For the drying of liquids of liquid suspensions, two types of dryers can be used: film drum dryers for duties in the region of 600 lb/h for a larger dryer of about 4 ft diameter X 10 ft face length or large spray dryers (as in Fig. 4) with drying rates of approximately 22,000 lb/h. Where tonnage production is required, the drum dryer is at a disadvan-
tage. However, the thermal efficiency of the drum dryer is high in the region of 1.3-1.5 lb steam/lb of water evaporated and for small to medium production runs, it does have many applications.
Drum dryers usually are steam-heated, although work has been done involving the development of units for direct gas or oil heating. Completely packaged and capable of independent operation, these dryers can be divided into two broad classifications, ie, single-drum and double-drum.
Double-drum machines normally employ a "nip" feed device with the space between the drums capable of being adjusted to provide a means of controlling the film thickness. Alternatively, and in the case of the single-drum types, a variety of feeding methods can be used to apply material to the drum. The most usual is the simple "dip" feed. With this arrangement, good liquor circulation in the trough is desirable in order to avoid increasing the concentration of the feed by evaporation. Again, for special applications, single-drum dryers use top roller feed. While the number of rolls is related to the particular application and the material being handled, in general this method of feeding is used for pasty materials such as starches. Where the feed is very mobile, rotating devices such as spray feeds are used.
It will be seen from the Figure 5 drawings that there are a number of different feeding arrangements for drum dryers, all of which have a particular use. In practice, these variants are necessary owing to the differing characteristics of the materials to be dried and to the fact that no universally satisfactory feeding device has yet been developed. This again illustrates the need for testing, not only in support of theoretical calculations for the determination of the best dryer size but also to establish where a satisfactory film can be formed.
It must be emphasized that the method of feeding the product to the dryer is of paramount importance to selection or design. There are, of course, certain materials that are temperature-sensitive to such a degree that their handling would preclude the use of an atmospheric drum dryer. In such cases, special subatmospheric equipment may provide the answer, although the capital cost in relation to output generally would restrict its use to premium grade products.
As an alternative, the spray dryer offers an excellent solution to many drying problems. Many materials that would suffer from thermal degradation if dried by other methods often can be handled by spray drying owing to the rapid flash evaporation and its accompanying cooling effect. The continuous method of operation also lends itself to large outputs and with the correct application of control equipment, to low labor cost as well.
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