Using ventilation technology to control air contaminant is a common tool for
an environmental engineer. An appropriate ventilation design can provide a high
quality living and working environment for occupants. Therefore, to quantify
emission from a building, the ventilation rate and the gas concentrations in
exhaust section must be known (Kavolelis, 2003). In
fact, the acceptable level for indoor air quality is specified by many countries.
There are accurate measurements techniques for ventilation rate with forced
ventilation have been presented by Berckmans et al.
(1991) and Young et al. (1999). Including
the standard test and sampling methods are developed by many research institutes.
However, some real environmental aspects didnt consider in those methods
or standards, such as the obstacles inside the indoor environments. Ventilation
for contaminant control falls into two general categories, general exhaust and
local exhaust ventilation that have been conducted by Sterling
et al. (1985). Fume hoods and external hoods for controlling contaminant
dispersal in industrial workplaces are typical examples of local exhaust ventilation
(ASHRAE, 1991). These are effective where the location
of the contaminant source is known and fixed. In contrast, when contaminant
source locations are not known or when it is not practical to keep the source
in one location, general exhaust ventilation is used appropriately to remove
air from the entire ventilated space. Most health care facilities may have many
sources and the source locations usually are not known. Furthermore, CDC
(1994) investigated the exposed individual receivers (i.e., patients or
medical staff) need to move around so that their locations are not always predicable
in relation to the source locations. The contaminant control efficiency of ventilation
systems is evaluated by measuring their ability to remove airborne contaminants
from a space. The rate at which contaminants are removed is compared to the
rate that would occur if the clean incoming air were instantly and completely
mixed with the air in the room (perfect mixing). Recently, the indoor contaminant
removal efficiency has been studied widely and accumulated many practical measuring
techniques. In addition, mathematical models are the best tools available for
prediction purpose in the field of air quality management. Subramanian
and Natarajan (2006) developed a Gaussian Plume Model to determine the concentration
of pollutants from point source emissions. Chen et al.
(1988) used PHOENICS package to simulate the indoor air flow and the distribution
of airborne contaminants. The results show that the increasing ACH may enhance
the ventilation efficiency. The influence of opening locations (toward the outdoor
and indoor) with different wind velocities, air exchange rates and average indoor
temperature are analyzed, via CFD numerical simulations. It shows that, in different
wind conditions, roofs with outdoor and indoor openings have different capabilities
in air exchange and thermal environment. (Wen et al.,
2008). In addition, tracer gas measurements are widely used (Vant
and Heitlager, 1994; Boulard and Draoui, 1995; Snell
et al., 2003; Jiang and Chen, 2003), but
they mostly rely on the assumption that tracer is perfectly mixed in space and
the measurement point is representative of average ventilation efficiency within
the space. Therefore, measurements errors are unavoidable due to imperfect mixing
of tracer gas in the measured volume. Garrison et al.
(1989a, b; 1991) applied
the N2, CO2 and SF6 tracer gas to examine the
ventilation efficiency. They concluded that the greater density gas such as
CO2 and SF6 will stay at the bottom of the test chamber
and the location of inlet/outlet vents play an important role to determine the
ventilation efficiency. Thus, the location of exhaust vent is suggested to locate
near the contaminate sources. The similar results are also found by Shen
and Chang (1994). Chung and Hsu (2001) used a full
scale test chamber and tracer gas technique investigated the ventilation efficiency
under different air change rate and relative vent locations, the indoor air
quality may influenced by inlet/outlet relative locations larger than increasing
the fresh air supply rate.
When a certain amount of fresh air is supplied into occupant space, the efficiency
of ventilation is a measure of the ventilation system's ability to remove airborne
contamination in the space being ventilated at a given air change rate. The
high efficiency ventilation systems are needed for most health care facilities,
especially in high clean standard required areas, such as operating rooms. However,
the high air change rate does not guarantee the high ventilation efficiency
of a ventilated space. Basically, the airflow pattern will also play an important
role in obtaining high ventilation efficiency in a ventilated space. In principal,
airflow pattern may be characterized in terms of short circuiting, perfect mixing
and displacement flow which are general terms used to describe the nature of
ventilation flow pattern within a space (Sandberg and Sjoberg,
1983). When supply air enters an occupant space and exhaust directly through
the exit diffuser without mixing with the contaminant in the space at large,
it is a short circuiting effect. The short circuiting effect is a very inefficient
form of ventilation and results in the cumulative of the contaminants in the
occupant spaces. Often, the inappropriate displaced locations of supply and
exhaust diffusers are the major reason to cause the short circuiting airflow
pattern. Therefore, the obstacles effect must be considered in the whole
evaluation process for more understanding the indoor ventilation efficiency.
