Cooling Tower Approach and Range

Range, approach, heat rejection (gpm × 500 × range BTU/hr).

Run the calculator

Example

You enter

Entering water T (°F) 95
Leaving water T (°F) 85
Ambient wet-bulb (°F) 75
Design flow (gpm) 300
Fan kW (optional) 15

You get

Range 10
Approach 10
Efficiency 0.5
Heat rejection 1500000
Approach flag in-range (5-10 °F)
Range flag in-range (8-12 °F)
Fan kW per ton 0.12

Details, formula, and sources

Range, approach, heat rejection (gpm × 500 × range BTU/hr), and fan kW per ton against typical 5-10 °F approach / 8-12 °F range targets.

Range = T_in − T_out. Approach = T_out − T_wb. Efficiency = range / (range + approach) = range / (T_in − T_wb). Heat rejection (BTU/hr) = gpm × 500 × range. Fan kW per ton = fan_kW × 12000 / heat_rejection.

Cooling Technology Institute (CTI) ATC-105 standard practice by name; ASHRAE Handbook (HVAC Systems and Equipment) cooling-tower chapter by name.

CTI guides free at cti.org outreach; ASHRAE Handbook licensed.

Estimate. AHJ and licensed mechanical contractor govern. ACCA Manual J / D / S supersede rules of thumb.

Field names used by the API: T_in_F, T_out_F, T_wb_F, gpm, fan_kW, range_F, approach_F, efficiency, heat_rejection_BTU_hr, approach_flag, range_flag, fan_kW_per_ton

Constant 500 (8.34 lb/gal × 60 min/hr × cp = 1 BTU/lb-°F) ≈ 500 BTU/(hr · gpm · °F)physical-fact derivation for water
Approach target band 5-10 °FCTI engineering practice
Range target band 8-12 °FCTI engineering practice