In the second case, a wetted pad is used to cool the entering airstream. The pads are specially designed to retain water on the surface of the pads to ensure that it does not carry over to the finned coil, minimizing the risk of coil damage.
The use of a small amount of water to precool the air entering the heat exchanger lowers the required airflow and fan power compared to air-cooled units, while also lowering the fluid temperature back to the system. In the most efficient adiabatic systems, the air is cooled close to the wet-bulb temperature. Such substantial depression of the air temperature results in a significant increase in dry cooling capacity and energy efficiency compared to dry-only designs. Once the ambient temperature begins to approach freezing, or during times of reduced load, the unit can be switched to operate in a dry-only mode, thus decreasing water usage.
Adiabatic heat rejection system controls are designed to be flexible, intelligent, and customer-friendly, taking full advantage of the dual operating modes. At a customer-selected design point, such as a cooler ambient dry bulb and a lower heat load, the unit can turn off the recirculating water and switch to operation in dry mode. Additionally, the low volume recirculating water sump automatically drains when freezing temperatures are experienced, negating the need for sump heaters.
The recirculating design of an adiabatic heat rejection system consumes less water than an evaporative cooling tower. Note that some designs use a “once through” wetting system which can substantially increase water usage, as compared to the recirculating type system. However, such designs may also violate local codes banning the use of once-through cooling so local code officials should be consulted.