Document Ed0V2NZGqe10p48V3aX9xk42x

1172 CHAPTER 47 1958;Guide 3. Drying .tests, should ;be coiiducted:in prototype or laboratory units of the most promising equipment available. Sometimes a pilot plant is justified. 4. Summary of tests to evaluate quality and samples of the dried products. Some items may overshadow the operating cost, or the investment cost, such as: 1. Product quality, which should not be sacrificed. . 2. Dusting', solvent or other product losses. 3. .Space.limitation. . . ........ 4. Bulk density of product which may affect packaging cost. A comprehensive bibliography for special reading will be found in the article by S. J. Friedman.7 Radiant Drying " In artificial systems, radiating surfaces, heated by steam, electricity or other-means, afford a good .method of. heat distribution,and control. Ra diant heating sets up convection .currents, and in low-temperature dryers Industrial Drying Systems 11,73 use during the war. for.the drying of blood plasma, and. has since, been expanded to other fields'such as the preservation of bacteria: and, other micro-organisms. This has come to be known as freeze, drying or. drying, by sublimation. The material to be dried is first frozen,and thenplaced in a high vacuum chamber connected to extremely low; temperature con-: densers. The water is removed by vaporizing from the solid; directly to the gas without ever becoming liquid. Convection Drying (Direct Dryers) A typical convection dryer is composed of .an. air conditioning, system,: material handling system, and the dryer housing^ The normal:practice is to recirculate as much as 95 percent of the air, which is desirable from the standpoint of heat economy. There.is an optimum point;for the.perr, centage of recirculation, depending on the temperature and evaporation rate. ' ' . ,.\ Y. A limited amount of convection drying takes place in almost any clryer such as those described in the preceding piaragraphs. .Hdweyef; to be classified as a convection dryer the principal source of heat is the heated air or other gases circulated in the diyer. There are a number of mechan- WtT lUTCXUl only about one-third to one-half of.the total heat for evaporation is actually supplied to the material by radiation; At high temperatures the. radiation output increases rapidly, according to the fourth-power, law: . The total radiation may be computed by the equations and tables given in Chaptdr'5. In general, fins and irregular surfaces do not increase radiation, hence, 'the area to, be .used in calculations is the'area of a smooth-surface , envelope enclosing the radiating elements. ' .. A certain amount, of air circulation is required through a radiant dryer in order to carry off the vapor. . ............ . Radiant heat, from infra-red. lamps, has been . accepted; by certain in dustries as. practicable for their specific problems. An. example of success ful application is found in the drying of lacquers., . ni Conduction Drying '.'J ; Dryingtolls or drums, Fig. 6,5 flat-surfaces, open kettles and immersion heaters are examples of the direct-contact method. Intimate contact ;gf the material with the heating surface is important, and in some cases agita tion is desirable to increase the uniformity of heating or to prevent' over heating. . ' 1 A rather interesting method of conduction drying was put into practical. I Fig. 7. Cross Section and Longitudinal Section Through Circulation Dryer5 ical means of accomplishing this circulation' of air or gases, each of which has some particular virtue. Brief descriptions of some important types of convection dryers follow: Rotary Dryers. These dryers are cylindrical drums which cascade the material being dried through the air stream. (See Fig. 7). The dryers may be heated directly or indirectly,, and the air circulation may be paral lel or counter-flow. A variation is the rotating louver type diyer, which introduces the air beneath the flights thus securing very intimate contact. Cabinet and Compartment Dryers., These are generally considered, batch dryers wherein each charge is dried to completion before removal.6. A wide range includes types from the heated loft with, only, natural ,conT yection, and usually poor and non-uniform drying, to the .self-contained mats with forced draft and property designed baffles which, give positive results. It is also possible to evacuate some of the systems for low tem perature drying of delicate or hygroscopic materials. These dryers are usually loaded with material spread irftrays to increase the. exposed surface, the trays are loaded directly into the dryer or may be stacked on trucks which are wheeled in. (See Fig. 8) ., Tunnel Dryers. Tunnel dryers are a modification of the compartment aDrer, and as a rule are continuous or semi-continuous in operation. Heated ?lr dr combustion gas is usually circulated by means of fans, although a ew natural draft units are still-in use. The material is handled on trays J *ks on trucks, and moves through the diyer either, intermittently or ontinuously. The air flow may be parallel, counter-flow or acombina-