How DDGS rotary dryers work
Grain-based distilleries separate the stillage after distillation into wet cake and thin stillage; the thin stillage is evaporated to syrup and mixed back with the wet cake. The wet cake, at about 60 to 65% moisture, is dried to about 10% to make distillers dried grains with solubles (DDGS) for animal feed. Wet feed is back-mixed with dried product to reduce stickiness. Two dryer types are common. Direct-fired rotary drums use a gas, oil, coal or biomass furnace with inlet gas at about 450 to 650°C and exhaust at 100 to 130°C. Indirect steam tube dryers heat the material through rotating tube bundles, with a sweep of air removing vapour at about 90 to 105°C. The exhaust carries sticky, odorous organic fines through cyclones to the ID fan and a scrubber or thermal oxidiser.
Where the FD fan sits
- Air intake
- FD (combustion air) fan
- Furnace
- Mixing chamber
- Rotary drum inlet
The fan sits at the furnace and supplies combustion air to the burner or grate, with dilution air or recirculated exhaust mixed in a chamber before the drum to bring the gas to the inlet temperature. The fan pressure covers the burner or grate, the furnace and the mixing chamber. In steam tube dryers there is no furnace, and the sweep air enters the drum through openings at the feed end, drawn in by the ID fan.
Air at the FD fan
| Air temperature at the fan | Ambient, 20 to 45°C |
| Gas to the drum | 450 to 650°C, downstream of the fan |
| Dust | Ambient air; filtered intake |
| Pressure | Burner or grate and furnace |
| Fuel | Natural gas, oil, coal or biomass |
| 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 DDGS rotary dryers
| Blade form | Backward curved |
| Material of construction | Mild steel |
| Drive and arrangement | Direct driven, variable speed |
| Bearings and cooling | Standard bearings |
| Flow control | Variable speed drive with firing rate |
| Also specified | Intake filter |
What this FD fan has to cope with
Inlet temperature and fire risk
Too hot an inlet scorches DDGS and can start fires in the drum. Combustion and dilution air are measured and controlled together to hold the inlet temperature, and the fan is interlocked with the outlet temperature so firing is cut back if the outlet rises.
Recirculated gas is not the FD fan's job
Some dryers recirculate exhaust to lower oxygen and save fuel. That gas is humid, sticky and odorous, and is handled by a separate recirculation fan or taken from the ID fan discharge, not by the clean air FD fan.
Biomass and coal firing
Distilleries often fire biomass or coal in the dryer furnace. These need undergrate air at higher pressure than a gas burner and carry ash into the drum, which can affect DDGS quality. The fan arrangement and pressure follow the furnace type.
Energy and control
Firing rate follows the stillage flow from the distillery, which is steady for long periods. Speed control on the combustion air fan holds the air to fuel ratio and saves power against damper control. Where exhaust is recirculated in place of fresh dilution air, fuel use falls and oxygen in the drum drops, which also lowers the fire risk.
ID fan and FD fan in DDGS rotary dryers
| ID fan | FD fan | |
|---|---|---|
| Position | After the cyclones, before the scrubber or oxidiser | At the furnace |
| What it handles | Humid odorous exhaust with sticky fines | Ambient combustion air |
| Temperature at the fan | 90 to 130°C | Ambient |
| Main design concern | Sticky deposits and fire risk | Inlet temperature |
| Control | Drum suction | Firing rate |
See also the Induced Draft (ID) Fan for DDGS rotary dryers.
Questions buyers ask
Do steam tube DDGS dryers have an FD fan?
Not a combustion air fan. Steam heats the material through the tube bundle, and a small sweep air flow enters the drum through openings at the feed end, drawn in by the ID fan to carry the evaporated water away.
What is the FD fan on a direct-fired DDGS dryer?
It is the combustion air fan of the furnace that makes the drying gas. It supplies combustion air and often dilution air, handling ambient air only. Mixed furnace gas then enters the drum at about 450 to 650°C.
Why is the DDGS dryer inlet temperature limited?
High inlet temperatures scorch the product, darkening it and lowering protein quality and feed value, and overheated deposits in the drum can catch fire. The inlet temperature is set by the product and held by controlling combustion and dilution air together.
Information needed to quote an FD fan for DDGS rotary 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.
- Furnace. Fuel and furnace type, combustion and dilution air flows.
- Inlet temperature. Drum inlet temperature and any recirculation.
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.