How fertilizer rotary dryers and coolers work
Granular fertilizers such as DAP, complex NPK grades, granulated SSP and ammonium sulphate leave the granulator with 2 to 5% moisture and are dried in a co-current rotary drum, then screened and cooled. Hot gas from an oil, gas or coal fired furnace enters the dryer at typically 200 to 400°C, lower for heat-sensitive grades, and leaves at about 80 to 100°C. The exhaust passes cyclones and a wet scrubber, often with a separate ID fan for the dryer. The product is then cooled in a rotary or fluid bed cooler with ambient or chilled air, from about 70 to 90°C down to 40 to 50°C, so it does not cake in storage. Cooler air goes to its own cyclone and fan.
Where the FD fan sits
- Inlet louvre and filter
- FD (combustion air) fan
- Furnace
- Rotary dryer
- Cooler air fan
- Rotary or fluid bed cooler
The combustion air fan sits at the dryer furnace and supplies the burner and any dilution air. The cooler air fan pushes ambient or chilled air into the cooler, counter-current to the product in a rotary cooler, or through the distributor plate of a fluid bed cooler. On fluid bed coolers the fan pressure must cover the plate and the bed, typically 300 to 500 mmWC, compared with 100 to 200 mmWC for a rotary cooler.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C, or 15 to 25°C where the cooler air is chilled |
| Air after the fan | Unheated for the cooler; preheated only if the dryer furnace uses recovered heat |
| Dust | Clean air, but intake air in a fertilizer plant carries fine product dust and ammonia |
| Humidity | High monsoon humidity reduces cooling and promotes caking; some plants dehumidify cooler air |
| Pressure at the fan outlet | 100 to 200 mmWC for rotary coolers and burners, 300 to 500 mmWC for fluid bed coolers |
| 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 fertilizer rotary dryers and coolers
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel with a corrosion-resistant coating suited to the fertilizer plant atmosphere |
| Drive and arrangement | Direct coupled or belt driven |
| Bearings and cooling | Standard bearings with good sealing against plant dust |
| Flow control | Variable speed drive; cooler air held at a constant flow per tonne of product |
| Also specified | Inlet filter and outdoor intake; inlet silencer where the fan is near operators |
What this FD fan has to cope with
Humid air and caking
In humid weather the cooler air cannot cool the product below its critical relative humidity point, and the stored fertilizer cakes. The fan duty is checked at the worst monsoon condition, and chilled or dehumidified air is considered for grades prone to caking. Cooler air flow is also checked against the granule size.
Corrosive plant atmosphere
Fertilizer dust and ammonia settle on the fan, motor and coupling and corrode them from outside. A coating system suited to the atmosphere, sealed bearings and an intake from a clean area extend fan life. Motors with a suitable enclosure and a paint system rated for ammonium salts are specified, and the fan base is kept clear of spilt product, which absorbs water and corrodes the steel beneath.
Fluid bed cooler pressure
A fluid bed cooler needs higher pressure than a rotary cooler, and the bed pressure varies with load. A fan with a steep curve keeps the bed fluidised when the product rate rises. The selection point is checked at both the minimum and maximum bed loads, and the plate pressure drop is confirmed with the cooler supplier.
Energy and control
The cooler air fan runs whenever product flows and moves a large volume of air. Controlling it on product rate rather than running it at full speed saves power on lower-capacity grades. Routing the warm cooler exhaust to the dryer furnace as combustion or dilution air recovers its heat, which reduces fuel and lets the combustion air fan run slower.
ID fan and FD fan in fertilizer rotary dryers and coolers
| ID fan | FD fan | |
|---|---|---|
| Position | After the dryer cyclones, before the scrubber | At the dryer furnace and at the cooler air inlet |
| What it handles | Exhaust with hygroscopic dust and NH₃ | Clean ambient or chilled air |
| Temperature at the fan | 80 to 100°C | Ambient or chilled |
| Main design concern | Caking and corrosion on the impeller | Cooling capacity in humid weather |
| Control | Drum suction | Burner firing rate and product rate |
See also the Induced Draft (ID) Fan for fertilizer rotary dryers and coolers.
Questions buyers ask
What is the FD fan on a fertilizer dryer?
It is the combustion air fan for the dryer furnace or burner. On the same line there is also a cooler air fan, which supplies ambient or chilled air to cool the granules before storage. Both handle clean air. The cooler fan is usually the larger of the two.
Why is chilled air used in fertilizer coolers?
Some grades cake if they are stored warm or if cooled with humid air. Chilled, drier air cools the product further and keeps it below its critical relative humidity, reducing caking in storage. The extra cost of chilling is weighed against caking complaints, rework and bag losses. Some plants chill only during the monsoon months.
How much pressure does a fluid bed cooler fan need?
Typically 300 to 500 mmWC, covering the distributor plate, the product bed and the ducting. A rotary cooler usually needs much less, about 100 to 200 mmWC. The fluid bed figure varies with bed depth and plate design, so it is confirmed with the cooler supplier before the fan is selected, and a margin is kept for higher product rates.
Information needed to quote an FD fan for fertilizer rotary dryers and coolers
- 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.
- Cooler type. Rotary or fluid bed cooler, product rate and inlet and outlet product temperatures.
- Air conditioning. Whether cooler air is chilled or dehumidified, and the design ambient condition.
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.