Cooling Towers Explained | Cooling water System | Understanding Cooling Tower Design and Operation

Cooling towers are heat transfer devices designed to cool water for reuse. They cool hot water by bringing it into direct contact with air, using countercurrent or crossflow patterns. A cooling tower contains wood or plastic slats, called fill, that direct airflow and the flow of water falling from the top of the tower. The downward-flowing water coats the fill and forms a film, thereby increasing the surface area for contact between the cool air and hot water. Hot water transfers heat to the cooler air it contacts in the tower. This process results in both sensible heat loss and vaporation. Sensible heat is heat that can be measured or felt. When water changes to vapor, the vapor takes heat energy with it, leaving behind the cooler liquid. Evaporation, which accounts for 80 to 90% of the heat loss, is the most critical factor in cooling tower efficiency. It is affected by relative humidity (the amount of water in a given quantity of air at a given temperature), temperature, and wind velocity. Other factors that affect cooling tower efficiency are tower design, water contamination, and equipment problems. Cooling towers can be described as psychrometry devices. Psychrometry is the study of cooling by evaporation.
Temperatures in a cooling tower are closely controlled. The temperature difference (ΔT) between the inlet air temperature (wet bulb) and the outlet water temperature is referred to as the approach to tower. The temperature difference between the hot and cold water is referred to as the cooling range. Cooling tower capacity is defined as the amount of water a cooling
tower can cool.
There are two ways to measure temperature: dry-bulb temperature (DBT) and wet-bulb temperature (WBT). Wet-bulb temperature takes into account the relative humidity, whereas dry-bulb temperature does not. Wet-bulb temperatures usually are lower than dry-bulb temperatures. The wet-bulb temperature, perhaps the single most important factor in cooling tower performance, can be described in several ways:
• The lowest theoretical temperature to which water can be cooled in the tower.
• The temperature of the air saturated with water (also referred to as the dewpoint of air).
• A theoretical temperature that cannot be reached, only approached.