How grain and maize dryers work
Paddy, maize and other grains are harvested at about 18 to 25% moisture and must be dried to about 12 to 14% for safe storage. Most commercial plants use mixed flow or column dryers, in which grain flows down between air ducts or perforated screens while heated air passes across it, followed by a cooling section using ambient air. Heat comes from a rice husk or biomass furnace, either through a heat exchanger so that only clean air reaches the grain, or directly through a spark arrestor. Drying air is held at about 50 to 70°C for paddy and seed, and up to about 100°C for feed maize. Exhaust air leaves at 30 to 50°C, close to saturation, carrying chaff, dust and maize hairs.
Where the ID fan sits
- Furnace and heat exchanger
- Hot air plenum
- Grain column
- Exhaust plenum
- Settling chamber or cyclone
- Exhaust (ID) fan
- Atmosphere
On dryers built for suction operation, the exhaust fan sits on the exhaust plenum of the column, usually after a settling chamber or low-pressure cyclone that drops the heavier chaff, and discharges to a stack or louvred outlet. It works against the grain column and the collector, typically 50 to 150 mmWC, and the column resistance changes with grain type, depth and moisture. Pressure-type dryers have no exhaust fan; the drying air fan alone pushes air through the grain.
Gas at the ID fan
| Air temperature, normal | 30 to 50°C at the exhaust plenum |
| Maximum or upset | Up to about 80°C if grain flow stops with the heat on |
| Dust at the fan | Chaff, husk, maize hairs and fine grain dust; all combustible |
| Moisture and dew point | Close to saturation from the wet zone; condensation in the casing in cool weather |
| Pressure at the fan | Low, typically 50 to 150 mmWC, set by the grain column |
| 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. |
ID Fan selection for grain and maize dryers
| Blade form | Backward inclined or backward curved, with open blades that pass chaff |
| Material of construction | Mild steel with a corrosion-resistant coating; galvanised or stainless steel where condensate persists |
| Drive and arrangement | Belt driven arrangement 9, or direct drive with a variable speed drive |
| Bearings and cooling | Pedestal bearings sealed against dust; no cooling needed |
| Flow control | Variable speed drive set from exhaust temperature or plenum pressure |
| Also specified | Casing drain, cleaning door, and earthing against static |
What this ID fan has to cope with
Chaff and hairs wrapping the impeller
Maize hairs and fine husk catch on blade edges and build up on the back plate, unbalancing the impeller and cutting flow. Open blades without sharp internal corners, a settling chamber ahead of the fan, and a clean-out at each change of grain keep the wheel in balance through the season.
Condensation in the casing
Exhaust air from the wet zone is close to saturation, and in cool night air the fan casing drops below its dew point. Water mixes with grain dust into a paste that hardens on the blades and corrodes mild steel. A casing drain, a protective coating and insulation on exposed ducts reduce it.
Dryer fires
Chaff and fines collect in the plenums and can be ignited by sparks from a direct-fired furnace, or by overheating when grain flow stops with the heat on. Temperature alarms on the exhaust, a spark arrestor upstream, and a fan that can be stopped and isolated quickly limit the damage.
Energy and control
Grain dryers run in campaigns after each harvest, often round the clock for several weeks, and then stand idle. The exhaust fan is one of the largest power users in the plant. Speed control from exhaust air temperature lets the fan reduce flow as the grain approaches target moisture, saving power and furnace fuel. Recirculating part of the warm, less humid exhaust from the lower column cuts fuel use further.
ID fan and FD fan in grain and maize dryers
| ID fan | FD fan | |
|---|---|---|
| Position | On the exhaust plenum, after the settling chamber | After the furnace heat exchanger, before the hot air plenum |
| What it handles | Humid exhaust with chaff and dust | Warm drying air |
| Temperature at the fan | 30 to 50°C | 50 to 100°C |
| Main design concern | Chaff build-up and condensation | Air temperature and distribution |
| Control | Exhaust temperature | Plenum pressure |
See also the Forced Draft (FD) Fan for grain and maize dryers.
Questions buyers ask
What does the exhaust fan do on a paddy or maize dryer?
It pulls the moist air out of the grain column after it has picked up water from the grain, and holds the dryer at slight suction so dust and chaff do not leak out. On suction-type grain dryers it is the main fan; on pressure-type dryers the drying air fan does the work and there may be no exhaust fan at all.
Why does the grain dryer exhaust fan corrode?
The exhaust air is cool and nearly saturated. At night or in cool weather the casing falls below the dew point, and the water mixes with grain dust into a corrosive paste that sits on the casing floor. A casing drain, a protective coating or galvanised casing, and insulation on exposed ducts all help.
How much pressure does a grain dryer fan need?
Usually a low pressure, typically 50 to 150 mmWC, but a large volume. The resistance depends on the grain, its depth in the column and its moisture: small, wet grain such as paddy packs more densely than maize. The fan is sized for the highest-resistance grain the dryer will handle.
Information needed to quote an ID fan for grain and maize dryers
- 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.
- Dryer type and capacity. Column, mixed flow or recirculating batch dryer, tonnes per hour, and the grains handled.
- Operating mode. Suction or pressure operation, and whether exhaust recirculation is used.
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.