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Users Information Manual

ENERGY RECOVERY UNITS

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CONTENTS

IMPORTANT SAFETY INFORMATION ……… 4

GENERAL INFORMATION …………………… 5Initial Mechanical Check & SetupAir Seal AdjustmentsWheel-to-Air Seal Clearance

AIR FLOW BALANCING & CHECKING………… 6Controls

ROUTINE MAINTENANCE & HANDLING …… 7Lifting Hole LocationsCleaningWheel Drive Components ……………… 7

INSTALLATION CONSIDERATIONS…………… 7AccessibilityOrientation & Support

OPERATION ……………………………………… 8Start Up ProcedureDiameter Seal AdjustmentHub Seal Adjustment

SERVICE ………………………………………… 9Segment Installation & ReplacementSegment RetainerWheel Drive Motor & Pulley ReplacementBelt Replacement ……………………… 10

DESIGN CONDITIONS & CONTROLSTRATEGIES …………………………………… 11

Standard Temperature ControlFan Only ModeEconomizer ModeCooling ModeHeating Mode

VENTILATION OF OCCUPIED SPACESIN INDUSTRIAL APPLICATIONS ……………… 11

CROSS LEAKAGE IN ERV VENTILATIONSYSTEMS ………………………………………… 12

MOISTURE TRANSFER AND FUNGALGROWTH IN ENTHALPY WHEELS …………… 12

SILICA GEL DESICCANT………………………… 13

ARI PERFOMANCE CERTIFICATION ………… 14

Owner should pay particular attention to the words: NOTE, CAUTION, and WARNING. NOTES areintended to clarify or make the installation easier. CAUTIONS are given to prevent equipment damage.WARNINGS are given to alert owner that personal injury and/or equipment damage may result if installationis not handled properly.

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IMPORTANT SAFETY INFORMATION

ONLY QUALIFIED PERSONNEL SHOULDPERFORM INSTALLATION, OPERATION, ANDMAINTENANCE OF EQUIPMENT DESCRIBED INTHIS MANUAL.

AAON package units are designed for safe operationwhen installed, operated, and maintained within designspecifications, and the instructions set forth in thismanual. It is necessary to follow these instructions toavoid personal injury or damage to equipment orproperty during equipment installation, operation, andmaintenance.

RISK OF DAMAGE, INJURY, AND LOSS OF LIFE- Improper installation, adjustment, alteration,service or maintenance can cause propertydamage, personal injury, or loss of life. A qualifiedinstaller or service agency must performinstallation and service.

WARNING

RISK OF INJURY FROM MOVING PARTS -Disconnect all power before servicing to preventserious injury resulting from automatic starts. Unitmay have multiple power supplies.

WARNING

IMPORTANT!

This equipment is protected by a standard limitedwarranty under the condition that initial installation,service, and maintenance is performed accordingto the instructions set forth in this manual. Thismanual should be read in its entirety prior toinstallation, and before performing any service ormaintenance work.

Units described in this manual are available withmany optional accessories. If you have questionsafter reading this manual in its entirety, consultother factory documentation, or contact your salesrepresentative to obtain further information beforemanipulating this equipment, or its optionalaccessories.

NOTE

RISK OF ELECTRICAL SHOCK -Before attempting to perform any service ormaintenance, turn the electrical power to the unitOFF at disconnect switch(es). Unit may havemultiple power supplies.

WARNING

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GENERAL INFORMATIONThe units are designed as self-contained heating,cooling or combination units using refrigerant, chilledwater, natural or propane gas, electric resistance,steam or hot water as shown on the unit rating plate.

This AAONAIRE® unit has been equipped with anenergy recovery heatwheel. This booklet is furnishedto assure the energy recovery feature will be properlysetup to perform in accordance with the jobspecifications for your particular application.

