How dye and pigment spray dryers work
Reactive, acid, direct and disperse dyes, optical brighteners and some organic pigments are spray dried from a filtered and often desalted press cake or solution at 25 to 45% solids. Most dye dryers are co-current with a rotary atomiser or nozzles. Hot air at about 180 to 300°C comes from a direct gas or diesel burner, or from an air heater fed by a thermic fluid heater, which is common in Indian dye plants. The exhaust leaves at about 85 to 110°C carrying intensely coloured fines, and passes cyclones and a bag filter or a wet scrubber. Dye feeds often contain sodium chloride or sulphate from salting out, and plants change shade and product often, so cleaning and cross-contamination matter as much as drying.
Where the ID fan sits
- Drying chamber
- Cyclones
- Bag filter or wet scrubber
- Exhaust (ID) fan
- Stack
The fan follows the final collector. With a bag filter it sees little dust; with a wet scrubber after cyclones it handles saturated gas with coloured droplets. It holds the chamber at slight suction so dye dust does not escape, which is critical because even small leaks stain the plant and product. The suction covers the chamber, cyclones and collector, usually 200 to 400 mmWC.
Gas at the ID fan
| Air temperature, normal | 85 to 110°C after a bag filter; 50 to 60°C after a scrubber |
| Maximum or upset | Up to about 150°C if feed is lost |
| Dust at the fan | Fine, intensely coloured dye fines; some dyes are combustible dusts |
| Moisture and dew point | Humid exhaust, dew point 45 to 60°C |
| Corrosive species | Chlorides and sulphates from salted dyes; acidic or alkaline condensate depending on the dye class |
| 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 dye and pigment spray dryers
| Blade form | Backward curved, smooth blades with no pockets |
| Material of construction | SS 316 for chloride-bearing exhaust; SS 304 for salt-free dyes |
| Drive and arrangement | Belt or direct driven, variable speed |
| Bearings and cooling | Bearings outside the air stream with a good shaft seal |
| Flow control | Variable speed drive on chamber pressure |
| Also specified | Wash nozzles, drain, large access doors, earthing and explosion venting where the dust is combustible |
What this ID fan has to cope with
Cross-contamination between shades
Residue of one dye in the fan can tint the next batch. A fan with smooth internals, wash nozzles and doors large enough to reach every surface can be cleaned quickly between shades, which matters more than a small gain in efficiency.
Chloride pitting
Dyes salted out with sodium chloride carry chloride fines that deposit and, with condensate at shutdown, pit SS 304. SS 316, a drain at the low point and washing after salted campaigns prevent pitting.
Combustible dye dust
Many organic dyes and intermediates are combustible, and deposits can self-heat at the outlet temperature. The fan is earthed, rubbing contact is avoided, and the dryer explosion protection covers the fan duct.
Energy and control
Dye plants run short campaigns with frequent shade changes and washes, so the fan spends much of its time at part load or idle. A variable speed drive suits this pattern. Raising feed solids by membrane filtration and recovering heat from the exhaust cut the air volume and fuel per kg of dye more than any change to the fan itself.
ID fan and FD fan in dye and pigment spray dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After bag filter or scrubber | After the intake filter, before the heater |
| What it handles | Humid exhaust with coloured, salty fines | Filtered ambient air |
| Temperature at the fan | 85 to 110°C | Ambient |
| Main design concern | Shade changeover and chloride corrosion | Steady flow through heater |
| Control | Chamber pressure | Drying air flow |
See also the Forced Draft (FD) Fan for dye and pigment spray dryers.
Questions buyers ask
What material should a dye spray dryer exhaust fan be?
SS 316 where the dyes carry chloride from salting out, because chloride deposits and condensate pit SS 304. For salt-free dyes or pigments SS 304 is common. Mild steel is not suitable where frequent washing is needed.
How is a dye spray dryer fan cleaned between shades?
By washing through nozzles in the inlet and casing with the drain open, then opening the inspection doors to check that no colour remains. The fan should have smooth internals and no pockets that hold residue.
Is dye dust explosive?
Many organic dyes and intermediates are combustible dusts. The dryer explosion protection should cover the exhaust path, and the fan is earthed and built to avoid sparks from rubbing contact.
Information needed to quote an ID fan for dye and pigment spray 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.
- Dye classes. Dye classes dried, salt content and any combustible dust test results.
- Changeover. How often shades change and the cleaning method 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.