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UTILIZING VENTILATION EFFICIENCY FACTORS TO PREDICT
INDOOR CARBON DIOXIDE CONCENTRATION IN FIELD
MEASUREMENTS OF TAIWAN’S OFFICE BUILDINGS
Yen-Yi Li, Che-Ming Chiang, Po-Chen Chou, Chi-Ming Lai, Yu-Feng Tu
Department of Architecture, ArchiLife Environ-Control Research Center,
National Cheng-Kung University, Tainan, 70101 Taiwan, R.O.C.
ABSTRACT
The ventilation criteria of Building Code in Taiwan stipulate only the least air exchange volume per square
meter, an index of air exchange efficiency. However, the indoor air pollutants from many investigated
buildings are found to be with concentrations higher than several exposure criteria. We consider that the lack
of including “effective ventilation factors” in current Building Code can be in part explained as contributing to
the above phenomenon. Therefore, a more comprehensive index to reflect the air exchange efficiency should
be considered for inclusion in the Building Code.
A trace-gas technique was used to measure the ventilation efficiency in field studies. The ACH (air change
per hour) value, mean age of air, air exchange efficiency, and CO2 concentrations were measured
simultaneously. We also utilize the ACH values, air exchange efficiency, and CO2 concentrations to construct
the theory calculation.
A simplified, predictive equation is established after the field data is compared with the values calculated
from the above theoretical equation. The air exchange efficiency is shown to be the critical factor to build the
EACH (Effective ACH) from ACH. The simplified predictive formulae enables us to calculate what the
acceptable number of people are for the room based on HVAC ventilation efficiency even before the
building is occupied.
KEYWORDS
ACH value, Tracer-Gas Technique, Air Exchange Efficiency, Field Measurements, Office Building
INTRODUCTION
Taiwan is an island located in subtropical region with high temperature and humidity year-round according to
the data of National Central Weather Bureau. The average temperature is 34.5 ¢J in summer, and even in
spring and fall the temperature is 28 ¢J . Therefore, the application of HVAC system becomes essential to
control the thermal comfort in most building.
According to the investigation experience in Office Building survey, the indoor air pollutants from many
investigated buildings are found to be with concentrations higher than several exposure criteria. We
considered it could be lack of fresh air exchanging, so we improved the air exchange rate to get good indoor
air quality (IAQ). However, it seems that only increase the intake fresh air volume couldn’t improve the IAQ
of all the area. We consider that the lack of including “effective ventilation factors” in current Building Code
in Taiwan can be in part explained as contributing to the above phenomenon. Therefore, a more
comprehensive index to reflect the air exchange efficiency should be considered for inclusion in the Building
Code.
METHODS
The IAQ and Ventilation Efficiency field measurements in this study can be divided into three main
categories: the building selection include the performance of HVAC system, the IAQ survey include
concentration levels, population distribution density levels and thermal comfort and the ventilation efficiency
measurements. Methods for each are described briefly below.
Test building Selection
To realize different HVAC system and different building construction types affected the IAQ and
ventilation situations, six different buildings were chosen to be the test objectives. Table 1 is the brief overview
of the buildings. The performance of HVAC system can be evaluated via current operational situations, air
exchange rate, air distribution, and thermal comfort conditions:
Table 1.
The details of six test buildings
ID No. Bldg. A Bldg. B Bldg. C Bldg. D Bldg. E Bldg. E
Location Kaohsiung Kaohsiung Tainan Tainan Taipei Taipei
Construction RC RC RC SRC RC SRC
Age, years 18 16 14 15 19 10
FCU AHU FCU +
HVAC type (Air Handling FCU FCU AHU
(Fan Coil Unit) Unit) Natural
Peripheral Peripheral Peripheral
Room location Inner zone Inner zone Inner zone
zone zone zone
Test date Oct. 1997 April 1998 April 1998 May. 1998 Dec. 1997 Mar. 1998
Layout Open plan Cellular Open plan Cellular Cellular Open plan
Indoor Air Quality Survey
To assess the IAQ situation and evaluate the ventilation efficiency, it is necessary to take a Standard
Operation Procedure in field survey. It can be briefly illustrated as follows.
