Calculation sequence
Step-by-step method
- Subtract cold-water temperature from hot-water temperature for range.
- Subtract entering wet bulb from cold-water temperature for approach.
- Compare approach with the entered design value and tolerance.
CommissioningIntermediateCentral Plant
Cooling-Tower Performance Calculator
Calculate tower range, approach, effectiveness, and deviation from an entered design approach.
Why it matters: Range and approach distinguish heat-rejection load from the tower's ability to approach ambient wet-bulb temperature.
Calculated result
Engineering evaluation
Measured cooling-tower approach (A)7 °F
- Cooling-tower range (R)
- 10 °F
- Temperature effectiveness (E)
- 58.82 %
- Relative approach deviation from design (informational) (δArel)
- 0 %
Engineering interpretation
Measured range is 10 °F, approach is 7 °F, and temperature effectiveness is 58.82%. The relative approach deviation is informational; the status color uses the entered absolute approach tolerance. Compare only at the documented flow, load, fan operation, and ambient test basis.
Engineering knowledge center
Understand the calculation—not just the answer
Governing relationships
R = Th−Tc
A = Tc−Twb
E = R/(Th−Twb)×100
δArel = (A−Ad)/Ad×100
within entered absolute criterion when A ≤ Ad+T
Current substitution
R = 95−85 = 10 °F; A = 85−78 = 7 °F; E = 58.82%.
Equation legend
Inputs and calculated quantities
- Th
- Entering hot-water temperature — °F or °C
- Tc
- Leaving cold-water temperature — °F or °C
- Twb
- Entering-air wet-bulb temperature — °F or °C
- Ad
- Approved design approach — °F or K
- T
- Authorized approach tolerance — °F or K
- A
- Measured cooling-tower approach — °F
- R
- Cooling-tower range — °F
- E
- Temperature effectiveness — %
- δArel
- Relative approach deviation from design (informational) — %
Applications
How this calculation is used
Building design
Check scheduled range and approach relationships.
Construction
Evaluate startup readings and sensor placement.
Commissioning
Compare measured tower performance with the approved test basis.
Quality controls
Assumptions and limitations
Assumptions
- Entered values represent the same stable operating condition.
- Measurements and reference values use consistent units and boundaries.
Limitations
- This is a screening and field-verification tool, not an automatic acceptance decision.
- Project criteria, manufacturer data, governing standards, and measurement uncertainty remain controlling.
Field use
Verification procedure
- Confirm the approved reference values and test boundary.
- Stabilize the system and record calibrated field measurements.
- Review calculation assumptions, uncertainty, and supporting evidence before disposition.
Common engineering mistakes
- Comparing readings taken at different loads, modes, or reference conditions.
- Treating a calculated status as proof of code, contract, or manufacturer compliance.
Technical basis
References and source standards
- ASHRAE Handbook—FundamentalsCurrent handbook chapters covering fluid flow, thermodynamics, psychrometrics, measurement, and engineering analysis; the project reviewer must verify the licensed edition used for approval.
- ASHRAE Commissioning ResourcesCommissioning-process context for defining test methods, acceptance criteria, evidence, and owner requirements.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement