Modulating outdoor air via OAVOS
An OAVOS generally ranges in size between 113 to 1982 cubic metres per minute (CMM) (4000 to 70,000 cubic feet per minute [CFM]) and consist of many important components, which help minimize operating costs, while still providing the best possible space control.
The biggest problem with the old OAVOS was the constant volume of outdoor air as it rendered them expensive to operate, while over drying spaces in colder weather. Newer systems, however, closely monitor the indoor and outdoor conditions and introduce only as much outdoor air as is needed to control the indoor conditions at the set point levels.
This control strategy, facilitated by advances in microprocessor capabilities and better sensing devices, has a dramatic impact on lowering the operating costs, while providing more comfortable conditions for patrons in cold weather. This type of control was not previously available.
An effective OAVOS will introduce only as much outside air as is needed to provide the best possible space conditions at the lowest possible operating conditions.
All commercial buildings must bring in outdoor air as mandated by local building codes. These codes are generally based on the recommended values from American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), Standard-62, Ventilation for Acceptable Indoor Air Quality (IAQ). The Centers for Disease Control (CDC) has also recently published some suggested outdoor air requirements in its Model Aquatic Health Code (MAHC), which are substantially higher than ASHRAE values. Regardless of which is used, heating cold outdoor winter air to 27 to 29 C (82 to 85 F) is very expensive.
ASHRAE guidelines also recommend all natatoriums be maintained at a slight negative pressure. While moisture will always try to pass through the building envelope due to a pressure differential (it is critical an effective vapour barrier is in place as a preventative), maintaining negative space pressure assures potentially damaging pool air with inherent moisture and chemicals is not pushed through the natatorium’s walls or doors leading to the building’s non-pool areas.
Consequently, the amount of exhaust air must always exceed the HVAC system’s outdoor air. The exhaust air is humid and warm, making it extremely energy rich and ideal for energy recovery, which can be used to preheat the code-required outdoor air. This can reduce outdoor air heating costs by 60 to 75 per cent.
Most natatorium HVAC systems and equipment can easily use heat recovery because the outside air and exhaust air streams are in close proximity. This makes accomplishing heat recovery easy on just about any project.
Another recent technological advancement is the development of a glycol runaround loop (GRAL) for heat recovery. The GRAL uses corrosion protected fin/tube coils and circulates a glycol fluid for heat exchange between exhaust and outdoor air streams. It is a simple and highly efficient form of heat recovery (compared to other methods) and has several additional advantages. The coils, for instance, which manage the condensate quite effectively, are compact, resulting in equipment with an extremely small footprint. The smaller unit size makes it attractive for retrofits and a valuable space-saver in new construction projects.
Further, relative to other forms of heat recovery, the coils are lightweight, can be cleaned and serviced easily, and are fully corrosion protected. Finally, the low-energy requirement for the heat recovery loop’s fan makes the overall annualized system efficiency higher compared to other forms of heat recovery.
Chemical source capture devices and UV
HVAC systems using 100 per cent ventilated air are not immune to air quality issues. To ensure optimum IAQ, the following design criteria should be provided regardless of the humidity control system being considered:
- A properly designed and performing air delivery system is an absolute must for proper space (i.e. environment) control and IAQ;
- Supply good quality air from the dehumidifier/OAVOS down onto the deck and into the breathing zone. If supply air does not get to where the people are, there will be IAQ complaints; and
- Incorporate new technologies (e.g. ultraviolet germicidal irradiation [UVGI] or a source capture exhaust system) for controlling chloramines.
Chloramines are byproducts of chlorine (Cl) molecules that attach to contaminants in the swimming pool water. Chloramines, especially trichloramines, are heavier than air and hover just above the water surface in the swimmers’ breathing zone. Chloramines are also suspected in respiratory ailments associated with swimming pools such as Lifeguard Lung (a respiratory ailment caused by prolonged exposure to chloramines in natatoriums).
To combat this, a source capture/exhaust plenum-type device was recently developed to draw and exhaust chloramines directly off of the swimming pool surface. The exhaust air from the source capture/exhaust system can be ducted directly outdoors or exhausted through a dedicated air path in the OAVOS.
Another great benefit of the glycol runaround loop heat recovery method mentioned earlier is the exhaust air heat recovery coil can be mounted on the unit or remotely, allowing energy to be recovered from the exhaust air regardless of where the airstream is located. There is no energy penalty for using this source capture/exhaust technology. In fact, its effectiveness is currently being considered by code officials so indoor swimming pool facilities employing a source capture/exhaust system can operate at lower minimum outdoor air requirements than other natatoriums.
The chloramines source capture/exhaust system performance can be optimized when located at the swimming pool’s edge, nearest the natatorium’s return air grille, which helps draw chloramines toward the evacuation system.
Another option to fight chloramines as well as other biological contaminants that foul swimming pool water is the use of a UVGI system. Ultraviolet-C (UV-C) radiation can sanitize pool water, air and surfaces, and reduce chlorine use. It works by destroying the deoxyribonucleic acid (DNA) of algae, bacteria, cysts, and viruses as the pool water circulates past a UV lamp. The highly concentrated electromagnetic energy destroys organic matter and eliminates the formation of chloramines.
UVGI is a trusted form of sanitizing. Many large metropolitan areas use it to disinfect drinking water, while various waterparks also use it to sanitize water as per National Swimming Pool Foundation™ (NSPF™) standards. A registered sanitizer (e.g. chlorine or bromine [Br]) must be used in conjunction with the UV system (to sanitize water outside of the UV system chamber/piping) as recommended by health authorities in Canada and the United States.
The bottom line is, however, these methods help improve IAQ by reducing chloramines and lessening the need for 100 per cent outdoor air at all times, which saves energy. Instead, outdoor air can be reduced to levels recommended by ASHRAE.