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 ID fan sits
- Hot gas generator
- Rotary dryer
- Cyclones
- Dryer ID fan
- Venturi scrubber
- Tail gas scrubber
- Stack
Most plants place the dryer fan after the cyclones and before the venturi scrubbers, where the gas is still above its dew point at 80 to 105°C and the fan runs dry. Others put it after the scrubbers, where it handles saturated, acidic gas and is built for wet duty. The granulator and cooler have their own exhaust fans. Suction at the dryer fan covers the dryer and cyclones, typically 200 to 350 mmWC; with scrubbers ahead of it, 400 to 700 mmWC.
Gas at the ID fan
| Gas temperature, normal | 80 to 105°C after the cyclones |
| Maximum or upset | Up to 150°C if dryer feed is lost |
| Dust at the fan | Fine, sticky DAP or NPK dust, typically several g/m³ after cyclones |
| Moisture and dew point | High moisture from drying; dew point about 55 to 65°C |
| Corrosive species | Ammonia, fluorides and acidic phosphate dust that attack mild steel when wet |
| 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 DAP granulation plants
| Blade form | Radial tipped or backward inclined flat blades, which shed sticky dust |
| Material of construction | SS 304L or 316L impeller, or mild steel with wear liners where the gas stays dry; rubber-lined for wet positions |
| Drive and arrangement | Belt or direct drive, arrangement 9 or 8 |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on dryer outlet suction |
| Also specified | Wash nozzles, inspection doors, insulated casing and casing drain |
What this ID fan has to cope with
Build-up of sticky dust
DAP and NPK dust takes up moisture and sticks to the impeller, especially near the dew point and with NPK grades containing potash. Uneven build-up throws the rotor out of balance within days. Insulation, flat blades and a wash routine keep the fan running.
Corrosion in wet and acidic positions
Fluorides and phosphoric acid scrubbing liquor attack mild steel. Fans after the scrubbers, and the bottom of casings where condensate collects, need stainless or rubber-lined construction. Insulation keeps dry positions above the dew point.
Grade changes
Switching from DAP to NPK changes gas volume, moisture and the stickiness of the dust. A fan sized only for DAP may lack the suction or margin for other grades, so the full grade range should be specified.
Energy and control
Granulation plants run continuously, but switch grades and output with season and demand. Dryer gas volume changes with grade, moisture and output. Speed control on dryer suction saves power at reduced rates and limits air ingress at the dryer seals, which would add to the scrubber load and to the fuel used.
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 Forced Draft (FD) Fan for DAP granulation plants.
Questions buyers ask
Where should the dryer ID fan be placed in a DAP plant?
Usually after the cyclones and before the scrubbers, so it handles dry gas above the dew point. Placing it after the scrubbers is also common, but then the fan must be built for wet, acidic service.
Why does fertiliser dust build up on DAP dryer fans?
DAP and NPK dust is hygroscopic and becomes sticky near its dew point. It builds up unevenly on the blades and causes vibration. Flat blades, insulation and water washing reduce it.
What fans does a DAP granulation plant need?
Typically a dryer ID fan, a granulator exhaust fan, a cooler fan, a dedusting fan for screens and conveyors, and an FD combustion air fan for the hot gas generator.
Information needed to quote an ID fan for DAP granulation plants
- 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.
- Grades. Range of DAP, NPK or other grades produced, with output for each.
- Fan position. Whether the fan is before or after the scrubbers, and the scrubbing liquor used.
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.