Battling condensation in natatoriums
Some natatorium operators mistakenly believe that maintaining the 50- to 60-per cent RH recommended by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Handbook will guarantee condensation prevention. This is not the case, however. Window, ceiling, and wall condensation typically point toward an air distribution problem, which means the temperature of those surfaces is not being held high enough to prevent condensation from forming. Windows are most vulnerable to condensation as they typically have the lowest insulation value. However, a more serious problem is any amount of wall condensation that may indicate insufficient insulation or a vapour barrier breach that could help form and harbour condensation inside the wall cavity during cold weather. This must be addressed immediately to avoid serious building structure deterioration.
Condensation can be predicted, especially with metal duct. Surfaces colder than the ambient dew point temperature will experience condensation, much like a cold can of soda on a summer day. Likewise, indoor pools inherently have a high dew point temperature. For example, a 27.7 C (82 F) plus 60 per cent RH will produce a 19.4 C (67 F) dew point temperature. If the metal surface temperature falls below the dew point temperature, condensation will form. Windows and skylights are notorious for condensation because their surface temperatures may drop well below dew point due to low insulating R-values in winter.
Therefore, it is important to ensure dry air is dispersed across these surfaces, especially downward with ample velocity to reach the bottom of window and wall surfaces, which may be difficult when ductwork is often mounted 6 m (20 ft) or more above the finished floor. Linear dispersion and the Coandă effect, named after Henri Coandă who discovered air dispersed at velocity along a surface tends to stay attached to a surface, facilitating good air dispersion. However, architects should ensure horizontal window mullions protrude outdoors, instead of indoors, so as not to interrupt the process. Experts recommend ductwork positioning as close as 0.3 m (1 ft) from glass, walls, and ceilings (if possible). For airflow, ASHRAE recommends 0.9 to 1.5 m3/min per m2 of glass (3 to 5 cubic feet per minute [CFM]) per sf).
Flaking stainless steel duct replaced with fabric air dispersion
One example illustrating how natatorium environments can affect metal duct is a municipal indoor pool in Northern California. The natatorium’s originally specified galvanized steel duct became corroded. It was replaced with stainless steel duct; however, it only lasted three years. Dripping condensation and metal flakes falling into the pool water was a continuous battle. Further, the natatorium’s three walls of windows suffered condensation and visibility challenges on colder days.
Finally, the city switched to fabric duct with permeability that allows about 15 per cent of the airflow to penetrate the fabric, while the remainder is dispersed through four linear rows of air nozzles on the 810-mm (32-in.) diameter, 46-m (150-ft) long perimeter configuration. The 19-mm (3/4-in.) diameter nozzles, positioned at three, four, eight and 9 o’clock on the fabric duct easily cover the windows 6 m (20 ft) away with conditioned air. The fabric is still performing to its specifications 12 years after installation, making for a very happy facility manager and clientele.
Other natatorium metal ductwork throughout North America could be suffering from corrosion that is visibly difficult to detect. For example, corrosion may have caused ductwork to fall into a Sandusky, Ohio resort’s indoor pool resulting in injuries.
Further, metal air distribution could also be harbouring microbial growth that may never be detected, but affect indoor air quality—especially for asthma and allergy sufferers. This is another reason facilities must maintain strict RH, temperature, and air distribution set points 24-7.
