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# The Velocity Pressure (VP) Method

DUCT SEGMENT CALCULATIONS The Velocity Pressure (VP) Method is based on the fact that all frictional and dynamic (fitting) losses in ducts and hoods are functions of the velocity pressure and can be calculated by a loss coefficient multiplied by the velocity pressure. Loss coefficients for hoods, straight ducts, elbows, branch entries, contractions, and expansions are shown in Figures 5-13 through 5-16. Figure 5-1 shows the application of these coefficients. For convenience, loss coefficients for round elbows and entries are also presented on the calculation sheet (see Figure 5-3). Friction data for this method are presented as Tables 5-5 and 5-6. These tables give the loss coefficients per foot of galvanized and commercial steel, aluminum, PVC, and stainless steel duct. The equations for these tables are listed on S~ V1 v] VP, VP3 h + VP (1) e d (1) See 3.5.1 and 3.5.2 (2) See 5.9.1 FIGURE 5-1. System duct calculation parameter location duct 1 L ~I duct 2 h f Hf LVP’ — SP2 hen = Fen VP2 hel = Fel VP2 duct 3 5-4 Industrial Ventilation these tables and also on the calculation sheet (see Figure 5-3)YI) These equations and the resultant tables have been designed to be no more than 4% different from the “exact” values of the Colebrook-White equation and were designed to err on the high side ofthe normal velocity range of exhaust venti lation systems. For convenience, two data sets determined from the same equations were used to generate the friction tables. These tables are possible because, for a specific diameter, the friction loss coefficient changes only slightly with velocity. Each table lists the friction coefficient as a function of diameter for six different velocities. The error in using these data with velocities plus or minus 1000 fpm is within 6%. If desired, a linear interpolation between velocity values can be performed. In Chapter 1, an equation was presented for flexible duct with the wires covered. No data are presented here for this type of material due to the wide variability from manufacturer to manufacturer. Perhaps an even more important reason is that these data are for straight duct losses, and flexible duct, by its very nature, is seldom straight. Typically, bends in flexible duct can produce extremely large losses which cannot be predicted easily. Be very careful to keep the flexible duct as straight and as short as possible.