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It can be seen from the equation that the followingfactors increase the evaporation rate: Increasing water temperatureLowering air temperatureLowering air relative humidityHigh activity/agitationOnce equipment has been selected and installed, anychange of the variables that increases the evaporationrate can result in equipment no longer being suitable forthe new larger load.Occupant LoadSwimmers are not usually considered occupants as theyare submerged in the water. Swimmers and their wateragitation are included in the Activity Factor. Spectators,especially in facilities that host swim meets can totalseveral thousand, and add a significant moisture load,(see Table 3).It is important to understand that when a facility ishosting a swim meet the Activity Factor of the wateris considerably reduced. Typically there is only oneswimmer per lane and while they agitate the waterconsiderably, the overall agitation is much less than adensely occupied pool during a public swim.Table 3 – Occupants Latent LoadActivity LevelLb/h perSpectatorQuietly Seated0.155Moderate Activity0.205Enthusiastic0.250Highly Enthusiastic0.530To evaluate the dehumidification load during swimmeets an Activity Factor of 0.65 is used to calculate theevaporation rate. The total number of spectators andcompetitors on the pool deck must also include in theload. Codes also generally require that each spectatorbe provided with 7.5 CFM of outdoor air. The loadimpact of the outdoor air must also be calculated.Seresco recommends that facilities hosting swimmeets size equipment based on the larger of the twomain operating modes, normal operation load or swimmeet load.6Natatorium Design ManualOutdoor AirThe introduction of outdoor air is essential to maintaininggood air quality in any facility. The impact of this outdoorair ventilation on a natatorium changes with the weatherand the geographic location of the facility. Introducingoutdoor air during the summer generally adds moistureto the space and in the winter removes moisture from thespace. For maximum dehumidification load calculationsthe Summer Design conditions are considered.Construction codes generally require that outdoorair be introduced into a commercial building duringoccupied hours. ASHRAE Standard 62-2004 Table6.1 recommends the introduction of outdoor air into anatatorium at the following rates: 0.48 CFM/ft2 of pool and (wet) deck area as base line 7.5 CFM per spectator added to the base line duringswim meetsASHRAE interprets the wet deck area as no more thansix foot perimeter around the pool. The purpose ofthis outdoor air is to help dilute chemicals off-gassedfrom water. A predictably dry portion of the deck willnot factor into the IAQ issues. Additionally, outdoor airrequires considerable heating in the winter.More is not better. Exceeding code requirements is notrecommended. In winter it will significantly increase theoperating expenses and in summer may increase thedehumidification load.Load Estimation SoftwareSeresco has developed software that calculates allmoisture loads in a matter of minutes. Figure 2 gives asnapshot of the basic data that would generally need tobe entered to calculate a load.Figure 2 – Sample of Seresco’s Load Estimating Software

Humidity Control TechnologyHow to Remove MoistureEngineers typically consider one of three approaches toventilate a natatorium. It is important to understand thecapabilities and limitations of each approach in orderto select the best system for the application. Serescomanufactures all three styles of unit and will help youevaluate what is right for your application. Energyconsumption, geographic location, budget and desiredcontrol of space conditions are among some of the keydecision making criteria. Refrigeration based dehumidifiers Chilled water with reheat air handler Outdoor air ventilation (usually with heat recovery)Refrigeration Based DehumidifiersRefrigeration based dehumidifiers are by far the mostcommon and popular method of removing moisturefrom the space. These are the NE Series style unit builtby Seresco. The units are designed and developedspecifically for dehumidifying indoor pools. They offer yearround humidity control at the lowest operating costs.A major benefit of this approach is year round humiditycontrol with low operating costs. These systems useboth sides of the refrigeration cycle (evaporator andcondenser) simultaneously. Contrast this to traditionalair