Energy is required to provide the effluent's stack exit velocity. In the absence of a thorough wind model for the facility, accepted design practice recommends that exhaust air be ejected upward at 3,000 fpm (15.24 m/s or 34 mph). This will compensate for shadowing effects from the stack itself resulting from a horizontal wind velocity of approximately 22 mph. The accepted design practice further assumes that 22 mph is an "average" wind velocity for all facilities and that higher wind velocities will not have a stack shadow effect because the effluent will be diluted by the higher wind speed itself. However, the literature also points out that: "higher exit velocities may be necessary due to the proximity of adjacent structures, topographical features, or other unique situations." [Laboratory Control and Safety..., 1994]
Accepted design practice gives little guidance to the energy engineer who wishes to ensure that effluent is discharged upward at a high enough velocity but at the same time to use the smallest amount of possible fan power and energy. [Pericich, 2001]