VENTILATION EFFICIENCY MEASUREMENT
The ventilation efficiency validation for a living or working environment is very important to insure the indoor air quality. Therefore, the précising measuring technique for ventilation efficiency is studying often and widely. The common and economic way is the concentration decay method by tracer gas which is adopted by the study. In addition to the obstacles within the interior space, the location of the sampling point and air supply volume are investigated as well.
The tracer gas measurements were often used to study indoor airflow patterns
and indoor ventilation efficiency. For the safety and convince reasons, the
following guidelines for choosing appropriate tracer gas are suggested:
||Similar density to air
||Not normally present in the atmosphere
||Neither be flammable nor explosive
||Not easily be absorbed or sink
||Easily be detected at low concentration
||To a good order of accuracy
The decay concentration measuring technique is the easiest way to evaluate
the indoor ventilation efficiency by CO2 gas. First, injecting the
tracer gas into the interior space and mixing well after certain of time, the
CO2 concentration will record continuously during the testing period.
Then the on-site air change rate may calculate from the recording CO2
values using the following mass balance equation.
where, V is indoor volume (m3), C is indoor tracer gas concentration, Cin is supply tracer gas concentration, Q is air flow rate (m3 sec-1), F is tracer gas releasing rate (m3 sec-1), t is time (sec).
The decay method was adopted in this study to evaluate the ventilation efficiency for a perfect mixing room. The concentration of tracer gas will approach a peak level C(0) after the CO2 released at a certain time. Let the tracer gas releasing rate (F) equal to zero, then the air change rate may obtain from the slope of the following integral form:
where, C(0) is the initial concentration of tracer gas, C(t) is the concentration at time t. According to Eq. 2, the air exchange arte may obtain from the slop of log y-axis and time x-axis.
where, A is the slope, minus for the value of air exchange rate, b is the constant.
Equation 3 can be used to calculate the ACHc The slope of ACHc for the tracer gas concentration calculation are shown in Fig. 1.
Mixing factor: The major purpose of general ventilation is adopt fresh
air to dilute the contaminated air inside and exhaust the mixture air Therefore,
the level of air mix will play an important role for determining the air exchange
||The slope of ACHc for the tracer gas concentration
Used the measured air volume divided by the space volume, a local exchange
rate may obtain (ACHa). On the other hand we may use the CO2
concentrations detected on the space to calculate the local ventilation rate
(ACHc). Due to the obstacles within the real interior space, the
ACHa will greater than ACHc usually for a not well-mixed
air. The K value is defined as:
A full scale test chamber was used to study the ventilation efficiency by tracer gas technology with various obstacles design. The CO2 sampling points are installed at the test chamber for measuring the CO2 levels. The obstacle design is used to simulate the real living or working environment. In order to analyze test data quantitatively, the different percentage area of vertical and horizontal obstacles are designed for test as shown in Fig. 2.
Test chamber: A full-scale test chamber was used to undertake the experimental
program. The well-controlled chamber size is 4.2 m height x2.5 m width x2.5
m height as depicted in Fig. 3.
|| The obstacle arrangement in the test chamber
||Complete test chamber configuration with obstacle inside
The SA means the inlet for supply air and the EA means the outlet for exhaust
Ventilated equipments: The various air volume fan is used to supply adequate supplied air volume to the inside of chamber. For recording the air flow rate, the air velocities were measured by hot wire anemometers in positive pressure room. Three thermal anemometer sensors were mounted on each of three iron bars which were mounted vertically to form a traverse plane. The sensors were spaced uniformly across the height and width of the room. Detailed configuration and measured locations are shown in Fig. 3. The uncertainty of velocity calibration and A/D conversion are considered to be negligible. The velocity measurements were in the range of 6 to 10 m sec-1 and averaging nine readings reduces uncertainty to ±1 % approximately. At lower velocity the higher uncertainty will obtain.
CO2 measuring and recording: The CO2 sampling
points are located in M1 to M8. The CO2 sensor measuring ranged from
zero to 5000 ppm with analogy output. The CO2 transmitters were connected
to a 24 VAC supplier without an external rectifier. The CO2 concentrations
were measured by CO2 monitors with an accuracy of ±1.5% and
the monitors were calibrated with each use. First the CO2 was injected
into the negative air pressure zoom as a contaminant source. After the concentration
of CO2 gas reached an appropriate level (2000 PPM), the injection
was stopped. A small propeller fan was installed in the negative air pressure
zoom to ensure the CO2 gas well mixing. Mixing was confirmed by collection
and analysis of CO2 monitors. The perfect mixing of indoor air was
achieved approximately 10 min in negative air pressure room. Then the supply
and exhaust fans were turned on and the opening was opened simultaneously. The
measurement of contaminant of CO2 gas began and the duration of
concentration sampling was about 15 min.
||The location of CO2 sampling points and recording
In order to evaluate the effect of contaminant from negative air pressure area
into positive air pressure area through opening when the opening was opened
and resulted in instantly air pressure balance, total of eight or four CO2
sampling points were used. The complete configuration of the experimental
setup is shown in Fig. 4.
