When trying to cool something down, there tend to be four main ways that one may use to aid the transference of heat. They are radiation, evaporation convection, and conduction. In cooling water treatment, we are predominantly concerned with the cooling effect of evaporation.
In our every day lives, we're all familiar with the way evaporation cools. When we work up a sweat, our bodies use this process to keep us from heating up, and it's the evaporation of the sweat from our skin that actually removes heat, cooling our bodies down as it does. This process of cooling by evaporation is known as an endothermic reaction.
In cooling tower water treatment, a cooling water system is designed to remove heat from processes or equipment by transferring it to water. The heat absorbed by the water must then be dissipated to allow the water to be reused. In an open, evaporative cooling water system, this occurs in the cooling tower or evaporative condenser.
A cooling tower is designed to maximise contact between air and water and, in doing so, encourage some of the water to evaporate.
The evaporation of just a small proportion of the water cools the remainder. In fact, evaporating just 2% of the water will achieve a 9°C temperature drop. When water is lost by evaporation in this way the mineral content of the remaining water becomes more concentrated. The higher the concentration factor the less water the system uses as fresh make-up).
Cooling by evaporation like this is relatively energy efficient and effectively conserves water by allowing hot water to be cooled and reused, rather than discarded. However, without effective water treatment and regular maintenance, significant problems can arise:
• Cooling by evaporation increases the dissolved solids of the water - raising the likelihood of corrosion, scale and deposition problems occurring;
• The increase in water temperature increases the potential for corrosion and scale;
• The retention of warm water in an open evaporative cooling water system increases the tendency for biological growth;
• A cooling tower is in effect an air-washer and will scrub corrosive gases, microbial nutrients, micro-organisms and dust from the atmosphere which if untreated can lead to increased corrosion, fouling and microbial problems;
• An evaporative cooling system is potentially a good growth environment for legionella bacteria and the spray produced in a cooling tower is the ideal means of generating the aerosols necessary to cause legionnaires' disease if adequate controls are not maintained.