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1094 CHAPTER 44 1957 Guide stance) perforated panel supply outlets are very effective for mechanically cooled installations, because of their low induction characteristics. For further design information on outlet design and the throw and induction characteristics, see Chapter 30. Control of Direction With very few exceptions, directional outlets are necessary in order to maintain ventilation or make-up air to the space even when the workers do not wish to have the air directed into their work zones. In this way, the air can be directed down in the summer when it is needed, and up in the winter for ventilation, heating, and make-up purposes. It will be possible, too, to direct the air toward one or another work station as needed. From the production standpoint, directional control may be 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 make-up air must be maintained, an outlet similar to E will permit volume adjustment to the work zone without re ducing the total air supply to the building area. At least one manufacturer makes a double deflection grille with gang-operated, horizontal face bars (similar to D) which accomplishes the above objective by directing the air down to the work zone or toward the ceiling. 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 from several manufacturers. 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 foundry areas and at the work locations in kitchens. CONTROL OF ENVIRONMENTAL GASES AND VAPORS, DUSTS AND FUMES 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- Control of the Industrial Environment 1095 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- Fiq. 3. Dibectional Outlets for Spot Cooling 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 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: Q0 X 10 (MAC) -(SAC) (9) where Qa = quantity of air circulated, cubic feet per minute. Qc = rate of generation of contaminant, cubic feet per minute.