Why Wet Bulb Temperature Matters More Than You Think in Cooling Tower Selection
Oct 22, 2026
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Many buyers are surprised to learn that cooling towers are not sized based on the air temperature you see on a weather app.
The critical parameter is wet bulb temperature - and misunderstanding this concept is one of the most common causes of undersized cooling towers.
What Is Wet Bulb Temperature?
Wet bulb temperature represents the lowest temperature that water can reach through evaporative cooling. It is always lower than or equal to the dry bulb temperature (the ordinary air temperature shown on weather reports).
The difference between dry bulb and wet bulb temperature depends on humidity:
Low humidity: Large difference - evaporation is more effective
High humidity: Small difference - evaporation is less effective
100% humidity: No difference - evaporation stops entirely
This is why a cooling tower in a dry desert climate can achieve a lower outlet water temperature than the same tower in a humid coastal area, even if the air temperature is identical.
Why Wet Bulb Temperature Determines Tower Size
A cooling tower removes heat by evaporating a portion of the spray water. The rate of evaporation - and therefore the cooling capacity - is directly limited by the wet bulb temperature.
The key design parameter is the approach, which is the difference between the outlet water temperature and the wet bulb temperature:
Approach = Outlet Water Temperature − Wet Bulb Temperature
A smaller approach requires a larger tower. For example:
Cooling water from 37°C to 32°C at a 28°C wet bulb temperature → approach = 4°C → moderate tower size
Cooling water from 37°C to 30°C at a 28°C wet bulb temperature → approach = 2°C → significantly larger tower
Reducing the approach by just 1-2°C can increase the required heat exchange area by 20-40%, depending on the operating conditions.
Common Mistakes with Wet Bulb Temperature
Mistake 1: Using dry bulb temperature instead
Some buyers provide the local summer air temperature (e.g., 38°C) as the design condition. This is incorrect. A tower sized for a 38°C dry bulb temperature may be severely undersized if the actual wet bulb temperature is 30°C or higher.
Mistake 2: Using an average value instead of a design value
The average wet bulb temperature for a region is not suitable for tower sizing. Design should be based on the 1% or 2% annual exceedance value - the wet bulb temperature that is exceeded only 1-2% of the time during the year. This ensures the tower can handle even the hottest, most humid days.
Mistake 3: Ignoring seasonal variation
Some facilities operate year-round and assume that winter conditions are representative. A tower that performs well in winter may be completely inadequate in summer. Always design for the worst-case summer condition.
Mistake 4: Assuming one value applies to an entire region
Wet bulb temperature can vary significantly within a single province or city, especially in areas with varying elevation, proximity to water bodies, or urban heat island effects. Always obtain site-specific data when possible.
How to Obtain the Correct Wet Bulb Temperature
ASHRAE Handbook - Fundamentals: Provides detailed climate data for locations worldwide, including 0.4%, 1%, and 2% annual exceedance wet bulb temperatures.
Local meteorological data: National weather services often publish historical wet bulb temperature records.
On-site measurement: For critical applications, a portable wet bulb thermometer or psychrometer can provide site-specific data over a monitoring period.
Consult the manufacturer: A professional cooling tower supplier can help you identify the appropriate design wet bulb temperature based on your location and operating schedule.
The Cost of Getting It Wrong
An undersized cooling tower does not simply run "a little warm." The consequences can be severe:
Production losses: Process equipment may overheat and shut down during peak summer conditions.
Increased energy consumption: The tower runs at maximum fan speed continuously, driving up electricity costs.
Premature equipment failure: Operating beyond design conditions accelerates wear on fans, motors, and pumps.
Costly retrofit or replacement: Adding capacity to an undersized tower is often more expensive than sizing correctly from the start.
Practical Guidance
When requesting a quotation for a closed-circuit cooling tower, always provide:
Design wet bulb temperature (with source, if available)
Inlet water temperature
Outlet water temperature
Flow rate
Process fluid type
A professional manufacturer will use these parameters to perform a thermal calculation and confirm that the selected model meets your requirements at the design wet bulb temperature.
If a supplier does not ask for the wet bulb temperature, this is a red flag.
Conclusion
Wet bulb temperature is not just a technical detail - it is the foundation of proper cooling tower sizing. Taking the time to confirm the correct value before ordering can prevent costly problems later.
When in doubt, consult ASHRAE climate data or ask your cooling tower supplier to help you determine the appropriate design wet bulb temperature for your location.
Planning a new cooling tower project? Share your location and operating parameters with our engineering team, and we will help you determine the correct design wet bulb temperature and select the right tower.
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