or W = 2S (P + 15) / C
when S = D² L, which is a common proportion.
Here W = weight in tons.
P = working pressure as on gauge.
S = heating surface, in square feet.
D = diameter, in feet.
L = length, in feet.
C = a constant divisor, depending on the class of
riveting, etc. For boilers to Lloyds' rules,
and with iron shells having 75 per cent.
strength of solid plate, C = 13,200.
This formula, if correct--and it is almost strictly so--would give the
relative weight of boilers per sq. ft. of heating surface, for 105 lb.
and 150 lb. total pressure, assuming we wish to increase the efficiency
10 per cent, as follows: