Cooling Tower Approach Calculator
Determine cooling range, approach temperature to ambient wet bulb, tower thermal effectiveness percentage, and chiller compressor energy savings.
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Cooling Tower Approach & Chiller Plant Thermodynamics
A cooling tower can never cool water below the ambient air wet-bulb temperature. The difference between the leaving cold water temperature and the ambient wet bulb is known as the approach. The temperature difference between entering hot water and leaving cold water is the range.
Key Thermal Performance Metrics
- Thermal Effectiveness (η):
η (%) = [ Range / (Range + Approach) ] × 100
Typical well-designed industrial towers achieve 55% to 75% effectiveness. - The 1.5% Chiller Rule:
For water-cooled centrifugal chillers, every 1°F reduction in condenser entering water temperature lowers compressor lift and reduces chiller energy consumption by approximately 1.5% to 2.0%.
Frequently Asked Questions
Can a cooling tower cool water below the ambient dry-bulb temperature?
Yes! Because cooling towers rely on evaporative cooling, they cool water below the ambient dry-bulb temperature, down toward the ambient wet-bulb temperature.
What is a typical design approach for an HVAC cooling tower?
Most standard commercial HVAC cooling towers are selected for an approach between 7°F and 10°F (4°C to 5.5°C) above design wet bulb (e.g. 85°F cold water at 78°F wet bulb).
Why does a closer approach require a dramatically larger cooling tower?
As approach approaches zero, the driving thermodynamic force vanishes. Reducing the approach from 7°F down to 4°F can require nearly double the physical tower fill volume and fan power.