By David R. Olson, PE
Recently, I walked into a large gleaming natatorium. Only once before had I visited an indoor pool as large and impressive as this. What a spectacular facility this was! I could only imagine the starters signal while numerous finely tuned swimmers shot off the starting blocks at once, in search of gold and glory. I immediately noticed how muggy the air was. I pondered how this might affect the competing swimmers. The air within this natatorium did not remind me of a warm spring day in a mountain meadow. Not that it should, but the air was anything but fresh.
I started looking more closely at the many metal surfaces in the building. The purpose of my visit was to observe the operation of the installed mechanical dehumidification system. Despite the young age of the facility, I noticed rust starting to form on much of the exposed metal. I then looked up and saw what appeared to be condensation on the metal surfaces surrounding the clerestory windows high above the water surface. I observed that the return air was exiting the natatorium by a large return grille high in the wall at one end of the enormous pool enclosure. This made me wonder how this pool operator accommodated the foul air rising off the pool water surface before and after the inevitable pool chemical shock. Did they even concern themselves with this particularly common condition?
Much has been written regarding pool natatorium dehumidification and associated energy cost. Less focus has been given to acceptable indoor air quality. Traditional recommendations regarding air distribution in natatoriums appear to favor limiting pool evaporation and condensation at exterior building surfaces. Most, if not all of these papers disregard or minimize outdoor air ventilation and indoor air quality from the discussion. Successful indoor swimming pool environments require large quantities of outside air to satisfy the need for acceptable indoor air quality. It is the challenge of the pool designer to deliver this objective to their clients with a functioning, energy conscious mechanical system.
Swimming pool water chemistry is very difficult to maintain at acceptable levels. Biological contamination increases in proportion to bather loads. When the pool water chemistry becomes challenged, harmful chloramines are released at the water surface. These contaminates are detrimental to the swimmer’s who enjoy the pool activities. The chloramines must be effectively removed from the natatorium. Spoiled air may not be removed from the pool area without the reintroduction of replacement outdoor air. In most climates, it is expensive to introduce this air into natatoriums.
The American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) have developed a standard with recommendations for outdoor air ventilation for various building types and uses. ASHRAE recommends minimum outdoor air ventilation quantities of 15 cfm per natatorium occupant plus 0.5 cfm per square foot of pool water surface and wetted deck. This can become a staggering amount of outdoor ventilation airflow.
ASHRAE recommends 4 to 6 air changes per hour for natatoriums without spectator areas, and 6 to 8 air changes an hour for natatoriums with spectator facilities. Today’s typical municipal pool facility includes many water activities intended to stimulate the enjoyment for the bathers. Recent pools have included sprays and fountains, waterfalls and slides. These features increase the humidity level present in the natatorium. Primarily due to these enhanced water features, pool environments should be designed with minimum 6 air changes an hour for lap pools and 10 air changes an hour for activity leisure pools. Minimum outdoor air will typically range from 25% to 35%, even with the increased air circulation.
Huge improvements have been made in the science of pool water chemistry and the implementation of methods to control it (See “Addition by Addition”, Athletic Business, October, 2002). However, many public pools do not possess this state of the art, high tech supplemental water chemistry control systems. The water chemistry in these pools is very difficult to control. Heavy chlorine odors result from the release of chloramines at the pool water surface. At this time the pool water is typically shocked with large doses of chlorine. Unfortunately, the air quality suffers radically during this period of super chlorination.
Additional outside air is necessary during the period of time leading up to the shock event, due to extreme levels of air contamination within the natatorium air volume. The swimming pool must be closed while super chlorination is being undertaken. Therefore, it is advantageous for the air handling systems to accommodate 100% outdoor air ventilation in order to more rapidly purge the natatorium of contaminated air. Pool shock may be required during any condition of outdoor weather. The pool air handling equipment must be capable of conditioning this elevated air quantity.
Chloramines tend to accumulate just above the water surface, right in the pool users breathing zone. Yet, much of the popular literature suggests that supply air should not be directed at the pool water. Despite the increased surface evaporation that will naturally occur, supply air must be delivered (pushed) into the area dominated by contaminates in order to assist the exhaust system in removing (pulling) the undesirable air.
