Document 50wwyKj484MqoM2OqvYqoL368

HEATINC VENTILATING AIR CONDITIONING CUIDE 1941 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. Either conditioning or drying are frequently made continuous processes 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 the second general classification, that is the control of the rate of chemical reactions, occurs in the manufacture ofrayon. The pulp sheets are conditioned, cut to size, and passed through a mercerizing process. It is essential that during this process close con trol of both temperature and relative humidity should be maintained. Temperature controls the rate of reaction directly; while the relative humidity maintains a constant rate of evaporation from the surface, of the solution and gives a solution of known strength throughout the mercerizing period. Another well-known example of this class is the drying of varnish which is an oxidizing process dependent upon temperature. High relative humidities have a retarding action on the rate of oxidization at the surface and allow the gases to escape as the chemical oxidizers cure the varnish film from the bottom. 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 so as to hold the surface of the dough open to allow the COt gases formed by the fermentation 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 CHAPTER 38. INDUSTRIAL AIR CONDITIONING skin cured and colored, after which 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 fixed and are 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, arid only through close atmospheric control can the best quality of the leaf be developed. CONTROL 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 pans for pills, gum and nuts, a heavy sugar solution is added to the turiibling 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 ap pearance unsightly; if too fast, the coating will chip through 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. , ELIMINATION OF STATIC ELECTRICITY The presence of static electricity is very detrimental to the satisfactory 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. Fortunately, this hazard is easily eliminated by increasing the relative humidity. 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 45 per cent or more.. As some machines consume a great deal of. power which is converted directly into heat, the temperature 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 681