Round ductwork provides maximum air-carrying capacity for the pressure loss incurred. Round ductwork is also stiffer than rectangular and therefore much quieter during operation. Round ducts usually have lower installation costs, are easier to insulate, and can be sealed more easily than rectangular ducts. Drawbacks to incorporating round ductwork include expensive fittings and takeoffs and larger heights for a given cross-sectional area than needed for rectangular ductwork. A combination of a rectangular plenum and round branches can offer the designer a good compromise when round ductwork is not possible everywhere. [Shepard et al., 1995]
