7. Ventilation and Indoor Air-Quality.docx

7. Ventilation and Indoor Air-Quality

1. The Basic Concerns of IAQASHRAE Standard 62 defines acceptable indoor air quality as air

in which there are no known contaminants at harmful concentrationsas determined by cognizant authorities and with which a substantialmajority (80% or more) of the people exposed do not expressdissatisfaction. With acceptable indoor air quality, not only areoccupants comfortable, but their environment is free of bothersomeodors and harmful levels of contaminants. Maintaining thermalcomfort is not just desirable and helpful in assuring a productivework environment, but in many cases also has a direct effect on thehealth of the building occupants. Factors other than thermal comfortthat are controlled by the HVAC system involve maintenance of aclean, healthy, and odor-free indoor environment. These factors areoften what is intended by the term indoor air quality or IAQ.Maintaining good indoor air quality involves keeping gaseous andparticulate contaminants below some acceptable level in the indoorenvironment. The contaminants include such things as carbon dioxide,carbon monoxide, other gases and vapors, radioactive materials,microorganisms, viruses, allergens, and suspended particulatematter.

Contamination of indoor spaces is caused by human and animaloccupancy, by the release of contaminations in the space from thefurnishings and accessories or from processes taking place insidethe space, and by the introduction of contaminated outdoor air. Thecontaminants may be apparent, as in the case of large particulatematter or where odors are present, or they may be discernible onlyby instruments or by the effect that they have on the occupants.Symptoms such as headaches, nausea, and irritations of the eyes andnose may be a clue that indoor air quality in a building is poor.Buildings with unusual number of occupants having physical problemshave come to be described as having sick building syndrome. Emphasison comfort and health in the workplace and increased litigation inthis area place a great responsibility on contractors, buildingowners, employers, and even HVAC engineers to be well informed,technically competent, and totally ethical in any actions affectingindoor air quality. Good indoor air quality usually costs money, andthe economic pressure to save on initial and operating costs cansometimes cause poor decisions that lead to both human suffering andeven greater money costs.

2. Common Contaminants

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2.1 Carbon Dioxide and Other Common GasesCarbon dioxide is an exhaled by-product of human (and all

mammals) metabolism, and therefore CO2 levels are typicallyhigher in occupied spaces than for outdoor air. In heavilyoccupied spaces such as auditoriums, CO2 levels will often be amajor concern. This is not because of any direct health risk, butbecause CO2 is an easily measurable indicator of theeffectiveness of ventilation of the space. As such, it givesmeasurable indicator of the effectiveness of ventilation of thespace. As such, it gives at least an indirect indication ofpotentially unacceptable levels of more harmful gases. TheEnvironmental Protection Agency (EPA) recommends a maximum levelof 1000 ppm (1.8 g/m3) for continuous CO2 exposure, specificallyfor school and residential occupancy, and as a guideline forother building types.

Incomplete combustion of hydrocarbon fuels and tobacco smokingare two significant sources of carbon monoxide. Buildings withinternal or nearby parking garages and loading docks are morelikely to have a high levels of CO. HVAC outdoor air intakes atground level where heavy street traffic occurs can drawunacceptable levels of CO into the building’s air system.Improperly vented and leaking furnaces, chimneys, water heaters,and incinerators are often the source of difficulty. Carbonmonoxide is a toxic gas, and levels near 15 ppm can significantlyaffect body chemistry. The reaction of human to different COlevels varies significantly, and the effects can be cumulative.Headaches and nauses, are common symptoms in those exposed toquantities of CO above their tolerance.

Sulfur oxides are the result of combustion of fuels containingsulfur and may enter a building through outdoor air intakes orfrom leaks in combustion systems within the building. Whenhydrolyzed with water, sulfur oxides can form sulfuric acid,creating problems in the moist mucous membrane that may causeupper respiratory tract irritation and induce episodic attacks inindividuals with asthmatic tendencies.

Nitrous oxides are produced by combustion of fuels with air athigh temperatures. Ordinarily, these contaminants are brought inwith outdoor air that has been contaminated by internalcombustion engines and industrial effluents, but indoorcombustion sources frequently contribute significant amounts.

