Transcript Tracer Gas Decay Test

Objective
Finish with:
• Heat Flux & Relative Humidity Measurement
• Pressure measurement
Start tracer gas measurement
Heat flux sensor
The flow of heat through the transducer
creates a minute temperature difference
between its surfaces. A multi-element,
semi-conductor thermopile, consisting
of hundreds of Bi/Te elements generates
a D.C. voltage via the Seebeck effect.
The resulting signal is directly
proportional to the heat flux through the
transducer.
Semiconductor thermopile can be
replaced of thermocouple thermopile
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Heat flux sensor
Thermocouple thermopile
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Other methods for heat flux
measurements
Material / wall assembly testing
Conductivity (R value) measurement:
- Hot box
- Hot plate
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Relative Humidity: Why
Measure?
• Comfort
• HVAC operation effectiveness
• Dust mites, mold
300 m
• Accumulation of water molecules on surfaces
• Drying of liquid water
• Effects on other physical processes in buildings
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Measurement Techniques
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Psychrometer
(± 3 – 7% accuracy)
Mechanical sensors
(± 5-10% accuracy)
Thin-Film Polymer
(± 2 – 5% accuracy)
Chilled Mirror
(± 0.2 – 2 oK accuracy on Tdew)
Other
• Summary: ASHRAE Handbook Ch 14 – Table 3
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Thin-Film Polymer Sensors
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See ASHRAE PDS IV – pg. 2-12
Most common RH sensor
Water molecules adsorb/desorb w/ changes in RH
Material props change (R or dielectric constant)
Current converts change to V or capacitance
Change related to RH
Accuracy order 2-5% RH
Fast response, low cost, good accuracy
Contamination can be an issue
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Polymer: Resistance Sensors
• Use change in resistance of hygroscopic polymer
• Works well for 20% - 90% RH
• Absorption/desorption time takes ~minutes
• Sensitive to contaminants
– Need frequent calibration
– Not a trivial process
Source: Wiederhold. 1997. Water Vapor Measurement
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Polymer: Capacitance sensors
• Porous electrode absorbs water
• Faster response
– Thin
– Response time ~seconds
• Calibration issues
Source: Harriman. 2001. ASHRAE
Humidity Control Design Guide for
Commercial Buildings
– Need to be initially calibrated
– Same contamination issues
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Chilled Mirror
• Directly measures dew point
• Calibration standard
– Very accurate over large range
– Slow, expensive, contamination issues
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Source: Wiederhold. 1997. Water Vapor Measurement
Absolute vs. Pressure
Difference
• Absolute = barometric pressure
– Reference is a vacuum
• Differential ≈ gauge pressure
– Reference is user defined (often atmospheric)
Buildings:
– We are almost always only interested in pressure
difference.
– Get in the habit of saying pressure with respect to
reference
• i.e. House is +10 Pa w.r.t. outside, Garage is -5 Pa w.r.t.
outside
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Pressure Sensors
• Easiest and very accurate is a
manometer
– But, hard to
• measure low p
• Make electronic recording
• Piezoelectric sensors
• Diaphragm with strain gages
– Some issues
• Drift (auto-zeroing)
• Hysteresis
• Require power
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Some Sensors
• Setra (pressure input → voltage output)
– http://www.onsetcomp.com/solutions/products/energy
/_sensor.php5?snid=101
• Energy Conservatory DG-700
– http://www.energyconservatory.com/products/product
s4.htm
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Tracer Gas Tests
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?
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Non-toxic
Environmental friendly
Colorless and odorless
Easily detectable
Inert
No other sources
Common Tracer Gases Used
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Carbon Dioxide
Nitrous Oxide
Freon
Helium
Sulfur Hexafluoride
Application 1:Quantification of outside air
Volumetric Air Measurements
• Standard Test
– ASTM E741 - 00(2006)
• 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
– 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
º
º
Vspace·dC/d = V·Cin-V·Cout+N
0
º
If Cin = constant & Vspace/V = ACH
dC/(Cin-Cout) = ACH· d
º
º
V
V
Integrate:
…….
Cin
N
Cout
Source
ACH =1/Δ·{ln[Cout(=Δ)- Cin)]- ln[Cout(=0)- Cin)]}
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
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
º
V4
º
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V1
V3
V5
V3 or V6
V2
E
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V1
V2
• Tools
• Mass balance on tracer gas
• Mass (flow) balance on air
• Measured concentrations in each space
vdA
Equations
dC1 º
º
º
º
V
 V2Cout  V3C2  E  V3  V1 C1
1 dt
dC2
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º
º
V
 V4Cout  V3C1  vd AC2  V3  V5 C2
2 dt
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V1  V5  V2  V4
C1 ( ), C2 ( )
• How many unknowns? Equations?
• Flow direction for interzonal flow
• Air exchange rate for spaces
•Sums of flows
Comments
• 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
• 5 unknown flows
– Overall flow balance can be used to get four unknown
flows
– Measured tracer gas concentrations can be used to
eliminate two more flows
– Additional data needed for solvable system
• Often use multiple tracer gasses
Single zone Example:
Coffee Houses
-1
Air Exchange Rate (h )
2.5
SF6
CO2
2
1.5
1
0.5
0
1
2
3
4
Test Number
5
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Lohaus and Waring (2006) ArE 381E Course Project
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
º
V = N / (Cout - Cin)
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º
V
V
Cin
You need to get to steady state injection
N
Source
Cout
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 test
Tracer gas level at beginning of test
Air change effectiveness (E)
E=
avg age of air – Exhaust
avg age of air – age of air in breathing zone
avg age of air =
E = < 1.0 (less than perfect mixing)
E = 1 (perfect mixing)
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 )