Document QXB9V6p5mVdQNJ5RYV9wn0jzo

HEATING VENTILATING AIR CONDITIONING GUIDE 1943 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 sub-surface drying. Drying ceases when the equilibrium moisture content has been reached. The final moisture content of the product depends on the relative humidity of the air in contact with it. Equilibrium is established when the vapor pressure of the moisture in the air and. the vapor pressure of the moisture Table 2. Factors Influencing Drying Factor Temperature Humidity Air Velocity Air Direction Thickness of Material Dbting Period < Constant Bate, Unsatarated Surface Sub-Surface Increase in temperature increases drying rate Increase in temperature in creases drying rate, because with decreased viscosity, capil lary flow is increased. Drying rate increases as humidity is decreased r 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 film becomes thinner No effect Drying rate- is not affected by the Drying rate varies inversely as thickness the square of the thickness in the material are equal. The equilibrium moisture content varies with the hygroscopic properties of the material, (see table of Regain of Hygro scopic Materials, Chapter 39).. The drying of a slab of .whiting is shown in Fig. 5 and illustrates the principles referred to previously. The factors affecting the variations of drying rates during the periods mentioned are outlined 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 intermediate; state of unsaturated surface drying does not-occur and (he 752 CHAPTER 41. DRYINC SYSTEMS drying is of the sub-surface type during practically the whole of the falling 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 falling' rate period being confined solely in practice, to unsaturated surface drying. Temperature GENERAL RULES FOR DRYING 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. increases the drying time very much or causes a change of the physical properties of the material. It is often necessary to add humidity to the air in the initial stage Of drying. Lumber case-hardens, cracks, and warps if the outside is dried too fast. Ceramics crack if not heated through before drying commences. Elastic materials warp while others crack if not evenly dried. Many paints case-harden if not dried under high humidity. Oh the other hand, in the case of those materials whose physical or chemical properties require that they'be dried at relatively low tem peratures, high humidity tends to retard drying in the first stage and may even stop it altogether in the final stage. Where drying temperatures below 120 to 140 F are used, the drying rate may be highly dependent on. atmospheric humidity conditions. In such instances it is often desirable to dehumidify the air entering the dryer during periods of high atmos pheric humidity; where a high degree of uniformity is required, it is often possible to secure complete independence of atmospheric conditions by recirculating the air in a closed system which includes a suitable dehu-. midifier. For this purpose absorptive dehumidifying systems have the advantage of accomplishing the desired reduction of humidity without appreciably elevating or lowering the dry-bulb temperature of the air; 753