One of the most important factors to be considered in designing a stack is the pressure required to force the air for combustion through the bed of fuel on the grates. This pressure will vary with the nature of the fuel used, and in many instances will be a large percentage of the total draft. In the case of natural draft, its measure is found directly by noting the draft in the furnace, for with properly designed ashpit doors it is evident that the pressure under the grates will not differ sensibly from atmospheric pressure.
Loss in Stack—The difference between the theoretical draft as determined by formula ([24]) and the amount lost by friction in the stack proper is the available draft, or that which the draft gauge indicates when connected to the base of the stack. The sum of the losses of draft in the flue, boiler and furnace must be equivalent to the available draft, and as these quantities can be determined from record of experiments, the problem of designing a stack becomes one of proportioning it to produce a certain available draft.
The loss in the stack due to friction of the gases can be calculated from the following formula:
|
in which
| ΔD | = | draft loss in inches of water, | |
| W | = | weight of gas in pounds passing per second, | |
| C | = | perimeter of stack in feet, | |
| H | = | height of stack in feet, | |
| f | = | a constant with the following values at sea level: | |
| .0015 | for steel stacks, temperature of gases 600 degrees Fahrenheit. | ||
| .0011 | for steel stacks, temperature of gases 350 degrees Fahrenheit. | ||
| .0020 | for brick or brick-lined stacks, temperature of gases 600 degrees Fahrenheit. | ||
| .0015 | for brick or brick-lined stacks, temperature of gases 350 degrees Fahrenheit. | ||
| A | = | Area of stack in square feet. [Pg 240] | |
24,420 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters, Equipped with Babcock & Wilcox Chain Grate Stokers in the Quarry Street Station of the Commonwealth Edison Co., Chicago, Ill.