Cooling Tower Efficiency

Approach

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     As an essential factor in the water cooling process, the cooling tower approach would be the difference between the cold water temperature and wet bulb temperature (WBT) illustrated in Figure 1. The capability of the cooling tower is a measure of how close the tower can bring the water temperature to the WBT of the entering air. The smaller the approach value, the more efficient a cooling tower will be according to the formula.

F.4    Approach = Cooling water outlet Temperature – Wet Bulb Temperature

Wet Bulb Temperature

     The wet-bulb temperature is the temperature of the air when measured with a wet bulb thermometer from which water is evaporating. Today, WBT is calculated based on measurements of relative humidity and temperature. This is essentially a measurement of how much water vapor the atmosphere can hold at current weather conditions. A lower WBT means the air is drier and can hold more water vapor than it can at a higher WBT. Each location throughout the US has a unique design (worst case) wet bulb temperature that is published by organizations such as ASHRAE and can be obtained easily.

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Interpreting Results

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     Occasionally calculating the cooling tower efficiency can provide useful data on how well a cooling tower is performing. From the formula, the efficiency of a cooling tower is negatively correlated with an elevated wet-bulb temperature. As the WTB raises with temperatures, hotter climates result in lower cooling tower efficiencies. When performing an efficiency calculation, it is important to factor in the time of year for the reasoning above. Computing cooling tower efficiency in colder months will present a greater value than in the middle of summer. Water treatment programs are issued specifically to each cooling tower to help maintain high efficiency. Maintaining sufficient chemical residuals and concentration cycles will prevent corrosion and scale formation which hinders heat transfer efficiency. As previously mentioned, flow rate and heat load are two parameters that affect range. If the flow rate was to decrease, the range would typically increase (F.3). At a given rate of air moving through a cooling tower, the extent of heat transfer that can occur will depend upon the amount of water surface exposed. Plugged spray nozzles and distribution methods will decrease this surface area. Moving back to the original formula, range and approach play a large factor in overall efficiency. As the approach increases, efficiency inversely decreases.

     Periodically computing the efficiency of a cooling tower not only allows water treaters to ensure their programs are working but can help identify key mechanical errors when they occur. Understandably there are many parameters that contribute to efficiency and the thought process has been quite extensive but does help to better understand cooling towers and the role they play.