Radon, a naturally occurring radioactive gas resulting fromthe decay of radium, has received a great deal of attentionrecently, especially in areas where concentrations have been

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found to be very high. The primary concern with radon is thepotential for causing lung cancer. Radon gas may enter abuilding from the soil through cracks in slab floors and basementwalls, or through the water supply, or from building materialscontaining uranium or thorium. The rate of entry from the soildepends on pressure differences, and therefore pressurization ofa space is one means of reducing radon levels in that space.Other preventive measures include the ventilating of crawl spacesand under-floor areas and the sealing of floor cracks. Forsafety, radon levels should be kept low enough to keep theexposure of occupants below 4 picocuries per liter of air.

2.2 Volatile Organic Compounds (VOCs)A variety of organic chemicals species occur in a typical

modern indoor environment, resulting from a combustion sources,pesticides, building materials and finishes, cleaning agents andsolvents, and plants and animals. Fortunately, they usually existat levels that are below recommended standards. Some occupants,however, are hypersensitive to particular chemicals, and for themmany indoor environments create problems. Formalldehyde gas, oneof the more common VOCs, is irritating to the eyes and the mucousmembranes. It seems to have caused a diversity of problems inasthmatic and immunoneurological reactions and is considered tobe a potential cancer hazard. Formaldehyde, used in themanufacture of carpets, pressed board, insulations, textiles,paper products, cosmetics, shampoos, and phenolic plastics, seemsto enter buildings primarily in building products. These productscontinue to outgas formaldehyde for long periods of time, butmostly during the first year. Acceptable limits are in the rangeof 1 ppm as a time-weighted 8-hour average. For homes, levels of0.1 ppm seem to be a more prudent upper limit.

2.3 Particulate MatterA typical sample of outdoor air might contain soot and smoke,

silica, clay, decayed animal and vegetable matter, lint and plantfibers, metallic fragments, mold spores, bacteria, plant pollens,and other living material. The sizes of these particles may rangefrom less than 0.01 m. (10-8 m) to the dimensions of leaves andinsects. Fig. 4-5 shows the very wide range of sizes of particlesand particle dispersoids along with types of gas cleaningequipment that might be effective in each case.

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When suspended in the air, the mixture is called an aerosol.As outdoor air is brought into an indoor environment, it may beadditionally contaminated by human sources and activities,interior furnishings and equipment, and pets. Microbial andinfectious organisms can persist and even multiply when indoorconditions are favorable. Environmental tobacco smoke (ETS) hasbeen one of the major problems in maintaining good indoor airquality, and concern has been heightened by increased evidence ofits role in lung diseases, particularly cancer. Allergies are acommon problem in a modern society, and the indoor environmentmay contain many of the particulates found outdoors. In addition,some occupants may be sensitive to the particulates foundprimarily indoors, such as fibers, molds, and dust from carpetsand bedding.

3. Methods to Control ContaminantsThere are four basic methods to maintain good IAQ in buildings:

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a. Source elimination or modificationb. Use of outdoor airc. Space air distributiond. Air cleaning

3.1 Source Elimination or ModificationOf the four basic methods listed above, source elimination or

modification very often is the most effective method for reducingcontaminants not generated directly by the human occupants or thenecessary activities in the space.

3.2 Use of Outdoor AirFigure 4-6 is used to help define the various terms involved

in the air flow of a typical HVAC system. Supply air is that airdelivered to the conditioned space and used for ventilation,heating, cooling, humidification, or dehumidification.Ventilation air is that portion of supply air that is outdoor airplus any recirculated air that has been treated for the purposeof maintaining acceptable indoor air quality. Indoor spacesoccupied for any length of time require the intake of someoutdoor air to maintain air quality.

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Outdoor air is air taken from the external atmosphere andtherefore not previously circulated through the system. Someoutdoor air may enter a space by infiltration through cracks andinterstices and through ceilings, floors, and walls of a space orbuilding, but generally in air-conditioned buildings most outdoorair is brought into a space by the supply air. It is usuallyassumed that outdoor air is free of contaminants that might causediscomfort or harm o humans, but this is not always so. In somelocalities where strong contaminant sources exist neat abuilding, the air surrounding a building may not be free of thecontaminants for which there are concerns.

Recirculated air is that air removed from the conditionedspace and intended for reuse as supply air. It differs fromreturn air only in that some of the return air may be exhaustedor relieved through dampers or by fans. Makeup air is outdoor airsupplied to replace exhaust air and exfiltration. Exfiltration isair leakage outward through cracks and interstices and throughceilings, floors, and walls of a space or building. Some air maybe removed from a space directly by room exhaust, usually withexhaust fans. There must be a balance between the amount of air

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mass entering and the amount leaving a space as well as betweenthe amount of air mass entering and leaving the entire air supplysystem. Likewise there must be a balance on the mass of anysingle contaminant entering and leaving a space and entering andleaving the entire air supply system.

