Document QXneDLVz1mZnZ5erwDxwKKdZE

American Society of Heating and Ventilating Engineers Guide, 1937 drier decreases as a transfer of heat to the material being dried takes place Where part or all of the heat is supplied by steam coils or other means within the drier itself, the drier is known as a constant temperature drier Driers using little air for heating medium with a high temperature drop are difficult to hold at uniform temperatures; the more air used, the easier it is to secure accurate control of temperature and humidity. Drier's may be classified as shown in Table 1. y 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 inta 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 surface so rapidly that the surface remains thoroughly wet, and evapora tion proceeds at a constant rate, precisely as from a free water surface. The material assumes a temperature corresponding to the wet-bulb temperature of the surrounding air, or slightly higher, due to radiation and conduction from dry surfaces adjoining the material. The constant rate period continues until a time when the moisture no longer comes to the surface as fast as it is evaporated. This point is called the critical moisture content. As the drying proceeds, a period of uniform falling rate is entered. During this period, the surface of the material is gradually drying but, and the rate of drying falls as the remaining wet "surface decreases in area. This period is also known as unsaturated surface drying. As drying continues, the surface is completely dry and the water from the interior evaporates and comes through the surface as vapor. As the plane of water recedes, the diffusion of the vapor becomes more difficult and hence the period is known as varying falling rate period, or subsurface drying. As drying progresses another point called equilibrium moisture content is reached, where the vapor pressure of the moisture in the air and the vapor pressure of the moisture in the material are equal, and drying ceases. The drying of a slab of whiting is shown in Fig. 2 and illustrates the principles pointed out above. The factors affecting the variations of drying rates during the above periods are pointed out in Table 2. Omissions in the Cycle Many solids, such as lumber, are so dry at the beginning of the drying operation that the constant rate period of free surface evaporation does not occur. Frequently the surface of the material is dry enough so that no surface drying can take place, in which case only the final stage of sub surface drying is involved. In other instances, the critical moisture con 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 . 742 Chapter 41--Drying Systems intermediate state of unsaturated surface drying does not occur and the drying is of the sub-surface type during practically the whole of the fading rate period. With other kinds 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 Table 2. Factors Influencing Drying Drying Period Temperature Constant Rate, Unsaturated Surface Sub-Surface Increase in temperature increases drying rate Increase in temperature in creases drying rate, because with decreased viscosity, dif fusion increases Humidity No effect until equilibrium con Drying rate increases as humidity tent is reached; drying then is decreased ceases Air Velocity Drying rate varies approximately as th*e 0.6 power of the velocity No effect Air direction Thickness of Material Drying rate increases the more nearly the air blows perpendicular to surface; for dead air film becomes thinner No effect Drying rate is not affected by the Drying rate varies inversely as thickness the square of the thickness falling rate period being confined solely in practice, to unsaturated surface drying. MOISTURE Moisture in the solid may be in either of two forms: 1. Capillary or free. 2. Hygroscopic or chemically combined. Free moisture is contained in the capillary spaces between the particles or fibers of the materials. The loss of this moisture changes only the weight of the material. Chemically combined or hygroscopic moisture is intimately associated with the physical nature of the material and its removal changes both the physical characteristics as. well as the chemical properties. The amount of hygroscopic moisture a material can contain is limited. This limit is called the fiber saturation point. When material is dried below this point, care must be exercised to avoid physical changes in the material, such as shrinkage, hardening, etc. All hygroscopic materials have definite equilibrium moisture contents dependent on temperature and humidity. Materials are frequently dried to a lower moisture content than those of equilibrium conditions in use, and allowed to regain the necessary moisture after leaving the drier to equalize the moisture in the material. Fig. 3l shows the equilibrium moisture content of wood. lU. S. Department of Agriculture Bulletin No. 1136. 743 v/