New Ventilation Standards for IAQ

CII-Godrej GBC 13

Green Business Directory Green Buildings

WHAT IS IAQ?

The world focus has shifted from theenvironment to ‘Invironment’. This is

a new terminology, being usedincreasingly to focus on the Indoor AirQuality (IAQ) and its effect on humanhealth. While the outdoor environmentcontinues to be of concern, the indoorenvironment is receiving increasedattention as more information has becomeavailable on the presence and effect ofindoor contaminants.

Indoor air quality, as defined byASHRAE, is that which provides acceptablecomfort level to 80 percent of the peopleexposed to it.

The origins of poor IAQ issue are wellknown. An emphasis on energyconservation after the oil embargo of 1970sresulted in tighter buildings with re-circulated air for building ventilation andminimum amounts of fresh air beingbrought into commercial buildings. Thisminimized the amount of air to be heatedor cooled and hence conserved on energy.

However, the combination of “tight”buildings with little or inadequate fresh airventilation produced an indoorenvironment with relatively high levels ofchemical contaminants, bacteria, fungi anddust. It is a well-recognized fact now, thatindoor air in an air-conditioned/mechanically ventilated space can beseveral times more polluted than outdoorair. The larger concentration of indoor airpollutants, combined with the fact thatmost people spend 85 to 90% of their timeindoors, make them susceptible toillnesses related to these airbornecontaminants.

Pollutants contributing to poor IAQ

Sulphur, nitrogen dioxide, carbonmonoxide produced by combustion andemission, high pollen counts, pesticides,chemical compounds, all contribute tooutdoor pollution. Indoor air will contain allof the pollutants of the outdoor air as wellas those generated indoors by theoccupants and their activities.

New ventilation standards for Indoor Air Quality (IAQ) vs. energyconservation: enthalpy wheels meet the challenge

Deepak PahwaB.Sc.Engg.Member ASHRAE

Absttact: In recent years, the attentionof environ-mental researchers has beenfocused on indoor air pollution, as aresult of reports of symptoms orspecific diseases that occur mainly inair-conditioned and mechanicallyventilated buildings. Studies haveproved that level of contaminants in theindoor air can be often several timeshigher than outdoor air. This combinedwith the fact that people tend to spend90% of their time indoors, proves thepoint that a person’s major source ofexposure to airborne contaminants canbe indoors.

Poor indoor air quality leads to anincreased incidence of health relatedsymptoms, which in turn can lead toan increase in absenteeism and a lossof productivity.

“The solution to the problem of pollutionis dilution” or increased ventilation,runs contrary to the energyconservation guidelines being followed

by air-conditioning designers for thebuildings. However new standardsand guidelines being dictated byASHRAE standard 62-1989 for IAQ,establishing generally 20 cubic feetper minute (cfm) per person asrecommended outdoor air requirement,has set new challenges for equipmentmanufacturers to meet the needs ofthe building owners and designers formatching the IAQ requirements withenergy conservation needs.

This paper examines the above issuesin greater detail and also how ‘enthalpywheels’ have effectively provided thesolution for improving IAQ by curtailingenergy costs.

The advancement in the manufactureof enthalpy wheels and the resultantstrides in efficiency and features arediscussed alongwith the methods ofphysical integration of the “wheels”into the HVAC system.

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The indoor air contaminants which canbe hazardous to health includeEnvironmental Tobacco Smoke (ETS),formaldehyde, radon, asbestos, VOCsemanating from solvents, paints,varnishes, carpets causing long term andshort term illnesses. Biologicals likebacteria, viruses, fungus due to presenceof high humidity, directly affect the healthof the occupants. Odours and dust cancause significant discomfort, feelings ofunpleasantness.

In a conditioned space, since freepassage of air is limited, pollutants tend toaccumulate resulting in higherconcentration of some contaminants thanoutdoor ambient air. Most of the pollutantsthat we find indoors can be sourced tocommonly found items around us.

Air contaminants can be classifiedinto three (3) broad groups

Contaminants Sources• Gases & vapour Human beings

- CO2 Cigarette Smoke- Butyric Acid Road & highways- Carbon monoxide Adjacent parking

lots and garages- Nitrogen dioxide Industrial area- VOCs Paints, wood panelling,

office equipment, airfresheners, cleaningagents, pesticide sprays.

• Inert particles Man made fibres, dust,etc.

• Microorganism Damp corners,- Fungus Behind insulation,- Bacteria Virus. Under carpets.- Mold Evaporative/desert/swamp

coolers, cooling towers,air washers, humanbeings.

