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.
- Efficiency is defined as the ratio of actual cooling to the maximum possible cooling (from hot inlet to wet-bulb temperature), per ASHRAE and CTI standards.
- The NTU formula uses the Merkel simplified method, which assumes equal liquid-to-gas (L/G) ratio and neglects evaporation losses in the energy balance.
- Wet-bulb temperature is the theoretical lower limit for the cold water outlet temperature in an evaporative cooling tower.
- A positive approach temperature is physically required; a zero or negative approach is thermodynamically impossible in a real tower.
- Typical cooling tower efficiencies range from 55% to 80% for well-designed systems (ASHRAE Handbook — HVAC Systems and Equipment).
- Heat rejection in tons: 1 ton of refrigeration = 12,000 BTU/hr (ASHRAE definition).
- References: ASHRAE Handbook — HVAC Systems and Equipment (Chapter: Cooling Towers); Cooling Technology Institute (CTI) Standard STD-201; Merkel, F. (1925). Verdunstungskühlung.