What are the advantages of a round duct over a rectangular duct?
Air flow is calculated to provide specific amounts of airflow. This can be challenging due to space limitations. Sometimes you need to move a lot of air through a small space. Rectangular duct can be sized taking full advantage of the all space available. Think of putting a round peg in a square hole. The square duct uses all the available space for air flow. If ceilings have limited space a shallow but wide duct can be built to allow the correct amount of air in the reduced space. Rectangular duct has to be carefully designed because is is subject to collapse due to clogged filters etc in the line. The reduced pressure can implode the sides. The opposite is also true as the supplies can expand and round out the sides.Round ducts are easy to build and come in predetermined sizes and metal thicknesses. If the space is available it is the cheapest way to go because it can be mass produced and cut to size in the field. All the conceivable transitions are available off the shelf. So overall if the biggest concern is cost, and it always is, the round duct is a cheaper choice where possible. They are much stronger in high static and low pressure conditions thus reducing the fear of collapse as discussed in the previous paragraph.Often in commercial design systems the main ducts are rectangular and branch ducts are round allowing the maximum air flow to the branches. But not always. Some are rectangular throughout with 6′ flex connectors to the grilles and registers. Value engineering will often change as much as possible to round. Most large mechanical contractors will re-design a system and submit their bids accordingly.