L13-Indoor Air Quality 1 - Uwaterloo.ca

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11/5/2014Chapter 14Indoor Air Quality1Indoor EnvironmentandBuilt Environment Buildings Transportationsvehicles21

11/5/2014Indoor Air Quality (IAQ)Indoor air quality (IAQ) is the quality of airin an indoor environment. Thermal comfort– Temperature,– Relative Humidity Indoor Air Pollutant3Why is IAQ Important? We spend over 90% of our time in indoor environments IAQ is much poorer than outdoor air 2-100 times worse in USA/Canada National Institute for Occupational Safety and Health(NIOSH) ranked Occupational lung dysfunctions (includinglung cancer, pneumonoconioses, and occupational asthma)the top occupational diseases and injuries. 30% of newly constructed or remodeled facilities have IAQproblems. Illnesses related to indoor air pollution have beenclassified into two categories: Sick building syndrome (SBS) Building related illness (BRI) Indoor contaminants are responsible for half of all illnesses Liability issues2

11/5/2014Illnesses related to indoor air pollution SBS: Sick building syndrome– SBS is defined as the discomfort or sicknessassociated with poor indoor air quality with clearidentification of the source substances.– Examples: irritation to eyes, noses, or throat,fatigue, and nausea. BRI: Building related illness– BRI is defined as a recognized disease caused by knownagents that can be clinically identified.– Examples: asthma, legionella, hypersensitivity, andhumidifier fever. Approximately one million buildings in the United States are5sick buildings with 70 million occupants.Indoor airpollutants AsbestosDust/Aerosol/PMAsbestosCombustion related contaminantsFormaldehydeVOCs: Volatile Organic CompoundsBio‐aerosol: Allergen, mold , dust miteRadon63

11/5/2014Threshold Limit Values Threshold limit values (TLVs) refer to upper limit of theconcentrations of indoor air pollutants under which itis believed to be safe for all working occupants withoutimpacting their health.1. Time‐Weighted Average Threshold Limit Value (TLV‐TWA) Usually 8 hours 24 hours or Annual2. Short ‐Term Exposure Limit Threshold Limit Value ‐ (TLV‐STEL): Say 15 minutes3. Threshold Limit Value ‐ Ceiling (TLV‐C): At any instance7Concentration8‐Hour Time‐Weighted AverageExposure LimitTWA Limit8‐hr timeweightedaverageReal time measurementTime (hr)84

11/5/2014Exposure Limit Guidelines American Conference ofGovernmental IndustrialHygienists (ACGIH) alsopublishes– Threshold Limit Values forChemical Substances andPhysical Agents and BiologicalExposure Indices.– Updated annually WHO (2010) guidelines forindoor air quality9Threshold values of typical indoor air pollutants in work placesPollutantFormulaOdorGases (In ppmv unless stated otherwise)Sharp pungentAmmoniaNH3NoneCarbon dioxideCO2Carbon monoxideCONoneFormaldehydeHCHOPungentRotten eggsHydrogen neO3Particulates (Unit in mg/m3 unless stated otherwise)AsbestosCoal dust, anthraciteCoal dust, bituminousGrain dust (Oat, wheat, barley)Graphite (non fiber)Iron oxide particles and fume, inhalableLeadWelding ‐0.3150.1 fiber/ml0.40.94.02.05.00.055.05

11/5/2014Normalized Air Contaminant ConcentrationIn order to quantify the effect of the combinedeffect, the normalized concentrationMeasured concentrationsThreshold limits 1: Do something!11Example 14.1 In a welding shop, the measured concentrations of CO, CO2and welding fumes are 10 ppmv, 1,500 ppmv and 3.5 mg/m3,respectively, each below the recommended TLV‐TWA. Is this working environment safe to the workers daily based onnormalized concentration?SolutionAir PollutantTLV‐TWACO255,000CO2Welding fumes50.40Measured 701.0Since the CN 1, it is not safe for the workers to be there daily (8 hours).126

11/5/2014Clean room13Clean room ISO 146441‐1 100.1. upper limit of particle number concentration,number/m3 the clean room class number– N 1, 2, .9 threshold particle diameter in μm, – 0.1, 0.2, 0.3, 0.5, 1 and 5 μm. According to ISO 146441‐1, in a class N cleanroom,the number concentration of particles greater than cannot exceed .147

