How indirect fired hot air generators with heat exchanger work
An indirect fired hot air generator keeps the flue gas and the drying air apart. A furnace burning wood, coal, husk, briquettes or oil fires into a bank of tubes or plates, and clean ambient air is blown or drawn across the other side, leaving at 80 to 250°C for tea, spices, food, pharmaceutical and other products that must not see smoke. Tea withering and CTC dryers, cardamom and pepper dryers, and grain dryers are typical users. The flue gas leaves the exchanger at about 200 to 300°C, because the exchanger must keep a temperature difference to the air, and goes through a cyclone to a flue gas ID fan and chimney. Two separate fan systems run side by side.
Where the ID fan sits
- Furnace
- Tube or plate heat exchanger (flue side)
- Cyclone
- ID fan
- Chimney
The fan sits after the exchanger and a cyclone, at the chimney base. It holds the furnace and the flue side of the exchanger under suction, typically 60 to 150 mmWC including the tube bank and cyclone. The clean air side runs at positive pressure from its own fan, so a cracked tube leaks clean air into the flue rather than smoke into the drying air. This is the main reason the flue side is put under induced draft.
Gas at the ID fan
| Gas temperature, normal | 200 to 300°C after the exchanger |
| Maximum or upset | 350 to 450°C if the process air fan stops while the furnace is firing |
| Dust at the fan | Fly ash after a cyclone, varying with the fuel |
| Moisture and dew point | Depends on fuel; not usually limiting at these temperatures |
| Corrosive species | SO₂ on coal and furnace oil; tar from wood at low fire |
| Density at the design temperature | About 0.67 kg/m³ at 260°C, 55% 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 indirect fired hot air generators with heat exchanger
| Blade form | Radial tipped or backward curved, depending on the fuel and the cyclone in front |
| Material of construction | Mild steel to about 350°C; heat-resisting steel impeller where gas regularly exceeds that |
| Drive and arrangement | Direct coupled arrangement 8 or belt driven |
| Bearings and cooling | Pedestal bearings with cooling disc rated for the upset temperature |
| Flow control | Variable speed drive or damper on furnace draft |
| Also specified | Interlock with the process air fan, inspection door and casing insulation |
What this ID fan has to cope with
Process air fan trip
If the clean air fan stops while the furnace is firing, nothing cools the exchanger and the flue gas reaches the ID fan far hotter than normal. The exchanger tubes can also fail. An interlock that cuts fuel and holds the ID fan running to clear the furnace protects both. The fan is rated for this excursion.
Exchanger leaks and smoke in the product
Cracked tubes or failed seals allow flow between the two sides. With the flue side under suction and the air side under pressure, the leak carries clean air into the flue. If the ID fan is oversized or the air fan undersized, the balance can reverse at some loads. The two fans are selected and controlled together.
Fouled tubes and rising gas temperature
Soot, ash and tar on the flue side reduce heat transfer, so the drying air temperature falls, the operator fires harder and the flue gas at the fan gets hotter. Regular tube cleaning, and a gas temperature indication at the fan, keep the fan within its rating and the generator efficient.
Energy and control
Indirect generators lose more heat up the chimney than direct units because the flue gas leaves the exchanger hot. A variable speed ID fan holding furnace draft saves fan power at part load, but the larger gain is heat recovery: an air preheater on the combustion air, or a second exchanger pass, lowers the flue gas temperature and reduces both fuel and the hot gas volume at the fan.
ID fan and FD fan in indirect fired hot air generators with heat exchanger
| ID fan | FD fan | |
|---|---|---|
| Position | After the exchanger flue side and cyclone | Before the exchanger air side |
| What it handles | Flue gas with fly ash | Clean ambient air for the dryer |
| Temperature at the fan | 200 to 300°C, up to 450°C upset | Ambient, or 80 to 250°C if draw-through |
| Main design concern | Hot gas and suction below the air side | Pressure above the flue side |
| Control | Furnace draft | Dryer air flow or temperature |
See also the Forced Draft (FD) Fan for indirect fired hot air generators with heat exchanger.
Questions buyers ask
Why is the flue side of an indirect hot air generator kept under suction?
So that any leak in the heat exchanger carries clean air into the flue, not smoke into the drying air. The ID fan holds the flue side below atmosphere while the process air fan keeps the air side above it. This protects tea, spice and food products from smoke taint.
What temperature does the ID fan on an indirect hot air generator handle?
Usually 200 to 300°C after the exchanger, higher than on a boiler, because the exchanger must keep a temperature difference to the hot air it produces. The fan should also be rated for the excursion if the process air fan trips, often 350 to 450°C.
Can one fan handle both the flue gas and the drying air on an indirect generator?
No. The two streams must stay separate, which is the point of an indirect generator. The flue side needs its own ID fan, selected for hot gas and fly ash, and the clean side its own process air fan, selected for clean air volume. They are sized and controlled together so the pressure balance across the exchanger stays right at every load.
Information needed to quote an ID fan for indirect fired hot air generators with heat exchanger
- 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.
- Fuel and exchanger. Fuel fired, exchanger type and the flue gas temperature at the exchanger outlet.
- Clean air side. Process air volume and pressure, and whether that fan pushes or pulls through the exchanger.
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.