How thermic fluid heaters for textile stenters work
In a dyeing and processing house, a central thermic fluid heater supplies oil at about 250 to 290°C to the stenters, where heat setting of polyester blends and drying and curing of finished fabric need air at 160 to 220°C in each chamber. The heater is usually fired on wood, briquettes or coal on a grate, sometimes on oil or gas. Several stenters, a dryer or two and sometimes a calender share the same circuit, so the heat demand jumps when machines start, change fabric or stop for a lot change. The flue gas leaves the coils at 330 to 380°C and passes an air preheater, a multicyclone and often a wet scrubber or bag filter to the ID fan and chimney.
Where the FD fan sits
- Inlet screen
- FD fan
- Air preheater
- Undergrate and overfire air ducts
- Grate
The FD fan sits beside the heater and pushes ambient air through the air preheater to the undergrate plenum, with a branch to overfire air nozzles above the bed on many units. In a processing house the heater room is often close to the stenter hall, and the fan inlet can draw lint-laden air. The fan works against the preheater, the plenum, the grate and the bed, typically 70 to 130 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C; warmer if drawn from inside a hot heater room |
| Air after the preheater | 120 to 180°C, downstream of the fan |
| Dust | Clean air, but cotton and polyester lint from the processing hall can be drawn in |
| Pressure at the fan outlet | Typically 70 to 130 mmWC |
| Load pattern | Short peaks as stenters start, long periods at part load |
| Density at the design temperature | About 1.11 kg/m³ at 45°C, 93% 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 thermic fluid heaters for textile stenters
| Blade form | Backward curved, for a stable curve and efficiency on clean air |
| Material of construction | Mild steel throughout |
| Drive and arrangement | Direct mounted on the motor shaft or direct coupled |
| Bearings and cooling | Standard bearings; no cooling needed |
| Flow control | Variable speed drive from the fluid outlet temperature controller, ramped to avoid lifting the bed |
| Also specified | Inlet screen or filter against lint, and a separate damper for overfire air |
What this FD fan has to cope with
Lint on the impeller
Fibre from the processing hall collects on the impeller blades and inlet screen, and on a fan running at part speed it builds into felt that throws the wheel out of balance and chokes the inlet. Ducting the inlet from outside the building and fitting a cleanable screen solves it.
Blowing the bed off the grate
A controller that drives the FD fan straight to full speed when stenters start lifts fine ash and burning fuel off the grate into the coils and the collector. Ramping the speed, and raising overfire air and fuel feed together, gives the extra heat without carry-over.
Hot heater room air
Heater rooms in processing houses run hot, sometimes above 45°C near the coils and higher in summer. A fan drawing room air delivers less mass flow than one rated at 30°C ambient, and the heater runs short of air exactly when demand peaks. The fan is selected at the real inlet temperature, or the inlet is ducted from outside.
Energy and control
A processing house heater may switch between low and high fire dozens of times a shift. A variable speed drive on the FD fan, linked to the firing rate with an oxygen check, cuts fan power and stops the heater from running at high excess air during the long low-load periods. A clean air preheater raises combustion air temperature and reduces the fuel and air needed for the same heat.
ID fan and FD fan in thermic fluid heaters for textile stenters
| ID fan | FD fan | |
|---|---|---|
| Position | After the scrubber or bag filter | Before the air preheater |
| What it handles | Flue gas, often wet after a scrubber | Clean air, sometimes with lint |
| Temperature at the fan | 60 to 80°C wet, or 180 to 230°C dry | Ambient to 45°C |
| Main design concern | Draft during load swings and wet corrosion | Fast but smooth response |
| Control | Furnace draft | Fluid outlet temperature |
See also the Induced Draft (ID) Fan for thermic fluid heaters for textile stenters.
Questions buyers ask
What is the FD fan on a textile thermic fluid heater?
It is the combustion air fan that blows air under the grate, and on many units above it as overfire air, through the air preheater. It handles clean ambient air at moderate pressure. Its speed follows the fluid outlet temperature, which follows the stenter load.
Why does the FD fan inlet choke with fibre in a dyeing unit?
The heater room is usually close to the processing hall, and air drawn from inside carries cotton and polyester lint. The lint collects on the inlet screen and impeller. Taking the inlet from outside the building and fitting a cleanable screen keeps the fan clean and balanced.
How fast should the FD fan respond to a stenter start?
Fast enough to stop the fluid temperature dropping too far, but ramped so the bed is not disturbed. In practice a speed ramp over tens of seconds, with fuel feed and overfire air raised together, is typical. The ID fan must follow at the same pace to hold furnace draft.
Information needed to quote an FD fan for thermic fluid heaters for textile stenters
- 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.
- Air supply. Whether the fan draws from inside the heater room or from outside, and the inlet temperature.
- Overfire air. Share of air supplied above the grate and the nozzle pressure 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.