How to Calculate Cooling Tower Approach Temperature
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Hydronics April 14, 2026 9 min read

How to Calculate Cooling Tower Approach Temperature

Introduction

Cooling tower approach temperature is the primary metric for evaluating tower performance in chilled-water HVAC systems, yet many engineers rely on it only during commissioning and neglect periodic recalculation during operation. A 3°F (1.7°C) increase in approach temperature raises chiller condensing pressure enough to add 4.5–6% to compressor power draw (using the ASHRAE Applications Handbook Chapter 14 rule of thumb of 1.5–2% per °F). On a typical 500-ton chiller running through cooling season, this drift can translate to several thousand dollars per year in additional energy. Tracking approach over time transforms it from a commissioning metric into an early indicator of fill fouling, fan, and flow problems.

Approach temperature is bounded by the laws of thermodynamics: evaporative cooling can never reduce water temperature below the ambient wet-bulb temperature, making wet-bulb the absolute floor for leaving cold water temperature. Engineers use approach as the primary specification for tower sizing (a smaller approach requires a larger, more expensive tower with more fill media) and as the main diagnostic for identifying fouling, low flow, or fan degradation during operation. ASHRAE Applications Handbook (Chapter 14, Cooling Towers) targets approaches of 5–8°F (2.8–4.4°C) for comfort cooling applications, with 4–5°F used on premium installations and 8–10°F accepted on cost-driven projects.

What Is Cooling Tower Approach and Why Engineers Need It

Cooling tower approach is defined as the difference between the leaving cold water temperature (CWT) from the tower basin and the entering ambient wet-bulb temperature (WBT). It represents how closely the tower cools the water toward the thermodynamic minimum. A tower with a 5°F approach and a design wet-bulb of 76°F delivers 81°F supply water; the same tower serving a chiller designed for 85°F condenser water supply at 76°F design wet-bulb would have a 9°F approach.

Engineers use approach to specify towers in design and to diagnose tower problems in operation. During specification, approach drives the size-cost trade-off: every 1°F closer approach roughly adds 8–12% to fill volume. During operation, drift from commissioning baseline flags fouling, fan degradation, or low water flow before they show up as chiller alarms.

Understanding the Formula Step by Step

Approach (°F or °C) = Cold Water Temperature (CWT) − Wet-Bulb Temperature (WBT)

Cold Water Temperature (CWT) is the temperature of water leaving the tower basin after evaporative cooling. This is the supply temperature to the chiller condenser. It is measured at the tower sump outlet or at the condenser water pump inlet. Wet-Bulb Temperature (WBT) is the psychrometric condition of ambient air entering the tower, measured away from the tower to avoid recirculation effects. It represents the evaporative cooling limit at current atmospheric conditions.

The formula yields approach in the same temperature unit as the inputs. A positive approach (CWT > WBT) is physically required: water cannot be cooled below wet-bulb by evaporation alone. An approach of zero would represent a thermodynamically perfect tower of infinite size. Practical towers range from 4°F (2.2°C) approach for large, well-maintained towers to 15°F (8.3°C) or more for undersized or fouled equipment.

Worked Example 1: Design Condition Verification

A cooling tower is specified for 7°F approach at a design wet-bulb of 78°F, delivering 85°F supply water to the chiller condenser. At commissioning, the engineer measures: CWT = 84.3°F, WBT = 77.4°F. Approach = 84.3 − 77.4 = 6.9°F, within 0.1°F of specification.

In metric: CWT = 29.1°C, WBT = 25.2°C, approach = 3.9°C (equivalent to 6.9°F). Design supply water is 29.4°C (85°F), so the tower is delivering 0.3°C below spec. Acceptable variance for commissioning.

Worked Example 2: Diagnostic After Fill Fouling

A tower with a design 7°F approach is tested during summer peak: CWT = 88.6°F, WBT = 76.8°F, approach = 11.8°F — about 70% above design. Entering condenser water is 88.6°F instead of the design 85°F, forcing 3.6°F (2.0°C) higher condensing temperature than the chiller was selected for.

In metric: CWT = 31.4°C, WBT = 24.9°C, approach = 6.5°C. ASHRAE Applications Handbook (Chapter 14) gives roughly 1.5–2% chiller power increase per °F of elevated condenser temperature; at 3.6°F over design, the compressor draws 5.4–7.2% more power. For a 500-ton centrifugal chiller at 0.55 kW/ton (275 kW design draw) running 3,200 hours per year at $0.13/kWh, that is roughly $6,200–8,300/year in additional energy.

