Below is an explanation on how DSS gets a 37.25 inch long round port in a box 24 inches deep without using elbows.
For this demonstration I'm using a 6 inch round port with a external diameter of 6.5 inches and a internal surface area of 28.25 inches.
Steps involved for calculating port.
1. How far away to make the end of the port form the back wall. (No it's not the diameter of the port). The port area is 28.25 inches so we need the area between the port end and back wall equal or a tad more than the port area. So surface area of a cylinder is...
area = (2pi x radius) x height
since we know the area...
28.25 = (6.28 x 3) x height
1.49= height (distance from port end and back wall)
Next a port extension is needed with an ID that will create a surface area of 28.25 inches between the port and the port extension. I use sketch-up as it takes much less time than doing it by hand. In this case a port extension with a 9 inch ID works well.
Finally the port extension length is calculated like any other slot port extension to create the needed 37.25 inches of total port length.