How kiln exhaust waste heat thermic fluid heaters work
Lime shaft kilns, rotary kilns, ceramic tunnel and roller kilns and some cement kiln bypasses release exhaust gas at 250 to 500°C that is wasted at the stack. A waste heat thermic fluid heater places a coil bank in this exhaust duct, heating thermic oil to typically 250 to 300°C for dryers, spray dryers or process heating nearby. The coil cools the exhaust by 100 to 200°C. Because the heater adds resistance to a kiln that was balanced on its existing fan, a new or upgraded ID fan after the heater, or a booster fan, provides the extra draft. A bypass duct with dampers lets the kiln keep running when the heater or the oil circuit is out of service.
Where the ID fan sits
- Kiln
- Exhaust duct
- Thermic fluid heater coil
- Dust collector
- ID fan
- Stack
The fan sits after the heater and the kiln dust collector, often replacing the existing kiln fan. It handles exhaust cooled by the coil to about 180 to 280°C, with kiln dust that the collector did not remove. The suction it works against is the kiln draft, the coil, the collector and ducts, typically 150 to 300 mmWC. In bypass mode the gas skips the coil and reaches the fan hotter, so the fan is sized for the bypass temperature as well.
Gas at the ID fan
| Gas temperature, normal | 180 to 280°C after the heater coil |
| Maximum or upset | 250 to 500°C in bypass mode or on loss of oil circulation |
| Dust at the fan | Lime, clinker or ceramic dust; low after a bag filter, moderate after a cyclone |
| Moisture and dew point | Moisture from fuel, product drying and calcination |
| Corrosive species | SO₂ from fuel; fluorides and chlorides from some ceramic bodies and glazes |
| Density at the design temperature | About 0.71 kg/m³ at 230°C, 59% 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 kiln exhaust waste heat thermic fluid heaters
| Blade form | Backward curved after a bag filter; radial tipped with liners after a cyclone |
| Material of construction | Mild steel for normal duty; heat-resisting steel impeller where bypass gas is above about 350°C |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on kiln draft |
| Also specified | Bypass and isolation dampers, dilution air damper, inspection door |
What this ID fan has to cope with
Kiln draft disturbed by the heater
Adding the coil changes the resistance the kiln sees, and the oil circuit switching on and off changes it again. If the fan does not follow, kiln draft swings and the burning zone shifts. Control from kiln draft, not heater outlet, keeps the kiln undisturbed.
Bypass and oil circuit trips
If the thermic oil pump stops, the coil must be bypassed at once or the stagnant oil in the hot coil cracks and forms coke. In bypass the fan sees much hotter gas, often above 350°C. Materials, shaft cooling and bearings sized for the bypass case, or a dilution air damper interlocked with the bypass, protect it.
Dust and coil fouling
Kiln dust deposits on the coil and raises resistance over weeks, and on ceramic kilns fluoride-bearing deposits can harden. The fan is selected for a fouled coil, so the kiln draft is held at the end of the run, and sootblowers or cleaning access on the coil keep the rise in check.
Energy and control
Kilns run steadily, and most of the fan energy is set by the added resistance of the coil. A variable speed drive on kiln draft saves power compared with a damper. The oil heated in the coil replaces fuel in a separate heater, so the extra fan power is typically small compared with the heat recovered.
Questions buyers ask
Can the existing kiln ID fan handle a new thermic fluid heater?
Sometimes, if it has spare pressure and the motor has spare power. More often the added coil resistance needs a higher-pressure fan, a new impeller in the existing casing, or a booster fan. Measure the existing fan flow, pressure and kiln draft first, so the change is based on site data rather than drawings.
What happens to the ID fan when the thermic fluid heater is bypassed?
The gas skips the coil and reaches the fan hotter, by 100 to 200°C. The fan must be built for this temperature, with materials and bearings to suit, or protected by a dilution air damper that opens with the bypass. The control must also hold kiln draft steady while the dampers move.
Does the kiln exhaust dust affect the thermic fluid heater ID fan?
Yes. After a cyclone the fan sees moderate dust and needs liners or a radial wheel; after a bag filter it sees little. The coil also fouls with kiln dust, which raises the fan's working pressure over time. Both are part of the selection for a waste heat thermic fluid heater on a kiln.
Information needed to quote an ID fan for kiln exhaust waste heat thermic fluid heaters
- 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.
- Kiln exhaust. Exhaust flow, temperature, dust load and the existing kiln fan data.
- Heater and bypass. Coil resistance when clean and fouled, and the bypass arrangement.
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.