Wet Bulb Temperature and Cooling Tower Performance

What does Wet Bulb Temperature Mean?

Wet-bulb temperature accounts for both heat and humidity, unlike the standard temperature measurement you see on your weather app. It reflects what that combination means for the human body’s ability to cool down.

Wet-bulb temperature is the lowest level of temperature that can be obtained through evaporative cooling of a ventilated surface covered with ice or wet with water.

It indicates a temperature that is near thermodynamic or real temperature. It is the lowest level of temperature reachable under present ambient conditions brought by water evaporation.

Wet Bulb Temperature and Cooling Tower Performance

Evaporative cooling towers enable many buildings across the globe to enjoy moderate interior temperatures. They serve as the final heat transfer step that moves heat from the building interior to the surrounding environment. In addition to their extensive application throughout large residential, commercial and industrial HVAC systems, there are numerous process cooling applications that employ evaporative cooling towers as an effective means of heat rejection.

Delta Cooling Towers, Inc. is a globally recognized manufacturer of corrosion-resistant cooling towers, air strippers, and tanks fabricated of HDPE to provide extended life service. The company posted an article entitled “Understanding Wet Bulb Temperatures And How It Affects Cooling Tower Performance”. The original post is on this page of the company website, and all credit for the article goes to them. We share it below also, slightly edited for format on this forum. From the article…

A cooling tower primarily uses latent heat of vaporization (evaporation) to cool process water. Minor additional cooling is provided by the air because of its temperature increase. Cooling tower selection and performance are based on water flow rate, water inlet temperature, water outlet temperature, and ambient wet bulb temperature. Ambient wet bulb temperature and its effect on performance is the subject of this article. Ambient wet bulb temperature is a condition measured by a device called a psychrometer. A psychrometer places a thin film of water on the bulb of the thermometer that is twirled in the air. After about a minute, the thermometer will show a reduced temperature. The low point when no additional twirling reduces the temperature is called the wet bulb temperature. The measured wet bulb temperature is a function of relative humidity and ambient air temperature. Wet bulb temperature essentially measures how much water vapor the atmosphere can hold at current weather conditions. A lower wet bulb temperature means the air is drier and can hold more water vapor than it can at a higher wet bulb temperature. For example:

Since cooling tower cells cool water by evaporation, the wet bulb temperature is the critical design variable. An evaporative cooling tower can generally provide cooling water 5° – 7° higher above the current ambient wet bulb condition. That means that if the wet bulb temperature is 78°F, then the cooling tower will most likely provide cooling water between 83° – 85°F, no lower. The same tower cell, on a day when the wet bulb temperature is 68°F, is likely to provide 74° – 76°F cooling water. When selecting a cooling tower cell, the highest or the design wet bulb temperature your geographical area will encounter must be used. The highest wet bulb temperatures occur during the summer when air temperatures and humidity are highest. For example, in Indianapolis, Indiana, the design wet bulb temperature is 78°F. Historically Indianapolis can expect less than one hour per year that the conditions exceed a 78°F wet bulb. Typically, 6,000 hours a year will have a wet bulb of 60°F or lower meaning that a cooling tower cell designed for a 78°F wet bulb will be able to make 65-67°F water for 6,000 hours per year for nearly 70% of the year. Most cooling towers are capacity rated at a “standard” wet bulb temperature of 78°F. That means on the days when the wet bulb temperature is 78°F, the tower will produce its stated capacity. In other words, a tower rated to produce 135 tons of cooling will produce 135 tons of cooling at a 78°F wet bulb temperature. At a higher wet bulb temperature, the tower cell capacity to produce colder water decreases. Every location has a unique design (worst case) wet bulb temperature that is published by organizations such as ASHRAE and can be obtained easily.

What does it mean when your cooling tower water temperature is higher than the normal 5-7°F above the current wet bulb temperature?

Your cooling load may be larger than the rated capacity of your cooling tower.
Your cooling tower may have lost efficiency
Due to scale build-up on the tower heat exchange surfaces.
Due to loss of airflow across the heat exchange surfaces.
Due to improper water flow from clogged nozzles or pump performance

What can you do to improve your tower performance?

Add tower cell capacity
Check for the efficiency losses described above
Replace the heat exchange surfaces with new clean fill
Check for proper airflow
Check the water flow is at the design
Check that nozzle is not clogged or broken

Cooling tower performance is tied to ambient wet bulb conditions. Higher wet bulb temperatures occur in the summer when higher ambient and relative humidity occurs. Initial system design and proper system maintenance are critical to be certain your cooling tower is providing the desired cooling.

For more information, or to discuss your own heat transfer challenges, contact a product application specialist. Combine your own knowledge and experience with their application expertise to develop an effective solution.

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