Working Principle of Cooling Tower Cooling

Date:Mar 28, 2020

Cooling tower is a kind of equipment that uses the contact of water and air to disperse waste heat generated in industry or refrigeration and air conditioning through evaporation.

Basic principles of cooling tower

Dry (low enthalpy) air enters the cooling tower from the air inlet network after being twitched by the fan.  High-temperature water molecules with high partial pressure of saturated steam flow to air with low pressure, and hot and humid (high enthalpy) water is sprinkled into the tower from the water sowing system.  When water droplets come into contact with air, on the one hand, due to the direct heat transfer between air and water, on the other hand, due to the pressure difference between the surface of water vapor and air, evaporation occurs under the action of pressure, which takes away the heat from water, i.e. evaporation heat transfer, thus achieving the purpose of cooling down the temperature.

cooling tower

Working Process of Cooling Tower

For example, the working process of a circular counter-current cooling tower is as follows: the hot water main engine room pumps circulating water pressure into the cooling tower's water sowing system through pipes, horizontal throats, curved throats and central throats at a certain pressure, and evenly spreads the water on the filler through small holes in the water sowing pipe;  Dry air with low enthalpy enters the tower from the bottom air inlet net under the action of the fan, water film is formed when hot water flows through the surface of the filler to exchange heat with the air, hot air with high humidity and high enthalpy is pumped out from the top, cooling water drips into the bottom basin, and flows into the host through the water outlet pipe.  Under normal circumstances, the air entering the tower is dry air with low wet bulb temperature. There is obviously a concentration difference and kinetic energy pressure difference between water and air. When the fan is running, under the action of static pressure in the tower, water molecules continuously evaporate into the air to become water vapor molecules, and the average kinetic energy of the remaining water molecules will decrease, thus lowering the temperature of circulating water.

From the above analysis, it can be seen that the evaporative cooling has nothing to do with the air temperature (commonly called dry bulb temperature) being lower than or higher than the water temperature. As long as water molecules can continuously evaporate into the air, the water temperature will decrease.  However, the evaporation of water into the air will not continue indefinitely.  When the air in contact with water is unsaturated, water molecules evaporate into the air continuously, but when the air on the water-air contact surface reaches saturation, the water molecules do not evaporate out, but are in a dynamic equilibrium state.  The number of water molecules evaporated is equal to the number of water molecules returned to the water from the air, and the water temperature remains unchanged.  It can be seen from this that the drier the air in contact with water, the easier evaporation will be and the water temperature will be lowered.

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