The consequences of poor Indoor AirQuality in a work environment can betwofold:

1. The effect on the health of theindividual

2. The subsequent or related economiceffect by loss of productivity andincreased absenteeism.

“The solution to pollution isdilution”!

There are two basic solutions tomitigate the unacceptable levels ofairborne pollutants in the workplace:addressing the source of pollution andaddressing the level of contaminants inthe air. These may be referred to as ‘sourcecontrol’ and ‘removal’ respectively.

Source control, though the preferredapproach, may not be often practical.Source control measures are pollutantspecific and may include use of lowformaldehyde emitting materials, banningof cigarette smoking, prevention of radonentry through sealing of foundations,eliminating use of asbestos and storing ofpaints and chemicals outside the occupiedspace. Controlling relative humidity willprevent microbial contamination.

Removal of contaminants from abuilding or reducing its concentrationwithin a work space can be accomplishedby passive or active ventilation.

Passive ventilation refers to airexchanged through doors, windows orother openings by natural forces. In mostair-conditioned buildings, these openingshave been reduced to the minimum toconserve energy.

Active ventilation systems providecontinuous ventilation to which passiveventilation may add but not subtract whenpollutants are evenly mixed throughout aspace and the source rate is constant; theconcentration of airborne pollutants will beinversely proportional to the ventilationrate, that is, doubling the ventilation willhalve the concentration!

An existing ventilation system that isinadequate because of design flaws, poormaintenance or expanded use of a buildingis often associated with poor indoor airquality. Mitigation can often requireredesign or maintenance. In cases wherethe outdoor air ventilation provision of an

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The shift of focus to address totalindoor environmental quality needs of

offices and workplaces to include higherventilation and fresh air needs along withother issues like ergonomics, light, noise,decoration, and ambience has forced worldbodies such as ASHRAE to relook at theprevailing standards. In 1983, ASHRAEauthorised an early review of their standard62-1981 on ventilation. The requiredstandard 62-1989 recommended fresh airintake of 15 to 20 cfm per person where 5cfm was considered adequate by theindustry. CO2 levels, which have beenrecognised by ASHRAE as the surrogateventilation index (being the onlyeconomically and practically measurablevariable), should not exceed 1000 ppm.

The following are some of thestandards in force, or underimplementation, for ventilation rates forbuildings.

HVAC system is not being used, theremedy is obvious - increase ventilation.

Regulatory Body

OSHA

ASHRAE

NBCC

Country

US

US

Canada

Sweden

France

Japan

Standard

29 cfr1910.1033(Proposed)

62-1989

Ventilation

15 cfm/person7 people/1000ft2for office

0.5 AC/HR

0.5 AC/HR

0.5 AC/HR

15 cfm / person

CO2 (not exceeding)

800 ppm

1000 ppm

1000 ppm

1000 ppm

1000 ppm

1000 ppm

Regulations and Guidelines pertaining to IAQ

Codes for new homes / construction

Various organizations have established recommended levels for CO2 concentrations inindoor spaces.

The Dilution Principle :concentration of pollution is inverselyproportional to ventilation rate

Air changes per hour

ASHRAE 62-1989 IAQ standard:“ventilation for acceptable Indoor airquality”.

Conc

entra

tion

of p

ollu

tant

00.50 1.0 2.0 3.0tight

buildingaveragebuilding

leakybuilding

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• Maintain space humidity between 30%- 60% RH all year.

• Curtail peak electrical demand charges.

• Reduce or eliminate the use of CFCs.

The solution options: Energyrecovery devices

As market needs for control ofhumidity, energy, IAQ, continue to rise, itis imperative to integrate heat/energyrecovery devices to airconditioning designto keep all these requirements in mind.

ASHRAE equipment handbook 1988refers to six types of air to air heatexchange devices. There are some whichare sensible only and some are total heatexchangers (sensible and latent heat orenthalpy). The twin tower loop is a totalheat exchanger. The rotary exchanger orheat wheel can be either a sensible only ora total heat device. The rest are essentiallysensible heat exchangers in which transferof latent heat, if any, is incidental.

Types of Recovery Devices1. Rotary Energy Exchangers

2. Coil Energy Recovery Loop

3. Twin-Tower Enthalpy Recovery Loop

4. Heat Pipe Heat Exchangers

5. Fixed Plate Exchangers

6. Thermosyphon Heat Exchangers

Reproduced below are the recommended ventilation rates under the ASHRAE 62-89standard.

Ventilation VentilationApplication Rate/person Application Rate/person

Office space 20 cfm Smoking lounge 60 cfm

Restaurants 20 cfm Beauty salon 25 cfm

Bars/Cocktail 30 cfm Supermarkets 15 cfm

Hotel rooms 30 cfm/room Auditorium 15 cfm

Conference rooms 20 cfm Classrooms 15 cfm

Hospital rooms 25 cfm Laboratory 20 cfm

Operating rooms 30 cfm General retail 15 cfm

Source : ASHRAE Standard 62-1989

Increased ventilation standard vsenergy management: The challenge

As the recommended levels of outsideair brought into conditioned space hasbeen increased by 4 times (to 20 cfm from5 cfm per person), much higher latent andsensible loads are imposed on the cooling/heating equipment. This translates in twoways : (1) an improved indoorenvironment, and, (2) significantly higherutility bills for the owners.

Introduction of even a small quantityof air into an HVAC system raises physicalplant requirements dramatically, bringingto fore a new dimension of balancingenergy needs with the IAQ standard. In factthe HVAC designers are faced with severalparameters which need to be incorporatedin response to the regulations and guidlineslaid down by market needs.

HVAC System “Wish List” for the’90s• Efficiently handle increased outdoor air

percentage (20 vs. 5 CFM/person) withhumidity control.

• Minimise first cost, operating andmaintenance costs.

• Decouple the outdoor air load so thatconventional packaged HVACequipment can be used effectively.

• Retrofit into existing system design.

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The ability to transfer both sensible andlatent heat makes the enthalpy wheel farmore effective in energy recovery.

It is found that the total heat recoverydevice typically recovers nearly threetimes as much energy as the sensible heatrecovery device.

The chart below compares typicaleffectiveness and pressure drop data fordifferent recovery devices.

It is seen that the enthalpy wheel hasthe highest effectiveness and leastpressure drop at any face velocity, makingit the most appropriate choice for energyrecovery in comfort ventilation.

Enthalpy wheels: The best optionsfor IAQ enhancement

The enthalpy wheel is a cylinder,usually 4 to 10 inches deep, packed with aheat transfer medium that has numeroussmall air passages, or flutes, parallel to thedirection of airflow. The flutes are triangularor semi-circular in cross-section. Thestructure, commonly referred to as thehoneycomb matrix, is produced byinterleaving flat and corrugated layers of ahigh conductivity material, usuallyaluminium, surfaced with a desiccant.Stainless steel, ceramic, and synthetic

materials may be used, instead ofaluminium, in specific applications. Theflutes in most wheels measure between1.5 mm to 2.0 mm in height. The surfacearea exposed to airflow in a wheel liesbetween 300 to 3300 m2/m3, dependingupon the configuration.

In a typical installation, the wheel ispositioned in a duct system such that it isdivided into two half moon sections. Staleair from the conditioned space is exhaustedthrough one half while outdoor air is drawnthrough the other half in a counter flowpattern. At the same time, the wheel isrotated slowly (2 to 20 RPM). Sensible heatis transferred as the metallic substrate picksup and stores heat from the hot air streamand gives it up to the cold one. Latent heatis transferred as the medium condensesmoisture from the air stream that has thehigher humidity ratio through adsorptionby the desiccant (with a simultaneousrelease of heat) and releases the moisturethrough evaporation (and heat pick up) intothe air stream that has the lower humidityratio.

The psychrometrics of recovery isexplained in the figure below. In simplesensible recovery (Figure A), cold air isheated from 1 to 2 while hot air is cooled

Performance compar isonEnthalpy Wheels vs Other Recovery Devices

EFFECTIVENESS(%)

PRESSURE LOSS(IN WG.)

90

80

70

60

50

300 400 500 600 700 800 900 1000

0.5

1.0

1.5

2.0

2.5

EFFECTIVENESS PRESSURE LOSS

FACE VELOCITY (FPM)

HEAT WHEELCOIL TYPE

HEAT WHEELCOIL TYPE

PLATE TYPE HEAT PIPE

PLATE TYPE

HEAT PIPE

TWIN TOWER

1 2

34

12

34

Dry Bulb Temperature


Figure A: Sensible Heat Recovery

Figure B: Sensible Heat Exchanger recovering latent heat

Hum

idity

Rat

ioHu

mid

ity R

atio

1

2 3

4


Figure C: Total Heat Recovery

Hum

idity

Rat

io

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from 3 to 4. In this case, the cold airtemperature is above the dew point of thehot air and no condensation takes place inthe media.

Figure B illustrates a sensible processin which condensation occurs in the hot airstream along with evaporation in the coldone. In this case, latent heat transferenhances the overall wheel effectiveness.Figure C depicts the total heat recoveryprocess of the enthalpy wheel, assumingmass flow rates in the air streams are thesame, and latent and total heateffectiveness are equal.

