Selection

The application ratings given in Table 1 in which are listed the approximate mean evaporative rates for products under a generic classification are based on APV experience over a number of years in the design and selection of drying equipment. It should be appreciated, however, that drying rates vary considerably in view of the variety of materials and their widely differing chemical and physical characteristics. Furthermore, drying conditions such as temperature and the moisture range over which the material is to be dried have a very definite effect on the actual evaporative rate. It is important, therefore, when using the figures quoted that an attempt is made to assess carefully the nature of the product to be handled and the conditions to which it may be subjected in order to achieve greater accuracy. With these factors in mind, it is hoped that the foregoing observations on drying techniques along with the appropriate tables and curves will provide a basis for making an assessment of the type, size, and cost of drying equipment.

In making a preliminary assessment for dryer selection, there are a number of further points to consider:

1. What is the nature of the upstream process? Is it feasible to modify the physical properties of the feed, eg, mechanical dewatering to reduce the evaporative load?

2. Does the quantity to be handled per unit time suggest batch or continuous operation?

3. From a knowledge of the product, select the type(s) of dryer that it appears would handle both the wet feedstock and the dried product satisfactorily and relate this to the equipment having the highest application rating in Table 1.

4. From a knowledge of the required evaporative duty, ie, the total mass of water to be evaporated per unit time and by the application of the approximate Eav figure given in Table 1, estimate the size of the dryer.

5. Having established the size of the dryer on an area or volumetric basis, refer to the appropriate curve in Figures 13 and 14 and establish an order of cost for the particular type of unit.

Although a great deal of fundamental work has been carried out into the mechanics of drying, which provides a basis for recommendations, it is most desirable for pilot plant testing to be done. This is necessary not only to support theoretical calculations but also to establish whether a particular dryer will handle the product satisfactorily. In the final analysis, it is essential to discuss the drying application with the equipment manufacturer who has the necessary test facilities to examine the alternatives objectively and has the correlated data and experience from field trials to make the best recommendation.

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