Research Objectives: The ultimate goal of this research is to develop
reliable and practical integrated testing and modeling VOC methodology. This
methodology will be used for rating products, and for predicting the long-term
VOC concentration changes over the products’ service life. In the future, this integrated prediction
methodology will be extended to applications for other gaseous and particulate pollutants.
The CHAMP model will also be useful for devising moisture control strategies
that reduce risk for potential mold growth. The latter is a significant factor
contributing to poor IAQ in some buildings. Future effort will also include
modeling the outdoor-indoor transport of semi-volatile organics (e.g., from
pesticides), particulate matter, and combustion-related gaseous such as SO2,
NOx, and O3. Thus, while the significance of the current
project may be broader, yet the project is currently defined and focused on
specific objectives. The objectives of the present effort are:
·
Develop
an extraction method to determine a total amount of VOC in the
material.(subtask 1a)
·
Develop a method for material characterization
that generates storage and transport coefficients for the material.(subtask 1b)
·
Develop
a model for simulating the combined heat, air, moisture and pollutant (CHAMP)
transport in building envelope systems, that includes VOC diffusion as well as
the effects of air transport through wall cavities and cracks (task 2)
·
Benchmark
the integrated approach that includes measured VOC extraction and material
characterization into the prediction. Compare this prediction with data
measured in a full-scale experiments on a typical wood frame wall assembly
(subtask 1c).
Approach:
Sub Task A.
Develop experimental technique to
determine the total VOC content in the material
Increasing ambient
temperature and reducing total air pressure to a low vacuum level are
recognized as factors that can accelerate diffusion processes. Yet the
increased temperature is also recognized as a factor affecting amount of gas
dissolved in the solid phase i.e., modifying the partition coefficient of VOC.
Therefore, the work on development of extraction method must be performed in
stages involving separately effects of increased temperature and that of
vacuum.
Generally, to accelerate the
desorption process all specimens will be subject to a mechanical extraction.
The specimen will be placed in a sealed space of a grinding chamber and milled
to crude powder grade (method a). The inlet of the chamber will be connected
with a clean gas supply and the outlet with a sampling system that uses sorbent
tubes connected in serial (for details of connection consult Figure 1). The sorbent tubes will be analyzed by using a
GC/MS.
A. B.
Figure 1. Schematic of the
mechanical extraction set-ups A. 5 Stage mechanical grinding, B. Simplified
Method – Ball mill grinder.
The schematic – Fiqure 1 A – Is the conceptual
design for a multi stage grinder that would be able to handle different kinds
of materials. The staging allows the unquie abililty to test the most optim
grinding size that will assist in extraction of the VOC. Air samples will be
able to be taken at each stage of the grinding.
Fiqure 1 B. Is the design
used by Little in his experiment to extract VOCs from floor tiles. The tiles
were exposed to liquid nitrogen in order to freeze tile and this made the tile brittle. The tile was inserted
in the sealed ball grinder.
The recreating this test will
help expand the current knowledge base, prior to exploring other methods for
extraction.
Figure 2. Schematic of the
Thermal / Vacuum extraction set-ups
Fiqure 2 is the schematic
layout of the thermal and vacuum extraction concept. The focus will look at the
rapid aging by the combination of thermal and pressure to extract compounds.
Methods to be investigated
will include:
a)
Thermal extraction: The method includes both the mechanical and thermal
extraction because the grinding chamber is placed in an insulated box with
adjustable and controlled temperature. The temperature required for the thermal
extraction will be dependent on the boiling point of the VOCs contained in the
specimen. See Figure 2.
b)
Vacuum extraction method: The grinding chamber will be connected to a reference
chamber. Both chambers will have carefully calibrated volume and will be
provided with instruments to measure and record total gas pressure. Vacuum will
be applied stepwise by evacuating the reference chamber. See Figure 2.
Knowing
the chamber volumes and pressure before and after opening the connecting valve
one will be able to measure the pressure difference caused by the VOC diffusing
out of the specimen. In case of VOC with a high molecular weight determination
of the difference between expected air pressure (pressure equalization between
both chambers) and the total gas pressure may be used to determine the rate of
diffusion under vacuum.
