Document G6o2r52d2qpQMynbxaYBwG8a4

150 CHAPTER 11 1965 Guide And Data Book will be variation, depending on the vision of the observer, the color of the dust, and the contrast available. Dark field micro scopic techniques will reveal particles smaller than these, to a limit of approximately 0.4' micron. The smaller submicroscopic dusts can be sized and compared by the use of the elet>' tron microscope. Other sizing techniques may take into account velocity of samplings in calibrated devices and actual settlement measurements in laboratory equipment. The elec tron microscope and various sampling instruments, including the cascade inpacter, have been used with some success for the sizing of liquid particulates, including fogs and mists:r Concentrations of these materials arc usually reported on a weight basis. Permanent Atmospheric Impurities The size of the air'contaminant, its physical properties, and its toxic effect on man are of prime importance in the design of ventilation systems and the application of air cleaning equipment. In actual practice, the need-for air handling and air cleaning is established by the toxic, flammable, or nuisance qualities of the contaminant. Considerable evidence and data are available from experience and research to indicate the toxic actions and the permissible exposure level of many com mon materials. However, it must be emphasized that, for the. most part, these exposure levels are based upon the industrial exposure for a few hours.daily, and they must not be applied or extrapolated to include the air pollution situation where, a broad spectrum of people are exposed continually. Accord ingly, air contaminants may be grouped as industrial air contaminants and air pollutants. INDUSTRIAL AIR CONTAMINANTS Many industrial processes release air contaminants as dusts, fumes,- smokes, mists, and as fogs, vapors, and gases. Un less control is applied, these materials will be dispersed through the industrial atmosphere and will be transported by air from the source to remote locations in the plant, exposing many workers to their toxic effects. The ventilation engineer' must have an understanding of the process controls and venti lation controls which can be applied for their removal at the source; see Chapter 34 of the 1964 Guide And Data Book. * It is recognized, however, that absolute control cannot be' maintained and that the individual is able to assimilate small quantities of various toxic materials without permanent in jury. This is the basic concept which is applied to the entire science of industrial hygiene, and it is the basis for the stand ards of exposure level which have been developed. .< Permissible levels of exposure vary considerably -for'.dif ferent contaminants. Tables showing maximum allowable concentration values have been prepared by several sources,' notably the American Standards Association. Industrial toxi cologists today, however, advocate use of threshold limit val ues, which are time-weighted average concentrations for a normal workday, rather than maximum allowable concentra tions. In the United States, these values are annually re viewed, revised, and published by the Committee on Thresh old Limits of the American Conference of Governmental Industrial Hygienists. The latest available values (1863) are reproduced in Table 2. These limits are suggested guides, not hard-and-fast, rigid values. While they are based on working . experience, laboratory research, and medical data, they are subject to constant revision. The engineer should refer to the latest standards available when evaluating a workroom exposure. In applying these threshold limits, the following factors must be considered: 1. The duration of exposure is 8 hr per day for 5 or 6 days a week. 2. The measurements taken must be indicative of toe concen tration in the breathing tone of the exposed person. Average con centrations can be misleading in many instances. 3. The average exposure should not exceed the threshold limit value for an 8 hr day and, for shorter exposures, should not neatly exceed the threshold limit value. For example, it cannot be assumed that, if 100 parts per million is safe for an 8 hr exposure, 800 parts per mulioo is permissible for one hour. . 4. The threshold limit values presented are upper limits, and it is always desirable to operate well below these levels. 