Rotary Drum Dryers, Incinerators

Rotary drum dryers, calciners, kilns, and incinerators tumble bulk material or pieces peripherally and lengthwise downhill, thus exposing all load surfaces, even 

Rejects are costly Even if you can recycle the material, you cannot recover the cost of the labor, machine time, or fuel put into the rejected piece. All have to be bought again. If the job is on a rush delivery schedule, you cannot buy the lost time again. More than one business has gone down the drain because they let minor dips in product quality slip through to their customers, and the customers never came back therefore, add reputation as another cost of rejects. 

Heat transfer in low-temperature rotary drums is largely by convection because radiation is naturally less intense in this temperature range. If the drum diameter is 5 ft (1.5 m) or more, radiation from triatomic gases can be helpful. However, many low-temperature rotary dryers use so much excess air (for moisture pickup) that the triatomic gas concentration is diluted signiflcantly. 

The lifters should not be used too close to the burner flame (1) because flame contact with the granules may be harmful and (2) because the hfe of the shelves would be shortened. Lifter flights have been as wide as 10% of drum inside diameter, but the greater widths require sturdier eonstruction to carry a deeper pile, which obstructs gas flow. Many short, closely spaced flights make it difficult for maintenance persons to walk through the cold drum to inspect it. Parts 4 and 5 of figure 4.4 show the use of suspended chains to heat up when hanging across the hot gas stream, and then heat the load in the bottom of the dram by conduction (contact). 

Care must be exercised in operating rotary drams so that the hot gas velocity is not too high relative to the size and weight of the granules, as that may cause carry-over into the exhaust (particulate emissions). 

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Fig. 1.10. Rotary drum dryer/kiln/furnace for drying, calcining, refining, incinerating granular materials such as ores, minerals, cements, aggregates, and wastes. Gravity moves material cocurrent with gases. (See fig. 4.3 for counterflow.)...

Fig. 4.3. A rotary drum dryer, kiln, incinerator, or furnace transports granular loads left to right) by gravity and rotation, counterflow to the burner gases and induced air. Parallel flow orco-current flow (fig. 1.10) can be used with some load materials and processes.

Inclined rotary drum furnaces, kilns, incinerators, and dryers often use long type F or type G flames (fig. 6.2). If drying is involved, substantially more excess air than normal may be justified to provide greater moisture pickup ability. (See fig. 1.10.)... 

Continuous dryers, ovens, incinerators, and furnaces take any of a variety of forms such as rotary drum, tower, shaft, tunnel oven, multihearth (Herreshoff) kiln, and fluidized bed. As with all continuous furnaces, their design is very dependent on how the load(s) can be moved through the furnace (or occasionally, how the furnace can be moved over the loads). 

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