How evaporative cooling works
An evaporative cooler (swamp cooler, desert cooler, evaporative air cooler) uses the heat needed to evaporate water to lower the temperature of air. It has no compressor or refrigerant, just a fan, a pump and wet media.
The physics in one paragraph
Turning liquid water into vapour takes energy, roughly 2,400–2,500 kJ for every kilogram evaporated at everyday temperatures. When warm air passes through wet pads, that energy comes from the air itself, so the air cools and gains moisture. The total heat content of the air stays nearly constant: sensible heat (temperature) is exchanged for latent heat (moisture). The process can continue only until the air is saturated, and the lowest temperature it can reach is the wet-bulb temperature.[1]
How close a cooler gets to the wet bulb is its effectiveness (or saturation efficiency): ε = (Toutdoor − Toutlet) / (Toutdoor − Twet bulb). DOE reports that aspen-pad coolers are about 70% effective, modern coolers with thick media reach 75–93%, and 12-inch media can exceed 90%.[1] Try it in the outlet temperature calculator.
Direct, indirect and two-stage coolers
| Type | How it works | Adds moisture to room air? |
|---|---|---|
| Direct | A fan pulls outdoor air through wet media straight into the home. This is the simplest and most common residential type. | Yes |
| Indirect | A separate, evaporatively cooled air stream chills an air-to-air heat exchanger. Supply air is cooled through the exchanger walls and never touches water. The moist secondary stream is exhausted outside. | No |
| Two-stage (indirect/direct) | An indirect stage pre-cools the air, then a direct stage cools it further. The result is cooler air with less moisture than a single-stage unit. DOE cites ≥95% effectiveness (PG&E 2004). | Less than direct |
Source for the descriptions: DOE Building America Solution Center.[1] Indirect and two-stage units cost more but extend the climates where evaporative cooling is useful. DOE suggests indirect cooling may be best where design wet bulbs reach about 70–74 °F (21–23 °C).[1]
Pad and media types
- Shredded aspen (wood fibre): inexpensive and common in older and portable units, with lower effectiveness (about 70% per DOE).
- Rigid media (treated cellulose or similar, 8–12 inch): higher effectiveness and longer life when maintained. DOE notes modern direct coolers use 8- to 12-inch media of cellulose, fiberglass, plastic foam or aspen.[1]
Features that matter
- Vent-only mode: runs the fan without the pump, using the cooler as a whole-house fan in mild weather without adding humidity.[1][2]
- At least two fan speeds (DOE recommendation).[1]
- Mounting: ground- or wall-mounted units are easier to maintain and avoid roof leaks compared with traditional roof down-flow units.[1]
Worked example
Outdoor air at 40 °C (104 °F) and 15% RH at sea level has a wet bulb of about 20.4 °C (68.7 °F), a depression of about 19.6 °C. An aspen cooler (ε ≈ 0.70) gives 40 − 0.70 × 19.6 ≈ 26.3 °C (79 °F) outlet air. A 90% rigid-media cooler gives about 22.3 °C (72 °F). Check these in the calculator.
Sources
- U.S. DOE Building America Solution Center (PNNL), "Evaporative Cooling Systems"
- U.S. DOE / NREL, "Cooling Your Home with Fans and Ventilation", DOE/GO-102001-1278 (2001)
- ASHRAE Handbook—Fundamentals (2017), Ch. 1 Psychrometrics: latent heat and adiabatic saturation.