How fish meal dryers work
Raw fish or fish offal is cooked, pressed to separate oil and stickwater, and the press cake, at about 50 to 55% moisture, is dried to about 8 to 10% before milling. Two main dryer types are used. Direct-fired rotary dryers take hot gas from a furnace at 400 to 600°C at the inlet and give a darker, flame-dried meal. Indirect dryers, with steam-heated discs, tubes or hot air from a steam coil, give a lower temperature meal of higher quality. In both, the dryer exhaust leaves at about 80 to 100°C, saturated with water vapour and carrying meal fines, fat droplets, ammonia and amines. A cyclone recovers the fines and the vapour then goes to a condenser, scrubber or incineration for odour control.
Where the FD fan sits
- Air intake
- FD fan
- Burner or grate
- Furnace
- Mixing chamber
- Rotary dryer
The fan sits at the furnace and delivers combustion air to an oil or gas burner, or under the grate of a solid fuel furnace. Secondary or tempering air that brings the gas down to the dryer inlet temperature is either drawn in by the ID fan through a damper or supplied by a second branch of the FD fan. The pressure needed is typically 150 to 400 mmWC for a burner and less for a grate.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 40°C |
| Dust | Clean air; salt-laden in coastal plants |
| Pressure at the fan outlet | 150 to 400 mmWC for oil or gas burners; less for grate furnaces |
| Fuel | Furnace oil, diesel, gas, coal or biomass |
| Turndown | Follows the catch; long part-load periods |
| 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 fish meal dryers
| Blade form | Backward curved, for efficiency and a stable curve |
| Material of construction | Mild steel with marine-grade coating; galvanised or stainless fittings |
| Drive and arrangement | Direct coupled |
| Bearings and cooling | Standard bearings; sealed against salt air |
| Flow control | Variable speed drive or damper linked to burner firing rate |
| Also specified | Inlet filter and silencer; corrosion protection on fasteners |
What this FD fan has to cope with
Salt air corrosion
Plants near the harbour draw salty air that corrodes mild steel casings, fasteners and motor terminals. A marine-grade paint system, stainless fasteners and a sheltered inlet extend the life of the fan.
Air-fuel ratio at part load
When the dryer runs at part load, a fixed-speed fan with a damper gives poor control of excess air and the meal colour suffers from smoky or overheated gas. Speed control linked to the burner gives steadier combustion.
Inlet temperature overshoot
A sudden drop in feed raises the dryer outlet temperature and risks scorching the meal or a fire. The combustion air and fuel must be cut back quickly, which the fan control must allow without losing flame stability.
Energy and control
On a direct-fired fish meal dryer the fuel cost far exceeds the fan cost, so the FD fan's value is in good combustion. Holding the right excess air at every firing rate, with a speed-controlled fan, saves fuel and gives a more consistent meal colour. Running the furnace at the lowest stable excess air also reduces the gas volume through the ID fan and the odour system.
ID fan and FD fan in fish meal dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the fines cyclone, before the condenser or scrubber | At the furnace, feeding the burner or grate |
| What it handles | Saturated, oily, salty vapour with fines | Ambient air |
| Temperature at the fan | 80 to 100°C | Ambient |
| Main design concern | Corrosion, fat deposits and odour containment | Combustion control and salt air |
| Control | Dryer suction | Burner firing rate |
See also the Induced Draft (ID) Fan for fish meal dryers.
Questions buyers ask
Does a steam-heated fish meal dryer need an FD fan?
No. A steam disc or tube dryer is heated by steam from a boiler, so its combustion air fan is on the boiler. The dryer itself needs only a vapour extraction fan and sometimes a small purge air fan.
What is the FD fan on a fish meal plant used for?
On a direct-fired rotary dryer, the forced draft fan is the combustion air fan of the furnace. It supplies air to the burner or grate, and sometimes tempering air to the mixing chamber.
How do you protect a combustion air fan in a coastal fish meal plant?
Use a marine-grade coating, stainless fasteners, a sheltered air intake and motors with good ingress protection. Inspect the casing and impeller for corrosion at each shutdown.
Information needed to quote an FD fan for fish meal 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.
- Burner or furnace. Burner type, firing rate and air pressure at the burner, or grate details.
- Site. Distance from the sea 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.