Direct-indirect driers

Driers may be classified by the method of transferring the heat to the wet solid. Direct driers employ a stream of hot gas passing over or through the wet solid which vaporizes the liquid and carries it away, whereas indirect driers rely on the transfer of heat through a metal wall. The choice of drier depends upon many factors which include ... 

Method of supplying the heat necessary for evaporation of the moisture. In direct driers, the heat is supplied entirely by direct contact of the substance with the hot gas into which evaporation takes place. In indirect driers, the heat is supplied quite independently of the gas used to carry away the vaporized moisture. For example, heat may be supplied by conduction through a metal... 

Figure 9.4. Types of dryers cited in Tables 9.1 and 9.2. (a) Tray or compartment, (b) Vacuum tray, (c) Vertical agitated batch vacuum drier, (d) Continuous agitated tray vertical turbo, (e) Continuous through circulation, (f) Direct rotary, (g) Indirect rotary, (h) Agitated batch rotary (atmos or vacuum), (i) Horizontal agitated batch vacuum drier, (j) Tumble batch dryer, (k) Splash dryer. (I) Single drum, (m) Spray, (n) Fluidized bed dryer, (o) Pneumatic conveying (mostly after Nonhebel and Moss, 1971).

Heat may be transferred directly as in the burning of solid fuel mixed with the particulates in the sintering of ores or indirectly as in the combustion of fuel to produce hot gases in pellet hardening. External heat transfer may also take place across a metallic surface as in drum and belt driers and flakers. 

Novatec has developed two systems which can be applied to drying fillers. One is an indirect gas fired heater which can be used with any drier to improve the process economy. About 80% of the heating cost can be saved by the use of these heaters. Novatec also offers a portable drying/conveying system which conveys particulate materials through the drier and delivers them directly to the next process step. 

Over the years, many failures of plant systems and components have been attributed to solenoid-operated valve (SOV) problems. Several events have occurred in which SOV failures affected redundant safety components, multiple trains of safety systems, or multiple safety systems. SOVs are in wide-spread use in each nuclear power facility. They are used in safety-related systems indirectly as pilot operators working with control system fluid (such as pneumatic or hydraulically operated isolation valves) and directly in fluid systems (such as to vent the reactor vessel head or to supply air to the starting system for emergency diesel generators). Many SOVs are also used in non-safety-related systems that can significantly affect safety systems (such as plant instrument air drier systems). 

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