Naughton, ("HVAC Systems… Part 1," 1990) quotes Gerbig (1984), who points out, "Pressurized plenum designs may reduce system static requirements up to 1.0 in. WG (249 Pa) when compared to ducted HEPA filter systems. The primary benefit of ducted HEPA filters is the precise balance of the cleanroom unidirectional velocity profile or parallelism. Precise balance also provides the flexibility to have mixed clean room velocities."
Energy savings result when a cleanroom uses mixed velocities. The average velocity is reduced when production equipment air-flow velocity is 90 fpm (0.457 m/s) and the balance of the cleanroom area air flow is 60 fpm (0.305 m/s). The amount by which the velocity is reduced is simply proportional to the areas of mixed velocity. Even though mixed velocities can be provided with a pressurized plenum design, a ducted design is flexible for future changes. The designer is cautioned that ducted systems require "... a more tedious and iterative balancing procedure with higher maintenance operating costs." [Naughton, "HVAC Systems… Part 1," 1990]
High- and low-velocity cleanrooms
Cleanroom laminar flow velocity