1.Walk-through Inspection
This task is to understand the building/HAVC system and its surroundings. Efforts should be made to look
for the inherent characteristics that can result in IAQ problems, for example, outdoor-air intake, frequency of
duct cleaning etc. Factors, which can adversely affect the performance, should also be noted. For example,
combustion devices, copy machines, office cleaning frequencies, cleaning agents, interior decoration
materials… etc. Results should be recorded in the checklists or via photographs.
2.Chemical Pollutants Measurements
Carbon monoxide (CO), Carbon dioxide (CO2), and Suspended Particles (PM10, counted by those are
smaller than 10 mm in aerodynamic particle size) were monitored continuously for a 24-hour period. Their
detection principles are illustrated in Table 3 respectively.
Formaldehyde (HCHO) and Total Volatile Organic Compounds (TVOC) samples from investigated
locations were collected continuously and sent to the Multi-Gas Monitor, which is based on the photo-
acoustics detection principle.
Table 2.
Performance of apparatus for monitoring chemical contaminants
Item Method for analyzing Range Precision
CO Electrochemical oxidation 0~150 ppm < ±1 ppm
CO2 Non-dispersion infrared absorption 0~5000 ppm < ±50 ppm
3
PM10 Light scatter at a fixed angle 0.001~10mg/m < 10 %
Formaldehyde Photo-acoustics infrared spectroscopy ~0.04 ppm < ±1.55 %
TVOC Photo-acoustics infrared spectroscopy ~0.02 mg/m 3
< ±1.55 %
3.Thermal Comfort
On-situation measurements of air temperature, relative humidity, and velocity were accomplished to
evaluate the indoor thermal conditions for a 24-hour period. Performances of the apparatus are listed in Table
3.
Table 3.
Performance of apparatus for thermal comfort evaluation
Item Method for analyzing Range Precision
Temperature Resistance -40~115¢J <±0.3¢J
Relative humidity Capacitance 0~100%RH <±0.1%
Velocity Hot-wire 0~30m/s <±2%
Ventilation Efficiency Measurements
1.Air Exchange Rate Measurement
Air exchange rate was measured with the tracer gas decay method. An adequate amount of Sulphur
Hexafluoride (SF6) gas is injected directly into the air supply main duct. After injection, about 1.5 hours is
allowed for mixing of the tracer gas. Samples from investigated locations and the return duct are collected
continuously and sent one after another into the Multi-Gas monitor that is based on the photoacoustic
detection principle. The Air exchange rate is then obtained by plotting the measured concentrations in
logarithm scales against time in hours. Those data are fitted with a straight line and the slope of the line is the
Air exchange rate (ACH). [Shaw, 1991]
2.Air Exchange Efficiency
The Air Exchange Efficiency (AEE) can present the air-flow-pattern of the ventilation room. To decide the
AEE depended on the major two factors in the office building with HVAC system:
- Relative location of the supply and extract devices
- Momentum of the jet
Table 4.
Performance of Room ventilation situation and the Air Exchange Efficiency representation
Situation Air-Exchange Efficiency The Evaluation with the Age-of-air
Unidirectional Local mean age-of-air in Exhaust
100¢M
50¢M¡ã
Flow ¡×Room average age-of-air¡Ñ 2
Local mean age-of-air in Exhaust
Perfect Mixing 50¢M
¡×Room average age-of-air
Local mean age-of-air in Exhaust
Short- Circuiting 50¢M
0¢M¡ã
¡ÕRoom average age-of-air
For ventilation measurement, the Concentration Decay Method is chosen for the standard operation
procedure. The Air Exchange Rate (ACH), Room average age-of-air and Local mean age-of-air were
measured at the same time. The AEE value was also calculated with the age-of-air indicators.
