Every branch duct or pipe shall be connected to the main duct or pipe at an angle not exceeding 45 degrees measured from the centerline of the main duct or pipe. Every bend, turn or elbow in any duct or pipe shall be constructed so as not to reduce the cross sectional area of the duct or pipe. Every change in cross section of any such duct or pipe shall be by means of a transition fitting having a slope or taper of not more than 1 inch for each 4 inches in length of the said duct or pipe.