A historical perspective

Use of rotary heat exchangers incomfort air conditioning dates back to midfifties with folded wire mesh pads. Thesedevices were essentially sensible heatdevices. Wheels with the familiarhoneycomb matrix were introduced in themid sixties. The medium was asbestospaper impregnated with lithium chloride.Due to inherent absorption properties ofasbestos and lithium chloride these rotorshad a short life and in the late seventiesasbestos was replaced by kraft paper;however, lithium chloride continued toremain the preferred desiccant due to itsease of impregnation of media.

In the mid seventies, two new enthalpywheel models hit the market and continueto be offered till date. The oxidisedaluminium wheels offered by somemanufacturers, has corrugated aluminiumfoil wound on a mandrel and braced by steelstrips on the sides. The assembly is dippedinto a bromide solution to cause thealuminium to oxidise and form a layer ofalumina - a known desiccant. Such wheelshave heat transfer characteristicscomparable to the others at a lower cost.However, they have a weaker structuralintegrity and suffer from a desiccantmigration problem. The other type ofwheel uses silica gel as desiccant which is

bonded to the aluminium substratethrough a coating process. The matrix issupported by spokes and rim assembly.

In the 1980s, considerable advanceswere being made in the fabrication of silicaand other compounds for thesemiconductor industry. A derivative ofthese innovations was the developmentof molecular sieves - synthetic zeolites thatcould be designed at the molecular level.At the same time, manufacturingprocesses had been developed to allowthe bonding of a breathable layer ofdesiccant to metal or plastic surfaces.These technologies have influenced thenewer generation of enthalpy wheels. Thechronology of wheel development isgraphically depicted.

Enthalpy wheels :

Historical perspective

Wheels 1960 1970 1980 1990 1995Mesh TypeAsbestos-LiClOxidized AluminiumKraft Paper - LiClSilica Gel - AluminiumMolecular Sieve CoatingsDesiccant Mixture Coatings

Advancements in enthalpy wheels... What you see!

The new generation of enthalpy wheelshave several features which have distinctadvantages over others, which need to becarefully studied before selecting thecorrect wheel for the application.

• Selective adsorption which eliminatescross contamination of bacteria and airborne contaminates.

In certain application areas such ashospitals, hotels, clean rooms and animalhouses requiring stringent control of IAQ,where 100% fresh air is normally therequirement, designers are apprehensiveof using the heat wheel for fear of crosscontamination due to carryover of bacteria,germs or foul odours from the exhaust to

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the incoming air. The new generationwheels using 3Å/4Å (molecular sieve)mixtures as the desiccant; however wouldallow even the smallest diameterpollutants to blow over, because the poresize of the desiccant will essentially allowmolecules smaller than 3Å in diameter,5000 times smaller than the diameter ofthe human hair to pass into the fresh airsupplies. Water molecules, 2.8Å indiameter, can enter and exit the sieve. Asa result, the contaminations remain in theexhaust air stream.

• In-built purge sector eliminates crosscontamination. Cross contaminationgenerally refers to a mixing of air

between supply and exhaust airstreams. In rotary heat exchangers, thisoccurs through leakage and carryover.Carryover occurs each time a portionof the matrix passes the seals dividingthe supply and exhaust air stream, asthe exhaust air still inside the flutes ispushed back into the room by theincoming outdoor air. To eliminate

carryover, a purge sector is constructed,which flushes out the flutes before theyenter the supply air side.

With effective purge arrangementssome manufacturers are able to limit crosscontamination to .04% of the exhaust airconcentration by volume.

• Models of heat wheels using noncontact seals have a distinct advantageof larger life and effective sealing dueto the use of four pass labyrinth seal.

• The choice of desiccant is the keyelement in the enthalpy wheeltechnology. Silica gel, activatedalumina and molecular sieves are thedesiccants currently being offered onenthalpy wheels. Molecular sieveshave a relatively higher sorptioncapacity at low concentration levels ofwater vapour, which does not increasesignificantly with increase in relativehumidity. However the decrease inadsorption capacity of molecular sievewith increase of temperature is muchsmaller compared to the other twodesiccants. Both silica gel and activatedalumina have adsorption capacity twiceas much as molecular sieve at 100%RH. These characteristics influencewheel design and determine moisturetransfer effectiveness of the wheel atdifferent temperature and humidityconditions of the two air streams.

