Psychrometrics - the science behind humidity control

Feb. 12, 2014

Do you know what a Psychrometric Chart is? If you are involved with saving energy in complex systems, you should.

Ever See the Movie “Psycho”, Go to a “Psychiatrist”, Have your Child Tested by a Psychometrist or Encounter an Engineer Claiming to be a Psychrometrist?

Holmes: How many of you know the difference between Psychrometrics and Psychometrics? How many have taken a Thermodynamics class? Maybe I should ask how many have passed a Thermodynamics class. Do you know what a Psychrometric Chart is, Watson?

Watson: I learned how to use a Psych Chart in engineering school but am not sure when I would use it the real world. How is it used? Do many people really understand it?

Holmes: Every space that requires humidity control including museums, libraries, surgery suites, research labs, clean rooms, indoor swimming pools, ice arenas and more operate in accordance with the principles of Psychrometrics and the Psychrometric Chart. Hopefully the engineers who design the mechanical systems that control the humidity in such spaces understand Psychrometrics. Luckily, I did my Master’s Thesis on Humidity and the Psychrometric Chart and later taught the theory for Purdue so I have always been able to understand and solve humidity problems I have encountered in buildings.

But I can’t remember ever meeting a single person out in the field who understood the physics behind humidity control. The standard approach is to just try to overwhelm the problem with mechanical systems and waste a lot of energy in the process.

Watson: Can you give me an example where you actually used it?

Holmes: Sure. We used the information from the chart to help us understand and solve humidity problems in a Natatorium while cutting the energy costs in half.

Natatorium (Indoor Pool)
After we had signed a contract to manage 20 buildings in a public school system, the first thing we did was to get all of the electric, gas, oil, water and sewer bills for the previous five years. The five largest buildings accounted for 75% of the total utility costs so those were the ones we started with. In looking at bills from one of the two largest buildings, a high school, we noticed that its annual costs had jumped well over $100,000 from one year to the next. We had to find out why.

Watson: I love a good mystery and problem solving. What did you do?

Holmes: Our first step was to install permanent instrumentation in the pool to monitor the air temperature and humidity, water temperature and all of the major energy systems including the make-up air system. A make-up air system is one that brings in fresh air according to codes to keep the environment healthy, safe and comfortable for swimming. Unfortunately from the cost standpoint, fresh air is very expensive to heat in the winter. (The pool wasn’t air conditioned during the warm weather months.) There are a lot of health problems associated with indoor swimming pools so it was critical to maintain the proper environment.

By spending a lot of time in the facility and along with the monitored data, it was pretty easy to see that the mechanical systems were really screwed up. Although the original design wasn’t too bad, it had been compromised during the bidding process in order to bring the costs down to the point where the pool could be built. The school maintenance staff was already spread too thin when they had to assume responsibility for the new pool and they really weren’t qualified to operate such sophisticated systems.

The biggest problem however was the temperature control system.

Watson: It seems like temperature control systems cause a lot of problems.

Holmes: In my experience, improper control is the # 1 cause of energy waste in non-industrial buildings. This one was set up to bring in tremendous volumes of fresh air 24 hours a day whether the pool was in use or not. After consulting with one of the nation’s top pool designers, we modified the control system and were able to reduce the fresh air intake by 80% during unoccupied periods while meeting all health codes. In the process we discovered that the excessive volumes of cold, winter air had dropped the humidity in the pool too low. It was unhealthy for swimmers to be inhaling the dry air. When we reduced the volume of cold air to that required by code, a side benefit was that the humidity rose above 40%, into the required range.

Watson: So you used Psychrometrics to fix the problems and everybody was happy?

Holmes: Not exactly. Yes we fixed the technical problems but then we had to deal with the people problems that resulted from the changes.

The first problem was purely psychological; the huge fan had been noisy while the smaller one was very quiet. When someone noticed how quiet it was, many assumed that fan had failed and called our office. We would call up the pool on our computer, make sure everything was working properly, and assure the caller that all was well.

Record Cold Spell

Soon after we had made our changes the temperature dropped to minus twenty during a December cold spell and stayed there for several days; very unusual for Central Indiana. Thank goodness for Global Warming; without it the temperature might have been forty below. After a couple of sub-zero days, large ice cycles started to form on the bricks on the outside of the building. The schools called the building’s architect to see what the problem was. He came, looked at the building and announced that the ice cycles were caused by the changes we had made in the pool operation; we were going to ruin his beautiful building. He cited some codes that we were violating when in fact, we had corrected some initial violations through the changes we had made after consulting the pool expert. The building’s architect was local, a friend of mine and a good architect; he had done some very nice work in the community. But the key word is “Architect”. Architects have enough training in mechanical and electrical systems to be dangerous but generally leave most of those details to the mechanical & electrical (M&E) engineering firms they hire.

I knew it wasn’t anything we had done and in this case my friend should have known better, too. This was a building designed to house an indoor pool. We had monitored data showing the indoor humidity stayed between 40-50% as required for indoor pools. That wasn’t the problem. The problem was with the vapor barrier in the walls. They had either been designed or constructed improperly.

When it is humid and warm on one side of a wall, there’s a high vapor pressure, and when it is cold and dry on the other, there’s a low vapor pressure. The result is a huge force trying to drive the vapor, or water into the walls. The walls should have been designed and built with a vapor barrier such that no significant amount of water should have been able to penetrate them. And for any small amounts that did, they should have an escape route so the moisture could get to the outside without damaging the bricks. Wall design for pools in Canada and colder climates is an exact science, but the Midwest doesn’t normally experience such severe temperatures and something had gone wrong. We hadn’t done anything to cause it. Maybe the schools should have called Al Gore to complain.

Watson: Thanks. That helps me understand how those vapor pressure values on the Psych Chart apply to a real building.

I guess some people just don’t like change.

Holmes: Very astute for a young pup, Watson. “That’s the way we have always done it” is one of the most powerful attitudes in the world and very difficult to change.

Watson: Any more examples?

Holmes: One project where the humidity got so high in an Ice Arena it began to rain and create stalagmites on the ice. Another about a new air conditioning system that resulted in mold in an elementary school. I’ll tell you more about them another time

Press here to read more about the Natatorium project.

Tell us about your experiences, both good and bad with energy professionals, what has worked and what hasn’t. Send us your comments, thoughts and suggestions on how to improve our profession so we can all continue to learn from each other. Thanks – Holmes & Watson.

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