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76 never draw them. They are included here for instructional purposes.

36. Related Mathematics on Tangent Tees and Tee Joints.—Altering the Main Pipe Size.—In a blow-through system, whenever a branch is taken from the main pipe, the diameter of the main must be reduced beyond that point. Also, if a branch pipe is added on to the main of an exhaust system, the diameter of the

main must be increased. By so doing, the original velocity and static pressures are preserved and the system exerts an equal "pull" at all points.

Rule for Altering the Size of Main.—The area of the main must be increased, or diminished, by an amount equal to the square inches of cross-sectional area of the branch. In other words, the cross-sectional area of the main must at all times be equal to the combined cross-sectional area of all its branches.