conditioning units that use only the evaporator anddump condenser heat outdoors. These units use theevaporator to dehumidify (and cool when needed)but also put heat back into the pool and/or backinto the process air. This approach is unique to theThe Energy CycleDry Airnatatorium industry. The system can be simultaneouslydehumidifying (cooling) the air and then reheating it (and/or the pool water) to deliver dehumidified and reheatedair to the space, and warm water to the pool. Not asingle BTU on either side of the refrigeration cycle iswasted. COPs are close to 8 and EERs greater than 25!Figures 3 and 4 illustrate schematically how warm humidair passes through the dehumidifying coil and is cooledto below its dew point. Once air is cooled below its dewpoint, moisture condenses out of the air. Depending onthe space temperature requirements the hot gas fromthe compressor can be used to reheat the air or berejected to an outdoor condenser. Compressor hot gascan also be used to heat the pool water. Figure 4 showsan example of these components in a vertical unit.Figure 3 – Packaged Mechanical Refrigeration SystemsOutdoor CondenserHot gasPoolWaterHeaterCool gasWater topoolWater frompoolDryAirPlug FanCompressorHot gasReheat Coil(Condenser)Dehumidifier(Evaporator)DryAirHumid AirHumid AirRecycledHeatEnergyLiquidThermostaticExpansion ValveHumidity Control Technology7

have 100% outdoor air modes for milder weather. It isimportant to note that the space conditions may not bemaintained in summer as the outdoor air may not bedry enough.Figure 4 – Vertical DehumidifierOUTDOORAIRSUPPLYAIRThe outdoor ventilation style units have several attractivefeatures if your facility is located in a suitable geographicarea. They have a lower first cost than a refrigerantbased approach and are also considerably simplerunits. With heat recovery between the exhaust air andoutdoor air streams and controls that properly sequencethe outdoor air, this can be a very attractive means toventilate a natatorium.Electrical PanelReceiver withSight GlassBlower MotorBlowerCompressorRETURNAIRPool Water HeaterAir FiltersEvaporatorReheat coilDrain PanChilled Water with Reheat Air HandlerThese units are not very common and generally onlyfound on applications that have abundant chilled wateryear round and installing a refrigerant based systemproves problematic. The dehumidification processof these systems is essentially the same as withcompressorized systems. The chilled water supplytemperature must be low enough to cool the air belowits dew point at the chilled water coil and condensemoisture out of the air. Reheat is required for most ofthe year. Systems that cannot use captured condenserheat for reheating air require the operation of a heatingcoil while the chilled water coil is in use. If reclaimed heatis not available for reheat, this approach does not meetmost energy codes. This approach is not recommendedif chilled water is not available year round.Outdoor AirVentilation SystemsWhile not technically a dehumidifier, becauseonly outdoor air is used, this approachis considered in areas that are dry and/or cool outdoors year round. Solutionscan be a very basic, from a single pass ofoutdoor air that is introduced at one end ofthe space and exhausted at the other to amore sophisticated system that has variableoutdoor air and heat recovery. The singlepass system is rare due to the prohibitiveoperating costs.A more typical approach is the Seresco NVseries style ventilation unit. These units provideventilation air, heat recovery, heating and also8Natatorium Design ManualThere is a misconception that because these systemsprovide more outdoor air than a refrigeration basedunit that the spaces as a result will have much betterair quality. This is not correct. The airside designrequirements are the same for a Natatorium regardlessof which solution is chosen. Supply air CFM, minimumoutdoor air CFM and minimum exhaust air CFMrequirements are identical for all systems.Some manufacturers promote more outdoor air thancodes require in order to ensure good IAQ. Seresco doesnot recommended this approach because it increasesbuilding operating costs and still may not have any suchimpact on the space if the air distribution is not donecorrectly. A better solution is to stick with the ASHRAEguidelines and ensure the air distribution is correct.System comparisons and features are highlighted intable 4. Note: The table assumes all units are designedwith the same airside characteristics.Table 4 – System ComparisonRefrigerationChilledWaterOutdoor AirVentilationOperating CostLowestHIghestMediumFirst CostHighestMediumLowestYear round humidity controlYesYesWeatherdependantMeets/exceeds EnergyStandard 90.1YesNoNoDehumidification energy recoveryYesNoNoHeat recovery on Exhaustand Outdoor airOptionOptionOptionFree summertime ACYesYesNoFree summertime poolwater heatingYesNoNo

