Stack Design. The stack height calculations are for the effective
stack height rather than the actual height; this is the distance from the top
of the stack to the centerline of the opening of the stack where the flue gas
enters. Air and gas flow losses through the inlet air duct, air heater (air
side), windbox, furnace and passes, air heater (gas side) or economizer, gas
cleanup equipment and other losses through duct and breaching should be
plotted and overcome with the fans. The kinetic discharge head, the friction
losses at the entrance to the stack, and friction losses in the stack should
be provided by the natural draft of the stack. Barometric pressures adjusted
for altitude and temperature must be considered in determining air pressures.
The following stack design parameters must be determined.
a) Extreme and average temperatures of ambient air and gas
Heat losses in stack (to find mean stack temperature).
Altitude and barometric corrections for specific volume.
d) Gas weight to be handled. (Consider infiltration of air into
e) Stack draft losses due to fluid friction in the stack and
kinetic energy of gases leaving stack.
Minimum stack height to satisfy dispersion requirements of gas
Economical stack diameter.
h) Stack height for required draft. (Where scrubbers are used,
the temperature may be too low for sufficient buoyancy to overcome the stacks
internal pressure losses and provide adequate dispersion of flue gas into the
Example Stack Calculations. Items a) through i) regarding stack
calculations were taken from The Babcock & Wilcox Company (1978), chapter 17.
a) Preliminary Selection: For convenience in making a tentative
selection of optimum stack dimensions, for sea level and 80 degrees F (26.7
degrees C), the necessary data are given graphically in Figure 38, Figure 39,
Figure 40, and Figure 41. The use of these graphs to establish stack
dimensions is illustrated by the following examples: