How rice mills and paddy dryers work
A modern rice mill cleans, dries, dehusks, whitens, polishes and grades paddy, and the mill itself is full of fans: husk aspiration, bran collection, polisher air and dust collection. Paddy arriving wet at harvest, often 18 to 24% moisture, is dried to about 14% before storage or milling. Small mills use recirculating batch dryers; larger mills use column or LSU dryers. Hot air at typically 45 to 80°C for raw paddy comes from a husk-fired furnace, either direct with a spark separator or indirect through a heat exchanger. The husk furnace has its own combustion air fan and, on indirect systems, an ID fan pulling flue gas through the exchanger and a cyclone. Many mills also run a husk-fired boiler.
Where the FD fan sits
- Husk furnace combustion air fan
- Husk furnace
- Heat exchanger
- FD (hot air) blower
- Dryer plenum
- Paddy column
The hot air blower draws air through the heat exchanger shell, or from the mixing chamber of a direct husk furnace, and pushes it into the dryer plenum. It therefore handles warm air at 45 to 80°C for raw paddy. On recirculating batch dryers the blower is built into the dryer. The combustion air fan is a separate small fan at the furnace grate, handling ambient air.
Air at the FD fan
| Air temperature at the fan | 45 to 80°C at the hot air blower |
| Air after the fan | Same, into the dryer plenum |
| Dust | Clean warm air; husk sparks on direct systems |
| Humidity | Ambient humidity in harvest season |
| Pressure at the fan outlet | Typically 50 to 150 mmWC through the grain column |
| Density at the design temperature | About 1.04 kg/m³ at 65°C, 87% 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 rice mills and paddy dryers
| Blade form | Backward curved or axial for high volume |
| Material of construction | Mild steel |
| Drive and arrangement | Belt driven |
| Bearings and cooling | Standard bearings |
| Flow control | Damper or variable speed drive |
| Also specified | Spark arrestor on direct systems |
What this FD fan has to cope with
Uneven drying
Poor air distribution leaves wet pockets in the bin or column, where paddy heats and discolours in storage. A blower with enough pressure for the full grain depth, and a plenum that spreads air evenly, give uniform flow. Moisture checks at several points after each pass confirm it.
Sparks into the dryer
On direct-fired dryers, husk sparks carried from the furnace can ignite dust and husk in the dryer. A spark separator between the furnace and the blower inlet is essential, and the blower casing should have an inspection door to check for embers after a disturbance.
Over-drying and cracking
Too much hot air, or air that is too hot, dries the outer layer of the grain too fast and causes fissures that break in milling, lowering head rice yield. Controlling air temperature and flow through the blower, with tempering between passes, protects the grain.
Energy and control
Paddy drying is seasonal and the moisture of incoming paddy varies from day to day. Speed control on the hot air blower matches air flow to the paddy moisture and the number of dryers in use. Tempering the paddy between passes lets moisture move from the core to the surface, so the dryer needs less hot air for the same result.
ID fan and FD fan in rice mills and paddy dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the furnace heat exchanger and cyclone | Between the furnace and the dryer plenum |
| What it handles | Husk flue gas with silica ash | Warm drying air |
| Temperature at the fan | 150 to 250°C | 45 to 80°C |
| Main design concern | Silica erosion | Uniform air through the grain |
| Control | Furnace draft | Drying air temperature |
See also the Induced Draft (ID) Fan for rice mills and paddy dryers.
Questions buyers ask
What is the FD fan in a rice mill paddy dryer?
It is the hot air blower that pushes warm air from the husk furnace or heat exchanger through the paddy. The husk furnace also has a smaller combustion air fan under the grate. Both handle air rather than flue gas. The blower is the larger of the two and sets drying uniformity.
What air temperature is used for drying raw paddy?
Typically 45 to 80°C depending on the dryer type, kept low to avoid grain cracking and to protect head rice yield. Parboiled paddy can take higher temperatures, which is why LSU dryers in parboiling plants run hotter. Air temperature is set by the dryer type and paddy variety.
Does the paddy dryer blower handle hot air?
Yes. It usually draws air through the heat exchanger or from a mixing chamber and handles warm air at 45 to 80°C. Its bearings and drive are selected for that temperature, and the motor is kept out of the hot air stream. Bearing temperature is checked during the season.
Information needed to quote an FD fan for rice mills and paddy 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.
- Dryer capacity. Tonnes per batch or per hour and moisture reduction.
- Air flow. Air flow and pressure at the plenum.
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.