5. Two substances with similar threshold limit values may be entirely different in effect on the workman, and may have dif ferent actions at higher concentrations. In some instances, the threshold limit value has been placed at a level which minimi^ irritation and provides a wide margin of safety. In other cases, toe maximum value is close to the concentration at which severe or irreversible damage may be encountered. It is obvious that there are wide limits of individual sus ceptibility. There also may be considerable-variation in work habits between individuals, and, in some instances, exposure to contaminants may vary considerably in the day to day manufacturing operations. The effects on health of selected toxic materials are summerized in the Hygienic Guide Series of the American Indus-, trial Hygiene Association1 in a manner that may prove more practically useful. Guides for about 150 industrial compounds have been published, showing recommended maximum atmospheric concentration (8 hr), severity of hazards, short' exposure tolerance, atmospheric concentrations immediately hazardous to life, significant properties, industrial hygiene practice (recognition, evaluation, and control), and specific procedures. Each guide condenses into two pages a vast amount of published and unpublished research on the ma terial under consideration. FLAMMABLE GASES AND VAPORS5-3-4 The use of flammable materials is widespread in our society. Flammable gases and vapors may be found in such locations as sewage treatment plants, sewage and utility tunnels, dry cleaning plants, and automobile garages, as well as in indus trial finishing process plants. Adequate ventilation is a primary requirement for minimising or preventing fires and explosions of these materials. The need for good ventilation is not removed by the use of other precautions such as the eUmination of the ignition sources, safe building construction, and the use of automatic alarm and extinguisher systems. The ability of a flammable liquid or gas to form explosive mixtures is determined by its vapor pressure and volatility, or rate of evaporation. A convenient method of expressing these properties is to refer to the flash point of the material involved. The flash point may be defined as the temperature to which a combustible liquid must be heated to produce a flash when a small flame is passed across the'surface of the liquid. Depending upon the test methods, this may be either the open cup or closed cup flash point, usually expressed in Fahrenheit degrees. The higher the flash point the more safely can the liquid be handled. 1-iqnids with flaah points under 70 F should be regarded as highly flammable. In practical usage, the air-vapor or air-gas mixture must be in the explosive range before it will be ignited by a source of ignition. The explosive range is defined as being between the upper and lower explosive limits which are expressed as per cent by volume in air. Concentrations of material above or ^..Contaminants . 151 below the lower explosive range will not explode. .Table 3 -gay of toe common explosive materials, giving their -Tor and tower explosive limit m percent by volume, the limed cup flash point in Fahrenheit degrees, and the NBFU !wficattons by number. Where mixtures of flammable gases 'Svapots are present/the reader is referred to other publica tions tor more detailed information on evaluating the hazard. ' The designer of. equipment for the control of combustible materials is referred to Chapter 10 with respect to combustible anesthetics, and to Chapter 35 of the 1964 Guide And Data Boos. In ventilation systems for-the control of gases and vapors, the engineer must consider the following:" ' 1 Most safety authorities and fire underwriters prefer to limit the concentrations to 20 to 25 percent of the tower explosive limit of the material. The resulting safety factor of 4 or 5 allows lati'tude for imperfections in air distribution and variations of tem perature or miixture. It is also a further safeguard against un predictable pf unrecognized sources of ignition. In only.rare in stances should there be a resort to operation at concentrations above the upper explosive limit. It must be recognized that, in order to reacn the upper explosive limit, the flammable gas or vapor must'pass through toe active explosive range, in which ; any source of ignition will cause an explosion. In addition, a drop ' in gas concentration due to unforeseen dilution or reduced evaporation'rate will result in operation in the dangerous explo sive range. .. 2. In occupied places where ventilation is applied for proper heaith control, the danger of an explosion mil be minimized. In most instances, flammable gases ana vapors are also toxic, and the marimirm allowable concentrations are far below toe lower ex- filosive' limit of the material. As an example, proper ventilation or acetone vapors will keep the concentration below 1000 parts per million. This is equivalent to 0.1 percent, by volume. The lower explosive limit for acetone is 2.5 percent by volume. Thus, proper ventilation to reduce the health hazard will result in con centrations far below the lower explosive limit. 