Drying drum (rotary drum dryer)

01

Choose direct or indirect heating and the gas-flow direction

FEECO, a rotary dryer maker, describes direct rotary dryers that tumble material in a rotating drum in a stream of drying gas, set at a slight slope so gravity moves the material, with lifting flights that pick it up and drop it through the gas to maximise heat transfer; material and combustion gases can run co-currently (parallel) or counter-currently. Indirect dryers seal the drum from the combustion gases and heat material through the shell; FEECO describes them as less efficient but needed for inert-atmosphere processing and for ultra-fine material that would be carried into the exhaust. It says rotary dryers “typically operate at temperatures between 200°F and 2200°F (93 to 1200°C)”. [2]

The US EPA's AP-42 chapter on hot mix asphalt plants describes the same choices in aggregate drying: batch plants dry aggregate in a gas- or oil-fired rotary dryer whose flights shower the aggregate inside the drum; parallel-flow drum mix plants feed aggregate at the burner end and add asphalt cement midway in a lower-temperature zone; counterflow drum mix plants move material against the exhaust gases and put the mixing zone behind the burner flame. It reports that about 85% of plants then being built were counterflow drum mix designs, that "Most plants have the capability to use either gaseous fuels (natural gas) or fuel oil", and that counterflow plants can process recycled asphalt pavement at up to 50% with little or no effect on emissions. [1]

Editorial note: for an asphalt plant, state whether the drum only dries (batch plant) or dries and mixes (drum mix), and the recycled asphalt share; for other materials, give the supplier the maximum temperature the product tolerates and any dust or atmosphere constraints that point to indirect heating.

Give the supplier the material, feed rate and particle size, inlet and target moisture, maximum product temperature, bulk density and stickiness, required outlet temperature, fuel and burner preference, exhaust handling (baghouse, scrubber) and emission limits, site space and whether the drum must also mix (asphalt drum mix), plus power supply and controls.

02

Size the drum from capacity and moisture load

FEECO's sizing chart gives standard direct dryers from 0.9 m diameter, 6–9 m long, about 7 t/h and 5.5 kW, through 2.4 m by 15–21 m at about 73 t/h and 55–90 kW, to 4.3 m by 21–31 m at about 227 t/h and 225–300 kW, sprocket-driven up to 2.7 m and gear-driven above; capacities vary with the material and are based on granular fertilizer at 60 lb/ft³ with up to 10% moisture removal. Its listed features include diameters of 1–4.6 m and capacities of 1–181+ t/h. [2]

For asphalt plants, AP-42 notes that stockpiled aggregate moisture “typically stabilizes between 3 to 5 percent by weight” and that the rotary dryer is the largest ducted emission source, emitting evaporated water, particulate matter and combustion products. [1]

Editorial calculation: drying 200 t/h of aggregate to near zero moisture from 5% wet-basis moisture removes about 200 × 0.05 / 0.95 ≈ 10.5 t of water per hour, which the burner, drum and exhaust system must handle. Feed rate and moisture are assumed examples; give the supplier your wettest expected feed.

Burner tuning matters, but dryers differ from furnaces. DOE's Process Heating Tip Sheet #2 (Check Burner Air to Fuel Ratios, November 2007) says “Most high temperature direct-fired furnaces, radiant tubes, and boilers operate with about 10% to 20% excess combustion air at high fire”, that about 1 ft³ of air releases about 100 Btu with natural gas, propane or fuel oil, and that “For most systems, 2% to 3% of oxygen” with 10–50 ppm of combustibles in the flue gas indicates ideal operating conditions. It adds that processes that evaporate moisture or solvents need large amounts of excess air to ensure drying rates and carry vapours out, and that cutting excess air to minimal levels can slow the process and create an explosion hazard. Its worked example: at 2,400 °F flue gas, reducing excess air from 25% to 10% raises available heat from 22% to 29%, a fuel saving of 24%. [3]

Editorial note: ask the dryer supplier to state the design excess air or exhaust oxygen for drying, the burner turndown, and the expected fuel per tonne of water evaporated, rather than applying furnace air-fuel targets. [3]

03

Dryers sold for minerals, sand, fertilizer, biomass and manure

Listings include rotary dryers for minerals and ore, sand and slag, fertilizer granulation lines, biomass and sawdust pellet lines, and manure. None of the suppliers reviewed lists a complete asphalt plant; ask each for drum size, flight design, burner and fuel, drive and the capacity basis for your material.

04

Budget burner energy per tonne from feed moisture and mix temperature

A study by D. Peinado and co-authors from Carlos III University of Madrid and Intrame SA analysed the rotary dryer of a hot mix asphalt plant, firing heavy fuel oil, using mean operating values from commissioned plants. For the reference case (aggregate inlet moisture 3%, outlet 0.5%, mix leaving at 170 °C, exhaust gas at 100 °C, 8% filler), the burner needed 63.1 kWh per tonne of mix from 5.65 kg of fuel; energy efficiency was 89.63% and exergy efficiency 18.12%, and the flue gases were the main loss, at around 7%. [4]

The authors give 1–7% by mass as the inlet moisture range met in operating plants in Europe, and report that a 4% increase in water content raised the energy needed by the burner by 60%: about 8.21 kWh more per tonne for each 1% of moisture, and 2.62 kWh more for each 10 °C of mix temperature. Raising the air ratio from 1.25 to 4 times stoichiometric (2.25 in the reference case) cut energy efficiency by about 8%. Flue gas temperature cannot be lowered freely, to avoid reaching the dew point in the gas cleaning system, especially with high-sulphur fuel; the authors propose controlling exit gas temperature through the drum's rotational speed. [4]

Editorial calculation: at the paper's example production of 200 t/h, 63.1 kWh/t is about 12,620 kW of burner input (200 × 63.1), and a feed 2% wetter than the reference adds about 3,280 kW (200 × 2 × 8.21). An air ratio of 2.25 is 125% excess air ((2.25 − 1) × 100), far above the 10–20% DOE gives for “Most” high-temperature furnaces and boilers. Ask each supplier for burner rating and fuel per tonne at your wettest feed and target mix temperature. [4] [3]

05

Shell, lifting flights, burner, riding rings, drive, seals and exhaust handling

FEECO supplies the mechanical rows; AP-42 and DOE add the burner and exhaust points specific to asphalt and drying service.

06

Drum size and burner duty: FEECO chart values and the UC3M fuel figure

FEECO's chart gives the size, capacity and power ranges on a fertilizer basis (60 lb/ft³, up to 10% moisture removal); the fuel figure comes from the UC3M asphalt-dryer study.

07

Send the material, moisture and plant data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Material: [type; size; bulk density; temperature limit]
Rate: [t/h wet feed]
Moisture: [inlet; outlet]
Configuration: [direct or indirect; parallel or counterflow; drum mix]
Fuel: [gas; oil; other]
Exhaust: [collector; emission limits]
Site: [space; power]

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