Objectives

Finish with tracer gas measurement

Blower door and duct bluster measurements

Tracer gas and IAQ ApplicationsQuantification of outside airAir distribution system efficiency Air change EfficiencyContaminant removal effectiveness Leak detection House/chamber/duct/Duct flow Re-entrainment of exhaust air into ventilation systemSimulate toxic pollutant distributionMany other applications

A Good Tracer Gas?Non-toxicEnvironmental friendlyColorless and odorlessEasily detectableInertNo other sources

Common Tracer Gases UsedCarbon DioxideNitrous OxideFreonHeliumSulfur Hexafluoride

Application 1:Quantification of outside airVolumetric Air MeasurementsStandard TestASTM E741 - 00(2006)Available by the UT library website

Test Method for Determining Air Change in a Single Zone by Means of aTracer Gas Dilution

ASTM E741 Test MethodDifferent methods:

Concentration Decay (or concentration increase)Constant Injection

Concentration Decay MethodInject predetermined volume of gas into room

Mix room air to get uniform concentration

Monitor gas concentration decay

Aim for 10 samples over measured time

Use reactor model to predict concentrations

Theoretical BasisSpace balanceVoldC/d = VCin-VCout+N0 If Cin = constant & Vol/V = ACH dC/(Cin-Cout) = ACH d Integrate:

. ACH =1/{ln[Cin(=)- Cout)]- ln[Cin(=0)- Cout)]}

Concentration Decay MethodAir Change Rate:

In the case of zero inlet concentrate and perfect mixing in the space ACH =- (ln C2 - ln C1)/ (in hours)

C1 = Tracer Gas Concentration at start of testC2 = Tracer Gas Concentration at end of test

Tracer gas result[minutes]

Decay TestAdvantagesDont need to release precise amountDont need to measure volume (if Cout = 0)DisadvantagesNeed to keep building well-mixedRecontamination from buffer spacesHouse needs to stay in one condition for entire test

Single zone Example:Coffee Houses Lohaus and Waring (2006) ArE 381E Course Project

How do you estimate uncertainty?Use standard error of slopeFollow ASTM E741ACH < 10%

Advanced Tracer Gas TestingMulti-zone flowsEasiest Use several unique tracer gasesHarder Use flow and mass balances

Consider Two-Zone Building(non-perfect mixing)V1V2V4V3V1V2EvdAV6V5ToolsMass balance on tracer gasMass (flow) balance on airMeasured concentrations in each space

EquationsHow many unknowns? Equations?Flow direction for interzonal flowAir exchange rate for spacesSums of flows

Solution procedure Reduce mass balance to one equation by solving C2 equation for C1 and substituting into C1 equation2nd order ODESame thing for C1 equation6 unknown flowsOverall flow balance can be used to get two unknown flowsMeasured tracer gas concentrations can be used to eliminate two more flowsAdditional data needed for solvable systemWe need to use multiple tracer gasses!Or we need to measure flows with flow meters!

Example 2 Air distribution system efficiencyHow well is outside supply air distributed to breathing zones in occupied areas?

Air exchange efficiency

ASHRAE Standard 129 Measuring Air Change EffectivenessUses Tracer Gas TechniquesAge-of-Air Measurements

Why Worry About Good Mixing?Poor MixingOccupant complaintsASHRAE Standard, Ventilation for Acceptable Indoor Air QualityASHRAE Standard is based on amounts of outside air getting to breathing zone not to supply air louvers

Short circuiting airflow patternsWhere a significant portion of supply air flows directly to the exhaust, bypassing the occupied portion (breathing zone) of the ventilated space.

Air Exchange EffectivenessThe definition is based on a comparison of the age of air in the occupied portions of the building to the age of air that would exist under conditions of perfect mixing of the ventilation air.

Age of AirThe age of the air at a give location is the average amount of time that has elapsed since the air molecules at that location entered thebuilding.Amount of time outside air has been in an area

Two Methods of determinationStep-up constant tracer gas injectionTracer gas concentration decay

How to measure Age of Air? Step down method:

Injection and mixing Air in the room is marked with tracer gas (injection and mixing) Ventilation turned onAge of Air MeasurementsLocations of interest In the exhaust (C)

Constant Injection

V = N / (Cout - Cin) You need to get to steady state injection

Constant InjectionAdvantagesCan determine time-dependence of air exchange ratesDisadvantagesNeed to keep building well-mixedRecontamination from buffer spacesNeed to have mass flow controllerNeed to measure volume (for ACH())

How to measure Age of Air and Air Exchange EffectivenessAge of air at a location =Air change effectiveness (E)E = < 1.0 (less than perfect mixing)E = 1 (perfect mixing)avg age of air =

Significance of Air Exchange EffectivenessASHRAE Standard 62.1-2004 Ventilation for Acceptable Indoor Air Quality - Outside air requirements = QA/E as E decreases, OA should increase

US Green Building Council LEED Rating requires an E > 0.9 in all ventilated zones

Tracer Gas Instrumentation

Tracers which we useSF6 Gas analyzer

ppm with IR absorption or photo-acoustic IRppb with GC/ECD

CO2 Tracers gas analyzer ( CO2 sensor )

*Infiltration/Leakage Measurements (Calibrated Fans)Flow going through fan is calibrated to pressure rise across fanIf you measure pressure, than you know flowRequires very smooth inlet and outlet conditionsOften have to restrict flow to get measurable pressure signal for low flowsExamples:Blower door and Duct Blaster*

Fan PressurizationIn 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

Quantitative Blower Door How many points should you measure? How can you estimate uncertainty?http://www.energyconservatory.com/products/products1.htm#pd16

Test Data1000 ft2 house, 10 ft high ceilings

Two-point test1688 CFM at 51 Pa1048 CFM at 24.5 PaWhat is ACH50 (50), C?

Answers50= 1 ACHC = 131.07 CFM/Pann = 0.65

Reporting blower door dataFlow (or air exchange rate) at a given pressureEquivalent Leakage Area

What are dimensions of c?

CommentBlower doors do not measure air exchange rateWhy not?What do they measure?

*Blower Door Misc. All windows and doors to outside closedWhy? What about buffer spaces?HVAC offHard to do on windy days Watch out for buckling linoleum floors and fireplacesFlow direction change/ring change can be discontinuousRemember to record before and after reference pressures with fan covered*

More Blower Door TestsMeasure component leakageWhat fraction of leakage is due to a certain component?Tape off component and repeat testWhat are issues with this approach?Series leakage paths (AKA pressure diagnostics)Air leaks to attic and then to crawlspace and then to outside

Residential Component Leakage

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

IGERT: Indoor Environmental Science & Engineering***********