the cooling tower

A cooling tower is a device that uses the contact between water and air to dissipate waste heat generated in industry or in refrigeration and air conditioning through evaporation. Waste heat generated in industrial production or refrigeration processes is generally directed away with cooling water. A certain amount of water is drawn from natural water bodies such as rivers, lakes, and seas as cooling water. The cooling process equipment absorbs waste heat to increase the water temperature and then discharges it into rivers, lakes, and seas. This cooling method is called direct current cooling. When direct current cooling conditions are not available, a cooling tower is required for cooling. The function of the cooling tower is to exchange heat between the cooling water carrying waste heat and the air in the tower, so that the waste heat is transferred to the air and dispersed into the atmosphere.

1. Basic working principle of cooling tower

After being pumped by the fan, dry (low enthalpy) air enters the cooling tower from the air inlet network; high-temperature water molecules with high saturated steam partial pressure flow to the air with low pressure, and humid (high enthalpy) water is sprinkled into the tower from the water seeding system. When water droplets come into contact with air, on the one hand, due to direct heat transfer between air and air, on the other hand, due to the pressure difference between the water vapor surface and the air, evaporation occurs under the action of pressure, which takes away the latent heat of evaporation, taking away the heat in the water, that is, evaporation heat transfer, thereby achieving the purpose of cooling.

2. Working process of cooling tower

Take the working process of circular countercurrent cooling tower as an example:

Hot water is pumped from the main engine room through the pipe, horizontal throat, curved throat, and central throat at a certain pressure to the water spreading system of the cooling tower, and the water is evenly spread on the filler through the small holes on the water spreading pipe; dry low-humidity air enters the tower from the bottom air inlet net under the action of the fan, and the hot water forms a water film when it flows through the surface of the filler to exchange heat with the air. The hot air with high humidity and high humidity is extracted from the top, and the cooling water drips into the bottom basin and flows into the main engine through the outlet pipe.

Generally speaking, the air entering the tower is dry air with low wet-bulb temperature. There is an obvious concentration difference and kinetic pressure difference of water molecules between water and air. When the fan is running, under the action of the static pressure in the tower, water molecules continue to evaporate into the air and become water vapor molecules. The average kinetic energy of the remaining water molecules will decrease, thereby reducing the temperature of the circulating water. From the above analysis, it can be seen that evaporative cooling has nothing to do with the air temperature (usually the dry-bulb temperature) being lower or higher than the water temperature. As long as the water molecules can continue to evaporate into the air, the water temperature will decrease. However, the evaporation of water into the air will not continue endlessly. When the air in contact with water is unsaturated, water molecules continue to evaporate into the air, but when the air on the water-gas contact surface reaches saturation, the water molecules cannot evaporate, but are in a dynamic equilibrium state. The number of water molecules evaporated is equal to the number of water molecules returned from the air to the water, and the water temperature remains unchanged. From this, it can be seen that the drier the air in contact with water, the easier it is to evaporate and the easier it is to reduce the water temperature.