Indoor Air QualityGood indoor air quality can be a challenge in an indoorpool. However, designers that follow ASHRAE guidelinesas well as those addressed in this manual shouldhave every expectation of a great space conditionand pleasant overall experience for the patrons of thefacility. There are steps a design engineer must takein their design to minimize the chances that a patronexperiences a foul smelling pool odor and stingingeyes. There are many factors that impact the IAQ in anatatorium: pool water chemistry problems, inadequateoutdoor air, air stagnation, poor air distribution, highhumidity, mold, mildew, condensation and corrosion.Four of the five key factors having the mostdirect impact on Indoor Air Quality (IAQ) areunder the control of the design engineer. Pool water chemistryPoor air distribution: No Airflow in the Breathing Zone.Outdoor air ventilationExhaust airAir change rateChemical off gassing from the pool water isthe pollutant that causes all the IAQ issuesin a natatorium. A successful HVAC designwill adequately remove these chemicals andprovide good IAQ by properly addressing thefour key airside design requirements.Pool Water Chemistry is the key variable thatimpacts IAQ and is not under the responsibilityof the design engineer.Poor water chemistry is the single biggestsource of indoor air pollution and corrosionproblems in a Natatorium.Ultraviolet light (UV) treatment of poolwater has shown to have a very positiveChlorine smellWhen you walk into an indoor pool and smell a strongchlorine odor, you naturally think it’s caused by toomuch chlorine in the water. The odor is actually causedby chloramines (combined chlorines) off gassing fromthe pool water surface. Chloramines are formed in thepool water when there is insufficient free chlorine in thepool to address the nitrogen-containing compoundsbrought into the pool water by the swimmers. Thesenitrogen compounds are naturally-occurring andcontained in sweat, urine, body oils and other proteinsthat get released into the pool water. If the introductionof these nitrogen compounds outpaces the introductionof free chlorine the chlorine becomes combined with thenitrogen compounds rather than fully oxidizing them.The chloramine (combined chlorine) levels increase inthe water, resulting in an increase in chloramine offgassing, which creates the odor of chlorine in the room.There are three different types of chloramines that canform: monochloramine, dichloramine and trichloramine.Trichloramine is the most volatile and will off-gasmost quickly.The powerful chlorine smell that is often associatedwith indoor pools is NOT the result of too much freechlorine in the water; it is TOO LITTLE free chlorine thatis the culprit!Pool Water ChemistryGood pool water chemistry by the buildingoperator is critical in order to achieve levelsof human comfort and health. Also bymaintaining optimal pool water conditionsyou will have the best possible indoor airquality and ensure optimal performancefrom the mechanical system.impact on the water chemistry and can help reduce,if not totally eliminate chloramines. This approach toenhance water treatment is gaining popularity and aspositive data continues to come should become moreand more common.To avoid chloramines it is imperative to maintain properfree chlorine and pH levels as outlined in table 5.Table 5 – National Spa and Pool Institute Recommended Levels forWater QualityPool Water Chemistry Parameters Recommended by NSPIPoolsWhirlpoolsDesirable RangeDesirable RangepH7.4 – 7.67.4 – 7.6Alkalinity80 – 100 PPM80 – 100 PPMFree Chlorine2.0 – 3.0 PPM3.0 – 4.0 PPMCombinedChlorine0 PPM0 PPMDissolved Soli ds100 – 300 PPM100 – 300 PPMTotal Hardness225 – 250 PPM175 – 275 PPMIndoor Air Quality9

