Proceedings of Healthy Buildings 2009
Paper 523
The Ventilation Effectiveness Evaluation of an Underfloor Air Distribution System Installed in a Classroom Renata Maria Marè1, Luana Conceição de Oliveira1, and Brenda Chaves Coelho Leite1. 1
Civil Construction Engineering Department of the Polytechnic School of the University of São Paulo, Brazil. Corresponding email:
[email protected]
SUMMARY This study aims to evaluate the ventilation effectiveness of an Underfloor Air Distribution System in terms of its ability in removing indoor air contaminants. The system is operating in a classroom, located at the Civil Engineering Department of the Polytechnic School of the University of São Paulo – Brazil, thus in a non-steady state condition. The concentration levels of indoor air-borne particles were measured, as well as the air temperature, air velocity, airflow and relative air humidity under six different air conditioning system operational conditions, varying the intake airflow, air temperature, and relative air humidity. The concentration levels of air contaminants were measured at the breathing zone for seated people (1.10m) and in the exhaust simultaneously. The results allow identifying under which operational conditions the best ventilation effectiveness occurs. KEYWORDS Ventilation effectiveness, Air conditioning, Underfloor air distribution system, Indoor air quality. INTRODUCTION The ventilation effectiveness of an air conditioning system could be verified by its capacity of renovating air and of removing indoor air contaminants, expressed by the contaminant removal effectiveness index (Mundt et al. 2004). This index is considered satisfied if the concentration level of air pollutants in the air exhaust is greater than the concentration at the breathing zone. Many authors point the ventilation effectiveness of an underfloor air distribution system as having better results if compared with the overhead system (Cermak, Melikov, 2006). In the overhead system, the contaminant removal effectiveness index is at maximum 1, and for underfloor systems it can reach 1.3. There are still many doubts about the effectiveness of this system because the air distribution is made underfloor, where relevant particle deposition can be found. Furthermore, the influence of the position, kind and temperature of the pollutant source is known, besides resuspension over the ventilation effectiveness in this kind of system (Mundt, 2001). These matters show the importance of thoroughly study this system (Abe et al. 2006). The underfloor air distribution system utilizes an underfloor plenum to deliver the conditioned air directly to the occupied zone. The air exhaust is normally made through the upper plenum. There are many studies showing the advantages of this system over the overhead system (Leite, 2003), if well-engineered, such as easy layout reconfiguring; acceptance of individual control systems for thermal comfort; air supplied at higher temperatures, which implies reduced energy use; and regarding ventilation effectiveness and indoor air quality, the air can be supplied directly at the breathing zone (1.10m), thus the thermal stratification and the natural buoyancy are advantages for the indoor air contaminants removal. This study intends to provide data about the ventilation
Proceedings of Healthy Buildings 2009
Paper 523
effectiveness of an underfloor air distribution system in terms of removing indoor air particles, and to determine what are the best system operational conditions for promoting it. CHARACTERISTICS OF THE CASE STUDY SYSTEM The research was carried out in a classroom with a rectangular geometry of 180 m2 with raised floor (plenum of 28 cm height). The main heat sources are due to 49 people in light activities, and 25 personal computers. The most significant solar radiation is at the windows side of the room (Error! Reference source not found.). The air conditioning system is an indirect expansion system which supplies a 7.5 cooling ton coil capacity. A 5,500 m3/h airflow capacity fan supplies the plenum in a pressurized way. The filtered air is supplied to the classroom through 77 swirled air jet diffusers (Ø 200mm) and a fan forces the air exhaust through sixteen ceiling grilles. The supplied air is at higher temperatures than those observed in overhead systems. To reach higher temperatures, the air is reheated at an air mixture box with bypass of the returned air. Regarding the automation and control system, it has the purpose of inducing the control of air temperature, airflow and relative air humidity. This system operates under five main control routines related to: the fan frequency, to control the differential pressure between the plenum and the ambient, and consequently altering the supply airflow; the exhaust fan frequency, to control the return airflow; the modulation of the chilled water three-way valve; the return bypass and the outside air dampers modulation. METHOD The adopted procedures were based in ISO 7726 (1998) and ASHRAE 55 (2004) standards, and in LEITE (2003) and IKEDA (2008), considering thermal comfort conditions of 21°C ≤ T ≤ 26°C for air temperature (at 0.60m height), 0.10 < v < 0.30 m/s for relative air velocity, and 50% for relative air humidity. Measurements started in the period between winter and spring, 2008, in six alternate days, varying the ambient air temperature setpoint according to the interval presented above. The air temperature and velocity were measured in 14 points of the ambient (Figure 1), and at six different levels: 0.10, 0.60, 1.10, 1.70, 2.00 and 2.35m.
Figure 1. Classroom and Map with 14 measurement points The particle concentrations were measured in eight points of the ambient, at 1.10 m and 2.60 m high simultaneously, that are respectively the breathing zone for seated people and the air exhaust to determine the contaminant removal effectiveness index. RESULTS AND DISCUSSION To reach the setpoints of air temperature at 0.60m level, some variable setpoints combinations were established for the air conditioning system operation, whose results at the ambient are presented in Table 1 (for the occupied classroom):
Proceedings of Healthy Buildings 2009
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Table 1. Operational Conditions - Occupied Classroom Variable / Temperature Setpoint (°C) Outside Air Temperature (°C) Outside Relative Air Humidity (%) Differential Air Pressure - ∆P (Pa) