In the DX type of central air conditioning plants the air used for cooling the room is directly chilled by the refrigerant in the cooling coil of the air handling unit. Due to this heat transfer process is more efficient, since there is no middle agency involved for the heat transfer resulting in higher cooling efficiency.
In case of the chilled water system, the cooling effect from the refrigerant is first transferred to the chilled water, which is then used to chill the air used for cooling the room. There is some loss of the cooling effect when it is being transferred from the refrigerant to the chilled water and from there to the air due to which the chilled water systems have lesser cooling efficiency. The chilled water acts as the secondary medium for cooling the room air in air handling unit.
Further, the chilled water has to flow long distance along the whole building. On its way it tends to get heated due to friction of flow and also due to surrounding heat absorption. The chilled water also has to be pumped by the pump, which adds more heat to it. Thus as the chilled water flows from the chiller to the air handling unit and again back to the chiller, apart from the heat from air, it also absorbs lots of additional heat, which leads to high increase in its temperature. The chiller has to remove this additional heat from the water due to which its efficiency decreases or rather additional power is required to remove this additional heat. In the DX type of central air conditioning plants the refrigerant travels only through the small distances and there is no pump involved so the additional heat absorption is less, which makes the DX plants even more efficient.
