This section is from the book "A Treatise On Architecture And Building Construction Vol4: Plumbing And Gas-Fitting, Heating And Ventilation, Painting And Decorating, Estimating And Calculating Quantities", by The Colliery Engineer Co. Also available from Amazon: A Treatise On Architecture And Building Construction.
48. Currents of fluid should always be guided in such a manner that they do not interfere or oppose one another's motion. The resistance offered by opposing currents in the common T connection, as shown at A in Fig. 8, is so great that its use should never be permitted. By making the corners a and b round, as shown at B, the evil is lessened, but only to a small degree.
The effect of the opposing currents may be reduced to a minimum by interposing between them a dividing ridge or partition d, as in the twin elbow C. If the current moves in the opposite direction to that shown by the arrows, the form C will still be preferable to B or A.
49. When the flow in one of the branchesfis to continue in the same direction as in the main g, the other branch should be curved as shown at e, in the style D.
When a branch is to be taken from a main, as in Fig. 9, it should be attached at the smallest practicable angle. If the situation will not permit the form shown, and the branch must be joined at a right angle, then the connection should be made with a gusset, as at a, Fig. 10.
50. The connection of the flat wall pipe to the round pipe, shown in Fig. 11, is a faulty one. The change of direction which the current of air is required to make, is very abrupt; the dead end C will also operate to retard the current by forming an eddy at that point. The connection should be made similar to that shown in Fig. 12.