How NPK granulation plant dryers work
In an NPK granulation plant, phosphoric acid, ammonia and sulphuric acid react in a pipe reactor or pre-neutraliser, and the slurry is sprayed with solid potash, urea and recycle onto a rolling bed in the ammoniator-granulator. The granules then pass a rotary dryer, screens, crushers and a cooler, with recycle ratios typically 3 to 6 times the product rate. The granulator off-gas carries free ammonia and steam and is scrubbed with phosphoric acid; the dryer exhaust carries dust, ammonia and fluorides and goes through cyclones to its own scrubber; a third dedusting system serves screens and elevators. Each system has its own exhaust fan. Dryer inlet gas is typically 250 to 400°C, reduced on urea-based grades.
Where the FD fan sits
- Outdoor intake
- FD (combustion air) fan
- Burner and furnace
- Dilution air
- Rotary dryer
- Cooler air fan and cooler
The combustion air fan sits at the furnace and feeds the burner, which fires furnace oil, natural gas or, on some plants, coal. Secondary or dilution air brings the gas to the dryer inlet temperature set for the grade. A separate cooler air fan pushes ambient or conditioned air through the rotary or fluid bed cooler. On some plants the cooler exhaust, after its cyclone, is returned to the furnace as dilution air.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the fan | Unheated; may be mixed with warm cooler exhaust as dilution air |
| Dust | Clean intake air, but the plant atmosphere carries NPK dust and ammonia |
| Humidity | High humidity affects the cooler more than the burner |
| Pressure at the fan outlet | Typically 150 to 400 mmWC for the burner; higher 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 NPK granulation plant dryers
| Blade form | Backward curved for clean air |
| Material of construction | Mild steel with a coating suited to an ammonia and fertilizer dust atmosphere |
| Drive and arrangement | Direct coupled or direct mounted |
| Bearings and cooling | Sealed bearings; no cooling needed |
| Flow control | Variable speed drive on the burner management system |
| Also specified | Outdoor intake with filter; flow measurement for air-fuel ratio |
What this FD fan has to cope with
Wide turndown between grades
DAP-based grades need hot gas, urea grades much cooler gas. If the fan is sized only for the hot grades, it runs throttled and unstable for the cool ones. Speed control and a curve that stays stable at low flow keep the burner steady. The burner supplier's turndown range is the starting point.
Intake contamination
Fan intakes inside the granulation building draw NPK dust and ammonia, which corrode the impeller and foul the burner. An outdoor intake with a filter avoids this. Fertilizer dust also coats the motor fins and raises its temperature. A clean intake and a motor enclosure suited to the atmosphere protect both the fan and the drive, and periodic cleaning of the motor keeps it cool.
Reusing cooler exhaust
Returning cooler exhaust as dilution air saves fuel, but it carries fines and must pass a cyclone first. The ducting and dampers are designed so that fines do not reach the burner. The extra resistance of the return ducting and the warm air temperature are added to the fan duty, and an isolation damper lets the furnace run on fresh air when the cooler is down.
Energy and control
Fuel and fan power both follow the grade being produced. A variable speed drive on the combustion air fan holds the air-fuel ratio across the wide range of firing rates. Reusing cooler exhaust as dilution air recovers its heat and reduces the fresh air the combustion air fan must supply.
ID fan and FD fan in NPK granulation plant dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the dryer cyclones, before the scrubbers | At the dryer furnace burner |
| What it handles | Dryer exhaust with NPK fines, NH₃, chlorides and fluorides | Clean ambient air |
| Temperature at the fan | 80 to 100°C | Ambient |
| Main design concern | Sticky dust and corrosion | Burner turndown across grades |
| Control | Dryer suction | Burner firing rate |
See also the Induced Draft (ID) Fan for NPK granulation plant dryers.
Questions buyers ask
What does the FD fan do in an NPK plant?
It supplies combustion air to the dryer furnace burner. The cooler has its own air fan, and dilution air may come from the same fan, a separate one, or from the cooler exhaust. All of these handle air rather than flue gas, so they are mild steel fans selected at ambient density with a coating suited to the plant atmosphere.
Why does an NPK dryer burner need good turndown?
Different grades need very different dryer inlet temperatures, especially urea-based grades, which need lower temperatures to avoid melting and sticking. The burner and its air fan must operate steadily across that range. A speed-controlled fan on a burner management system keeps the air-fuel ratio correct at the low end, where a damper-throttled fan becomes unstable.
Can the cooler exhaust be used as combustion air?
It can be used as dilution or secondary air after a cyclone removes the fines. This recovers heat from the product and reduces fuel consumption. The return duct needs isolation dampers and good insulation. The combustion air fan or dilution system is then sized for the mixture of fresh and recovered air, at the warmer mixed temperature.
Information needed to quote an FD fan for NPK granulation plant 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 range. Fuel, maximum and minimum firing rates and air pressure required at the burner.
- Heat recovery. Whether cooler exhaust is returned to the furnace, and at what temperature.
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.