The AAONAIRE® heatwheel option is designed torecover energy that would normally be lost through theventilation required by today's codes and standards forcomfort and health. The benefits of energy recoveryare significant in that 35 to 40 percent of the unitheating and cooling capacity can be achieved bycollecting this otherwise lost energy from the exhaustair and returning this energy to the building. The costof removing humidity in the summer is also greatlyreduced by the use of the desiccant coating on theenergy wheel.

The Energy Recovery Cassette consists of a frame,wheel, wheel drive system and energy transfersegments. Segments are removable for cleaning orreplacement. The segments rotate through counterflowing exhaust and outdoor air supply streams wherethey transfer heat and/or water vapor from the warm,moist air stream to the cooler and/or drier air stream.This energy recovery process can reduce coolingdesign loads by up to 4 tons per 1000 CFM of outdoorair ventilation while also reducing heating demand andhumidification requirements. Operating savings,reduced demand charges and first cost equipmentsavings provide a rapid payback to the building owner.

The initial set-up and servicing of the heatwheel is veryimportant to maintain proper operating efficiency andbuilding occupant comfort.Normal maintenance requires periodic inspection offilters, the cassette wheel, drive belts, air seals, wheeldrive motor and its electrical connections.Wiring diagrams are provided with each motor. Whenwired according to wiring diagram, motor rotatesclockwise when viewed from the shaft/pulley side.

By carefully reviewing the information within thismanual and following the instructions, the risk ofimproper operation and/or component damage will beminimized.It is important that periodic maintenance be performedto help assure trouble free operation. Shouldequipment failure occur, contact a qualified serviceorganization with qualified, experienced HVACtechnicians to properly diagnose and repair thisequipment.

INITIAL MECHANICAL CHECK & SETUPOutdoor units equipped with outside air intake willhave an outside air hood. The outside air hood mustbe opened prior to unit operation.Remove shipping screws from each side of the hood inthe “closed” position. Lift hood to the “open” position,seal flange, and secure with sheet metal screws.Outdoor air intake adjustments should be madeaccording to building ventilation, or local coderequirements.After the unit installation is complete, open thecassette access door and determine that the energywheel rotates freely when turned by hand. Applypower and observe that the wheel rotates atapproximately 30 RPM. If the wheel does not rotatewhen power is applied, it may be necessary to readjustthe "diameter air seals".

AIR SEAL ADJUSTMENTSPile type air seals across both sides of the energywheel diameter are factory adjusted to provide closeclearance between the air seal and wheel. Racking ofthe unit or cassette during installation, and / ormounting of the unit on a non level support or in otherthan the factory orientation can change sealclearances. Tight seals will prevent rotation.

WHEEL-TO-AIRSEAL CLEARANCETo check wheel-to-seal clearance; first disconnectpower to the unit. In some units the heatwheelassembly can be pulled out from the cabinet to viewthe airseals. On larger units, the heatwheel may beaccessible inside the walk-in cabinet.A business card or two pieces of paper can be used asa feeler gauge, (typically each .004" thick) by placing itbetween the face of the wheel and the pile seal.Using the paper, determine if a loose slip fit existbetween the pile seal and wheel when the wheel isrotated by hand.

To adjust air seal clearance, loosen all seal plateretaining screws holding the separate seal retaining

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plates to the bearing support channels and slide theseal plates away from the wheel. Using the paperfeeler gauge, readjust and retighten one seal plate at atime to provide slip fit clearance when the wheel isrotated by hand.Confirm that the wheel rotates freely. Apply power tothe unit and confirm rotation.

AIRFLOW BALANCING & CHECKINGHigh performance systems commonly have complexair distribution and fan systems. Unqualified personnelshould not attempt to adjust fan operation, or aircirculation, as all systems have unique operatingcharacteristics. Professional air balance specialistsshould be employed to establish actual operatingconditions, and to configure the air delivery system foroptimal performance.

ControlsA variety of controls and electrical accessories may beprovided with the equipment. Identify the controls oneach unit by consulting appropriate submittal, or orderdocuments, and operate according to the controlmanufacturer’s instructions. If you cannot locateinstallation, operation, or maintenance information forthe specific controls, then contact your salesrepresentative, or the control manufacturer forassistance.

