Drying 711 a constant volume core, it causes the material to warp, check, crack or therwise change its structure. Moreover, the reduced moisture content in the hardened outer layer increases the resistance to diffusion. In the end, the superficial hardening, combined with the decrease in diffusive movement, make the layer on the surface practically impervious to the flow of moisture, either as liquid or vapor. This is called case hardening All these problems can be minimized by reducing the drying rate, thereby flattening the moisture gradient into the solid. Since the drying behavior presents different characteristics in the two periods--constant-rate and falling-rate--the design of the dryer should recognize these differences, i.e substances that exhibit predominantly a constant-rate drying are subject to different design criteria than substances that exhibit a long falling-rate period Since it is more expensive to remove moisture during the falling-rate period than during the constant-rate one, it is desirable to extend as long possible the latter with respect to the former. Particle size reduction is a practical way to accomplish this because more drying area is created An analysis of the laws governing drying is essential for a good dryer design, therefore, it is important to note that, due to the complex nature of solid phase transport properties, only in a few simple cases can the drying rate (and drying time)be predicted with confidence by the mathematical expres sions reported above. In these cases, one usually deals with substances that exhibit only, or primarily, constant-rate drying For materials that present a non-negligible falling-rate period, the of specific mathematical equations is subject to a high number of uncert ties and simplifying assumptions are generally required It is clear that the purely mathematical approach for designing a drying plant is not possible, given the present state of knowledge 3.0 EQUIPMENT SELECTION Several methods of heat transfer are used in the dryers where all the heat for vaporizing the solvent is supplied by direct contact with hot gases and heat transfer by conduction from contact with hot boundaries or by radiation from solid walls is negligible, the process is called adiabatic, ordirect drying In indirect or nonadiabatic drying the heat is transferred by conduc tion from a hot surface first to the material surface and then into the bulk This chapter discusses only indirect drying The problem of equipment selection can be very complex; different factors must be taken into consideration, for example, working capacity, ease of cleaning, hazardous material, dryer location and capital cost(see Fig. 2)Drying 711 a constant volume core, it causes the material to warp, check, crack or otherwise change its structure. Moreover, the reduced moisturecontent in the hardened outer layer increases the resistance to diffusion. In the end, the superficial hardening, combined with the decrease in diffusive movement, make the layer on the surface practically impervious to the flow of moisture, either as liquid or vapor. This is called case hardening. All these problems can be minimized by reducing the drylng rate, thereby flattening the moisture gradient into the solid. Since the drying behavior presents different characteristics in the two periods-constant-rate and falling-rate-the design of the dryer should recognize these differences, Le., substances that exhibit predominantly a constant-rate drying are subject to different design criteria than substances that exhibit a long falling-rate period. Since it is more expensive to remove moisture during the falling-rate period than during the constant-rate one, it is desirable to extend as long as possible the latter with respect to the former. Particle size reduction is a practical way to accomplish this because more drying area is created. An analysis of the laws governing drying is essential for a good dryer design, therefore, it is important to note that, due to the complex nature of solid phase transport properties, only in a few simple cases can the drying rate (and drying time) be predicted with confidence by the mathematical expres￾sions reported above. In these cases, one usually deals with substances that exhibit only, or primarily, constant-rate drying. For materials that present a non-negligible falling-rate period, the use of specific mathematical equations is subject to a high number of uncertain￾ties and simplifying assumptions are generally required. It is clear that the purely mathematical approach for designing a drying plant is not possible, given the present state of knowledge. 3.0 EQUIPMENT SELECTION Several methods of heat transfer are used in the dryers. Where all the heat for vaporizing the solvent is supplied by direct contact with hot gases and heat transfer by conduction from contact with hot boundaries or by radiation from solid walls is negligible, the process is calledadiabah'c, or direct drying. In indirect or nonadiabatic drying, the heat is transferred by conduc￾tion from a hot surface, first to the material surface and then into the bulk. This chapter discusses only indirect drying. The problem of equipment selection can be very complex; different factors must be taken into consideration, for example, working capacity, ease of cleaning, hazardous material, dryer location and capital cost (see Fig. 2)