How Does an Evaporative Cooling System Work?
Evaporative cooling uses roughly a sixth of the electricity of refrigerant air conditioning, costs a fraction to install, and delivers pure fresh air. It also stops working almost entirely on a humid day. Both of those statements are true, and the whole engineering decision lives in knowing which one applies to your site.
This guide covers the physics that decides whether evaporative cooling will work at all, the difference between direct, indirect and two-stage systems, an honest comparison against air conditioning, pad types and maintenance, water hygiene, and how to size a unit.
- TL;DR
- How Evaporative Cooling Works
- Direct, Indirect and Two-Stage
- Live Climate Suitability Calculator
- Benefits of Evaporative Cooling
- Evaporative Cooling vs Air Conditioning
- The India and GCC Picture
- Where It Can Be Used
- Cooling Pad Types and Maintenance
- Water Hygiene and Legionella
- Choosing and Sizing a Cooler
- FAQs
- Sources and Further Reading
TL;DR
Key takeaways
- Evaporative cooling works by adiabatic saturation: water evaporating into an air stream absorbs latent heat, so the air gets cooler and wetter with no refrigerant and no compressor.
- Wet bulb temperature is an absolute floor. No evaporative system can cool air below it, no matter how good the equipment is. The available cooling is the wet bulb depression.
- The practical suitability test is a design wet bulb of about 20°C or lower, which roughly corresponds to ambient relative humidity below 40%.
- Direct systems add moisture to the supply air, reaching 80 to 90% saturation effectiveness with good cellulose media. Indirect systems add none but reach only 60 to 80%. Two-stage combines both.
- Power consumption is roughly one sixth of equivalent refrigerant cooling, because you are only running a fan and a small pump.
- It cannot hold a setpoint. Supply temperature floats with the weather, which rules it out wherever a specified temperature and humidity must be maintained.
- In India the story is seasonal, not regional. A Delhi cooler drops air by about 16K in dry May and under 4K during the monsoon.
- Water hygiene is a real duty. Bleed off, cleaning and drain down are not optional extras; they are what keep the system safe and the pads alive.
How Evaporative Cooling Works
Evaporating water takes energy. About 2,450 kJ per kilogram at typical conditions, and that energy has to come from somewhere. In an evaporative cooler, it comes from the air itself.
Hot dry air is drawn through a continuously wetted pad. Water evaporates into the air stream, and the latent heat of that evaporation is taken directly out of the air, so the air temperature falls. The moisture that evaporated is now in the air, so its humidity rises.
The key physical point
Total heat in the air stream barely changes. The process is adiabatic: sensible heat (temperature) is converted into latent heat (moisture) at almost constant enthalpy. Nothing is removed from the air; heat is only changed from one form into another. This is fundamentally different from a refrigeration coil, which actually extracts heat from the air and rejects it outside the building. It is also exactly why evaporative cooling is so cheap to run and so limited in what it can achieve.
- tdb ambient dry bulb temperature
- twb ambient wet bulb temperature
- ε saturation effectiveness of the pad, typically 0.80 to 0.90 for good rigid cellulose media
The bracket (tdb − twb) is the wet bulb depression, and it is the entire cooling resource available. Effectiveness only determines how much of that resource you capture. No equipment, at any price, can go below the wet bulb temperature in a single direct stage. That single constraint explains everything else in this article.
Direct, Indirect and Two-Stage
Select a tab to see how each arrangement works and what it does to the air.
Figure 1: The three configurations. Watch whether the water ever touches the supply air stream, because that is the entire difference between direct and indirect.
Live Climate Suitability Calculator
This is the calculation that should be done before anything else. Pick a location and season, or enter your own conditions, and see what each system type would actually deliver.
The result that reframes the whole technology
Set the calculator to Delhi in May: wet bulb 23.7°C, a 16K temperature drop, supply air at about 26.6°C and 79% relative humidity. Now switch to Delhi in August: the same machine manages under 4K and pushes supply humidity to 95%. Nothing about the equipment changed. Only the weather did. This is why an evaporative cooler in North India is a seasonal appliance that gets covered up during the monsoon, and it is the single most important thing to understand before specifying one. Note also that even at its best, Delhi in May produces air at 26.6°C and 79% RH. That is relief, not comfort conditioning, and it is exactly why the technology dominates factory floors and is almost absent from offices.
Benefits of Evaporative Cooling
- Very low energy consumption. There is no compressor. You run a fan and a small circulating pump, nothing else. A 10,000 m³/h unit consumes roughly 1.5 kW in total, against something like 9 to 11 kW for a refrigerant system delivering comparable sensible cooling. That is a factor of six or seven.
