How detergent spray drying towers work
Detergent powder is made by mixing surfactant (usually neutralised LABSA), sodium sulphate, soda ash, silicate and other builders into a slurry of about 60 to 70% solids, then pumping it at high pressure to nozzles near the top of a tall counter-current tower. Hot gas from an oil or gas-fired furnace, typically 250 to 350°C, enters through a ring main near the bottom and rises against the falling droplets, which dry into hollow beads. The powder leaves the cone at about 70 to 90°C and goes to an airlift that cools it. The exhaust leaves the top at about 85 to 110°C, laden with fine detergent dust and a blue haze of surfactant mist, and passes cyclones and a wet scrubber or bag filter before the stack. The tower runs at a slight suction.
Where the ID fan sits
- Counter-current tower
- Tower top outlet
- Cyclones
- Exhaust (ID) fan
- Wet scrubber
- Stack
On many towers the exhaust fan follows the cyclones and pushes into a wet scrubber, so it handles hot gas with the fines the cyclones miss. Where a bag filter is used, the fan follows the filter and sees much less dust; where the scrubber comes first, the fan handles saturated gas with alkaline droplets. The suction holds the tower top slightly below atmospheric, and covers the cyclones and, where upstream, the scrubber; typically 200 to 400 mmWC.
Gas at the ID fan
| Gas temperature, normal | 85 to 110°C after the cyclones; 55 to 65°C if the fan follows the scrubber |
| Maximum or upset | Up to 150°C if the slurry feed is lost |
| Dust at the fan | Fine, sticky detergent fines, often 0.5 to 2 g/m³ after cyclones |
| Moisture and dew point | High moisture from slurry water; dew point around 55 to 65°C |
| Corrosive species | Alkaline dust and droplets (carbonate, silicate); combustion products from the furnace |
| Density at the design temperature | About 0.95 kg/m³ at 100°C, 79% 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 detergent spray drying towers
| Blade form | Backward curved plate or radial-tipped, open shape that sheds sticky fines |
| Material of construction | Mild steel after cyclones; SS 304 or coated steel where the fan handles scrubber gas |
| Drive and arrangement | Belt or direct driven, with a variable speed drive |
| Bearings and cooling | Pedestal bearings with cooling disc; shaft seal against dust and moisture |
| Flow control | Variable speed drive on tower top pressure |
| Also specified | Wash nozzles, large inspection doors, casing drain and earthing |
What this ID fan has to cope with
Sticky build-up on the blades
Detergent fines are hygroscopic and the surfactant part softens when warm and moist, so they coat the blades and casing. Uneven deposits throw the rotor out of balance. An open blade shape, wash nozzles and a regular wash routine at tower stops keep it under control.
Tower and duct fires
Organic surfactant deposits on tower walls and in the exhaust duct can ignite if the tower overheats. The ID fan is interlocked with the furnace and slurry pumps, and the tower fire procedure defines whether the fan stops or runs to clear smoke.
Wet alkaline gas after the scrubber
Where the fan follows the scrubber, it sees saturated gas with alkaline droplets that dry into crusts on the impeller. Stainless or coated parts, a mist eliminator before the fan and a casing drain prevent crusting and corrosion.
Energy and control
Detergent towers run in campaigns between formulation changes and wall-cleaning stops. Tower output and outlet temperature follow the slurry rate, so a variable speed drive on the exhaust fan saves power against damper control. Raising slurry solids and reducing air leakage at the tower door and cone cut the gas volume per tonne of powder.
ID fan and FD fan in detergent spray drying towers
| ID fan | FD fan | |
|---|---|---|
| Position | After cyclones, before or after the scrubber | At the hot gas furnace |
| What it handles | Moist exhaust with sticky detergent fines | Ambient combustion and dilution air |
| Temperature at the fan | 85 to 110°C | Ambient |
| Main design concern | Sticky deposits and fire risk | Uniform inlet gas temperature |
| Control | Tower top pressure | Firing rate and inlet temperature |
See also the Forced Draft (FD) Fan for detergent spray drying towers.
Questions buyers ask
Where is the ID fan on a detergent spray drying tower?
Usually after the cyclones and before the wet scrubber, so the fan handles hot gas with residual fines. Some plants place it after the scrubber or after a bag filter, which changes what it sees: wet alkaline gas or very little dust.
Why do detergent tower exhaust fans go out of balance?
Hygroscopic detergent fines and surfactant mist stick to the blades in the warm, humid exhaust. The deposits grow and break off unevenly. Regular washing and a blade shape that sheds deposits reduce how often the fan needs balancing.
What causes the blue haze from a detergent tower?
Fine droplets of surfactant and organic matter that evaporate in the hot zone and recondense as a sub-micron mist. Cyclones do not catch them, so a wet scrubber or other control is needed, and the ID fan pressure must cover it.
Information needed to quote an ID fan for detergent spray drying 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.
- Gas cleaning order. Whether the fan follows cyclones, a bag filter or the wet scrubber, with each pressure drop.
- Formulation. Surfactant level and builders, which affect stickiness and the exhaust moisture.
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.