Direct expansion (DX) units involve a direct heat exchange between the refrigerant and the air to be treated, without an intermediate heat exchange, i.e., refrigerant-air. In contrast, common large-scale central air conditioning systems, composed of chilled water units and air handling unit (AHU) terminals, use a different heat exchange method than DX systems, which is: refrigerant-water-air. This means the refrigerant absorbs heat from water to produce chilled water, which is then transported through piping to the room's fan coil units and other terminal units. The unit's fan circulates air, enabling heat exchange between the indoor air and the chilled water within the coils, producing the "cool air" that is blown out.
