Document kDBK0XwEd1Qp2NoG5ZwynD5Zq

EXHAUST SYSTEMS, GENERAL ij COMFORT VENTILATION The New York State Department of La bor has examined the economics of air re circulation in detail0 General Ventilation Contaminant control by dilution Although local exhaust systems have sev eral inherent advantages, as compared with general ventilation--for example, removal of contaminant before it spreads throughout the room, economy of air flow, less heat loss, etc--there are some operations where local exhaust systems are impracticable With such operations the contaminant is usually a solvent vapor, but occasionally it is a fume or dust Local exhausts may be unseated because there is a multitude of sources of vapor, or the source may be ex tensive as in a large coating process, or the amount of ductwork to connect all die nec essary hoods may be forbidding Such operations can sometimes be con trolled by diluting the general room at mosphere with outdoor air fast enough to keep the concentration of toxic vapor m the room air within safe limits Solvents appear in synthetic varnishes and lacquers, in cements and adhesives, m liquid coatings for fabrics and other mate rials, in cleaning operations, and in many other processes The basic purpose of sol vents in these applications is to evaporate into the atmosphere, leaving behind some physically changed substance, which is the desired end product Thus, the very heart of the process involves polluting the air with vapor. The aim of die engineer is to keep this vapor concentration as low as possible, certainly below the toxic limit. The rate of solvent evaporation can easi ly be ascertained as can the chemical nature of the solvent It is known that a weight of a given volume of vapor from a liquid when it evaporates is proportional to its molecu lar weight It is possible, then, to calculate how much air must be mixed with this va por to hold the concentration down to safe limits. Hemeon,00 using not the man- mum acceptable concentration for the com pound, but what he calls the ventilation de sign concentration which has incorporated in it a safety factor based on toxicity, odor, and experience, has published a table from which general ventilation can he calculated (see Table 28-Q). This table is based on this formula Cfm required = lb solvent evap ^ 400 ^ 10* nun mw solvent V D C where V D C is the Ventilation Design Concentration, and mw stands for molecu lar weight Neither the maximum allowable concen tration (MAC) nor the threshold limit value (TLV) should be used for calculat ing VDC not only because vapor dilution in the working space is bound to bo uneven, but also because concentrations must al ways be maintained below the MAC or TLV in order to provide a factor of safety In turn, this factor of safety depends on whether the solvent vapor is to be con trolled because of its inherent toxicity or its disagreeable odor. For example, suppose one gallon of methyl ethyl ketone is evaporated per hour One pint of methyl ethyl ketone requires a ventilation rate amounting to 31,000 cu ft of air. One gallon would then require (31,000)8, or 248,000 cu ft of air If this is needed per hour, the ventilation rate would be 4,133 cfm It is important to note that this example assumes there is perfect mixing of the clean air with the solvent vapor, but in practice this does not occur The ventilation rate calculated is therefore a minimum and should be increased depending upon other factors involved such as type and location of air diffusers, location of people in the room, and relative toxicity of the vapor The size of the room where the work is done does not enter the calculation This is at variance with the common practice of specifying ventilation requirements in terms of "number of room air changes per min ute" which, of course, directly involves "Stem, Arthur C and Donald P O'Neil "Recirculation from Industrial Exhaust Systems." Monthly Review, vol. 30 (May, June, July 1951) New York State Depart ment of Labor #0Hemeon, W. C. L. Plant and Process Ventilation, 2nd ed New York City, The Industrial Press, 1963 28-48