Page 366 - Fundamentals of Air Pollution 3E

I. Plume Rise 321

Impact estimates by specific models are required to meet some regulatory
requirements.

I. PLUME RISE

Gases leaving the tops of stacks rise higher than the stack top when they
are either of lower density than the surrounding air (buoyancy rise) or
ejected at a velocity high enough to give the exit gases upward kinetic
energy (momentum rise). Buoyancy rise is sometimes called thermal rise
because the most common cause of lower density is higher temperature.
Exceptions are emissions of gases of higher density than the surrounding
air and stack down wash, discussed next. To estimate effective plume
height, the equations of Briggs (1-5) are used. The wind speed u in the
following equations is the measured or estimated wind speed at the physical
stack top,

A. Stack Down wash

The lowering below the stack top of pieces of the plume by the vortices
shed downwind of the stack is simulated by using a value h' in place of
the physical stack height h. This is somewhat less than the physical height
when the stack gas exit velocity u s is less than 1.5 times the wind speed u,
1
m s" .

where d is the inside stack-top diameter, m. This h' value is used with
the buoyancy or momentum plume rise equations that follow. If stack
down wash is not considered, h is substituted for h' in the equations.

B. Buoyancy Flux Parameter
For most plume rise estimates, the value of the buoyancy flux parameter
4
3
F in m s~ is needed.

2
where g is the acceleration due to gravity, about 9.806 m s , T s is the stack
gas temperature in K, T is ambient air temperature in K, and the other
parameters are as previously defined.