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Oasis Iceberg
11
2024
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07
How much make-up water does the closed cooling tower require?
After prolonged operation and use, the closed cooling tower requires make-up water—so how much water should be added? Let’s find out together!
After prolonged operation and use, the closed cooling tower requires water replenishment. But how much water should be added? Let’s find out together!
A closed cooling tower uses an internal water circuit that requires no replenishment, as there’s no evaporation or water loss. However, the external circuit—specifically the spray water—continuously evaporates and gets consumed over time, so it must be topped up regularly to ensure the system operates smoothly.
The amount of water replenished varies depending on the cooling capacity—when process equipment generates more heat and the outdoor air temperature is high, more spray water is needed for evaporation. This increased evaporation helps remove heat through greater latent heat of vaporization, naturally leading to higher water consumption. Conversely, if less heat needs to be removed by the cooling tower, the amount of spray water required—and thus consumed—is also reduced.
Additionally, the long-term consumption of spray water is directly related to the heat-transfer capacity of closed cooling towers. If the surface area is sufficiently large, air cooling alone can meet the requirements—effectively functioning as an air cooler—eliminating the need for spray water altogether. However, this approach would increase the initial investment and also raise the energy consumption of the operating fans. Typically, a rational and economically optimal value is chosen based on the golden ratio principle, balancing reduced capital costs with minimized operational expenses. In winter, when ambient temperatures are mild, air cooling alone is sufficient to meet cooling needs without any spray water usage. Meanwhile, during summer's full-load operation—essential for maintaining process temperatures—the system relies on the latent heat of vaporization from the spray water, transferring thermal energy into the outdoor atmospheric environment.
Closed cooling towers, also known as evaporative condensers, are increasingly being used in industrial cooling applications. Many customers, when selecting a closed cooling tower, aren’t sure about the water consumption of these systems. Today, we’ll walk you through how to calculate the make-up water requirement for a closed cooling tower.
The primary consumptions of a closed cooling tower include electrical power usage and circulating water consumption. Specifically, the electrical power consumption is the sum of the motor power, spray pump power, and electric heater power, while the circulating water consumption is divided into evaporative losses, drift losses, and drain losses.
The photo shows the Oasis Ice Peak closed cooling tower being loaded for shipment.
Calculation of supply water:
The circulating water or spray water in the cooling tower decreases due to the following reasons.
1 The cooling method of a cooling tower works by evaporating a portion of the circulating water or spray water into the atmosphere. As a result, the circulating water or spray water gradually decreases.
2 The fan forcibly drives air through the tower, causing fine water droplets to splash out—resulting in reduced water consumption. This phenomenon is known as "water drift."
3 To ensure the quality of circulating water and spray water, a portion of the circulating and spray water is replaced—this process is known as water dilution. The amount of water replaced must be replenished with make-up water.
The amount of water supplied is calculated according to the following formula.
1 Calculate evaporation rate (WE) kg/h
WE = (tw1 - tw2) × L × Cp ÷ 2520
tw1: Inlet water temperature (℃), tw2: Outlet water temperature (℃),
L: Circulating water flow rate (kg/h), Cp: Specific heat capacity of water at constant pressure (Kcal/kg℃),
The latent heat of vaporization of water (kJ/kg°C)
The inlet-outlet water temperature difference is 5°C, and the evaporation rate is approximately 0.54% of the circulating water volume.
2 Water Diffusion Rate (WD)
Depending on the structure of the tower and variations in wind speed, it’s roughly around 0.001% of the circulating water volume.
3 Water Dilution Ratio (WB)
Depending on changes in airborne pollutant levels, the quality of make-up water, concentration multiples, and other factors, the typical amount of circulating water used in air-conditioning systems is around 0.08% of the total circulation volume.
Water Supply Volume (△L)
ΔL = WE + WD + WB = 0.54 + 0.001 + 0.08 = approximately 0.621%
We recommend replenishing the water at 0.7–1.5% of the circulating water volume, where "circulating water" refers to the hourly circulation rate specified during the design of the closed cooling tower.
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