Document m3E32goaKOGnJJ8a1emnGL4B

822CHAPTER 45 -' 1948 Guide . Conditioning and Drying In general, the exposure of materials to desirable humidities for treat ment may be coincidental with the manufacture or processing of the materials, or they may be treated separately in special enclosures. This latter treatment may Seclassified as conditioning or drying. ' The purpose of conditioning or drying is usually to establish a desired condition of moisture content and to regulate the physical properties of the material. When the final moisture content is lower than the initial one, the term drying is applied (See Chapter 47). If the final moisture content is to be higher, the process is termed conditioning. In the case of some textile products and tobacco, for example, drying and conditioning may be com bined in one process for the dual purpose of removing undesirable moisture and accurately regulating the final moisture content. Frequently con ditioning or drying is made a continuous process in which the material is conveyed through an elongated compartment by suitable means and subjected to controlled atmospheric conditions. Control of Rate of Chemical Reactions A typical example of control of the rate of chemical reactions occurs in the manufacture of rayon. The pulp sheets are conditioned, cut to size, and passed through a mercerizing process: It is essential that during this process close control of both temperature and relative humidity should be maintained. The temperature controls the rate of reaction directly, while the relative humidity maintains a constant rate of evaporation from the surface of the solution and obtains a solution of known strength throughout the mercerizing period. Another well-known example in this class is the drying of varnish which . is an oxidizing process dependent upon temperature. High relative humidities have a retarding effect on the rate of oxidization at the surface and allow the internal gases to escape freely as the chemical oxidizers cure the varnish from within: This produces a surface free from bubbles and a film homogeneous throughout. Desirable temperatures for drying varnish vary with the quality. A relative humidity of 65 per cent is beneficial for obtaining the best processing results. Control of Rate of Biochemical Reactions In the field of biochemical control, industrial air conditioning has been applied to many different and well-known products. All problems involving fermentation are classed under this heading. As biochemistry is a subdivision of chemistry, subject to the same laws, the rate of reaction may be controlled by temperature. An example of this is the dough room of the modern bakery. , Yeast develops best at a temperature of 80 F. A relative humidity of 65 per cent is maintained to hold the surface of the dough .open to allow the carbon dioxide gases formed by the fermentation to pass through and produce a loaf of bread, when baked, of even, fine texture without large voids. The curing of fruits, such as bananas and lemons, also comes under this classification. Bananas require, a cycle of temperatures and relative .humidities for ripening. The.starches in the pulp of the fruit must be changed and the skin cured and colored, after which the fruit is cooled to maintain as low a rate of metabolism as possible.' Ideal storage conditions : range between 56 and 60 F with about 75 per cent relative'humidity and ventilation at the rate of three or four air changes per hour. ( j '' Industrial Air Conditioning 823 The curing of lemons is an entirely different, problem. Bananas are cured for a quick market, while lemons are held for a future.market. The process, therefore, varies in the temperature used. Temperatures from 54 to 59 F have been found to be best suited for this process. , A high relative humidity of 88 to, 90 per cent is necessary to hold shrinkage to a minimum and, at the same time, develop the rind so it will be sufficiently tough to permit handling. Tobacco from the field to the finished cigar, cigarette, plug or pipe tobacco, , offers another interesting example of what may be done by industrial air conditioning in the control of color, texture-and flavor. In the processing of tobacco, control of regain, of chemical and biochemical reactions is involved, and only through close atmospheric control can the best quality of leaf be developed. Control- of Rate of Crystallization The rate of cooling of a saturated solution determines the size-of the crystals formed. Both dry- and wet-bulb temperatures are of importance, as the one controls the rate of cooling, white the other, through evapora tion, changes the density of the solution. .. In the coating pans for pills, gum and nuts, a heavy sugar solution is. added to the tumbling mass. As the water evaporates, each separate piece is covered with crystals of sugar. A smooth,, opaque coating, is only accomplished by blowing into the kettle the proper-amount of air at the right dry- and wet-bulb temperatures. The wet-bulb depression deter mines the rate of moisture pick-up or drying by the air and may be termed the drying head. Of equal importance is the uniformity at which the. wetbulb is maintained. If this is allowed to vary, poor results wilt follow due to checking and cracking of the unfinished coating. Elimination of Static Electricity The presence of static electricity is very detrimental to the staisfactory and economical processing .of many light materials, such as textile fibers, paper, etc. It is also extremely dangerous where explosive atmospheres or materials are present. Fqrtunately, this hazard'is.miriimizi'.by,^in creasing the relative humidity if the material being processed is not damaged thereby. '.. . V' , .. In attempting to eliminate static electricity, it must be borne in mind that for successful elimination the air that actually comes in contact with the material in the machine must be at a relative humidity of 50 per cent or more. As some machines consume a great deal of power which is converted directly into heat, the teihperature in the machine may be considerably higher than the temperature adjacent to the machine where the relative humidity is normally measured. In such cases, the relative humidity in the machine. will be appreciably lower than that elsewhere in the room, and' it may be necessary to maintain a room relative humidity of 65 per cent, or even more, before the desired results can be obtained inside the equipment. CALCULATIONS The methods for determining the heating and cooling loads for the various industrial processes are similar to those outlined in Chapters 14 and 15. The several other chapters previously cited call attention to additional phases of calculation which may be heeded. Because of the