RESULTS
All the results of the field survey in different six buildings are as Table 5 shows. The ACH (air change per
hour) value, mean age of air, air exchange efficiency, and CO2 concentrations were measured simultaneously.
As figure 1 shows, when the HVAC system introducing outdoor air the EACH (Effective ACH, EACH =
ACH ¡Ñ AEE) value is higher than the no outdoor-air intake situation. To compare with the ACH and added
CO2 concentration (Figure 2), the EACH value has more representation than only ACH in describing the
ventilation efficiency. We also utilize the ACH values, air exchange efficiency, and CO2 concentrations to
construct the theory calculation. The result equations are as follows:
Cadd = 1000 ⋅ EACH −2 (open outdoor air inlet) (1)
−1 .5
Cadd = 500 ⋅ EACH (close outdoor air inlet) (2)
Where Cadd is the added CO2 concentration by the occupies.
A simplified, predictive equation is established after the field data is compared with the values calculated
from the above theoretical equation. The air exchange efficiency is shown to be the critical factor to build the
EACH (Effective ACH) from ACH.
Added CO 2 Conc. per meter per person
Added CO 2 Conc. per meter per person
16000 14000
14000 -2 12000
y = 1000x y = 500x-1.7
12000
10000
10000 R 2 = 0.6987 R 2 = 0.5252
8000
8000
6000
6000
4000
4000
2000 2000
0 0
0.0 0.5 1.0 1.5 2.0 2.5 0.0 0.2 0.4 0.6 0.8 1.0 1.2
EACH Value (open) EACH Value (close)
Figure 1: The added CO2 conc. per meter per person and the EACH relationship
Added CO2 Conc. per meter per person
Added CO2 Conc. per meter per person
12000 12000
10000 10000
8000 8000
6000 6000
4000 4000
2000 2000
0 0
0.0 1.0 2.0 3.0 4.0 5.0 0.0 0.5 1.0 1.5 2.0 2.5
ACH Value (open) ACH Value (close)
Figure 2: The added CO2 conc. per meter per person and the ACH relationship
DISCUSSION
The furniture location and HVAC system type are the critical factors for the air exchange efficiency. The
ACH and AEE value constructed EACH value could be the effective design criteria In the architecture design
procedure. The simplified predictive formulae enables us to calculate what the acceptable number of people
are for the room based on HVAC ventilation efficiency even before the building is occupied.
ACKNOWLEDGEMENT
We are especially grateful to ARCHILIFE Research Foundation for support of Numerical and
experimental facilities.
REFERENCE
ASHRAE, (1989), ASHRAE Handbook ¡AFundamental Volume¡AAmerican Society of Heating,
Refrigerating and Air-conditioning Engineering Inc., Atlanta, GA.
ASHRAE, (1989). ASHRAE Standard 62,Ventilation for Acceptable Air Quality¡AAmerican
Society of Heating, Refrigerating and Air-conditioning Engineering Inc., Atlanta, GA.
Shaw, C. Y., Magee, R. J., Shirtliffe, C. J. and Unligil, H., 1991, Indoor Air Quality Assessment in an
Office-Library Building: Part 1-Test Methods, ASHRAE Transactions. Vol. 97, No. 2, 129-135.
A.I.V.C. Guide to Ventilation. (1996). A Guide to Energy Efficient Ventilation. Martin W Liddament
Che-Ming Chiang, Yen-Yi Li, Po-Cheng Chou, Chi-Ming Lai, (1999). CFD Simulation to Predict
Natural Ventilation Efficiency in a Dwelling Bedroom with the Central Horizontal Pivot Window,
INDOOR AIR 99, Vol. 4.
Che-Ming Chiang, Chi-Ming Lai, Po-Cheng Chou, Yen-Yi Li, (1999). The Influence of An
Architectural Design Alternative (Transoms) on Indoor Air Environment in Conventional Kitchens
in Taiwan. BUILDING AND ENVIRONMENT.
