Reflections on the Dog Days of Summer

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By Andrew Wengerd, PE, Senior Mechanical Engineer

The dog days of summer are a time when water in a crystal clear swimming pool can be refreshing or seem oppressive depending on the relative humidity conditions. I’m reminded of the incredible energy of water as it changes phase in evaporation and condensation.

Outdoors, water is helping to regulate the temperature of the planet in natural ways. At the 2014 ASHRAE conference in New York City, Dr. Mankiewicz of the Gaia Institute reminded the audience of the cooling potential of water. This cooling potential is available because of the phase change of water. There is also heat transfer that takes place during the phase change, although the energy associated with the phase change is often overlooked.

One phase changes happens with the transpiration occurring in plants and trees.  According to USGS, transpiration is simply the release of water from plant leaves. Much like we release water vapor when we breathe, plants do too. Trees draw water and nutrients from the soil with their roots; and some of this water is returned to the air by transpiration. As the tree releases water it evaporates, producing a cooling effect that can be experienced under the canopy of a tree.

Another phase change of water is the evaporation of the morning dew. The evaporation of water requires energy. This phase change produces a cooling effect as the energy is absorbed with the phase change.  The phase change of 0.6 mm dew over ½ acre can be the equivalent of 10 tons of cooling. 

Inside a building, mechanical equipment such as fans, cooling coils, and chillers are required to remove the moisture from air and create comfortable conditions. Recently our mechanical engineers have designed for low humidity laboratory environments. Our air conditioning calculations for treating summer time outdoor air are focused on removing moisture from the air and the energy required to do so.

During the dog days of summer we can appreciate the ways water is cooling and refreshing our lives.