Air-side economizer uses outside air (OA) to offset mechanical cooling.
AHU mixing dampers vary OA flow from minimal required flow rate (for people in a space) up to 100% OA.
Different control systems for the economizer:
Return-Air Temperature Economizer
Toa<Tra —> at min OA preheat coil keeps Tma=Tsa; increase OA to maintain Tma=Tsa
Toa=Tra => 100% OA
Toa>Tra => minimum ventilation OA and cooling coil keeps Tma=TSa
Variation of QA Fraction and MA Temperature with QA Temperature
(Return Air Temperature Economizer Cycle) — j _g
Advantage: energy savings.
Disadvantages: high VAV energy fan, large OA ducts, humidity control, expensive control system, complexity, and maintenance.