Rotary Dryers

Another type of dryer that is very much in evidence in the chemical and process industries is the continuous rotary dryer. This machine generally is associated with tonnage production and as a result of its ability to handle products having a considerable size variation, can be used to dry a wide range of materials. The principal sources of thermal energy are oil, gas, and coal. While typical inlet temperatures for direct-fired dryers using these fuels is in the order of 1200°F, in certain instances they may be as high as 1500-1600°F depending largely on the nature of the product handled. Where feed materials are thermosensitive, steam heating from an indirect heat exchanger also is used extensively. These dryers are available in a variety of designs but in general, can be divided into two main types: those arranged for direct heating and those designed for indirect heating. As seen from Figure 7, certain variants do exist. For example, the direct—indirect dryer uses both systems simultaneously.

Where direct heating is used, the product of combustion are in intimate contact with the material to be dried while in the case of the indirect system, the hot gases are arranged to circulate around the dryer shell. Heat transfer then is by conduction and radiation through the shell.

With the indirect—direct system, hot gases first pass down a central tube coaxial with the dryer shell and return through the annular space between the tube and the shell. The material being cascaded in this annulus picks up heat from the gases and also by conduction from direct contact with the central tube. This design is thermally very efficient. Again, while there are a number of proprietary designs employing different systems of airflow, in the main these dryers are of two types: parallel and countercurrent flow. With parallel flow, only high-moisture-content material comes into contact with the hot gases, and, as a result, higher evaporative rates can be achieved than when using countercurrent flow.

In addition, many thermosensitive materials can be dried successfully by this method. Such an arrangement lends itself to the handling of pasty materials since the rapid flashing off of moisture and consequent surface drying limits the possibility of wall buildup or agglomeration within the dryer. On the other hand, countercurrent operation normally is used where a low terminal moisture content is required. In this arrangement, the high-temperature gases are brought into contact with the product immediately prior to discharge where the final traces of moisture in the product must be driven off.

In both these types, however, gas velocities can be sufficiently high to produce product entrainment. They therefore would be unsuitable for low-density or fine-particle materials such as carbon black. In such cases, the indirect-fired function-type dryer is more suitable since the dryer shell usually is enclosed in a brick housing or outer steel jacket into which the hot gases are introduced. As heat transfer is entirely by conduction, conventional flighting and cascading of the material is not used. Rather, the inside of the shell is fitted with small lifters designed to gently turn the product while at the same time maintaining maximum contact with the heated shell.

Another type of indirectly heated dryer that is particularly useful for fine particles or heat-sensitive materials is the steam-tube unit. This dryer can be of the fixed-tube variety equipped with conventional lifting flights designed to cascade the product through a nest of square section tubes, or alternatively, a central rotating tube nest can be used. Figure 8 shows a fixed-tube rotary dryer, which normally has an electrical vibrator fitted to the tube nest in order to eliminate the possibility of bridging of the product with consequent loss of heat-transfer surface. Since the heat exchanger is positioned within the insulated shell in this type of dryer, the air rejection rate is extremely low and thermal efficiencies are high. In general, this design is suitable only for free-flowing materials.

A considerable amount of work has been done on the development of various types of lifting flights, all designed to produce a continuous curtain of material over the cross section of the dryer shell. Other special configurations involve cruciform arrangements to produce a labyrinth path. The object is to give longer residence times where this is

Parallel flow

Indirect/direct firing

Figure 7. Typical rotary dryer arrangements.

Indirect/direct firing

Counter-current flow

Indirect firing conduction type Materials Heat flow —

Figure 8. Fixed-tube rotary dryer.

necessary. When the diffusional characteristics of the material or other process considerations call for extended residence times, these machines no doubt will continue to find application.

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