Direct evaporative cooling

Estimate leaving dry-bulb temperature from entering dry/wet bulb and specified saturation effectiveness.

How this calculation works

The entering dry-bulb and thermodynamic wet-bulb temperatures define the temperature reduction available to this simplified direct evaporative process. Enter both temperatures, the saturation effectiveness for the actual operating airflow and the absolute air pressure. The calculator multiplies the dry-to-wet-bulb difference by effectiveness and subtracts the result from entering dry bulb. Effectiveness is supplied by the user; the model does not predict pad performance or choose equipment. Absolute pressure supports a separate inlet-state validation using the ASHRAE humidity relationship. An impossible dry/wet-bulb pair, including one implying negative humidity, is rejected. This approximately adiabatic temperature estimate does not calculate outlet humidity or water consumption. Fan heating and water-quality effects are also outside the model, so the leaving temperature is only one part of an equipment assessment.

Inputs and units

Entering dry bulb (°C)
Entering wet bulb (°C)
Saturation effectiveness (%)
Absolute air pressure (kPa): Used to check whether the dry/wet-bulb pair implies a physical humidity ratio.

Method and formula

Tleave = Tdry−ε(Tdry−Twet), where ε is effectiveness/100. Inlet validation uses Ws = 0.621945 pws(Twet)/(p−pws(Twet)) and W = [(2501−2.326Twet)Ws−1.006(Tdry−Twet)]/(2501+1.86Tdry−4.186Twet), with temperatures in °C, p in Pa, and W in kg/kg dry air; negative W is rejected.

Worked example

Example inputs

Entering dry bulb: 35 °C
Entering wet bulb: 20 °C
Saturation effectiveness: 80 %
Absolute air pressure: 101.325 kPa

Calculation steps

At 35 °C dry bulb and 20 °C wet bulb, available temperature difference = 35 − 20 = 15 K. The inlet pair is checked at 101.325 kPa.
Dry-bulb reduction = (80 / 100) × 15 = 12 K. Leaving dry bulb = 35 − 12 = 23 °C.

Example results

Leaving dry bulb: 23 °C
Dry-bulb reduction: 12 K

Assumptions

Direct single-stage evaporative process with approximately adiabatic contact; wet bulb is thermodynamic.

Limitations

No outlet humidity, water consumption, fan heat or water-quality assessment.
The input effectiveness must apply at the operating airflow.
Preliminary educational check; verify inputs and equipment data before design or operation.

Sources

EnergyPlus Engineering Reference: Evaporative Coolers: Direct Evaporative Cooler Special Research Model, equation 16.168: leaving dry-bulb temperature
PsychroLib official equation references: GetSatVapPres and moist-air property references to ASHRAE Fundamentals 2017 chapter 1; saturation fit coefficients