Concluding my series on general drying systems, in this column I will continue my discussion of rotary dryers.
Rotary Tube Furnace Dryers. An indirect dryer that allows a high degree of temperature control, a rotary tube furnace (RTF) dryer consists of a muffle furnace with a steel drum passing through it. Tumbling or rolling flights rather than the lifting flights such as those in the cascade rotary dryer are fitted to the inside of the drum. In operation, the particles are exposed to the drum surface, which is heated from the outside by a suitable heat source such as gas burners or electric elements. The internal flights tumble and mix the product, constantly exposing new surfaces to the heated drum surface. In addition there is a high degree of conduction between the product particles to enhance operation efficiency. This same principle is the basis for indirect calciners and reactors, which will be a topic for a future column.
In all rotary dryers the speed of advancement of the material -- and hence its retention time in the dryer -- is determined by the rotational speed of the drum as well as its angle of inclination. By varying these parameters, the residence time can be controlled accurately. The amount of material in the drum at any one time -- the drum fill -- is relatively low as a percentage of the cross-sectional area or total volume of the drum. They are typically of the order of 8 to 15 percent of the total volume of the drum.
Rotary dryers are continuous processing machines that can effectively process feeds that are classified as powders, granules, nonfriable agglomerates and large solid particles. Some -- the direct cascade and tube furnace, for example -- are able to cope with wide variations in the feed such as particle size and moisture. Depending on the configuration of the particular dryer, the feed and carrier inlets, discharges (or both) need to be well sealed to prevent the introduction of cold air into the system or the expulsion of hot, dust-laden air to the atmosphere. They operate at varying feed rates from several pounds to hundreds of tons per hour.
Process FlexibilitySome of these dryer designs increase productivity because they are able to integrate specific process requirements. For example, integral cooling is easily accommodated in all of the types described except steam tube and multipass cascade units. For high temperature or corrosive applications, internal refractory linings can protect the fabric of direct-cascade units. Hammers, chains and knockers will relieve the buildup of sticky products to the flights and internals of cascade, multipass and tube furnace dryers.
Heat sources include steam, electricity, coal, liquid fuels and gas. In the United States, however, most large-scale rotary cascade and louver dryers use gas due to the relatively high cost of electricity and the costs associated with treatment of the products of combustion for coal and heavy fuel oil burners to meet EPA requirements. Indirect rotary tube furnaces use gas frequently and use electricity more often than conventional cascade type dryers. Obviously, steam tube dryers use steam but may also use other thermal oils.
As discussed above, rotary dryers have a dynamic airflow that can principally be either co-current or countercurrent. In either instance, an induced-draft exhaust fan normally achieves extraction of the carrier. For conventional direct rotary dryers such as the cascade or louver, dust collection systems are essential. The technology for these systems includes primary and secondary collection devices such as cyclones, bag houses, scrubbers and/or electrostatic precipitators.
Control of these systems is either by PLC or solid-state controllers. The system traditionally modulates the inlet temperature (energy) based on the discharge temperature. Controls for the modulation of feed also can be incorporated into the design architecture.
LimitationsRotary dryers do have some significant limitations, especially if the technology is misapplied. Wet and sticky products cause clogging of the inlet and transfer section of the dryer drum. Flights often are clogged, reducing their carrying capacity and the volume of the curtains. Chains and knockers can mitigate this somewhat, but industry spends far too much valuable processing time digging out "blocks" of built-up material.
Like the hammers and chains, processing large particles causes noise, and the impact of the particles from the fall may cause size reduction. Cascade dryers have difficulty providing accurate temperature control, particularly if there is a variation in the feed characteristics. This results in variations in the dried product characteristics and, most commonly, the final moisture. Direct cascade dryers are simple machines with aggressive material handling. This can result in significant wear and high maintenance costs. In addition, a large rotating mass such as a drum has numerous high maintenance aspects and components. Well designed, engineered and maintained, these are controlled. This, however, is more the exception than the rule, and quick-fix patches do nothing to improve the reputation of these systems.
Because the dryer drum is large and rotating, insulation is frequently omitted for cost or other reasons. Although insulation is only really effective in the first 25 to 30 percent of the length due to the rapid decrease of temperature, not having insulation compromises the system's thermal efficiency, with high losses associated with the drum shell. Large volume machines require large real estate.
There is no other technology that can offer the same complete benefits to a high volume process with wide variations in the feed. Rotary cascade dryers are the "donkeys" of the drying industry: They are relatively low cost machines that are effective when applied correctly for the specific operation.
The rotary dryer family offers a comprehensive choice in processing. Each has a specific niche and when properly applied offers significant overall advantages for the application.