Evaporative cooling and swamp cooler systems field guide

What evaporative cooling is, and where it works

Evaporative cooling lowers the temperature of air by evaporating water into it. Liquid water turning to vapor takes heat to do it, and it takes that heat out of the air passing by. The air comes out cooler and wetter than it went in. That is the whole trick, and it is the same physics a cooling tower uses to reject a chiller's heat, covered in the cooling tower guide. The difference is what you do with the cooled stream.

Because the cooling comes from evaporation, it only works when the air is dry enough to take on more water. Hot dry air in Phoenix or Albuquerque has a lot of room for vapor, so a swamp cooler can pull the temperature down hard. The same machine in Houston in August does almost nothing, because the air is already close to saturated and there is nowhere for the water to go. Evaporative cooling lives and dies on that one climate fact.

This is a Southwest and high-desert technology for a reason. Where the air is dry, it cools cheaply with a fan and a pump and no compressor. Where the air is humid, it just makes a warm, sticky room. Get the climate match right and the rest of this guide is detail. Get it wrong and no amount of good equipment saves the install.

How does an evaporative cooler work?

An evaporative cooler pulls warm dry air across a wetted surface, and as some of the water evaporates into the air, it absorbs heat and the air temperature falls. The heat does not disappear. It goes into changing liquid water to vapor, which is latent heat, so the air loses sensible heat (temperature you can read on a thermometer) and gains moisture in trade. Total heat content stays close to the same. The process rides nearly along a line of constant wet-bulb on the psychrometric chart.

That trade is the key to everything. A refrigerated air conditioner removes heat from the space and dumps it outside through a compressor and a condenser. An evaporative cooler does not remove heat from the building at all. It converts sensible heat into latent heat right there in the airstream by adding water vapor, and then, in a direct system, blows that cooler, wetter air into the room.

So the energy bill is small because there is no compressor doing the work, just a fan moving air and a small pump wetting the pad. The catch comes back to the air. You only get the temperature drop if the incoming air is dry enough to accept the vapor, and you are adding that vapor to the space. Hold those two ideas together and every limit in this guide follows.

The wet-bulb is the floor

The coldest an evaporative cooler can drive the air is the wet-bulb temperature of the incoming air, and it never reaches it. The wet-bulb is the temperature air would reach if you evaporated water into it until it was fully saturated. Saturated air cannot take any more vapor, so evaporation stops, and so does the cooling. The wet-bulb is the floor.

What you actually get is the wet-bulb depression, the gap between the dry-bulb and the wet-bulb, multiplied by how good the cooler is. The drier the air, the wider that gap, and the bigger the drop you can win. In dry desert air the dry-bulb might be 100°F with a wet-bulb near 65°F, a 35°F spread, and a good cooler captures most of it. In humid air the dry-bulb might be 95°F with a wet-bulb of 78°F, a spread of only 17°F, and even a perfect cooler has almost nothing to work with.

Field practice is to look up the design wet-bulb for the location before anything else. A common working rule is that you want a wet-bulb depression of roughly 15°F to 20°F or more for evaporative cooling to earn its place. Below that, the math stops being worth the water. The exact threshold depends on what the space needs and the manufacturer's saturation rating, so treat the rule as a starting screen, not a spec.

Direct evaporative cooling: the swamp cooler

Direct evaporative cooling is the swamp cooler. A blower pulls outdoor air through a wetted pad, the air gives up heat to evaporate the water, and the cooled, humidified air blows straight into the space. There is no heat exchanger and no separation between the water and the air you breathe. The air that touched the wet pad is the air in the room.

Because it adds moisture, a swamp cooler raises the humidity of the space every minute it runs. In a dry climate that is often fine, even welcome, since the starting humidity is so low. The two things people miss are that the supply air is wet, so it cannot be ducted long distances without giving up cooling, and that the air has to leave the building somewhere, which is the relief requirement covered later.

The direct unit is the cheapest and simplest evaporative machine there is. Residential window and rooftop coolers, shop coolers, and spot coolers on a loading dock are almost all direct units. It is also the one that fails hardest in the wrong climate, because everything that makes it cheap, the open path from wet pad to room, is exactly what dumps humidity into a space that may already have too much.

