Cooling Tower Efficiency Calculator
Calculate cooling tower efficiency (effectiveness), range, approach temperature, and heat rejection rate based on inlet/outlet water temperatures, wet-bulb temperature, and flow rate.
Formulas Used
Range (°F):
Range = Thot,in − Tcold,out
Approach (°F):
Approach = Tcold,out − Twb
Cooling Tower Efficiency (Effectiveness):
η = Range / (Range + Approach) × 100 = (Thot,in − Tcold,out) / (Thot,in − Twb) × 100
Heat Rejection Rate:
Q (BTU/hr) = GPM × 500.4 × Range
where 500.4 = 8.34 lb/gal × 60 min/hr × 1 BTU/(lb·°F) (specific heat of water)
Q (Tons) = Q (BTU/hr) / 12,000 | Q (kW) = Q (BTU/hr) × 0.000293071
Number of Transfer Units (NTU) — Merkel simplified:
NTU = Range / Approach
(Valid approximation when the operating line and equilibrium line are approximately parallel.)
Assumptions & References
- Water is assumed to have a specific heat (Cp) of 1.0 BTU/(lb·°F) and a density of 8.34 lb/gal.
- References: ASHRAE Handbook — HVAC Systems and Equipment (Chapter: Cooling Towers); Cooling Technology Institute (CTI) Standard STD-201; Merkel, F. (1925). Verdunstungskühlung.