Document DDyJ325EwZe57MokzrkLKXgbn

656 CHAPTER 31 - 1950 Guide where ht = regain in static pressure, feet of fluid flowing. ' Oi and tit = mean velocities in inlet and outlet duct sections, respectively, feet per second. . The static pressure regain in a gradually expanding transition, followed by an after section, may be expressed as: or (16) where ci = an experimentally determined constant depending on nature of construction. Curves have been developed showing the static pressure regain and the theoretical efficiency of conversion in abrupt expansion, and in diverging sections in smooth circular ducts.`*,u DUCT DESIGN The discussion of duct design in this chapter refers to ducts in fan sys tems for central heating, ventilating and air conditioning systems. Addi tional data for heating ducts used in residences are to be found in Chapter 18 (Gravity Warm Air Systems) and Chapter 19 (Mechanical Warm Air Systems). The design of ducts in industrial exhaust systems is discussed in Chapter 45. The following general rules should be followed in design: 1. The air should be conveyed as directly as possible at the permissible velocities to obtain the desired results with greatest economy of power, material and space.. 2. Sharp elbows and bends should be avoided. Carefully designed splitters and turning vanes should be used to reduce the elbow or outlet pressure loss. 3. Diverging transformation pieces should be made as long as practicable. As shown in the section on area changes, losses in sudden enlargements are high, and abrupt enlargements should be avoided. The included angle of divergence for enlargements should not exceed 20 deg. Losses in contractions are low, but the in cluded angle of convergence should not be larger than 60 deg. ' 4. Special-care should be taken to avoid restriction of flow in elbows or trans formation pieces. . 5. Rectangular ducts should be made as nearly square as possible. Good practice limits the ratio between the long side and the short side 3 to 1. In no case should this ratio exceed 10 to 1. 6. Ducts should be constructed of smooth material, such as steel or aluminum sheet metal. For ducts made from other materials, for example masonry, proper allowance for the surface friction coefficient should be made. Procedure for Duct Design The general procedure for design is outlined as follows: 1. Study the plan of the building and draw in roughly the most convenient system of ducts, taking cognizance of the building construction, avoiding all obstructions in steel work and equipment, and at the same time maintaining a simple design.. 2. Arrange the positions of duct outlets to insure the proper distribution of air. 3. Divide the building into zones and proportion the volume of air necessary for each zone. 4. Determine the size of each outlet, based on the volume as obtained in the pre- Air Duct Design 657 ceding paragraph, for.the proper outlet velocity and throw. In case of some ceiling diffusers, determine size of outlet for proper throat velocity and radius of diffusion. 5. Calculate the sizes of all main and branch ducts by one of the three methods of sizing air supply systems in common use, the velocity reduction method, the equal friction method or the static regain method. 6. Calculate the losses for the duct offering the greatest resistance to the flow of air, using the A.S.H.V.E. Friction Charts, Figs. 1 and 2, and the other data given in this chapter. Recommended Design Velocities The air velocities given in Table 4 have been found to give satisfactory results in engineering practice. Where the higher velocities are used, the ducts should be cross-braced to prevent breathing, buckling or vibra tion, and should be constructed of heavier gage metal. At the higher velocities, it is particularly important to design the ducts for minimum Tablb 4. Recommended and Maximum Duct Velocities Recommsitobd Velocities, fpm Maximum Velocities, fpm DsSHONATIOir 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, Industrial. Public Buildings Buildings 600 500 500 800 900 1200 250 300 350 300 350 . 350 450 500 600 500 600 700 500 500 500 500 700 800 1000 900 1000-1600 1300-2000 1600-2400 1700 500 500 1000 1400 1500-2200 1700-2800 700-900 1000-1300 1200-1800 800-1200 1100-1600 1300-2200 600 600-900 800-1000 700-1000 800-1300 1000-1801 500 600-700 800 650-800 800-1200 1000-1603 * These velocities are for total face area, not the net free area; other velocities in table are for net free area.. resistance. As high velocities at one point offset the effect of proper design in all other parts of the system, emphasis should be placed on the impor tance of air..velocities, elbow, design, location of dampers, fan connections, grille and register approach connections, and similar details. For industrial buildings, noise is seldom given much consideration, and main duct veloci ties as high as 2800 or 3000 fpm are sometimes used, but when these veloci ties are used due consideration should be given to duct design, resistance pressure, fan efficiencies and motor horsepower. For department stores and similar buildings, 2000 to 2200 fpm are sometimes used in main ducts where noise is not objectionable and space conditions warrant it. Where high velocity diffusing outlets are used, the duct velocity should be, if possible, equal to, or somewhat lower than the throat (neck) velocity of the diffuser, in order to utilize the effect of higher static pressure in the duct for equalization of air discharge. ', _ The velocities in main ducts, and particularly in branch ducts and branch risers, should be correlated to the throat (neck) velocity of the air outlets, and manufacturers' data should be consulted for permissible throat velocity for the particular type of application. - If it is necessary to'use a duct velocity that is twice the velocity for an outlet mounted on the side of the duct, a collar with directing vanes should