Regarding cooling towers, this trick seems simple, but it’s effective at exposing the tactics used by design firms to deceive people!

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What is a cooling tower used for? A cooling tower is a structure used for cooling water, and it is commonly found in factories such as power plants, chemical plants, and cement plants where precise control of water temperature is required. Working principle of a cooling tower: It utilizes the air blown in and the water sprayed from above to create convection, thereby removing heat sources; part of the water evaporates during this convection process, carrying away the corresponding latent heat of vaporization. Thereby reducing the temperature of the water. Two important concepts related to cooling towers: I learned about two terms in textbooks a long time ago – dry-bulb temperature and wet-bulb temperature. I understood the concept of dry-bulb temperature back then, but I was never able to truly comprehend wet-bulb temperature; I simply remembered that it is measured by covering the mercury bulb on the thermometer with a damp cloth. It wasn’t until one summer day, much later on, that I finally understood this issue. That day I called the sales department of a company that manufactures cooling towers, intending to ask about various technical matters such as the basic design requirements for cooling towers and equipment pricing. At first, I explained to them the purpose of cooling as well as the process parameters; but then a question from them sent me back to square one: “Mr. X, what are the dry and wet bulb temperatures in your area during this season?” I remember my mind going blank for a moment; I had never measured this before... I guess the other person picked up on it and realized how confused and embarrassed I was. They immediately told me that measuring it was quite simple: I needed to find two mercury thermometers. One could be used to take the temperature directly, without any special treatment, while the other should be wrapped in a small piece of gauze and dipped into water. Once the temperature stabilized, both readings could be obtained – the reading taken directly was the dry-bulb temperature, while the one obtained with the gauze-wrapped thermometer was the wet-bulb temperature. I quickly pressed on, asking: What is this measurement you mentioned used for? Seeing that I didn’t know much, the other person began to explain: So why is this measurement necessary? Let’s start with dry-bulb and wet-bulb temperatures. The dry-bulb temperature is the actual temperature of the air in contact with the surface of the mercury bulb; it’s what we refer to as the air temperature ； The wet-bulb temperature is the temperature of the mercury bulb when water is attached to its surface; the evaporation of this water removes heat from the bulb, so this temperature is lower than that of the dry-bulb temperature. By how much exactly? This depends on two factors: dry air and humid air. The rate of water evaporation differs in these two conditions. For example, the weather in the north is relatively dry, so the rate of water evaporation is higher, resulting in a lower wet-bulb temperature. Conversely, the wet-bulb temperature in the south is slightly higher. More technically speaking, the rate of water evaporation is related to the humidity of the air; the higher the humidity, the lower the rate of evaporation and the less heat that is removed, which in turn results in a smaller difference between the dry-bulb and wet-bulb temperatures ； The lower the air humidity, the greater the rate of water evaporation, and thus the more heat is carried away. This results in a larger difference between the dry-bulb and wet-bulb temperatures; therefore, the pattern of changes in this temperature difference can be used to indicate the current level of air humidity. Oh, I see. So what is the relationship between these wet and dry bulb temperatures you mentioned and cooling towers? Realizing that I didn’t understand these concepts, the other person immediately started explaining them to me again: I won’t explain dry-bulb temperature; it’s actually simple to understand. Think about it – when gauze is soaked in water and placed around a mercury bulb, the humidity of the air around that bulb is actually saturated, meaning the humidity level is at its maximum. When I take the wet-bulb thermometer into air that isn’t saturated, some of the moisture on the mercury bulb will vaporize, absorbing heat in the process. Meanwhile, the air transfers heat to the water; this is a form of heat transfer, and for water, it’s sensible heat. When this sensible heat equals the heat required for the water to vaporize, the temperature stabilizes. The value we read is the wet-bulb temperature. What’s the use of this? It’s very useful when designing cooling towers, as we use the difference between dry-bulb and wet-bulb temperatures to estimate the cooling capacity of cooling towers in a given area. In other words, the cooling effect of the cooling towers we manufacture cannot be lower than this value; in reality, it’s usually 3-5 degrees higher than this difference, because the measured dry-bulb and wet-bulb temperature difference represents an ideal value. Therefore, the cooling efficiency of our cooling towers is generally slightly lower than this value. I see, it’s the wet-bulb and dry-bulb temperatures that determine the cooling effect. An experienced traditional Chinese medicine doctor teaches you how to avoid being deceived. Most suppliers of cooling towers in China design them based on standard parameters: the maximum allowable outlet water temperature is equal to the atmospheric wet-bulb temperature τ. The difference between the outlet water temperature t2 (which is usually 32°C) and the return water temperature t1 (which can be 42°C) and the wet-bulb temperature τ is referred to as the proximity value. The lower this value, the higher the energy consumption of the cooling tower, the more filler is required, and the larger the size of the cooling tower. Therefore, for standard cooling towers, this value is set at 4°C. The atmospheric wet-bulb temperature is generally taken as 28°C with a relative humidity of 77%. Therefore, you should carefully check the design parameters provided by the supplier, namely the atmospheric parameters such as atmospheric pressure, relative humidity, wet-bulb temperature, and proximity. Ensure that these values are standard; some manufacturers may cut corners by using values such as 60% for relative humidity and 26°C for wet-bulb temperature, while claiming that these are the actual local parameters. Ask him directly what the proximity is Are the relative humidity of the atmosphere in the design data (standard value: 77%) and the wet-bulb temperature (standard value: 28°C), along with the proximity value (design value: 4°C), standard values? By now, basically no one dares to trick you.