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SAN JOAQUIN DELTA COLLEGE Shght 7 4 3 2 Moderate 8 5 4 3 High 1I 8 7 6 However, these data are difficult to obtain and are subject to error, and the cfm obtained cannot be extrapolated to a future change in conditions which may require lower or higher ventilation rates A more rational method is to use data on rate of contammant release to determine rate of exhaust necessary to continuously dilute the contaminant to safe levels,where d=density of parent liquid mol wt =molecular weight of parent liquid TLV=threshold limit value of the vapor in ppm K=factor of safety The formula is Equation 1 2 cfm =(403)( )(g 10b)(pints of liquid evaporated/mm)(K) (enol wt)(TLV) Where the evolution rate is expressed in pounds/minute,the formula becomes Equation 1 3 cfm=(387)(106)(lb of liquid evaporated/mm�[K� (mol wt)(TLV) Equation Q 2) or (1 3) above are only applicable to volatile organic liquids Dilution ventilation for gases can also be designed, but determination of the rate of generation is much more difficult that with liquids if the rate can be determined with reasonable accuracy, it should be expressed in terms of pounds/mmute, and equation (1 3) should be used The table below describes a simplified method of calculating dilution ventilation for substances for which TLV s. have not been established,using the general formulas and safety factor data shown Dilution CFM Formula Equations Slightly toxic cfm = 1 x j2d)(pmts eva orated per min substances mol wt Moderately toxic cfm= 2 x l 06 d rots evaporated per mm K substances mol wt Highly toxic cfin=(8 x 106 (d)(pints evaporated per minn)(KK) substances mol wt d=specific gravity of liquid mol wt=molecular weight of liquid Rate of Contaminant Generation The amount of liquid evaporated per minute must be determined as accurately as possible Several methods for such determinations are available Usually process control data are the most reliable sources For example,in a coating operation,the amount of solids applied to the product can be determined, and this information, coupled with production rate and coating material composition data, can be used to calculate pints of volatile material released per minute Often,operating and supervisory personnel can supply fairly reliable information on rate of use of volatile materials Purchasing department data over sufficiently large period of time can also be utilized to determine the average amount of volatile material per month This can be translated into pmts evaporated per working minute Where an operation is Page #9