Functions: Heating and cooling
Water-cooled chillers tend to offer a significant energy efficiency advantage when compared to using separate PTAC units. Although they are centralized systems, modern units come equipped with variable speed compressors that can increase or decrease their output 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 of 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 being that a boiler is used instead of a chiller. Hydronic piping and fan-coils are used to distribute heat indoors.
HVAC systems that implement a cooling tower, chiller and boiler can be classified into two main categories:
Two-tube systems use the same hydronic piping circuit for heating and cooling, which means the chiller and boiler cannot operate simultaneously. In other words, the entire building must be in either heating mode or cooling mode.
Four-tube systems have separate hydronic piping for the chiller and boiler, allowing both units to operate simultaneously. If some areas of the building need cooling while others need heating, this system configuration allows for that. Of course, the need to install two piping systems increases initial costs.
