How Evaporative Coolers Work (Physics, Parts and Real Numbers)

An evaporative cooler works by letting hot, dry air give up its heat to evaporate water. A blower pulls outdoor air through wet pads; the water that evaporates takes roughly 1,060 BTU of heat with every pound, and the air comes out 15 to 30 F cooler in a dry climate. There is no compressor and no refrigerant, which is why a swamp cooler uses a fraction of the electricity of an air conditioner. The trade-off is that it adds moisture and it can only cool as far as the outdoor wet bulb temperature allows.

This page explains the physics in practical terms, walks through every part of a typical unit, and shows the arithmetic you can use to predict what a cooler will actually do at your house.

The physics: trading sensible heat for latent heat

Air carries heat in two forms. Sensible heat is what a thermometer reads. Latent heat is energy stored in water vapor. When liquid water evaporates, it needs energy to change phase, and in a cooler that energy comes from the air passing over the wet pad. The air's sensible heat drops (it gets cooler) while its latent heat rises by the same amount (it gets more humid). The total energy in the air, its enthalpy, stays nearly constant. Engineers call this an adiabatic saturation process.

Two numbers make the whole thing predictable:

Latent heat of vaporization

About 1,060 BTU per pound of water at typical cooler temperatures

Weight of water

8.34 lb per gallon, so one gallon evaporated absorbs about 8,840 BTU

Sensible cooling of moving air

BTU/h = 1.08 x CFM x temperature drop (F)

Because the process trades one kind of heat for another, the air can only keep cooling until it is saturated. That saturation point is the wet bulb temperature. Dry air has a large gap between its dry bulb and wet bulb temperatures (the "wet bulb depression"), so it has lots of room to cool. Humid air has a small gap and very little room. That single fact explains why coolers are excellent in Phoenix in June and disappointing in Houston in any month. For a deeper look at the climate side, see do swamp coolers work in humidity.

The key formula: predicting supply air temperature

No pad is perfect. The fraction of the wet bulb depression a pad actually captures is its saturation efficiency. The supply temperature is then:

Typical efficiencies: aspen (excelsior) pads about 60 to 80% depending on thickness, packing and age; 8 in rigid cellulose media about 85 to 90%; 12 in rigid media up to about 90 to 95%. Efficiency falls as pads scale up with minerals or dry out in spots.

You can run your own numbers with the cooling temperature calculator, which converts temperature and relative humidity to wet bulb and supply temperature.

Inside the cabinet: every part and what it does

A residential downdraft or side-draft cooler from Phoenix Manufacturing, Champion/Essick Air or similar makers has the same basic anatomy. Knowing the parts makes maintenance and troubleshooting far easier.

Water side

• Sump (reservoir pan): holds 2 to 5 gallons in most residential units. The water settles near the wet bulb temperature.
• Float valve: a ballcock that refills the sump from a 1/4 in or 3/8 in supply line, keeping the level roughly 1 in below the overflow. See float valve adjustment.
• Recirculating pump: a small submersible pump (often 1/70 to 1/40 hp) that lifts water to the top of the pads.
• Distribution system: a spider (distributor) with tubes feeding troughs over each pad, or a trough with drilled holes. Clogged holes are the most common cause of dry streaks.
• Bleed-off line or purge pump: sends a little water to drain continuously, or dumps the sump on a timer, so dissolved minerals do not concentrate. Details in bleed-off and water quality.
• Overflow standpipe and drain: protects the roof and cabinet if the float sticks.

Air side

• Pads and pad frames (louvers): aspen pads held by pad retainers, or rigid media blocks. The pads are the heat exchanger.
• Blower wheel: a squirrel-cage centrifugal wheel, typically 15 to 20 in diameter in whole-house units.
• Motor: usually a two-speed 1/3 to 1 hp motor, belt-driven through a sheave (adjustable pulley) on the motor and a larger pulley on the blower shaft. Some units are direct drive.
• Belt and idler: belt tension and pulley alignment control blower speed and noise. See belt and pulley adjustment.
• Discharge: straight down into the duct (downdraft) or out the side (side-draft), usually through a roof jack or wall opening.

Airflow and the open-window rule

A swamp cooler is a once-through system. It does not recirculate indoor air; every cubic foot it delivers is outdoor air that must leave the house somewhere. If it cannot leave, three things happen: static pressure rises and the blower delivers less air, the indoor humidity climbs toward saturation, and the room starts to feel sticky instead of cool.

The usual rule of thumb is to provide 1 to 2 sq ft of open window or relief-vent area for every 1,000 CFM of cooler output, placed on the far side of the house from the supply registers so cool air sweeps through living spaces. Barometric up-duct relief dampers into a vented attic do the same job without open windows, which is useful for security and dust. Our guide to venting and relief air covers placement in detail.

How much cooling a cooler actually delivers

Because the supply air is 100% outdoor air, the useful number is the temperature difference between supply air and the room air you are replacing, multiplied by the airflow.

The motor drawing this airflow uses roughly 500 to 900 watts on high speed, plus about 50 to 100 watts for the pump. A central air conditioner delivering similar room cooling typically draws 3,500 to 5,000 watts. That gap is the entire economic case for evaporative cooling; see swamp cooler operating cost for the monthly math.

Use the water usage calculator for your own airflow and climate.

Why pads, airflow and water must be in balance

Technicians commonly find coolers that are "working" but delivering 80 F air on a 100 F day. Almost always, one of three balances is off:

1. Pad wetting. Dry streaks act as bypass paths. Air takes the easiest route through dry spots and arrives warm. Clear the distribution holes and confirm even flow over the full pad face.
2. Face velocity. Pads are rated for an air speed through their face, typically 400 to 600 feet per minute for aspen and around 400 to 500 for rigid media. Too fast and water droplets carry over into the duct; too slow (blocked relief, closed windows) and capacity drops.
3. Water quality. Scale on pads reduces wetted surface and efficiency. Bleed-off keeps mineral concentration down so pads last a full season or more.

If your unit is underperforming, the swamp cooler not cooling guide walks through the checks in the order pros use.

Direct, indirect and two-stage designs

Everything above describes a direct evaporative cooler, where air touches the wet pad. An indirect cooler uses a heat exchanger: a secondary airstream is evaporatively cooled and then cools the supply air through a wall, without adding moisture. Two-stage (indirect-direct) units chain the two to deliver lower temperatures than a direct cooler alone, sometimes a few degrees below the incoming wet bulb. These cost more and are less common in homes. The full comparison is in direct vs indirect evaporative cooling.

Where the physics sets hard limits

• Humidity: once the outdoor dewpoint climbs into the mid 50s F and above, as during the Southwest monsoon, the wet bulb depression shrinks and supply air warms. See swamp coolers during monsoon season.
• Indoor humidity: a properly vented home typically sits at 50 to 70% RH with a cooler running. That is comfortable for most people in arid regions but too damp for some materials and some health conditions; see swamp coolers and indoor humidity.
• Freezing weather: the water system must be drained and the duct covered before the first freeze. See how to winterize a swamp cooler.

Within those limits, an evaporative cooler is a simple, efficient machine: a fan, a pump, a float and a wet pad, doing exactly what a desert breeze over a lake does, only on purpose.