TABLE OF CONTENTS Duct-sizing considerations for cold-air distribution systems are essentially the same as those for conventional designs. The reduction in supply air volume offers the designer more flexibility in selecting duct sizes. Typically, ducts are sized to the same pressure loss criteria used for conventional systems. This approach reduces duct size and cost, as well as fan energy, and often allows the use of round or oval ducts where space limitations would otherwise have required rectangular duct.
Round and oval ducts are inherently strong and rigid and are generally the most efficient and economical ducts for air distribution systems (ASHRAE 1996, chapter 16). Round and oval ducts are easier to install and to seal against leakage and have better sound control characteristics than rectangular ducts.
Cold-air duct systems may also be "oversized," using duct sizes typical of conventional systems. This oversizing reduces duct pressure loss, minimizes noise, and maximizes fan energy savings. However, the increased duct surface area per unit air volume leads to a higher supply air temperature rise than with standard sizing. This approach is particularly well suited to applications where noise control is a primary consideration, such as courthouses, theaters, and concert halls.
The trade-off between reducing duct size and reducing duct pressure loss is a design decision that depends on space limitations, ductwork costs, and energy costs. If the load requirements of a conventional system exceed the original design calculations, reducing the supply air temperature may be a means of increasing the distribution system capacity.
