Round ducts have a smaller pressure drop per unit area of all duct types and are generally the most cost effective. An example in the Fundamentals Handbook [ASHRAE, 2001] chapter on Duct Design shows that using round instead of rectangular ducts saves about 20% in metal weight [Tsal, et al., 1988]. Round ducts with spiral construction are available in extended lengths. A system with long sections has fewer joints with lower pressure losses and less leakage than a system constructed of many shorter sections. Round fittings are commercially available and provide a convenient connection to flexible duct; they are easier to insulate and to seal. Slip joints are the most economical to install. Disadvantages of round ducts include: they require more clear height for installation; larger sizes may cause shipping and handling difficulties.
In addition to their economic advantages, round ducts have an extremely important acoustical advantage. The HVAC Systems Handbook [ASHRAE, 2000] shows that low-frequency sound is well contained a round duct. Therefore, sound has ample time to attenuate naturally. In practice, fixing a noise problem in rectangular ductwork is more difficult than in round ductwork.
If round ducts cannot be installed in a particular laboratory because of height limitations, it is important to try to keep the maximum aspect ratio of the rectangular ducts as close to 2:1 as possible. Rectangular main ducts can also be used with round branches.