How dal and pulse mill dryers work
Dal mills dehusk and split pigeon pea, black gram, green gram, chickpea and lentil. To loosen the husk, the grain is conditioned with edible oil and water, then dried, often over several wetting and drying cycles. Many mills still sun dry on open yards, but larger mills use mechanical dryers: column or bin dryers, or continuous LSU type dryers. Drying air is heated to typically 60 to 90°C by an indirect heat exchanger fired with rice husk, pulse husk or wood, or by steam. The grain must stay below about 60°C or it cracks and the split quality falls. Exhaust air leaves at 35 to 50°C, humid and carrying husk fragments and bran powder, and passes a cyclone before discharge.
Where the ID fan sits
- Grain column or LSU dryer
- Exhaust plenum
- Cyclone
- ID fan
- Exhaust stack
The fan sits after the cyclone so the heavier husk fragments are removed first; the fan then handles fine bran powder and light fibre. It works against the grain column or LSU section, the plenum, cyclone and ducting, typically 80 to 180 mmWC. In some smaller mills the only fan is a supply blower and the dryer exhausts freely; an exhaust fan is added when dust in the mill, or dryer pressure control, becomes a problem.
Gas at the ID fan
| Gas temperature, normal | 35 to 50°C, humid exhaust air |
| Maximum or upset | 70 to 90°C if the grain column empties or the heater overruns |
| Dust at the fan | Fine bran powder and husk fibre after the cyclone; several hundred mg/Nm³ is common |
| Moisture and dew point | Close to saturation early in each drying cycle; condensation in cold ducting at night |
| Other | Oily dust from oil-treated grain; combustible |
| 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 dal and pulse mill dryers
| Blade form | Radial tipped or open backward curved, which shed fibre and oily dust |
| Material of construction | Mild steel with an internal coating, or galvanised; stainless steel where condensate stands |
| Drive and arrangement | Belt driven, arrangement 9, for easy speed adjustment |
| Bearings and cooling | Standard pedestal bearings; no cooling needed |
| Flow control | Variable speed drive or inlet damper on dryer suction |
| Also specified | Inspection door, casing drain, and an earthed construction against static on dry husk dust |
What this ID fan has to cope with
Oily husk build-up
Oil-conditioned grain gives off husk and bran that is slightly sticky, and in humid exhaust it packs on the blade backs and the scroll. Build-up unbalances the wheel and cuts flow. Self-cleaning radial blades, an inspection door and regular cleaning in the off season keep the fan running.
Condensation in the casing
Early in a drying cycle the exhaust is near saturation, and in cool weather it condenses in the duct and casing. Mixed with bran, the water forms paste and rust. Insulated ducting, a casing drain and a fan located inside the building reduce this. Drain it daily.
Fire in the exhaust system
Husk and bran dust are combustible, and sparks from a husk-fired heater with a leaking exchanger can reach the dryer. A tight heat exchanger, spark screen on the hot air duct and a fan that can be isolated quickly limit the risk. Keep the cyclone hopper emptied.
Energy and control
Dal mills dry in batches and seasonal campaigns, with each batch starting wet and finishing nearly dry, so the moisture removed and the useful exhaust flow fall through the cycle. A variable speed drive lets the operator reduce exhaust flow toward the end of each pass, which saves both fan power and husk fuel. On mills running only part of the year, a belt drive and pulley change to the actual duty is a simple first step.
ID fan and FD fan in dal and pulse mill dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclone, before the exhaust stack | Before the heat exchanger, feeding the plenum |
| What it handles | Humid air with husk and bran dust | Clean ambient air |
| Temperature at the fan | 35 to 50°C | Ambient |
| Main design concern | Build-up and condensation | Steady flow and clean drying air |
| Control | Dryer suction | Drying air flow |
See also the Forced Draft (FD) Fan for dal and pulse mill dryers.
Questions buyers ask
What type of exhaust fan is used on a dal mill dryer?
A radial tipped or open backward curved centrifugal fan placed after a cyclone is usual on a dal mill dryer. Radial blades shed the oily husk and bran that build up on closed backward curved wheels. Belt drive makes it easy to match speed to the dryer, and an inspection door allows quick cleaning.
Why does my pulse dryer exhaust fan lose flow over the season?
Usually because husk and bran are building up on the blades and in the scroll, or the cyclone and duct are partly choked. Cleaning the fan and duct restores flow. Persistent build-up may need a different blade form. Check the cyclone hopper and duct bends first, since they often choke before the fan itself.
Is an ID fan necessary on a pulse mill dryer?
Not always. Many small dryers use only a supply blower and vent freely. An induced draft fan is added when dust in the mill, uneven drying or emissions from the dryer need to be controlled. It also allows the drying air flow to be set more precisely through each pass.
Information needed to quote an ID fan for dal and pulse mill 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. Column, bin or LSU dryer, and the grain holding capacity per batch.
- Heat source. Type of heater and fuel, and whether heating is direct or indirect.
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.