Transitions are a very important component of many air handling systems because they are used to change from round duct work to rectangular and vice versa. Since both types of duct work are frequently used on railway equipment and there are many instances where the two must interface with one another, it is important to be able to fabricate transitions for these applications. In addition to straight transitions, it is very common to see transition elbows that not only change the duct type but also change direction of the air at the same time.
There are many excellent sheet metal textbooks available that address the issues of how to lay out a desired part on sheet metal so that it can be cut out and bent to the shape required. If you want to do anything complex or just want to understand the rules behind the layout of parts for cutting and bending, I suggest you consult one of these books. This note is only meant as a simple description of how to create transitions.
To start out, measure the round and rectangular ducts to get an idea of their size and configuration. Also note their orientation and the space available to fit the transition into. Note that it is not possible to make the sheet metal go around sharp turns nor is it possible to change between very large ducts to very small ducts in a short length. Nor is it wise to make the air try to bend around sharp corners or do unnatural acts (with or without a Collie dog). Thus, the transition should be of a reasonable length and not have any sharp angles, funny bends, etc. The less radical the transition, the easier it will be to fabricate and the happier the air will be.
Here's the basic concept behind a transition. You start out with four rectangular pieces of sheet metal, as if you were forming a rectangular duct the same as the one you are transitioning to/from. Then, you angle these inwards to meet the round duct. At the same time, you bend a couple of triangle shaped pieces on the side of each rectangle, so as to form a curved part at the opposite end from the rectangular duct. To actually form the round duct, you attach a strip of metal, formed into a circular ring, around the now roughly curved portion of the duct and weld it in place (its easier to do than describe). Some tabs are left on the triangular shaped pieces (which should have bases that are equidistant around the circumference of the circle) so that they can be bent up and the circular ring welded to them.
If you need a transition elbow, I suggest you make the elbow in the square part of the duct because it is easier. It is, however, possible to make it in the round duct or even as part of the transition, if you are into that kind of thing.
The drawing Mechanical_TransEl can be downloaded and viewed with Acrobat. This drawing shows how a particular transition elbow was laid out for cutting.
The number of triangular shaped pieces on each side of the rectangular sheet was chosen as one. This gives a total of twelve sides leading into the the round duct. If you have a large diameter duct, you may wish to increase this number to two (20 sides) or even three (28 sides) triangles. The number of the sides is governed by the circumference and the fact that no side should be more than an inch or two in length at the round end.
All of the information about side length, etc. is laid out in a drawing (a CAD program helps). You'll get a chance to use Pythagoras and some of your other plane geometry rules. I print a copy of the drawing on some thin cardstock and the cut it out and tape it together to work out the bugs before the sheet metal is cut. Cardstock of the same thickness as a file folder is perfect for this job. Or, if you used a CAD program to draw the transition, you can print it on some 8-1/2" x 11" sheets of file card material on your laser printer. Doing this will help you work out any topolgy problems before you start making scrap out of galvo.
The transition should be fabricated out of sheet metal that is thin enough to comfortably work with but thick enough to retain its stiffness and shape, and constrain the air. Since the transition will have a number of formed edges, you will want to be able to bend it easily. Fortunately, the formed edges lend strength to the sheet so that it is possible to use thinner metal than would be employed for a straight duct. I prefer #20 or #22 ga. galvo or #16 or #18 ga. aluminum. The galvo has the added benefit of being weldable with a spot welder whereas the aluminum lends itself nicely to assembly with pop rivets.
Some of the bends can be made on a brake but you may have to hand bend them using sheet metal worker's forming pliers, a sharp edge on a piece of steel and a hammer, or whatever else you can think of. When bending the triangles on the round end of the transition, you'll want to start with the bends to the center of the piece and work your way out.
After final assembly is complete, you can cover up any holes that are likely to leak air with PSA covered aluminum tape. It conforms nicely to curves and edges and sticks agressively. It comes in quite handy for making everything air tight.