The parts of a swamp cooler

A direct cooler is a box with a fan, a pad, and a way to keep the pad wet. Knowing the parts by name is how you talk to a parts counter and how you read a failed unit on a roof.

Water sits in the sump at the bottom, kept at level by a float valve the same way a toilet tank works. A small pump lifts water to a distribution header, often called the spider on residential units, which dribbles it across the top of the pads so it runs down and soaks them. Air is pulled through the wet pads by the blower and pushed into the space. A bleed or purge line continuously wastes a little water to keep minerals from concentrating in the sump, which is the same idea as blowdown on a cooling tower.

What is the difference between direct and indirect evaporative cooling?

Direct evaporative cooling wets the supply air itself, so it cools and humidifies the space. Indirect evaporative cooling cools the supply air through a heat exchanger, so the space gets cooler air without the added moisture. That one difference, whether the water touches the air you deliver, decides everything else about the two systems.

In an indirect unit, the evaporation happens on a secondary, or scavenger, airstream. That wet stream is cooled the usual way, then it runs through one side of an air-to-air heat exchanger. The supply air passes through the other side, gives up heat to the cool wet stream, and comes out colder. The wet exhaust air is thrown away outdoors. The supply air never sees the water, so its moisture content does not change. You get sensible cooling with no humidity penalty.

The price of that is depth of cooling and cost. An indirect stage cannot pull the supply air as far down as a direct stage, because it is cooling through a wall instead of by direct contact, and it costs more for the heat exchanger and the second fan. Indirect makes sense where you cannot tolerate added humidity, like a server space, a hospital area, or anywhere the indoor humidity is already a problem. Direct makes sense where dry air and a tight budget meet.

Indirect-direct two-stage cooling

A two-stage, or indirect-direct, cooler runs an indirect stage first to pre-cool the air without adding moisture, then a direct stage to finish the job. Pre-cooling the air sensibly in the first stage lowers its wet-bulb before it ever reaches the wet pad, and a lower entering wet-bulb means a lower achievable temperature out the other end. The two stages together reach deeper than either one alone.

The reason this works is the wet-bulb floor. The direct stage can only chase the wet-bulb of the air entering it. By dropping the dry-bulb sensibly in the indirect stage first, the air arrives at the direct pad with a depressed wet-bulb, so the direct stage finishes colder. A well-built two-stage unit can deliver air approaching the original wet-bulb while adding far less moisture to the space than a straight direct cooler would.

This is the high-performance end of evaporative cooling, and it shows up where the load justifies it: larger commercial spaces, schools, and dry-climate buildings that need real cooling capacity without a compressor. It costs more and uses more water and fan power than a simple swamp cooler, but in the right climate it competes with refrigerated air at a fraction of the energy.

What saturation efficiency can an evaporative cooler reach?

Saturation efficiency, also called saturation effectiveness, is how close the cooler drives the air to the wet-bulb, written as a percentage of the full wet-bulb depression. At 100 percent the air leaves at the wet-bulb, which never happens. The number tells you how much of the available drop the pad actually captures.

As a general guide, thin aspen pads commonly land around 80 to 85 percent, and deep rigid media, the corrugated cellulose pads sold under names like CELdek, commonly run 85 to 90 percent and higher. The exact figure depends on the media depth and the air velocity through it, since slower air and thicker media give the water more contact time. Rigid media holds up far longer than aspen and resists channeling, which is why most commercial and high-performance units use it.

Treat published efficiency as a manufacturer rating at a stated face velocity, not a fixed property. Push more air through the same pad and the efficiency drops, because the air spends less time against the water. When you size for a temperature target, use the manufacturer's saturation efficiency at the actual design airflow, and remember that a scaled or dust-loaded pad runs well below its rated number no matter what the catalog says.

Do swamp coolers work in humid climates?

