Area of influence:
5L Downwind 2L Upwind 0.5L Crosswind
5L In all directions -
H = Height of building L = Lesser of height or width of building
1Compare H+1.5L to plume elevation (stack height above grade + momentum plume rise 2L
2Compare H+.5L to physical stack height above grade
FIG. 5.8.13 Selection of downwash algorithms in the ISCST model. (Reprinted, with permission, from Trinity Consultants, Inc., 1989, Atmospheric diffusion notes, Issue no. 13, Dallas, Tex. [June].)
proach is called the good engineering practice (GEP) of stack height design (U.S. EPA 1985) and retroactively covers all stacks built since December 31, 1970. The following equation determines the GEP stack height:
Hgep = the GEP stack height
H = the maximum height of an adjacent or nearby structure
L = the lesser dimension of the height or projected width of an adjacent or nearby structure
The projected width of a structure is the exposed area perpendicular to the wind as shown in Figure 5.8.14. Environmental engineers should check all structures within 5L of the stack for their possible effect on the plume. The range of influence for a given structure is defined by Tickvart (1988) as 2L upwind of the structure, 5L downwind, and 0.5L on the sides parallel to the wind flow.
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