- Low capital cost. No refrigerant circuit, no compressor, no condenser, no charge, no F-gas paperwork. Installation is simpler and faster.
- 100% fresh air, always. An evaporative system does not recirculate. Every cubic metre delivered is outdoor air, so indoor air quality and dilution of contaminants are inherently good. In a welding shop or a kitchen this is a genuine process advantage, not just a comfort one.
- No refrigerant at all. Zero GWP, zero ODP, nothing to leak, nothing to phase down, no charge limits, no A2L handling requirements.
- Works well with open buildings. Because it uses 100% fresh air and pressurises the space, it tolerates open doors and loading bays, which defeat a refrigerant system entirely.
- Simple to maintain. Pads, a pump, a fan and a water sump. No refrigerant technician, no gauges, no certified handling.
Evaporative Cooling vs Air Conditioning
| Factor | Evaporative Cooling | Refrigerant Air Conditioning |
|---|---|---|
| Energy consumption | Very low, fan and pump only | High, compressor dominates |
| Capital cost | Low | Substantially higher |
| Performance in humid air | Poor to useless | Unaffected |
| Performance in dry air | Excellent | Excellent |
| Temperature setpoint control | Not possible, output floats with weather | Precise |
| Humidity control | None. Direct systems add humidity | Dehumidifies as it cools |
| Fresh air | 100% by design | Requires a separate ventilation provision |
| Refrigerant | None | Regulated, phasing down, leak management |
| Water consumption | Continuous, plus bleed off | None (air cooled) |
| Open doors and loading bays | Tolerates them | Defeated by them |
| Maintenance | Pads, sump, water hygiene, moderate frequency | Refrigerant circuit, filters, coils, skilled labour |
| Noise | Large fans moving high air volumes; unit noise moderate but airflow noise in the space is noticeable | Compressor noise outdoors, quieter indoors |
| Best fit | Industrial and semi-open spaces in dry climates | Any climate where a setpoint must be held |
Reading manufacturer efficiency claims
Evaporative cooling marketing quotes very large savings against air conditioning, and figures in the range of 60 to 75% energy reduction for two-stage systems are attributed to ASHRAE in industry literature. Treat all such numbers as conditional. They are entirely dependent on climate and application, and they compare an evaporative system doing what it can do against a refrigerant system doing considerably more, namely holding a setpoint and controlling humidity in any weather. The honest comparison is not "which uses less energy" but "can evaporative cooling meet this brief at all?" If it can, the energy saving is real and large. If it cannot, the comparison is meaningless.
The India and GCC Picture
The received wisdom is that evaporative cooling suits North and West India and not the coasts. That is broadly right but too coarse, because the more important variable is the season, not the region.
| Location and season | Ambient | Wet bulb | Supply air | Verdict |
|---|---|---|---|---|
| Jodhpur, May | 42°C, 18% RH | 22.4°C | 25.3°C at 78% RH | Strong fit |
| Riyadh, July | 45°C, 15% RH | 23.0°C | 26.3°C at 76% RH | Strong fit |
| Jaipur, May | 42°C, 20% RH | 23.1°C | 25.9°C at 79% RH | Good for industrial |
| Delhi, May | 43°C, 20% RH | 23.7°C | 26.6°C at 79% RH | Good for industrial |
| Bengaluru, April | 35°C, 30% RH | 21.4°C | 23.4°C at 84% RH | Workable |
| Hyderabad, May | 40°C, 25% RH | 23.4°C | 25.9°C at 82% RH | Workable |
| Dubai, June | 44°C, 30% RH | 27.8°C | 30.2°C at 83% RH | Weak, humidity too high |
| Delhi, August | 34°C, 72% RH | 29.5°C | 30.1°C at 95% RH | Useless |
| Kolkata, May | 36°C, 65% RH | 30.0°C | 30.9°C at 94% RH | Unsuitable |
| Chennai, May | 38°C, 60% RH | 30.7°C | 31.8°C at 93% RH | Unsuitable |
| Mumbai, May | 34°C, 72% RH | 29.5°C | 30.1°C at 95% RH | Unsuitable |
Conditions are representative seasonal values for illustration. Use actual design weather data for your site and the correct design percentile before making a decision. The point of the table is the pattern, not the individual figures.
Three regional conclusions
North and West India, dry season: a strong fit for industrial and semi-open buildings from roughly March to June. Jodhpur, Jaipur, Ahmedabad, Delhi and Nagpur all offer 18 to 20K of wet bulb depression at peak. The monsoon ends it. The same installation becomes near-useless from July, which is why hybrid designs, evaporative for the dry months and refrigerant cooling for the humid ones, are common on serious industrial projects rather than evaporative alone. Coastal and eastern India, Mumbai, Chennai, Kolkata, is genuinely unsuitable year-round for comfort purposes. The GCC is not uniformly dry: inland Riyadh is excellent, but coastal Dubai in June carries enough humidity that direct evaporative cooling delivers little and adds a lot of moisture. Do not treat "desert climate" as automatically meaning "suits evaporative cooling."
Where Evaporative Cooling Systems Can Be Used
- Industrial sheds and factory floors. The classic application. High ceilings, high heat loads, open doors, workers who need relief rather than a setpoint, and a client who will not pay to air condition a volume that size.
- Warehouses and logistics buildings. Large volumes, loading bays constantly open, goods that need a temperature ceiling rather than precise control.
- Spot cooling of hot work areas. Foundries, forging, glass and heat treatment, where cooling the whole building is impossible and cooling the operator is the objective.
- Poultry, livestock and greenhouses. Where the humidity gain is neutral or even beneficial, and the economics of refrigerant cooling never work.
- Kitchen and process make-up air. Replacing large exhaust volumes with tempered fresh air cheaply.
- Pre-cooling condenser and turbine intake air. Evaporatively cooling the air entering an air cooled condenser or a gas turbine raises its capacity and efficiency on the hottest days, which is when you most need it.
- Two-stage systems in commercial buildings in genuinely dry regions, where the reduced humidity gain makes conditioned occupancy possible.
Cooling Pad Types and Maintenance
The pad is where the whole process happens, and pad quality maps directly onto saturation effectiveness, which maps directly onto how much cooling you get.
| Pad type | Construction | Effectiveness | Notes |
|---|---|---|---|
| Rigid cellulose (honeycomb) | Corrugated cellulose sheets bonded into a rigid block with cross-fluted channels | 80 to 90% | The industry standard. Cross-fluting forces air and water to mix turbulently. Long life if kept clean and wet |
| Rigid synthetic / plastic | Similar geometry in plastic or glass fibre | 70 to 85% | More resistant to decay and aggressive water, slightly lower effectiveness than cellulose |
| Loose fibre (aspen, wood wool) | Packed natural fibre in a frame | 50 to 70% | Cheap, common in domestic desert coolers, decays quickly, replaced rather than cleaned |
| Spray or mesh pads | Water sprayed onto an open mesh | 50 to 70% | Low pressure drop, lower effectiveness, higher carryover risk |
Why the honeycomb geometry matters
A rigid cellulose pad is not just an absorbent block. It is built from corrugated sheets bonded at opposing angles, so the channels cross. Air entering is forced to change direction repeatedly and mix turbulently with the wetted surface instead of taking a straight path through. That is what lifts effectiveness from around 60% for a loose pad to 85 or 90% for a good rigid one. Thickness and face velocity matter too: a thicker pad gives more contact time and higher effectiveness, at the cost of more pressure drop and therefore more fan energy. The pad specification is a performance decision, not a consumable choice.
Maintenance Essentials
- Keep the pad evenly and fully wetted. Dry streaks are air bypass paths: air passing through a dry section is not cooled at all and simply dilutes the cooled air. Check the water distribution header for blocked holes at every service.
- Manage scaling. As water evaporates, dissolved minerals stay behind and concentrate. In hard water areas they deposit on the pad as scale, which blocks channels, raises pressure drop and destroys effectiveness. Bleed off is the control, and hard water needs a higher bleed rate.
- Clean on condition, not on calendar. Inspect for scale, dust caking on the entering face, algae and physical decay. A pad that has gone soft or lost its structure has lost its geometry, and no amount of cleaning restores it.
- Replace when effectiveness has gone. Pads are consumables. Life depends heavily on water quality and how well bleed off has been managed, so replace on inspection rather than a fixed interval, and expect shorter life on hard water.
- Run the fan after the pump stops. Drying the pads at the end of a run cycle limits microbial growth and slows decay. Many units do this automatically; verify it is enabled.
- Drain down when out of season. In North India that means the entire monsoon and winter. Standing water in an idle sump for months is the single worst thing you can do to the installation.
Water Hygiene and Legionella
Any system with a recirculating water sump deserves a hygiene plan, and this one is no exception. It is worth being accurate about the level of risk rather than either ignoring it or overstating it.
A proportionate view of the risk
Guidance summarised by BSRIA and CIBSE makes two useful points. First, the relatively low water temperature in an evaporative cooler sump is unlikely to promote bacterial growth, unlike the warm water in a cooling tower. Second, even where bacteria do become established, they are unlikely to transfer into the air stream provided air velocity stays below the point at which aerosols are generated. The risk is therefore generally lower than for an open cooling tower, but it is not zero, and it rises with neglect. The core control measure is straightforward: periodically draw off some or all of the circulating water and replace it with fresh water, which prevents solids building up in the sump and keeps water quality acceptable.
