How parboiled paddy LSU dryers work
In a parboiling plant, paddy is soaked in hot water, steamed with low pressure steam from a husk-fired boiler, and then dried from about 30 to 35% moisture down to 14%. LSU (Louisiana State University) column dryers do most of this work: the wet paddy flows down a tall bin through rows of inverted V-shaped air ducts, and hot air at typically 90 to 120°C is blown in through alternate rows and leaves through the others. Paddy passes the dryer several times with tempering in between. The hot air is made in a husk-fired furnace and heat exchanger, so the grain sees clean air, or on some plants by steam coils fed from the boiler. The furnace flue gas is pulled through the exchanger by an ID fan.
Where the FD fan sits
- Fresh air inlet
- Heat exchanger (air side)
- FD (hot air) blower
- Hot air plenum
- LSU inlet duct rows
- Paddy column
The blower draws fresh air across the shell side of the heat exchanger and pushes the heated air into the plenum feeding the inlet duct rows of the LSU column. It therefore handles air at the drying temperature, typically 90 to 120°C. The pressure covers the exchanger shell, the plenum and the paddy column, typically 100 to 200 mmWC. The combustion air fan at the furnace grate is a separate small fan on ambient air.
Air at the FD fan
| Air temperature at the fan | 90 to 120°C after the heat exchanger |
| Air after the fan | Same, into the LSU plenum |
| Dust | Clean air; smoke only if exchanger tubes leak |
| Humidity | Ambient air; part of the dryer exhaust is sometimes recirculated |
| Pressure at the fan outlet | Typically 100 to 200 mmWC through the exchanger and grain column |
| Density at the design temperature | About 0.92 kg/m³ at 110°C, 77% 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 parboiled paddy LSU dryers
| Blade form | Backward curved for efficiency on clean warm air |
| Material of construction | Mild steel, with insulated casing |
| Drive and arrangement | Belt driven or direct coupled, motor clear of the hot casing |
| Bearings and cooling | Cooling disc on the shaft; bearings rated for warm surroundings |
| Flow control | Variable speed drive or damper on plenum temperature |
| Also specified | Inspection door and a temperature sensor at the fan outlet |
What this FD fan has to cope with
Uneven drying through the column
If the blower cannot hold pressure as the column fills, the upper or lower duct rows starve and moisture varies across the batch, which shows as broken grains. A fan with a steady pressure curve and correct plenum design keeps the duct rows evenly supplied. Checking the static pressure in each duct row at commissioning confirms the distribution.
Bearing heat
Air at 90 to 120°C heats the shaft and bearings, and grease life shortens through a long season. A cooling disc, an insulated casing and bearings mounted clear of the scroll keep bearing temperatures in range. Grease intervals are also shortened in the season. A temperature probe on each bearing gives early warning before a failure stops the dryer in mid-batch.
Recirculation and humidity
Recirculating part of the warm exhaust saves husk but raises the humidity of the drying air. The blower is sized for the mixed air temperature and the higher volume that recirculation brings. Recirculation is taken from the lower stages, where the exhaust is warmer and drier, and the share is reduced in humid weather to keep drying times steady.
Energy and control
The hot air blower runs continuously through the parboiling season. Speed control on plenum temperature and paddy moisture saves power, and recirculating part of the dryer exhaust from the lower, drier stages cuts husk consumption. Tempering the paddy between passes lets the dryer use less air for the same moisture removal.
ID fan and FD fan in parboiled paddy LSU dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the heat exchanger and multicyclone | After the heat exchanger on the air side |
| What it handles | Husk flue gas with silica ash | Clean heated drying air |
| Temperature at the fan | 180 to 280°C | 90 to 120°C |
| Main design concern | Ash erosion and leak-safe suction | Even air to the duct rows |
| Control | Furnace draft | Plenum temperature |
See also the Induced Draft (ID) Fan for parboiled paddy LSU dryers.
Questions buyers ask
Does the LSU dryer blower handle hot air?
Yes. In the usual parboiling layout the blower draws air through the heat exchanger and pushes it into the dryer, so it handles air at 90 to 120°C. Bearings, casing and motor placement are chosen for that temperature. Some plants heat air with steam coils from the parboiling boiler instead of a husk furnace; the blower position is the same.
Why do parboiled rice grains break after LSU drying?
Uneven or too rapid drying is a common cause. If the blower does not keep the duct rows evenly supplied, parts of the column over-dry. Correct air flow, temperature and tempering between passes reduce breakage. Temperature and moisture readings across the column help locate the problem.
Is the combustion air fan the same as the hot air blower?
No. The combustion air fan supplies the husk furnace grate. The hot air blower moves the drying air through the exchanger to the paddy. They are separate fans with very different duties. The combustion air fan is small and takes ambient air. The hot air blower is much larger and handles heated air. Both are usually belt driven.
Information needed to quote an FD fan for parboiled paddy LSU 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.
- LSU column. Column height, number of duct rows and air flow per column.
- Recirculation. Whether dryer exhaust is recirculated and at what share.
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.