ROUTINE MAINTENANCE & HANDLINGHandle cassettes with care. All cassettes should belifted by the bearing support beam. Holes are providedon both sides of the bearing support beams to facilitaterigging as shown in the following illustration.

Lifting Hole Locations

Routine maintenance of the Energy RecoveryCassettes includes periodic cleaning of the EnergyRecovery Wheel as well as inspection of the Air Sealsand Wheel Drive Components as follows:

CleaningThe need for periodic cleaning of the energy recoverywheel will be a function of operating schedule, climateand contaminants in the indoor air being exhaustedand the outdoor air being supplied to the building.The heatwheel is “self-cleaning” with respect to dryparticles due to its laminar flow characteristics. Smallerparticles pass through; larger particles land on thesurface and are blown clear as the flow direction isreversed. Any material that builds up on the face of thewheel can be removed with a brush or vacuum. Theprimary need for cleaning is to remove oil basedaerosols that have condensed on energy transfersurfaces.

A characteristic of all dry desiccants, such films canclose off micron sized pores at the surface of thedesiccant material, reducing the efficiency by whichthe desiccant can adsorb and desorb moisture andalso build up so as to reduce airflow.

In a reasonably clean indoor environment such as aschool or office building, measurable reductions ofairflow or loss of sensible (temperature) effectivenessmay not occur for several years. Measurable changesin latent energy (water vapor) transfer can occur inshorter periods of time in applications such asmoderate occupant smoking or cooking facilities. Inapplications experiencing unusually high levels ofoccupant smoking or oil based aerosols such asindustrial applications involving the ventilation ofmachine shop areas for example, annual washing ofenergy transfer may be necessary to maintain latenttransfer efficiency. Proper cleaning of the energy

Do not alter factory wiring. Deviation from thesupplied wiring diagram will void all warranties,and may result in equipment damage or personalinjury. Contact the factory with wiringdiscrepancies.

WARNING

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recovery wheel will restore latent effectiveness to nearoriginal performance.

To clean, gain access to the energy recovery wheeland remove segments. Brush foreign material from theface of the wheel. Wash the segments or small wheelsin a 5% solution of non-acid based coil cleaner oralkaline detergent and warm water.

Soak in the solution until grease and tar deposits areloosened (Note: some staining of the desiccant mayremain and is not harmful to performance). Beforeremoving, rapidly run finger across surface of segmentto separate polymer strips for better cleaning action.Rinse dirty solution from segment and remove excesswater before reinstalling in wheel.

CAUTION !Do Not use acid based cleaners, aromaticsolvents, steam or temperatures in excess of170°F; damage to the wheel my occur!

Air SealsFour adjustable diameter seals are provided on eachcassette to minimize transfer of air between thecounter flowing airstreams.

To adjust diameter seals, loosen diameter sealadjusting screws and back seals away from wheelsurface. Rotate wheel clockwise until two opposingspokes are hidden behind the bearing support beam.Using a folded piece of paper as a feeler gauge,position paper between the wheel surface anddiameter seals.

Adjust seals towards wheel surface until a slightfriction on the feeler gauge (paper) is detected whengauge is moved along the length of the spoke.Retighten adjusting screws and recheck clearancewith “feeler” gauge.

Wheel Drive ComponentsThe wheel drive motor bearings are pre-lubricatedand no further lubrication is necessary.The wheel drive pulley is secured to the drive motorshaft by a combination of either a key or D slot and setscrew.The set screw is secured with removable locktite toprevent loosening. Annually confirm set screw issecure. The wheel drive belt is a urethane stretchbelt designed to provide constant tension through thelife of the belt. No adjustment is required. Inspect thedrive belt annually for proper tracking and tension. Aproperly tensioned belt will turn the wheel immediatelyafter power is applied with no visible slippage duringstart-up.

