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 ID fan sits
- Drying chamber
- Cyclone
- Bag filter or wet scrubber
- Exhaust (ID) fan
- Stack
The fan follows the final collector and holds the chamber at a slight suction. In smaller plants a cyclone and wet scrubber are common, so the fan may see saturated air with extract droplets; with a bag filter it sees little dust. The suction covers the chamber, cyclone and collector, typically 150 to 300 mmWC. These are compact fans, often mounted on the dryer structure.
Gas at the ID fan
| Air temperature, normal | 80 to 95°C at the fan inlet; 45 to 55°C after a wet scrubber |
| Maximum or upset | Up to about 130°C if feed is lost |
| Dust at the fan | Fine, hygroscopic plant extract; low after a bag filter |
| Moisture and dew point | Humid exhaust; condensate stains and ferments if left |
| Solvent | Traces of ethanol if the extract is not fully stripped |
| Density at the design temperature | About 0.97 kg/m³ at 90°C, 81% 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 herbal extract spray dryers
| Blade form | Backward curved, smooth and easily cleaned |
| Material of construction | SS 304 or SS 316 wetted parts |
| Drive and arrangement | Direct driven, compact, variable speed |
| Bearings and cooling | Bearings outside the air stream |
| Flow control | Variable speed drive on chamber pressure |
| Also specified | Wash nozzles, drain, quick-release inspection door, earthing, flameproof motor where alcohol is present |
What this ID fan has to cope with
Changeover between extracts
Residue of one extract can taint the next, which is unacceptable for nutraceutical and Ayurvedic products. A fan with smooth internals, a large door and wash nozzles lets operators clean and inspect it during each changeover.
Sticky hygroscopic deposits
Many plant extracts contain sugars, gums and saponins that absorb moisture and stick. Deposits in the fan grow quickly when the outlet temperature drops. Holding outlet temperature and washing at each stop prevent build-up.
Residual alcohol
Concentrates from hydro-alcoholic extraction may carry some ethanol if stripping is incomplete. The vapour can reach flammable levels in the chamber. The fan is earthed, a flameproof motor is used where the area is classified, and solvent is stripped before drying.
Energy and control
Herbal dryers run batch campaigns of hours to days between extracts, with cleaning in between. Fan energy is modest, and a variable speed drive lets one fan cover different extracts and outlet temperatures without dampers. Higher feed solids from the evaporator save more energy than any fan change, because they cut the water the dryer must evaporate.
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 Forced Draft (FD) Fan for herbal extract spray dryers.
Questions buyers ask
What exhaust fan does a small herbal extract spray dryer need?
Usually a compact stainless backward curved fan on a variable speed drive, sized for the drying air plus evaporated water at the outlet temperature, with pressure for the cyclone and bag filter or scrubber. Exact size comes from the dryer heat balance.
How is a herbal spray dryer fan cleaned between products?
Through wash nozzles in the inlet and casing with the drain open, followed by inspection through a quick-release door. Smooth internals with no pockets or ledges make sure no residue of the previous extract remains, and the cleaning is recorded as part of the changeover.
Is alcohol a risk in herbal extract spray dryers?
It can be. Hydro-alcoholic extracts must be stripped of solvent in the evaporator before drying. If residues remain, the exhaust may contain flammable vapour, so the fan is earthed, built to avoid rubbing contact and fitted with a motor rated for the hazardous area where one is classified.
Information needed to quote an ID fan for herbal extract spray dryers
- 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.
- Product range. The extracts dried, carrier use and whether any are potent or classified.
- Solvent. Extraction solvent and the residual level in the concentrate.
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.