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. 2000 × 34½ The total coal burned at rating = ---------- = 8624 pounds. 8 The coal per square foot of grate surface per hour at rating = 8624 ---- = 22 pounds. 400 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, _Inches_ Boiler 0.4 Furnace 0.6 Flues 0.1 Turns 0.1 --- Total 1.2 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), 1.5 H = ----- = 211 feet. .0071 Its diameter from curve in Fig. 33 is 96 inches if unlined, and 102 inches inside if lined with masonry. The cross sectional area of the flue should be approximately 70 square feet at the point where the total amount of gas is to be handled, tapering to the boiler farthest from the stack to a size which will depend upon the size of the boiler units used. Correction in Stack Sizes for Altitudes--It has ordinarily been assumed that a stack height for altitude will be increased inversely as the ratio of the barometric pressure at the altitude to that at sea level, and that the stack diameter will increase inversely as the two-fifths power of this ratio. Such a relation has been based on the assumption of constant draft measured in inches of water at the base of the stack for a given rate of operation of the boilers, regardless of altitude.