Boilers or industrial furnaces that have a well-defined stack and emissions resulting from products of combustion are the most common sources of pollutants modeled. The hot plume emitted from the stack rises until it has expanded and cooled sufficiently to be in volumetric and thermal equilibrium with the surrounding atmosphere. The height at which the plume stabilizes is referred to as the effective plume height (H) and is defined as:
where (hs) is the physical stack height portion and (AH) is the plume rise portion as shown in Figure 5.8.5. The plume rise is the increase in height induced by both the momen tum and buoyancy effects of the plume. The momentum component of the rise is the physical speed at which the effluent is ejected from the stack, while the buoyancy component is due to the thermal characteristics of the plume in relation to ambient air. In the modeling, pollutants are assumed either to emit from a point directly above the stack at the effective plume height or to gradually rise over some distance downwind of the emission point until the effective plume height is reached.
The standard plume rise formula used in most EPA air dispersion models is based on a review of empirical data performed by Briggs (1969). (The next section includes a more detailed description.) The Briggs formula takes into account both momentum and buoyancy factors as well as meteorological conditions.
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