11/5/2014Example 14.2: Cleanroom class An ISO 146441 1 Class 2 cleanroom is 3 meter heighwith a total area of 100 m2, what is the maximumamount of particles that are greater than 100 nm? Solution:0.1;2Substitute 100 100.1.10 0.10.1.100 Since the volume of the room is 300 m3, the totalnumber of particles larger than 100 nm in diametercannot exceed 30,000 in total.15ISO ClassNumber(N)Maximum particle number concentration (#/m3) forparticle sizes 0.1 μm 0.2 μm 0.3 μm 0.5 μm 1 μm 5 μmISO Class 1 102ISO Class 2 10024104ISO Class 3 1,000237102358ISO Class 4 10,0002,3701,0203528310,2003,52083229ISO Class 6 1,000,000 237,000 102,00035,2008,320293ISO Class 7352,00083,2002,930ISO Class 83,520,000832,000 29,300ISO Class 935,200,000 8,320,000 293,000ISO Class 5 100,000 23,7008

11/5/2014IAQ Control Source control Very important to IAQ Ventilation: Ventilation controls the indoor airquality by bringing fresh air into an indoor environment supply or reduce the heat and moisture and dilute gaseous and particulate pollutants indoors. IAQ control devices Indoor air cleaner Indoor (de)humidifier17Exhaust9

11/5/2014 In general, three types ofvariables are of concern andcan be controlled byventilation in an indoorenvironment: temperature, relative humidity, and air pollutants. Ideally a minimum ventilationrate must be maintained inorder to control all theseparameters at desired levelsfor an indoor environment. Practically many buildingsminimum ventilation ratesare based on the temperaturecontrol due to the energyconcern.Moist Air Moist Air Dry air Water vapor For air at certain pressure and temperature Water vapor pressure– Partial pressure of water vapor Saturated air2010

11/5/2014Dew Point The dew point is thetemperature at which thewater vapor in air (at constantpressure) condenses intoliquid water at the same rateat which it evaporates. At temperatures below the dew point, watervapor condenses and leaves the air. The condensed water is called dew when itforms on a solid surface.21 The maximumsaturation pressureof the water vapor inmoist air varies withthe temperature ofthe air vapor mixtureSaturated Fraction of Water Vapor by Mass2211

11/5/2014Humidity Absolute humidity: the mass of water vaporper unit volume of moist air– Not useful for heat and mass transfer Relative humidity is the ratio of the partialpressure of water vapor in an air‐watermixture to the saturated vapor pressure ofwater at a prescribed temperature.– The relative humidity of air depends ontemperature and the pressure of the system ofinterest.23How Relative Humidity Affects IAQ Too low, dry air, evaporation from surface(including your body)– Dry skin, dry eyes, dry throat . Discomfort!– 50‐60% most comfort Too high,condensation,mold2412

11/5/2014Acceptable Temperature RangesASHRAE 62.1-2004 Dry bulb at 30% relative humidity: Winter: 68.5 ºF – 76.0ºF (20.28 oC- 24.44 oC) Summer: 74.0ºF – 80.0ºF (23.33 oC-26.67 oC)Minimum Ventilation Rate Consider a control volume, where temperature,relative humidity and air pollutant levels areuniform.– This assumption is acceptable if the control volume issmall enough or the error is acceptable. This control volume could be an entire room or azone within. When the control volume is a room, the airwithin is assumed completely mixed. Admittedly this is a bold assumption, but it hasbeen widely used in guiding the HVAC industry.2613

11/5/2014Model SchematicDiagramQ Volumetric flow rate;V Volume of the paceh total enthalpyw moisture contentc pollutant concentration supply air exhaust air stands for generation rate Perfect mixing the properties in the exhaust air are thesame as in the room air.;,Mass balance for dry air where is specific volume of moist air, which isdefined herein as the volume of the moist air (dry airplus the water vapor) containing one unit of mass of“dry air”. we use specific volume of the air instead of the densitybecause specific volume the of the air defines thevolume of the mixture (dry air plus the water vapor)containing one unit of mass of “dry air”.2814

