COLUMN ENGINEER'S NOTEBOOK TABLE 1 Rule-of-thumb design velocities for transfer assemblies. FIGURE 3 Inexpensive return air sound boot. DUCT SIZING APPLICATION RULE OF THUMB FPM NO. INLET LOCATION DISCHARGE LOCATION 1 Return Air Plenum Return Air Plenum Lined 5 ft Boot (Figure 3) 800 2 Return Air Plenum Return Air Plenum Flex Duct Both Sides 750 3 Return Air Plenum Return Air Plenum Single Elbow (No Turning Vanes) 700 4 Return Air Plenum Return Air Plenum Double Elbow Both Sides (No Turning Vanes) 575 5 Ceiling Grille Return Air Plenum Flex Duct to Perforated Face Grille 500 6 Ceiling Grille Ceiling Grille Flex Duct to Perforated Face Grilles 350 7 Return Air Plenum Toilet Makeup. Ceiling Grille Flex Duct to Perforated Face 325 Wall Lined Transfer Boot many floor-to-floor partitions, such as acoustical partitions, tenant separation walls, and rated corridor walls. Where there are multiple partitions between the air handler or return air shaft and the most remote rooms, return air transfer openings and sound boots must be sized for even lower velocities and get progressively larger (Figure 4) to ensure that the overall pressure drop remains low to www.info.hotims.com/54426-11 46 ASHRAE JOURNAL ashrae.org MARCH 2015 The velocities are intended to result in a 0.08 in.w.c. (20 Pa) pressure drop across the transfer assembly including pressure drop of entrance, exit, duct, and grilles. Return air plenum is assumed to be 0.02 in. w.c. (5 Pa) relative to the space for Application 5 and 0.05 in. w.c. (12 Pa) for Application 7. Note that these are for a single return air transfer - for multiple boots in series (e.g., cascading from one room to another before it gets to the shaft), velocities must be even lower so the total pressure drop does not exceed 0.08 in. from furthest room to shaft or fan room to ensure exterior plenum walls are positively pressurized relative to the outdoors. www.info.hotims.com/54426-33