The initial reason the modular concept took hold was a frequent lack of space when it came time to replace a large chiller. Often, these chillers were installed in a confined mechanical room in the basement at the beginning of construction, and the building was then built around it. There was barely enough room to get people to the chiller for maintenance, let alone replacing the chiller itself.
An airside economizer is typically used on a packaged rooftop or tied to an indoor AHU, allowing filtered outside air into the space when outdoor temperatures drop below 55°F (12 °C) (the common supply air temperature of indoor spaces) thus alleviating the need for the refrigeration cycle to be running. Interior space is being cooled yet the refrigeration system is not running, hence the name Free Cooling.
