1.1.4.5

Unstable or Neutral ‑Momentum Rise.For situations where the stack gas temperature is less than or equal to the ambient air temperature, the assumption is made that the plume rise is dominated by momentum. If ΔT is less than (ΔT)

_{c}from Equation (1‑10) or (1‑11), the assumption is also made that the plume rise is dominated by momentum. The plume height is calculated from Equation (5.2) (Briggs, 1969, p. 59):Briggs (1969, p. 59) suggests that this equation is most applicable when v

_{s}/u_{s}is greater than 4.

1.1.4.6

Stability Parameter.For stable situations, the stability parameter, s, is calculated from the Equation (Briggs, 1971, p. 1031):

As a default approximation, for stability class E (or 5) ¶θ/¶z is taken as 0.020 K/m, and for class F (or 6), ¶θ/¶z is taken as 0.035 K/m.

1.1.4.7

Stable ‑ Crossover Between Momentum and Buoyancy.For cases with stack gas temperature greater than or equal to ambient temperature, it must be determined whether the plume rise is dominated by momentum or buoyancy. The crossover temperature difference, (ΔT)

_{c}, is determined by setting Briggs' (1975, p. 96) Equation 59 equal to Briggs' (1969, p. 59) Equation 4.28, and solving for ΔT, as follows:If the difference between stack gas and ambient temperature, ΔT, exceeds or equals (ΔT)

_{c}, plume rise is assumed to be buoyancy dominated, otherwise plume rise is assumed to be momentum dominated.

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