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: f W² C H [Delta]D = -------- (26) A³ in which [Delta]D = 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. [Illustration: 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.] This formula can also be used for calculating the frictional losses for flues, in which case, C = the perimeter of the flue in feet, H = the length of the flue in feet, the other values being the same as for stacks. The available draft is equal to the difference between the theoretical draft from formula (25) and the loss from formula (26), hence: f W² C H d^{1} = available draft = KH - -------- (27) A³ Table 53 gives the available draft in inches that a stack 100 feet high will produce when serving different horse powers of boilers with the methods of calculation for other heights. TABLE 53 AVAILABLE DRAFT CALCULATED FOR 100-FOOT STACK OF DIFFERENT DIAMETERS ASSUMING STACK TEMPERATURE OF 500 DEGREES FAHRENHEIT AND 100 POUNDS OF GAS PER HORSE POWER FOR OTHER HEIGHTS OF STACK MULTIPLY DRAFT BY HEIGHT ÷ 100