- Bleed off continuously or periodically to control dissolved solids. This is the same measure that protects the pads from scaling, so hygiene and performance point the same way.
- Keep air velocity within the design range across the pad, and fit drift eliminators where specified. Aerosol generation is the transfer mechanism that matters.
- Clean the sump and drain down during shutdown periods, and never leave stagnant water in an idle unit.
- Include the installation in the building water safety plan alongside cooling towers, calorifiers and other water systems, and follow the local regulatory regime.
- Use potable quality make-up water where the system is direct, since that water goes into air people breathe.
Choosing and Sizing a Cooler
Evaporative systems are sized on airflow, not on tonnage, because they do not deliver a defined cooling capacity. This is a genuine mental shift for engineers used to refrigerant equipment.
Evaporative systems use far higher air change rates than air conditioning, typically 20 to 30 ACH against 6 to 8 for a conditioned space, and up to 60 for spot cooling of hot processes. That is not an oversight; it is the mechanism. Because the supply air is only moderately cool, a large part of the comfort effect comes from air movement across the skin, which increases evaporative heat loss from the occupant. High airflow is doing real work, not just distributing cool air.
The Selection Checklist
- Design wet bulb first. Before anything else. If the design wet bulb is above about 20°C, understand precisely what you will and will not achieve, and get the client to agree it in writing.
- Airflow, not tonnage. Size on air change rate for the application, then confirm the fan can deliver it against the actual system pressure including pads and any ductwork.
- Relief air path. The single most common installation failure. All that supply air must leave the building. Provide deliberate relief openings, powered exhaust or open bays, sized for the flow.
- Water availability and quality. Confirm supply, drainage for bleed off, and hardness. Hard water changes the maintenance regime and the pad life.
- Noise. Large air volumes mean large fans and noticeable air movement noise in the space. Acceptable in a factory, often not acceptable in an office, and worth checking against the acoustic criteria for the space rather than assuming.
- Seasonal strategy. Decide now what happens during the humid months, whether that means shutdown, ventilation-only operation, or a hybrid with refrigerant cooling.
Where This Knowledge Leads
Evaporative cooling is a good test of whether an engineer understands psychrometrics or only equipment. Anyone can quote a manufacturer's temperature drop. Knowing that the drop is bounded by the wet bulb, that the same unit performs completely differently in May and August, and that the resulting supply condition may be relief rather than comfort, is the difference between specifying a system that works and one that gets switched off in its first monsoon.
Frequently Asked Questions
Sources and Further Reading
- ASHRAE Handbook: Fundamentals, ASHRAE, Psychrometrics chapter, for the adiabatic saturation process and the moist air property relationships used in this article's calculator.
- ASHRAE Handbook: HVAC Systems and Equipment, Evaporative Cooling chapter, for equipment types, applications and performance definitions.
- CIBSE Journal CPD Module 135, Direct evaporative cooling for comfort applications, for the effectiveness definition, typical commercial effectiveness of 80 to 90%, and the water hygiene position summarised from BSRIA guidance.
- BSRIA guidance on evaporative cooling and water systems, for sump management, bleed off and hygiene practice.
- Peer-reviewed literature on wetted media performance, which reports saturation effectiveness of roughly 70 to 90% for rigid media pads depending on thickness and air velocity, with cellulose outperforming corrugated plastic.
- Manufacturer selection data for pad thickness, face velocity, pressure drop and water consumption. Independent verification of quoted savings against a defined baseline is advisable before using them in a design report.
Calculation basis for this article's tools
The climate calculator computes moist air properties from the Magnus relation for saturation vapour pressure, finds wet bulb temperature by iterative solution of the constant-enthalpy condition, and models direct evaporative cooling along the constant wet bulb line using the standard effectiveness relation. Indirect cooling is modelled as a sensible-only process at 70% of the wet bulb depression, within the 60 to 80% range reported for indirect equipment. Two-stage performance applies the direct relation to the state leaving the indirect stage. Design conditions listed for each location are representative seasonal values for illustration only; real projects must use actual design weather data at the appropriate design percentile. The sizing tool applies air change rates typical of evaporative applications and estimates fan power at an assumed 250 Pa and 60% combined efficiency, which will differ with the actual unit and system pressure. The equivalent refrigerant load comparison assumes a 10K sensible effect and 1.0 kW/TR, both of which vary in practice. These are teaching and first-pass tools, not a substitute for manufacturer selection or a full psychrometric analysis.
This article was last reviewed on 1 August 2026.
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