Duct Shape Selection
Erfan Akbarnia
Senior Engineer expert at EPCC projects, operation and maintenance fields. fluid flow, heat transfer & thermodynamic process specialist (Piping, Process Equipment, HVACR & Utility systems and equipment)
Round duct work is preferable to rectangular or flat oval ductwork when adequate space is available for the following reason.
1- Mass of round ductwork is less than rectangular. Attached figure shows the relative weight of rectangular duct to round duct for duct pressures from ±125 to ±2500 Pa when the equivalent diameter of the rectangular duct is the same as the round duct diameter.
Equivalent diameter is defined as the diameter of a rectangular duct that has equal resistance to flow for equal flow and length.
2- Perimeter of round ducts is less than rectangular ducts. For rectangular duct aspect ratios from 2 to 4, the increase is approximately 30 to 55%. This increase results in increased insulation, including possible thickness to offset the additional heat transfer. For a comprehensive study of round and rectangular ducts as they affect system performance, consult McGill (1988).
3- Round ducts have an excellent resistance to low-frequency breakout noise (Schaffer2011).
4- Duct rumble can occur in rectangular duct systems (Paulauskis 2016).
