Swamp Cooler Math

Outlet air temperature calculator

Estimate the temperature of the air leaving a direct evaporative cooler. This is the outlet, or supply, air. It is not the temperature your room will settle at.

Formula

Toutlet = Toutdoor − ε × (Toutdoor − Twet bulb), where ε is the cooler's effectiveness (also called saturation efficiency). The same definition is used by the US DOE Building America Solution Center, which notes that newer coolers with thick media reach 75–93% and that aspen-pad coolers are about 70% effective.[1]

Worked example (from DOE)

Outdoor 105 °F (40.6 °C) with a 60 °F (15.6 °C) wet bulb gives a 45 °F (25 °C) depression. With 90%-effective media: 105 − 0.9 × 45 = 64.5 °F (18.1 °C).[1] Our automated tests check that the calculator reproduces this arithmetic.

Outlet air vs indoor temperature

Air path diagramHot outdoor air passes through wet pads, leaves the cooler at the outlet temperature, warms as it absorbs heat inside the home, and exits through open windows or relief vents. Outdoor airhot, dry Wet padswater evaporates Outlet aircoolest point Roomsair warms from sun,people, appliances exits via windows / relief vents

Supply air picks up heat from sunlight through windows and walls, from people, cooking and electronics, and from ducts in hot attics. So rooms are warmer than the outlet temperature. How much warmer depends on airflow and the building. More airflow means a smaller rise, which is why evaporative coolers move far more air than air conditioners. We do not predict indoor temperature here because it needs a building heat-load calculation.

About the uncertainty range

The range recalculates the outlet temperature at the humidity and effectiveness extremes you set. It reflects input uncertainty only. It does not cover clogged or dry pad sections, air bypassing the media, or heat gained in ducts, all of which make real outlet air warmer. See diagnosing poor cooling.

Two-stage (indirect/direct) coolers deliver cooler air with less added moisture than a single-stage direct cooler, and DOE cites effectiveness of 95% or more for them.[1] This calculator models a single-stage direct cooler. See direct vs indirect.

Sources

  1. U.S. DOE Building America Solution Center, "Evaporative Cooling Systems": effectiveness formula, media effectiveness ranges, worked example.
  2. ASHRAE Handbook—Fundamentals (2017), Ch. 1: psychrometric equations (wet bulb, humidity ratio).