Document oNR35nepdJNGzKqMaXr6dpJw

162 CHAPTER 8 1957 Guide more safely can the liquid be handled. Liquids with flash points under 70 F should be regarded as highly flammable. Upper and lower limits of flammability of gases and vapors, and the flash 2 points of the corresponding liquids are given in Table 6. g Methods for estimating the flammable limits of mixtures of gases or ? vapors must be applied with caution; the reader is referred to other publi- 2 cations for this information.12'18 Design of equipment for the control of combustible anesthetics is out- 7- lined in Chapter 7. Construction of equipment for handling air contain- 7 ing flammable substances, or operating in atmospheres so contaminated, is discussed in Chapter 45. It is customary to report the concentrations of flammable gases or vapors .' in percent by volume, or volume percent. Comparison with concentra- Tari.it. 5. Limits fob Mineral Dusts SUBSTANCE Threshold Limit Values A.C.O.LB. 1956 mppcf* 50 5 50 50 50 20 5 20 50 50 | liiln Total dust (below 5% free SiOi).................................................................... 50 20 20 50 * mppcf--million particles per cubic foot of air, standard light field count. t ,. Table 5 values reprinted by permission from AJH.A. Archives of industrial Health, August 1956, VoL n. pp. 186-189. tions on the part per million scale used in chemical, medical or industrial hygiene literature is readily made by the conversion: 1 percent = 10,000 ppm (parts of contaminant per million parts of air, by volume, or in other words, cubic feet of contaminant per million cubic feet of air). It will be .: noted in Table 6 that nearly all of the substances listed have lower explosive )' limits above 1.0 percent, while the maximum allowable concentration& for % gases and vapors in Table 3 are below 1000 ppm or 0.1 percent in most cases. Therefore, control of toxic or injurious vapors to levels below their : maximum allowable concentrations for health usually requires much more effective ventilation than for the prevention of a fire hazard. COMBUSTIBLE DUSTS A dust explosion is essentially a sudden pressure rise caused by the very rapid burning of airborne dust. The primary explosion often originates from a small amount of dust in suspension exposed to a source of ignition and the pressure and vibration it creates may be sufficient to dislodge large accumulations of dust on horizontal ledges or surfaces of the building - Air Contaminants 163 and equipment, thereby creating a secondaiy explosion of great force. Thus the air conditioning engineer ,is involved for two reasons: (1) to obtain a movement of dust-laden air into exhaust hoods or openings, and through ventilating or pneumatic conveying ducts, in a manner that will prevent accumulation of highly flammable dust at points where it could ignite inside the equipment; and (2) to so design process ventilation as to prevent the escape of dust which might settle on horizontal surfaces and become a potential source of disaster at some distance from the dusty operation. (See Chapter 45). Intensity of a dust explosion depends upon: chemical and thermal properties of the dust; particle size and shape; concentration in air; propor tion of inert dust in the air; moisture content and composition of the air; size and temperature of the ignition source; and degree of dispersion of the dust cloud. Investigations on the explosibility of dusts require determina tion of the maximum pressure developed during explosion of a known air concentration, as well as determination of the rate of pressure rise. In vestigators frequently experience difficulty in obtaining dust suspensions of uniform dispersion, and this should be kept in mind when comparing results from several sources.14 Minimum explosive concentrations of airborne dusts already tested range from 0.01 to 0.5 oz per cubic foot, or 10 to 500 grams per cubic meter of air. Maximum pressures generated have been reported as high as 500 psi, although they are more likely to be of the order of 50 psi. Investiga tions on the flammable characteristics of dusts are currently made at 0.1 and 0.5 oz per cubic foot.15-21 ATMOSPHERIC POLLEN Properties of pollen grains discharged by weeds, grasses and trees and responsible for hay fever, are of special interest to designers of air cleaning equipment (see Allergic Disorders in Chapter 7, and Air Cleaning, Chap ter 33). Whole grains and fragments transported by the air range chiefly between 10 and 50 microns in size, but some have been measured as small as 5 microns, and others over 100 microns in diameter. Ragweed pollen grains are fairly uniform in size within the range of 15 to 25 microns. Pollen grains can be removed from the air more readily than the particles of dust prevalent in outdoor air or produced by dusty processes, since the latter predominate in the range of 0.1 to 10 microns in size. Most grains are quite hygroscopic and therefore vary in weight with the humidity. Illustrations and data on individual pollen grains are available Jn the botanical literature.22'2824 Geographical distribution of plants known to produce hay fever is also recorded.26, 26 The quantity of pollen grains in the air is generally estimated by exposing an adhesive-coated glass plate outdoors for 24 hr, and then counting cali brated areas under the microscope. Methods are available for determinln the number of grains in a measured volume of air,26 27 28 but their greater accuracy has not caused them to replace the more simple gravity slide method used for most pollen counts. Counting technics vary some what, but the daily pollen counts reported in local newspapers during the bay fever season usually represent the number of grains found on 1.8 sq cm of a 24-hr gravity slide. Hay fever sufferers may notice the first symptoms when the pollen count 13 10 to 25, and in some localities the maximum figures for the seasonal Peak may approach 1000 for a 24-hr period, depending upon the sampling