Document NGKQ2N7LX3Vzz3EgzGjV6bj0p

714 Chapter 41 1945 .Guide ' tent of a wet solid is sufficiently low that sub-surface drying starts almost immediately after the conclusion of the constant rate -period. Thus the intermediate state of unsaturated surface drying does not occur and the drying is of the sub-surface type during- practically the. whole of the falling rate period. With other lands of material',-particularly thin sheets,' such as newsprint paper, sub-surface drying may occur at such a low moisture content that it is not encountered in commercial work, the falling rate period being confined in practice to unsaturated surface drying. ` Table 2. Factors Influencing Drying Factor' Temperature , Humidity Air Velocity Air Direction Thickness of Material Drying-Period Constant Bate, Unsaturated Surface Sab-Surface Increase in temperature in Increase in temperature increases creases drying rate, because drying rate with decreased viscosity, capil lary flow is increased. Drying rate increases as humidity is decreased No effect until equilibrium con tent is reached; drying then ceases Drying rate varies approximately as the 0.6 power of the velocity No effect Drying rate increases, the more nearly the air blows perpendicular to surface; for dead air him becomes thinner No effect Drying rate is not affected by the Drying rate varies inversely as thickness the square of the thickness GENERAL RULES FOR DRYING Temperature The highest temperature possible should be used because of faster drying and smaller requirements for ventilation. The amount of moisture that can be carried by a pound of air increases rapidly, with rise in tem perature as shown in the humidity chart of Fig. 6. Too high a tempera ture may-cause spoilage.of materials; many materials calcine or change their chemical properties if heated too hot; gypsum and glauber salts lose some of the chemically combined water, fall apart, and change their chemical properties. Too high or rapid rise in temperatures in drying lumber or.ceramics may create a liquid vapor tension within the material so high that the cells explode, causing permanent injury to the fiber. If too high a temperature is used on some chemicals, they begin to react exothermally; a temperature rise and chemical action from within will burn the materials, e.g., bakelite products, gunpowder, etc. During the constant rate period of drying, the material heats only to the wet-bulb temperature of the surrounding air, consequently high temperatures will not injure the material in this stage. Humidity Moisture in the drying air may be very important. - Many materials tend to case-harden, dry on the outside, forming a skin which retards the moisture flow from the inside to the surface, or stops it completely, and so Drying Systems 8o Oo U31VM 81 M3d nia 715 8I