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698 CHAPTER 51 1959 Guide Control of the Industrial Environment 699 50 fpm for light activity in a fixed position, and- particularly An air supply with mechanical refrigeration offers the Control of Direction. With very few exceptions, direc in pilasters, control panels, tables, equipment, or in the floor when the individual is seated at a desk or bench. The maxi greatest relief. Such systems are expensive in first cost and tional outlets are necessary in order to maintain ventilation itself. Such outlets have been quite successful in welding and mum velocity around the worker should not exceed 200 fpm this has retarded their use. However, air supply with me or make-up air to the space even when the workers do not foundry areas and at the work locations in kitchens. for continuous exposure. With a high work level and inter mittent exposure (relief stations), velocities of 1000 to 2000 chanical refrigeration is finding greatly increased use in pre cision work and testing areas, in areas requiring constant 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 CONTROL OF ENVIRONMENTAL GASES AND fpm have been used successfully. Great care must be exercised ambient conditions for product uniformity or control, and needed, and up in the winter for ventilation, heating, and VAPORS, DUSTS, AND FUMES in using such high velocities, however, to avoid the undesirable effects of air at high temperature. Ordinarily the temperature of the air supplied to the worker for convective cooling should be about SO F. Evapo where the increased efficiency of the worker is reflected in a reasonable return on the investment. It is essential that the outdoor air, (with or without cool ing) be brought as directly as feasible to the proximity of the 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 In general, the systems for the control of these atmos pheric contaminants in industrial plants will be of three types: rative cooling ventilation systems will usually provide this. Preferably, the air should be directed to the front and torso of the body. Impingement on the back of the head, neck, and shoulders should be minimised. The supplied air may require tempering in winter to raise it to an acceptable temperature. However, in areas of high beat load, the tem perature of the air supplied during the winter may have to be lower than room temperature to provide adequate relief for the worker. . People vary considerably, in their tolerance to air motion, temperature, and humidity, and this tolerance varies with the season. It is essential, therefore, that the air supply out work station. Impingement upon hot equipment or mixing with hot ambient air should be mimmired Supply ducts passing through hot areas should be insulated. The use of aluminum ducts will minimize heat gain from radiant sources. For large work areas provided with a high rate of ventilation the local relief can best be obtained by locating the outlets as close to the floor as posible, consistent with the work and structural interferences. Design of Outlets for Local Relief In the design of outlets for local relief it is of great im 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. Directionaliring and damper control should be designed for easy ad justment from the floor by chain, sash cord, or pole. Where 1. Local exhaust systems will be indicated where the con tamination 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 rhia type system is discussed in Chapter 52, 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, re moving 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 lets for most local relief ventilation systems be adjustable in direction, and permit reduction in outlet velocity. A level portance to consider the following: location; discharge veloc ity, discharge volume, and control of direction. The influence the type of system should be made on the Hactu of economic comparisons. of air motion which feds comfortable and refreshing in hot weather may feel disagreeable and drafty in the winter. of these items will be discussed in the paragraphs which follow. Design of Dilution Systems Types and Design Requirements of Systems Location. The outlets should be kept as close to the worker as posable in order to minimise mixing with the warmer The first step in the design of a system employing the dilution method is to determine as exactly as possible the The supply air can be provided by local man-cooling fans; by outdoor air introduced directly or after dehumidification or cooling (evaporative or mechanical); and by combinations air in the space. In most areas the outlets can and should be brought down to the 7-ft level. Outlets at floor level can be used with success in many cases. nature and extent of the contaminating load. This will often be difficult, and may require construction of pilot production models. Often, however, the required data will be available of outdoor and recirculated air either direct or after con Discharge Velocity. The discharge velocity may be as High from production records, showing the weight or volume rate ditioning, as required. aa necessary to obtain the desired velocity at the work of loss of the contaminating agent to the atmosphere, or it Local man-cooling fans should be used with caution other station, as outlined above. Outlet velocities of 3000 to 4000 may be estimated from parallel operations in other plants, or