Calculate evaporative cooler outlet temperature, effectiveness, or required wet bulb in °C or °F, with airflow capacity, ideal water use, and sensitivity results.

Conventional single-stage direct cooler with unchilled, unfrozen liquid water. Effectiveness is approach to inlet wet bulb, not electrical efficiency. This screening model does not size equipment or predict room comfort.

Optional settings

Blank omits capacity and ideal evaporation; zero means no airflow.

Evaporative Cooling Efficiency Formula

The following formula is commonly used to calculate the saturation effectiveness (often called “efficiency”) of a direct evaporative cooler—how close the outlet air temperature gets to the inlet wet-bulb temperature.

eff = (DB - OT) / (DB - WB)
  • Where eff is the evaporative cooler effectiveness (as a fraction; multiply by 100 to get percent)
  • DB is the inlet (ambient) dry-bulb temperature
  • OT is the outlet (supply air) dry-bulb temperature
  • WB is the inlet (ambient) wet-bulb temperature

To calculate evaporative cooling effectiveness for a conventional single-stage direct cooler, subtract the outlet temperature from the inlet dry-bulb temperature, then divide by the available temperature drop (inlet dry-bulb minus inlet wet-bulb). The outlet lies between the inlet wet- and dry-bulb temperatures for effectiveness from 0 to 100%. At a saturated inlet, the denominator is zero and effectiveness cannot be measured by this ratio. This calculator assumes unchilled, unfrozen liquid water; effectiveness is not electrical efficiency or COP.

What is evaporative cooling?

Optional airflow estimates use fixed standard-air sensible heat capacity and approximate latent heat/water density. They describe sensible air-side temperature change and ideal evaporation, not net total room cooling capacity or total system water use. A solved required wet bulb still needs a separate pressure/humidity feasibility check.

Evaporative cooling is a process by which heat is transferred from an object or air through the evaporation of water. When water evaporates, it requires heat in order to do so. As a result, it takes heat away from whatever it was in contact with, which then causes cooling on that object.

How to calculate evaporative cooling efficiency?

Example Problem:

The following example outlines how to calculate the evaporative cooling effectiveness (“efficiency”) of a machine.

First, determine the inlet dry-bulb temperature. In this example, the dry-bulb temperature is measured to be 50 °F.

Next, determine the inlet wet-bulb temperature. In this example, the wet-bulb temperature is measured to be 35 °F.

Next, determine the outlet temperature. For this machine, the outlet temperature is measured to be 40 °F.

Finally, calculate the evaporative cooling effectiveness using the formula above:

eff = (DB – OT) / (DB – WB)

eff = (50 – 40) / (50 – 35)

eff = 10 / 15 = 0.667 = 66.7%