The physics of outdoor air conditioning and evaporative cooling

Abstract

In outdoor air conditioning (AC) systems, a fine spray of water mist is used to evaporate water and extract heat from the air. Misting systems use high pressure nozzles to produce water drops 5 μm in diameter. This dramatically increases the surface area of the water–air interface for a given volume of water facilitating heat transfer from air to water. Small drops stay suspended in the air for a sufficient amount of time to allow complete evaporation. An example of a misting system in operation at a Café in Montville, Queensland, Australia is discussed. As well as being cheaper to run, misting systems are less harmful to the environment since they reduce the dependence on indoor AC. This is an important topic to discuss with students in relation to climate change. A classroom experiment suitable for senior secondary and first-year university students is described in which a fan is used to blow air across a shallow container of water placed on an electronic balance. The experiment was performed throughout the day and night during periods of low and high humidity. Data collected from this experiment indicated that evaporation was much higher during the day than at night. In view of climate change and global warming, heat, energy and evaporation are important subjects to teach students, especially considering that these students will soon be adults making important decisions around sustainability and environmental protection in their day-to-day lives, and develope future policies to mitigate the impact of climate change.