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 ID fan sits
- Pipe reactor and granulator
- Rotary dryer
- Dryer cyclones
- Dryer ID fan
- Venturi and tail gas scrubbers
- Common stack
The dryer ID fan usually sits between the cyclones and the venturi scrubber, in a dry but dusty duty. The granulator fan sits after the granulator scrubber, handling saturated gas with ammonia and acid mist, and is rubber-lined or FRP. The dedusting fan follows a cyclone or bag filter on the screen and crusher hoods. On many plants all three discharge into a tail gas scrubber and a common stack, so each fan's pressure includes its share of that final scrubber.
Gas at the ID fan
| Gas temperature, normal | 80 to 100°C after the dryer cyclones |
| Maximum or upset | Up to 140°C when the dryer loses feed during a granulation upset |
| Dust at the fan | NPK fines, several g/Nm³ after cyclones; sticky and low-melting on urea-based grades |
| Moisture and dew point | Moderate on the dryer fan; saturated at about 60 to 75°C on the granulator scrubber fan |
| Corrosive species | Ammonia, fluorides from phosphoric acid, chlorides from potash |
| 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 NPK granulation plant dryers
| Blade form | Radial tipped or open backward curved for the dryer and dedusting fans; backward curved for the clean, wet granulator scrubber fan |
| Material of construction | SS 316 or lined mild steel for the dryer fan; rubber-lined mild steel or FRP for the granulator scrubber fan |
| Drive and arrangement | Belt driven or direct coupled, arrangement 9 or 8 |
| Bearings and cooling | Bearings outside the gas stream; shaft seal against ammonia and dust |
| Flow control | Variable speed drive on dryer suction; damper on the smaller fans |
| Also specified | Wash nozzles, casing drain and inspection doors on every fan |
What this ID fan has to cope with
Sticky dust on urea-based grades
Grades containing urea form low-melting eutectics with ammonium salts and produce dust that smears onto hot blades. Lower drum inlet temperatures, efficient cyclones and a fan with wash nozzles keep the build-up under control. Grades with high urea content may need more frequent washing. The fan casing door is sized for manual cleaning.
Chloride and fluoride corrosion
Potash adds chlorides and phosphoric acid adds fluorides, and both concentrate in deposits on the fan. Chlorides pit SS 304, so SS 316 or a lined construction is the safer choice where deposits stay damp. The casing drain must also stay open, because a pool of chloride-rich condensate at the bottom of the scroll is where corrosion usually starts.
Granulator scrubber fan in wet gas
The granulator fan sees saturated gas with acid mist and occasional slurry carry-over. A mist eliminator before the fan, a lined or FRP casing and a drain at the lowest point protect it. Rubber-lined fans are kept below the lining's temperature limit, so a high gas temperature trip from the granulator protects the lining during upsets.
Fan balance across the system
When the three fans share a tail gas scrubber, a change in one alters the others. Each fan is selected on its own branch losses plus the shared scrubber at full plant load. Dampers in each branch let the operator rebalance when one section is offline, and the fan curves are checked for stable operation in that state too.
Energy and control
NPK plants change grades often, and the recycle ratio and evaporation load change with them. A variable speed drive on the dryer ID fan follows these swings more efficiently than a damper. Lowering the recycle ratio through better granulation control reduces the dryer gas volume and the power of every fan in the loop.
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 Forced Draft (FD) Fan for NPK granulation plant dryers.
Questions buyers ask
How many fans does an NPK granulation plant need?
Typically at least three exhaust fans: one for the granulator scrubber, one for the dryer, and one for dedusting screens, crushers and elevators. The cooler has its own air system, and the dryer furnace has a combustion air fan. Some plants combine dryer and cooler gases into one fan. Each fan duty is set by its own collector, scrubber and branch losses.
What material is used for the NPK dryer exhaust fan?
SS 316 or lined mild steel, because the dust carries chlorides and fluorides and absorbs moisture. The granulator scrubber fan, which handles saturated gas, is usually rubber-lined or FRP. Mild steel is only suitable where the fan stays well above dew point at all times. Inspection of the casing bottom at each shutdown shows whether the chosen material is coping with the plant's grades.
Why does the NPK dryer fan build up more on some grades?
Urea-containing grades form low-melting compounds that become sticky at dryer temperatures. The dust then smears onto the blades. Grade-specific drum temperatures and casing wash nozzles help control this. Plants making high-urea grades often schedule a fan wash at each grade change. A record of vibration against grade helps identify the worst grades.
Information needed to quote an ID fan for NPK granulation plant 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.
- Grade list. All grades produced, including urea-based grades, with dryer inlet and outlet temperatures.
- Scrubbing system. Scrubber layout for granulator, dryer and dedusting gases and whether they share a tail gas scrubber.
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.