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Calculating Cooling Tower Efficiency

About Cooling Towers

Installing the appropriate size and type of cooling tower helps to ensure that it transfers heat efficiently for your particular system. So do other external factors, such as climate. The term “efficiency” is common knowledge, but how exactly is that calculated for a specific cooling tower? Knowing how to determine the efficiency allows you to have confidence the tower is functioning properly and address any changes in it over time to determine if there are any issues.  


The formula for calculating cooling tower efficiency is fairly simple:


Efficiency = Range / (Range + Approach) X 100


However, being able to plug numbers into the equation involves first calculating both the Range and the Approach. Both of these variables are measured in degrees, but the tools needed differ slightly.


Cooling Tower Approach and How to Calculate


Essentially, the term Approach refers to the difference between the Cold Water Temperature (CWT) and the Wet Bulb Temperature (WBT). Thus, calculating this involves simple subtraction. But, how does one measure these two temperatures?  


The CWT is simple. This is the temperature of the water that collects in the cold water basin (aka collection basin). Measuring this temperature involves a simple thermometer.


Wet-bulb temperature is measured using a psychrometer. This device collects a thin water film on the thermometer’s bulb and then twirls it around to reduce the temperature. The point at which twirling no longer results in further reducing the temperature is the wet-bulb temperature. Several factors affect this temperature, including both the external air temperature and the humidity in the area. Because WBT tells us the amount of water vapor the air can hold, drier areas tend to have a lower WBT. The atmosphere in those areas can hold more water vapor than humid areas.


Cooling Tower Range and How to Calculate


Calculating the cooling tower range is very simple. The concept of Range refers to the difference in temperature between the hot water that enters a cooling tower and the cooled water that collects after the cooling process. Thus, the formula is simply HWT – CWT.


Putting it All Together


Now that we have both the Range and the Approach, it is simply a matter of plugging those numbers into the original efficiency equation. Because of how the Wet Bulb Temperature factors into this equation, it’s important to note that efficiency in a cooling tower is negatively correlated to that WBT. So, in hotter climates where the temperature outside it is higher, efficiency tends to be less. When thinking about any cooling system, this is generally the case. Systems must work harder to bring the temperature down.


How to Increase Efficiency in a Cooling Tower


Knowing the efficiency of a cooling tower is just the first step in ensuring your tower runs as effectively as possible. There are several strategies to bring that efficiency up over time. One of these is by assessing and optimizing blowdown that occurs within the tower. Blowdown (also called bleed-off) refers to the occurrence of mineral-rich water being flushed out of a cooling tower to be replaced by freshwater. The reasoning behind this process is to consistently decrease the mineral levels in the water. Minerals produce build-up and scaling when above their saturation point. So, using bleed-off keeps those levels below the threshold and build-up to a minimum.


When discussing blowdown, it’s helpful to define the term Cycles of Rotation (COC). This refers to the number of cycles that water travels through in a cooling tower before being discharged. While the formula for calculating COC is a little complicated, it’s important to note that the higher the COC, the less blowdown will occur. This is ideal when thinking about maintaining an effective heat transfer during its operation. However, it’s a balancing act between this and also preventing scaling on the tower’s components. Determining the ultimate COC to find that balance takes time, measurements, and constant observation.


Water treatment is another strategy for increasing cooling tower efficiency. Water entering the tower will differ in makeup depending on the source. Impurities picked up along the way lead to issues like scaling and corrosion. Thus, creating an effective water treatment plan can limit this and keep efficiency high.


Lastly, switching from static to dynamic driver speeds throughout the tower also helps to increase efficiency. The concept of driver speeds refers to the motors used to power fans that move water throughout the cooling process. The drivers used in older cooling towers were static, meaning they consistently ran at one high speed. This speed works well when there’s the need to have full power heading to the fans. However, there are also times when this high power isn’t needed. For this reason, dynamic drivers are the better option which varies the power sent to the fan motors depending on the need.


Achieving Optimum Efficiency and Maintaining It are Two Different Things


Energy efficiency when it comes to cooling towers is not simply reached once and then forgotten. Because the factors impacting efficiency are ever-changing, you must regularly monitor and take the proper measurements to calculate efficiency over time. Big manufacturers often have teams of staff collaborating on this. This is a great option when there is expertise and experience on the team – people who have spent years learning best practices, product knowledge around cooling tower installation and maintenance.  


Others look to industry experts to step in and take the lead. At Cooling Tower Experts, we offer services that range from planning and installation to ongoing maintenance and repairs. We’ve gathered years of experience working with cooling towers of all sizes and designs. We’ve got the tools and knowledge to get your towers running efficiently and keep them doing so for years. If you’ve got a project you’re ready to start or need some assistance with an existing unit – we would love to help out.  Send us a message or give us a call at (904) 680-1677 to get started.