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Branch Entry Velocity Correction

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. The following steps will establish the overall pressure loss of a duct segment that starts at a hood. Figure 5-2 shows a simple one-hood ventilation system. The use of a calculation sheet can be very beneficial when performing the calculations manually. Figure 5-3 shows the details of the calculations for each component of the system. There is also a profile through the system showing the magnitude and relationships of total, static, and velocity pressures on both the “suction” and the “pressure” sides of the fan on Figure 5-2. It should be noted that YP is always positive. A