How to speed up the evaporation and heat dissipation of cooling tower water

How to speed up the evaporation and heat dissipation of cooling tower water

In the process of water cooling in the cooling tower, the water is formed into very small water droplets or very thin water film, which expands the contact area of water and air and prolongs the contact time, which strengthens the evaporation and vaporization of water and takes away a large amount of heat in the water. Therefore, the process of water cooling in the cooling tower is the process of conduction heat dissipation and evaporation heat dissipation.

　　 From the theory of molecular motion, the evaporation of water on the surface is caused by the thermal motion of molecules, and the motion of molecules is irregular. The speed of each molecule is different and the fluctuation range is large. When the kinetic energy of some water molecules on the water surface is to overcome the cohesive force of the water inside, these water molecules escape from the water surface and enter the air, which is evaporation. As the water molecules with larger kinetic energy escape, the average kinetic energy of the remaining water molecules decreases, and the temperature of the water also decreases, so that the water is cooled. This is the main reason for evaporation and heat dissipation. Therefore, evaporation and heat dissipation are the result of the movement of water molecules.

　　The evaporation and heat dissipation of water can be carried out when it is boiling, or at a temperature below the boiling point, and most of the evaporation and heat dissipation in nature are evaporation at a temperature below the boiling point. The cooling of hot water in the cooling tower is a phenomenon of evaporative heat dissipation under the condition of lower than the boiling point. The water molecules escaping from the water surface may collide with each other, or the escaping water molecules collide with the existing water molecules in the air, and then they may re-enter the water. If more water molecules escape in a unit time than those returned to the water surface, then the water will continue to evaporate, the water temperature will continue to drop, and the water will be cooled.

The surface evaporation of water is carried out when the water temperature is lower than the boiling point. At this time, there is a vapor pressure difference on the intersection of water and air. It is generally believed that in the contact between water and air, there is a layer at the interface. The extremely thin saturated gas layer is called the water surface saturated gas layer. The water first evaporates into the saturated atmosphere and then diffuses into the air.

　　 Let the temperature of the saturated gas layer on the water surface be t1 and the temperature of the water surface be tf. The smaller the water droplet or the thinner the water film, the closer t1 and tf are. Suppose the partial pressure of saturated water vapor in the saturated gas layer on the water surface is p1, and the partial pressure of water vapor in the air far from the water surface at the temperature t (t is the dry bulb temperature) is p2, then the partial pressure difference between them is: △p=p1-p2

This △p is the driving force for the evaporation and diffusion of water molecules into the air. As long as there is p1﹥p2 (that is, △p is a positive value), then the surface of the water must evaporate and the water will cool down, and the temperature tf with the water surface is high It does not matter whether it is below the air temperature t above the water surface. If the heat consumed by evaporation is represented by H, then under the condition of p1﹥p2, the heat of evaporation H always runs from the water surface to the air, and the heat in the water always decreases.

　　 In order to speed up the evaporation and heat dissipation of water, the following two measures should be taken in the cooling tower:

　　 1. Increase the contact area between hot water and air. The larger the contact area, the more opportunities for water molecules to escape, and the faster the evaporation and heat dissipation. The contact between water and air is mainly carried out in the water spray filler in the cooling tower. On the one hand, the water droplets formed in the water spray filler should be as small as possible and the water film should be thinner; on the other hand, the filler itself should be required. The thinner the better, that is, the larger the area of the filler, the better (the thinner the filler, the larger the total area).

2. Increase the speed of the water film (or water droplets) in the filler, so that the water vapor molecules escaping from the water surface quickly diffuse into the air outside the cooling tower, and the driving force of the diffusion is kept constant, that is, it is not △ p drops down. If the escaped water vapor molecules are not quickly eliminated, the partial pressure p2 of water vapor in the air will increase, and the value of △p=p1-p2 will become smaller (the evaporation driving force is reduced), which is not conducive to evaporation. Therefore, a certain amount of air volume and wind speed must be maintained.