Objectives

Finish with tracer gas measurement

Blower door and duct bluster measurements

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

A Good Tracer Gas?

• Non-toxic

• Environmental friendly

• Colorless and odorless

• Easily detectable

• Inert

• No other sources

Common Tracer Gases Used

• Carbon Dioxide

• Nitrous Oxide

• Freon

• Helium

• Sulfur Hexafluoride

Application 1:Quantification of outside airVolumetric Air Measurements

• Standard Test

– ASTM E741 - 00(2006)• Available by the UT library website

• Test Method for Determining Air Change in a Single Zone by Means of a

Tracer Gas Dilution

ASTM E741 Test Method

• Different methods:

– Concentration Decay (or concentration increase)

– Constant Injection

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

Theoretical Basis

Space balance

V V

Cin NSource

Cout

Vol·dC/d = V·Cin-V·Cout+N

0 If Cin = constant & Vol/V = ACH

dC/(Cin-Cout) = ACH· d

Integrate:

…….

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

Concentration Decay Method

Air Change Rate:

In the case of zero inlet concentrate and perfect mixing in the space

ACH =- (ln C2 - ln C1)/Δ (in hours)

C1 = Tracer Gas Concentration at start of test

C2 = Tracer Gas Concentration at end of test

Tracer gas result

[minutes]

Decay Test

• Advantages– Don’t need to release precise amount

– Don’t need to measure volume (if Cout = 0)

• Disadvantages– Need to keep building well-mixed– Recontamination from buffer spaces– House needs to stay in one condition for

entire test

Single zone Example:Coffee Houses

0

0.5

1

1.5

2

2.5

1 2 3 4 5 6Test Number

Air

Exc

hang

e R

ate

(h-1) SF6

CO2

Lohaus and Waring (2006) ArE 381E Course Project

How do you estimate uncertainty?

1. Use standard error of slope

2. Follow ASTM E741– ΔACH < 10%

Advanced Tracer Gas Testing

• Multi-zone flows– Easiest – Use several unique tracer gases– Harder – Use flow and mass balances

Consider Two-Zone Building(non-perfect mixing)

V1

V2

V4V3

V1 V2E vdA

V6

V5

•Tools• Mass balance on tracer gas• Mass (flow) balance on air• Measured concentrations in each space

º

ºº

º

º

º

Equations

25621342

2

1132621

1

CVVACvCVCVdt

dCVol

CVVECVCVdt

dCVol

dout

out

)(),( :mesure

and

21

63544251

CC

VVVVVVVV

•How many unknowns? Equations?• Flow direction for interzonal flow• Air exchange rate for spaces

•Sums of flows

Solution procedure

• Reduce mass balance to one equation by solving C2 equation for C1 and substituting into C1 equation– 2nd order ODE– Same thing for C1 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!

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

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.

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.

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

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)

Constant Injection

V = N / (Cout - Cin) º

Vº

Vº

Cin NSource

Cout

You need to get to steady state injection

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())

How to measure Age of Air and Air Exchange Effectiveness

Age of air at a location =Average tracer gas level during testTracer 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 – Exhaustavg age of air – age of air in breathing zoneE =

avg age of air =

Significance of Air Exchange Effectiveness

• ASHRAE 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 use

SF6 Gas analyzer

– ppm with IR absorption or photo-acoustic IR– ppb with GC/ECD

CO2 Tracers gas analyzer ( CO2 sensor )

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

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

Quantitative Blower Door

• How many points should you measure?

• How can you estimate uncertainty?

http://www.energyconservatory.com/products/products1.htm#pd16

Test Data

• 1000 ft2 house, 10 ft high ceilings

• Two-point test– 1688 CFM at 51 Pa– 1048 CFM at 24.5 Pa

• What is ACH50 (λ50), C?

Answers

• λ50= 1 ACH

• C = 131.07 CFM/Pan

• n = 0.65

Reporting blower door data

• Flow (or air exchange rate) at a given pressure

• Equivalent Leakage Area

– What are dimensions of c?

Comment

• Blower doors do not measure air exchange rate– Why not?

• What do they measure?

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

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

Residential Component Leakage

Component Range Average

Walls 18-50% 35%

Ceilings 3-30 13

Windows/Doors 6-22 15

Fireplaces 0-30 12

HVAC 3-28 18

Other Vents 2-12 5