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TECH | One Engineer's Opinion: Who in the World would try to Air Condition a Screened Porch?? I would!

 September, 6th 2017     Comments     Views

By W. Bruce Longino, P.E., LEED AP

The concepts I am going to discuss in this article have the potential to save more energy and money than anything I have written about in the last 25 years of writing for The HVAC Insider.  However, I am going to have to “Geek Out” in order to explain the concepts to you.

A good friend asked me how to condition a large screened porch for a restaurant. After thinking about it I recommended an evaporative cooler. The restaurant did not buy the concept. However, by the end of this article I hope you will.

In thermodynamics [I actually loved these courses] we learned about the “partial gas laws”. I believe Dalton developed these laws. One of the laws states that in a non-reactive mixtures of gases, each gas acts independently of the other gases. In the HVAC industry we deal with mixtures of Oxygen, Nitrogen and Water Vapor. This mixture if dry by weight is approximately 78% nitrogen, 21% oxygen and 1% rare gases. If water is added, we can have up to about 2% by weight.  It is amazing how only 2% water can make us miserable with high humidity.

Each gas has its own vapor pressure. I had a hotel owner told me years ago that my PTACs were causing his vinyl wall paper to come off of the walls. Water vapor from outside was permeating the brick and block wall to get inside because the water vapor inside was low because of my PTACs. However, the water vapor couldn’t get through the vinyl wall paper so it stopped there. The room temperature was below the dew point of the outside air, so the water vapor condensed and created mold with the wall paper glue. Then the wall paper peeled off the wall. I tell you this to demonstrate how powerful the force of vapor pressure is.

Now back to my screened porch. If water vapor can force its way through brick and block, it will penetrate screen like it is not even there. Let’s say it is a cooling design day in Atlanta of 94 dry bulb and 74 wet bulb. Then we take the air of the screened porch through an evaporative cooler.  If the cooler is 100% efficient the air will come out 74 DB, 74 WB and 100% saturated with water vapor. This is a very uncomfortable condition in a building. However, on our screened porch the water vapor will leave the porch at 700 feet per minute (only a GEEK like me would know that number).

So on our porch, the air through the evaporative cooler has dropped the DB temperature 20 degrees. The bad effects of the increased humidity doesn’t occur because “Elvis has left the building” and so has our water vapor.

Admittedly the market to condition a screened porch is limited. However, there are thousands of warehouses, gyms and open air markets that this concept could be used on. Warehouses typically leave garage doors open, so the water vapor can escape easily. Gyms typically have louvers, and again water vapor can easily leave the buildings. Open air markets are “open air”. 

When I started Georgia Tech 40 years ago our basketball coliseum and my favorite restaurant for the next 4 years, The Varsity, only used evaporative cooling. When I was in either venue I was always comfortable because there was a lot of air motion.

Now we all know tab water is expensive to use. Therefore, I recommend collecting and filtering rain water to at least supplement tab water. The use of multiple rain barrels would be good for retrofit. On new projects cisterns should be used. This should be done for every existing and new cooling tower.

Many warehouses and gyms use mechanical refrigeration. Evaporative cooling use a fraction of the electricity.

I hope you consider using evaporative cooling on projects that have a means for the water vapor to escape. The owner will appreciate the lower power bills. If this technique doesn’t work for you at least try to collect rain water as make-up for cooling towers.

W. Bruce Longino, P.E., LEED AP, may be reached by email at