Document pmmbY9xDz7ZrjjXprzDew3opa

702 CHAPTER 32 1953 Guide 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-broken 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 resistance. Since high velocities at one point offset the effect of proper de sign in all other parts of the system, emphasis should be placed on the im- Tahi.e 4. Recommended and Maximum'Duct Velocities Recommended Velocities, ppm Maximum Velocities, fpm Designation Outside Air Intakes* Filters* Heating Coils* Air Washers Suction Connections Fan Outlets Main Ducts Branch Ducts Branch Risers Schools, Residences Theaters, Public . Buildings Industrial Buildings Residences Schools, Theaters, Public - Buildings Industrial Buildings 500 500 250 300 450 500 500 800 , 900 1200 350 300 350 350 600 500 600 700 500 500 500 700 800 1000 1000-1600 1300-2000 1600-2400 500 900 1700 500 500 - 1000 1400 1500-2200 1700-2800 700-900 1000-1300 1200-1800 800-1200 1100-1600 i300-2200 600 600-900 800-1000 700-1000 800-1300 1000-1800 500 . 600-700 800 650-800. 800-1200|l000-1600 * These velocities are for total face area, not the net free area; other velocities in table are for net free area. portance of air velocities, elbow design, location of dampers, fan connec tions, grille and register approach connections, and similar details. For in dustrial 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, resis tance 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. 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 be used to straighten the flow of air from the outlet. Sometimes it is desirable to mount the outlet flush with the side of the duct, in which case Air Duct Design 703 the duct velocity should be kept below twice that of the outlet velocity; and even then an outlet larger than normally required should be used,'as the entire outlet area will not be effective. Manufacturers' selection tables base sizing of outlets on required volume of air, temperature differential, and distance of throw or radius of diffusion. In following their recom mendations, maxima should be avoided. See Chapter 31 for a discussion of air outlets. DESIGN METHODS The design of the-air transmission system is generally the last step in the design of the heating, ventilating, or air conditioning system, but it should always be kept in mind that the type of air transmission used will, to some extent, depend on the type of equipment used, as well as on the purpose of the system. Various factors such as zoning and zone control, and their influence on the transmission and air distribution system, are briefly discussed in Chapter 30 (Central Systems for Air Conditioning). The methods used for the design of duct systems reflect, to some degree, certain developments in the arts of heating, ventilating and air conditioning, and it took a long time before empirical methods gave way to more refined and scientific calculations. Some engineers prefer speed and simplicity to scientific exactness, but experience is then needed and proper judgment must be exercised to prevent mistakes. Both the Velocity Reduction Method and the Equal Friction Method take no account of the static regain resulting from the difference between the velocity of fan discharge and velocities of pipe discharge, and are therefore, to some degree, approximate methods. However, they are more easily applied than the static regain method which is based on proper theory,.but is subject to' an assumption (based on tests) regarding the efficiency of conversion from kinetic energy'to static regain. 1.. Velocity Reduction. Method When this method is used, arbitrary velocities for the various sections of the ducts are selected, with the highest velocity at the fan outlet, and lower velocities down the run as various branch ducts are taken off the main duct. Since the quantities of air that are to be delivered through each section of the duct are known, the area of each duct section can be easily determined by using the formula: 0, V, (17) where A = duct area in square feet. Q. = air quantity in cubic feet per minute. Vm = air velocity in feet per minute. , To find the total static pressure against which the fan must operate, the static pressure loss of each section is calculated separately, and the total loss found by adding the individual losses of the sections of the duct which has the highest resistance. This may be the duct with the longest run, but not necessarily so. The velocity method has the advantage that the duct area can be deter mined very easily.- It should be used only for simple layouts. The air velocities given earlier in this chapter are helpful in choosing proper ve-