No. Swamp coolers do almost nothing in humid air and they make the space muggy doing it. The cooling comes from evaporating water into the air, and humid air is already near saturation, so very little water evaporates and very little heat is removed. What does evaporate raises the indoor humidity, which is the opposite of comfort on a sticky day.

The practical breakpoint is relative humidity, but the honest measure is the wet-bulb depression. Below roughly 30 percent relative humidity the drop can be large, often 20°F to 30°F in true desert air. Around 50 percent humidity many units manage only about a 10°F drop, and above roughly 55 to 60 percent the cooling falls off to almost nothing while the room gets damp. The same model that wins in Phoenix is a mistake in Houston, and the equipment is not the difference. The air is.

Monsoon season is where this bites people who run swamp coolers in the Southwest. The cooler that worked all dry summer suddenly blows warm wet air when the humidity spikes in July and August, and the fix is to shut the evaporative side down on the humid days and not fight it. A space that has to stay cool through humid weather needs refrigerated or indirect cooling, not a direct swamp cooler.

Once-through air and the relief requirement

A direct evaporative cooler is a once-through machine. It pulls in fresh outdoor air, cools it, blows it into the space, and that air has to leave somewhere. This is the single biggest install difference from refrigerated air conditioning, which recirculates the same air in a closed loop. A swamp cooler does not recirculate. It pushes a continuous river of outdoor air through the building.

That means you have to give the air a way out. In a house you crack windows or open relief vents on the far side of the space from the cooler, sizing the opening so the cooler can push its full airflow through without backing up. Too little relief and the cooler chokes, pressure builds, doors get hard to open, and the cooling collapses. Open the windows in the rooms you want cooled and you also steer the cool air where it should go, since the air follows the relief path.

Get the relief right and the once-through nature is an advantage, because the space is flushed constantly with fresh air rather than breathing the same recirculated air all day. Get it wrong and even a correctly sized cooler in a perfect climate will underperform. The number one swamp cooler complaint that is not a humidity problem is a relief problem.

How much water does an evaporative cooler use?

An evaporative cooler uses water two ways: the water it evaporates to make the cooling, and the water it bleeds off to control minerals. The evaporated portion is the cooling itself and cannot be avoided. As a rough order of magnitude, residential and small commercial coolers commonly run a few gallons per hour up to the mid-teens, scaling with airflow and how hard the cooler is working, and larger commercial units run much more.

The bleed is the part people forget when they tally water. As pure water evaporates, the dissolved minerals it leaves behind concentrate in the sump, exactly like cycles of concentration on a cooling tower covered in the cooling tower guide. A bleed line continuously wastes a small flow of the concentrated water and lets the float replace it with fresh, holding the mineral level down. Adding a bleed roughly increases total water use over evaporation alone, sometimes substantially, depending on how hard the local water is.

Use the manufacturer's water rate at the design airflow for any real number, and adjust for local water hardness, because the bleed needed to control scale in hard water is far higher than in soft water. The honest tradeoff is right here: the bleed costs water but saves the pads and the sump from scaling solid. Shut the bleed off to save water and you buy a scaled-up cooler instead.

Scale, hard water, and treatment

Hard water is the slow killer of evaporative coolers. Every gallon that evaporates leaves its calcium and other minerals behind, and in hard-water country that builds up fast on the pads, in the sump, on the float, and in the pump screen. Scaled pads lose saturation efficiency because the mineral crust blocks both water and airflow, so the cooler quietly gets worse over a season even when nothing has broken.

The bleed is the first line of defense, the same logic as blowdown on a tower. It caps how concentrated the sump water gets, which caps how fast scale forms. The harder the supply water, the more bleed you need, and the more often you will still be cleaning and replacing pads. Rigid media tolerates scaling better than aspen and is easier to flush, but nothing makes hard water stop depositing minerals.

Beyond the bleed, treatment ranges from periodic acid cleaning of the sump and pads, to inline scale-control devices, to feeding softened water on systems where the hardness is severe. Match the treatment to the water you actually have, and confirm any chemical approach against the manufacturer's guidance and the local water authority, since both the dosing and the discharge of bleed water can be regulated.