3. Proper location of. exhaust and supply ventilation equip ment depends upon the manner in which the contaminant is given off and upon other problems of the-process, and only secondarily upon the relative density of .flammable vapor. Again using acetone as an example: -> density of air = 1.0 density of acetone vapors = 2.0 2.5% TM 0.025 parts acetone ** 0.025 X 2 = 0.05 97.5% = 0.975 parts air = 0.975 XI - 0.975 1.025 ,. The density of air-acetone mixture at tower explosive limits is then 1.025. It can be seen that the specific gravity of the explosive mixture is substantially the same as that of air alone, and it is apparent that cross drafts, moving equipment, and tem perature differentials may cause sufficient reiving to-produce explosive concentrations and disperse these throughout the atmosphere, with no regard to heavier than air vapors. The engineer is cautioned to provide proper exhaust and supply air patterns to control the hazardous material, preferablyat its source, or to take into account drafts, equipment move ment, and convective forces, to provide good distribution of exhaust and supply air for general dilution and exhaust. COMBUSTIBLE DUSTS8-6 Many mineral and organic dusts are capable of producing dust explosions. Often, a primary explosion results from a ' small amount of dust in suspension, which has been exposed to a source of ignition; the pressure and vibration created may then be sufficient to dislodge large accumulations of dust on horizontal surfaces, thereby creating a secondary explosion. In general, dust clouds require high temperatures and a Table 4 is representative of explosive concentrations, ignition temperatures, explosive temperatures, 'and pressure rise; however, it must be recognized that some discrepancies in obtaining test results are caused by -the difficulty in obtain ing dust suspension of uniform dispersion. This should be kept in mind when comparing results from several sources.* Explosive dusts are potential hazards whenever the dust escapes, uncontrolled, to disperse in the atmosphere or settle on horizontal surfaces such as-beams and ledges. - Proper ventilation design will involve the principles discussed in Chapter 35. of the 1964 Guide And Data Book. Special care must-be taken to insure that the ventilation systems and equipment chosen will prevent the pocketing of dust inside of the equipment itself. In many instances, magnetic traps are used to separate tramp iron from other metallic and organic dusts being conveyed in production equipment and duct work.' When local exhaust ventilation is used, it is often desirable to rental! separation equipment as close to the source of dust as possible to prevent the transport of the quantities of dust in the exhaust system. AIR POLLUTION7 8 The total particulate load in the atmosphere includes all forms of air contaminants outlined in this chapter. The sources of these materials may be natural, as with the perma nent atmospheric impurities, or man made, where the materi als may arise from domestic, commercial, and industrial combustion process along with the air contaminants ex hausted for health control. Contaminant sizes will cover the broad range illustrated in Fig. 1. The larger particles will settle out of the air rapidly, in the vicinity of the source. The smaller particles will remain suspended for tong periods of time, settling many miles from the source, or becoming a permanent contamination in the air.. It has been common practice to evaluate atmospheric dust loads in terms of soot fail in tons per square mile per month. Such data published for the cities in this country range from 20 to 200 tons per square mile per month. To the ventilating and- air-conditioning engineer, however, these values have little significance since they do not express any measure of the suspended material which' must be cleaned from the ventila tion air before it can be.used. Soot fall data are most useful in indicating the effectiveness of smoke abatement and proper combustion methods, and, to some extent, serve as comparative indices of such control programs. ; In selecting air cleaning equipment for ventilation systems, the engineer requires data of the type illustrated by Table 5. Utilizing the air loading in terms of grains per thousand cubic feet, or milligrams per cubic meter, he may use-technical literature provided by the air cleaner supplier or the sup plier's .representative in selecting equipment which will do the job adequately, with the minimum of power consumption and maintenance. It is apparent from the stable that there is a considerable variation in the atmospheric loading between rural and suburban districts, and many industrial develop ment areas. On the one hand, simple air filters may be all that is required for proper air cleaning, in other instances, it may be necessary to resort to heavy duty industrial gas cleaning equipment to supply air that is clean enough for human use, or for process requirements. (Continued on p. 164)