Evaporative Cooler Outlet Temperature Calculator

Understanding the evaporative cooler outlet temperature calculator — concept and principle

A direct evaporative cooler (swamp cooler) blows outdoor air through wet pads, where evaporation lowers the air temperature. This calculator estimates the supply air temperature from the outdoor dry-bulb and wet-bulb temperatures and the cooler's saturation efficiency. It helps you decide whether evaporative cooling suits your climate and how much cooling to expect.

The pad closes 80% of the 13 K gap to the wet-bulb temperature, so air leaves at 24.6 °C.

How it is calculated

T_out = T_db − η · (T_db − T_wb)

T_db: dry-bulb (ordinary) air temperature entering the cooler, °C.
T_wb: wet-bulb temperature of that air, °C.
η: saturation (or cooling) efficiency in percent, the fraction of the wet-bulb depression the cooler achieves.

The difference T_db − T_wb is called the wet-bulb depression; it is the maximum cooling available.

The principle

As water evaporates into the air stream, the heat needed for evaporation comes from the air itself, so its temperature falls while its moisture rises. The total heat content (enthalpy) stays almost constant, so the air moves along a line of nearly constant wet-bulb temperature. If the air became fully saturated it would leave exactly at the wet-bulb temperature; real pads have limited contact time and area, so they achieve only part of that drop. The efficiency η captures that fraction.

Worked example

Inputs: T_db = 35 °C, T_wb = 22 °C, η = 80%.
Wet-bulb depression: 35 − 22 = 13 K.
Achieved cooling: 0.80 × 13 = 10.4 K.
T_out = 35 − 10.4 = 24.6 °C.

Each point is the calculator re-run with only “Wet-bulb (°C)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Wet-bulb (°C)	Outlet air (°C)
5.5	11.4
11	15.8
16.5	20.2
22	24.6
27.5	29
33	33.4
44	42.2

How to read the result

The bigger the wet-bulb depression, the better evaporative cooling works, so it performs best in hot, dry climates and poorly in humid weather. Typical direct coolers with good media reach saturation efficiencies of roughly 70–90%. Because the supply air is more humid, the cooler needs a constant flow of fresh air and an exhaust path such as open windows. The outlet can approach but never go below the wet-bulb temperature.

Common mistakes

Entering relative humidity in place of the wet-bulb temperature.
Expecting the same performance on humid days.
Forgetting the added moisture, which can make indoor air feel clammy.
Assuming η stays constant with dirty or partly dry pads.

Keep in mind: Results are idealised and ignore fan heat, pad condition and air leakage. Equipment selection should use manufacturer data, and building ventilation should follow local codes and professional advice.

Frequently asked questions

How do I get the wet-bulb temperature?

Measure it with a sling psychrometer, or estimate it from temperature and relative humidity with a wet-bulb calculator.

Is an indirect evaporative cooler different?

Yes. It cools air through a heat exchanger without adding moisture to the supply air, usually with lower efficiency.

Does an evaporative cooler use much water?

It evaporates water continuously, roughly in proportion to the cooling delivered, plus some extra bleed water to limit mineral build-up.

Sources: ASHRAE Handbook — HVAC Systems and Equipment, Evaporative Air-Cooling Equipment chapter · ASHRAE Handbook — Fundamentals, Psychrometrics chapter

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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