It is very important to direct dry supply airflow at exterior wall and glass surfaces within pool enclosures. Mechanical systems are expected to overcome the frosting of glass and dripping of skylights. This can be reasonably accomplished by directing dry supply air onto these surfaces. This has two effects. It reduces the relative humidity of the airflow closest to the cold surfaces and it raises the temperature of those surfaces with the warmest airflow.
Supply air should be delivered from duct systems located high above the pool decks. About a third of the available supply airflow is directed towards the exterior wall and glass surfaces. During periods of cold outdoor temperatures, the supply air cools and falls as it releases heat to the glass. The balance of the supply air quantity is aimed towards the pool water surface and interior walls. This assists in removing the airborne contaminants and provides fresher air within the pool users breathing zones. The net result of this strategy is drier exterior wall and glass surfaces and improved air quality at the pool water surface.
Chloramines will have the least effect if they are removed closest to their source. Return/exhaust air should be taken close to the pool deck surface. Multiple return air inlet points are preferred. This can be accomplished either by dropping multiple return air ducts down to wall grilles from overhead exposed duct systems or up from buried or tunnel located return ducts below the pool deck. Architectural features typically dictate the quantity and locations of the return grilles. Return air must be drawn from all sides of the pool area in order to maintain good air quality throughout the natatorium. This method allows effective removal of airborne contaminants near their source, while assisting the air handling system to circulate and mix the natatorium air volume.
Non-insulated aluminum duct systems and corrosion resistant hangers are typically used within natatoriums. The ductwork is normally field painted. Exterior ductwork is insulated for thermal protection. Duct liner should not be used, due to its tendency to absorb moisture and become a potential habitat for mold and fungi. Aluminum sidewall supply registers are spaced along the perimeter of the supply duct. When necessary, air is directed close to skylights with branch ducts and registers.
Dehumidification is a breeze, so to speak, in the western United States. The outdoor air is very dry year around. It is perfect for absorbing excessive humidity within the swimming pool environment. Humid and rainy days are rare, with most moisture coming in quickly passing summer afternoon storms and winter snow storms. In other areas, some form of mechanical dehumidification is necessary. The outdoor air in these regions has a higher relative humidity than that desired within the natatorium. Regardless of where the project is located, fresh outdoor air is necessary to maintain acceptable indoor air quality. All projects must include some form of heat recovery to help control energy expenditures by the pool operators. A bypass damper is necessary so that return air may be blended directly into the supply air tunnel. This is required to maintain space relative humidity at the 50% level.
Energy efficiency may be accomplished with heat wheels, plate type air to air heat exchangers and refrigerant filled heat pipe. While the effectiveness of heat pipe and plate type heat exchangers is roughly the same, the ease of cleaning heat pipe is much more attractive. Natatoriums are surprisingly dirty environments. Regardless of which style of heat exchanger is chosen, it is essential that pre-filters be installed on the pool exhaust air stream, up stream of the heat exchanger. Failure to do so will result in heat exchangers that are clogged with lint. Another advantage of heat pipe is the ability to adjust the tilt, thereby seasonally fine tuning the heat recovery function.
Air temperatures within natatoriums are usually maintained at 2 degrees F above the temperature of the predominant body of water. Most whirlpools in commercial facilities are maintained at 104 degrees F. Many leisure pools are kept around 86 to 90 degrees F these days. Normally, even with pools this warm, the maximum indoor air temperature is set at 86 degrees F. A clever way of doing some summertime cooling of the entering air is to install evaporative cooling media in the exhaust air stream upstream of the heat exchanger. This will raise the exhaust air relative humidity from the 50% set point to about 90%. This will result in roughly a 10 degree F drop in exhaust air temperature. This energy reduction is then transferred to the supply air stream via the heat pipe without increasing the relative humidity of the incoming air stream.
Pool heat recovery equipment requires the capacity to perform air delivery at an average of 8 to 10 air changes per hour. Natural gas heating is used to supplement the heat recovery performance, allowing the delivery of 100 % outside air under all conditions of anticipated outdoor temperature. During much of the life of this equipment, full capacity is not necessary.
