53. The shape which should be given to an air pipe or conduit depends upon several considerations.

1. To carry a given volume of air, the circular form has the following advantages: (a) It requires less material to construct it. (b) It offers less resistance from skin friction. (c) It requires but little bracing to maintain it in proper shape.

2. To carry the greatest volume in the least space, the square form should be used.

For example, if it be required that a pipe should have an area of 254 square inches, it will be found that the desired area will be afforded by a round pipe 18 inches in diameter, or a square pipe 16 in. xl6 in. The circumference of the round pipe will be 56 1/2 inches, while that of the square pipe will be 64 inches. But the square pipe may be run through a space which is 2 inches less each way than that required for the round pipe of equal capacity.