Document nmVbQGJz1w6EO53BB5LxOxreX

American Society of Heating and Ventilating Engineers Guide, 1936 temperature of 80 F with a relative humidity of 65 per cent to hold the surface of the dough open, so that the C02 gases formed by the fermenta tion may pass out and produce a loaf of bread, when baked, of even, fine texture without large voids. Another example of a similar nature is found in the curing of macaroni. The flour and water mixture is fermented and dried. As it is necessary to have a definite amount of water present to carry on a fermentation process, the moisture must, be removed in a relatively short period to stop fermentation and prevent souring and in such a manner as to avoid setting up internal strains in the mixture. Best results are obtained with the correct cycles of both temperature and humidity. The curing of fruits, such as bananas and lemons, also come under this classification. Bananas are treated somewhat differently and to accom plish the required results, a cycle of temperatures and relative humidities is used. The starches in the pulp must be changed and the skin cured and colored. Then the fruit is cooled to maintain as slow a rate of metabolism as possible. Ideal conditions range between 55 to. 57 F, and in no case should the temperature go below 49 F, as the starches then become indigestible. 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 F to 59 F have been found to be best suited for this process. A high relative humidity, 88 to 90 per cent is necessary to hold shrinkage to a minimum and, at the same time, develop the rind so that it will be sufficiently tough to stand 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, the first three classifications of industrial air, conditioning are involved, and only through close atmospheric control can the best quality of the leaf be developed. CONTROL OF RATE OF CRYSTALLIZATION The rate of cooling of a saturated solution determines the size of the crystals formed. Both temperature and relative, humidity are of im portance, as the one controls the rate of^cooling, while the other, through evaporation, changes the density of the solution. In-the coating kettles 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 temperature and relative humidity. If the cooling and drying is too slow, the coating will be rough and semi-translucent, and the appearance unsightly; if too fast, the coating will chip thru to the interior. Only by balancing temperature, relative humidity, and volume of air to the sugar solution can the proper rate be obtained and a perfect coating assured. The foregoing is presented as typical of a few of the problems met with in industrial air conditioning. They are far from complete but, with the 716 Chapter 40--Air Conditioning for Industrial Processes help of a few of the natural laws, may act as a stimulus to the imagination . and assist in solving others. CONDITIONING AND DRYING In.general, the exposure of materials to desirable conditions 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 be classified 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. 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 combined in one process for the dual purpose of removing undesirable moisture and accurately regulating the final moisture content. Either conditioning or drying are frequently made continuous processes in which the material is conveyed through an elongated compartment by suitable means. ATMOSPHERIC CONDITIONS REQUIRED The most desirable relative humidity during processing depends upon the product and the nature of the process. As far as the behavior of the material itself and its desired final condition are concerned, each material and process represents a different problem. The best relative humidity may range up to 100 per cent. Similarly the most desirable temperature may range between wide limits for different materials and treatments. Extremes in either relative humidity or temperature require relatively expensive equipment for maintaining these conditions and controlling them automatically. Also, in departments where people are working, their health, comfort, and productive efficiency must be considered. A compromise often is desirable. , It is generally considered that relative humidities below 40 per cent are on the dry side, conducive to low regains, a brittle condition of fibrous materials, prevalence of static electricity, and a tendency toward dryness of the skin and membranes of human beings. At the other end of the scale, humiditiej above 80 per cent are relatively damp, conducive to high regains, extreme softness, and pliability. Table 2 lists desirable temperatures and humidities for industrial pro cessing. In using this table, care must be taken in qualifying the process. In preparing many materials, conditions are not maintained constantly, but different temperatures and humidities are held for varying lengths of time. GENERAL REQUIREMENTS In general, air conditioning apparatus for industrial purposes must be capable of absorbing heat from various sources such as machinery power, electric lights, people, sunlight and chemical reaction; of warming or cooling to any desired degree, and of giving or permitting ample air supply at all times. Refrigeration may or may not be required, depending upon natural conditions, the required relative humidity and the maximum 717