355. To find the necessary number of tubes to give any amount of heating surface.
Formula.
N = S
Ldπ,
when N is the number, S the required surface, L the length, d the diameter, both in feet, and π = 3.1416. (See Table 10.)
No. 11.
356. To find the mean cylinder pressure for any percentage of admission.
Formula.
13.5√a – 28,
where a is the percentage of admission. (See Table 11.)
As to the internal arrangement of the barrel of the boiler, we must of course have the length of tubes the same as that of the barrel, (that is, in the general plan of boiler, some makers have moved the back flue plate ahead). The length of tubes will of course be the same as the distance between the tube sheets. The number is governed by their diameter and by the proper clearance, which is found by the formula,