Solution of a Problem—The stack diameter can be determined from the curve, Fig. 33. The height can be determined by adding the draft losses in the furnace, through the boiler and flues, and computing from formula ([30]) the height necessary to give this draft.
Example: Proportion a stack for boilers rated at 2000 horse power, equipped with stokers, and burning bituminous coal that will evaporate 8 pounds of water from and at 212 degrees Fahrenheit per pound of fuel; the ratio of boiler heating surface to grate surface being 50:1; the flues being 100 feet long and containing two right-angle turns; the stack to be able to handle overloads of 50 per cent; and the rated horse power of the boilers based on 10 square feet of heating surface per horse power.
The atmospheric temperature may be assumed as 60 degrees Fahrenheit and the flue temperatures at the maximum overload as 550 degrees Fahrenheit. The grate surface equals 400 square feet. The total coal burned at rating = 2000 × 34½⁄8 = 8624 pounds. The coal per square foot of grate surface per hour at rating = 8624⁄400 = 22 pounds.
The atmospheric temperature may be assumed as 60 degrees Fahrenheit and the flue temperatures at the maximum overload as 550 degrees Fahrenheit. The grate surface equals 400 square feet.
For 50 per cent overload the combustion rate will be approximately 60 per cent greater than this or 1.60 × 22 = 35 pounds per square foot of grate surface per hour. The furnace draft required for the combustion rate, from the curve, Fig. 34, is 0.6 inch. The loss in the boiler will be 0.4 inch, in the flue 0.1 inch, and in the turns 2 × 0.05 = 0.1 inch. The available draft required at the base of the stack is, therefore,
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Since the available draft is 80 per cent of the theoretical draft, this draft due to the height required is 1.2 ÷ .8 = 1.5 inch.
The chimney constant for temperatures of 60 degrees Fahrenheit and 550 degrees Fahrenheit is .0071 and from formula ([30]),
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