Document wKBZ4k7br9QrL4y0beVXL4Yz6

176 CHAPTER 10 1948 Guide 'i- The ability of a' -flammable liquid to form 'explosive mixtures is de` termineddargfely by'its vapor pressure, volatility;'drt&te of evaporation. ; Flash pointis a convenient method of expressing this'property in terms of the temperature scale. It may be defined as the temperature to which a combustible liquid must be heated to produce .a flash of flame when a small flame is passed across the surface of the liquid. The. higher the flash point, the more safely can the liquid be handled.. Liquids- with flash points, under 70 F should be regarded as highly flammable. :v:. The tipper and lower limits of flammability of gases and'vapors, and the flash points of the corresponding liquids are given in Table 6.- - Methods for estimating the flammable limits of mixtures of gases or vapors must be applied with caution; the reader is; referred|to other publications for this information12-13. Design of equipment for the control of combustible anesthetics is outlined in Chapter 13. Construction of equipment for handling air containing flammable substances, or operating in., atmospheres so con taminated, is discussed in Chapter 46. It is-customary to report the concentrations of flammable gases or vapors in per cent by volume, or volume per cent. Comparison with concentrations on the part per million scale used in chemical, medical or industrial hygiene literature is readily made by the conversion: '1 per cent = 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 i.O per cent, while the maximum aUowabie concentrations for gases and vapors in Table 3 are. below 1000 ppm or 0.1 per cent in most cases. Therefore, control of toxic or injurious vapors in workrooms to levels below their maximum allowable concen trations 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 6n::horiz6ntal. ledges or surfaces of the building and equipment,. thereby creating a- secondary explosion of great force. Thus the air conditioning engineeris 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 46). 'The intensity of a dust explosion depends upon: the chemical and thermal properties of the dust; the particle size and shape; the concen tration in air; the proportion of inert dust in the'air; the moisture content and composition of the air;.the size and temperature of the ignition source; and the degree of dispersion of the dust cloud. Investigations on the explosibility of dusts require a determination of the maximum pressure developed during an explosion of a known air'concentration, as well as | Air, Contaminants m determination of the rate of pressure rise'. Investigators frequently experience difficulty in obtaining dust suspensions of uniform dispersion;and .this should be kept, in mind when comparing results from several . sources14. .: . : The 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: ' Investigations on the flammable characteristics of dusts are-currently ipade at 0.1. and 0.5 oz-per cubic foot1M1. - ' ATMOSPHERIC POLLEN ! The; properties of pollen grains discharged by weeds, grasses, and trees arid responsible for hay fever are of special interest to engineers who design equipment for their removal from indoor air (see Allergic. Dis orders in Chapter 13, and Air Cleaning Devices, Chapter 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 parti cles of . dust prevalent in outdoor air and . found near dusty industrial 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 in the botanical literature a- u. -The geographical distribution of plants known"to-produce hay fever is also recorded ... 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 calibrated areas under the microscope. Methods are available for determining the number of grains in a measured volume of air 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 somewhat, but the daily pollen counts reported in local newspapers during the hay 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 wfienthe pollen count is 10 to 25, and in some localities the maximum figures for the seasonalpeak may approach 1000 for a 24-hr period, depending upon the sampling and reporting' methods of the laboratory. Translatio'h.of: gravity(counts by special formulas to a volumetric basis, or the number of grains per cubic'yard or per cubic foot of air, is still -uncertain,beeaiuse of the com plexity of the modifying factors. When such information is important, it is best obtained directly by a volumetric instrument. The number of pollen grains per cubic-yard of air evidently.; varies fromJ2. to 20 times the number found on 1 sq cm of a 24-hr gravity slide, depending on grain diameter, shape, specific gravity, wind velocity, humidity -and physical placement of the collecting plate29,30, 31. AIRBORNE BACTERIA Study of the occurrence and significance of micro-organisms in .the' atmospheres of the indoOr world is currently absorbing the energies of-.a