The basic equation for contaminant concentration in a space isobtained using Fig. 4-6, making a balance on the concentrationentering and leaving the conditioned space assuming completemixing, a uniform rate of generation of the contaminant, anduniform concentration of the contaminant within the space and inthe entering air. All balances should be on a mass basis;however, if densities are assumed constant, then volume flowrates may be used. For the steady state case,

QtCe+N=QtCs (1)where:Qt = rate at which air enters or leaves the spaceCs = average concentration of a contaminant within the spaceN = rate of contaminant generation within the spaceCe = concentration of the contaminant of interest in the enteringair

Equation (1) can be solved for the concentration level in thespace Cs or for the necessary rate Qt at which air must enter thespace to maintain the desired concentration level of acontaminant within the space. This fundamental equation may beused as the basis for deriving more complex equations for morerealistic cases.

Example No. 1A person breathes out carbon dioxide at the rate of 0.30 L/min.The concentration of CO2 in the incoming ventilation air is 300ppm (0.03 percent). It is desired to hold the concentration inthe room below 1000 ppm (0.1 percent). Assuming that the air inthe room is perfectly mixed, what is the minimum rate of flow ofair required to maintain the desired level?Given:N = 0.30 L/minCs = 1000 ppm = 0.1 percent = 0.001Ce = 300 ppm = 0.03 percent = 0.0003Required: Qt

Solution:From Equation (1):

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Qt=N

Cs−CeQt=

0.30 L/min(0.001−0.0003) (60 s /min)

Qt=7.1 L /s=15 cfm

In most HVAC systems emphasis is placed on maintaining theoccupied zone at a nearly uniform condition. The occupied zone isthe region within an occupied space between the floor and 72 in.(1800 mm) above the floor and more than 2 ft (600 mm) from thewall or fixed air conditioning equipment. In most cases perfectmixing of the supply air with the room air does not occur, andsome fraction S of the supply are rate Qs bypasses and does notenter the occupied zone, as shown in Fig. 4-7. Because of this,some of the outdoor air in the room supply air is exhaustedwithout having performed any useful reduction in the contaminantsof the occupied zone.

The effectiveness Eoa with which outdoor air is used can beexpressed as the fraction of the outdoor are entering the systemthat is utilized:

Eoa=Qo−QoeQo (2)

where:Qo = rate at which outdoor air is taken inQoe = rate at which unused outdoor air is exhausted

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From Fig. 4-8, with R equal to the fraction of return air Qr thatis recirculated, the rate at which outdoor air is supplied to thespace Qos is

Qos=Qo+RS {Qos¿ (3)The amount of unused outdoor air that is exhausted Qoe is

Qoe=(1−R )SQos (4)Combining equation (2), (3), and (4) yields

Eoe=1−S1−RS (5)

Equation (5) gives the effectiveness with which the outdoor airis circulated to the occupied space in terms of thestratification factor S and the recirculation factor R. S issometimes called the occupied zone bypass factor. Using thissimple model with no stratification, S would equal to zero andthere would be total mixing of air, and the effectiveness Eoa wouldbe 1.0. Note also that as the exhaust flow becomes small, Rapproaches 1.0 and the effectiveness again approaches 1.0. Thissimple model neglects the effect of infiltration and assumes thatthe occupied space is perfectly mixed air.

Example No. 2For a given space it is determined that due to poor location ofinlet diffusers relative to the inlet for the air return, and dueto partitions around each work space, about 50 percent of thesupply air for a space is bypassed around the occupied zone. Whatfractions of the outdoor air provided for the space are

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effectively utilized as the recirculation rate is changed from0.4 to 0.8?Given:S = 0.5R = 0.4 to 0.8Required:Eoa

Solution:Eoe=

1−S1−RS

For R = 0.4Eoe=

1−0.51−(0.4) (0.5)

=0.625

For R = 0.8Eoe=

1−0.51−(0.8) (0.5)

=0.833

Standard 62 describes two methods by which acceptable indoorair quality can be achieved. The first of these procedures, theVentilation Rate Procedure, prescribes the rate at which outdoorair must be delivered to a space and various means to conditionthat air. A sample of these rates is given in Table 4-2, fromStandard 62, and are derived from physiological considerations,subjective evaluations, and professional judgments. TheVentilation Rate Procedure prescribes:

The outdoor air quality acceptable for ventilation Outdoor air treatment when necessary Ventilation rates for residential, commercial,

institutional, vehicular, and industrial spaces Criteria for reduction of outdoor air quantities when

recirculated air is treated by contaminant-removalequipment.