Off-gassed chloramines also have a strong attraction tothe airborne humidity which will combine with moisturein the air. Consequently, any condensation of the spacehumidity will become corrosive. An example of this cansometimes be seen at pools with chloramines problemswhere the ladder rails outside the water corrode whilethe portion under water does not.The proper balance of outdoor air and room exhaustair along with air movement at the water surface is alsocrucial to ensuring chemical concentration levels aremaintained within acceptable levels.Pool Water pH LevelsHigh pH levels (alkaline range) encourage scaleformation, which reduces pool water heater efficiency.With low pH levels the water is acidic and corrosive. Thismay damage the metal parts in pumps, water heatersand piping. Maintaining pH levels between 7.2 and 7.6will ensure the longest life for the pool equipment.Total Dissolved SolidsIt is imperative that pool water filters are routinely backwashed and that total dissolved solids monitored. Highquantities of total dissolved solids will overrun the freechlorine levels and form chloramines.A proper pool water test kit should monitor theparameters seen in table 5.Humidity and CorrosionAs previously mentioned, off-gassed chloramineshave a strong attraction to the airborne humidity andwill combine with moisture in the air. Consequentlyany condensation of the space humidity will becomecorrosive. It is critical that the space humidity levels becontrolled such that condensation is prevented as it willdamage the building and mechanical system. Serescounits are built for a pool environment. All electrical andrefrigeration components are out of the pool air andin a protective service vestibule. The components incontact with the pool air stream are protected with thebest possible corrosion resistant paints, coatings andmaterials. Seresco units are built to last!By design, indoor pool environments arewarmer and have higher moisture levelscompared to traditional spaces.Engineers and architects must understand theconsequences of this and pay special attention to thespecial requirements on the entire HVAC system andbuilding envelope.10Natatorium Design ManualOutdoor Ventilation AirThe amount of outdoor air to be introduced to the facilityis determined by construction codes. Most codes adoptASHRAE Standard 62. Outdoor air is critical towardsdiluting airborne chemicals and maintaining good indoorair quality.Facilities that introduce outdoor air per ASHRAEStandard 62 and have proper air distribution have thebest IAQ. More outdoor air than required by ASHRAE Standard62 is not recommended (except water parks) Outdoor air requires a significant amount of heatingenergy in the winter and must be included in heatload calculations. Heat recovery should be considered between theexhaust air and outdoor air streams. Exceeding code amounts is not recommended,as it creates extremely high operating expenses.Engineers must ensure condensation is avoided.Wintertime outdoor air is cold and in summerit can be very humid. Both scenarios can leadto condensation. Introduce the outdoor air at the factory providedintakes on the air handlers Locate outdoor air intakes away from sources ofairborne contamination such as exhaust fans orplumbing vents. The outdoor air must be preheated to 65 F – If morethan 20% of the total airflow is outdoor air or if thewinter design temperature is below 10 F A certified air-balancing contractor must balance thesystem airflow.All Seresco units are equipped with an outdoor airconnection, filter and balancing damper. Motorizeddampers and time clocks are also available.Exhaust AirASHRAE recommends the room be maintainedat 0.05-0.15” WC negative pressure relative tosurrounding spaces.Ten percent more exhaust air than outdoor air is a goodrule of thumb.More exhaust air than recommended by ASHRAE willnot reduce or stop moisture migration through thebuilding envelope to outdoors in cold weather. Vapormigrates based on vapor pressure differential. Thereis effectively a 10” WC pressure differential betweenindoors and outdoors on cold winter days. There is noamount of negative airside pressure you can put into aspace to stop vapor migration. The building envelope

must be designed by the architectto adequately deal with this byplacing vapor retarders in appropriatelocations in the entire buildingenvelope.Figure 5 illustrates how the strategiclocation of the exhaust grille can alsosignificantly impr

water heating requirement . in cold climate applications it is important to ensure no more outdoor air be introduced than what the codes require . more is not better in this case as it causes the rH levels can plummet to as low as 20%, costing the operator in dearly in increased air and pool water heating costs .