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 ID fan sits
- Grate and furnace
- Thermic fluid coils
- Air preheater
- Multicyclone
- Wet scrubber or bag filter
- ID fan
- Chimney
The fan sits at the end of the gas path, after the multicyclone and the wet scrubber or bag filter that pollution control boards in textile clusters commonly require. With a venturi or packed scrubber, the gas reaches the fan cooled to 60 to 80°C and saturated; with a bag filter it stays at about 180 to 230°C. Suction at the fan is typically 150 to 300 mmWC, much of it in the scrubber.
Gas at the ID fan
| Gas temperature, normal | 60 to 80°C after a wet scrubber; 180 to 230°C after a bag filter |
| Maximum or upset | Up to 350°C if the scrubber water fails or the air preheater is bypassed |
| Dust at the fan | Fine ash after a scrubber or bag filter; heavier after a multicyclone alone |
| Moisture and dew point | Saturated with droplets after a scrubber; dew point about 60 to 70°C |
| Corrosive species | Acidic scrubber water from SO₂ on coal firing; chlorides from some wood and briquettes |
| Density at the design temperature | About 1.01 kg/m³ at 80°C, 84% 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 thermic fluid heaters for textile stenters
| Blade form | Backward curved or radial tipped, with smooth blades that shed wet ash |
| Material of construction | SS 304 or SS 316, or mild steel with rubber or FRP lining, after a wet scrubber; mild steel after a bag filter |
| Drive and arrangement | Direct coupled arrangement 8, or belt driven on smaller heaters |
| Bearings and cooling | Pedestal bearings outside the gas stream, with a shaft seal against moisture |
| Flow control | Variable speed drive on furnace draft with fast response |
| Also specified | Casing drain, inspection door, and a temperature trip to protect a lined fan if scrubber water fails |
What this ID fan has to cope with
Load swings upset the draft
When two stenters start together the fluid temperature drops and the firing rate rises sharply. If the ID fan responds slowly, the furnace goes positive and smoke leaks from the fuel door; if it overshoots, cold air is pulled through the bed. A speed drive with a tuned draft loop and the furnace pressure tapping in a stable position keeps the furnace at a steady small suction.
Wet ash and corrosion after the scrubber
Ash carried over from the scrubber as a wet slurry sticks to the blades and dries unevenly when the fan stops, causing unbalance at restart. The scrubber water also turns acidic on coal. Stainless or lined construction, a mist eliminator, and washing the wheel at weekly stops keep the fan running.
Hot gas through a lined fan
If the scrubber pump trips while the heater is firing, gas at 300°C or more can reach a rubber or FRP lined fan in minutes. A temperature trip on the fan inlet linked to an emergency water spray or the fuel feed protects the lining.
Power failure and fluid overheating
On a power failure the circulating pump, the ID fan and the FD fan stop together while a solid fuel bed is still burning. The fan cannot help here; the heater needs its emergency cooling or a diesel-backed pump. The fan is restarted only after the furnace has been purged of smouldering gas.
Energy and control
Stenter loads follow the shift and the lot plan, and the heater spends most of its time well below rating with frequent short peaks. A variable speed ID fan tracking furnace draft saves a large share of its power against damper control. Recovering heat from the stenter exhaust and insulating fluid lines reduce the firing rate, and each reduction comes straight off the ID fan duty as well.
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 Forced Draft (FD) Fan for thermic fluid heaters for textile stenters.
Questions buyers ask
Why does smoke come out of the fuel door when stenters start?
The heater fires harder as the fluid temperature drops, and the gas volume rises faster than a slow draft loop can open the ID fan. The furnace goes positive for a few seconds. A variable speed ID fan with a faster draft control loop, and enough fan margin above full fire, prevents this.
Which material is used for the ID fan after a wet scrubber on a textile thermic fluid heater?
Stainless steel, usually SS 304 or SS 316 where scrubber water is acidic, or mild steel with rubber or FRP lining. The gas after the scrubber is saturated and carries droplets with ash, so plain mild steel corrodes and gets coated quickly. A drain and a mist eliminator are also needed.
Can one ID fan serve a thermic fluid heater feeding several stenters?
Yes. The heater is one combustion system whatever the number of stenters. The fan is sized for the heater at full fire with the scrubber or filter at its highest resistance, and a speed drive lets it follow the large swings in demand as stenters start and stop.
Information needed to quote an ID fan for thermic fluid heaters for textile stenters
- 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.
- Stenter load. Number of stenters and other consumers on the circuit, and how their demand varies through a shift.
- Pollution control. Whether a wet scrubber, bag filter or multicyclone is fitted, and the gas temperature at the fan inlet.
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.