Air-Side Economizer

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

Economizer

Return Air Temperature Control

Xoa = romimum (ie. zь7oj; tma < r — neai db

Xoa = varies to maintain tma at setpoint (53° F)

Xoa — 100%; tma = toa; cooling coil cools to setpoint (S3‘ F)

Xoa = minimum (25%); tma > 76°F; cool to setpoint (b3"F) — j _-j

 

Toa<-16°F — 16"F < toa < 53°F

 

H3°F < t0a < 76°F

»oa > 76°F

 

Air-Side Economizer

Control action:

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

CO

Air-Side EconomizerE

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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.

Posted in Fundamentals of Heating. Ventilating, and Air-Conditioning


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