How seaweed and agar dryers work
Seaweed is dried at two points in the chain. Raw weed such as Gracilaria, Gelidium or Kappaphycus is often sun dried, but mechanical belt or rotary dryers are used where weather or volume demands it, drying from about 80 to 90% moisture to 15 to 20%. In agar and carrageenan plants, the extracted gel, pressed sheets or semi-refined chips are dried in belt, tray, rotary or flash dryers, and the final powder is often finished in a fluid bed dryer. Air is heated by steam coils or an indirect fired air heater, typically to 70 to 120°C, and leaves at 45 to 70°C, humid and carrying salt, fines and a marine odour. Supply fans push air through the heaters; exhaust fans draw it out through cyclones or scrubbers.
Where the ID fan sits
- Belt, rotary or flash dryer
- Exhaust hood or outlet
- Cyclone or scrubber
- Exhaust (ID) fan
- Stack
The fan sits after the cyclone or scrubber, at the end of the dryer exhaust. On a flash or fluid bed dryer for agar powder it must keep the dryer and cyclone under suction and carries most of the system pressure, often 200 to 400 mmWC. On belt and tray dryers for raw weed it handles a large volume at low pressure, typically 30 to 100 mmWC.
Gas at the ID fan
| Air temperature, normal | 45 to 70°C at the fan |
| Maximum or upset | About 100°C if the feed stops with the heaters on |
| Dust at the fan | Fine seaweed or agar dust with sea salt |
| Moisture and dew point | Humid; condensate forms in cold ducts and at shutdown |
| Corrosive species | Chlorides from sea salt; sulphurous odour from decaying weed |
| Density at the design temperature | About 1.06 kg/m³ at 60°C, 88% 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 seaweed and agar dryers
| Blade form | Backward inclined or radial, easy to wash |
| Material of construction | SS 316 for wetted parts, or mild steel with a chloride-resistant lining or coating |
| Drive and arrangement | Belt driven or direct coupled, with a variable speed drive |
| Bearings and cooling | Bearings outside the air stream with a shaft seal |
| Flow control | Variable speed drive on dryer suction or humidity |
| Also specified | Wash nozzles, casing drain and stainless fasteners |
What this ID fan has to cope with
Hygroscopic salt deposits
Fines from raw seaweed carry sea salt that absorbs moisture and forms a damp, sticky layer on the impeller. It grows unevenly and causes vibration, and corrodes the steel beneath. A wash connection, a drain and a cleaning routine between batches keep the wheel clean.
Chloride pitting
Condensate in the fan is a dilute brine, which pits mild steel within months and can attack SS 304 at welds and crevices. SS 316 or a lined casing, full welds and keeping the fan warm and drained give the best life.
Agar dust in the exhaust
On agar or carrageenan powder dryers, fines that pass the cyclone are product loss and a nuisance, and they swell into a gel where condensate forms. A high-efficiency cyclone or bag filter before the fan and a warm casing help.
Energy and control
Seaweed processing is seasonal and depends on the harvest, so dryers run hard for months and then stand. A variable speed drive on the exhaust fan, set on dryer suction or exhaust humidity, cuts power during part-load runs and reduces heat loss. Recirculating part of the exhaust air on belt dryers, where the outlet air is not saturated, saves steam or fuel.
ID fan and FD fan in seaweed and agar dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone or scrubber | Before the air heater |
| What it handles | Humid salty air with fines | Ambient coastal air |
| Temperature at the fan | 45 to 70°C | Ambient |
| Main design concern | Chloride corrosion and salt deposits | Steady flow and salt air protection |
| Control | Dryer suction or humidity | Air flow |
See also the Forced Draft (FD) Fan for seaweed and agar dryers.
Questions buyers ask
What material is used for a seaweed dryer exhaust fan?
SS 316 is preferred because the exhaust carries sea salt and moisture, which form a chloride condensate. Mild steel with a chloride-resistant lining or coating is an alternative, but it needs regular inspection.
Why do seaweed dryer fans become unbalanced?
Salt-laden fines absorb moisture and stick to the impeller in uneven layers. Washing the fan through built-in nozzles and draining the casing remove the deposits before they cause vibration.
Does the agar powder dryer need a different exhaust fan?
An agar or carrageenan powder flash or fluid bed dryer needs a higher pressure fan, since it pulls through the dryer and a cyclone or bag filter, and the fan must handle fine, gel-forming dust. Belt dryers for raw weed need a low pressure, high volume fan.
Information needed to quote an ID fan for seaweed and agar 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.
- Dryer and product. Raw seaweed, agar gel, carrageenan chips or powder, and the dryer type.
- Salt content. Whether the weed is washed before drying and the expected salt in the fines.
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.