Filtration Basics

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Filtration 101

ENGINEERING

This issue of EngineeringSystem Solutions providesa tutorial on the various

types of filtration and theirapplication in HVAC systems. Inpreparing materials for thisarticle, we relied heavily on our

sister company, American AirFilter (AAF).

As the only major HVACmanufacturer with direct ties to amajor supplier of filtrationproducts, McQuay has uniqueaccess to expert advice on filtrationand its application in HVACequipment. But the relationshipgoes beyond free advice. McQuayand AAF work closely together in

assessing the requirements of ourcustomers and developing newfiltration technologies to meet theneeds of the HVAC industry.

Feel free to contact your local AAFrepresentative for assistance withyour filtration needs or visitwww.aafintl.com for moreinformation. Contact your localMcQuay representative for

information on the manyfiltration options available inMcQuay equipment, or visitwww.mcquay.com.

Hugh CrowtherDirector of Applications

McQuay International

S Y S T E M S O L U T I O N S

Edition No. 18 September 2003

In addition to everything from heat loadcalculations to acoustics and buildingcodes, filtration is among the long list ofitems that HVAC engineers are requiredto understand. For most buildingapplications, ASHRAE Standard 62.1,

Ventilation for Acceptable Indoor AirQuality, defines minimum filtrationlevels. However, the amount and type offiltration should be given carefulconsideration for each building projectbecause it can significantly impact bothoccupant comfort and equipmentperformance. This newsletter provides thebasics of filtration what is available, howit is rated, when to apply the various typesof filtration and when to get expertadvice.

What are we trying to filter?Filtration is used to control particulate,gaseous and biological contaminants.Some contaminants enter from outsidethe building, such as fly ash, diesel fumesor viruses. Other contaminants aregenerated within the building, such asman-made fibers, tobacco smoke andbacteria.

Figure 1 shows the range of particle sizes.A typical cubic foot of air has 2.5 billionparticles in it! While 99% of the

individual particles are smaller than 1micron, 70% of total weight comes fromparticles larger than 1 micron. This createsa need for two-part filtration one step toremove the bulk of the material and asecond step to remove fine particulate.

Gaseous contaminants normally passthrough a particulate filter, so they requiredifferent filtration technology. Gaseous

filtration is usually accomplished usingsome form of chemical treatment. Twocommon methods are carbon filters,which are effective for hydro-carboncontaminants, and PotassiumPermanganate (sometimes referred to as

Purple Pellets), which is effective forinorganic compounds such as HydrogenSulfide.

Chemical filters will not removeparticulate, and particulate filters will notremove chemicals. Therefore, both typesmay be required depending on theapplication. Biological contaminants areessentially particulate, so they can becontrolled by an appropriate particulatefilter. Ultraviolet lights provide anothermethod for controlling biological

contaminants. When applied properly,they can be effective at killing pathogens.However, in much the same way aschemical filters, ultraviolet lights do notremove particulates, so other forms ofparticulate filtration are required.

Why do we filter air?We do not filter the air we breatheoutside, so why is necessary to focus somuch attention on HVAC filtration? Theoriginal focus of HVAC filtration was onprotecting the HVAC equipment and

avoiding unsightly staining of diffusers andceilings. More recently, occupant healthhas become the main focus. From ahuman comfort and health point of view,buildings have much higher populationdensities and pollutant sources, somaintaining acceptable indoor air qualityis more of a challenge. Dirty equipmentcan become a source of pollutants


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particularly biological and mold canpropagate well in a dirty, moistenvironment.

Process and healthcare applications alsorequire a higher standard of filtration.Cleanroom classification is often based onthe filtration level. A class 10,000 (ISOClass 7) clean room has 10,000 particles,0.5 microns or less per cubic foot. A class10 (ISO Class 4) clean room has only 10

particles per cubic foot. The latter requiresan advanced filtration and system design.

Health care facilities need to controlbiological contaminants. In critical areassuch as operating rooms and intensive careunits (ICU), the filtration should be ableto stop most pathogens.

