Document 85aMERmQQQvvdMzyBkdDYQ05K

HEATING VENTILATING AIR CONDITIONING GUIDE 1941 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. 4. 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 RELATIVE HUMIDITY IN ATMOSPHERE. PER CENT Fig. 3. Relation of Equilibrium Moisture Content in Wood to the Relative Humidity of Surrounding Air exothermally; a temperature rise and chemical action from within will bum 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. On 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; 702 CHAPTER 40. DRYING SYSTEMS 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 drier 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; for this reason after-cooling is not required, and reheating is reduced to a minimum. Complete descriptions of such dehumidifying systems are given in Chapter 23 on Cooling and Dehumidification Methods. Air Circulation As noted under Mechanism of Drying, air velocity is more important in the first two stages of drying than in the last, and for this reason zone drying in continuous driers is frequently considered. It permits accurate regulation of temperature, humidity, and velocity in the different zones. High velocity results in more rapid drying, more even distribution of temperature and consequently more even drying in the first period. Too high a velocity may be detrimental because of excessive power needed for creating it, or because the material may blow away if it is light and fluffy. In the drying of paints, varnishes, and enamels, high velocity or improper distribution of the air even with the use of filters, may cause dust already in the drier, to be blown against the material, ruining the finish. Table 3 presents data on drying of various materials. EQUIPMENT FOR DRYING Equipment for drying may be divided into the following classes: 1. Heat and humidity supply. 2. Methods of handling. 3. Ovens. The heat and humidity supply for low temperature work up to 250 F is often steam; steam coils either in the oven or outside, heat the 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 bustion space allowed to insure complete combustion of fuel. Humidity can be obtained in driers by the use of steam spray, humidifiers, qr recirculation. Methods of handling of material have been indicated in Table 1. For low temperature work up to 200 F ovens and driers are commonly built of two thicknesses of insulating board (fireproof preferred), with air space between. As the temperature increases materials better able to 703