“It may be observed, however, that priming to any extent is so costly and detrimental that much consideration needs to be exercised before dispensing with the provisions ordinarily made to prevent it.
“For circular tubular boilers, having a working pressure of from 60 to 80 lbs. per square inch and to be used for compound engines, the following proportions represent current practice.
“1st. One square foot of fire grate area to every indicated horse power of the engine.
“2d. 28 square feet of heating surface[73] to 1 square foot of fire grate area.
[73] The heating surface here referred to includes the total interior surface of the tubes, the sides, backs, crowns and tube plates of the combustion boxes, and that part of the furnace that is above the level of the fire bars, but does not include the front tube plate (i. e., the tube plate in the smoke box).
“3d. 61⁄2 to 8 cubic feet of steam space to each square foot of fire grate area.
“4th. 8 to 10 square feet of tube surface to the total heating surface in single ended boilers.
“5th. 81⁄2 to 10 is about the ratio of tube surface to the total heating surface in double ended boilers.
“6th. The diameters of boiler tubes should be about one-half inch for each foot of length of tube. If less, the tube is liable to choke. About 14 cubic feet of steam (of from 60 to 80 lbs. pressure) should be made for each square foot of fire grate area.
“Each square foot of fire grate will burn from 13 to 18 lbs. of steam coal per hour. About 11⁄2 cubic feet of live steam (of the above pressure) is required for each indicated horse power.”