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 FD fan sits
- Ambient air inlet
- Furnace heat exchanger
- Mixing chamber
- FD (drying air) fan
- Hot air plenum
- Grain column
The drying air fan is placed after the heat exchanger and mixing chamber, so it pulls ambient air across the exchanger and delivers warm air into the hot air plenum of the column. In this position it handles heated air, typically 50 to 100°C, and is selected for that density. It works against the exchanger on its suction side and the plenum and grain column on its discharge, typically 80 to 200 mmWC in total on mixed flow dryers.
Air at the FD fan
| Air temperature at the fan | 50 to 100°C after the heat exchanger |
| Maximum or upset | Up to about 120°C if the mixing damper sticks or the furnace overfires |
| Dust | Clean through an indirect exchanger; ash and sparks if direct-fired without a good spark arrestor |
| Pressure at the fan outlet | Typically 80 to 200 mmWC |
| Volume | Large, sized for the wettest grain at full capacity |
| Density at the design temperature | About 1.00 kg/m³ at 80°C, 83% 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 grain and maize dryers
| Blade form | Backward curved or aerofoil, for efficiency on a large volume |
| Material of construction | Mild steel, coated |
| Drive and arrangement | Belt driven arrangement 9, or direct drive with a variable speed drive |
| Bearings and cooling | Pedestal bearings with a heat slinger for the upper temperatures |
| Flow control | Variable speed drive on plenum pressure; air temperature held by the furnace and mixing damper |
| Also specified | High-temperature cut-out, inlet damper, and spark detection where the furnace is direct-fired |
What this FD fan has to cope with
Smoke and sparks in the drying air
A cracked heat exchanger tube, or a direct-fired furnace with a poor spark arrestor, lets smoke and sparks reach the grain, where they cause taint and fires. Regular inspection of the exchanger, a spark detector at the fan outlet and a high-temperature trip on the plenum are the standard protection.
Overheating the grain
If the furnace overfires or the mixing damper sticks, the fan delivers air hot enough to crack paddy or damage seed germination. A temperature controller on the plenum acting on the furnace and the fresh air damper, independent of the fan speed, protects the grain.
Uneven air in the plenum
A fan discharging straight into a short plenum sends more air to one side of the grain column, so some grain over-dries while the rest stays wet. Turning vanes or a diffuser after the fan, and a plenum long enough for the flow to settle, even out the distribution.
Energy and control
Drying air is the largest energy flow in the dryer. Matching air temperature and flow to the incoming grain moisture, rather than running full flow all the time, saves both fan power and furnace fuel. A variable speed drive lets the operator turn flow down as the grain dries late in the season, and a separate small fan for the cooling section avoids pushing hot air where it is not needed.
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 Induced Draft (ID) Fan for grain and maize dryers.
Questions buyers ask
What temperature does the drying air fan handle on a grain dryer?
Where the fan follows the furnace heat exchanger, it handles warm drying air, typically 50 to 70°C for paddy and seed and up to about 100°C for feed maize. The fan is selected for the lower density of warm air, with bearings protected by a heat slinger at the higher temperatures.
Can a paddy dryer use direct heat from a husk furnace?
It can, but only with a good spark arrestor and clean, well-burnt flue gas, because smoke and sparks can taint or ignite the grain. Indirect heating through a heat exchanger is safer for milling paddy and essential for seed. Indirect heating usually adds a separate flue fan for the furnace side.
Why does one side of my grain dryer dry faster?
Uneven air distribution in the hot air plenum is the usual cause: a fan discharging straight into a short plenum sends more air to one side of the column. Turning vanes, a diffuser or a longer plenum even out the flow. Uneven grain depth or blocked screens have a similar effect and should be checked too.
Information needed to quote an FD fan for grain and maize dryers
- 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.
- Heat source. Husk or biomass furnace, direct or indirect heating, and the heat exchanger resistance.
- Drying temperatures. Required drying air temperature for each grain, and the cooling section 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.