The energy case in a dry climate

The reason anyone runs evaporative cooling is energy. A direct cooler moves air with a fan and wets a pad with a small pump, and that is the entire electrical load. There is no compressor, which is the part of a refrigerated air conditioner that eats most of the power. In a dry climate a swamp cooler can deliver real comfort cooling for a fraction of the energy a comparable refrigerated system would draw.

That gap is why evaporative cooling never went away in the desert and why it keeps drawing interest for large loads. The savings are largest exactly where the air is driest, because that is where the cooler does the most work per gallon and per watt. Two-stage indirect-direct units widen the range where the energy case holds, since they reach deeper cooling without a compressor.

The honest framing is that you are trading energy for water and giving up some control over humidity and the lowest achievable temperature. In a dry climate with available water, that trade is heavily in evaporative cooling's favor. In a humid climate, or where water is scarce or expensive, the trade can flip, which is the decision the rest of these sections lay out.

Data-center evaporative and adiabatic cooling, and the WUE tradeoff

Evaporative and adiabatic cooling have become a major tool for data centers in dry climates, because the energy savings show up directly in PUE, the ratio of total facility power to IT power. Direct evaporative and adiabatic designs let a facility reject heat with far less compressor work, and published direct-evaporative data-center designs commonly report PUE in the range of about 1.05 to 1.2 in favorable climates. That is the free-cooling and water-side economizing win, related to the free-cooling ideas in the cooling tower guide.

The catch is water, and the metric that tracks it is WUE, water usage effectiveness, the liters of water per kilowatt-hour of IT energy. Evaporative cooling cuts energy but spends water to do it, so a design that drives PUE down often drives WUE up. The right answer is not one number. It is the balance of energy and water for the specific climate, and that balance is heavily location-dependent.

A useful way to hold this: in a hot dry region with cheap available water, evaporative or adiabatic cooling can cut both energy use and, counting the power plant's own water, sometimes net water too, while in a water-stressed region the same design trades a scarce resource for an abundant one. ASHRAE's data-center thermal guidance, from TC 9.9, sets the temperature and humidity envelope the equipment has to stay inside while you make that call. Run the energy-versus-water tradeoff for the actual site, not a generic case.

Evaporative pre-cooling of condensers and intakes

Evaporative pre-cooling boosts a refrigerated system instead of replacing it. By wetting the air just before it hits a condenser coil or an air handler intake, you lower the entering air temperature, and the refrigerated equipment behind it works less for the same output. It is a way to borrow the dry-climate evaporative advantage without committing the whole building to evaporative cooling.

On an air-cooled condenser, misting or a wetted pad ahead of the coil drops the air temperature the condenser rejects heat into, which lowers head pressure and lifts capacity and efficiency on the hottest afternoons, exactly when the plant is straining. On an air handler's outdoor-air intake, a pad pre-cools the ventilation air before the cooling coil, cutting the coil's load. The air handler guide covers the coil side that pre-cooling feeds.

The same rules apply that govern any evaporative stage. It only helps when the air is dry, it uses water and needs a bleed to control scale, and a neglected pre-cool pad scales up and starts hurting airflow instead of helping. Done right in a dry climate, pre-cooling is a low-cost way to claw back capacity from equipment that is already installed.

Commercial and rooftop evaporative units

Commercial evaporative cooling scales the same physics up into large rooftop and packaged units, often moving tens of thousands of CFM through deep rigid media. These serve warehouses, manufacturing floors, gyms, and other high-ceiling dry-climate spaces where the ventilation rate is high and the once-through fresh-air flush is a feature rather than a problem.

At this size the water side stops being an afterthought. A large unit evaporates a lot of water and needs a real bleed and a maintenance plan to keep the media and sump from scaling. Many commercial units are two-stage indirect-direct to reach deeper cooling, and some are paired with a small refrigerated stage to cover the humid days that would otherwise leave the space uncomfortable.

