In order to use the Coanda effect to hold the dehumidifier’s warm, dry supply air against an exterior window or door and prevent them from sweating, ductwork must be positioned approximately 1 foot or less from windows.
James Hogan is product development engineer at IAQ equipment manufacturer Dectron Internationale, Roswell, Ga., a maker of heat-recovery dehumidifiers for commercial indoor pools under the Dry-O-Tron® brand. In his 12 years at Dectron, Hogan has seen it all and has advised hundreds of natatorium building owners with IAQ problems. Hogan also is the director of the Dectron Installation & Service School, Niagara Falls, N.Y., which offers a curriculum for HVAC contractors and service technicians.
He has seen natatoriums with rainforest conditions that literally rain on the occupants, a ceiling that has collapsed into the pool, and metal surfaces corroded from the volatile environments of pool chemicals and excessive humidity.
According to Hogan, four essential factors must work in synergy for a natatorium to function with the desired 50 percent rh and optimum indoor air comfort: building envelope, mechanical equipment, air distribution, and water chemistry. If any of the four are neglected or designed or installed improperly, it affects the other three factors, and the natatorium will probably never function up to its potential.
“Poorly designed or installed ventilation systems are quite common in dysfunctional natatoriums,” Hogan said. “It almost always comes down to ductwork problems.”
A lot of rules of thumb that might work in more typical HVAC systems don’t work for natatoriums. “The issue here is people who say, ‘Don’t worry, that air will find a way.’ To keep condensation from happening unexpectedly, you have to use the Coanda effect. Projecting the air from 15 feet away just does not work,” Hogan said.
“People don’t face this problem in their everyday life because, in their homes, humidity can drop quite low. But in a natatorium, the humidity just does not go down.”
TOP PROBLEMS
Hogan addressed five problems in designing HVAC for indoor pool areas:1. Failure to use the Coanda effect.The main problem in natatorium design, he said, deals with something called the Coanda effect. “Using the Coanda effect in natatorium ductwork design and installation helps ensure that the air runs along the length of the windows,” Hogan said. What is the Coanda effect? Put your arm under a running faucet; the water appears to hold tight to the surface as it runs down the arm. “When a moving fluid nears a surface, it tends to follow that surface.”
Likewise in air distribution, he said, the Coanda effect can be used to hold the dehumidifier’s warm, dry supply air against an exterior window or door and prevent it from sweating. In order to do this, ductwork must be positioned approximately 1 foot or less from the windows.
The diffusers usually should be as wide as the surface to be covered, he continued. The best results come from linear diffusers with steerable vanes to allow for some misalignment.
“Window condensation is common in natatoriums, especially in winter months,” Hogan said. “It usually results from not positioning ductwork close enough, or without a sharp enough diffuser angle to the windows, to evenly distribute warm, dehumidified air across them.
“Remember, surfaces other than windows and doors may go below dew point, such as uninsulated wall sections, skylights, and tie-rods or other metal penetrations.”
2. Elbows and tees too close to the air handlers.Return air ducts that take a deviated path via an elbow or tee positioned too close to the air handler will create a swirling back-up that prevents evenly distributed air across the heat exchangers, Hogan pointed out.
“This is the same effect that piles up leaves and paper directly behind a moving pickup truck cab. The air swirls as it goes past the cab, causing the wind to blow back toward the cab. The same thing happens as air flows through an elbow or tee.” The resulting dehumidifier operation will obviously be at less-than-the-expected capacity.
According to Hogan, minimum straight lengths of full-sized return ducts are required for 100 percent capacity, as outlined in a dehumidifier owner’s manual. Similar minimum straight lengths of supply duct are also required to prevent flow reductions.
This applies to all air-handling equipment, he said, and it can be found in information from Sheet Metal and Air Conditioning Contractors’ National Association and Air Movement and Control Association International. “Natatoriums are closely sized to the load,” he said; their systems need to run 45-50 minutes per hour or the windows will sweat.