While selecting the enthalpy wheelsfor any application, therefore, the followingpoints should be carefully scrutinized:choice of desiccant, selectivity, flute

Return Air Exhaust Air

Supply Air Outdoor Air

H2O

CH3

NH3

H2S

SO2

CO

Outdoorair

Supplyair

Purgesection Wheel

rotation

Returnair

Exhaustair

MEDIA

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dimension, purge sector, sealarrangement, efficiencies, pressure drops,structural strength of the rotor. Thoughmanufacturers give detailed data onperformance, which should be consultedfor a given application, there are a few othercharacteristics of the manufacturingprocess which must be known to make awiser choice.

Advancements in enthalpy wheel ...What you don’t see!1. Desiccant technology, in the recent

years has made considerableadvancements and wheels arecurrently available coated withdesiccants with the distinguishingfeatures, such as :

• Desiccants with high diffusion rates

• Desiccants with selective adsorptioncharacteristics

• Desiccant mixtures which combinehigh diffusivity with selectivity

• Desiccants which are adhered tosubstrate using water based/nonmasking adhesives with pollutioncontrol considerations.

2. High quality substrate webs utilisingsimultaneous double sided coatingmethods.

3. Structural rigidity of the honeycombmedia/matrix by using state-of-the-artsurface winding techniques in place ofcentre winding techniques.

4. Highly polished and finished surfacesenabling distortion free production oflarge diameter rotors for use withcontact less seals.

These very recent 1995 developments,in manufacturing techniques, haveenabled the new generation rotors to haveall advantages of the previous wheels plusmore to provide the best recoveries,rigidity and reliability, with minimumpressure drops.

Integrating the enthalpy wheel inHVAC systems

The most widespread application ofenthalpy (heat) wheels is forpreconditioning fresh outside air before itis introduced to a building. The system caneasily be tapped into an existing ventilationsystem. A portion of the air that wouldnormally be recirculated through thesystem is exhausted through the wheeland fresh air is introduced into the buildingin its place. Operating in virtually anyclimate zone, a single desiccant wheeloperated with just a small motor to rotatethe wheel can deliver fresh air on a yearround basis that is generally within 3-7degrees and 10% RH of inside conditions,regardless of what outside conditions are(without any type of mechanical coolingor heating). The cost to provide high levelsof fresh air ventilation becomes minimalcompared to the normal heating coolingrequirements of the building. The potentialbenefits are numerous.

• Current standards for outside airventilation can be met or exceededwith minimal energy cost impact on thebuilding.

• Incoming outside air is dehumidifiedby the desiccant wheel, allowing therest of the ventilation system to rundry. As a result, indoor humidities aremaintainable well below the conditionsthat would favour the growth of mould,mildew and other microbialcontamination.

• The need for cooling capacity thatnormally would be required todehumidify and cool outside air iseliminated. This is typically 30 to 50%of total system capacity. In most cases,the cost of the energy wheels is almostless than the cooling capacity it isreplacing. The first cost of a building’scooling system can actually be reducedwith a wheel system.

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• Many utilities charge extra for theelectrical energy used during peakcooling periods. A wheel system cansignificantly reduce peak demandcharges.

• In the winter, wheel systems canpreheat and humidify incoming colddry air.

• Because the system is capable ofrecovering 80% of the heating orcooling energy that is exhausted froma building, the cost of fresh airventilation is reduced. Annual savingscan range from US$1 to $2 annually foreach cfm of fresh air ventilation.

• Given that the cost of the system issimilar to the cost of conventionalheating and cooling capacity, thesystem has an immediate payback. Inretrofit applications, where coolingcapacity is already in place, paybackwould typically take place in 1 to 3years.

The enthalpy wheels are available inseveral sizes and configurations and arebeing integrated in small, compactstandardized units for installation in hotels,restaurants, discotheques, bars, pubs,offices, nursing homes, as unitary systemsand are being sold as energy saving freshair preconditioners to handle smaller loads.

Typical Unitary Energy RecoveryVentilator (ERV)

Typical installation option: ceiling mounted

Larger heat wheels are being integratedin packaged AHUs or designed in modularsystems for integrating into HVACsystems to cater to larger fresh air loadsfor hospitals, animal laboratories andhotels.

Typical Installation: ‘Heat RecoveryWheel Integrated with HVAC for NewAreas

Conclusion

There have been changes in the air!The rules have changed for the way thebuildings have to be designed and built.The demands for stringent indoor airquality, additional fresh air ventilation,concerns about humidity and microbialcontamination and the need to find nontoxic replacements for CFCs have posed achallenge to the technical creativity anddesign finesse of the engineers, to findsolutions to these needs. The desiccantenthalpy wheel has remarkablysuccesscully addressed the market needsof the 90’s and has integrated the task ofproviding indoor air quality with efficientuse of energy.

New Ventilation Standards for IAQ

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