How DAP granulation plants work
In a DAP or NPK granulation plant, phosphoric acid is partly neutralised with ammonia in a pre-neutraliser or pipe reactor, and the slurry is sprayed onto a rolling bed of recycled product in a rotary ammoniator-granulator, where more ammonia is added. The moist granules go to a co-current rotary dryer heated by a hot gas generator firing oil, gas or coal, with inlet gas at 250 to 450°C and outlet at 85 to 110°C. The dried product is screened, oversize crushed and recycled, and product cooled. The granulator and dryer off-gases carry ammonia, fluorides and fine fertiliser dust. They pass cyclones, then venturi scrubbers using phosphoric acid and a tail gas scrubber, before the ID fans and stack. The FD fan supplies combustion air to the hot gas generator.
Where the FD fan sits
- Inlet screen or filter
- FD fan
- Burner or furnace
- Hot gas generator
- Dilution chamber
- Rotary dryer
The fan stands beside the hot gas generator at the feed end of the dryer. It delivers combustion air to the burner, or undergrate and overfire air on a coal-fired furnace, and dilution air to the mixing chamber that sets the dryer inlet temperature. Outlet pressure is typically 200 to 500 mmWC for oil or gas burners and lower for grate firing.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Dust | Plant air carries fertiliser dust; screen or filter needed |
| Pressure at the fan outlet | Typically 200 to 500 mmWC |
| Turndown | Firing varies widely with grade, product moisture and output |
| Hot gas to dryer | 250 to 450°C after dilution, downstream of the fan |
| 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 DAP granulation plants
| Blade form | Backward curved, for efficiency on clean air |
| Material of construction | Mild steel with a coating suited to ammonia and fertiliser dust |
| Drive and arrangement | Direct coupled, arrangement 4 or 8 |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive on firing rate, with separate dilution air control |
| Also specified | Inlet screen or filter and a weatherproof, corrosion-resistant motor |
What this FD fan has to cope with
Fertiliser dust and corrosion
Fertiliser dust in plant air is corrosive when it picks up moisture. It settles on fan casings and motors, and on the impeller if not filtered. Coatings, an inlet filter and a sealed motor reduce the damage.
Dilution air upsetting combustion
Taking dilution air from the combustion air fan means a change in dryer temperature disturbs the flame. Separate control of dilution air keeps the burner steady. Separate fans are simpler to tune.
Low firing on a large fan
Fans sized for the highest moisture grade run throttled on drier grades. Speed control gives stable air at low firing and saves power. It also keeps burner air velocity in range at low load.
Energy and control
The combustion air fan runs whenever the dryer fires, and heat demand changes with grade and product moisture. Speed control on firing, with oxygen trim, avoids excess air that would add to the dryer gas volume, raise dust carry-over and increase the dryer ID fan power. Oil or gas firing is usually the larger cost.
ID fan and FD fan in DAP granulation plants
| ID fan | FD fan | |
|---|---|---|
| Position | After the dryer cyclones, before the scrubbers | Before the hot gas generator |
| What it handles | Humid dryer gas with sticky fertiliser dust | Ambient air |
| Temperature at the fan | 80 to 105°C | Ambient |
| Main design concern | Dust build-up and corrosion | Turndown with grade changes |
| Control | Dryer outlet suction | Firing rate and dilution |
See also the Induced Draft (ID) Fan for DAP granulation plants.
Questions buyers ask
Is the FD fan on a DAP plant the combustion air fan?
Yes. It supplies air to the burner or furnace of the dryer hot gas generator, and often dilution air. It handles ambient air, not process gas. Granulator and cooler fans are separate duties.
Why does a DAP dryer need dilution air?
Combustion gas is far hotter than the dryer can use. Dilution air brings it down to 250 to 450°C at the dryer inlet, depending on grade and dryer design.
Does the DAP dryer combustion air fan need corrosion protection?
The fan handles clean air, but the air in a fertiliser plant carries ammonia and dust that are corrosive when wet. Coatings and a suitable motor enclosure are advisable.
Information needed to quote an FD fan for DAP granulation plants
- 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.
- Hot gas generator. Fuel, burner or furnace type, and air pressure at full firing.
- Dilution. Dryer inlet temperature range by grade and whether dilution air comes from this fan.
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.