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 FD fan sits
- Inlet
- FD fan
- Heat exchanger
- Hot air duct
- Dryer plenum
- Grain column
The blower sits before the heat exchanger, so it handles cool ambient air and the exchanger runs under positive pressure. That arrangement is deliberate: if the exchanger tubes leak, clean air leaks into the flue gas side, not smoke into the drying air. Downstream the blower must overcome the exchanger, ducting, plenum and the grain column itself, typically 80 to 200 mmWC depending on grain depth and size.
Air at the FD fan
| Air temperature at the fan | Ambient, 25 to 45°C |
| Air after the heater | 60 to 90°C, downstream of the fan |
| Dust | Mill air with husk and bran dust; inlet screened |
| Pressure at the fan outlet | Typically 80 to 200 mmWC |
| Humidity | Higher ambient humidity in the monsoon lengthens drying but changes the fan duty little |
| Density at the design temperature | About 1.13 kg/m³ at 40°C, 94% 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 dal and pulse mill dryers
| Blade form | Backward curved for efficiency on clean air |
| Material of construction | Mild steel |
| Drive and arrangement | Belt driven or direct coupled |
| Bearings and cooling | Standard pedestal or motor bearings |
| Flow control | Variable speed drive, or an inlet damper on smaller dryers |
| Also specified | Inlet screen drawn from a clean area, and an air flow or pressure indicator at the plenum |
What this FD fan has to cope with
Smoke and odour in the grain
If hot air is drawn through a leaking exchanger, or a direct-fired heater is used, smoke taints the dal and spoils its colour. Placing the blower before the exchanger so it pressurises the air side, and checking the tubes each season, keeps the drying air clean.
Uneven drying through the column
Grain size and depth vary between pulses and batches, and air takes the easiest path. A fan with a rising pressure curve holds flow when the column is deep or packed with small grains, and plenum pressure readings show when air is bypassing. Rotate the grain regularly.
Husk fibre on the impeller
The mill floor is dusty, and a blower drawing from inside the building collects husk fibre on the blades. An inlet screen and an intake from outside the dehusking area prevent unbalance and loss of flow. The screen is checked weekly in the season, since a choked screen reduces drying air without any obvious sign at the fan.
Energy and control
Grain drying temperature and flow are set by the pulse being processed and how many wetting cycles remain. A variable speed drive allows lower flow for small grains and late in a pass, and reduces husk fuel as well as fan power. In mills that switch between sun drying and mechanical drying with the weather, efficient part-load running of the blower matters more than the peak efficiency figure.
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 Induced Draft (ID) Fan for dal and pulse mill dryers.
Questions buyers ask
What fan is used for hot air in a dal mill dryer?
A backward curved centrifugal blower is usual. It pushes ambient air through an indirect heat exchanger fired with husk or wood, and the hot air then passes through the grain. The fan sees only ambient air. A screened inlet keeps husk fibre off the impeller, and a belt drive makes speed changes easy.
Should the hot air blower be before or after the heat exchanger?
Before. The forced draft fan then handles cool air and pressurises the air side of the exchanger, so any tube leak sends clean air into the flue gas rather than smoke into the drying air and grain. It also keeps the blower out of the hot air, so standard bearings and mild steel are sufficient.
What air temperature is used to dry pulses?
Typically 60 to 90°C air, with the grain kept below about 60°C. Hotter air dries faster but cracks the grain and lowers the yield of whole dal. The right temperature depends on the pulse and the number of passes remaining. Chickpea and pigeon pea tolerate different conditions, so mills usually set temperature by experience and check the grain temperature at the dryer outlet.
Information needed to quote an FD fan for dal and pulse mill 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.
- Pulses handled. Pulses dried, grain depth and the moisture removed per pass.
- Heater. Heat exchanger type and its air side pressure drop.
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.