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 FD fan sits
- Air intake and filter
- FD (supply) fan
- Steam coils or indirect air heater
- Dryer inlet
- Belt, rotary or flash dryer
The fan sits before the air heater, so it handles ambient air and stays small and cool. On a belt dryer, several supply fans feed the zones; on a flash or fluid bed dryer, a single fan supplies the heater and the dryer. The pressure covers the filter, the heater and the dryer inlet, from about 50 mmWC on a belt dryer to 300 mmWC or more on a fluid bed.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 40°C, humid coastal air |
| Air after the heater | 70 to 120°C, downstream of the fan |
| Dust | Filtered air; salt aerosol in coastal plants |
| Humidity | High ambient humidity reduces drying capacity |
| Pressure at the fan outlet | From about 50 to 300 mmWC depending on dryer type |
| 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 seaweed and agar dryers
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel with marine coating, or galvanised; stainless fasteners |
| Drive and arrangement | Direct coupled or belt driven, with a variable speed drive |
| Bearings and cooling | Standard bearings, sealed against salt air |
| Flow control | Variable speed drive holding air flow |
| Also specified | Inlet filter and corrosion protection for outdoor installation |
What this FD fan has to cope with
Salt air attack
Plants on the coast draw air laden with salt aerosol, which corrodes casings, fasteners and motors. A marine-grade coating, stainless fasteners and a sheltered intake reduce the damage.
Humid air reduces capacity
In the monsoon the humid inlet air carries more moisture, and the dryer removes less from the product. A supply fan with speed margin can raise the air flow, or the inlet temperature can be raised, to hold throughput.
Fluidisation on the agar powder dryer
A fluid bed dryer needs a steady air flow to keep the bed fluidised without blowing fines away. A supply fan with flow control holds the bed stable as the filter loads.
Energy and control
The supply fan runs for every hour the dryer runs, and on belt dryers there are several of them. High-efficiency wheels, variable speed drives and the right air flow for each zone save power. Heat recovery from the exhaust to the inlet air is limited by the salt and fines in the exhaust, so a recovery coil must be cleanable.
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 Induced Draft (ID) Fan for seaweed and agar dryers.
Questions buyers ask
Does the seaweed dryer supply fan handle hot air?
No. The supply fan is normally placed before the steam coils or the indirect heater, so it handles ambient air. This keeps the fan small and avoids high temperature construction.
How is the air heated in an agar dryer?
Usually by steam coils from the plant boiler, or an indirect fired air heater. Direct firing is avoided because combustion products would contaminate a food ingredient.
What pressure does a fluid bed dryer supply fan need for agar powder?
Typically 250 to 400 mmWC, to overcome the filter, the heater, the distributor plate and the fluidised bed. The exact figure depends on bed depth and the distributor design.
Information needed to quote an FD fan for seaweed and agar 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. Steam coil or indirect heater type and the air side pressure drop.
- Site. Coastal location and whether the fan is indoors or outdoors.
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.