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710 Chapter 41 1945 Guide Conduction or Direct Contact drying Drying rolls or drums, flat surfaces,, open kettles and immersion heaters are examples of the direct-contact method. Intimate contact of the material with the heating surface is important, and in some cases agitation is desirable to increase the uniformity of heating or to prevent overheating. Greatest resistance'to heat transfer occurs on the air side of the material being dried. The rate of heat transfer from the surface of the heated material to theair, and hence the rate of drying, may be increased by: (a) forced convection or air circulation and (b) vacuum operation to lower the boiling point of the liquid being evaporated. Drying Systems 7H. Simple drying ovens are often used for drying smaller quantities of material. The heat and humidity supply for low temperature work up to 250 E is Often steam; steam cOils either in the oven or outside, heat die air used for drying. Circulation of heated oil is used to a limited extent, but the danger of leaks is serious, for if the. oil is hotter than the flash point, a fire may start if the oil is released to the atmosphere. In many cases where steam is not available, direct or indirect-fired heaters are used with gas or oil as fuel. Indirect heaters should be carefully selected from a standpoint of.long life and efficiency. The heat exchange surface should be adequate . in area and easily accessible for . cleaning and removal. For extremely high temperatures, alloy surface may be used. With direct-fired equip ment care must be used in the selection of burners and sufficient com Convection or Air Drying The circulation of heated air or other gases over the material being dried is termed convection drying. Some convection drying occurs in practically all types of dryers, but if the main source of heat is from the air or gases, the dryer may be called a convection dryer.. Typical forms or examples may be enumerated: 1. Rotary drum dryers, (Fig. 1). 2. Tunnel or oven dryers, batch type. 3. Tunnel dryers, conveyor type, (Figs: 3 and 4). 4. Tower or column dryers. 5. Through-circulation dryers, (Fig. 2). In any type of convection dryer the heat transfer and hence the drying depends primarily upon the surface area of material exposed to the air. and the velocity of the air over the surfaces (see Dryer Calculations, later). A general classification of several types of dryers is given in Table 1. The chief basis of classification in this table is that Of intermittent or , batch operation as oppOsed to continuous operation. Another important basis of classification would be the method of handling the material to be dried. In ah effort to secure maximum contact of the air with the prod-, uct, as well as uniformity of heating, the effectiveness of cascading the material in an inclined drum or of blowing heated air through a bed of granular or pre-formed material is at once apparent, and where con tinuous drying at high capacity is required, these types are preferred. Extensive experimental studies on both types are available, (see References). bustion space allowed to insure complete combustion of fuel. Humidity can be obtained in dryers by the use of steam spray, humidifiers, or recirculation. . For low temperature work up to 200 F ovens and dryers are commonly built of two thicknesses of insulating board (fireproof preferred), with air space between. As the temperature increases materials better able to withstand the heat must be used. Metal lined ovens are easy to keep clean, and many high temperature dryers up to 1000 F are made of metal panels with insulation between. Care should be taken to avoid through . metal (metal extending through the oven wall from inside to out). Batch type ovens are entirely closed while in use and control of air leakage is easily taken care of. In the continuous dryer where the ends are open, heat and air leakage becomes important. Warm air leaking out of the ends of ovens means a heat loss, and often the temperature and humidity outside the oven becomes unbearable. For this reason, inclined or bottom entry ovens are used, as the warm air leakage can be more easily con trolled. See Figs. 3 and 4. MECHANISM OF DRYING The modern theory of drying may be summed up as follows: Assuming uniform velocity and distribution of air at a constant temperature and humidity over the surface to be dried, the drying cycle will be divided into two distinct stages: 1. Constant rate period. 2. Falling rate period. The constant rate period occurs while the material being dried is still very wet, and continues as long as the water in the material comes to the