Quick answer: An indirect evaporative cooler typically reaches about 60–80 % of the wet-bulb depression without adding moisture to the supply air, so it suits dry and moderate climates. Two-stage systems add a direct stage after the indirect one, and dew-point (regenerative) designs can cool towards the outdoor dew point.
Two air streams pass through a heat exchanger: on the wet side, water evaporates into a secondary (exhaust) air stream and cools the plates; on the dry side, the supply air gives up heat to those plates and leaves cooler at the same moisture content. Indirect cooling suits dry and moderate climates and spaces that must not get more humid, such as data centers, offices and some factories.
Quick answer
- Supply air: cooler, same absolute humidity (RH rises only because it is cooler).
- Effectiveness: about 60–80 % of (dry bulb − wet bulb).
- It cools; it does not dehumidify.
How the two air streams work
| Stream | Path | What happens |
|---|---|---|
| Primary (supply) air | Dry channels of the heat exchanger | Gives up sensible heat through the plate; leaves cooler with no added water |
| Secondary (working) air | Wet channels, with water sprayed or wicked onto the surfaces | Water evaporates, absorbing heat from the plate; air leaves warm and humid, exhausted outside |
The two streams never mix in normal operation. That is the key difference from a direct (swamp) cooler, which evaporates water into the supply air itself.
Performance
Indirect effectiveness = (T_supply,in − T_supply,out) ÷ (T_supply,in − T_wb,secondary)
Example: outdoor air at 35 °C with a wet bulb of 19 °C, effectiveness 70%: T_supply = 35 − 0.70 × (35 − 19) ≈ 23.8 °C, with the same moisture content as the outdoor air. Its RH rises from 20 % to about 38 %, only because the air is cooler.
| Configuration | What it achieves |
|---|---|
| Indirect only | Cooler air, no added moisture; supply stays above the outdoor wet bulb |
| Indirect + direct (two-stage, IDEC) | Indirect pre-cools, direct finishes; supply can go below the outdoor wet bulb, with less moisture added than direct alone |
| Dew-point (regenerative) indirect | Part of the cooled product air is diverted into the wet channels; supply can approach the outdoor dew point |
| Indirect as pre-cooler for mechanical cooling | Reduces the compressor load in hot hours |
See evaporative cooling efficiency for the climate comparison table.
Indirect adiabatic cooling: same thing, different name
"Indirect adiabatic cooling" is the term often used in Europe and in data centers for the same process: the heat exchanger is cooled adiabatically (by evaporation) on the secondary side, and the supply air is cooled without added moisture. "Indirect-direct adiabatic cooling" is the two-stage version.
Process diagram (in words)
Outdoor (secondary) air ──► wet channels ──► warm, humid exhaust ──► outside
│ evaporation cools the plates
Outdoor or return (primary) air ──► dry channels ──► cooler supply air (same g/kg) ──► spaceAdvantages and disadvantages
| Advantages | Disadvantages |
|---|---|
| Cooling without adding moisture to the supply | Less cooling than direct in the same air |
| Much less energy than compressors in dry climates | Two fans; larger heat exchanger |
| Keeps outdoor contaminants out of the space (data centers) | Water use and treatment |
| Works as a pre-cooler to cut compressor load | Weak in humid climates |
Choose indirect evaporative cooling for dry or moderate climates when the supply air must stay at the same moisture content.
Indirect evaporative air conditioning
Combined with a small compressor stage, indirect evaporative units can act as air conditioning: the indirect stage does most of the cooling in dry hours and the compressor tops up on the hottest or most humid days. For homes, small indirect or two-stage coolers exist for dry climates; in humid climates, conventional air conditioning remains the usual choice.
Where indirect evaporative cooling is used
| Application | Why indirect |
|---|---|
| Data centers | Keeps outdoor air and its humidity and pollutants out of the white space; see data center free cooling |
| Offices and schools in dry climates | Cooling without humid air; often as a pre-cooler |
| Factories and warehouses | Cooling where products should not get more humid |
| Make-up air for kitchens and process areas | Pre-cooling large outdoor-air volumes |
Related: data center free cooling.
The US GSA's technology evaluation of indirect evaporative coolers reports on their performance in federal buildings (GSA).
Energy and water
- Energy: fans (two air streams, so more fan power than a direct cooler) and a pump; far less than a compressor in dry conditions.
- Water: about 1.5 litres evaporated per kWh of heat removed on the wet side, plus bleed-off to control scale. The US DOE FEMP covers water efficiency for water-intensive processes (DOE FEMP, BMP 13).
- Maintenance: water treatment, heat exchanger cleaning, and draining in freezing weather.
Limits
- In humid climates, the wet-bulb depression is small, so the cooling is small.
- It does not remove moisture. If the space has internal moisture sources (people, processes, wet goods), humidity can still rise.
- Leaks between the wet and dry sides add moisture to the supply air; check the heat exchanger condition.
Indirect cooling plus dehumidification
Where a space needs both cooling and humidity control (for example a warehouse holding moisture-sensitive goods in a hot, moderately humid climate), a common arrangement is indirect evaporative pre-cooling for the heat and a dehumidifier for the moisture. Size the dehumidifier for the internal and infiltration moisture load at the cooled space condition. Our industrial dehumidifiers and ceiling-mounted and ducted units cover that side.
FAQ
Frequently asked questions
How does indirect evaporative cooling work?
Water evaporates on one side of a heat exchanger, cooling the plates; supply air on the other side is cooled by the plates without touching the water.
Does indirect evaporative cooling add humidity?
No moisture is added to the supply air. Its RH rises only because the same water content at a lower temperature means higher RH.
What is the difference between direct and indirect evaporative cooling?
Direct coolers evaporate water into the supply air; indirect coolers keep the supply air separate and cool it through a heat exchanger.
Is indirect evaporative cooling good for humid climates?
Not on its own. The cooling depends on the wet-bulb depression, which is small in humid air.
Sources
- ASHRAE Handbook—HVAC Applications, Evaporative cooling.
- US GSA, Indirect evaporative cooler technology evaluation.
- US DOE FEMP, Best management practice 13: other water-intensive processes.
