Document qmdaVJ2G9ObO16akbB0bMeqQM

HEATING VENTILATING AIR CONDITIONING CUIDE 1943 organic and inorganic materials when in equilibrium at a dry-bulb tem perature of 75 F and various relative humidities. The effect of relative humidity on regain of hygroscopic substances is clearly indicated. The effect of temperature is. comparatively unimportant. In the case of cotton, for instance, an increase in temperature of 10 F has the same effect on regain as a decrease in relative humidity of one per cent. Changes in temperature do, however, affect the rate of absorption or drying. Sudden changes in temperature cause temporary fluctuations in regain even when the relative humidity remains stationary. The regain or moisture content affects the physical properties of textiles to a marked degree, changing the strength, pliability and elasticity. The fact that the regain of textiles will come into equilibrium with the conditions of' the surrounding air and vary with its temperature and relative humidity is the fundamental basis for the control of physical qualities during manufacture. During the preparation processes in a cotton mill, the cotton fibers should be in a condition to be easily carded. These preliminary processes are carried out best in a relative humidity 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.1 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 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 >The Present Status of Textile Regain Data, by A. E. Stacey, Jr. Manufacturers, 1927). 726 (National Association of Cotton CHAPTER 39. INDUSTRIAL AIR CONDITIONING 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 regxi-. 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 of rayon. 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 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 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 727