C.M Chiang, R.P. Lai, (1998). The diagnostic methods of relationship between indoor air quality
and HVAC facilities in office buildings. The Research Report of the Architecture and Building
Research Institute, Taiwan.
Table 5 The detail results of six test buildings
Kaohsiung Kaohsiung Tainan Tainan Taipei Taipei
Building A Building B Bldg. C Bldg. D Building E Building F
Factors
Fall Summer Summer Summer winter Spring
2F 3F 5F 2F 1F 2F 2F 7F 10F 3F 5F 6F
FCU +
HVAC System FCU FCU AHU AHU AHU FCU FCU FCU AHU AHU AHU
Ducts
Construction Type RC RC RC SRC RC SRC
Ages 1982 1984 1986 1985 1981 1990
Area 70 104 525 341 525 735 853 722 722 176 377 129
Population Number 4~5 9~10 11 13 28 13 52 92 62 36 40 1
Density 0.06 0.09 0.02 0.04 0.05 0.02 0.06 0.13 0.09 0.20 0.11 0.01
High 966 658 628 642 932 776 809 600 1222 1512 1374 732
CO 2 Average 562 545 467 452 760 579 598 420 607 524 437 456
(ppm) SD 166 32 105 122 118 129 145 50 324 405 224 76
High 0.10 0.24 0.07 0.07 0.07 0.12 0.07 0.03 0.02 0.02 0.01 0.03
PM 10 Average 0.06 0.08 0.05 0.05 0.05 0.06 0.05 0.02 0.02 0.01 0.01 0.02
(mg/m3) SD 0.02 0.06 0.02 0.01 0.01 0.02 0.01 0.01 0 0.01 0.00 0.01
ACH open 3.92 2.42 1.50 1.30 1.74 0.78 2.32 2.10 1.89 2.78 2.80 2.98
(h-1) close 1.45 1.05 1.25 1.10 1.49 0.41 2.15 1.65 1.32 1.65 1.45 1.90
open 20.6% 36.0% 36.7% 89.0% 22.6% 31.8% 24.0% 81.9% 39.6% 26.9% 32.4% 15.3%
AEE
close 24.5% 34.3% 35.6% 67.8% 15.4% 29.3% 13.0% 65.5% 37.1% 45.2% 55.4% 17.5%
open 0.808 0.872 0.551 1.157 0.393 0.248 0.557 1.720 0.749 0.749 0.906 0.457
EACH
close 0.355 0.360 0.445 0.746 0.229 0.120 0.280 1.080 0.490 0.746 0.804 0.333
High 26.5 27.2 27.1 26.1 27.2 25.1 25.2 20.6 19.5 27.9 26.4 26.6
Temp Average 25.9 26.6 26.0 24.9 26.2 24.3 23.4 19.1 18.5 27.3 25.5 24.8
( ¢J ) SD 0.8 0.5 1.1 1.4 0.9 0.8 1.1 0.4 0.6 30.6 0.6 33.6
High 67.2 70.8 66.1 70.0 64.3 46.0 59.1 53.8 55.7 78.0 65.5 84.5
RH Average 56.9 64.9 55.3 62.8 55.9 45.3 53.4 49.5 51.7 64.9 50.4 64.7
¡]¢M¡^ SD 10.6 2.3 7.4 6.6 6.5 0.5 4.0 2.9 2.7 21.8 9.7 24.8
High 0.03 0.51 0.08 0.05 0.05 0.06 0.08 0.08 0.21 0.07 2.07 0.17
V Average 0.03 0.03 0.01 0.03 0.02 0.03 0.05 0.07 0.10 0.02 0.31 0.05
(m/sec) SD 0.01 0.10 0.01 0.01 0.02 0.02 0.02 0.01 0.03 0.02 0.43 0.04
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