If a cooling tower is not holding temperature, the real question is whether it is performing as expected for the actual duty, site wet-bulb, airflow condition, water quality, and mechanical condition on the day. Tower Thermal’s own FAQ, service, and engineering pages make that clear: wet-bulb sets the practical lower bound for leaving-water temperature, while recirculation, fouling, water chemistry, and maintenance quality all affect real performance.
What does cooling tower efficiency mean?
Cooling tower efficiency is a simple way to compare the actual cooling achieved by the tower against the maximum cooling theoretically available under current ambient conditions. In practical terms, it tells you how close the tower is getting to the entering air wet-bulb temperature. That matters because Tower Thermal identifies wet-bulb as the dominant environmental factor in tower performance, and defines approach as the difference between leaving-water temperature and wet-bulb.
This is a useful performance check, but it should not be treated as the only measure that matters. A tower still has to meet the required cold-water temperature, stay stable as loads change, and remain maintainable over time. Tower Thermal’s design and engineering pages focus on those same points: performance, reliability, whole-of-life cost, layout, access, and site constraints all need to be considered together.
The three temperatures you need
To calculate the efficiency of a cooling tower, you need three temperature readings:
Hot water temperature
The temperature of the water entering the tower.
Cold water temperature
The temperature of the water leaving the tower.
Wet-bulb temperature
The entering air wet-bulb temperature at the tower.
Tower Thermal’s sizing guidance asks for hot and cold water temperatures, project location, and design wet-bulb because those values drive tower selection and performance expectations.
Range and approach explained
Before using the formula, it helps to define two core terms.
Range is the drop in water temperature through the tower:
Range = Hot Water Temperature - Cold Water Temperature
Approach is how close the tower gets to the ambient wet-bulb temperature:
Approach = Cold Water Temperature - Wet-Bulb Temperature
A lower approach means the tower is cooling the water closer to the practical ambient limit. Tower Thermal’s FAQ states that wet-bulb sets the practical lower bound for leaving-water temperature, and that approach is leaving water minus wet-bulb.
Cooling tower efficiency formula
Once range and approach are clear, the efficiency calculation is straightforward.
Cooling Tower Efficiency (%) = [(Hot Water Temp - Cold Water Temp) / (Hot Water Temp - Wet-Bulb Temp)] x 100
You can also write the same formula as:
Cooling Tower Efficiency (%) = [Range / (Range + Approach)] x 100
That second version works because the total possible cooling is the difference between the hot water temperature and the wet-bulb temperature, which is simply the range plus the approach.
Example calculation
Say your readings are:
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Hot water entering the tower: 35°C
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Cold water leaving the tower: 29°C
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Entering air wet-bulb: 24°C
First calculate the range:
35 - 29 = 6°C
Then calculate the approach:
29 - 24 = 5°C
Now calculate efficiency:
Efficiency = (6 / (35 - 24)) x 100
Efficiency = (6 / 11) x 100
Efficiency = 54.5%
That means the tower is achieving about 54.5% of the maximum theoretical cooling available under those conditions.
Why this calculation matters
This calculation gives you a quick way to judge whether the tower is broadly in line with expectations or whether something needs attention. It is especially useful when condenser water temperatures are rising, approach is worsening, or the plant is working harder than usual to hold setpoints. Tower Thermal’s maintenance page lists those exact symptoms as signs that the tower may need inspection and corrective service.
It is also useful when you are deciding whether the problem is operational or structural. If the efficiency has dropped, the cause may be fouled fill, poor water distribution, airflow problems, scaling, corrosion, biological growth, or mechanical wear. Tower Thermal’s chemical treatment and maintenance pages link those issues directly to reduced heat transfer, reduced efficiency, water loss, vibration, alarms, and unplanned shutdowns.
What affects cooling tower efficiency in the real world?
The formula is simple. Real sites are not.
The biggest external factor is wet-bulb temperature. When wet-bulb rises, the tower’s achievable leaving-water temperature rises with it. That is why a tower that performed well on one day may appear to struggle on another, even if the tower itself has not changed. Tower Thermal’s FAQ is explicit that wet-bulb is the dominant factor and sets the practical lower bound for leaving-water temperature.
The next major factor is airflow quality. If hot, saturated discharge air is pulled back into the tower inlet, performance drops because the tower is no longer seeing the entering-air condition it was designed around. Tower Thermal flags recirculation from poor placement, wind effects, nearby walls, or multiple towers as a direct cause of reduced performance.
Then there is water quality. As water evaporates, dissolved minerals stay behind. If blowdown and chemical control are not managed properly, scale, corrosion, and biological fouling build up and reduce heat rejection efficiency. Tower Thermal’s chemical treatment page makes that point directly and ties stable treatment to better heat transfer performance and fewer shutdowns.
Finally, mechanical condition matters. Fan faults, drive issues, vibration, basin issues, leaks, degraded fill, and structural wear all affect the tower’s ability to reject heat properly. Tower Thermal’s maintenance and repair page lists higher condenser water temperatures, worsening approach, unusual noise, motor or gearbox faults, and recurring alarms as clear warning signs.
How to improve cooling tower efficiency
Start with the basics. Confirm that the temperature readings are accurate and taken at the same operating condition. Then inspect the tower for obvious airflow and water distribution issues.
After that, the main improvement levers are usually:
Planned maintenance
Routine inspections and maintenance keep the tower operating correctly, reduce wear on key components, and stop small issues becoming bigger failures.
Chemical treatment
A stable treatment program reduces scale, corrosion, and biological growth, protects heat transfer performance, and helps avoid unplanned shutdowns.
Better controls and monitoring
Tower Thermal’s BMS and controls options are designed to improve visibility, automate operation, maintain setpoints, reduce energy use, and flag issues early through alarm reporting and fault detection.
Engineering review
If performance has dropped and the cause is not obvious, design and engineering review can help isolate the real issue and define the right upgrade or repair scope. Tower Thermal positions this as the point where performance, reliability, and whole-of-life cost are decided.
Performance testing
Where thermal performance needs to be verified properly, Tower Thermal provides site performance testing to CTI standards, detailed reporting, and upgrade pathways where required.
One reading is useful. A trend is better.
A single efficiency result gives you a snapshot. A trend gives you something you can act on.
If the calculated efficiency is slipping over time, or if the same plant needs more fan speed, more water, or more operator intervention to achieve the same result, that usually points to a real problem. Tower Thermal’s BMS and controls pages focus on monitoring key operating points such as temperatures, fan status, alarms, and run hours so issues are identified early rather than after performance drops.
Efficiency should be tied back to the whole system
A cooling tower should not be judged in isolation. Tower Thermal’s broader capabilities include supply, installation, ongoing service, refurbishment, performance testing, audits, heat exchangers, water treatment and filtration, and controls integration. That matters because a tower can only perform properly if the wider system supports it. Poor water quality, poor control, bad layout, or an unsuitable configuration can all undermine thermal performance.
Need help checking cooling tower performance?
Tower Thermal supports cooling tower projects from selection through to installation, testing, repair, and ongoing service across HVAC, refrigeration, building services, mining, oil and gas, industrial power, food and beverage, and data centres. Its products are designed, engineered, QA tested, and assembled in Australia, and the business supports both Australian and export markets.
If your cooling tower is running hot, approach is worsening, or the system is getting harder to control, the next step is to review the actual operating data and test the tower against real site conditions. The fastest way to do that is to send through the flow rate, hot and cold water temperatures, project location, wet-bulb, water quality, and site constraints so the right performance check or upgrade path can be defined.
