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Cooling Tower Efficiency

As a general rule of thumb, Cooling Tower Efficiency = Range / (Range + Approach) X 100, but how is the formula interpreted and utilized?

A cooling tower is a heat rejection device that rejects waste heat to the atmosphere through the cooling of a coolant stream, usually water to a lower temperature. They represent a relatively cost-effective and dependable means of removing low-grade heat from cooling water and although there are several types, operate under similar principles. As water usage and energy costs continue to be a pressing topic, monitoring the efficiency of cooling towers can help plant engineers and managers ensure their equipment is operating as designed.

How efficient a cooling tower is operating can be calculated using the formula below. Taking a step back, it is important to understand all relative parameters and where they are derived from.

F.1    Cooling Tower Efficiency = Range / (Range + Approach) X 100

Range

Simply put, range is the difference between the water temperature entering a cooling tower and that leaving. This is determined by the load on the cooling tower and the circulation rate. Cooling tower range will not change if pump circulation and heat loads are constant. This is a direct function of the amount of water circulated and the head load. Increasing the range because of added heat demand requires an increase in the cooling tower size. Cooling towers are often specified to cool a certain flow rate from one temperature to another at a given wet bulb temperate.

F.2    Range (or ΔT) = Inlet Temperature – Cooling water outlet Temperature

Another way to calculate range is from the formula below:

F.3    Range °F = Heat Load Btu/min / (GPM X 8.33)

Approach

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.

Interpreting Results

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.

Jed Kosch

List of Resources:

[1] SPX Cooling Technologies, Inc. (n.d.). Cooling Tower Performance. SPX Cooling Technologies. Retrieved October 3, 2021, from https://spxcooling.com/wp-content/uploads/TR-016.pdf

[2] R. (2021, May 7). Easy Guide to Cooling Tower Efficiency & How To Increase it. Industrial Manufacturing Blog | Linquip. https://www.linquip.com/blog/increasing-cooling-tower-efficiency-calculation/

[3] Cooling Tower Efficiency Calculations. (n.d.). IDC Technologies. Retrieved October 3, 2021, from https://www.idc-online.com/control2/Cooling_Tower_Efficiency_Calculations.pdf

[4] apeirosmarketing. (2021, September 11). Calculating Cooling Tower Efficiency. Cooling Tower Experts. https://coolingtowerllc.com/calculating-cooling-tower-efficiency/

[5] Cooling Tower Efficiency. (n.d.). The Engineering Toolbox. Retrieved October 3, 2021, from https://www.engineeringtoolbox.com/cooling-tower-efficiency-d_699.html

[6] Cooling Tower. (n.d.). Bureau of Energy Efficiency - Government of India. Retrieved October 3, 2021, from https://beeindia.gov.in/sites/default/files/3Ch7.pdf

Figure 1: Relationship of temperatures related to cooling tower operation.

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