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 ID fan sits
- Heater
- Plenum under bed
- Green malt bed
- Kiln top
- Glass tube heat recovery
- Exhaust (ID) fan
The exhaust fan sits at the top of the kiln or after the heat recovery unit, drawing air from above the bed. In kilns with a main fan below the floor, the exhaust leaves under the main fan's pressure, and an exhaust fan is only used to overcome the heat recovery unit. Pressure at the exhaust fan is typically 30 to 100 mmWC.
Gas at the ID fan
| Air temperature, normal | 25 to 35°C in free drying; up to 70 to 85°C in curing |
| Maximum or upset | About 100°C on dark malt curing |
| Dust at the fan | Rootlets, husk fragments and malt dust |
| Moisture and dew point | Near saturated in free drying |
| Corrosive species | Mild organic acids |
| 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. |
ID Fan selection for malt kilns
| Blade form | Axial or backward inclined, for large flow at low pressure |
| Material of construction | SS 304 or aluminium wetted parts |
| Drive and arrangement | Direct coupled with a variable speed drive |
| Bearings and cooling | Bearings outside the air stream |
| Flow control | Variable speed drive on kiln schedule |
| Also specified | Drain and inspection door |
What this ID fan has to cope with
Saturated air in free drying
For the first half of the cycle the air leaves the bed at 25 to 35°C and almost saturated, so water condenses on the duct walls and the fan casing for hours at a time. A stainless or aluminium wheel, a sealed casing with a drain at the lowest point and a duct that falls back towards the kiln stop water collecting in the fan.
Rootlets and husk on the wheel
Dried rootlets and husk fragments lifted from the top of the bed stick to the wet blades and to the glass tubes of the heat recovery unit. Wet deposits grow unevenly and unbalance the wheel. An open blade shape, an inspection door and washing the fan when the heat recovery tubes are washed keep both clean.
Duty that changes through the cycle
Air volume, temperature and humidity change from free drying to break point to curing, so a fan chosen for one stage runs far from its best point in the others. Selecting the fan across the whole cycle and fitting a variable speed drive that follows the kiln programme keep it stable and efficient.
Energy and control
Kilning is the largest heat and power user in a maltings, and each kiln runs an 18 to 24 hour cycle almost continuously. Heat recovery from the exhaust air to the incoming air saves a large share of the heat, and the exhaust fan has to overcome the recovery unit. A variable speed drive on the exhaust fan that follows the kiln programme, rather than a damper, saves power in the stages that need less air.
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 Forced Draft (FD) Fan for malt kilns.
Questions buyers ask
Does a malt kiln need an exhaust fan?
Many malt kilns use only a main fan below the floor, which pushes air up through the bed and out of the kiln. Larger kilns with glass tube heat recovery often add an exhaust fan to overcome the resistance of the recovery unit and to control the kiln top pressure.
What temperature is the malt kiln exhaust air?
During free drying the exhaust is cool, about 25 to 35°C, and close to saturation. After the break point it warms quickly, reaching 70 to 85°C in curing for pale malts and higher for darker or specialty malts. The fan must cope with both conditions.
Why do malt kiln exhaust fans corrode?
The exhaust air in free drying is saturated and condenses inside the fan for hours, and the condensate carries mild organic acids from the malt. Mild steel rusts quickly under these conditions, which is why stainless steel or aluminium wetted parts are preferred.
Information needed to quote an ID fan for malt kilns
- 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.
- Kiln data. Kiln batch size, bed depth and the drying schedule.
- Heat recovery. Heat recovery unit type and 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.