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' HEATING VENTILATING AIR-CONDITIONING GUIDE 1943 practice limits the ratio between the long side and the short side to 3 to 1. In no case should this ratio exceed 10 to 1. 6. Wherever possible, ducts should be constructed of smooth material such as sheet metal. Where masonry ducts are used, proper allowance for the surface coefficient should be made. 7. The use of furred spaces, spaces between joists, etc., should be avoided unless lined with sheet metal. Procedure for Duct Design The general procedure for designing.a duct system is outlined in the several items listed herewith: 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 preceding paragraph, for the proper outlet velocity and .throw. 5. Calculate the sizes of all main and branch ducts by either of the following two methods: , . Velocity Method. Arbitrarily fix the velocity in the various sections, reducing the velocity from the point of leaving the fan to the point of discharge to the room; In this case the pressure loss of each section of the duct is calculated separately and the total loss found by adding together^the losses of the various sections of the continuous run. .- Friction Pressure Lass Method. Proportion the duct for equal friction pressure toss per foot of length. 6. Calculate the friction for the duct offering the greatest resistance to the flow of air, which resistance represents the static pressure which must be maintained in the fan outlet or in the plenum space to insure distribution of air in the duct system. The duct having the greatest resistance will usually be that having the longest run, although not necessarily so. Air Velocities The air velocities given in Table 3 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 vibration. High velocities at one point in the system offset the effect of proper design in all other parts of the system; hence the importance of-air velocities, elbow design, location of dampers, fan connections,, grille and register approach connections, and similar attention to details. For industrial buildings-, noise is seldom given much consideration, and main duct velocities as high as 2800 or 3000 fpm are sometimes used but, when these velocities 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. Wherever velocities-higher than those shown in Table 3 are used, it is essential that the ducts should be of heavier gages, have addi tional bracing and be carefully constructed for a minimum resistance* Where the high velocity diffusing outlets are used, the duct velocity should not be less than the throat velocity of the diffusers, as dynamic losses occur wherever velocities are stepped up dr down. One recent 616 CHAPTER 32. AIR DUCT DESICN trend in grille design is toward the use of much higher grille and branch duct velocities. Some installations have been made with velocities as high as 1600 fpm in branches and through the net area of grilles, but many of these have proven unsatisfactory because of noise and drafts. Grille manufacturers publish selection tables which size the grilles for volume of air, temperature differential and distance of throw. In following these tables, maximums should be avoided and the manner in which the duct connects to the grille should be given careful consideration. Most of the selection tables are based on. straight approach to the grille. Elbow connections to supply grilles should be provided with turning vanes to equalize the face velocity. See Chapter 31 for a discussion of grilles. Fan outlet velocities ate discussed in Chapter 30 and will not be dealt with here except to indicate that fan . noises should be given proper consideration. Main Trunk Ducts Main trunk ducts with branches are commonly used to convey the air from the fan to the grille or register outlets in preference to individual ducts from the fan to these outlets. The velocities in these ducts and branches vary according to the nature of the installation and the degree of quietness desired. The recommended velocities in Table 3, with good construction, should give satisfactory results. The maximum velocities indicated should not be used except in areas where noise is not a deciding factor. Velocity Method The velocity method of designing a duct system involves arbitrarily selecting velocities at various sections of the duct system with the highest velocities generally chosen at the fan and progressive lower velocities Table 3. Recommended and Maximum Duct Velocities Designation Recommended Velocities, fpm Maximum Velocities, fpm Residences Schools. Theaters, Public Buildings Industrial Buildings . Residences Schools. Theaters, Public Buildings Industrial Buildings Outside Air Intakes2 Filters2 Heating Coils2 700 250 450 800 1000 800 300 350 300 500 600 500 900 1200 350 350 600 700 - Air Washers Suction Connections Fan Outlets 500 500 500 700 800 1000 1000-1600 1300-2000 1600-2400 500 900 1700 500 500 1000 1400 1500-2200 1700-2800 Main Ducts Branch Ducts Branch Risers 700-900 1000-1300 1200-1800 800-1000 1100-1400 1300-2000 600 600-900 800-1000 ,700 800-1000 1000-1200 500 600-700 800 650 800-900 1000 aThese velocities are for total face area, not the net free area. 617