INSTALLATION CONSIDERATIONSAAONAIRE® Energy recovery cassettes areincorporated within the design of packaged units,packaged air handlers and energy recoveryventilators. In each case, it is recommended that thefollowing considerations be addressed:

AccessibilityThe cassette and all its operative parts; i.e.: motor,belt, pulley, bearings, seals and energy transfersegments must be accessible for service andmaintenance. This design requires that adequateclearance be provided outside the enclosure.Where cassettes are permanently installed in acabinet, access to both sides of the cassette must beprovided.

Orientation & SupportThe Energy Recovery Cassette may be mounted inany orientation. However, Care must be taken tomake certain that the cassette frame remains flatand the bearing beams are not racked.To verify, make certain that the distance betweenwheel rim and bearing beam is the same at each endof the bearing beam, to within 1/4 of an inch(dimension A & B). This amount of racking can becompensated for by adjusting the diameter seals.

If greater than 1/4 inch, racking must be correctedto ensure that drive belt will not disengage fromwheel.

Avoid Racking Of Cassette Frame

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OPERATIONCAUTION !Keep hands away from rotating wheel!!Contact with rotating wheel can cause physicalinjury.

Start Up Procedure1. By hand, turn wheel clockwise (as viewed from thepulley side), to verify wheel turns freely through 360ºrotation.

2. Before applying power to drive motor, confirm wheelsegments are fully engaged in wheel frame andsegment retainers are completely fastened.(See Segment Installation Diagram).

3. With hands and objects away from moving parts,activate unit and confirm wheel rotation. Wheel rotatesclockwise (as viewed from the pulley side).

4. If wheel has difficulty starting, turn power off andinspect for excessive interference between the wheelsurface and each of the four (4) diameter seals. Tocorrect, loosen diameter seal adjusting screws andback adjustable diameter seals away from surface ofwheel, apply power to confirm wheel is free to rotate,then re-adjust and tighten hub and diameter seals, asshown in hub seal adjustment diagram.

5. Start and stop wheel several times to confirm sealadjustment and to confirm belt is tracking properly onwheel rim (approximately 1/4” from outer edge of rim).

Diameter Seal Adjustment

Hub Seal Adjustment

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SERVICE

CAUTION !Disconnect electrical power before servicingenergy recovery cassette.Always keep hands away from bearing supportbeam when installing or removing segments.Failure to do so could result in severe injury tofingers or hand.

Segment Installation & ReplacementWheel segments are secured to the wheel frame by aSegment Retainer which pivots on the wheel rim andis held in place by a Segment Retaining Catch.

Segment Retainer

To install wheel segments follow steps one throughfive below. Reverse procedure for segment removal.

1. Unlock two segment retainers (one on each side ofthe selected segment opening.2. With the embedded stiffener facing the motor side,insert the nose of the segment between the hubplates.

Segment Installation

3. Holding segment by the two outer corners, press thesegment towards the center of the wheel and inwardsagainst the spoke flanges. If hand pressure does notfully seat the segment, insert the flat tip of a screwdriver between the wheel rim and outer corners of thesegment and apply downward force while guiding thesegment into place.4. Close and latch each Segment Retainer underSegment Retaining Catch.5. Slowly rotate the wheel 180º. Install the secondsegment opposite the first for counterbalance. Rotatethe two installed segments 90º to balance the wheelwhile the third segment is installed. Rotate the wheel180º again to install the fourth segment opposite thethird. Repeat this sequence with the remaining foursegments.

Wheel Drive Motor & Pulley Replacement1. Disconnect power to wheel drive motor.2. Remove belt from pulley and position temporarilyaround wheel rim.3. Loosen set screw in wheel drive pulley using a hexhead wrench and remove pulley from motor driveshaft.4. While supporting weight of drive motor in one hand,loosen and remove (4) mounting bolts.5. Install replacement motor with hardware kitsupplied.6. Install pulley to dimension as shown and secure setscrew to drive shaft.7. Stretch belt over pulley and engage in groove.8. Follow start-up procedure.

