Document ExKvVLw3G8mKqN5271XNb5mJj

830 CHAPTER 45 1946 Guide of 50 to 55 per cent. As the cotton fiber comes to the spinning operation, more flexibility is needed and the relative humidity is increased in this department. For many years, 65 per cent relative humidity was con sidered the optimum. To offset the extra work performed on the fiber as the spindle speed is increased; many cotton mills now carry 70 per cent. relative humidity in the spinning rooms *. Winding, warping and weaving are all processes calling for great flexibility and a consequent need for higher humidity. Other textile fibers, due to their different natural characteristics, are processed under relative humidities and temperatures applicable to each.. Rayons, on account of great loss of strength with the higher regains, should be processed in a relative humidity of 55 to 70 per cent. Acetate silk, another chemical fiber, with approximately 50 per cent of the regain of rayon, may be processed between 60 and 65 per cent relative humidity. All hygroscopic materials when in the state of absorbing moisture from the surrounding air produce a sensible heat rise to the air equivalent to the latent heat released by air to the material. This adiabatic conversion may account for a small percentage of the total heat load of the con ditioned space. 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 & desired condition of moisture content and to regulate the physical properties of the material. When the final moisture content i? 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 regu lating the final moisture content. Frequently conditioning 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 ol Rate of Chemical Reactions A typical example'of the second general classification, the 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 tem perature 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. - . . 1 .. . .. 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 Industrial' Air Conditioning 831 varnish vary with the quality. A relative humidity'of 65 per cent is beneficial for obtaining die 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 sO as to hold the surface of the dough open to allow the carbon dioxide gases formed by the fer mentation to pass through and produce a loaf of bread, when baked, of even, fine texture without large voids. Another example of a similar process is found in the curing of maca roni. 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 fer' mentation 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 comes 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 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 conditions range between 55 to 57 F and in no case should the temperature go below 49 F, as the starches then would become fixed and be 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 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, the first three classifications of air con ditioning 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 crys tals formed. Both dry- and wet-bulb, temperatures are of importance, as the one controls the rate of cooling, while the other, through evaporation, 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.