Objectives Finish air cleaner problem Measure ventilation rates Focus on smaller buildings

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Transcript of Objectives Finish air cleaner problem Measure ventilation rates Focus on smaller buildings

  • ObjectivesFinish air cleaner problem

    Measure ventilation ratesFocus on smaller buildings

  • Measuring air exchange ratesTwo general strategies to get Direct measurement Tracer gas (constant injection or decay)Apply well-mixed reactor modelIndirect measurement Blower doorApply infiltration model

  • Tracer Gas TestingRelease gas and measure concentrationUse model to estimate ventilation rateProperties of a tracerNontoxicNon-reactive (chemically inert)Not present at significant concentrations in typical environmentsEasy to measure over wide concentration range

  • Common tracersSF6ppm with IR absorptionppb with GC/ECDTracers of chance (eg. CO2)

  • Tracer Gas Decay TestSeed tracer gasMix wellAim for 10 samples over ~1 time constantUse reactor model to predict concentrations

  • Decay TestAdvantagesDont need to release precise amountDont need to measure volume (if you just want air exchange rate)DisadvantagesNeed to keep building well-mixedRecontamination from buffer spacesHouse needs to stay in one condition for entire test

  • Murray and Burmaster (1995) Risk Analysis

    ACH plots

    0.150.25

    0.20.5

    0.270.8

    0.31

    0.331.3

    0.391.4

    0.51.6

    0.581.9

    0.752.5

    0.95

    1.4

    2.5

    Residential

    Commercial

    % Less Then

    Air Exchange Rate (1/hr)

    Building Air Exchange Rates

    Sheet1

    ResidentialCommercial Buildings

    % less thanACH (1/h)% less thanACH (1/h)

    50.1550.25

    100.2250.5

    200.27450.8

    250.3651

    300.33831.3

    400.39871.4

    500.5921.6

    600.58951.9

    700.75982.5

    800.95

    901.4

    952.5

    Sheet2

    Sheet3

  • Constant InjectionContinuously release known amount of tracer gas into spaceContinuously mix airUse model to determine air exchange rate

  • Constant InjectionAdvantagesCan determine time-dependence of air exchange ratesDisadvantagesNeed to keep building well-mixedRecontamination from buffer spacesNeed to have mass flow controllerTypically uses more tracer gasNeed to measure volume

  • Common problemsContamination from buffer spacesHW #2 Problem 3 (last year)

  • SummaryTracer gas can be used to measure ventilation rateAssumes reactor modelConstant injection or decay

    Measures ventilation rate at time of testVaries with time, T ,wind

  • Fan Pressurization The Early YearsIn 1970s, smoke evacuation fans used to find air leakageInstall blower doorUse fan to create artificial pressure difference between inside and outsideUse smoke stick (or cigarette, etc.) to visualize flow patterns.Seal leaks

  • Ref: Home Energy Magazine, 1995

  • LimitationsFlow rate (can use multiple fans)Not good for complex leakage pathsMultizone buildingsDoes not give you air exchange rateIndependent of wind, T, timeUseful for comparison of different buildings

  • Blower Door (quantitative)

  • Procedure for Blower door testInstall blower doorUse fan to create artificial pressure difference between inside and outsideMeasure flow at several inside-outside pressure differencesFind n and C

  • Ref: Sherman and Dickerhoff (1994, 1998)

  • Residential Component Leakage

    ComponentRangeAverageWalls18-50%35%Ceilings3-3013Windows/Doors6-2215Fireplaces0-3012HVAC3-2818Other Vents2-125

  • Reporting blower door dataFlow (or air exchange rate) at a given pressureWhy is air exchange rate preferable?Leakage Area

    What units are c in?

  • Where does that leave us?ELA is at specific reference pressure.So, use LBL model and improvements to get infiltration and ventilation ratesASHRAE 2001 Handbook of Fundamentals 26.21-26.24Requires L, T, wind and shelter informationMore advanced forms require distribution of leakage

  • LBL ModelRef: Sherman (1992) Indoor AirQ = Ventilation air flow (CFM)AL = ELA from blower door test (in2)Cs = stack coefficient f(height) (CFM2/(in4 F))t = temperature difference (F)Cw = wind coefficient f(height, shielding) (CFM2/(in4 mph))Ref: ASHRAE Fundamentals (2001) ch. 26

  • SummaryIn order to measure air exchange ratesNeed to use direct tracer gas measurements to get a snapshot of Use reactor modelNeed to use a blower door to compare to other buildingsUse LBL (or other) model to get

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