How are filters rated?How is it that you can buy a HighEfficiency Particulate Air Filter (HEPA)

for your home furnace while a HEPAsystem for institutional projects is a majorundertaking? The answer is in the testingmethod. ASHRAE has an old and a newstandard for filter testing. The oldStandard 52.1, Gravimetric and Dust-spotProcedures for Testing Air Cleaning DevicesUsed in General Ventilation for RemovingParticulate Matter, provides familiarratings for most HVAC engineers. When

Table 1 Comparison of Clean Ratings From Federal Standard 209 and ISO 14644-1.

0.1 mm Particles 0.5 mm Particles 5.0 mm Particles

FS 209 IS0 FS 209 ISO FS 209 ISO FS 209 ISOClass Class Parts/ft3 Parts/m3 Parts/m3 Parts/ft3 Parts/m3 Parts/m3 Parts/ft3 Parts/m3 Parts/m3

1 10

2 100 4

1 3 35 1230 1,000 1 35 35

10 4 345 12,200 10,000 10 353 352

100 5 3450 122,000 100,000 100 3,530 3,520 29

1000 6 34,500 1,200,000 1,000,000 1,000 35,300 35,200 7 247 293

10,000 7 345,000 1.22 x 107 10,000 353,000 352,000 65 2,300 2,930

100,000 8 3,450,000 1.22 x 108 100,000 3,530,000 3,520,000 700 24,700 29,300

9 3.45 x 107 1.22 x 109 35,200,000 293,000

Note: Values shown are the concentration limits for particles equal to and larger than the sizes shown. C n = N(0.5/D)2.2where Cn = concentration limits in particles/ft3,N = FS 209 class, and D = particle diameter in mm.

Cn = 10N

(0.1/D)2.08

where Cn = concentration limits in particles/m3

, N = ISO class, and D = particle diameter in mm.Copyright 2003, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. www.ashrae.org. Images reprinted by permission from ASHRAE 2003 Applications Handbook.

Figure 1 Relative Particle Size


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a filter is referred to as being a 35%efficient pleated 2-inch throwaway, thefilter was rated using the 52.1 Standard.

Standard 52.1 initially rated filters ontheir ability to arrest material that wouldhave an adverse effect on the HVACequipment. The arrestance test uses adefined mixture of particles and is theaverage performance over the life of thefilter. The filter is able to arrest morematerial as it gets dirtier.

Over time, the tests evolved to include adust spot test to rate the filters ability tostop fine particles that could soil thebuilding and its contents (e.g. the stainsaround diffusers). The dust spot test usesatmospheric air which can vary from testlocation to test location. When a singleefficiency number is given (say 95%), it isthe average because there is a portion oftime when the filter is not as efficient asits nominal filter efficiency suggests. Infact, the filter goes through a wide rangeof efficiencies as it goes from clean todirty. This is why the filter manufacturersprovide a range of efficiencies in theircatalogs for each filter. A filter with a 25%dust spot efficiency will usually have anarrestance above 90%. The percentefficiency we refer to is the dust spotefficiency.

The new standard is ASHRAE Standard52.2, Method of Testing GeneralVentilation and Air Cleaning Devices forRemoval Efficiency by Particle Size.Standard 52.2 is focused on minimizinghealth risks from particulate matter. Thisstandard requires testing the filter with acontrolled aerosol (potassium chloride)instead of typical air. A particle counteris used to measure the arresting ability ofthe filter over 12 size ranges from 0.30

micron up to 10 microns. The filter isloaded in increments from clean to dirtywith a pre-defined dust mixture and thenre-tested. Once this is done, a chart can bedrawn that shows filter efficiencies fordifferent particle sizes from clean to dirty(Figure 2).

The performance of the filter is thencompared to a predefined set ofparameters called a minimum efficiency

reporting value (MERV). MERV valuesrange from 1 to 20. Table 2 shows acomparison of the MERV Rating versusthe Standard 52.1 method.

How Does ASHRAE Standard 62 Fit In?ASHRAE Standard 62.1 defines theminimum ventilation rates required toobtain acceptable indoor air quality. Thepremise is that the outdoor air is ofacceptable quality, and that ventilating abuilding with it will control indoorcontaminants through dilution. Table 3was developed by the U.S. EnvironmentalProtection Agency (EPA) and providesambient air quality standards for outdoorair. About 60% of the U.S. populationlives in non-attainment areas, whereconditions exceed the EPA table. Standard62 requires that the outdoor air be treatedif it does not meet the levels required bythe U.S. EPA table.

