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 FD fan sits
- Air intake
- Filter
- FD (supply) fan
- Thermic fluid air heater or burner
- Air disperser
- Drying chamber
The supply fan sits after the intake filter and before the heater, handling ambient air. A thermic fluid finned-tube air heater has a high air side pressure drop and limits the inlet temperature to about 250°C; a direct burner allows hotter air with less pressure loss, and the fan then also supplies the burner air. The fan pressure covers the filter, heater, ducts and disperser.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Air after the heater | 180 to 300°C, downstream of the fan |
| Dust | Filtered ambient air; dye plant air can carry coloured dust |
| Pressure | Filter, heater coil and disperser |
| Heat source | Thermic fluid heater coil, steam coil, or direct gas or diesel burner |
| 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 dye and pigment spray dryers
| Blade form | Backward curved for efficiency |
| Material of construction | Mild steel with coating, or stainless steel |
| Drive and arrangement | Direct driven with a variable speed drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on air flow |
| Also specified | Intake filter and inspection door |
What this FD fan has to cope with
Coloured dust in the intake
Dye plants have airborne coloured dust near packing and blending. If it enters the supply air it contaminates the next shade. The intake is placed away from these areas and filtered.
Heater coil pressure drop
Finned thermic fluid coils foul with dust over time, and the air side pressure drop rises. The supply fan duty includes a fouled coil, and the coil is cleaned on schedule so the fan does not run short of flow.
Inlet temperature limits
Heat-sensitive dyes lose strength or change shade if the inlet air is too hot. The supply fan and heater are controlled together to give a steady, limited inlet temperature, and the fan flow must not drop when the heater coil fouls.
Energy and control
The supply fan runs with the dryer through each campaign. Speed control holds the flow as filters and coils foul, without damper losses. Using exhaust heat to preheat the inlet air reduces the load on the thermic fluid heater or burner, and cleaning fouled heater coils lowers both the fan pressure and the heat transfer penalty.
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 Induced Draft (ID) Fan for dye and pigment spray dryers.
Questions buyers ask
What heats the air in a dye spray dryer?
Often a finned-tube air heater fed by a thermic fluid heater, or a steam coil, or a direct gas or diesel burner. Thermic fluid systems limit the inlet to about 250°C; direct burners allow hotter air.
Is the dye dryer FD fan the combustion air fan?
Only where a direct burner heats the air, in which case the supply fan also provides the burner air. With a thermic fluid or steam coil, it is simply the process air supply fan.
How much pressure does the supply fan need?
Enough for the intake filter, the heater coil when fouled, ducts and the air disperser. Thermic fluid coils add significant pressure drop, so the fan duty depends on the heater design.
Information needed to quote an FD fan for dye and pigment spray dryers
- 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.
- Heater. Heater type, air side pressure drop when clean and fouled, and inlet temperature.
- Intake location. Where the air is drawn from and the filtration required.
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.