Document 10kj5dJ82OB5JmjLMVeO2GGGo

746 CHAPTER 32 1956 Guide The loss for a sudden symmetrical contraction can be expressed as where Hc -- pressure loss due to sudden contraction (Fig. 10b) Co = loss coefficient based on orifice area A o Vo = velocity of air through orifice, feet per minute. The loss for a gradual symmetrical contraction can be similarly expressed H.,, (11) where the coefficient of loss, C2, depends on the included angle of the sides of the duct and the sharpness of the edges at the junction of taper to follow ing duct section. Pressure loss data for a variety of duct elements are given in Table 3. DUCT DESIGN The following discussion refers to ducts for commercial and industrial heating, ventilating, and air conditioning systems of the central station type. The design procedures given yield the static pressure required to ' overcome the resistance of the ductwork, including the supply outlets and return intakes. The fan selected for the duct system must not only pro duce this pressure but also the additional pressure required by the central equipment such as washers or spray chambers, heating or cooling coils, and filters. Pressure losses of these components should be obtained from the manufacturers'catalogs. Special duct design procedures for heating ducts used in residences can be found in Chapter 19, Gravity Warm Air Systems, and Chapter 20, Forced Warm Air Systems. The design of ducts in industrial exhaust systems is discussed in Chapter 46. General rules which should be followed in the design are: 1. The air should be conveyed as directly as possible at the permissible velocities to obtain the desired results with minimum noise and greatest economy of power, material and space. 2. Sudden changes in the direction or velocity of the air should be avoided. When sudden changes are necessary at bends, turning vanes should be used to minimise the pressure loss. 3. Diverging transition pieces should be made as gradual as practicable. As shown in the section on area changes, losses in abrupt enlargements are high and therefore such transitions should be avoided. The included angle of divergence for enlargement should not exceed 20 deg. Losses in contractions are low but the in cluded angle of convergence should not be greater than 60 deg. 4. Where the greatest air carrying capacity per square foot of sheet metal is desired, rectangular ducts should be made as nearly square as possible. Aspect ratios (ratio of width to depth) greater than 10 to 1 should be avoided. 5. Ducts should be constructed of smooth material, such as steel or aluminum sheet metal. For ducts made from other materials, proper allowance for the change in roughness should be made.' 6. Through the design procedures which follow, a reasonably precise estimate of. the flow resistances offered by the system can be obtained. However, it should be recognised that in actual installations, resistances may vary considerably from the Air Duct Design 747 calculated values because of variation in the smoothnessiof materials, types of joints used, and the ability of workmen to fabricate the system in accordance with the de sign. Fans and motors should therefore be selected to provide at least a slight factor of safety, and dampers should be installed in each branch outlet for balancing the system. Procedure for Duct Design The general procedure is as follows: 1. Study the plan of the building and arrange the positions of the supply outlets to provide proper distribution of air within each space. Select outlet sizes from manufacturers' catalog data. 2. Draw a sketch of the most convenient system.of ductwork, connecting the supply outlets and return intakes with the. central station apparatus, taking cog- Table 5. Recommended and Maximum Duct Velocities Recommended Velocities, tvu Maximum Velocxixss, tfu Designation Outside Air Intakes* Filters* Heating Coils* Air Washers Suction Connections Fan Outlets Main Ducts Branch Ducts Branch Risers Residences Schools, Theaters, Public Buildings Industrial Buildings Residences . Schools,: Theaters, Public Buildings; Industrial Buildings 500 250 450 500 500 300 350 500 600 800 : 900 1200 300 350. 350 500 600. 700 500 500 500 : 700 . 800 1000 1000-1600 1300-2000 1600-2400 500 900 1700 500 `500 1000 1400 1500-2200 1700-2800 700-900 600 500 1000-1300 1200-1800 600-900 800-1000 600-700 800 800-1200 1100-1600 1300-2200 700-1000 800-1300 1000-1800 650-800 800-1200 1000-1600 * These velocities are for total face area, not the net free area; other velocities in table are for net free area _ _______ 0 --......<vimuiu ,wi uusvructions in steel worjc ana equipment, and at the same time maintaining a simple design. , ' 3. Calculate the sizes of all main and branch ducts by one of the procedures given in the following section. 4. Determine the total pressure requirement of the supply and return duct sys tems. Although the loss in total pressure of each duet run connecting the fan and each supply outlet (or return intake) should be calculated and made the same for all runs, ordinarily only the pressure loss of the duct run apparently having the greatest resistance is referred to as the pressure loss of the duct system. Dampers are relied upon for balancing the system. Design Velocities It is not possible to give specific rules for selecting duct velocities. Since the fan horsepower increases approximately as the square of the velocity, Ahd noise generation increases with static pressure, velocities should, be kept low for quiet and economical operation. On the other hand, from Equation 3, at a given flow rate the duct size increases with decreasing velocity. For multi-story buildings, it is sometimes possible to reduce the height between floors by using very small ducts, thereby effecting, a con siderable reduction in building investment cost. Velocities as high as 6000 fpm in main ducts have been used in high-velocity duct systems. Spe cial outlets for such systems have recently been marketed; they incorporate a device for reducing the pressure and attenuating the noise before intro ducing the supply air into the conditioned space. High-velocity, duct sys-