than in light beat-load areas where the ambient temperature fpm may be necessary for remotely located outlets. Velocities by applying experienced engineering judgment. However ob is below the skin temperature. In particular, where there is of 1000 to 2000 fpm are the most frequently used for low tained, the determination of the nature and magnitude of the an elevated ambient temperature with or without high radi outlets (at the 7-ft level). When the supply air is cooled, contaminating load is an indispensable step in the proper ation load, the high velocity may add considerably to the the velocity through an outlet directly at or over the worker design of the corrective system. Designs based on number convective heat load and thus seriously increase the demand must be kept low (around 50 fpm). It should be kept in mind of air changes per hour, or other rule-of-thumb methods, are for sweating and evaporative cooling. that these recommended velocities are for conditions of max hopelessly inadequate, and lead either to unsuccessful opera A relief system employing outdoor air is to be preferred imum heat load. For more moderate weather and ambient tion or to excessive and unnecessarily high cost of installation. over-man-coolers and will provide excellent relief in many industrial areas. However, when the outdoor air temperature exceeds the skin temperature, the direct supply of outdoor conditions the workers will desire to reduce the velocities. Outlet dampers for velocity control (in the direction of the worker) should always be provided. The control must be 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. air is obviously reduced in effectivenes. Such a system is designed so as not to reduce the ventilation for the space. For cases involving diffusible vapor or gas contaminants, maxi properly used only in geographical areas where the periods of hot weather are of short duration. In geographic areas where outdoor humidities do not Discharge Volume. The outlet volume required will vary widely, depending upon whether the system is designed to provide highly localized spot cooling or is to provide general Fig. 3 .... Directional Outlets for Spot Cooling mum allowable concentrations (MAC) of commonly encoun tered gases and vapors have been established, and these data are tabulated in Chapter 7. From these data, and the previously, established rate of addition of the contaminant to the space, exceed 50 percent relative humidity at high daytime tem ventilation throughout a sizeable work area. Generally 1500 the volume of air required to dilute the addition to a tolerable peratures an evaporative cooling system offers greater relief to 2000 cfm per station will be adequate for moderate loads, a constant total volume of supply make-up air must be level can be calculated by the equation: for the worker in that the discharge air temperature can be and 3000 cfm per station for higher loads such as at hot maintained, an outlet similar to E will permit volume adjust _ _Qt X 10* lowered (within five degrees of the wet-bulb temperature) to obtain adequate convective body cooling. The sensible heat gain by the discharge air (through mixing) in a property metal furnace stations. With remote outlets large air volumes are required to insure adequate relief because of the mixing of the supply air with the wanner surrounding air through ment 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 where v (MAC)-(SAC) w designed system will be sufficient to reduce the relative hu midity (and vapor pressure) of the supply air and thus per mit the necessary evaporative loss from the worker's skin. Generally the supply of evaporatively cooled air should not exceed 20 to 50 percent of the total ventilation through the building. So limited, the relative humidity will be kept low enough to avoid distress to the workers and rusting of equip which it is projected. The air stream from a large outlet will maintain an appreciable core of air at the original supply air temperature for a considerable distance from the outlet. Small outlets and slot outlets have small cores which are rapidly dissipated through induction. In small enclosures or semi-enclosures (shields against radiant heat, for instance) perforated panel supply outlets are very effective for me 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. Easilyadjusted diffusers, which will provide a flat or down-blast air discharge pattern, are available. Q " quantity of air circulated, cubic feet per minute. Qe " rate of generation of contaminant, cubic feet per minute. (MAC) " maximum allowable concentration, ppm by vol ume. (SAC) = concentration in supply air, ppm by volume. ment under most weather conditions in the United States. For information on the design of evaporative cooling systems, Il see References 5 and 6 and Chapter 41. chanically cooled installations, because of their low induction characteristics. For further information on outlet design and the throw and induction characteristics, see Chapter 20. Where overhead installations are not possible because of structural interferences or other difficulty, outlets near the floor may be used. These may be in the form of grilles located The rate of generation of the contaminating vapor will often be available as a weight or volume of liquid evaporated into the space per unit time. These may be converted to the units of Equation 9 by applying the principle that a pound-mol of a 1