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Belt Replacement1. Obtain access to the pulley side bearing accessplate if bearing access plates are provided. Removetwo bearing access plate retaining screws and theaccess plate.2. Using hexagonal wrench, loosen set screw inbearing locking collar. Using light hammer and drift (indrift pin hole) tap collar in the direction of wheelrotation to unlock collar. Remove collar.3. Using socket wrench with extension, remove twonuts which secure bearing housing to the bearingsupport beam. Slide bearing from shaft. If notremovable by hand, use bearing puller.4. Form a small loop of belt and pass it through thehole in the bearing support beam. Grasp the belt atthe wheel hub and pull the entire belt down.

Note: Slight hand pressure against wheel rim willlift weight of wheel from inner race of bearing toassist bearing removal and installation.

CAUTION !Protect hands and belt from possible sharp edgesof hole in Bearing Support Beam.

5. Loop the trailing end of the belt over the shaft (beltis partially through the opening).6. Reinstall the bearing onto the wheel shaft, beingcareful to engage the two locating pins into the holesin the bearing support beam. Secure the bearing withtwo self locking nuts.7. Install the belts around the wheel and pulleyaccording to the instructions provided with the belt.8. Reinstall diameter seals or hub seal and tightenretaining screws. Rotate wheel in clockwise directionto determine that wheel rotates freely with slight dragon seals.9. Reinstall bearing locking collar. Rotate collar byhand in the direction the wheel rotates (see labelprovided on each cassette for wheel rotation).10. Lock in position by tapping drift pin hole withhammer and drift. Secure in position by tightening setscrew.11. Reinstall Bearing Access Cover.12. Apply power to wheel and ensure that the wheelrotates freely without interference.

Belt Replacement

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DESIGN CONDITIONS & CONTROL STRATEGIESStandard temperature controlThe unit can be configured with normal air flows andcontrols but still have the benefit of a large amount ofmakeup air, better humidity control and loweroperating cost than a unit without a heat wheel. Theenergy recovery unit operates in four (4) basic modes;fan only; economizer; cooling and heating. Each ofthese modes has specific functions as defined below.

Fan only mode: When the unit supply fan is started,and there is no call for cooling or heating, the uniteconomizer moves to its minimum position, theheatwheel is activated and the heatwheel fan isstarted. If the unit is equipped with heatwheel bypassdampers, these are closed.

Economizer mode: With the unit supply fan inoperation and a call for cooling is made, if the outdoorair temperature and humidity are below the enthalpysetpoint, the heatwheel exhaust fan is activated, theheatwheel is deactivated and the economizermodulates to maintain the mixed air setpoint. If the unitis equipped with heatwheel bypass dampers, theseare opened to accommodate the increase in outsideair volume.

Cooling mode: With the unit supply fan in operationand a call for cooling is made, if the outdoor airtemperature and humidity are above the enthalpysetpoint, the economizer moves to its minimumposition and mechanical cooling is activated. Theheatwheel is activated and the heatwheel exhaust fanis started. If the unit is equipped with heatwheelbypass dampers, these are closed.

Heating mode: Upon a call for heat, the heatingfunction is activated, the supply fan is activated andthe economizer moves to its minimum position. Theheatwheel is activated and the heatwheel exhaust fanis started. If the unit is equipped with heatwheelbypass dampers, these are closed.

Notice that in all four (4) basic above modes, theoperation of the heatwheel is determined by theposition of the economizer. With the exception of unitshutdown or a night setback mode, the heatwheelexhaust fan is in operation.When control systems are "by others", all of the abovemodes of operation must be considered.

Ventilation of Occupied SpacesIn Industrial ApplicationsGeneral ventilation of occupied spaces in Industrialfacilities is an excellent application for energyrecovery. It can have many significant benefitsincluding: odor control, a better working environmentfor employees, higher productivity, reduced risk fromexposure to volatile compounds and particulates in theindoor air, improved humidity control (for process andpeople) and reduced energy costs to condition theventilation air. General ventilation with energy recoveryis not a substitute for fume hood exhaust. The successof the industrial application depends on proper designand an understanding of the performancecharacteristics of the enthalpy wheel.