Criteria for variable ventilation when the air volume in thespace can be used as a reservoir to dilute contaminants.

For most of the cases in Table 4-2, outdoor-air requirementsare assumed to be in proportion to the number of space occupantsand are given in cfm (L/s) per person. In the rest of the casesthe outdoor-air requirements are given in cfm/ft2 [L/(s-m2)], andthe contamination is presumed to be primarily due to otherfactors. Although estimated occupancy is given where appropriate

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for design purposes, the anticipated occupancy should be used.For cases where more than one space is served by a common supplysystem, the Ventilation Rate Procedure provides a means forcalculating the outdoor-air requirements for the system. Roomsprovided with exhaust air systems, such as toilet rooms andbathrooms, kitchens, and smoking lounges, may be furnished withmakeup air from adjacent occupiable spaces provided the quantityof air supplied meets the requirements of Table 4-2.

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3.3 Air CleaningSome outdoor air is necessary in buildings to replenish the

oxygen required for breathing and to dilute the carbon dioxideand other wastes produced by the occupants. In many cases it isdesirable to clean or filter the incoming outdoor air. Incombination with the introduction of outdoor air, sourcereduction, and good air distribution, cleaning or filtration ofthe recirculated air can often provide a cost-effective approachto the control of indoor air contaminants. Design of a propersystem for gas cleaning is often the final step in assuring thatan HVAC system will provide a healthy and clean indoorenvironment.3.3.1 Gas Removal

The 1995 ASHRAE Handbook, HVAC Applications has adetailed discussion of the control of gaseous contaminantsfor indoor air. Industrial gas cleaning and air pollutioncontrol is discussed in the 1996 ASHRAE Handbook, HVACSystems and Equipment.

Contaminants may be removed from an airstream byabsorption, by physical adsorption, by chemisorptions, bycatalysis, and by combustion. In some cases particulatematter may also be removed as these processes take place.

Absorbers are commonly used in the life support systemsof space vehicles and submarines. Both solid and liquidabsorbers may be used to reduce carbon dioxide and carbonmonoxide to carbon, returning the oxygen to the conditionedspace. Air washers, whose purpose may be to controltemperature and humidity in buildings, not only removecontaminant gases from air stream by absorption, but canremove particulate matter as well. Contaminant gases areabsorbed in liquids when the partial pressure of thecontaminant in the air stream is greater than the solutionvapor pressure with or without additive for thatcontaminant.

Adsorption is the adhesion of molecules to the surfaceof a solid (the adsorbent), in contrast to absorption, inwhich the molecules are dissolved into or react with asubstance. Good adsorbents must have large surface areasexposed to the gas being adsorbed and therefore typicallyhave porous surfaces. Activated charcoal is the most widelyused adsorbent because of its superior adsorbing properties.It is least effective with the lighter gases such as ammoniaand ethylene and most effective with gases having high

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molecular mass. The charcoal may be impregnated with othersubstances to permit better accommodation of chemicallyactive gases.

Chemisorption is similar in many ways to physicaladsorption. It differs in that surface binding inchemisorption is by chemical reaction and therefore onlycertain pollutant compounds will react with a givenchemisorber. In contrast to physical adsorption,chemisorptions improves as temperature increases, does notgenerate heat (but may require heat input), is not generallyreversible, is helped by the presence of water vapor, and isa monomolecular layer phenomenon.

Catalysis is closely related to chemisorptions in thatchemical reactions occur at the surface of the catalyst;however, the gaseous pollutant does not reactstoichiometrically with the catalyst itself. Because thecatalyst is not used up in the chemical reaction takingplace, this method of air purification has the potential forlonger life than with adsorbers or chemisorbers, assumingthat an innocuous product is produced in the reaction. Thechemical reactions may involve a breakdown of thecontaminant in to smaller molecules or it may involvecombining the contaminant gas with the oxygen available inthe air stream or with a supplied chemical. Only a fewcatalyst appear to be effective for air purification atambient temperatures. Catalytic combustion permits theburning of the offending has at a temperatures lower thanwith unassisted combustion and is widely used in automobilesto reduce urban air pollution.

