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 ID fan sits
- Furnace or steam tube bundle
- Rotary drum
- Cyclones
- ID fan
- Scrubber or thermal oxidiser
- Stack
The fan usually follows the cyclones and pushes into a scrubber or thermal oxidiser, so it sees the fines the cyclones miss, often 0.5 to 2 g/m³. In steam tube dryers the gas volume is smaller and closer to saturation. The suction covers the drum, discharge hood and cyclones, typically 200 to 350 mmWC, and the fan discharge pressure must cover the downstream treatment.
Gas at the ID fan
| Gas temperature, normal | 100 to 130°C for direct-fired drums; 90 to 105°C for steam tube dryers |
| Maximum or upset | Up to 200°C or more during a drum fire or feed loss |
| Dust at the fan | Sticky, oily organic fines with syrup; 0.5 to 2 g/m³ after cyclones |
| Moisture and dew point | Very humid; dew point 70 to 85°C, especially in steam tube dryers |
| Corrosive species | Organic acids and VOCs; SO₂ from coal or biomass firing |
| Density at the design temperature | About 0.91 kg/m³ at 115°C, 76% 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 DDGS rotary dryers
| Blade form | Radial-tipped or open backward curved, shapes that shed sticky deposits |
| Material of construction | SS 304 or coated mild steel for acidic, humid exhaust |
| Drive and arrangement | Belt or coupled, variable speed |
| Bearings and cooling | Pedestal bearings with cooling disc |
| Flow control | Variable speed drive on drum suction |
| Also specified | Wash nozzles, drain, insulated casing, large inspection door, temperature and spark detection upstream |
What this ID fan has to cope with
Sticky deposits and imbalance
Syrup in DDGS fines makes them sticky and oily. They build up on the blades and in the casing and cause vibration within weeks. Back-mixing that keeps the feed dry enough, open blade shapes and regular washing through casing nozzles keep deposits under control.
Drum and duct fires
Overheated deposits in the drum or ducts can ignite and carry burning material to the cyclones and fan. Outlet temperature monitoring, spark detection and water deluge in the ducts, and a defined fan response during a fire, stop or contain it, protecting the downstream oxidiser or scrubber.
Condensation in steam tube dryer exhausts
The sweep air from steam tube dryers is nearly saturated, and condensation in the ducts and fan forms a sticky, acidic sludge with the fines. Insulated and heat-traced ducts, an insulated casing with a drain and a small sweep air margin prevent it.
Energy and control
DDGS dryers run with the distillery and follow the stillage flow, which is steady for long periods. A variable speed drive on the ID fan follows the load and holds drum suction. Steam tube dryers need much less air and allow heat recovery from the vapour, and evaporating more thin stillage to syrup reduces the water the dryer must remove.
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 Forced Draft (FD) Fan for DDGS rotary dryers.
Questions buyers ask
What is the difference between direct-fired and steam tube DDGS dryers?
Direct-fired drums pass hot furnace gas through the material and exhaust a large volume of gas at 100 to 130°C. Steam tube dryers heat the material through tubes and exhaust a small volume of nearly saturated air. The ID fan is larger in the first case and wetter in the second.
Why do DDGS dryer exhaust fans get dirty so fast?
Syrup in the product makes the fines sticky and oily. Fines that pass the cyclones build up on the blades and casing of the ID fan and cause vibration. Keeping the back-mix ratio right, washing regularly and using blade shapes that shed deposits keep the fan running.
How are fires prevented in a DDGS dryer exhaust system?
By limiting inlet temperature, back-mixing so wet feed does not stick, watching the outlet temperature, fitting spark detection and water deluge in the ducts, and cleaning deposits from the drum, cyclones and fan on a schedule.
Information needed to quote an ID fan for DDGS rotary 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. Direct-fired drum or steam tube dryer, and the downstream treatment.
- Product. Syrup ratio, feed and product moisture.
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.