When Approach Doesn't Tell You Everything

Approach is a powerful diagnostic but it has blind spots.

Recirculation. Hot, humid plume re-entering the inlet inflates apparent wet-bulb. Approach calculated against a fan-stack-influenced WBT reading can look acceptable while actual condenser supply temperature is well above design. Always measure WBT at an upwind free-air location, ideally 30 ft from the tower.

Water quality drift. Conductivity, scaling, and biological fouling change tower performance gradually. Approach catches the result, but only chemistry monitoring catches the cause early enough to prevent fill replacement.

Variable flow. Approach derived from one-time CWT and WBT measurements assumes design flow. A pump throttled to 70% flow shows lower approach (less heat to reject) and can mask cell isolation valves left half-closed. Always log condenser water flow alongside CWT.

Dry-bulb at low wet-bulb. In dry climates, evaporation drives CWT toward wet-bulb, not dry-bulb. Engineers used to humid-climate towers sometimes expect a Phoenix tower to deliver 75°F supply water on a 95°F day; with WBT around 65°F, that 10°F approach is actually loose, not tight.

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Common Engineering Mistakes

The most common error is measuring wet-bulb temperature at the air inlet face of the tower rather than at an upwind free-air location. Hot, humid exhaust air recirculating back to the inlet can raise the apparent wet-bulb by 3–6°F, making approach appear falsely high and causing engineers to over-specify tower size or condemn well-performing equipment. A second mistake is comparing approach temperatures measured at different wet-bulb conditions — a 5°F approach at 75°F WBT and a 5°F approach at 65°F WBT represent different absolute performance levels since lower wet-bulb conditions are easier to approach.

Conclusion

Cooling tower approach equals cold water temperature minus wet-bulb temperature. Tracking approach monthly against the commissioning baseline gives advance warning of fill fouling and fan or flow degradation, often months before the problem reaches the chiller alarm panel. Even a few degrees of approach improvement directly reduces chiller energy consumption and operating costs.

FAQ

What is the difference between approach and range in a cooling tower?

Approach is CWT minus wet-bulb (how close the tower gets to the thermodynamic limit). Range is hot water in minus cold water out (how much heat the tower removed per gallon). Approach is set by tower design and ambient conditions; range is set by load and flow rate. A tower can have a tight approach with low range (well-designed tower at part load) or a wide approach with high range (undersized tower at full load).

How often should I measure approach during operation?

Monthly during cooling season for trending against the commissioning baseline, plus a full performance test annually before peak summer load. Continuous monitoring through BAS is worthwhile on chiller plants over 1,000 tons because the energy stakes justify the instrumentation cost.

Why can a tower not cool water below the wet-bulb temperature?

Evaporative cooling extracts heat by evaporating a small fraction of the circulating water. The evaporation rate stops when the air leaving the tower reaches saturation at the water temperature. Wet-bulb is the temperature at which air can absorb no more moisture, so it sets the absolute floor for water leaving the tower. Reaching wet-bulb itself would require infinite contact area.

Why does outdoor dry-bulb temperature matter for approach measurement?

Wet-bulb depends on both dry-bulb and humidity, so a hot dry day and a cooler humid day can have the same wet-bulb, and the tower will deliver the same CWT at the same WBT regardless of dry-bulb. Dry-bulb matters for tower selection in arid climates where wet-bulb is much lower than dry-bulb (15–25°F spread is common in the desert Southwest), allowing smaller towers than the dry-bulb would suggest.

What approach should I specify for a new chiller plant?

For comfort cooling, 6–7°F approach at the 0.4% design wet-bulb is the cost-effective sweet spot. Going to 4–5°F approach typically increases tower cost by 25–40% and pays back only on plants running over 4,000 hours per year or where utility rates exceed $0.15/kWh. For data center applications using waterside economizers, specify 4–5°F approach to maximize free-cooling hours.

How does fan staging affect measured approach?

Two-speed and VFD-controlled fans reduce airflow at part load, which raises approach during low-load operation. This is normal and energy-optimal. Compare measured approach to design only at near-full-load conditions (over 80% of design heat rejection); part-load approach numbers are not directly comparable to commissioning data.

Can approach get worse without fill fouling?

Yes. Common non-fouling causes: drift eliminator damage that reduces effective fill area, fan blade pitch slipping (especially on adjustable-pitch fans without lockout torque), air bypass through gaps in casing or louvers, and water bypass through cracked distribution decks. A full visual inspection under operation often locates these problems faster than a chemistry workup.

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