The air temperature can be reduced through the phase transition from liquid water to water vapor (evaporation), capable of refreshing the air using much less energy than classic refrigeration (the one based on the vapor compression refrigeration cycle, as in traditional air conditioners).
Simple natural examples of evaporative cooling are the perspiration of our body, or the transpiration of trees and plants.
Evaporative cooling causes a drop in air temperature proportional to the sensible heat loss and an increase in humidity proportional to the latent heat gain.
The effectiveness of evaporative cooling is therefore inevitably linked to atmospheric relative humidity: the lower the relative humidity, the greater the effectiveness of the cooling. This implies a widespread use of evaporative cooling in places with very dry climates (for example in some regions of Australia or the United States). In areas with moderate humidity, this type of cooling is widely used in industrial plants, production environments, agricultural greenhouses, farms.
In many cases evaporative cooling has the added benefit of increasing air humidity and reducing airborne dust, which benefits the comfort of building occupants.
Unlike closed loop refrigeration, evaporative cooling always requires a source of water and must continually consume water in order to function.
We can distinguish direct and indirect evaporative cooling systems. The direct evaporative cooler (open circuit) is used to lower the temperature and increase the humidity of the air using the latent heat of evaporation, transforming liquid water into water vapour. Warm, dry air is transformed into cool, moist air. The heat from the outside air is used to evaporate the water. Relative humidity increases to 70-90%, which worsens the cooling effect of human perspiration. The humid air must be continually expelled outside, otherwise it becomes saturated and the evaporation process stops.
The indirect (closed loop) evaporative cooler uses direct evaporative cooling in addition to a heat exchanger to transfer cold energy (absorb heat) to the inlet air. The refreshed moist air from the direct evaporative cooling process never comes into direct contact with the conditioned supply air. The moist air stream is blown outside or used to cool other devices. Indirect cooling is an effective strategy for hot-humid climates that cannot afford increased moisture content of the treated air due to indoor air quality and human thermal comfort concerns.
Adiabatic cooling
Residential and industrial evaporative coolers use direct evaporation. The air is moved by a fan and a water pump is used to wet the evaporative cooling pads. To cool, air is drawn from the room and passed through the wet pads, the heat in the air evaporates the water from the wet surfaces which are constantly re-humidified to continue the process. Once cooled, the humid air is introduced into the building, where it is necessary to guarantee an air exchange to avoid saturation and therefore the loss of effectiveness of the process.
Misting systems
Misting systems work by forcing water, via a high pressure pump, to pass through misting nozzles with orifices of a few micrometres, thus producing a very fine mist. The droplets of water that create the mist are so small that they evaporate very quickly, reducing the temperature of the surrounding air in seconds. Nebulization is used for applications in the open air, in large partially covered spaces, sheds, greenhouses, farms and allows to cool, humidify and reduce dust and odors present in the air.