Addendum S has now been added toStandard 62 and requires that filters withno less than MERV 6 (or 25% efficiency)be used upstream of all cooling coils or

a Not to be exceeded more than once per yearbArithmetic mean.c Standard is attained when expected number of days per calendar year with maximal hourly average

concentrations above 0.12 ppm (235 g/ m3) is equal to or less than 1, as determined by Appendix H tosubchapter C, 40 CFR 50.

d Three-month period is a calendar quarter.

Copyright 2001, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. www.ashrae.org.Images reprinted by permission from ANSI/ASHRAE Standard 62-2001 Ventilation For Acceptable Indoor Air Quality.

100

90

80

70

60

50

40

30

20

10

0

0.1 1 10

Particle Diameter (micrometers)

FiltrationEfficiency(%)

Initial Efficiency

After 1st loading

After 2nd loading

After 3rd loading

After 4th loading

After 5th loading

Figure 2 Standard 52.2 Filter Efficiency Curve

Standard 52.2 (MERV)Approximate Standard 52.1

Particle Size RangeDust Spot Efficiency Arrestance

15 >95% N/a 1

14 90-95% >98% 1

11 60-65% >95% 2

8 30-35% >90% 3

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other condensate-producing devices.Addendum R requires that eachventilation system with an outdoor airintake have the necessary devices toclean the air prior to its introductioninto occupied spaces in locationswhere the outdoor air is notacceptable. For particulates, filters

with a minimum MERV of 6 arerequired. Addendum Z also addressesunacceptable outdoor air, requiring aminimum 40% efficient ozoneremoval system be included if theozone is too high.

If you elect to use the indoor airquality procedure of Standard 62,then you must identify indoorcontaminants and maintain them atthe same level as acceptable outdoorair. This will require some level offiltration, depending on thecontaminants.

HEPAs and ULPAsHEPA filters are tested in a differentmanner than other commercialparticulate filters. A HEPA filter is99.97% efficient at arresting a 0.3micron particle. They are tested byexposing the filter to Dioctyl-Phthalate (DOP) particles, which are0.3 microns in size. For every 10,000

particles, only 3 can get through thefilter. A ULPA filter is 99.999%efficient at stopping 0.3 micronparticles.

What about buildings andextraordinary events?Recent events have drawn attention tohow buildings will respond inextraordinary (terrorist) incidents.ASHRAE has teamed up with the

U.S. Army Corp of Engineers toevaluate buildings in thesecircumstances. At the January 2003meeting, ASHRAE published a 75-page report on their findings. Thiscan be downloaded from their websiteat www.ashrae.org.

Assessing a buildings vulnerability is a

topic on its own, but filtration canplay a key role in defending againstseveral kinds of biological andradiological particles. One of therecommendations of the ASHRAEreport is to consider filters with thehighest MERV feasible because mostbiological and radiological particlesare in the range of 0.1 to 10 microns.A filter with a MERV rating of 14 to20 can be very effective in defendingagainst these particles. During non-critical times, the building will benefitfrom cleaner air and a betterenvironment.

General Guidelines For FiltrationApplicationThe following are some guidelines inthe application of filters for HVACapplications.

What level of filtration is required?As a minimum, the designer should

meet the requirements of Standard62.1, taking into account the recentaddendums that address cooling coilfiltration and the quality of outdoor air.For healthcare applications, thedesigner should follow specificationsoutlined in the AIA guide to hospitaldesign or the new ASHRAE designmanual of healthcare design (Table 4).

For process applications, it becomesimportant to understand what size

particulate needs to be filtered. Filtersshould not be specified by just MERVor dust spot efficiency, but on theirability to arrest the particle size inquestion. For example, the particlesize for a paint line is around 4microns, so the filter should beeffective specifically around 4

microns. Different filters with thesame MERV number may be more orless efficient at arresting 4 micronparticles, so it is important to reviewthe filter effectiveness curves that areavailable from filter manufacturers.