Energy recovery wheels or enthalpy wheels havesome inherent exhaust air transfer due to the volumeof air carried by wheel rotation from one airstream tothe other. In addition, while wheels are highly resistantto fouling due to the counter flowing airflowarrangement, they can be plugged by large amounts ofsemi-volatile compounds or aerosols, which areallowed to impinge and/or condense on the wheelsurfaces. These characteristics affect the installationand application as follows:

1. Use energy recovery for general dilutionventilation of the occupied space, not forrecovering energy from dedicated, highlyconcentrated or toxic exhaust.Exhaust air transfer in the energy recovery systemresults in a small amount of the exhaust air, typicallyless than 5% for wheels operating in balanced flow,returning to the space. This amount of exhaust airtransfer is appropriate to handling general exhaust inan environment where continuous exhaust and supplyof outdoor air to the space achieves the requireddilution of contaminants. In space conditioningapplications, where the ventilation system is operatingto maintain acceptable indoor air quality, there shouldnot be contaminants in concentrations of concern. It isnot appropriate for recovering energy from highlyconcentrated machine exhaust, such as hoodsinstalled on the print heads themselves. Even a smallamount of exhaust air transfer in this case canincrease contaminants and odors in the space. This airis best exhausted directly outdoors and treated as maybe required by local code. If energy recovery is desiredin these environments, a “run around loop” approach issuggested.

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2. Take “return” air (air to be exhausted afterrecovering energy from it) from the occupied zone,not from areas containing a high concentration ofdusts or aerosols such as the hood.If necessary, provide supplemental filtration of thereturn air at the inlets to the duct system. The goal ofthe dilution ventilation is to preserve a healthful andcomfortable environment in the breathing zone. Supplyand return diffusers and grilles should be located toachieve this end. Ceiling returns located directly abovemachinery can provide additional benefits by directingcontaminants away from operators. In the industrialapplication this air may contain high levels of aerosols,which, once deposited and dried, would be difficult orimpossible to clean from ductwork, fans, dampers andwheels. Therefore a filter of appropriate efficiency isrecommended to be installed at the inlet or “returngrille”.

Experience in industrial applications from smallfacilities to large factories has shown that when thesetwo recommendations are observed, successfulapplication of energy recovery and its attendantbenefits is the result. On the other hand, ignoringthese common sense rules can result in reducedsatisfaction and/or equipment damage and amaintenance challenge.

Cross Leakage in EnergyRecovery Ventilation SystemsThe issue of cross leakage in rotary wheel basedEnergy Recovery used in space conditioningapplications is often misunderstood. As a result, manysystems are installed with purge sectors and theadditional fan capacity required to allow these sectorsto function when in fact they are unnecessary.Understanding the rationale for the purge sector, itshistory, its added first cost, and the associatedcontinuing cost of operation, the designer will rarelyspecify purge.

A purge sector minimizes the carry over cross leakagefrom exhaust into the supply airstream by shunting aportion of the supply air back into the exhaustairstream across the seal separating the exhaust andsupply. This is required for industrial processapplications where the exhaust contains contaminantswhich would be detrimental to the process.(Historically, heat wheels have been used primarily fordehumidification and process heat recovery.) Thevolume of air required for effective purge is listed at10% to 20% of rated flow by manufacturers ofindustrial process wheels. In addition to the cost ofproviding the sector, the system must move 10 to 20%more air than is required by the application in order topurge.

By contrast, in space conditioning applications, wherethe ventilation system is operating to maintainacceptable indoor air quality, there should be nocontaminants in concentrations of concern. Crossleakage in the energy recovery system results in asmall amount of the exhaust air, typically less than 5%for wheels operating in balanced flow, returning to thespace from which it came. This is not “contamination”as it is often labeled. It is air that effectively never leftthe space. The operating cost of moving this air is farless than that required to operate purge sector.

