Indoor air quality: Solving natatorium air dispersion and condensation challenges

by Dave Flaherty | October 14, 2020 2:31 pm

By Philip Daughtery

Indoor pools are challenging environments for design engineers, especially when considering the indoor air quality (IAQ) and the longevity of exposed metal components.

A natatorium’s corrosion-potential environment of high humidity, gaseous pool chemicals, and condensation is akin to a late-night horror
show for metal heating, ventilation, and air conditioning (HVAC) ductwork.

The toxic mix slowly but steadily disintegrates metal air distribution systems in natatoriums. The condensation that forms on metal duct and other metallic materials in a natatorium expedites the disintegration.

Considering the detrimental effects of corroding ductwork, fabric ductwork is being used in many new and renovated natatoriums. Unlike metal, fabric does not allow condensation to form because it uses strategies inherent in its technology to prevent it. Fabric duct can be specified as permeable, which allows air to pass through the fabric weave. This airflow through the entire duct surface prevents condensation on the material. Fabric duct also does not require special coatings to protect against corrosion or rust. The polyester fabric used in natatoriums should be UL-2518 classified. Primarily referenced for flame and smoke compliance, UL 2518 also requires a test demonstrating a fabric swath sealed in a 100 per cent relative humidity (RH) environment for 60 days does not propagate biological growth. Fabric duct can also be specified with an additional anti-microbial treatment for extra protection against mould and other microbial growths.

While condensation is typically blamed, the corrosion of metal surfaces in natatoriums could be caused by a variety of issues that may go well beyond the material’s vulnerability:

• Round metal duct typically should be double wall to thermally buffer the cold temperature air flow from the duct exterior that is exposed to humid conditions. Rectangular duct demands insulation either in the interior or wrapped around the exterior. Although it can be expensive and unsightly, skipping the insulation can allow otherwise avoidable condensation to form on the duct surface.
• Certain metals are less likely to corrode, such as stainless steel or aluminum or hot-dipped galvanized steel, and are better choices than electro-galvanized steel. However, an improperly maintained natatorium environment will eventually take a toll on any metal surface.
• Coatings on metal duct are necessary and helpful, but may require periodic maintenance as they too, can corrode or flake off into the pool water.
• Keeping strict RH and temperature set points is not a surety. New maintenance employees from staff turnovers do not always get the training their predecessors have in troubleshooting set point deviations. Mechanical equipment calibration, which goes beyond most natatorium staff expertise, needs quarterly checks to make sure the equipment is running as per the consulting engineer’s design.
• Indoor pools designed with materials unconducive to natatorium environments will foster a corrosive process that shortens ductwork lifecycles.

These points are all related to the four pillars of natatorium design—building materials, ventilation, mechanical equipment, and water chemistry—that must all work in synergy. If one or more of the four pillars are poorly designed or not maintained, it will be difficult to provide an acceptable natatorium environment. Consequently, many North American natatoriums do not operate properly[1].

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.[2]

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.

Air dispersion superiority with linear vents

Besides condensation, fabric can outperform metal diffusers in air dispersion performance. A fabric duct’s construction is unique in that it incorporates a variety of factory-designed, integrated vents (or nozzles) that span the duct length in a linear fashion and are specified at no extra charge. The linear design allows for a uniform dispersion of the mechanical system’s dry, conditioned air across windows, ceilings, and walls to prevent condensation.

In contrast, natatorium metal duct is typically round spiral duct with registers installed at intervals along the duct. Not only do those metal registers distribute uneven air streams that are stronger near each register and weaker between them, but the subsequent drafts may cause a chilling effect on swimmers[3]. Metal duct does have linear dispersion capabilities with slot diffusers, but those options are more costly than fabric ducts’ laser-cut linear vents. Metal slot diffusers can be cost-prohibitive because a typical natatorium perimeter ventilation configuration requires multiple linear vents aimed at dedicated targets, such as ceilings, walls, windows, and the pool surface.

Fabric duct is easily manufactured with multiple linear vents and orifice diameters engineered to provide the desired throws specified by the consulting engineer. For example, a duct run with the wall on the right side might have the following linear vent configuration and percentage of distribution dictated by orifice diameter: 10 o’clock for the ceiling (30 per cent); 2 o’clock and 5 o’clock for the upper and lower wall or windows (60 per cent combined), respectively; and the 7:30 mark for the pool surface (10 per cent). Each vent is individually designed to achieve the desired throws necessary. For instance, a maximum velocity of 9.1 metre/minute (30 FPM) is recommended across the pool surface, but vents covering the ceiling may need much higher velocities and longer throws. On-site installation of four linear configurations of corrosion-resistant aluminum slot diffusers which need to be individually balanced will increase project costs. Fabric duct is factory-designed and balanced when it arrives on site.

