In stationary boilers the chimney area should be one-fifth greater than the combined area of all the tubes or flues.
For marine boilers the rule is to allow fourteen square inches of chimney area for each nominal horse power.
The draught of a chimney is usually measured in inches of water. The arrangement most commonly made use of for this purpose consists of a U-shaped glass tube connected by rubber tubing, iron pipe, or other arrangement, with some part of the chimney in such a way that the draught will produce a difference of level of water in the two legs of the bent glass tube.
The “Locomotive” suggests that the unit for chimney construction should be a flue 81 feet high above the level of the grates, having an area equal to the collective area of the tubes of all the boilers leading to it, the boilers being of the ordinary horizontal return tubular type, having about 1 square foot of heating surface to 45 square feet of heating surface.
Note the above conditions, and, in case of changing the above proportions, it should be observed that the draught power of chimneys is proportional to the square root of the height, so we may reduce its area below the collective area of the boiler tubes in the same proportion that the square root of its height exceeds the square root of 81.
For example, suppose we have to design a chimney for ten boilers, 66 in. in diameter, each having 72 tubes, 31⁄2 in. in diameter, what would be its proportion?
The collective area of the 720 31⁄2-in. tubes would be 6,017 square inches, and if the chimney is to be but 81 feet high, it should have this area, which would require a flue 6 ft. 51⁄2 in. square.
But, suppose, for some reason, it is decided to have a chimney 150 feet in height, instead of 81 feet. The square root of 150 is 121⁄4; the square root of 81 is 9; and we reduce the area of the chimney by the following proportion: 12.25:9 = 6,017:4,420 square inches, which would be the proper area, and would call for a chimney 5 ft. 6 in. square, and similarly if any other height were decided upon.