The temperature in this graphic represents the surface temperature at the windows and the speed represents the velocity of the three supply grilles directed at these windows. Note the downward angle of the airflow and the lack of warm dry air at the top of the windows to prevent condensation occurring there.


After 18 years of experience in consulting engineering, James Harrison Jr. says that mechanical work on natatoriums is still one of the most challenging design applications in the HVAC industry.

According to Harrison, a mechanical engineer with GMB Architects and Engineers in Holland, Mich., pool projects can be challenging because of their huge evaporative rates. And while Harrison says he believes that his past designs have been successful, every new natatorium project is a test.

There was no exception to that rule when Harrison started his ninth pool project, a huge 16,500-square-foot natatorium at East Kentwood High School in a Grand Rapids, Mich., suburb.

The school's 9,650-square-foot indoor pool was expected to produce a humidity load of more than 550 pounds of moisture an hour.



Airflow in 3-D

There are no sure bets when it comes to designing and installing a mechanical system for a natatorium. There are a multitude of variables that must be analyzed to arrive at a final design concept. Consequently, consulting engineers are sometimes victimized by factors beyond their control, such as variances in ductwork and fitting manufacturer specifications.

Also, field airflow modifications due to architectural or structural changes, not to mention start-up, and testing and balancing problems, can change plans after the system is finally in operation.

To tackle this large indoor pool project, Harrison and construction officials with East Kentwood Public Schools accepted an offer to be the first to use Computational Fluid Dynamics analysis.

The CFD analysis, called Dectron 3-D, was offered to Harrison by Dan Kasper, a sales engineer with Michigan Air Products in Grandville, Mich. According to Dectron officials, it's the HVAC industry's only natatorium airflow-modeling consulting service. Dectron International developed it in conjunction with Flovent software from Flomerics Inc.

"The CFD analysis puts more science into the artistry of HVAC design and particularly how the airflow might function in different operation modes," said Michael Keller, Dectron's CFD project manager. "An airflow design might look good on paper, but application variables such as airflow over the pool surface and windows, drafts, temperature gradients, heating/cooling seasons, etc., can severely affect airflow, which is critical in natatorium comfort."

"I particularly liked the software's function of visualizing small sections of the space, such as the 800-seat spectator area, and actually seeing the temperature, humidity and airflow profiles in that particular area," said Harrison.

Once Harrison designed the East Kentwood natatorium using Microstation CAD software, he collected as much available data as possible, ranging from diffuser dimensions, duct sizes, window performance data, airflow velocities, return-air grille sizes and dozens of other product specifications.

For example, Harrison spent several hours compiling airflow data and supply diffuser sizing for airflow accuracy from the catalog of air-movement equipment supplier Titus. The data and natatorium final construction drawings were then submitted to Dectron's CFD analysis department, which took two weeks to build the 3-D space, input the information into Flovent, and package the results.

The result was a PowerPoint presentation with animated views of the airflow over the pool surface and windows, as well as concentrations, temperature and relative humidity changes within the natatorium's ventilation system.

Designers were able to measure how long it took supply air to reach certain locations. Seasonal variations were also considered because of inherent heating and cooling variations during those times. Any post-analysis changes could be quickly evaluated against relevant performance data.

Additionally, the analysis allowed for the impact of both short- and long-term ventilation system failure on the conditions within the natatorium, plus correct the diffuser location and operational performance.



The Dectron 3-D software measures how long the supply air took to get to a specific point.

Correcting problems

"The Dectron 3-D indicated there would be condensation problems near the top of the glass because of the angle of the supply diffusers," said Harrison. "Natatorium windows are particularly difficult areas for airflow because of the tremendous potential for heat loss and gain, and stagnant air pockets created by large window mullions."

Harrison contends that the verification of the rest of the design was invaluable to the project.

"Now the client has the assurance that the HVAC system will perform as designed and that air comfort in the space will be as anticipated," he said. "Also, if there are air comfort complaints in the future, we can eliminate the airflow design and look toward installation or equipment problems."

GMB credits its success on the East Kentwood project to the firm's experience in natatorium design and the CFD analysis. In fact, Harrison said he believes that an engineer designing a natatorium for the first time would find the CFD analysis an invaluable tool.

He also believes that the tool would be valuable for sales in the engineering marketing process by proving that ductwork location and diffuser orientation is more functional than a competing firm's design that was not 3-D modeled. GMB Architects and Engineers were pleased with the success of the Dectron 3-D, and plan to specify the service for a similar-sized natatorium at Zeeland High School in Zeeland, Mich.

This article and its images were supplied by Dectron International.