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Sheet M-105
PPDPDD PACE

HVACconcept

Economizer cycle: air-side vs water-side free cooling

One-line orientation

An economizer cycle shifts the cooling load from mechanical refrigeration to a “free” natural source — outdoor air (air-side) or a cooling tower (water-side) — whenever conditions allow, cutting compressor energy use.

Key points

  • Air-side economizer:
    • The AHU’s outdoor-air damper modulates to admit more cool outside air when outdoor conditions are suitable for cooling.
    • is favorable compared with return-air conditions, using dry-bulb temperature or enthalpy control.
    • Benefits: reduces mechanical cooling load; simultaneously increases ventilation (improved IAQ).
    • Climate suitability: most effective in climates with cool nights or cool shoulder seasons and large diurnal temperature swings (e.g., high-altitude or continental climates).
    • Limitation: in humid climates, outside air may be too warm or too humid for most of the year; enthalpy controls can prevent introducing air that adds latent load.
  • Water-side economizer (free cooling via cooling tower):
    • When outdoor wet-bulb temperatures are low, the cooling tower can produce water cold enough to bypass the chiller and feed the chilled-water loop directly.
    • The chiller compressor shuts off (or reduces load), and tower water does the work.
    • Benefits: major energy savings for buildings with year-round cooling loads (data centers, interior zones) in climates with cold winters.
    • Requires a heat exchanger (or direct connection with suitable water treatment) between the condenser water loop and the chilled-water loop.
  • Both types: “free” is a relative term — fans, pumps, and cooling tower continue to run, but compressor energy (the dominant load) is eliminated or reduced.

Confusions / comparison

Same pipework, two states — economizer mode sends the flow around the machine

Read each row left to right: normal cooling runs through the compressor, economizer mode runs around it. One line means a duct, two mean supply and return.

View diagram Hide diagram

Scroll horizontally to explore

Air-side and water-side economizers shown in both their normal and free-cooling states A two-by-two comparison. The rows are the two economizer types; the columns are the two states, normal cooling and economizer mode. Every cell shows the same chain: an inlet, a junction, the machine sitting on the straight run, a branch that arches over the machine, a second junction, and an outlet. In the normal-cooling column the straight run through the machine is the live route and the machine box is solid and labelled compressor running, while the arching branch is a dashed hairline carrying nothing. In the economizer column the arching branch is the live blue route, the machine box has gone dashed and muted and is labelled compressor off, and the straight run through it is the dashed hairline instead. The air-side row draws its routes as a single line, the convention for a duct, and runs from outdoor air to the space; in economizer mode its condition is that outdoor air is cooler than the return air, and cool outdoor air then cools and ventilates the space. The water-side row draws its routes as a pair of lines, the convention for hydronic supply and return, and runs from the cooling tower out as chilled water; its condition is that the wet-bulb is low enough that tower water beats the chilled-water setpoint. Below the grid, two labelled rows on the same left rail and the same column datum as the cells above: a row headed Stops, holding one dashed, muted chip reading compressor, and beneath it a row headed Keeps running, holding three plain chips reading fans, pumps and cooling tower. Normal coolingflow runs THROUGH the machineEconomizer modeflow runs AROUND itAir-sideair, through the AHUrefrigerationcompressor runningoutdoor airthe spacethe compressor cools the airoutdoor air cooler than the return airrefrigerationcompressor offoutdoor airthe spacecool outdoor air cools the space — and ventilates itWater-sidewater, in the hydronic loopchillercompressor runningcooling towerchilled waterthe compressor chills the waterwet-bulb low enough that tower waterbeats the chilled-water setpointchillercompressor offcooling towerchilled watertower water is already cold enoughSTOPScompressorSTILL RUNSfanspumpscooling tower
  • blue = the live route in economizer mode
  • dashed hairline = the branch not carrying flow

Only the COMPRESSOR is shed. Fans, pumps and the cooling tower keep running, which is why “free” is relative.

Where enthalpy control is used the air-side condition is humidity as well as temperature: warm, humid outdoor air can add latent load even when its dry-bulb temperature looks acceptable.

Air-side economizerWater-side economizer
Free cooling sourcecool outdoor aircooling tower (condenser water)
What it bypassesrefrigeration cycle (compressor off / reduced)chiller compressor
Best climatelarge diurnal swings; cool/dry shoulder seasonscool winters; consistent low wet-bulb temps
Key componentoutdoor-air dampers + controlscooling tower + heat exchanger
Limitationineffective in hot/humid climatesrequires cooling tower; complex piping
IAQ benefityes — more fresh airno direct IAQ benefit

→ HVAC distribution systems (VAV systems most commonly use air-side economizers) · HVAC system selection (operating cost criterion) · HVAC components (dampers control outdoor-air fraction).

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