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948 CHAPTER 44 1952 Guide Control of Temperature for Close Machining Tolerances Where tolerances must be held within 2 or 3 ten-thousands of an inch, as in the manufacture of precision instruments, tools, and high quality lenses, temperature variations may cause expansion and contraction of material to an extent that will seriously affect the quality of the work. This type of work usually requires close temperature control to assure accuracy and uniformity of the product. ' Usually the temperature level with respect to the product is not as important as controlling, the temperature within close limits. For this reason, conditions are usually selected within the comfort range. Control of Dew-Point for Protection of Polished Surfaces In the manufacture of certain metal articles, the presence of finger prints, tarnish, or etching can not be tolerated in the finished article. If these articles are manufactured under conditions of effective tempera tures that will cause the hands to perspire, an unsatisfactory product will result. 'Die salt and acid contained in body perspiration, when deposited on the highly polished article, can show corrosion and rust within a few hours if examined under a microscope. It is therefore important to maintain temperatures and relative humidi ties (dew-point) low enough to prevent sweating of the hands. In addi tion, the manufacture of polished surfaces usually requires a better-thanaverage job of air filtering to avoid abrasion of the surfaces. Control of Humidity for Reduction of Static Electricity - The presence of static electricity is often 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 minimized by in creasing the relative humidity to at least 55 percent, if the material being processed is not damaged thereby. - 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 55 percent or more. As some machines consume a great deal of power, which is converted directly into heat, the tempera ture 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 therefore be neces sary to maintain a room relative humidity of 65 percent, or even more, to maintain the desired humidity, Control of Conditions for Material Test Laboratories Laboratories having controlled conditions of temperature and humidity, are becoming more common, not only for the purposes of scientific re search, but also for routine testing and for quality production control. A control of temperature and humidity within fairly close plus or minus limits is usually required. Laboratories designed for scientific research may require control of conditions over a wide range, whereas the routine testing laboratory or quality control laboratory will usually be designed to maintain the A.S.T.M. Standard Conditions of 50 percent relative hu midity and 23 C (73.4 F) temperature. Industrial Air Conditioning 949 CALCULATIONS . The methods for determining the heating and cooling loads for the various industrial processes are similar to those outlined in Chapters 11 and 12. Some factors affecting heating or cooling requirements are given in Table 1. Because of the large number of motors and heat producing units usually found in an industrial application, it is particularly important that operating allowances, for the latent and sensible heat loads, be defi nitely ascertained and used in the calculations to determine the total design load. GENERAL REQUIREMENTS FOR HEALTH, SAFETY AND EFFICIENCY Control of Atmospheric Contaminants Safeguarding the health and maintaining the safety and efficiency of workers, require control of dusts, fumes, smokes, mists, fogs, vapors, and gases, and control of the effective temperature, which includes tempera-, ture, humidity and motion of air about the worker. General ventilation may be relied upon in some cases to control air contaminants. Chapter 10 gives information on natural ventilation. If mechanical ventilation is to be used, Chapter 32, Fans; Chapter 30, Air Distribution; Chapter 31, Air Duct Design; Chapter 33, Air Cleaning; and Chapter 34, Spray Apparatus, furnish information on a broad range of industrial design conditions. Specialists in the field of industrial hygiene should be consulted in case of doubt concerning the presence of airborne industrial hazards to health. Chapter 8, Air Contaminants; Chapter 6, Physiological Principles; and Chapter 7, Air Conditioning in the Prevention and Treatment of Disease, will be of help in determining the atmospheric conditions which should be maintained around the worker. Local codes, ordinances, or state labor laws must likewise be observed, particularly for ventilation requirements for hazardous trades. Comfortable conditions are desirable because they are likely to increase the efficiency of workers. For purposes of analysis, both sensible and latent heat should be included as contaminants of in dustrial atmospheres. A recent small scale survey has indicated that more than half of the air conditioning and ventilation installations in a typical' industrial plant were concerned 'with removal of either sensible or latent heat as a source of air contamination. Contaminant Control Systems In general, systems for control of atmospheric contamination in industrial plants will be of three types: 1. Local exhaust systems will be indicated where the contamination originates at concentrated areas and is characterized by low or imperceptible air motion, or where the contaminant is a dust, mist or fume-requiring a capture velocity exceeding 25 fpm. Design of this type system is discussed in Chapter 45, and will not be further treated in this chapter. 2. A system employing the dilution method will usually be indicated where the contamination originates at scattered points dispersed generally throughout the area. 3. Combination of local exhaust and dilution methods is often economical, since well designed exhaust hoods or openings, removing from the space that portion of the contamination load which is susceptible to such treatment, will often reduce greatly the air volumes required for dilution purposes. The choice of the type of system should be made on the basis of economic comparisons.: