Approach & Range, Explained

A question Brentwood occasionally receives from customers is: “Why is my tower not performing as designed?” Along with the inquiry, we are typically given the hot and cold water operating temperatures for their tower and an observation that the difference between the two is less than design specifications.

A common misconception is that a tower’s operating range is an indication of how the tower is performing. The range of a recirculating cooling tower that serves a steady-state process is completely independent of tower performance. This can be seen by evaluating the heat load equation for a typical process:


Approach and Range


The above equation shows that range (∆T) is a function of the heat load (Q) and mass flow (m) at the process heat exchanger, not the tower. If the pump speed (mass flow) and heat load remain constant, the range will remain constant as well. So, for a properly functioning cooling tower operating under steady-state conditions, wet bulb temperature will not affect the tower range. In theory, a change in wet bulb temperature will cause the water temperature exiting the tower to increase/decrease proportionally. The system will reach thermal equilibrium, maintaining a constant range at a higher/lower approach. What will reduce the cooling tower range is a decrease in heat load or increase in mass flow rate delivered to the tower. An example of this is winter operation. If the process heat load is reduced during winter months, this can cause a reduction in your tower range.

When compared to design conditions, approach is the true indicator of how your tower is operating. The entering wet bulb temperature is the lowest possible temperature the tower water can obtain. The lower approach a tower can reach, the more effectively it is using the available heat exchange. However, because reaching a lower approach increases the size of the required tower exponentially, low approaches quickly become cost prohibitive. To avoid significant upsets from small calculation errors, the minimum recommended design approach is 5°F. There are many factors that can cause a larger approach (reduced performance) for your tower. A few examples of these factors are an increase in process heat load, reduced airflow through the tower, and lower entering air wet bulb.

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