Document JZrNn51G2ZQG6MZaBaRwv9d6

934 CHAPTER 44 1951 Guide 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. In such cases, 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 temperature in the machine may be considerably higher than the tempera ture 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 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 AJS.T.M. Standard Conditions of 50 percent relative hu midity and 23 C (73.4 F) temperature. 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. The several other chapters previously cited call attention to addi tional phases of calculation which may be needed. 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 definitely 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. Industrial Air Conditioning 935 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 latentfheat should be included as contaminants of in dustrial atmospheres. A recent small scale survey has indicated that more than half 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 dilvlion 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 por tion 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. Design of Dilution Systems The first step in the design of a system employing the dilution method is to determine as exactly as possible the nature and extent of the contami nating load. This will often be difficult, and may require construction of pilot production models. Often, however, the required data will be available from production records, showing the weight or volume rate of loss of the contaminating agent to the atmosphere, or it may be estimated from parallel operations in other plants, or by applying experienced engi neering judgment. However obtained, the determination of the nature and magnitude of the contaminating load is an indispensable step in the proper design of the corrective system. Designs based on number of air changes per hour, or other, rule-of-thumb methods, are hopelessly inade quate, and lead either to unsuccessful operation or to excessive and un necessarily high cost of installation. 1. Gases and Vapors. Once having established the nature and magnitude of the contamination load, it is rarely necessary to completely remove contaminating agents from the atmosphere. For cases involving diffusible vapor or gas contami nants, maximum allowable concentrations (MAC) of commonly encountered gases and vapors have been established, and these data are tabulated in Chapter 8. From these data, and the previously established rate of addition of the contaminant to the space, the volume of air required to dilute the addition to a tolerable level can be calculated by the equation: vxw. ,. v (MAC) - (SAC) w