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-( American Society of Heating and Ventilating Engineers Guide, 1937 A splitter facing the direction of air flow cuts off the air and delivers the desired amount to the branch. uc 11 Why does a wide, shallow duct offer more resistance to the flow of air than does a square duct of equal cross-sectional area? The perimeter of the wide, flat duct is greater than that of the square-section duct, so the former has the greater frictional area which increases the resistance and thus reduces the volume at any given pressure. 12 9 What methods are used to keep large ducts from vibrating because of air pulsations, and from sagging because of their own weight? External bracing, such as standing seams, or structural shapes,_like tees or angles, should be placed across the top and bottom. Exterior braces or cross buckling of metal sheets in diagonal panels may be used for the sides of large ducts. 13 What velocities of air flow, should be used in the trunk ducts of a venti lating system in a, public building? . . .. From 1200 to 1600 fpm. 14 In a ventilating system in a residence, what is the recommended, air. velocity through supply registers and-grilles?' - 400 fpm. 382 Chapter 21 INDUSTRIAL EXHAUST SYSTEMS System Classification, Design Procedure, Requirements for Suction and Velocity, Hoods, Design of Duct Systems, Col lectors, Resistance of Systems, Efficiency of Exhaust Systems, Selection of Fans and Motors, Corrosion SOME type of exhaust and collecting system is necessary in almost every industry and the present chapter attempts to give general information relating to the design of factory exhaust systems in order that efficient and economical control of dusts and fumes may be achieved. SYSTEM CLASSIFICATIONS There are two general arrangements, the central and the group systems. In the central system a single or double fan is located near the center of the shop with a piping system radiating to the various machines to be served. In the group system, which is sometimes employed where the machines to be served are widely scattered, small individual exhaust fans are located at the center of the machine groups. The group arrangement has the advantage of flexibility. Exhaust systems are also classified by the means employed to collect dust or other material handled. The dust or refuse may be collected and controlled by enclosing- hoods, open hoods, inward air leakage, or by exhausting the general air of the room. With some classes of machinery it is not feasible to closely hood the machines and in these cases open hoods over or adjacent to the machines are provided to collect as much as possible of the dust and fumes. This class includes such machines as rubber mills, package filling machinery, sand blast, crushers,; forges, pickling tanks,, melting furnaces, and the unloading points of various types of conveyors. The open hoods should be placed as close to the source of dust or fumes as possible, with due regard to the movements of the operator. When the hood must be placed-at some distance above the machine it should be large enough to encompass an area of considerable extent as diffusion is usually quite rapid. ' Consideration must also be given to the natural movement of the fumes. For' those, that are lighter than air the hood should be over of above the machine and where a heavy vapor or dust-laden air at ordinary temperature, is to be removed, horizontal or floor connections are required. If it is attempted to remove heavy dust such as lead oxides by an over head hood the conditions may be worse than if no exhaust were used at all, owing to the rising air current carrying the dust up through the 383