This amount of cross leakage is appropriate tohandling bathroom exhaust in an environment wherecontinuous exhaust of the restroom achieves an airquality on a par with the adjacent space. It is notappropriate for recovering energy from toxicenvironments, laboratory fume hoods, operatingrooms, etc. These are not recommended applicationsfor rotary based technology without a purge sector.

In fact, many of these environments should nottolerate any cross leakage and as such should notutilize rotary technology as even well designed purgesectors do not achieve zero cross leak.If energy recovery is required in these environments, a“run around loop” approach is suggested.

The adjustable mechanical purge is capable ofreducing cross leakage to a fraction of one percent.Nevertheless, purge should only be specified based onan engineering evaluation of the cost to provide, thecost to operate and the specific needs of theapplication.

Moisture Transfer and Fungal Growth in DesiccantBased Enthalpy WheelsThere is evidence that fungi germinate when watercondenses onto surfaces of air handling systemswhere nutrients are present. Surfaces which remainwet for a period of 12 to 24 hours allow fungi and moldspores already present to “bloom”, resulting in apotential IAQ problem.

This knowledge has led to questions of whetherdesiccant energy recovery ventilation wheels, which infact transfer water from one airstream to another,could provide a medium for growth of mold and fungi.Such is not the case for AAONAIRE® technology, norhas it been reported in the literature for other enthalpywheels.

In silica gel based desiccant wheels, the watermolecules are transferred by sorption, individually,onto and off of the silica gel surface. Water is presenton the wheel in a molecular layer only. Condensationdoes not occur. AAONAIRE® desiccant wheelsexperience “dry” moisture transfer in that there is no

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bulk liquid water present which could support fungalgrowth or dissolve other chemical species. Thetransfer of water onto and off of the wheel’s desiccantsurfaces occurs in the vapor or gas phase. There areno “wet” surfaces and liquid water does not enter theair stream.

The sensible (non-desiccant coated) wheel can alsotransfer water through the different mechanism ofcondensation and re-evaporation, however; again,there is no accumulation of water, unless the frostingthreshold is violated through misapplication of thecomponent. In this case, the water is in the form offrost or ice which does not support fungal growth.Sensible (uncoated) wheels from all manufacturers areidentical in this regard.

Both moisture and nutrients are required to supportfungal growth. Therefore dirt accumulation on heatwheels is of potential concern. It is also true that anyheat wheel can accumulate semi-volatile compoundslike tars and grease which are deposited on surfaces.These surfaces can then become odor andcontaminant sources, in the same way that a filter orany other element of an air handling system canbecome a source of compounds accumulated overtime.

The heatwheel was designed to respond to theseissues over the life of the system by providing forcleaning and maintenance with washable desiccantsurfaces, removable segments and easy to accesscassettes. Many aspects of this technology arepatented and are unique in the industry.

Silica Gel DesiccantSilica gel is an inert, highly porous solid adsorbentmaterial that structurally resembles a rigid sponge. Ithas a very large internal surface composed of myriadmicroscopic cavities and a vast system of capillarychannels that provide pathways connecting theinternal microscopic cavities to the outside surface ofthe “sponge”.

The characteristic curve for adsorption of water onsilica gel is shown in Figure 1 (page 13), as % weightadsorbed versus relative humidity of the air stream incontact with the silica gel. The amount of wateradsorbed rises almost linearly with increasing relativehumidity until RH reaches about 60%. It then plateausout at about 40% adsorbed as relative humidityapproaches 100%. (The curve for molecular sieves, bycontrast, rises rapidly to plateau at about 20%adsorbed at 20% relative humidity. This helps toexplain why the molecular sieve is an excellent choicefor regenerated applications such as desiccant coolingand dehumidification systems which are designed toreduce processed airstreams to very low relative

humidity. On the other hand, silica gel has superiorcharacteristics for the recovery of space conditioningenergy from exhaust air.)