RESULTS AND DISCUSSION
The tracer gas technique is used to investigate the ventilation efficiency under various arrangements of interior obstacles. Total of 12 different test sets are scheduled for examining the vertical and horizontal obstacles within the test chamber with different inlet/outlet air volumes. The complete test parameters are listed in Table 1. The V means vertical obstacles and H means horizontal obstacles in Table 1.
In Table 1 illustrate the test parameters for test set A to L using both 6 and 12 ACH for four and eight sampling points. The CO2 concentration distribution for 6 ACH and eight sampling points test data without obstacle inside is shown in Fig. 5 using Eq. 1-3 to calculate the slope. The slope is -0.0741 that means the ACHc is 4.4 (0.0741x60) which less that 6 ACHa calculated by the actual air supply volume from the inlet vent.
The ACHc will affect by the obstacle area severely, especially in
the flow stream direction as listed in Table 2. The detailed
explanation of case number index is shown in Fig. 6. The larger
area of obstacles it will cause the less ventilation efficiency for the obstacle
standing vertically in the flow stream. The result is that the ACHc
from 4.44 will decay to 3.8 following the increase of obstacle area above from
floor and from 4.44 will decay to 2.73 following the increase of obstacle area
down from ceiling under local exchange rate (ACHa) is 6.
|| Complete test parameters
||The test ACHc values of obstacle orientate vertically
to the flow path at 6 and 12 ACHa
|| The ACHc for 6 ACH and eight sampling test case
|| Test case index illustrations
The result show same tendency at 12 ACHa condition, the ACHc
from 6.3 will decay to 5.35 following the increase of obstacle area above from
floor and from 6.3 will decay to 4.5 following the increase of obstacle area
down from ceiling.
||The test ACHc values of obstacle orientate horizontally
to the flow path at 6 and 12 ACHa
||Test results of different sampling locations for 6 ACHa
and vertical obstacles inside
The data in Table 2 also presented the obstacle above from
floor will affect ACHc more than it down from ceiling. Table
3 show the test results for the obstacle installed parallel with the flow
stream. Excluding the obstacle down from the ceiling the data of the parallel
obstacle test results show less influence than the vertical one. When increase
the obstacle area above from floor to 4/5 height, the ACHc only decrease
13.2% at 6 ACHa condition and decrease 5.7% at 12 ACHa
condition. It is because the inside obstacles cause less contaminant removal
from indoor to outdoor through the ventilation systems.
For evaluating the effect of number of sampling points on the ventilation efficiency, the four and eight sampling points are examined and the locations of four sampling points are also changed during the test process. Table 4 show the ACHc for eight and four sampling points test at 6 ACHa condition. Basically, the tested values of ventilation efficiency (ACHc) are quite similar in the test room. However, when the sampling point close to the air supply or exhaust vent the measuring CO2 level will fluctuate severely. Generally, the ventilation efficiency for using eight or four sampling points are similar at a closure area about 10 m2.
Table 5 lists the K values for a single compartment with
one inlet vent and outlet vent ventilation type. The K value is the ACHc
divided by ACHa that value will always bigger than one. Referred
to the test data, the upper part obstacle has greater influence on the ventilation
efficiency than that of lower part obstacle.
|| The weighting K value for the vertical obstacle cases
The bigger obstacles area show higher K value for the vertical obstacle
case. The K value may increase 3 to 6% when the 6 ACH case whereas increasing
1 to 4% for 12 ACH.
The study examined the ventilation efficiency using various interior obstacles design with different air change rates experimentally. The tracer gas technology was used for analysis indoor contaminant removal rate and ventilation efficiency. Referred to the test data, the initial CO2 concentration at 2000 and 3000 ppm did not affect the test results.
The effective ventilation efficiency may influence by the interior obstacles. Especially the obstacle located vertically at the flow path. The larger vertical area of the interior obstacles the more influence is observed in the tests. Finally, the K value is a weight factor for the obstacles surface to chamber vertical surface or horizontal surface had found in this study help for evaluating the actual ventilation efficiency or contaminant removal efficiency.
The sampling points and locations will affect the test results. For better recording and testing results, the sampling locations are suggested taking data around the flow domain and avoiding the vent area.
The authors gratefully acknowledge the financial support provided to this study by the Institute of Occupational Safety and Health of Taiwan under the grant No. IOSH97-H307.