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Objectives Finish with tracer gas measurement Blower door and duct bluster measurements.

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Presentation on theme: "Objectives Finish with tracer gas measurement Blower door and duct bluster measurements."— Presentation transcript:

1 Objectives Finish with tracer gas measurement Blower door and duct bluster measurements

2 Tracer gas and IAQ Applications Quantification of outside air Air distribution system efficiency –Air change Efficiency –Contaminant removal effectiveness Leak detection House/chamber/duct/… Duct flow Re-entrainment of exhaust air into ventilation system Simulate toxic pollutant distribution Many other applications

3 A Good Tracer Gas? Non-toxic Environmental friendly Colorless and odorless Easily detectable Inert No other sources

4 Common Tracer Gases Used Carbon Dioxide Nitrous Oxide Freon Helium Sulfur Hexafluoride

5 Application 1:Quantification of outside air Volumetric Air Measurements Standard Test –ASTM E (2006) Available by the UT library website Test Method for Determining Air Change in a Single Zone by Means of a Tracer Gas Dilution

6 ASTM E741 Test Method Different methods: –Concentration Decay (or concentration increase) –Constant Injection

7 Concentration Decay Method Inject 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

8 Theoretical Basis Space balance V V C in N Source C out V ol ·dC/d = V·C in -V·C out +N 0 If C in = constant & V ol /V = ACH dC/(C in -C out ) = ACH· d Integrate: ……. ACH =1/ Δ ·{ln[C in ( = Δ )- C out )]- ln[C in ( =0)- C out )]}

9 Concentration Decay Method Air Change Rate: In the case of zero inlet concentrate and perfect mixing in the space ACH =- (ln C 2 - ln C 1 )/Δ (in hours) C 1 = Tracer Gas Concentration at start of test C 2 = Tracer Gas Concentration at end of test

10 Tracer gas result [minutes]

11 Decay Test Advantages –Dont need to release precise amount –Dont need to measure volume (if C out = 0) Disadvantages –Need to keep building well-mixed –Recontamination from buffer spaces –House needs to stay in one condition for entire test

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

13 How do you estimate uncertainty? 1.Use standard error of slope 2.Follow ASTM E741 –ΔACH < 10%

14 Advanced Tracer Gas Testing Multi-zone flows –Easiest – Use several unique tracer gases –Harder – Use flow and mass balances

15 Consider Two-Zone Building (non-perfect mixing) V1V1 V2V2 V4V4 V3V3 V1V1 V2V2 E vdAvdA V6V6 V5V5 Tools Mass balance on tracer gas Mass (flow) balance on air Measured concentrations in each space º º º º º º

16 Equations How many unknowns? Equations? Flow direction for interzonal flow Air exchange rate for spaces Sums of flows

17 Solution procedure Reduce mass balance to one equation by solving C 2 equation for C 1 and substituting into C 1 equation –2 nd order ODE –Same thing for C 1 equation 6 unknown flows –Overall flow balance can be used to get two unknown flows –Measured tracer gas concentrations can be used to eliminate two more flows –Additional data needed for solvable system We need to use multiple tracer gasses! Or we need to measure flows with flow meters!

18 Example 2 Air distribution system efficiency How well is outside supply air distributed to breathing zones in occupied areas? Air exchange efficiency ASHRAE Standard 129 – Measuring Air Change Effectiveness Uses Tracer Gas Techniques Age-of-Air Measurements

19 Why Worry About Good Mixing? Poor Mixing Occupant complaints ASHRAE Standard, Ventilation for Acceptable Indoor Air Quality ASHRAE Standard is based on amounts of outside air getting to breathing zone not to supply air louvers Short – circuiting airflow patterns Where a significant portion of supply air flows directly to the exhaust, bypassing the occupied portion (breathing zone) of the ventilated space.

20 Air Exchange Effectiveness The 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.

21 Age of Air The 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 the building. Amount of time outside air has been in an area Two Methods of determination –Step-up constant tracer gas injection –Tracer gas concentration decay

22 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 on Age of Air Measurements –Locations of interest –In the exhaust (C)

23 Constant Injection V = N / (C out - C in ) º V º V º C in N Source C out You need to get to steady state injection

24 Constant Injection Advantages –Can determine time-dependence of air exchange rates Disadvantages –Need to keep building well-mixed –Recontamination from buffer spaces –Need to have mass flow controller –Need to measure volume (for ACH( ))

25 How to measure Age of Air and Air Exchange Effectiveness Age of air at a location = Average tracer gas level during test Tracer gas level at beginning of test Air change effectiveness (E) E = < 1.0 (less than perfect mixing) E = 1 (perfect mixing) avg age of air – Exhaust avg age of air – age of air in breathing zone E = avg age of air =

26 Significance of Air Exchange Effectiveness ASHRAE Standard 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

27 Tracer Gas Instrumentation

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30 Tracers which we use SF 6 Gas analyzer –ppm with IR absorption or photo-acoustic IR –ppb with GC/ECD CO 2 Tracers gas analyzer ( CO 2 sensor )

31 31 Infiltration/Leakage Measurements (Calibrated Fans) Flow going through fan is calibrated to pressure rise across fan –If you measure pressure, than you know flow Requires very smooth inlet and outlet conditions Often have to restrict flow to get measurable pressure signal for low flows Examples: –Blower door and Duct Blaster 31

32 Fan Pressurization In 1970s, smoke evacuation fans used to find air leakage 1. Install blower door 2. Use fan to create artificial pressure difference between inside and outside 3. Use smoke stick (or cigarette, etc.) to visualize flow patterns. 4. Seal leaks

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35 Quantitative Blower Door How many points should you measure? How can you estimate uncertainty?

36 Test Data 1000 ft 2 house, 10 ft high ceilings Two-point test –1688 CFM at 51 Pa –1048 CFM at 24.5 Pa What is ACH 50 (λ 50 ), C?

37 Answers λ 50 = 1 ACH C = CFM/Pa n n = 0.65

38 Reporting blower door data Flow (or air exchange rate) at a given pressure Equivalent Leakage Area –What are dimensions of c?

39 Comment Blower doors do not measure air exchange rate –Why not? What do they measure?

40 40 Blower Door Misc. All windows and doors to outside closed –Why? What about buffer spaces? HVAC off Hard to do on windy days Watch out for buckling linoleum floors and fireplaces Flow direction change/ring change can be discontinuous Remember to record before and after reference pressures with fan covered 40

41 More Blower Door Tests Measure component leakage –What fraction of leakage is due to a certain component? –Tape off component and repeat test What are issues with this approach? Series leakage paths (AKA pressure diagnostics) –Air leaks to attic and then to crawlspace and then to outside

42 Residential Component Leakage ComponentRangeAverage Walls18-50%35% Ceilings Windows/Doors Fireplaces HVAC Other Vents2-125


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