Heat recovery design handles humidity swimmingly
June 1, 2007
The idea of using the year-round dry outside air of the New Mexico desert to reverse the high humidity loads of a 13,000-sq-ft indoor swimming pool building sounds logical in theory. Mechanical engineer, Dave Kenney, P.E. isn’t quite old enough, however, to forget the chilling evaporative effects that dry 10% rh air had on his body as a junior swimmer at a local facility employing the same strategy 30 years ago. So when city officials of Taos, NM, floated the idea of saving indoor pool mechanical dehumidification costs on the new $3.1 million Youth and Family Center, Kenney - the project engineer from ABQ Engineering, in Albuquerque - disagreed.
Desert Dehumidification?While indoor air comfort concerns got the city’s attention, Kenney along with Jim Asperger, president of manufacturer’s representative, Crest Mechanical Systems Inc., and project architect, Jorge de la Torre, AIA, of de la Torre Architects, reversed the building committee’s opinion with the fact that a Dry-O-Tron® heat recovery dehumidifier from Dectron Inc., would save the city $22,800 annually. “Actually, mechanical dehumidification is turning out to be cheaper,” said Kenney. “Instead of heating makeup air at night at an estimated $6,500 annually, they’re recovering heat to warm outside air and provide free pool water heating at an additional savings of approximately $12,000 annually.”
The customized model DB-182 also extracts heat from exhaust air with a Smart Saver heat pipe option to pre-heat cool desert outdoor air ranging from -10° to 90°F year-round, while the pool space temperature remains a constant 84°. “We determined there would be savings in preheating the outside air, moving less air than would be used in an evaporative cooling system, and providing pool water heating with the features of this unit,” said Kenney.
A hidden savings is in the fact that moisture recovered from the return air in the dehumidification process is returned to the pool, thus saving water - a valuable commodity in the desert. Dectron estimates that the Taos project returns approximately the entire volume of the main pool, children’s pool, and spa - a total of 190,000 gal of water - as condensate per year.
Brian Greer, director of the Taos Youth & Family Center, is a believer in mechanical dehumidification now. “There’s a significant difference in the air comfort between the old and new facility, so I’m glad we finally decided on mechanical dehumidification,” he said, referring to the city’s 30-year-old former indoor pool that utilized outdoor air ventilation techniques for humidity control.
Deceptive WarmthThe Taos recreation center committee also entertained the idea of surrounding the indoor pool’s perimeter with overhead doors that when opened, could mimic an outdoor pool’s effect. “Even though it seems as if it’s warm here most of the time in the summer, you really can’t do both (an indoor/outdoor pool) because you’re still faced with the chilling effect of water evaporating too quickly off the skin with the dry, desert air,” said Asperger. “Also, overhead and sliding doors typically leak a lot of air because they don’t have tight air seals or seals that last for more than a few years.”
Instead of the large garage-style overhead doors, the facility now boasts huge thermal double-paned windows, bathing the facility in natural sunlight that aids in solar heating during the winter months.
Additional ChallengesAnother design challenge was the limited funds allotted for mechanical room space. Outdoor ground-level or rooftop equipment ideas were rejected in favor of hiding equipment aesthetically indoors. Thus, William Reed, president of HVAC contractor W.W. Reed Co., faced the challenge of fitting the 40-ft-long dehumidifier and accompanying ductwork connections into the tight 675-sq-ft mechanical room. Reed made onsite refinements to the air distribution design by taking the ductwork from the unit’s top discharge up through the mechanical room ceiling and making a right angle turn outdoors before re-entering the building at the designed 20-ft height of the natatorium ceiling perimeter ductwork.
Window condensation was also a design issue. To reach the bottom of the windows with the needed long throw from the overhead duct, Kenney used model DL drum diffusers by Titus. He also specified a Reznor-Thomas & Betts Corp. gas-fired makeup air unit for the shower areas, and Carrier Corp. high-efficiency packaged units for the offices and other areas of the recreation center.
Instead of the chilling effects of dry, desert air, Taos recreation center users are swimming in an environment of a constant 84° with a perfect 50% rh and indoor air comfort. ES