Document RpVZVGn11VXK4dn09GgvB359E

1114 CHAPTER 44 1958 Guide necessary to direct the relief ventilation so as not to disturb the product or upset,the performance of local exhaust hoods. Types of Outlets for Local Relief A few of the different types of outlets are shown in Fig. 3. In most cases, volume control is highly desirable.. Directionalizing and damper control should be designed for easy adjustment from the floor by chain, sash cord, or pole. Where a constant total volume of supply make-up air must be maintained, an outlet similar to E.will permit volume adjustment to the work zone without reducing the total air supply to the building area. A double deflection grille is available with gang-operated, horizontal face bars (similar to D) to accomplish the desired objective by directing the air down to the work zone or toward the ceiling. Such a grille could be installed on outlet F. Outlets A, B and C could be used without dampers for these applications. An inexpensive means of supplying summer relief air is shown in G. Easily-adjusted diffusers, which will provide a flat or down-blast air discharge pattern, are available. Where overhead installations are not possible because of structural inter ferences or other difficulty, outlets near the floor may be used. These may be in the form of grilles located in pilasters, control panels, tables, equipment or in the floor itself. Such outlets have been quite successful in welding and foundiy areas and at the work locations in kitchens. CONTROL OF ENVIRONMENTAL GASES AND VAPORS, DUSTS AND FUMES 1 In general, the systems for the control of these atmospheric contami nants in industrial plants will be of three types: 1. Local exhaust systems will be indicated where the contamination originates at concentrated areas and is characterized by low or imperceptible air motion, or where the contaminant is a dust, mist or fume requiring a capture velocity exceeding 25 fpm. Design of this type system is discussed in Chapter 45, and will not be further treated in this chapter. 2. A system employing the dilution method will usually be indicated where the contamination originates at scattered points dispersed generally throughout the area. 3. Combination of local exhaust and dilution methods is often economical, since well designed exhaust hoods or openings, removing from the space that portion of the contamination load which is susceptible to such treatment, will often reduce greatly the air volumes required for dilution purposes. The choice of the type of system should be made on the basis of economic comparisons. Design of Dilution Systems The first step in the design of a system employing the dilution method is to determine as exactly as possible the nature and extent of the contami nating load. This will often be difficult, and may require construction of pilot production models. Often, however, the required data will be avail able from production records, showing the weight or volume rate of loss of the contaminating agent to the atmosphere, or it may be estimated from parallel operations in other plants, or by applying experienced engi neering judgment. However obtained, the determination of the nature and magnitude of the contaminating load is an indispensable step in the proper design of the corrective system. Designs based on number of air changes per hour, or other rule-of-thumb methods, are hopelessly inade quate, and lead either to unsuccessful operation or to excessive and un necessarily high cost of installation. 1. Gases and Vapors. Once having established the nature and magnitude of the Control of the Industrial Environment' `' K H15 contamination load, it is rarely necessary to completely remove contaminating agents from the atmosphere. For cases involving diffusible vapor or-gas'contami nants, maximum allowable concentrations (MAC) of- commonly encountered gases and vapors have been established, and these data are tabulated in Chapter 8. : From these data, and the previously established rate of addition of the contaminant to the space) the volume of air required to dilute the. addition, to a tolerable .level can be calculated by the equation:. . . ' . ...... - Q - Qo X10* . (MAC)-(SAC) where Q. = quantity of air circulated, cubic feet per minute. Q, = rate of generation of contaminant, cubic feet per minute. (MAC) = maximum allowable concentration, ppm by volume. (SAC) = concentration in supply air, ppm by volume. DIRECTIONAL VERTICALLY SUMMER - WINTER OUTLET FOR MAKE-UP AIR SUMMER OUTLET Fig. 3. Directional Outlets fob Spot Cooling weiahf , generation of the contaminating vapor will often be available as a of .hqul<1 evaporated into the space per unit time. These may be a car oTa t0 - nnlts of Equation 9 by applying the principle that a pound-mol of turn ThaPr W1 occupy approximate^ 359 cu ft at standard pressure and tempera- where cfm (vapor) = W X 359 X t + 460 492 (10) u rate of generation of contaminant, pounds of liquid solvent per minute. - molecular weight. * -- air temperature, Fahrenheit.