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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