Cooling towers exploit a natural principle as simple as effective: the forced evaporation of a minimum amount of water, compared to the main mass. This happens by subtracting heat from the mass itself; which is therefore cooled (latent heat of evaporation).
The process is very simple and has been used throughout history. The ancient amphoras made of terracotta, a porous material, allowed water to exude outwards in minimal quantities. In this way, a process of evaporation was carried out. The water inside the container remained fresh even in presence of high ambient temperatures.
Cooling towers exploit the latent heat of evaporation
Cooling towers are able to carry out this process of water/air heat exchange in the most efficient way. The evaporation process happens using simple and effective components that, in general, require minimum maintenance.
We introduce two concepts in order to understand how heat subtraction works:
- Sensible heat. Amount of heat energy added to or subtracted from a physical element (e.g., a battery or finned coil) to change its temperature.
- Latent heat. This is basically based on the change in state that a substance can suffer after the addition or loss of heat. In the case of water, this can change from a liquid phase to a solid phase (ice) if the heat is removed when it reaches the freezing point. It can also pass from a liquid phase to a gaseous phase (vapor) if, when it reaches the boiling point, heat is introduced. Therefore, latent heat is defined as the heat that is introduced or removed to change the state of the water. In particular, in evaporative cooling systems, it is called latent evaporation heat.
A well designed cooling tower must give the water the largest possible contact surface with the air, so that the latent heat exchange is optimal.
To make this heat exchange possible, the cooling tower must be able to provide a very high contact surface between air and water. This is achieved by means of a heat exchange filler, which has been specifically designed for this purpose.
Knowing the quality of the water is an essential first step in choosing the right fill for a cooling tower. It is also important to know both the climatic and morphological conditions of the environment where it will be located.
A tower that, for example, receives clean industrial water but is located next to another industry with a dusty environment should be considered as having water loaded with solids.
This choice of fill will also influence the choice of fan, as the fan must be able to move the defined volume of air and overcome the system pressure drop, which will be different depending on the fill chosen.
In summary, in order to make the most of the latent heat in a cooling tower, it is essential to design it taking into account all the aspects that influence it. Only by taking into account all the variables will we ensure the maximum performance of the tower with the maximum durability of its components.