How malt kilns work
Germinated barley, called green malt, leaves the germination vessel at about 42 to 46% moisture and is dried in a kiln to about 4%. The kiln is a deep bed, typically 0.8 to 1.2 m, on a perforated floor, through which heated air passes. The schedule has a free drying stage at about 55 to 65°C, when the air leaves almost saturated, a break point when the bed dries out, and a curing stage at 80 to 85°C for pale malt, or higher for darker malts. Air is heated indirectly by steam, hot water or an indirect gas or oil fired heat exchanger, because direct firing can form nitrosamines. A large main fan moves the air, and glass tube heat recovery from the exhaust preheats the incoming air.
Where the FD fan sits
- Fresh air intake
- Glass tube heat recovery
- FD (main kiln) fan
- Heater
- Plenum under floor
- Green malt bed
The main fan sits below the kiln floor or beside the kiln, drawing fresh air through the heat recovery unit and pushing it through the heater into the plenum under the perforated floor. In curing, part of the exhaust air is recirculated and mixed with fresh air before the fan. The pressure covers the bed, floor and heater, typically 80 to 200 mmWC.
Air at the FD fan
| Air temperature at the fan | Ambient to 40°C in free drying; 40 to 60°C with recirculation in curing |
| Air after the heater | 55 to 85°C, higher for dark malts |
| Dust | Clean fresh air; some dust in recirculated air |
| Humidity | Recirculated air raises humidity in curing |
| Pressure at the fan outlet | Typically 80 to 200 mmWC |
| Density at the design temperature | About 1.09 kg/m³ at 50°C, 91% 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 malt kilns
| Blade form | Backward curved or axial, for large flow |
| Material of construction | Mild steel with coating, or galvanised |
| Drive and arrangement | Direct coupled with a variable speed drive |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive on kiln schedule |
| Also specified | Inlet filter and access to the heat recovery unit |
What this FD fan has to cope with
Changing bed resistance
The green malt bed is dense and wet at loading, then shrinks and opens as it dries, so its resistance falls through the cycle. A fixed-speed fan gives too little air at the start and too much later. A fan selected across the range, with a variable speed drive following the kiln programme, delivers the right air at each stage.
Uneven drying of the bed
If the air is not spread evenly in the plenum under the floor, one part of the bed reaches the break point early while another stays wet, and the malt is uneven in colour and moisture. A good plenum layout and a fan working near its best efficiency point give uniform flow across the floor.
Fouled heat recovery
Dust from the recirculated air fouls the glass tubes of the heat recovery unit, raising the pressure the main fan must overcome and reducing the heat recovered. Regular washing of the tubes, and a fan with enough pressure margin to ride through a fouled period, hold the air flow.
Energy and control
The main kiln fan is usually the largest motor in a maltings and runs for most of each cycle. A variable speed drive that follows the kiln programme saves a large share of its power, because the air volume needed after the break point is well below the free drying volume. Heat recovery on the fresh air and recirculation in curing reduce heat use further.
ID fan and FD fan in malt kilns
| ID fan | FD fan | |
|---|---|---|
| Position | Above the bed, after heat recovery | Below the floor, before the heater |
| What it handles | Saturated or warm exhaust with rootlets | Fresh and recirculated air |
| Temperature at the fan | 25 to 85°C | Ambient to 60°C |
| Main design concern | Condensation and deposits | Even flow through changing bed |
| Control | Kiln schedule | Kiln schedule |
See also the Induced Draft (ID) Fan for malt kilns.
Questions buyers ask
What is the main fan in a malt kiln?
It is the forced draft fan that draws fresh air through the heat recovery unit and pushes it through the heater and up through the green malt bed. It is usually the largest fan in the maltings and decides how quickly the kiln can dry a batch.
Why is air recirculated during malt curing?
After the break point the air leaving the bed is warm and far from saturated, so much of its heat would be wasted. Recirculating part of it to the main fan inlet saves heat, and the main fan then handles warmer air, typically 40 to 60°C.
How does a VFD help a malt kiln fan?
The kiln needs a large air flow in free drying and less after the break point and in curing. A variable speed drive lets the fan follow the programme instead of being throttled by dampers, which saves power and gives better control of the bed.
Information needed to quote an FD fan for malt kilns
- 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.
- Kiln data. Batch size, bed depth and air flow required at each stage.
- Recirculation. Share of recirculated air in curing.
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.