Physics teaches us that water, when evaporating in an adiabatic system, removes sensible heat from such system. The temperature thus decreases. In order for the water to evaporate, energy is required, specifically 2501 kilojoules for each kilogram of evaporated water. In the absence of an external energy source, the water absorbs the required energy from the environment, in this case from the surrounding air. As a consequence, the air is cooled. In simple terms, it can be stated that evaporation of water converts the sensible heat of the air (temperature) into latent heat (humidity). As a result, the system is cooled and humidified at the same time. This type of cooling is referred to in literature as evaporative cooling or adiabatic cooling.
Current ventilation guidelines recommend maintaining ventilation with 100% outdoor air 24/7, or as much as possible if that is not feasible, as an effective way to keep airborne pathogen concentration under control. Obviously, outdoor air needs to be filtered and conditioned to guarantee the indoor temperature and humidity set points; this, in turn, implies a higher HVAC-related primary energy consumption, because more outdoor air needs to be conditioned compared to the pre-COVID-19 situation, when the flow of outdoor air was typically 20% of the total supply air entering the space.
