Computed from Formula ([28]). For brick or brick-lined stacks, increase the diameter 6 per cent
Height and Diameter of Stacks—From this formula ([27]) it becomes evident that a stack of certain diameter, if it be increased in height, will produce the same available draft as one of larger diameter, the additional height being required to overcome the added frictional loss. It follows that among the various stacks that would meet the requirements of a particular case there must be one which can be constructed more cheaply than the others. It has been determined from the relation of the cost of stacks to their diameters and heights, in connection with the formula for available draft, that the minimum cost stack has a diameter dependent solely upon the horse power of the boilers it serves, and a height proportional to the available draft required.
Assuming 120 pounds of flue gas per hour for each boiler horse power, which provides for ordinary overloads and the use of poor coal, the method above stated gives:
For an unlined steel stack—diameter in inches = 4.68 (H. P.)2⁄5 (28)
For a stack lined with masonry—diameter in inches = 4.92 (H. P.)2⁄5 (29)
In both of these formulae H. P. = the rated horse power of the boiler.
From this formula the curve, Fig. 33, has been calculated and from it the stack diameter for any boiler horse power can be selected.
For stoker practice where a large stack serves a number of boilers, the area is usually made about one-third more than the above rules call for, which allows for leakage of air through the setting of any idle boilers, irregularities in operating conditions, etc.