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 ID fan sits
- Furnace or steam heating
- Rotary or disc dryer
- Fines cyclone
- ID fan
- Condenser or scrubber
- Odour treatment or stack
The fan sits after the fines cyclone and before the condenser or scrubber, so it keeps the dryer and the cyclone under suction and prevents odorous vapour leaking at seals and feed points. On a direct-fired dryer it carries the full flue gas flow plus the evaporated water; on an indirect dryer the flow is smaller and mostly vapour and leakage air. Suction at the fan is typically 100 to 250 mmWC.
Gas at the ID fan
| Gas temperature, normal | 80 to 100°C after the cyclone |
| Maximum or upset | 120 to 150°C on a direct-fired dryer if the feed is lost |
| Dust at the fan | Fine meal particles with fat; sticky rather than abrasive |
| Moisture and dew point | Near saturated; dew point close to the gas temperature |
| Corrosive species | Chlorides from sea fish, ammonia, amines and H₂S |
| Density at the design temperature | About 0.96 kg/m³ at 95°C, 80% 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 fish meal dryers
| Blade form | Radial or backward inclined flat blade, plain and easy to wash |
| Material of construction | SS 316 for all wetted parts; SS 304 only where the fish and water are low in salt |
| Drive and arrangement | Belt driven or direct coupled, arrangement 8 or 9 |
| Bearings and cooling | Bearings outside the gas, with a gas-tight shaft seal |
| Flow control | Variable speed drive on dryer suction |
| Also specified | Wash nozzles in the inlet and casing, a drain at the lowest point and an inspection door |
What this ID fan has to cope with
Fat and meal deposits
Fat droplets and meal fines that pass the cyclone stick to the blades and the casing and build into a heavy layer that throws the fan out of balance and goes rancid. Wash nozzles, a regular hot water or caustic wash and a drain clear the deposits before they cause vibration.
Chloride and ammonia corrosion
Sea fish brings salt, and decomposition produces ammonia, amines and sulphides. Condensate in the fan is aggressive to mild steel and attacks welds even in stainless. SS 316, full penetration welds, and keeping the fan warm and drained give long life.
Odour leaks
If the dryer or cyclone runs under positive pressure, odorous vapour leaks at the feed screw and the discharge valves, and the plant smells across a wide area. A fan with enough suction margin, and control on dryer pressure, keep every seal under suction.
Condensation at shutdown
When the dryer stops, the saturated vapour condenses in the casing and runs into the bearings unless the shaft seal and drain are right. Running the fan on air for a short time after the feed stops dries the casing.
Energy and control
Fish meal plants follow the catch, with heavy runs in the season and part-load or idle periods between. A variable speed drive on the ID fan, holding dryer suction, matches the fan to the actual evaporation and avoids drawing excess air that adds to the load on the condenser and odour treatment. Recovering heat from the vapour, for example to preheat stickwater, saves more energy than the fan itself.
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 Forced Draft (FD) Fan for fish meal dryers.
Questions buyers ask
What material should a fish meal dryer exhaust fan be made of?
SS 316 is normally used, because the exhaust is near saturated and carries salt, ammonia and sulphides that form an aggressive condensate. Mild steel fans corrode quickly on this duty, and SS 304 can pit where sea fish is processed.
Why does the ID fan on a fish meal dryer go out of balance?
Fat and fine meal that pass the cyclone stick to the impeller in the wet vapour and build up unevenly. Regular washing through nozzles in the casing, a drain and a plain blade shape keep deposits from building to the point of vibration.
Does the ID fan help odour control in a fish meal plant?
Yes. The fan keeps the dryer, cyclone and conveyors under suction so that odorous vapour is collected and sent to the condenser and odour treatment rather than leaking into the plant and the surrounding area.
Information needed to quote an ID fan for fish meal 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 type. Direct-fired rotary, steam disc or indirect hot air dryer, with evaporation rate in kg/h.
- Odour treatment. Condenser, scrubber or incineration after the fan, and its pressure drop.
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.