Fabric duct helps value-engineer a $48 million  aquatic centre

Metal is often misconceived as less expensive than fabric. Length for length, metal may indeed be less expensive; however, in the end, the required extras drive up the price. Fabric duct is factory-designed as a complete system with all the extras, such as suspension hardware and dispersion vents. Metal duct is rarely priced to include all the extras, such
as suspension hardware, boots, registers, insulation, and corrosion protection in natatoriums.

Another consideration is fabric duct installs 40 to 70 per cent faster than metal. For example, the recently constructed 8400-m² (90,000-sf) Aquatic Center at Mylan Park in Morgantown, W.Va., saved upwards of $55,000 because its mechanical contractor, A. Durer Inc. (ADI), Morgantown[4], value-engineered the ventilation to fabric duct from the originally specified polyvinyl chloride (PVC) coated metal duct. The multi-million dollar facility, which combines a separate competitive swimming/diving pool and community pool under one roof, is one of the most state-of-the-art indoor aquatic facilities in the state (see illustration A).

Fabric duct decreased labour costs and reduced trade stacking, because it was installed 60 per cent faster than metal duct. As a result, it shaved six to eight weeks off the ductwork installation. Further, the ability to fast-track allowed other trades to complete their work sooner than would have been possible. The fabric duct’s single-point anodized aluminum suspension system serving the Olympic-size competition pool and six-lane diving well was installed in just 10 days. Afterward, only five days were needed to hang the 300 m (980 ft) of fabric ductwork, consisting of 2.4 m (8-ft) diameter headers and branches ranging from 1220 to 1420 mm (48 to 56 in.). The 568,000 L (150,000-gal) community pool’s suspension system and 110 m (360 ft) of perimeter air dispersion was installed in less than a week.

The white fabric duct also eliminated additional costs for field-applied anti-corrosion coatings and paint to match the interior white décor scheme.

Fabric duct’s design flexibility helped solve the challenge of fitting the originally specified large diameter spiral ductwork through truss openings to create clean sightlines for spectators. The centre-positioned 2.4-m (8-ft) diameter trunk line runs between the trusses with smaller diameter branches to fit through open web truss members. The result was an esthetically pleasing layout with minimal cost changes.

Chloramines considerations

Besides condensation, chloramines, which are the heavy gases resulting from chlorine molecules bonding with body contaminants such as perspiration, urine, or fecal matter, are a major concern in natatorium ventilation. There are many incidents of poor indoor air quality, typically from chloramines, endangering respiratory systems both short-term and long-term.

In 2013, Caeleb Dressel, who later became a gold medal swimmer at the 2016 Olympics, was carted off to an emergency room with breathing issues later blamed on chloramine accumulation[5] at the Speedo Winter Junior Nationals at the North Carolina-based Greensboro Aquatic Center.⁵

Source capture units that come integrated into one side of the pool gutter (in new construction) or as a portable poolside plenum near one side of the pool, are excellent remedies for minimizing chloramines. They not only draw chloramines off the pool surface, but also exhaust them outdoors. Source capture units integrated into the gutter can also return the captured air to the mechanical air handler for energy recovery to efficiently pre-heat or pre-cool the ASHRAE Standard 62.1 mandated percentage of outdoor air required for natatoriums.

Source capture units also fit nicely into the overall design of return air and maintaining the building at a negative pressure so natatorium air is not pushed to other parts of the building. Return air grills work best at low positions to draw air off the occupied breathing zone on the pool surface and deck; however, high positions can function well, too, by eliminating stratification in the space. Multiple return grills, positioned low and high will help complete air changes.

The design and location of the air distribution system supply, return, and exhaust grills, as well as source capture units, should be integrated to achieve the most favourable results. The design capabilities and flexibility provided by fabric duct make it a great choice for the air distribution system in natatoriums.

Other benefits fabric duct offers versus metal

Besides the aforementioned benefits, fabric duct is approximately 90 per cent lighter than metal. This facilitates safer installation in new construction projects, especially with large diameter ducts that are difficult to hoist and install onto high ceilings. For projects retrofitted from metal to fabric, such as the earlier mentioned Northern California municipal pool, the roof’s structural integrity is not an issue. Likewise, older indoor pools originally designed without any ductwork can be retrofitted to fabric duct with no concerns about fortifying the roof to accommodate the weight of metal duct.

Keeping ductwork free of corrosion and evenly dispersing air across condensation-prone surfaces and down to the occupied breathing zone are key to improving natatorium indoor air quality.

Philip Daugherty is the engineering manager for FabricAir, a Lawrenceville, Ga.-based manufacturer of fabric air dispersion duct for heating, ventilation, and air conditioning (HVAC) applications. He has been with the company for eight years and is responsible for training, product innovation, installation guidance, and troubleshooting. The FabricAir engineering department can guide any aquatic manager or engineer through the natatorium ventilation design process for new construction as well as metal-to-fabric retrofits. Daugherty holds an engineering degree from the University of Georgia. He can be reached via email at phd@fabricair.com.

Source URL: https://www.poolspamarketing.com/trade/features/indoor-air-quality-solving-natatorium-air-dispersion-and-condensation-challenges/

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