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SAN JOAQUIN DELTA COLLEGE • performed for only part of a day, the time during which the vapor is being released is used to determine the rate of contaminant generation Effect of Distribution and Make-up Air One of the most important factors in the design of dilution ventilation, and the one most frequently overlooked, is the need to supply sufficient and properly directed make-up air to replace the air exhausted- Ideally, xhaustedIdeally, the make-up air will be positively introduced and distributed in such a manner that the source of contaminant is situated between the worker and the exhaust outlet,the exhaust is located as close as possible to the source of contamination, all the air will pass through the zone of contamination,and the contaminant will be diluted to the design levels as soon as it is generated Under such conditions, the K factor in the formula could be 1, since no factor of safety would be required However,ideal distribution conditions can never be assumed and actual conditions will vary considerably Average conditions may be assumed to be those where the toxicity of the contaminant is moderate,the exhaust fan or fans are placed reasonably close to the operation releasing vapors,make-up is by infiltration through doors,windows,and walls so that a reasonable amount of dilution occurs as the contaminant is generated, and the worker is not too near or in the zone of concentrated contaminant release Under such circumstances, the average K factor may be taken as 4 so that the average concentration in the workroom is 114 of the TLV and the concentration in any part of the workroom will probably not be higher that the TLV However, average distribution conditions are probably as elusive as ideal conditions and the need arises for a actual conditions,whether poor,average,good, or excellent Poor distribution, such as may be caused by short circuiting the make- up air, inadequacy of infiltration as a sources of make-up au, location of the worker directly downstream form or in the evolving vapors,will require a higher factory of safety than 4 A lower factor may be used with good distribution, which will usually consist of a positive mechanical make-up air supply system with properly placed supply and exhaust fans Excellent distribution will be provided by properly located exhaust fans with a mechanical supply system directing tempered air in the optimum manner by means of diffusers, ductwork, or a perforated plenum This will make possible a further reduction in safety factor Effect of Toxicity The TLV or relative toxicity of the substance being controlled enters into the design directly since it is part of the calculation formula Where the TLV(in ppm)of a substance has been promulgated by a recognized authority such as the ACGIH or the American Standards Association,it is recommended that dilution ventilation rates be calculated using this TLV value in equation (1 2) and applying the safety factors shown in K Factors Table For other substances, the general equations in the Dilution CFM Formula Equations, as well as the K Factor Table should be used Dilution CFM Formula Equations Slightly toxic cfm=(I x 106 (d)(pints evaporated per min) K substances mol wt Moderately toxic cfm=(2 x 106)(d)( rots evaporated ver mm)K substances mol wt Highly toxic cfm=(8 x 106)(d)(pints evaporated per min)(K) substances moi wt d=specific gravity of liquid mol wt=molecular weight of liquid The general dilution ventilation equations have been derived form equation (1 2) using arbitrary values of 400 ppm as the TLV for slightly toxic substances,and 200 ppm for moderately toxic substances,and 50 ppm for highly toxic substances If more knowledge were available,a rational method for designing dilution ventilation for such mixtures could be formulated. . In the absence of such information,dilution ventilation rates should be calculated for each component of the mixture and the cfm values obtained added together to p ve a total efin rate A method which is occasionally used to af[brd greater safety is to assume that the entire volume of mixture being evaporated consists of the component with the highest cfin rate per pmt and to calculated cfiri accordingly Page #10