Open recirculating cooling system evaporation

In open recirculating systems evaporation is the major factor in heat disposal. In these evaporative systems circulating water is continuously scrubbed with air, therefore saturated with dissolved oxygen. In contrast, water in closed recirculating cooling systems usually will contain minimum dissolved oxygen, even though the systems may include vented expansion tanks. 

Open recirculated cooling water systems remove the heat picked up in plant by evaporative cooling. This may be done by a spray pond, for example, combining air conditioning needs with aesthetic considerations in industrial parks. The most common type of evaporative cooling, however, is effective in cooling towers of one type or another (Fig. 8.11). 

Open recirculating systems These are more amenable to inhibition since it is possible to maintain a closer control on water composition. Corrosion inhibition in these systems is closely allied to a number of other problems that have to be considered in the application of water treatment. Most of these arise from the use of cooling towers, ponds, etc. in which the water is subject to constant evaporation and contamination leading to accumulation of dirt, insoluble matter, aggressive ions and bacterial growths, and to variations in pH. A successful water treatment must therefore take all these factors into account and inhibition will often be accompanied by scale prevention and bactericidal treatments. 

Most cooling towers are open-recirculating types in which heat rejection mainly is accomplished by evaporation of water. There also are closed cooling systems (of which an automobile radiator would be typical), from which heat is dissipated by the forced flow of air across radiation sections. In some combination systems, heat from closed cooling circuits is transferred through heat exchangers into circulating water which is cooled by evaporative systems. 

Continuous water recirculation system, whereby cooling water picks up heat by being pumped through a process heat source or heat exchanger and then over a cooling tower, open to the air, whereupon the sensible heat gained is lost by an evaporation process. 

---