Low-velocity ductwork design is very important for energy efficiency in air distribution systems. Designers should be aware that, for a particular air-flow rate, doubling of duct diameter will reduce friction loss by a factor of 32 times. However, low-velocity systems occupy more space and have higher first costs; facility owners are often reluctant to provide the space for more expensive ductwork, but significant energy savings can be realized even when the ductwork is only increased by one standard size. The use of pressurized plenums also reduces friction losses in supply ductwork. [Shepard et al., 1995; Naughton, "HVAC Systems… Part 1," 1990]
