Drying chambers, types

Fig. 2.9. Basic types of freeze drying chambers. A, bell jar or vertical cylinder b, rectangular or cylindrical chamber with one (or 2) door(s) c, tunnel dryer, in which the trays are transported in and out by a system (shown as a carrier on a monorail).

All these plants are of the chamber type (see Fig. 1.88 (c)) with cooled and heated shelves and a condenser which can be separated from the drying chamber by a valve. Refrigeration and vacuum systems should be laid out for temperatures and pressures which can be expected under extreme experimental conditions, even if these extreme data may not be used in the production process. Pilot plants for pharmaceutical or medical products should be laid out differently than those used for food. 

Drying chamber shape predominantly is either conical or flat-bottomed. The flat-bottomed dryers remove the powder as it falls to the floor of the dryer by use of a rotating pneumatic powder discharger that functions as a vacuum cleaner. These dryers subject the product to significantly more heat than do the cone-bottomed dryers. While for many types of dry flavorings this additional heat is insignificant, thermally labile materials (e.g., natural flavorings - tomato, cheese, and numerous fruit juice based products) may suffer from the additional heat. 

Drying chambers are generally used for small quantities, and in cases where most of the work is done over night, without supervision. Fuel and steam consumption is therefore rather high. Figure 21 shows a standard chamber dryer with upward direction of the air. Figure 22, the same type with downward movement, which is some times desirable to reduce the speed of the air... 

Drying chamber damage This type of damage can occur when the chamber is subjected to significant vacuum when the exhaust fans are started up before the supply fans. 

The design of the spray drying chamber depends on the needed residence time (Table 23.12) as well as the type of atomizers nsed (Table 23.13). The mode of flow, e.g., concnrrent, counter-current, mixed flow, etc., depends on the desired characteristics of the product, as summarized in Table 23.14. Finally, Table 23.15 gives suggested spray dryer system layouts depending on the feedstock characteristics, e.g., presence of organic solvents, danger of fire or explosion, etc. 

Broad radial spray requires large drying chamber (cylindrical-conical type)... 

Spray Dryers. Spray dryers generally consist of a chamber through which heated air passes upward, countercurrent to the fall of finely divided droplets of the material to be dried. The spray of material is produced either by conducting the solution under pressure to spray heads or by turbine-type dispersers. The bottom of the drying chamber ordinariljf contains some form of conveyor for removing the dried material. Such dryers are employed in the manufacture of soap powder, milk powder, and similar materials. 

In the cocurrent flow configuration, the liquid spray and air pass through the drying chamber in the same direction, although spray-air movement in reality is far from cocurrent in initial contact. This type of contact is commonly used in a centrifugal atomization spray dryer. It can lead to product temperatures lower than those obtained by the other two flow patterns (Masters 1991). 

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