“You can have problems with capacity,” Hogan said. “The air begins to spin; any flow interruption causes air to spin. This prevents the air from moving the way you think it’s moving over the heat exchangers. In a house it’s only apparent in the hottest weather. In a natatorium it’s always the same.”
If it’s bad enough, he said, capacity can be cut by one-half. Ice damage, mold damage, and problems to window frames can result.
“The main thing is to get that straight, full extension of duct in the last section of the duct. Turning vanes and perforated plates can make it better, but they can have upfront and energy costs.” Not all architects and engineers are aware of this problem, he said, and allowing appropriate space in the mechanical room is not always their top priority.
A CFD analysis can provide a lot of information, including all surfaces that may go below dew point.
“If it’s down low, it will be used as central vacuum cleaning. The effect on the duct system and the unit is not good. I have several jobs where the filters look like a trash truck,” Hogan said. “There’s paper 2-3 inches thick on them.” Also, if the supplies are low to the floor, and there is a low return grille, “you won’t get any vertical mixing to speak of.”
Return grille location should generate an airflow of 10-50 fpm across the entire pool surface, he said. “Stagnant zones and excessive air speeds will increase evaporation rates that surpass the capacity of the dehumidifier.”
Return grilles should never be placed near a spa or hot tub because the human skin oil re- leased by these hot areas will accumulate on filters, heat exchangers, blowers, and inside ducts, thus affecting the HVAC system’s efficiency and performance.
4. Outdoor air delivery mistakes.Outdoor air should never be brought into a dehumidifier’s return duct, Hogan said. “The amount of outdoor air needed can be found from ASHRAE 62.1.” Inadequate outdoor air can lead to IAQ problems, and excessive outdoor air is expensive to heat and cool, and can reduce the dehumidifier’s ability to reclaim heat for the pool.
The perceived need to bring in excess outdoor air can actually be due to a separate problem. “Building operators tend to perceive a need for excessive outdoor air because they don’t realize they have problems from improper chemical storage, pool-water chemistry, or air distribution,” he said. “The room in which pool chemicals are stored should have its own, self-contained ventilation.”
The problem is common where people are used to working with residential equipment. “I’ve gotten jobs where the evaporator is a complete block of ice,” he said. It happens because “The contractor said he only wanted to take care of one set of filters.” It’s false economy. “The dehumidifier runs year-round, it uses latent heat exchange. If you bring in outdoor air, that will make the returning mixed air way off design conditions and limit the ability of the unit to dehumidify,” Hogan explained. “Outdoor air must be brought into its designated port.”
5. Poor water and airflow balancing.Both too much and too little airflow will reduce dehumidification and cooling capacities, Hogan said. “Dehumidifier operation at reduced capacity is difficult to hide and leads to unhappy customers.” Inadequate or recirculated airflow for outdoor condensers also leads to high refrigerant-pressure problems.
Too much water flow will cause erosion of the heat exchangers. Too little water flow promotes scaling of heat exchangers. “It is very important to have flows measured.
“With air conditioning,” he said, “you have so much excess capacity on all but the hottest days, but in a natatorium it’s the same year-round. It must be sized right. If it’s sized for 70 percent of what you need, you will only get 70 percent. It must be within ±10 percent of design conditions.
“People always fudge the duct,” he said. “Then there’s the value engineering. If you do airflow balancing, the balance technician will find that.”
On the water flow, a side stream of pool water goes to the dehumidifier. The biggest part of the load is the latent load. “A heat exchanger in these dehumidifiers recovers heat from the air and puts it back into the pool. Because pool water is very hard, calcium is added.
“If the water flow rate is too low, you won’t be able to heat the water and scale up the heat exchanger. If it’s too high, it can cause damage.” Sized and designed properly, he said, no additional heat is required; virtually all can be recovered.
“Please, please, please, read the manual,” Hogan said in closing. “If you don’t understand something, use the toll-free numbers, and we will explain it.”
For more information, visit www.dectron.com.
Publication date:05/17/2010
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