The decision on a commercial building usually comes down to the ventilation requirement and the climate. A space that already needs large amounts of outdoor air for ventilation is a natural fit for evaporative cooling, because the cooler is moving outdoor air anyway. A tightly sealed space that recirculates its air leans back toward refrigerated cooling, since the once-through model fights the building.

How do you size an evaporative cooler?

You size an evaporative cooler by airflow, in CFM, not by tons of cooling. A refrigerated system is rated by how much heat it removes, but an evaporative cooler is rated by how much air it moves through the space, because the cooling is the once-through air change, not heat pulled out of a closed loop. Sizing by tonnage is a category error that trips up people coming from the refrigerated side.

The common residential rule of thumb is to take the floor area in square feet, multiply by the ceiling height in feet to get the volume, and divide by two, which gives the CFM needed to change the air roughly every two minutes. That works out to something like 20 to 30 air changes per hour. A drier, hotter climate and a higher load push you toward the faster end. Treat the rule as a starting point and confirm against the manufacturer's sizing data and the actual heat load.

Two things sit alongside the CFM number. The unit has to be able to push that airflow against the building, which is the relief-air question, and it has to hit the temperature target, which is where the saturation efficiency and the design wet-bulb come in. A cooler sized for airflow but starved for relief, or specified without checking the local wet-bulb, is sized on paper and wrong in the space.

Evaporative cooling vs refrigerated air conditioning

The decision between evaporative and refrigerated cooling comes down to climate first, then water, energy, humidity, and cost. Refrigerated air conditioning works in any climate, controls humidity, and recirculates air in a closed loop, but it runs a compressor and uses far more energy. Evaporative cooling sips energy and adds fresh air, but only works where the air is dry and it spends water and, in the direct case, adds humidity.

There is no universal winner. In a hot dry climate with available water, evaporative cooling wins on operating cost and often on comfort, especially with a two-stage unit. In a humid climate it is not a contender for primary cooling, and refrigerated air is the answer. Many dry-climate buildings run both, leaning on evaporative cooling through the dry months and switching to or supplementing with refrigerated cooling when the monsoon humidity rolls in.

Match the table below against the project before defaulting to either one. The cheapest mistake to avoid is repeating last job's choice in a different climate.

Legionella and water hygiene

Any system that holds standing water and makes a fine spray or aerosol deserves a water-hygiene plan, and evaporative coolers hold standing water in the sump. Stagnant warm water can grow bacteria, including Legionella, and a system that aerosolizes water can in principle carry it into the air. The risk is generally considered lower for typical direct coolers than for cooling towers, because the sump water tends to run cooler and a wetted-pad direct cooler produces far less drift than a tower's spray, but lower is not zero.

The hygiene basics are the maintenance basics. Do not let water sit stagnant in the sump for long idle periods, drain the system when it will be off for the season, keep the sump and pads clean rather than letting biofilm and sediment build, and keep the bleed working so the water turns over. A neglected unit with a fouled, warm, stagnant sump is the worst case, and it is also the unit that is failing at its actual job.

Where a water-management program is required, ASHRAE Standard 188 frames building water systems and the risk assessment, and it is most often applied to cooling towers and similar evaporative equipment. Whether and how it applies to a given evaporative cooler depends on the building, the equipment, and the local requirements, so confirm the scope with the standard, the manufacturer, and the authority having jurisdiction rather than assuming.

Maintenance, seasonal startup, and winterizing

Evaporative coolers are simple, which fools people into neglecting them, and then they scale up, rot their pads, and freeze and crack over winter. The maintenance is seasonal and it is not optional in a hard-water climate. Most of a cooler's problems trace back to a skipped startup or a skipped shutdown.

At spring startup you clean or replace the pads, flush the sump of last year's sediment, check the float setting, free and prime the pump, confirm the bleed is open and flowing, and check the blower belt and bearings. Through the season you watch the pads for scale and channeling and keep the bleed working. At fall shutdown you drain the sump and the supply line completely, because a sump or line left full will freeze and crack, and you cover or button up the unit so it does not collect debris over winter.

