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FLUES FOR CONSTRUCTION OF A BOILER The internal pressure at which the boiler shell will rupture can be calculated ; but the external pressure which will collapse a flue can be determined only by experiment. External pressure tends to increase any imperfection of shape. For instance, if a flue is slightly oval, the external pressure tends to make it more flat. The strongest form to resist external pressure is evidently the circle. When considering the strength of flues length is very important.

If a lap joint is used the flue will not be a true cylinder, for this reason welded or butt joints are preferable.

Fairbain gives the formula, P = (806,000*t^(2.19))/(ld) for calculating the collapsing pressure of flues, 1 = length of flue in feet, d = diameter in inches, t = the thickness in inches, P = pressure per square inch. The exponent of t is often taken as 2 instead of 2.19 for convenience. This formula is empirical and was prepared from his experiments.

Hutton gives, P = (C*t^2)/(d*sqrt(L)). In which C is a constant, which is 600 for wrought iron and 660 for mild steel, L = length in inches, d = external diameter in inches, and t = thickness in thirty-seconds of an inch. Results by Hutton's formula agree more nearly to those by experiment than do Fairbain's.

If the flues are oval, d in the above formula = the major axis.

Flues are strengthened by putting in hoops at stated distances. These hoops are made of T iron or angle iron.