Cooling Tower Range, Approach and Condenser Water Flow Calculator
Work out a tower’s range, approach and heat rejection from its water temperatures and flow — or the condenser water flow a chiller needs.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Range (entering − leaving water)
- 10°F
- Approach (leaving water − wet bulb)
- 7°F
- Heat rejected
- 1,500,000BTU/h
Range 10 °F, approach 7 °F, rejecting 1,500,000 BTU/h.
- A tower’s rated (nominal) tons are its capacity at one rating condition: 3 gpm per ton cooled from 95 °F to 85 °F at a 78 °F wet bulb, which is 15,000 BTU/h. At other conditions the same tower rejects a different amount; use the manufacturer’s selection data.
- Heat rejected ÷ 15,000 BTU/h (nominal tower-ton basis)
- 100nominal tower tons
- Range ÷ (range + approach)
- 59%
Show the working
- Range = entering − leaving water95 − 85 = 10 °F
- Approach = leaving water − entering wet bulb85 − 78 = 7 °F
- Heat rejected = 500 × gpm × range500 × 300 × 10 = 1,500,000 BTU/h
Range and approach
The range is the temperature drop of the water through the tower: entering hot water minus leaving cold water. The approach is how close the leaving water gets to the entering air’s wet-bulb temperature. A tower can never cool water to the wet bulb, so the approach is always above zero.
Heat rejected by the water = 500 × gpm × range.
Condenser water flow for a chiller
The condenser water carries away the chiller’s cooling load plus its compressor energy: heat rejected = capacity × (1 + 1/COP). At a COP of 4, a 100-ton chiller rejects 1.5 million BTU/h, which at a 10 °F range is 300 gpm — 3 gpm per ton.
That is the origin of the nominal “tower ton”: cooling 3 gpm from 95 °F to 85 °F at a 78 °F wet bulb, or 15,000 BTU/h. A tower’s capacity at other conditions comes from the manufacturer’s selection data.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Calculate
- Tower range, approach and heat rejection
- Hot water entering the tower
- 95 °F
- Cold water leaving the tower
- 85 °F
- Entering air wet-bulb temperature
- 78 °F
- Water flow through the tower
- 300 gpm
Result
- Range (entering − leaving water)
- 10 °F
- Approach (leaving water − wet bulb)
- 7 °F
- Heat rejected
- 1,500,000 BTU/h
- Heat rejected ÷ 15,000 BTU/h (nominal tower-ton basis)
- 100 nominal tower tons
- Range ÷ (range + approach)
- 59 %
Range 10 °F, approach 7 °F, rejecting 1,500,000 BTU/h.
Assumptions
- Heat rejected by the water = 500 × gpm × range (water at about 8.33 lb/gal).
- Chiller heat rejection = capacity + compressor input = capacity × (1 + 1/COP).
Limitations
- Evaporation, drift and blowdown are not calculated.
- Tower capacity at other conditions comes from the manufacturer’s selection data; this tool does not predict the approach a tower can reach.
Frequently asked questions
Why is a cooling tower ton 15,000 BTU/h?
It includes the compressor heat: 12,000 BTU/h of cooling plus about 3,000 BTU/h of compressor energy for a chiller with a COP of about 4.
What approach can a tower reach?
It depends on the tower size and design and the conditions; the manufacturer’s selection data shows what a given tower can do.
Does this calculate evaporation or blowdown?
No. Those depend on the water treatment and cycles of concentration, which this tool does not cover.
Sources
- Notes for products (nominal capacity definitions) — Baltimore Aircoil Company. Nominal tons of cooling: the capability to cool 3 USGPM of water from 95 °F entering to 85 °F leaving at a 78 °F entering wet-bulb temperature.
- Best Practices Guide for Energy-Efficient Data Center Design (revised July 2024) — U.S. Department of Energy, Federal Energy Management Program. For a given cooling load, larger cooling towers have a smaller approach to the ambient wet-bulb temperature, allowing lower condenser water temperatures and better chiller efficiency (p. 26).
- University Physics Volume 2, §4.3 Refrigerators and Heat Pumps — OpenStax, Rice University (CC BY 4.0). Energy conservation W = Q_h − Q_c; refrigerator coefficient of performance K_R = Q_c ÷ W; heat pump K_P = Q_h ÷ W.
- NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.
Spotted an error? Report a problem with this calculator — see our corrections policy.