3.3.2 Particulate Removal – FilteringThe wide variety of suspended particles in both the

outdoor and indoor environments has been describedpreviously. With such a wide range of particulate sizes,shapes, and concentrations, it is impossible to design onetype of air particulate cleaner that would be suitable forall applications. Clean rooms in an electronic assemblyprocess require entirely different particulate removalsystems than an office or a hospital. Air cleaners forparticulate contaminants are covered in more detail in theASHRAE Systems and Equipment Handbook.

The most important characteristics of the aerosolaffecting the performance of a particulate air cleanerinclude the particle’s

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Size and shape Specific gravity Concentration Electrical properties

Particulate air cleaners vary widely in size, shape,initial cost, and operating cost. The major factorinfluencing filter design and selection is the degree of aircleanliness required. Generally, the cost of the filtersystem will increase as the size of the particles to beremoved decreases. The three operating characteristics thatcan be used to compare various types are Efficiency Air-flow resistance Dust-holding capacity

Efficiency measures the ability of the air cleaner toremove particulate matter from an air stream. Figure 4-8shows the efficiency of four different high-performancefilters as a function of particle size. It can be seen thatsmaller particles are the most difficult to filter. Inapplication with dry-type filters and with low dustconcentrations, the initial or clean filter efficiencyshould be considered for design, since the efficiency insuch cases increases with dust load. Average efficiency overthe life of the filter is the most meaningful for most typesand applications.

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The air-flow resistance is the loss in total pressure ata given air flow rate. This is an important factor inoperating costs for the system. Dust-holding capacitydefines the amount of a particular type of dust that an aircleaner can hold when it is operated at a specified air-flowrate to some maximum resistance value or before itsefficiency drops seriously as a result of the collecteddust. Typical engineering data (physical size, flow rate ata stated pressure drop) for the four filters shown in Fig.4-8 are given in Table 4-3. The design requirement willrarely be exactly one of the air-flow rate or pressure lossshown in Table 4-3.

In these cases one can assume that the pressure lossacross a filter element is proportional to the square of theflow rate. Thus, letting the subscript r stand for ratedconditions, the pressure loss p at any required rate offlow Q can be determined by

Δp=Δpr( QQr )2

(6)The mechanism by which particulate air filters operate

include Straining Direct interception Inertial deposition Diffusion Electrostatic effects

The common types of particulate air cleaners may be put inone or four groups:

Fibrous-media unit filters Renewable-media filters Electronic air cleaners Combination air cleaners

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Air cleaning has been used for many years to improve thequality of air entering a building, to protect componentssuch as heat exchanger coils from particulate contamination,and to remove contaminants introduced into the recirculatedair from the conditioned space. Properly designed HVACsystems utilize air cleaning along with source modification,dilution with outdoor air, and space air distribution togive optimum performance with lowest cost.

The performance of an air cleaning system can be studiedby using a model shown in Fig. 4-9. This is simplified modelin which filtration, exfiltration, and room exhaust areignored and the air cleaner is assumed to be located eitherin the recirculated air stream (location A) or in the supplyair stream (location B).

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Ventilation efficiency Ev, the fraction of supply airdelivered to the occupied zone, depends on the room shape,as well as on the location and design of the supplydiffusers and the location of the return inlets. Theventilation efficiency can be seen to be equal to (1 – S) inFigure 4-7 note that ventilation efficiency is not the sameas the effectiveness of outdoor air use, Eoa.

Assuming that densities do not vary significantly, volumebalances can be used in place of mass balances. This seemsto be a common assumption in air cleaning calculations, butcare should always be exercised to be sure significanterrors are not introduced. Making volume balances on theoverall air-flow rates, and on any one contaminant ofinterest, Fig. 4-9 can be used to obtain equations for therequired constant outdoor-air rates for constant-air-volumesystems:

Filter location Required outdoor airrate

A Qo=N−EvR QrEfCsEv(Cs−Co ) (7)

B Qo=N−EvR QrEfCs

Ev [Cs−(1−Ef)Co ] (8)

Standard 62 gives five additional equations for variable-air-volume systems. Equations (7) and (8) can be used as anengineering basis for air cleaner (filter) selection. Atypical computation might be to determine the requiredoutdoor air that must be taken in by a system to maintainthe desired air quality, assuming air cleaning to occur. Theequations can also be used to solve for space contaminantconcentration, required recirculation rate, or requiredfilter efficiency.

