Document YjpLkLYE6BpOMzzRpxr00Gw4k

HEATING VENTILATING AIR CONDITIONING GUIDE 1941 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 2 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 2 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 or down. One recent 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. Table 2. Recommended and Maximum Duct Velocities Designation Recommended Velocities, ppm Maximum Velocities, fpm Residences Schools, Theaters, Public Buildings Industrial Buildings Residences Theaters,Public Buildings' Industrial Buildings Outside Air Intakes3 700 800 1000 800 900 1200 Filters3 250 300 350 300 350 350 Heating Coils3 450 500 600 500 600 700 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 700-900 600 500 1000-1300 1200-1800 600-900 800-1000 600-700 800 800-1000 1100-1400 1300-2000 700 800-1000 1000-1200 650 800-900 1000 aThe8e velocities are for total face area, not the net free area. 582 CHAPTER 31. AIR DUCT DESIGN 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 30 for a discussion of grilles. Fan outlet velocities, are discussed in Chapter 29 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 2, 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 toward the duct openings to the room. To find the total static pressure against which the fan must operate, the static pressure loss of each section must be calculated separately and the total loss found by adding the indi vidual losses of the various sections of the run having the highest resis tance. Usually this is the longest run but in some cases a shorter run may have more elbows; transformations, booster heaters, etc., which will cause it to have a higher resistance pressure. This method requires judgment and experience in choosing the proper velocities to approach equal friction for all lengths of run but many engineers believe that the velocity method is handier to use than other methods and will give satisfactory results for most practical applications. The air velocities given earlier in this chapter are helpful in choosing proper velocities. Adjustable dampers or splitters are used to regulate air quantities delivered. Equal Friction Method The equal friction method of design is sometimes preferred because it does not require nearly so much judgment and experience in selecting the proper velocities in the various sections of a system. The usual procedure in this method of design is to select the main duct velocity to be con sistent with good practice from a standpoint of noise for a particular type of building. This velocity should be less than the fan outlet velocity. All main ducts and branch ducts are sized for equal friction by the use of Fig. 2 and Table 1 or Fig. 3. 583