Document 1Q4nMv05BzLakkjxpaRzYMdOE

1042 CHAPTER 47 1954 Guide Industrial Drying Systems 1043 Actual dryer operation is somewhat more complicated because even-if radiation and conduction losses are neglected, the wet-bulb temperature of the air remains constant only as long as surface evaporation of water is taking place. When sub-surface evaporation' is occurring, some heat from the air is used to heat the material and hence, there is a drop in wetbulb temperature. Fig. 10 illustrates the drying process in a tunnel dryer in which the air is flowing parallel to the product.* In design calculations using Equation 26, the following steps outline the procedure: 1. The unit drying rate, pounds of water per hour, is determined from the experi mental drying time curve and the amount of product to be dried per hour. The drying time may also be approximated from previous experience. 2. The experimental data or experience also determine the drying condition, i.e., point L Fig. 9. This fixes Hi. Where experimental data are lacking L may be ap proximated from Regain Tables (see Chapter 45 Industrial Air Conditioning) since the relationship between the vapor pressure in the product and in the air at equilibrium for the desired final moisture content must prevail in the dryer. The air temperature must be not greater than the maximum permissible product temperature. Fig. 10. Temperature and Moisture Conditions in a Tunnel Drter Parallel Flow Air and Product 3. The rate of air circulation N. must be determined and also the supply air con dition S. In the design of some dryers, such as rotary or tunnel types, it is customary to determine S first and then to calculate the air rate N. In other types of dryers, such as tray dryers or through circulation dryers, where a fixed air velocity is main tained, N is calculated first and then point S is found. Because of the many vanables involved, it is generally not possible to select S except on the basis of past ex perience or on the basis of experimental drying tests. 4. The prevailing outside air conditions establish point A and hence, the line ALThe percent of recirculated air can then be calculated. 5. The physical arrangement of the dryer must then be selected to handle the desired quantity of product, and at the same time circulate the calculated air quantity at the desired velocity. 6. Equation 26 can then be used to calculate the heat requirements. DRYING METHODS AND EQUIPMENT Drying systems are sometimes classified according to the method of heat transfer that is employed, since the entire problem of drying resolves itself into individual problems of heat transfer and the thermodynamics ot air and water vapor. The methods of heat transfer are radiation, con duction and convection. Many types of dryers have been built on these principles for different purposes. Drying systems can also be classified, according to the method of product handling, as batch operation, semi-continuous and continuous. 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 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 Chapter 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. Lacquers and similar surface films can be very effectively dried by radiation. Special electric lamp units have been developed which give off a high percentage of infra-red and similar heat rays. For continuous manufacturing processes these units are mounted in tunnels through which conveyors pass. For local applications, as for example paint drying in automobile repair shops, they may be mounted on portable racks. Ob jects of relatively large surface area in proportion to their weight, and fabricated materials having a rather high heat absorption, may be satis factorily heated by such a source. For drying, baking, pre-heating and de-hydrating, where a low tem perature infra-red heat source is desired, or where use of glass-enclosed radiant lamps is objectionable for safety reasons, electric heating units employing low temperature metal sheathed resistors, are available. Conduction Drying Drying rolls or drums, Fig. 11,38 f4la5t s6u*rfaces, 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 agita tion is desirable to increase the uniformity of heating or to prevent over heating. 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 the air, 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. A rather interesting method of conduction drying was put into practical 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 py sublimation. The material to be dried is first frozen and then placed in a high vacuum chamber connected to extremely low temperature con densers. The water is removed by vaporizing from the solid directly 10 Ine gas without ever becoming liquid. Radiant Drying Convection Drying (Direct Dryers) ASun drying, the oldest form known to man, is still practiced where the material is amenable to such treatment, where the necessary time can be allowed, and where there is little danger of rain or atmospheric pollution- A limited amount of convection drying takes place in almost any dryer such as those described in the preceding paragraphs. However, to be