Economizer
A mode of operation that uses favourable outdoor conditions to reduce or avoid mechanical refrigeration work.
Also called: Free cooling, Air-side economizer, Water-side economizer
In practice
Cooling equipment spends most of its energy moving heat uphill — taking heat from a warm room and rejecting it to a place that may be warmer still. When the outdoor condition is cool enough, some of that lifting is unnecessary, and an economizer is the arrangement that takes advantage of it. The refrigeration equipment does less work, or none at all, while the heat still leaves the building.
The common forms differ in what actually crosses between inside and outside. An air-side economizer brings outdoor air into the cooling path, which means the design has to deal with filtration, humidity and contamination. An indirect arrangement keeps the two air streams separate and exchanges heat between them. A water-side economizer works at the plant, using the condenser loop and the heat-rejection equipment to cool the chilled-water loop without running a chiller, or with it running at reduced load.
How many hours a site gets is a property of its climate and its design temperatures, not a feature of the equipment. Anything evaporative depends on wet-bulb temperature rather than dry-bulb, so a hot dry location can perform far better than its air temperature suggests, and a mild humid one worse. Allowing a warmer supply temperature widens the window in every mode, which is one reason supply-temperature strategy and economizer hours are argued about together.
The trade to keep visible is water. Evaporative arrangements buy thermal performance with water consumption, which is a genuine cost in a water-stressed region and belongs beside the energy number rather than in a footnote. A site can improve its energy ratio and worsen its water picture with the same change.
Scope of this definition
This is an educational summary of how the term is used in data center practice. It is not a standard, a specification or engineering advice, and where a real decision depends on it, the current adopted standards, verified site information and qualified professional review are the correct sources.