One
of the possibilities to be examined in establishing precision of the sorbent
tubes is the use of a clean inert gas (nitrogen) supplied to the reference
chamber to allow pressure equalization during which the VOCs will be adsorbed
onto the passive sampling sorbent tubes and use them for analysis with a
thermal desorber GC/MS system. After sampling and determination of the VOC
extraction during this vacuum step the reference chamber is evacuated to the
initial vacuum level and the next step with increased level of vacuum is
executed.
c)
Combination of methods a) and b).
d)
Mechanical extraction, step 2. To examine effect of increased surface area of
the specimen on VOC extraction the second grinding step will be applied,
bringing the test material to a much finer powder.
e)
Combination of method d) and a).
f)
Combination of method d) and b).
The methods a) through f)
cover a broad range of processes involved in the VOC extraction, and will be
analyzed to optimize test procedure with regard to an appropriate degree of
milling, temperature, vacuum level and number of applied vacuum steps an.
Figure 1 shows a bench top set up will be developed that includes air supply, a
sealed chamber, specimen heating element, vacuum pump, and air sampling
devices.
Sub Task B.
Develop an experimental procedure for material
characterization
Material characterization is needed for defining the type of materials
and their VOC adsorption, storage and removal capabilities as well as the input
of material functions to the computer based models (task 2 of this thrust).
Material characterization may be achieved through indirect measurements
such as pore-size and pore size-distribution or through direct measurements of
VOC storage and transport coefficients. In the future, when appropriate
correlation has been established, it is likely that either approach will be
acceptable; yet, currently the characterization work must be focused on the
direct measurements.
To complement the extraction procedure (subtask 1a) we are focused on a
most universal and precise approach to determination of physical
characteristics of VOC movement through building materials, namely a dual
chamber method.
Figure 2 shows a schematic of the Dual Chamber system.
Figure 2 Schematic diagram
of dual chamber system
As is shown in Figure 2, the tested material (specimen) separates two
chambers. A clean air is supplied to the
receiving chamber, while the air flow with constant amount of VOCs is
supplied to the supply chamber. At
this stage temperature in both chambers is the same but relative humidity may
or may not be different (depending on the test program). Uniformity of gas
concentration in each of these two chambers is ensured by a mixing fan. The
average concentration of VOC in each of these two chambers is established from
measurements performed of each of them.
Samples are taken from both inlet and outlet of each chamber and
analyzed by GC/MS or GC/FID. When the experiment is underway, the difference
between inlet and outlet of the supply chamber gives the amount of VOC entering
the specimen and their average gives the mean VOC concentration. Respectively
for the receiving chamber the difference gives the amount of VOC transferred
through the specimen and their average givers the mean value of VOC
concentration in the chamber.
Dual chamber method was selected to allow examining the transport of VOC
through building materials between the supply and the receiving chamber under either
transient or steady state solutions. This permits separating two important VOC
parameters from one experiment, which are the effective diffusion and partition
coefficients. Furthermore dual chamber method has better resolution for each of
these parameters (Yang, 2004), which is an important aspects when developing
fundamental understanding of methods for material characterization as well as
input of CHAMPS models.
Another variant of dual chamber method will be used to examine how much
valid are the current assumptions of constant effective diffusion and partition
coefficients. The VOC concentration in the receiving chamber will be increased
in five steps from clean air to 50% of VOC concentration in the supply chamber.
To avoid distortion of flow conditions with removal of VOC by the sampling
process the same amount of carrier gas and VOC will be supplied to the
receiving chamber with the make-up mix. This experiment will be only conducted
in a transient stage to compare with transient stage experiments where the same
VOC transport was measured and averaged over a wide range of VOC
concentrations.
Initially the work will be performed on single VOC and clean materials,
later materials with know VOC concentration (information from Task 1a) will be
used. The work will progress from a single VOC to multiple and interacting VOC
mixes.
Sub Task C.
Conduct
full-scale VOC experiments to validate the integrated testing and modeling
approach
Figure 3 shows
a schematic test of a wall assembly.
Figure 3. Schematic of test wall assembly
Full-scale chamber
experiments will be conducted on a typical wood-framed residential wall
assembly in a full-scale coupled indoor/outdoor environmental simulator
(C-I/O-ES).