Pre-filters vs. Final FiltersAs mentioned earlier, the bulk of theweight of particulate matter comesfrom particles larger than 1 micron.The purpose of a pre-filter is toremove as much of this mass aspossible. The final filter is then usedto remove the high count (but smallweight) of fine particulate. A typicalmaintenance arrangement will requirethe pre-filters to be changed quarterlyand the final filters to be changedannually (although this ultimately iscontrolled by usage).

Both filter banks can be held by thesame filter rack at the front of an airhandling unit, which can help to

minimize the unit length. The filtersare usually the first component afterthe mixing box, which is a commonarrangement for high-end office andinstitutional applications.

Some applications, such as health carebuildings, require that the final filtersbe the last component in the airstream so they can capture any dustthat is introduced into the airhandling unit through servicing or

infiltration. In this arrangement, thepre-filters remain at the front of theunit to protect the equipment.

It is strongly advised not to use a blowthrough fan arrangement with finalfilters in the final position. McQuaywill not build the equipment in thisconfiguration without a clearunderstanding of the application. Theissue is that the cooled air will mostlikely be saturated, and the pressure

Area Designation No. Filter Beds Filter Bed #1(%) Filter Bed #2(%)

All areas for inpatient care,treatment, diagnosis, and thoseareas providing direct service orclean supplies such as sterileand clean processing, etc.

Protective environment room

Laboratories

Administrative, bulk storage,soiled holding areas, foodpreparation areas, and laundries

2

2

1

1

30

30

80

30

90

99.97

Table 4. Filter Efficiencies for Central Ventilation and Air ConditioningSystems in General Hospitals

Notes:


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drop through the filters will lower thepartial pressure. This can result inmoisture condensing on the filters,creating an IAQ issue. Draw throughfan arrangements add the fan motorheat to the supply air and raise itabove dew point, makingcondensation less of an issue.

The designer should consider aplenum fan for filters in the finalposition. The plenum fan will evenlydistribute air across the entire filterbank and it may allow the airhandling unit to be shorter in length.In addition, an Arrangement 3 fan(typical scrolled fan) without ductingconnected to it has about the sameefficiency as a plenum fan.

What filter static pressure should

be used?As filters accumulate particles, theirstatic pressure increases. Filtermanufacturers provide both cleanand dirty filter pressure drops. Thechange can be significant. Considerclean 30% pre-filters and 95% bagfilters. The pressure drop is only 0.6in. w.c. when they are clean, but itclimbs to 2.0 in. w. c. when they aredirty. If the HVAC system isdesigned based on clean filters, the

airflow will not meet designconditions as soon as the filters beginto load up. In critical airflowapplications, such as spacepressurization and air change rates,this may not be acceptable.Alternatively, designing a systembased on dirty filters will provide therequired airflow all the time, but itwill be more costly to build andoperate.

For most applications the mean ofthe aggregate filters will providesatisfactory results. In addition,choose a fan with a very steep fancurve so changes in static pressureresult in only small changes in airvolume.

For critical applications, particularlywhere HEPA filters are used, thesystem should be designed for thedirty filter static pressure. It may also

be necessary to provide a method formodulating the fans to maintain thedesign airflow rate. Using the fan

clean and dirty and see if the spacepressurization remains acceptable.

What velocity is best?Filters have a maximum allowablevelocity. Most are rated at 500 fpm,which is the typical face velocity fora cooling coil in an air handling unit.Some packaged DX rooftop units

operate at much higher facevelocities. At a minimum, the filterface velocity should be ascertainedand confirmed to be acceptable. Itmay be economically justifiable touse a face velocity below 500 fpm,particularly for constant volumeunits with high operating hours(health care, pharmaceutical, etc.).Reducing the internal static pressuredrop can often provide enough fan

power savings to offset the highercapital cost. The increased filter areareduces the frequency of filterchanges as well. McQuays EnergyAnalyzer can be used to check theavailable annual energy savings.