Example No. 3

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A constant air volume system having a filter located in thesupply duct (location B, Fig. 4-9) and a filter efficiencyof 70 percent for environmental tobacco smoke (ETS) is to beused to assist in holding the particulate level of the ETSin an occupied zone to below 220 g/m3. Assume that anaverage occupant (including smokers and nonsmokers) producesabout 125 g/min of ETS, and that 20 cfm of outdoor air perperson is to be supplied. For a ventilation effectiveness of0.65 for the space, determine the necessary rate ofrecirculation assuming no ETS in the incoming outdoor air.Given:N = 125 g/minCs = 220 g/m3

Co = 0Qo = 20 cfmEf = 0.7Ev = 0.65Required:RQr

Solution:Solving Equation (8) for RQr

RQr=N+Ev Qo[ (1−Ef)C o−Cs]

EvEfCsFor each person this is

RQr=125 μg/min+(0.65 ) (20 cfm ) [(1−0.7) (0)−220 μg /m3 ](0.0283 m3/ft3)

(0.65 )(0.7 ) (220 μg/m3 ) (0.0283 m3 /ft3)RQr=15.6 cfm /person

The total rate of supply air to the room Qt = Qo + RQr = 20 +15.6 = 35.6 cfm/person. If we assume that there were about 7persons per 1000 square feet as typical for an office, theair flow to the space would be

QA=

(35.6 cfm /person) (7 persons )1000 ft2

=0.25 cfm /ft2

This would probably be less than the supply air-flow ratetypically required to meet the cooling load. A lessefficient filter might be considered. If the above filterwere used with the same rate of outdoor air but withincreased supply and recirculation rates, the air in thespace would be better than the assumed level.

Example No. 4

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For Example No. 3 assume that the cooling load requires that1.0 cfm/ft2 be supplied to the space, determine therecirculation rate per person QrR, and the concentrationlevel of the ETS in the space. Assume that the rate ofoutdoor air per person and the filter efficiency remainunchanged.Given:1.0 cfm/ft2, other remains unchanged except the required.Required:RQr

Cs

Solution:R QrA =

QtA −

QoA =1.0−

(7) (20 )1000

=0.86 cfm/ft2

RQr=(0.86 cfm /ft2) (1000 ft2 )

7 persons=123 cfm /person

Solving Equation (11) for Cs,

Cs=N+Ev Qo(1−Ef)CoEv (Qo+RQrEf )

Cs=125 μg/ (min−person)+00.65 [20+(123 ) (0.7) cfm /person ](0.0283 m3/ft3)

Cs=64 μg/m3

The extra recirculation of the air through the filter hasreduced the space concentration level of the tobacco smokeconsiderably with no use of extra outdoor air.

Example No. 5Assume that the office in Example No. 4 is occupied by 70persons and that a suitably efficient filter was the M-15filter of Fig. 4-8 and Table 4-3. Using this filter, designa system that has a pressure loss of no more than 0.30 in.wg in the clean condition.Given:Office of Example No. 470 person, M-15 filter, pn = 0.30 in. wg maximum.Required:Design of a system (size and number of filter)Solution:Table 4-5 gives the application data needed. There are foursizes of M-15 filters to choose from, and the rated cfm at0.35 in. wg pressure loss is given for each size. We must

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choose an integer number of filter elements. The totalsupply cfm required for 70 person is

Qs=(123+20 cfm /person) (70 persons )=10,000 cfm

It is desirable for the complete filter unit to have areasonable geometric shape and be as compact as possible.Therefore choose the 24 x 24 x 12 elements for a trialdesign. The rated cfm will first be adjusted to obtain apressure loss of 0.30 in. wg using Equation(6):

Qn=Qr(ΔpnΔpr )1/2

=2000( 0.30.35 )1 /2

=1852 cfm /element

Then the required number of elements is

n=QsQn

=10,0001852

=5.40 element

Since n must be an integer, use 6 elements and the completefilter unit will have dimensions of 48 x 72 in., areasonable shape. However, the filter unit will have apressure loss less than the specified 0.30 in. wg. Againusing Equation (6) the actual pressure loss will beapproximately

Δp=Δpr ( QQr )2=0.35(10,000 /6

2000 )2=0.24 in.wg

This is not an undesirable result and can be taken intoaccount in the design of the air distribution system.

- End -

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