MIL-HDBK-1003/6

Stack Design. The stack height calculations are for the effective

8.7.2

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

entering stack.

b)

Heat losses in stack (to find mean stack temperature).

c)

Altitude and barometric corrections for specific volume.

d) Gas weight to be handled. (Consider infiltration of air into

casing and ductwork as well as combustion air.)

e) Stack draft losses due to fluid friction in the stack and

kinetic energy of gases leaving stack.

f)

Minimum stack height to satisfy dispersion requirements of gas

emissions.

g)

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

atmosphere.)

Example Stack Calculations. Items a) through i) regarding stack

8.7.3

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:

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