In a direct-expansion (DX) system, the evaporator is in direct contact with the air stream, so the cooling coil of the airside loop is also the evaporator of the refrigeration loop. The term “direct” refers to the position of the evaporator with respect to the airside loop.
The term “expansion” refers to the method used to introduce the refrigerant into the cooling coil. The liquid refrigerant passes through an expansion device (usually a valve) just before entering the cooling coil (the evaporator). This expansion device reduces the pressure and temperature of the refrigerant to the point where it is colder than the air passing through the coil.
The components of the DX unitary system refrigeration loop (evaporator, compressor, condenser, expansion device and even some unit controls) may be packaged together, which provides for factory assembly and testing of all components, including the electrical wiring, the refrigerant piping, and the controls. This is called a Packaged DX system.
One of the most common reasons for selecting a DX system, especially a packaged DX system, is that, in a smaller building, it frequently has a lower installed cost than a chilled-water system because it requires less field labor and has fewer materials to install. Packaged DX systems that use air-cooled condensers can be located on the roof of a building, in a small equipment room, or even within the perimeter wall of the building.