Open vs Closed Circuit Cooling Towers
An indirect or closed circuit cooling tower involves no direct contact between the air and the fluid being cooled. Unlike the open cooling tower, the indirect cooling tower has two separate fluid circuits.
One is an external circuit in which water is recirculated
on the outside of the second circuit, which consists of tube bundles
(closed coils) which are connected to the process for the hot fluid
being cooled and returned in a closed circuit. Air is drawn through the
recirculating water cascading over the outside of the hot tubes, providing
evaporative cooling similar to an open cooling tower.
In operation the heat flows from the internal fluid circuit, through the tube walls of the coils, to the external circuit and then by heating of the air and evaporation of some of the water, to the atmosphere.
Operation of an indirect evaporative cooling tower is therefore very similar to the open cooling tower with one exception. The process fluid being cooled is contained in a "closed" circuit and is not directly exposed to the atmosphere or the recirculated external water.