The use of silica gel on rotary regenerators for energyrecovery ventilation applications involves a processcycle where the silica gel is alternately exposed toairstreams having nearly equal relative humiditysomewhere in the mid range of this curve (typicallybetween 40 and 60%). When the air stream with thehigher relative humidity passes over the silica gelcoated wheel, moisture is adsorbed from the airstream into the silica gel. Then when the air streamwith the lower relative humidity contacts the silica gel,moisture is desorbed (removed) from the silica gel andput into the air stream.

In this ventilation energy recovery application, thesilica gel has all of its surface area covered with atleast a monomolecular layer of water because it has agreater affinity for water than any other chemicalspecies. With all of the adsorption sites occupied bywater, the silica gel will not be able to transfer otherchemical species by adsorption and desorption in itsnormal form. Species that are soluble in water couldbecome dissolved in the adsorbed water and thenreleased when the water is desorbed but this processis limited by kinetics and does not present a veryefficient mechanism for contaminant transfer.

An example of this phenomenon is formaldehyde, agas which is very highly soluble in water.In the early 1980’s when energy recovery ventilatorswere being used to mitigate excessive formaldehydelevels in mobile homes, concern was expressed bysome people that enthalpy type heat exchangers thattransferred moisture as well as heat might alsotransfer excess amounts of formaldehyde gas due toits high solubility in water. Accordingly, tests wereconducted by the Lawrence Berkeley Laboratories ofthe U.S.D.O.E., on two enthalpy type exchangers todetermine whether this suspicion was justified. Resultswere presented in ASHRAE paper No. CH85-03 No. 3which reported that the rotary type enthalpy heatexchanger transferred formaldehyde with only 3-6%efficiency. They concluded that “formaldehyde transferbetween airstreams by processes other than airleakage does not seriously compromise theperformance of these enthalpy exchangers”.

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ARI Performance CertificationThe certified ratings program requires testing, ratingand independent verification of componentperformance at standard conditions and rated flow.Testing is in accordance with ASHRAE Standard 84.

ARI certified ratings include very complete information,some of it previously unavailable, to allow designers tofully characterize thermal and airflow performance. Inaddition to separate sensible, latent, and totaleffectiveness at two airflows for both summer andwinter test conditions, the standard requiresinformation on pressure loss as well as air leakage.Airxchange publishes ARI certified ratings for allenergy recovery ventilation components of theirmanufacture in accordance with the requirements ofthe ARI program. These ratings may be found on theARI website www.ari.org. Application ratings areprovided for the complete range of airflows and allAirxchange cassettes bear the ARI Certification Seal.With the ARI industry performance certificationprogram in place, engineers and buildingowners/operators no longer need accept selfcertification. It is important to point out that ratingsfrom non-participating manufacturers are difficult tocompare regardless of whether they are tested inhouse or by an “independent” testing agency. This is inpart because the latent performance of a given unitcan change significantly when tested at differentoutdoor air conditions and at less than rated airflows.A product can be made to look better by testing it“independently” at an easier condition. Lower relativehumidity (lower wet bulb) and less than rated airflowimproves the tested performance.

Also, self-certification does not include the necessaryperiodic verification tests and challenge proceduresprovided by the industry certification program.Specifications requiring ARI Certification inaccordance with the latest revision of ARI Standard1060 provide the best assurance that components andsystems will perform as designed.

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AAON, Inc.2425 S. YukonTulsa, Oklahoma 74107

Tel 918-583-2266Fax 918-583-6094

Download this manual,and others from:www.aaon.com

It is the intent of AAON to provide accurate and current specificationinformation. However, in the interest of product improvement, AAON,Inc. reserves the right to change pricing, specifications, and/or designof its products without notice, obligation, or liability.

AAON is a registered trademark of AAON, Inc.

Effective August 2006Supercedes August 1998

R86610 (Rev. A 8-06)