ABSTRACT
Variable air volume (VAV) systems are self-balancing, since the airflow through each section of ductwork is controlled by the VAV damper in each terminal unit served. The most efficient duct, in terms of flow capacity at a certain velocity with minimum pressure loss due to friction, is a circular duct. Friction losses in fittings are typically a much greater part of any duct system’s losses than the straight duct. The velocity reduction method involves the selection of some appropriate velocity at the fan outlet, then progressively reducing the velocity at each junction or branch duct. Supply air ductwork must be insulated to minimize heat gain or loss and, for cold air supply, prevent surface condensation. Fire and smoke control for occupant safety is a major design consideration for any building and is particularly important as part of duct systems design. There are four methods used for duct design and sizing: constant velocity, velocity reduction, static regain, and equal friction.
