Water-cooled chillers tend to offer a significant energy efficiency advantage when compared with the use of separate PTAC units. Although they are centralized systems, modern units come equipped with variable speed compressors that can ramp their output up and down depending on the current cooling load. Water-cooled chillers operate with two water circuits:
The chilled water circuit circulates throughout the building, and hydronic fan-coils are used to cool individual areas.
Another water circuit is used to reject heat through the cooling tower.
This system configuration uses fans in each individual fan-coil and in the cooling tower, and pumps in both water circuits. If all these components are equipped with variable speed drives, chilled water air conditioning can offer remarkable energy efficiency. The heating mode also uses hydronic piping and fan coils, with the difference that a boiler is used instead of a chiller. Hydronic piping and fan-coils are used to distribute heat indoors.
HVAC systems that deploy a cooling tower, chiller and boiler can be classified in two main categories:
Four-pipe systems have separate hydronic piping for the chiller and boiler, allowing both units to operate simultaneously. If some building areas need cooling while others need heating, this system configuration enables it. Of course, the need to install two piping systems raises upfront costs.