(Single, Double, T and Y's)
Also known as Lateral, 'Pant', T on taper and/or Y-branches, branches are still the same- a 'leg' is coming off the main line.
Typically, Single and Double Branches taper as the new line enters the main line. This is because the new line is bringing in more air. The exiting part of the branch (toward the fan) is therefore bigger to accommodate the increase in air. The new line typically enters at 30 or 45 degree so as to 'merge' into the air flow.
T's are called T's because the line enters the main line at 90 degree- forming a 'T'. T's are not the best air flow for negative systems as they increase static pressure to the point that the air can slide right by the side line and pull only from the straight end where the flow is 'easier'. T's however are often used in positive systems- where the air is being pushed.
Y's are exactly that- Y's with the two inlets diverging from the center line. Once again, the volume of the outlet is the sum of the two inlets. Typically the two inlets are the same size. If one needs to be smaller than the other, the branch is made with the two inlets equal to the largest requirement and then a reducer is used to reduce to the smaller requirement.
Incuts are used to create a branch on an existing trunk line. To create a new branch with an incut, a hole is cut in the pipe at the location of the branch. The incut is saddled over the hole and fastened with tech screws and caulked.
All US Duct branches are 18 gauge or heavier. This enables us to weld the branching part to the trunk and 'this' insures a very tight seal.