MIL-HDBK-1003/6
b)
General Data:
Type of Firing
Approx. Gas Weight lb/lb of Steam
Oil or Gas
1.15
Pulverized Coal
1.25
Stoker
1.50
Example Stack Calculations:
GIVEN:
Fuel
Pulverized Coal
Steam generated, lb/hr
360,000
Stack gas flow, lb./hr.
450,000
Stack inlet gas temp., degrees F
550
Stack exit gas temp. (assumed), degrees F
450
Stack draft required (from point of balance to stack
gas entrance), in. of water
1.0
Altitude of plant
Sea level
c) Diameter. Enter graph, Figure 38, with a stack gas flow of
450,000 lb/hr (56.7 kg/s). This will give a stack diameter (to nearest 6 inch
increment) of 14 ft 6 inch (4.42 m).
d) Height (approximate). Enter graph, Figure 39,, with a required
stack draft of 1 inch W.G. and an average gas temperature of (550 + 450)/2 =
500 degrees F (260 degrees C). This will give an approximate stack height of
160 ft (48.8 m).
e) Exit Gas Temperature, Figure 40. An approximate stack height
of 160 ft (48.8 m) and a diameter of 14 ft 6 inch (4.42 m) will result in a
stack exit temperature of 436 degrees F (224.4 degrees C), and an average
stack temperature of (550 + 436)/2 = 493 degrees F (256 degrees C).
FIND: Find the economical dimensions of the stack. Diameter of stack to
nearest 6 inch increment in feet; active height of stack in feet; stack exit
gas temperature, degrees F; total height of stack in feet.
f) Height (actual). Enter graph, Figure 39, with the stack draft
required increased by 10 percent as a safety factor; and using the average
stack temperature of 493 degrees F (256 degrees C), determine a new stack
height based on zero stack flow loss = 177 ft (53.95 m). For a stack flow
loss taken at the usual arbitrary value of 5 percent, the final actual stack
height = 177/0.95 = 186 ft (56.7 m). This is the active height above flue
entrance, to which must be added any inactive section required from foundation
to flue entrance.
160
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