Closed Circuit cooling tower is also known as Evaporative Cooling Tower. An evaporative cooling tower is a heat exchanger that transfers heat from circulating water to the atmosphere. Warm water from the heat source is pumped to the top of the tower and will than flow down through plastic or wooden shells. As it falls downward across baffles, the water is broken into small droplets. Simultaneously, air is drawn in through the air inlet louvers at the base of the tower and travels upward through the wet deck fill opposite the water flow. A small portion of the water is evaporated which removes the heat from the remaining water causing it to cool down 10 to 20°C. The water falls down into a basin and will be brought back into the production process from there. Some of the water is lost to evaporation and thus the fresh water is constantly added to cooling tower basin to make up the difference
Evaporation results in cooling…
On a warm day when you work or play hard, your body heats up, and you begin to sweat. Because your skin is more moist than the air, the sweat EVAPORATES and it ABSORBS heat from your body. By absorbing heat from your body, the temperature of your body is lowered. It is the evaporation or the change from a liquid to a vapor of the water on your skin which causes the skin to be cooled. If you stand in a breeze, you feel cooler, even though the temperature of the breeze will be the same as the temperature of still air. The breeze STEPS UP the EVAPORATION process of the sweat and more rapidly cools the body. It is not the breeze alone that makes you feel cooler. It is the increase in the rate of evaporation which makes the body feels cooler.
All cooling towers operate on the principle of removing heat from water by evaporating a small portion of the water that is recirculated through the unit. The heat that is removed is called the latent heat of vaporization. Each one pound of water that is evaporated removes approximately 1,050BTU’s in the form of latent heat. The amount of heat lost by the water depends on the temperature rise of the ambient air before it leaves the tower. This means that both the dry bulb and wet bulb temperatures of the air are important. When WBT = DBT, this condition corresponds to 100% relative humidity (RH) that implies the air is fully saturated. The air will no longer accept water and the lack of evaporation do not allow the wetted bulb to reject heat into the air by evaporation
Higher the difference between DBT and WBT, lower is the relative humidity or drier is the air. The lower relative humidity indicates greater capacity of air to absorb or hold water and shall result in efficient lowering of water temperatures.
Sensible Cooling…
The air temperature rises as it absorbs sensible heat from the water. This sensible heat transfer occurs, if the dry bulb temperature (DBT) of air is less than the DBT of water. This may account for 20% of the cooling.