As the exhaust air and supply air filters collect dirt, the airflow delivery tends to change. It is essential that the pressure relationships be maintained between the natatorium and the remainder of the building. The natatorium must be maintained at an air pressure that is less than the rest of the building. This way, odors are contained within the pool enclosure and do not drift to other portions of the building. A supply fan variable speed drive, in conjunction with a space pressure control system, will adjust the supply air volume to maintain a negative natatorium air pressure.
In more humid climates, the design solution should incorporate heat recovery, gas heating and mechanical dehumidification. The heat recovery enables ASHRAE minimum ventilation quantities, while saving roughly 50% of the energy that would otherwise be necessary to heat and dehumidify the incoming air. The gas heating allows for efficient heating of the balance of heating necessary for make up air tempering. Mechanical cooling is used for dehumidifying the incoming air and maintaining acceptable relative humidity levels in the natatorium. Beware of system alternatives that satisfy only the humidity and space temperature needs, while ignoring the indoor air quality enabled by increased outdoor air.
The construction of all natatorium heat recovery units must pay close attention to the corrosive nature of the pool environment. The inside of the heat recovery units must be constructed of corrosion resistant materials such as aluminum or 316 grade stainless steel. Heat recovery coils and fans are coated with a heresite coating to reduce the long term effects of corrosion.
Virtually all commercial natatorium projects today include direct digital temperature controls (DDC). These allow the pool operator to set and monitor all levels of pool air temperature and humidity. Quantities of outdoor ventilation air can be monitored and adjusted quickly and efficiently. The pool air circulating systems run continuously, day and night. Where multiple units condition a single natatorium, they are each controlled simultaneously. If one unit requires servicing, the other unit is able to deliver 50% airflow to the natatorium.
The pressure relationship of the natatorium to the balance of the building is critical. A constant volume exhaust fan maintains to pool airflow in constant proportion to the balanced condition in the rest of the building. A natatorium pressure sensor varies the quantity of supply air in order to maintain the space pressure negative to the adjacent locker and pool mechanical rooms. The locker rooms and pool mechanical rooms include separate and independent air handling systems, make-up air and exhaust, such that the corrosive pool air is not introduced into these areas.
In dry climates, the quantity of outside air is controlled from space relative humidity level. In humid climates, the outside air quantity remains more stable, at a minimum acceptable level, and dehumidifying is accomplished by cycling the self contained mechanical refrigeration equipment. Regardless of project location, minimum outdoor airflow must be maintained in compliance with ASHRAE standards. When the outdoor humidity level is low, the bypass damper downstream of the constant volume exhaust fan is opened to avoid over dehumidifying the natatorium.
Temperature is maintained by the fully modulating gas heating section in conjunction with the integral heat recovery system. The use of heat pipe allows the amount of heat recovery taking place to vary in fringe months by adjusting the tilt of the refrigerant filled heat pipes. During periods of warm weather, the exhaust air evaporative cooling section is sprayed with water, allowing indirect evaporative cooling of the incoming supply air stream.
Pool natatoriums may be designed in a practical and logical manner. The results are happy pool operators and facilities with good air quality. A high priority is mandated for acceptable indoor air quality. Air to air heat pipe heat recovery is used to enhance energy savings. Outside air is efficiently employed for dehumidifying wherever possible. Minimum outdoor air quantities must be in compliance with ASHRAE Standard 62, regardless of the climate outside the controlled pool environment. Fully modulating gas heating in concert with air to air heat recovery allows the most efficient solution to the natatorium challenge.
Supply air is best delivered to the pool enclosures from overhead, and return air is drawn from deck level where harmful chloramines tend to collect. Duct systems are corrosion resistant exposed and non-insulated painted aluminum. Supply air is delivered liberally across the pool water surface so that the highest quality air is maintained in the pool user’s breathing zone. Consider total circulated air quantities higher than typically recommended by respected authorities. This helps to accommodate and maintain the indoor environment despite the ever increasing activity levels found in pool enclosures today. Water sprays, fountains, water falls and slides release great quantities of humidity that must be controlled by the pool mechanical systems.
Operating costs can be enhanced by the use of heat recovery and the use of outdoor air wherever possible for natatorium dehumidification. However, energy cost should take a secondary priority to the condition of air within the natatorium. The pool user’s health and bathing experience must remain the highest priority. Revenues from happy swimmers will result in happier pool operators and facility operators.