The C-I/O-ES has three major
components: a 4.87 m by 3.66 m by 3.05 m high (16 ft by 12 ft by 10 ft) IEQ
chamber, a 1.98 m by 3.66 m by 3.05 m high (6.5 ft by 12 ft by 10 ft) outdoor
climate chamber, and a replaceable test wall assembly that separates the two
test chambers. Both chambers and their
respective HVAC systems use stainless steel interior surfaces and PTFE gaskets
to minimize pollutant emissions and adsorptions in the facility. The HVAC
systems for the IEQ and climate chambers are both controlled with a direct
digital control (DDC) system, providing accurate controls of temperature,
relative humidity and pressure in both chambers.
The wall assembly tested is a
3.66 m ´ 3.05 m high (12 ft ´ 10 ft) section and 0.2 m (8 inches) thick. As shown
in Figure 2, it is consisted of vinyl siding, water resistive barrier type P
(WRB) type P (polymeric fibers) , 0.012 m (˝") oriented strand board
(OSB), 0.15 m (6") mineral fiber insulation (fiberglas batt), 6 mil
polyethylene vapor barrier, 0.012 m (˝") gypsum wallboard (drywall), and
water-based paint.
A tracer gas method will be
used determine the leakage flow paths. With known leakage path the
characterization of flow resistance will be done with the pressure mapping.
Pressure sensors will be installed across each major leakage airflow paths, and
the measured pressure drop (DP) will be used in the Q = f(DP) equation to determine the airflow rate through the
path.
A test of VOC transport
through the wall assembly under conditions of simultaneous heat, air, moisture
and pollutant transport through the wall assembly will be conducted. These
tests include the following conditions:
Test 1: Wall assembly is exposed to air pressure difference
of 10 Pa (climate chamber pressurized), while 23 C and 50% RH conditions are
maintained on both sides of the assembly. As the VOC initial concentration and
air leakage path are known one may calculate and measure the VOC time dependent
concentration in the receiving chamber (IEQ chamber).
Test 2: Measurements of the air leakage effect: formaldehyde
and another single VOC (say, Ethyl benzene) are released at certain location(s)
outside or inside of the wall. For instance one may inject known amounts of VOC
to the frame wall cavity and measure VOC concentration inside the insulated
cavity in the frame wall. Performing these measurements with positive, negative
10 Pa or 0 zero pressure difference permits determination of the effect of air
flow on the total VOC transmission. Some of these tests will be performed over
a period long enough to include not only the transient but also
quasi-steady-state conditions. In such a case a constant VOC concentration will
be maintained on the supply side.
Test 3: Test of mixed-convection airflow effect: as in
the test 2, formaldehyde and another single VOC are released at certain
location(s). This time pressure difference is 5 Pa, but the climate chamber is
exposed to cold winter conditions (range of -15 to -20 C).
The details of test
conditions will be evaluated with the simulation tool before carrying out the tests.
The test scenarios may change based on the simulations to ensure the data for
model validation. These test results will be analyzed and if needed one of the
tests will be repeated.
Repeating the tests will require long time and application of large air
pressure difference to clean the wall from the remaining concentrations from
the previous test.
Air samples are taken from
different locations of wall cavity through sample tubes installed on the wall
assembly. Air samples are also taken
from both IEQ chamber and climate chamber and analyzed by GC/MS and HPLC (for
“light” compounds such as formaldehyde and acetaldehyde) to determine the
concentration of each VOC in both chambers.
Several details of these
tests will be decided on the basis of computer calculations. Those details
include air change rate for IEQ chamber if such is applied in addition to the
measured "background leakage" and frequency of gas concentration
measurements. (Mechanical mixing will be
used to uncouple the ACH from the requirement of well mixed air). Air change
rate of clean air provided to both chambers will also be measured. Finally,
based on measured VOC concentrations, air change rate, and surface area of the
test wall assembly one may determine the rate for each VOC transport measured
Temperature and relative humidity (RH) are measured by
thermocouples and relative humidity sensors to mapping the temperature and RH
in the wall assembly, to help determine the airflow pattern in/through the wall
assembly.
Comparison
between the measured results and those predicted by the CHAMP model will be
done as a benchmark validation. One should not expect the uncertainty in
comparing the VOC concentration to be better than 30% of the estimated value.
While the accuracy of VOC concentration measurement is within 15%, yet using Q
= Q(DP) relationship from another test and only measuring DP in the actual test
brings at least the same level of uncertainty. Other sources of uncertainty are
effects of temperature and humidity.