DX Units vs. Chillers
The immediate and most noteworthy difference between these two units is that the DX (direct expansion) unit cools air, and chiller units cool water. DX Units vary in use between supplemental or emergency building AC, or primary AC at tented events or relief structures. Chiller units cool water for use in other AC systems like chilled water air handlers.
DX Package Units
The DX Air Conditioner system uses a refrigerant vapor expansion and compression cycle to cool air coming in through a supply plenum and returns it to the area that needs cooling through the return. The unit effectively circulates refrigerant through a series of condensers and evaporators, which warm air moves through and is cooled down before being pushed through the return. Another key feature of DX Packaged units is the advantage of having the entire cooling system self-contained in one unit. This allows for the installation on top of or alongside a building without too large of a footprint, and are generally less expensive to install.
Air Cooled Chiller Units
Chiller units also use a vapor expansion/compression cycle for liquid refrigerant, much like the DX units. The refrigerant is continuously transformed from a liquid, to a vapor, and back again. This process cools down the refrigerant which is passed through an evaporator. Warm water coming through a 2-5 inch radius supply hose is passed over the evaporator and cooled off to the desired temperature. Typically the cooled water is used for existing air handler systems.