• Spring: replace or deep-clean the pads before first run.
• Spring: flush the sump of sediment and scale, clean the pump screen.
• Spring: check the float level, prime the pump, and confirm the bleed is open and flowing.
• Spring: inspect the blower belt, bearings, and wheel balance.
• Season: watch the pads for scale and channeling, confirm even wetting from the header.
• Season: verify the bleed keeps flowing so the sump water turns over.
• Fall: drain the sump and the water supply line fully so nothing freezes.
• Fall: cover or close up the unit and shut off and drain the water feed.

What to document

An evaporative install is only as good as the record of what was chosen and why, because the failures show up months later as a muggy room or a scaled pad and someone has to know what the design assumed. Capture the system type, the design airflow, the local design wet-bulb the selection was based on, the saturation efficiency used, the water and bleed rates, and the relief-air provision.

The single most useful artifact is a short table that states the type, whether it adds humidity, and the climate it is meant for, because that is the line that tells the next person whether the unit is right for the space at all.

Common mistakes

• Installing a swamp cooler in a humid climate, then blaming the equipment for the muggy, barely-cooled room.
• Giving the cooler no relief path, so the once-through air backs up and the cooling collapses.
• Shutting off or never installing the bleed, so the sump and pads scale solid in hard water.
• Neglecting the pads until they channel, rot, or crust over and the saturation efficiency falls off.
• Leaving the sump and supply line full over winter, so they freeze and crack.
• Expecting refrigerated-level cooling on a humid day, instead of switching to or supplementing with refrigerated cooling.
• Sizing by tons instead of CFM, or specifying without checking the local design wet-bulb.
• Ignoring water hygiene, letting a warm stagnant sump foul over an idle stretch.

Standards and references

Evaporative cooling design draws on the ASHRAE Handbook for the psychrometrics and the equipment, and on ASHRAE 90.1 where energy code applies, since evaporative and economizer cooling are part of how buildings meet energy requirements in dry climates. For data centers, ASHRAE's TC 9.9 thermal guidelines set the temperature and humidity envelope that any evaporative or adiabatic scheme has to keep the equipment inside.

Saturation efficiency, water consumption, and bleed rates are equipment-specific. Take them from the manufacturer's published data at the actual design airflow rather than from a rule of thumb, because they shift with media depth, face velocity, and water hardness. The design wet-bulb for the location comes from the climate data for the site, and it is the number that decides whether evaporative cooling is viable at all.

For water hygiene, ASHRAE Standard 188 frames water-management programs for building water systems and is most often applied to cooling towers and related evaporative equipment. Its application to a specific evaporative cooler, along with any bleed-water discharge and treatment chemical rules, depends on the equipment and the local authority, so verify the scope and the requirements rather than assuming a single standard covers it.

Units, terms, and conversions

Evaporative cooling carries a few names and units that show up differently across a spec, a manufacturer sheet, and a psychrometric chart, so the same idea can read several ways.

A direct evaporative cooler is also called a swamp cooler or a desert cooler. Airflow is given in CFM (cubic feet per minute) in North American data and in cubic meters per hour or liters per second in metric sources. Temperatures appear as dry-bulb and wet-bulb, in °F or °C, and the gap between them is the wet-bulb depression. Saturation efficiency and saturation effectiveness mean the same thing. Adiabatic cooling is the same constant-enthalpy process described from the thermodynamic side, the term that shows up most in data-center work.

Wet-bulb temperature

The temperature air reaches when saturated by evaporation; the floor an evaporative cooler approaches but never beats

Wet-bulb depression

The gap between dry-bulb and wet-bulb; the cooling potential available to an evaporative system

Saturation efficiency

How close the cooler drives the air to the wet-bulb, as a percent of the full wet-bulb depression

Direct evaporative (DEC)

Wets the supply air itself; cools and humidifies the space (the swamp cooler)

Indirect evaporative (IEC)

Cools the supply air through a heat exchanger; no moisture added to the space

Bleed / blowdown

Wasted flow that holds down mineral concentration in the sump, the same idea as tower blowdown

WUE / PUE

Water usage effectiveness and power usage effectiveness; the water-versus-energy scorecard for data-center cooling

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