Bag filters vs Cartridge filtersBag and cartridge filters have thesame efficiency, but bag filters takeup more space (they are 12 to 24inches longer than cartridge filters)and they can sag at part airflow

(VAV applications). In some cases,the bag filters must be hung fromhooks to stop them from collapsing.However, bag filters are about halfthe price of the equivalent cartridgefilter, and they have an abundance ofsurface area which can result in lessfrequent changes being required.

Where filters will be challenged andhave to be changed frequently,consider bag filters. For other

applications, particularly VAV, usecartridge filters.

Front loaded vs. Side loadedFront loaded filters fit into a rack thatlooks like a grid. The dirtier thefilters get, the more the pressure sealsthe filters in the frame. Because theygenerally seal better, front loadedfilters can offer better performance.Side loaded filters do not require anaccess section upstream to allow filter

changes. On smaller units, where it isdifficult for a person to enter theunit, side loaded filters can be an

Where is the best place for HEPAfilters?HEPA filter applications arespecialized. It is possible to installHEPA filters in the air handling unit,but this is not common. HEPA filtersgenerally are designed to operate at250 fpm (special high flow units are

available) and the air handling unitwill have to be designed for the lowervelocity. More often, the HEPA filtersare integrated into the space throughthe ceiling grid. As much purified airas possible is recirculated to minimizefilter loading , although this is notalways possible in health careapplications).

In most applications, the air handlingunit should have 30%, 60% and 90%

filter banks to minimize the load onthe HEPA filters. In many clean roomapplications, it is not unusual forHEPA filters to last the lifetime of theapplication (e.g. integrated circuitmanufacturing).

What about gas phase filters?The use of carbon filters andpotassium permanganate filters isgenerally limited to industrialapplications. On occasion, pooroutdoor air quality requires gas

filtration for conventional HVACapplications. These are specializedfilters that require expert assistance.They are also expensive to maintain.In many cases the customer would bebetter served by trying to avoid gasfilters. A common problem is truckand car exhaust fumes are entrained bythe intake of an air handling unit. Thedesigner should look for all possiblemethods to relocate the intake before

considering carbon filters.What about UV Lights?UV lights can kill bacteria, virusesand spores when applied properly.They can enhance medium rangeparticulate filters that are not efficientenough to arrest the pathogens. Inaddition, they can minimizebacteriological growth in the airhandling unit while it is shut down.Refer to Edition 15 of EngineeringSystem Solutions, UV Lights and AirHandling Equipment, onwww.mcquay.com for more

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For comments or suggestions, please call or write:

Chris Sackrison, EditorMcQuay International13600 Industrial Park BoulevardMinneapolis, MN 55441Phone: (763) 553-5419E-mail: [email protected] more information on McQuay products and services, or to speak with your local representative, call (800) 432-1342, or visit our web page at www.mcquay.com .2003 McQuay International

Electrostatic technologySmall scale electrostatic filters operate bycharging a grid that causes particulate toadhere to the grid. Once the grid is dirty,it requires cleaning. The advantage of thissystem is the low static pressure drop.They are particularly attractive for smallterminal units and residential applications.The disadvantage of electrostatic filters is

that their efficiency decreases as theybecome dirty. Disciplined maintenance isthe best method to maintain goodperformance.

On a larger scale, there are systems such asCosatron that are considered filterenhancers. These systems charge

particulate causing it to stick together sothe conventional mechanical filter canarrest the larger particle size. They are verypopular in casino applications where thereis a lot of tobacco smoke.

ConclusionFiltration is more than just looking afterthe equipment and avoiding stains onceiling tiles. It is a key component inachieving acceptable indoor air quality. Inaddition, filtration can play a critical rolein defending against extraordinary events.

Filtration costs money, both in terms ofcapital cost and operating cost. McQuaysEnergy Analyzer can be used to evaluate

the operating cost of the higher pressuredrops caused by more effective filters. Thiscan lead to a clear discussion with theowner about the minimum filtration, theadvantages of enhancing the filtration andthe capital and operating costs that gowith it.

Filtration in process applications canbecome very complex. This is a good timeto get assistance from filtrationprofessionals who can be a source ofinformation on what works in similarindustries. They can even performmockups and tests to make sure theproposed solutions can work as intended.

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Filtration Basics