Thickness in inches of a wrought-iron flue exposed to external pressure: I. Find (1) the product of the diameter of the flue in inches, (2) the length of the flue in feet, (3) the pressure of steam in lbs. per square inch, and (4) .000, 009, 091.

II. Extract the square-root of this product.

Suppose, for example, that a flue is 12 inches in diameter and 6 feet long, and that the pressure of steam is 40 lbs. per square inch: The product of 12, 6, 40, and 0.000, 009, 091 is about 0.02618, and the square-root of this number is 0.1618+, or about 3/32 of an inch. It will be observed that the thickness of a flue increases directly as the length. Thus, other things being equal, a flue that is 12 feet long must be twice as thick as one that has a length of 6 feet. In making long flues, it is common to strengthen them at intervals by bands, thus converting them into a series of short flues, so far as the strength is concerned.

Safe pressure in lbs. per square inch for a wrought-iron flue exposed to external pressure: Multiply the square of the thickness in inches by 110, 000, and divide this product by the product of (1) the diameter of the flue in inches, and (2) the length of the flue in feet.

Example.-What is the safe pressure for a flue 13 inches in diameter, 8 feet long, and 3/8 of an inch thick?

The product of 0.140, 625 (the square of the thickness) and 110, 000 is 15, 468.75. Dividing this by 15 times 8, or 120, the quotient is about 128.9 lbs., the pressure required. B.