11/5/2014the energy balancesensible heat for supply air and exhaust air (kJ/kg ofdry air). It implies that the difference between the sensibleheat of the supply air and that of the exhaust air equals tothe total sensible heat production plus the change ofsensible heat in the room air. The total sensible heat transfer rate, , of the ventilatedairspace is the sum of all heat loss or gain through andwithin the airspace, including the sensible heat productionrate by occupants and indoor equipment (e.g. stove, lights),and the heat transfer through building envelope. cam bepositive or negative, which are referred to as heating loador cooling load. 29Moisture mass balanceswherehumidity ratio (kg of water vaporper kg of dry air, kg/kg).Subscripts 's' and 'e' stand for supply andexhaust air, respectively. is the water vaporproduction rate in kg/s.3015

11/5/2014Specific Pollutant Mass Balanceis the mass production rate of the particulatepollutant in kg/s. In these two equations, the units ofpollutant concentration should be (kg/m3 air). In most engineering practice, the unit of gases is inppmv Therefore, unit conversions are necessary foraccurate calculation of ventilation rate with a unit ofm3 air/s ,31CO2 Production Rate From large scale statistical analyses, it has been determinedthat the carbon dioxide production rate of average humanand animals is 1 liter of CO2 24,600 J of total heat production (THP)under standard indoor conditions. Converting to the mass production rate of carbon dioxide inkg/s is24600the density of carbon dioxide (1.83 kg/m3 at 20 oCand 1 atm) the total heat production of occupants (kJ/s) 3216

11/5/2014;; 01133Psychrometric chart http://www.uigi.com/UIGI SI.PDF See handout3417

11/5/2014Example 14.3: Ventilation rate calculation A dinning room have 20 people each produces 200 W total heat. The carbon dioxide concentration in the supply air is 500ppmv. Assume the supply air temperature is 15 oC and 50%relative humidity. The room air is 22 oC and 60% relative humidity. If the required maximum CO2 concentration in the room is1000 ppmv, estimate the minimum ventilation rate based on the CO2concentration.35Solution Based on the supply air temperature of 15 oC and 50% relative humiditywe can get the specific volume of supply air using a psychrometric chart, 0.822 m3/kg dry air The specific volume of exhaust air at 22 and 60% relative humidity is 0.855 m3/kg dry air The total heat production rate by 20 people in the dining room is,THP 20 200 W 4000W or 4 kJ/s Then the mass production rate of CO2 is estimated using Equation (14‐9)41.832.98 10/22 2460024600 Then the minimum ventilation rate fan is determined using Equation(14‐6)2113618

11/5/2014Solution (continued)121 Since , the minimum ventilation rate should be calculatedbased on the exhaust air. However, we need to convert the unit ofconcentration from ppmv to kg CO2/m3 air.1000 31.83 10The exhaust ventilation rate is10632 1.8322332 3212.981000321.83106 30.311042 32 3210.8220.8555001000/37Indoor Air Cleaning ModelingSpace volume V;Indoor concentration ci ;Source: ;Outdoor concentration coFresh airMake upair filterRecirculatingair filter,11Exhaust air,1Recirculating air,3819

11/5/201411Removal rateInto the control volumeAccumulationrateMake up airfilter,Fresh airInternal cleaner11Exhaustair,1Source:Recirculatingair filter,Recirculating air,20

11/5/20141111111exp114111exp11111 1exp4221

11/5/2014Example 14.4 Consider a kitchen with a volume of 80 m3 where natural gas is used as a cooking fuel Assume after cooking the indoor fume concentration is1000 µg/m3. The HVAC system works at a fresh flow rate of 1 m3/s a recirculating flow rate 1/3 of that of the fresh air. An internal air cleaner has a flow rate of 0.1 m3/s; Ignore the outdoor and indoor source. The intake and recirculating filter efficiencies are thesame as 90% the internal air cleaner has a flow rate of 1 m3/s and an efficiency of 95%. Plot the indoor fume concentration over time.43Solution0 0, and0 .1000 exp75.35.4422

Indoor fume concentration e (s)Figure 14‐4. Calculated indoor air pollutantconcentration over time23

Indoor Air Quality (IAQ) Indoor air quality (IAQ) is the quality of air in an indoor environment. Thermal comfort - Temperature, - Relative Humidity Indoor Air Pollutant 3 Why is IAQ Important? We spend over 90% of our time in indoor environments IAQ is much poorer than outdoor air 2-100 times worse in USA/Canada

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