Understanding evaporative cooling without overpromising
Principle: evaporation, supply air, and physical limits
Evaporative cooling lowers air temperature by evaporating water. Air passes through a wet medium or a fine mist. The evaporation “takes” heat, and the cooled air is then supplied into the building. The limit is the wet-bulb temperature. The more humid the air is to begin with, the smaller the gain. And in direct systems, indoor humidity also rises.
Evaporative cooling vs air conditioning: what you can (and cannot) guarantee
Air conditioning aims for a stable setpoint. Evaporative cooling aims for a measurable drop that varies with outdoor humidity, internal gains, and air change rate. You can promise a temperature difference, not a guaranteed 24°C in the middle of a humid heatwave. Also keep in mind the “water” topic and sanitation treatment.
Evaporative free cooling: when outdoor air becomes an asset
In a well-ventilated building, outdoor air can become free fuel for cooling. When it is cooler and drier (night-time, shoulder season), you ventilate more, then reinforce the effect through evaporation. In coastal areas and during muggy periods, the benefit quickly falls off.
Calculating the temperature drop based on humidity
Key concepts to master: dry-bulb temperature, wet-bulb temperature, relative humidity
In evaporative cooling, air cannot drop below the wet-bulb temperature. Dry-bulb temperature is what you read on the thermometer. Wet-bulb temperature represents the temperature achievable through evaporation. Relative humidity tells you whether the air is already “loaded” with water. The higher it is, the smaller the gap between dry-bulb and wet-bulb temperatures. So the possible gain decreases.
Simple on-site calculation method: estimate the possible supply temperature
- Measure dry-bulb temperature and relative humidity. 2. Find the wet-bulb temperature using a psychrometric chart or an app. 3. Estimate supply temperature with realistic efficiency (often 0.6 to 0.8). Practical formula. Supply T ≈ dry T - η × (dry T - wet-bulb T). With η = 0.7, you get a quick order of magnitude.
Examples of real gains: dry air, average air, humid air (typical cases)
Assumption. Outdoor air at 32°C. The more humid the air is, the more the gain closes down.
| Typical case | Relative humidity (%) | Wet-bulb temperature (°C) | Estimated supply temperature (°C) | Drop (°C) |
|---|---|---|---|---|
| Dry air | 20 | 19 | 23 | 9 |
| Average air | 50 | 24 | 26 | 6 |
| Humid air | 70 | 27 | 28 | 4 |
Assessing water consumption and operating impact
What drives water consumption: airflow, temperature delta, humidity
In evaporative cooling, the water consumed is mainly the water that evaporates. It depends on three levers. The treated airflow: the higher it is, the more evaporation increases. The target temperature delta: the more you want to cool the air, the more you “load” the air with moisture. And outdoor humidity, which is the real deciding factor. When the air is already humid, the actual drop falls and more water is lost to purge rather than useful cooling.
Order-of-magnitude figures and sizing benchmarks: m³ of water, fan kWh, maintenance
| Airflow (m³/h) | Evaporated water (L/h) | Fan electricity (kW) |
|---|---|---|
| 5,000 | 10 to 30 | 0.2 to 0.5 |
| 10,000 | 20 to 60 | 0.4 to 0.9 |
Add a margin for bleed-off, often 10 to 30% depending on water quality. On the maintenance side, plan for cleaning, pump checks, replacement of media or nozzles, and simple monitoring of consumption.
Water quality, scaling, and hygiene: conditions to set with the client
Ask for hardness, conductivity, and the presence of particles. Without a framework, scaling cuts performance and the bill rises. Put it in writing: filtration, bleed-off, access for cleaning, and a maintenance plan. The goal is stable water, not “perfect” water. On hygiene, avoid stagnant water, secure draining, and formalize cleaning and disinfection frequencies.
Identifying the climates and buildings where it really works
Favorable climates in 2026: dry zones vs humid zones (the decisive factor)
Evaporative cooling performs well when outdoor air is dry. The larger the gap between air temperature and wet-bulb temperature, the more degrees you gain. In coastal areas or during very humid spells, the benefit drops sharply.
| Outdoor relative humidity (%) | Typical drop (°C) | Rule of thumb |
|---|---|---|
| 20 | 8 to 12 | Dry climate, renewed air |
| 50 | 4 to 7 | Warm shoulder season, inland |
| 70 | 1 to 4 | Humid conditions, coastline |
Suitable buildings: workshops, warehouses, large volumes, ventilated spaces
It works well in open volumes, with frequent door openings, high ceilings, and exhaust. The goal is to handle a significant sensible load without aiming for perfect airtightness. The better controlled the ventilation, the more stable the result.
Risky cases: homes, coastal areas, spaces sensitive to humidity
Be careful with homes and rooms occupied for long periods. The process adds moisture. In already humid areas, you can quickly leave the comfort zone and increase the risk of condensation or mold. The same caution applies to archives, electronics, timber, food products, and any space where humidity must remain controlled.
Securing implementation: sizing, settings, and commercial safeguards
Choose direct or indirect: comfort, indoor humidity, and usage constraints
Direct evaporative cooling cools by humidifying the supply air. It works mainly when outdoor air is dry. If the building is already humid, or if occupancy is dense, indirect systems limit the rise in indoor humidity and protect comfort. Also think about how the building is used. Continuous exhaust, fresh-air supply, and clean, monitored, maintained water management are required.
Sizing points: airflow rates, air changes, pressure losses, noise
First secure what can be measured: actual airflow, available pressure, and noise. Without that, a good unit becomes a bad installation.
| Benchmark | Fresh air (m³/h/person) | Air changes (vol/h) | Available pressure loss (Pa) | Target noise (dB(A)) |
|---|---|---|---|---|
| Starting order of magnitude | 30 to 60 | 4 to 10 | 80 to 200 | 35 to 45 |
Honest sales pitch: phrases to avoid, written conditions, and measurements to plan
Avoid “replaces air conditioning everywhere” or “guaranteed temperature.” Put the conditions in writing: favorable outdoor humidity, exhaust in operation, humidity limits, maintenance access. Plan temperature and RH measurements at supply and in the room, airflow verification, and a commissioning report. That is your safeguard when reality catches up with the brochure.
