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 FD fan sits
- Inlet filter
- FD (process air) fan
- Heat exchanger (air side)
- Hot air duct
- Dryer
In the blow-through arrangement the fan sits before the exchanger on cold air and pushes it across the tubes to the dryer, which keeps the air side above flue pressure and the fan at ambient temperature. Some tea and grain dryers use a draw-through fan after the exchanger instead; that fan handles hot clean air at 80 to 250°C and the air side may run below atmosphere, so the flue side must be held at stronger suction.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C, in blow-through arrangement |
| Draw-through arrangement | Hot clean air at 80 to 250°C at the fan |
| Dust | Filtered or screened ambient air; tea fluff and grain dust near the dryer |
| Pressure at the fan outlet | Exchanger air side, ducts and dryer bed, typically 80 to 250 mmWC |
| Pressure balance | Air side kept above flue side at every load |
| 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 indirect fired hot air generators with heat exchanger
| Blade form | Backward curved or aerofoil, for efficiency on clean air |
| Material of construction | Mild steel, with food-grade coating or stainless where the product requires |
| Drive and arrangement | Direct coupled or belt driven; bearing cooling only on draw-through fans |
| Bearings and cooling | Standard bearings for blow-through; cooling disc for hot draw-through duty |
| Flow control | Variable speed drive on dryer air flow or temperature |
| Also specified | Inlet filter or screen, and an interlock that trips the furnace if the fan stops |
What this FD fan has to cope with
Losing the pressure balance
If the process air fan is throttled back while the ID fan holds strong suction, the air side pressure at a weak exchanger joint can fall below the flue side and smoke enters the drying air. A minimum air flow on the process fan, and a pressure difference check across the exchanger, keep the balance right at every load.
Exchanger overheating on low air flow
The process air cools the exchanger tubes. Running it too low to reduce the drying air temperature, instead of reducing firing, overheats the tubes, distorts the tube plates and shortens their life. Control should turn down the fire first and reduce the air flow only within the exchanger maker's minimum, with a tube temperature or flue gas temperature alarm as a guard.
Tea fluff and dust at the inlet
Tea factories and grain dryers have fibre and dust in the air around the dryer. Drawn into the fan it fouls the impeller and the exchanger air side, reducing flow and heat transfer. Inlet screens and filters, taken from a clean location and cleaned regularly, prevent it.
Energy and control
The process air fan moves all the drying air and runs as long as the dryer runs, often through a whole season. A high efficiency backward curved or aerofoil wheel with a variable speed drive cuts its power considerably. Recirculating part of the dryer exhaust through the exchanger, where the product allows, raises the inlet temperature and saves fuel, and the fan is selected for the mixed air.
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 Induced Draft (ID) Fan for indirect fired hot air generators with heat exchanger.
Questions buyers ask
What is the FD fan on an indirect fired hot air generator?
It is the process air fan that blows clean ambient air across the heat exchanger to the dryer. It is usually much larger than the combustion air fan of the furnace. On some dryers the process fan is placed after the exchanger instead and draws hot air through it.
Should the process air fan push or pull through the heat exchanger?
Pushing keeps the fan on cold air and the clean side above the flue side, which protects the product if the exchanger leaks. Pulling puts the fan on hot air and can leave the clean side below atmosphere, so it needs a stronger flue side suction and a fan rated for hot air.
Why must the furnace trip when the process air fan stops?
The process air cools the exchanger. Without it, the tubes overheat within minutes and the flue gas reaching the ID fan rises far above normal. An interlock that cuts fuel when the process fan stops, while the ID fan keeps running to clear the furnace, protects the exchanger, the ID fan and the dryer, and is part of a safe design.
Information needed to quote an FD fan for indirect fired hot air generators with heat exchanger
- 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.
- Arrangement. Blow-through or draw-through, and the air temperature at the fan.
- Dryer. Product, dryer type, air volume and the hot air temperature 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.