Quick answer: Evaporative cooling efficiency (also called saturation or wet-bulb effectiveness) is how close a cooler brings the air to the wet-bulb temperature: efficiency = (outdoor dry bulb − supply air) ÷ (outdoor dry bulb − wet bulb). Good direct pad coolers reach about 70–90 %, indirect coolers about 60–80 %. In dry air the result is a large temperature drop: 40 °C air at 15 % RH can be cooled to about 23 °C. In humid air, the wet-bulb depression is small and the cooler adds humidity without much cooling. In dry climates, direct evaporative coolers can use about a quarter of the energy of central air conditioning (PNNL).
Quick answer
- Efficiency = (T_dry − T_supply) ÷ (T_dry − T_wet bulb).
- Direct pads: about 70–90 %; indirect: about 60–80 %.
- Water use: roughly 1.5 litres evaporated per kWh of cooling.
The formula and a worked example
ε = (T_db − T_supply) ÷ (T_db − T_wb)
- T_db: dry-bulb temperature of the entering air
- T_wb: wet-bulb temperature of the entering air
- T_supply: temperature of the air leaving the cooler
Example: outdoor air at 40 °C and 15 % RH has a wet-bulb temperature of about 20.4 °C. A pad with 85 % effectiveness gives: T_supply = 40 − 0.85 × (40 − 20.4) ≈ 23.3 °C, at about 76 % RH.
What the same cooler does in different climates
Assuming 85 % effectiveness:
| Outdoor air | Wet bulb | Supply air | Supply RH | Verdict |
|---|---|---|---|---|
| 40 °C, 15 % RH (hot desert) | 20.4 °C | 23.3 °C | about 76 % | Excellent cooling |
| 35 °C, 20 % RH | 18.9 °C | 21.3 °C | about 80 % | Very good |
| 32 °C, 40 % RH | 21.6 °C | 23.2 °C | about 87 % | Useful, humid supply |
| 28 °C, 50 % RH | 20.4 °C | 21.5 °C | about 90 % | Marginal, very humid supply |
| 32 °C, 70 % RH (humid tropics) | 27.3 °C | 28.0 °C | about 94 % | Little cooling, adds humidity |
The wet-bulb temperature is the floor for a direct cooler. The wet-bulb depression (dry bulb minus wet bulb) decides how useful it is.
When does evaporative cooling stop working?
It doesn't stop at a fixed temperature: it weakens as the wet-bulb depression shrinks. As a rule of thumb, when outdoor air is above about 60–70 % RH in warm weather (wet-bulb depression below about 5 K), a direct cooler gives little useful cooling and mainly humidifies. In cold weather it isn't needed. Check hours per year with a depression above about 8–10 K for your site.
How to increase evaporative cooling effectiveness
- Use thicker rigid media pads (about 150–300 mm) with even water distribution.
- Keep air speed through the pads at the maker's design value; too fast reduces effectiveness and can carry water droplets.
- Clean and descale pads; control bleed-off and water quality.
- Seal around pads so air can't bypass them.
- Pre-cool with an indirect stage (two-stage) in hot, dry climates.
- Shade the intake; avoid drawing air from hot roofs or exhausts.
Calculating evaporative cooling for a space
Cooling delivered (kW) ≈ airflow (m³/s) × 1.2 kg/m³ × 1.005 kJ/kg·K × (T_room − T_supply)
Example: 5 m³/s of 23 °C supply air holding a space at 29 °C: 5 × 1.2 × 1.005 × 6 ≈ 36 kW of cooling, evaporating about 36 × 1.5 ≈ 54 L of water an hour. A spreadsheet can repeat this for each hour of a weather file to give seasonal cooling and water use.
Typical effectiveness by type
| Type | Typical effectiveness | Adds moisture to supply air? |
|---|---|---|
| Direct, rigid media pads (thick) | about 80–90 % | Yes |
| Direct, thin fibre pads | about 60–80 % | Yes |
| Indirect (heat exchanger) | about 60–80 % of wet-bulb depression | No |
| Two-stage (indirect then direct) | Can supply air below the outdoor wet bulb | Less than direct alone |
Effectiveness falls with dirty or scaled pads, poor water distribution and air bypass. See indirect evaporative cooling explained.
Energy and water
- Energy: only fans and a water pump. PNNL's Building America Solution Center notes that in dry climates direct evaporative coolers can use about one quarter of the energy of central air conditioners (PNNL BASC).
- Water: evaporating 1 kg of water absorbs about 0.68 kWh, so about 1.5 litres evaporate for each kWh of cooling, plus bleed-off water to control scale. A cooler delivering 20 kW of cooling evaporates about 30 L/h.
The US GSA has evaluated indirect evaporative coolers for federal buildings (GSA).
Evaporative cooling and humidity control
Direct evaporative cooling is the opposite of dehumidification: it cools by adding water to the air. That matters when:
- greenhouses use pad and fan cooling: humidity rises through the house and becomes a problem when the pads stop at night; see greenhouse humidity by climate and the NSW Department of Primary Industries guide to evaporative cooling in greenhouses (NSW DPI);
- warehouses and factories hold moisture-sensitive goods;
- humid weather arrives: the cooler gives little cooling and humid supply air.
Do not run a dehumidifier against an open direct evaporative cooler; they cancel each other. Where both cooling and dry air are needed in humid climates, use mechanical cooling with dehumidification, or indirect evaporative cooling plus a dehumidifier for the moisture load. Our industrial dehumidifiers cover the moisture side.
How to estimate if it will work at your site
- Find the design dry-bulb and coincident wet-bulb temperatures for your location (local climate data or the ASHRAE design conditions).
- Calculate supply temperature with a realistic effectiveness (75–85 %).
- Check supply RH against what the space and its contents can accept.
- Check the hours per year with a useful wet-bulb depression (often above about 8–10 K).
- Include water supply, water treatment and bleed-off.
FAQ
Frequently asked questions
How do you calculate evaporative cooling efficiency?
Efficiency = (entering dry bulb − leaving air temperature) ÷ (entering dry bulb − entering wet bulb).
What is a good evaporative cooler efficiency?
About 80–90 % for good rigid media pads; 60–80 % for indirect coolers.
Does evaporative cooling work in humid climates?
Poorly. With a small wet-bulb depression, it cools little and adds humidity.
What temperature does evaporative cooling stop working?
There is no single temperature; it becomes ineffective when the air is humid (small gap between dry-bulb and wet-bulb temperatures), typically above about 60–70 % RH in warm weather.
How can the effectiveness of evaporative cooling be increased?
Thicker, clean pads with even wetting, correct air speed, no bypass, good water treatment, and an indirect pre-cooling stage.
How much water does an evaporative cooler use?
About 1.5 litres evaporated per kWh of cooling, plus bleed-off.
Sources
- PNNL Building America Solution Center, Evaporative cooling systems.
- US GSA, Indirect evaporative cooler technology evaluation.
- NSW Department of Primary Industries, Evaporative cooling in greenhouses.
- REHVA, Evaporative cooling guidebook.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
