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 FD fan sits
- Air intake
- FD (combustion air) fan
- Hot gas furnace
- Dilution and mixing
- Hot gas duct
- Ring main
- Spray tower
The FD fan sits at the furnace, taking ambient air and supplying both combustion and dilution air, sometimes through a separate dilution fan. The mixed hot gas flows through an insulated duct to the ring main and enters the tower through several ports near the bottom. The fan pressure covers the burner, the furnace, the hot gas duct and ring main and the small positive pressure needed at the tower ports; the ID fan does the rest.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Hot gas to the tower | 250 to 350°C at the ring main, downstream of the fan |
| Dust | Ambient air from a dusty powder plant; an intake filter is advisable |
| Pressure | Moderate; burner, furnace and hot gas duct |
| Fuel | Furnace oil, LDO, natural gas or LPG |
| 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 detergent spray drying towers
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Direct driven with a variable speed drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive with the burner, and a damper or separate fan for dilution |
| Also specified | Intake filter, silencer and flow measurement for air to fuel ratio |
What this FD fan has to cope with
Powder in the intake air
Detergent plants are dusty, and alkaline powder drawn into the fan coats the burner and the hot gas duct. Locating the intake away from packing and handling and fitting a filter keep it clean.
Inlet gas temperature swings
Uneven mixing of combustion gas and dilution air gives hot spots at the ring main ports, which scorch powder on the tower walls. Measured air flow and good mixing before the ring main hold a uniform inlet temperature.
Balancing with the ID fan
The tower must stay at slight suction. If the FD fan pushes more than the ID fan can draw, powder and hot gas escape at the doors. The two fans are controlled together around the tower top pressure.
Energy and control
The FD fan runs with the furnace. Combustion air follows the firing rate and dilution air sets the inlet temperature, so speed control on both saves fuel and power. Running the highest inlet temperature the formulation allows reduces the gas volume needed per kg of water evaporated, which shrinks the ID fan duty too.
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 Induced Draft (ID) Fan for detergent spray drying towers.
Questions buyers ask
What does the FD fan do on a detergent spray tower?
It supplies combustion and dilution air to the hot gas furnace. The resulting hot gas, typically 250 to 350°C, is ducted to the ring main at the bottom of the tower. It is the combustion air fan of the plant, not a fan that handles tower exhaust.
Why is the detergent tower inlet temperature limited?
Surfactants and some builders degrade or discolour at high temperature, and very hot gas at the ring main can scorch powder deposits on the tower wall. The inlet temperature is set by the formulation and controlled by dilution air.
Does the detergent tower FD fan need stainless steel?
No. It handles ambient air before the furnace, so mild steel is normal. The fan intake should be filtered and placed away from packing and handling areas, so alkaline detergent powder does not reach the burner.
Information needed to quote an FD fan for detergent spray drying towers
- 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.
- Furnace and fuel. Furnace type, fuel and burner, with combustion and dilution air flows.
- Tower inlet. Inlet gas temperature and the pressure at the ring main.
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.