Typically, large changes in duct size occur in connections from a filter bank to a duct trunk. Building a tapered connection in a length of less than 1.2 m (4 ft) (Figure 4) causes sharply angled (60°) sides in a four-sided connection, C=0.85 (C varies with angle and area ratio). To improve construction economy, while reducing pressure drop, install a plenum box on the filter bank, and add a cone, C=0.085, or bell mouth, C=0.03, for the duct entry (Figure 5).
Figure 7.4
Figure 7.5
