Document mmK5wZKk3gQNV3jxnk99YNJOJ

FILE NAME: Visible Dust (VDT) DATE: 1955 DOC#: VDT004 DOCUMENT DESCRIPTION: Book Excerpt - Plant and Process Ventilation I PLANT AND PROCESS VENTILATION | by W. C . HEMEON Engineering Dtreef or Industrial Hygiene Foundation o f America, Inc. Mellon Institute, Pittsburgh, Pa. Copyright 1955 TH E INDUSTRIAL PRESS New York 13, N. V. FE IN T E D IN T H E UN ITED STATES OF AMERICA FIRST EDITION p. 38 ! r p : I '|J If : !||; Iff ; li i If If ] M . Y-; : Lsst; ir : rt f p. 121 In "Song "8, P- 130 Figure 7 IOC j p. 131 Tails 7- cor 3 Par | P- 167 In ."SyEib sho L' p. m Line 13 (?* p. 276 In capti; fi to ' j p. 285 In 8th L '50 l in ; i . P- 301 last Won ! S P- 371 Caption i : ; 15-: P. 385 Bottom lj cap: :1 ! I; H h ft ) 14 O bjectives of Ventilation Still another factor of importance is the rate of contamination of the atmosphere. Although data are usually lacking on this rate, it never theless has a bearing on the problem equal in importance to the value of the maximum allowable concentration. This point is effectively illus trated in the following problem. Problem. A new process, which is to be installed in a plant, consists essentially of a rectangular tank 8 ft long by 3 ft wide, containing a solution which generates fumes that are described as extremely toxic to humans. The threshold limit atmospheric concentration is given as 0.001 milligram per cubic meter, equal to 0.00003 milligrams per cubic foot. (This is 100 times as toxic as lead, at 0.1 milligram per cubic meter.) It is known th at the process generates the dangerous fume a t a maximum rate of 3 milligrams per 100 sq ft of liquid area per hour, or 0-72 mgm per hr from the tank in question. The tank is to be exhausted by slots along each long side. What is a suitable minimum safe rate of exhaust in these circumstances. Solulion. Conventional exhaust ventilation rates for tanks (treated in a later chapter) would call for 100 to 200 cfm per sq ft of tank area In the absence of data, one might expect a ventilation specification of, say, 4800 cfm. However, the data permit one to calculate the air flow rate required to dihtfc the contaminant to a safe level. The escape rate is 0.72 mgm per hr. To dilute this flow to .00003 mgm per cu ft requires only .72/(.00003 X 60) = 400 cfm. This would be regarded as an insignificant source in the average industrial building. Even though extremely toxic as indicated by the maximum allowable concentration, the quantity evaporating is so minute as to be insignificant in most cir cumstances. in any case one can conclude that an exhaust at some level below 400 cfm would be entirely adequate. Appearance of Various D ust Concentrations The dust and fume concentrations given in the tables of threshold limit values can be roughly described in terms of sight perception. Some dusts may be present in potentially harmful concentration though not visible in ordinary interior illumination, and vice versa. For ex ample, quartz dust or lead oxide dust can be present in concentration well above the allowable limit yet not be apparent visually. Harmful concentration of manganese oxide dust on the other hand, would nearly always be apparent. Table i-8 provides a rough scale of dust concentration values in terms of the sense of sight These figures are based on the author's impressions Objectives of VenHIofion 15 and should not be regarded in any sense as other than an approximate description. It is clear by comparing this scale of values with threshold conce.ntracions of free silica dust (5 million particles per cubic foot) that hazardous concentrations are apparent visually only under the most favorable conditions of illumination. On the other hand, the figure most generally accepted as defining the upper limits of decent working conditions for miscellaneous non-metallic mineral dusts, SO million particles per cubic foot, will be visually apparent in nearly all common conditions of illumination. Concentrations of the toxic heavy metals like lead, with maximum permissible concentrations of 0.1 to 0.2 milligram per cubic meter, are seldom visually apparent until very toxic concentrations are attained. T A B L E 1-8 Sight-Perception D ust Scale (Visible Dust Concentrations in General Air) Perspective Lighting Short Distances (less than 10 to 15 ft) Long Distances (SO to 200 ft) Concentration, Million Particles per cu it Beam of sunlight, background relatively dark Bright sunlight b u t no beam effect Bright daylight "north" illumination (i.e., interior but no direct sun) Low intensity daylight Dirn artificial-light (night) iess- ihan 2 (no distance effect) 10-20 2-5 10-20 20-40 100-200 5-10 10-20 7.5-100 Noies: (a) Mica dust concentrations well below 1-2 million are readily visible in poor light and create an impression of greater dustiness than fact. (b) Dusts composed of abnormally large am ounts of coarse particles, or floes, appear to be more concentrated than actually (applies to very dusty, violent operations). (e) (n considering duels of heavy maals (e.g., lead oxide, manganese oxide) where results of measurement are expressed in terms of weight concentration, employ the figures above, changing the units to milligrams per 10 cubic meters of air. Thus the first item would apply to lead dust where concentration is "less than 2 milligrams per 10 cubic meters." (d) Degree of illumination and light contrasts affect visibility greatly, therefore the values listed are highly approximate.