temperatures. A variable speed drive lets one supply fan cover them all without throttling losses or damper wear, and makes the air flow a recorded setpoint in each product recipe, which helps repeatability between batches.
Information needed to quote an FD fan for herbal extract spray 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.
- Heater. Heater type and its pressure drop.
- Air flow range. Lowest and highest air flows across the products.
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.