How urea prilling towers work
In a prilling tower, molten urea at about 135 to 140°C from the evaporators is sprayed from a rotating bucket or shower heads at the top of a concrete tower 50 to 100 m high. The droplets fall against an upward stream of ambient air, solidify into prills and are cooled before reaching the tower base, where they are collected and sent to a fluid bed cooler. The air enters through louvres near the bottom and leaves at the top at typically 45 to 65°C, carrying fine urea dust, much of it submicron, and some ammonia. Older towers run on natural draft; most larger or upgraded towers use induced draft exhaust fans at the top, and plants meeting current emission limits add a wet dust scrubber at the tower top, with the fans after it.
Where the ID fan sits
- Air inlet louvres
- Prilling zone
- Tower top
- Dust scrubber (if fitted)
- Exhaust (ID) fans
- Discharge stacks
The fans sit on the tower top, usually several in parallel, either directly over the tower or after a wet scrubber that removes urea dust. Without a scrubber they handle the dusty tower exhaust directly; with one they handle saturated air carrying droplets of urea solution. The pressure needed is low, typically 50 to 150 mmWC without a scrubber and 150 to 300 mmWC with one, but the air flow is very large, often several hundred thousand m³/h per tower.
Gas at the ID fan
| Air temperature, normal | 45 to 65°C at the tower top |
| Maximum or upset | 70 to 80°C at high load in hot weather |
| Dust at the fan | Fine urea dust, often 50 to 150 mg/Nm³ without a scrubber, much lower after one |
| Moisture and dew point | Ambient humidity plus some water from the melt; saturated after a scrubber |
| Corrosive species | Ammonia and urea solution droplets, mildly corrosive to mild steel when wet |
| Density at the design temperature | About 1.06 kg/m³ at 60°C, 88% 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 urea prilling towers
| Blade form | Large backward inclined centrifugal, or axial fans with adjustable pitch on very large towers |
| Material of construction | SS 304L or 316L for the impeller and wetted parts; coated mild steel for dry casings |
| Drive and arrangement | Belt or direct drive with the motor outside the air stream, mounted on the tower top structure |
| Bearings and cooling | Standard bearings; weather protection for the tower top |
| Flow control | Variable speed drives, or variable pitch on axial fans, on prill temperature |
| Also specified | Wash water nozzles for the impeller, vibration monitoring and a structure check for the tower top |
What this ID fan has to cope with
Urea build-up and imbalance
Urea dust is hygroscopic. On the impeller it picks up moisture, hardens and builds up unevenly, then flakes off and the fan vibrates. Regular water washing, stainless blades that shed deposits and vibration monitoring keep the fans running between turnarounds.
Wet carry-over from the scrubber
After a wet scrubber, droplets of urea solution reach the fan, and the dissolved urea crystallises as the water evaporates. A good mist eliminator and drainable casing reduce the deposits.
Vibration transmitted to the tower
Several large fans on top of a tall concrete tower can excite the structure. Fans must be well balanced, mounted on a stiff base with isolators, and checked against the natural frequencies of the tower top.
Seasonal air demand
Prill temperature at the base rises in hot weather because the cooling air is warmer. Fans need margin for summer flows, or the plant must cut load. Speed control then lets the fans run slower for the rest of the year.
Energy and control
Prilling towers run continuously, but cooling air demand changes with ambient temperature and plant load. Running all fans at full speed in winter wastes power and over-cools the prills. Variable speed drives or variable pitch, controlled on prill temperature at the tower base, cut power at the cooler times of year and at reduced load.
Questions buyers ask
Why does a urea prilling tower need exhaust fans?
Natural draft alone limits the cooling air and so the tower capacity. Induced draft fans at the top raise the air flow, cool the prills reliably and are needed where a dust scrubber adds resistance.
What material should prilling tower fans be made of?
Stainless steel, usually SS 304L or 316L, for the impeller and wetted parts. Urea and ammonia are only mildly corrosive, but wet urea deposits attack mild steel and are easier to wash off stainless.
How do you control urea build-up on prilling tower fans?
Water wash nozzles at the inlet, used on a routine, plus smooth blades that shed deposits and good mist elimination after any scrubber. Vibration monitoring shows when washing is needed.
Information needed to quote an ID fan for urea prilling towers
- 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.
- Tower data. Production rate, tower height and diameter, and number of fans now fitted.
- Dust scrubber. Whether a top scrubber is fitted or planned, and its pressure drop.
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.