Understanding Cooling Tower Performance: Factors, Ratings, and Design Considerations

Jun 19, 2023 | H.E. Rieckelman

How do you evaluate the performance of a cooling tower? What factors impact how effective a cooling tower will be in a given application? When it comes to evaluating the performance of a cooling tower, it’s crucial to consider various factors that impact its effectiveness. In this post, we will explore the significance of ambient wet bulb temperature on cooling tower performance and how it influences sizing decisions.

Imagine yourself on a sunny day in Toronto, Canada, relaxing by the pool with a scorching temperature of 35C. As you jump into the pool to cool off, your body experiences relief as the water evaporates into the dry air. The water particles evaporate off your body, taking some of the heat with it. Cooling towers operate on the same principle but with a different purpose. Instead of cooling your body, they transfer heat away from the condenser water loop in a chilled water plant.

The key factor that affects cooling tower performance is the wet bulb temperature. It refers to the lowest temperature achievable through the evaporation of water under existing ambient conditions. The actual air temperature (dry bulb) and humidity determine the wet bulb temperature, varying across different regions. It’s essential to consider the design wet bulb temperature specific to your location when sizing a cooling tower system.

Now, let’s compare your poolside experience in Toronto to a scenario in humid Cancun, Mexico. In Cancun, although the air temperature remains the same, the higher humidity impedes evaporation, resulting in less heat being transferred from your body to the air. This analogy demonstrates how wet bulb temperature affects cooling tower performance—regions with different wet bulb temperatures require appropriately sized cooling towers to release heat effectively.

To evaluate cooling tower performance and determine their size, they are rated in tonnage, measured in BTU/hr. A refrigeration ton equals 12,000 BTU/hr, while a cooling tower ton equals 15,000 BTU/hr. The additional 3,000 BTU/hr accounts for the heat generated by the compressor in typical air conditioning applications, which the cooling tower dissipates.

The Cooling Tower Institute (CTI) establishes ratings for cooling towers based on specific design conditions: 95°F/85°F @ 78°F wet bulb, 10°F range, 7°F approach, and 3 GPM per Cooling Tower Ton. These parameters provide a standardized comparison among cooling towers. However, it’s important to note that selecting a cooling tower should involve considering the design wet bulb conditions specific to your region.

Approach and range are two crucial terms in cooling tower design. Approach represents the temperature difference between the water leaving the cooling tower and the ambient wet bulb temperature, indicating how effectively the tower can cool the water. Range, on the other hand, refers to the temperature difference between the entering and leaving water. Cooling towers are sized based on the region’s design wet bulb, rather than the dry bulb temperature, due to the evaporation process.

Adjusting the range impacts pump horsepower, energy consumption, and cooling tower size. Increasing the range reduces the GPM and pump horsepower but raises the average condensing water temperature, making the chiller work harder and consume more power. Balancing these factors is crucial in optimizing cooling tower design and operation.


Understanding the impact of ambient wet bulb temperature, ratings, and design considerations is essential when working with cooling tower systems for commercial and industrial applications. By considering these factors, you can make informed decisions to ensure optimal performance, efficiency, and cost-effectiveness in your cooling tower installations. Remember to take into account the specific wet bulb conditions of your region to select and size cooling towers appropriately.


By: Nash Mohammad, B.Eng