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 ID fan sits
- Husk furnace
- Tubular heat exchanger (flue side)
- Multicyclone
- Furnace ID fan
- Chimney
The fan sits after the heat exchanger and multicyclone, handling flue gas cooled to about 180 to 280°C with the husk ash that passes the cyclone. Its suction covers the furnace, the long tube bank of the exchanger and the cyclone, typically 100 to 200 mmWC. Because the hot air blower on the clean side pushes air through the exchanger shell at positive pressure, the ID fan holding the flue side under suction makes any tube leak run from air to gas, protecting the paddy from smoke and soot.
Gas at the ID fan
| Gas temperature, normal | 180 to 280°C after the heat exchanger |
| Maximum or upset | Up to 400°C with fouled tubes or heavy husk feed |
| Dust at the fan | Silica husk ash and unburnt char, typically 1 to 4 g/Nm³ after a multicyclone |
| Moisture and dew point | Low; husk is fired dry at about 10% moisture |
| Running pattern | Continuous through the parboiling season, often round the clock |
| Density at the design temperature | About 0.71 kg/m³ at 230°C, 59% 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 parboiled paddy LSU dryers
| Blade form | Radial tipped with replaceable liners, chosen for erosion life rather than peak efficiency |
| Material of construction | Mild steel casing; impeller in wear-resistant plate with liners at the blade tips and scroll cut-off |
| Drive and arrangement | Belt driven or direct coupled at moderate speed, arrangement 9 or 8 |
| Bearings and cooling | Pedestal bearings outside the gas stream with a shaft cooling disc |
| Flow control | Variable speed drive on furnace draft |
| Also specified | Inspection door for liner checks, casing drain and an ash clean-out at the bottom of the scroll |
What this ID fan has to cope with
Husk ash erosion
Silica ash that passes the multicyclone erodes the blade tips and the casing at the scroll cut-off, and on a round-the-clock season the wear is fast. A radial wheel in wear plate with bolted liners and a lower tip speed lets the fan run the season and be relined in the off-season.
Exchanger tube leaks
Heat exchanger tubes crack from thermal cycling and corrosion at the cold end. If the flue side is not held at stronger suction than the air side, smoke enters the drying air and discolours the parboiled rice. The ID fan is sized with margin to keep that pressure difference as the tubes foul.
Soot and ash fouling in the tubes
Husk ash and soot coat the exchanger tubes, raising the flue gas temperature at the fan and the resistance it works against. The fan is selected at the fouled condition, and periodic tube cleaning restores both heat transfer and fan margin. Soot blowing or manual brushing between batches, where the layout allows, also helps keep the gas temperature at the fan within its design range.
Energy and control
LSU dryers in parboiling plants often run the whole season with a variable paddy rate depending on the soaking and steaming schedule. A variable speed drive on furnace draft follows the husk firing rate and saves power at part load. Clean exchanger tubes reduce both husk consumption and ID fan power, and plants that also run a husk boiler sometimes use steam coils instead, which removes the furnace and its ID fan altogether.
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 Forced Draft (FD) Fan for parboiled paddy LSU dryers.
Questions buyers ask
Why does smoke get into parboiled paddy in an LSU dryer?
Usually a cracked heat exchanger tube combined with too little suction on the flue side. The hot air blower then pushes or the dryer pulls flue gas into the drying air. Repairing the tubes and keeping the furnace ID fan holding firm suction on the flue side stops it.
What type of ID fan is used for a husk-fired LSU dryer furnace?
A radial tipped fan in wear-resistant plate with replaceable liners, running at moderate speed, because husk ash is almost pure silica. A cooling disc protects the bearings from the 180 to 280°C flue gas. Belt drive allows the speed to be adjusted as the exchanger fouls. A clean-out door at the bottom of the scroll is useful.
How hot is the air in a parboiled paddy LSU dryer?
Typically 90 to 120°C, higher than for raw paddy, because parboiled paddy tolerates more heat. The flue gas reaching the ID fan after the heat exchanger is hotter, usually 180 to 280°C. The difference covers the exchanger's temperature approach. A rise in flue gas temperature at the fan over the season is a sign of tube fouling.
Information needed to quote an ID fan for parboiled paddy LSU 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.
- Furnace and exchanger. Husk firing rate, heat exchanger type and the flue gas temperature at its outlet.
- Dryer capacity. Number of LSU dryers served and tonnes of paddy per batch or per day.
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.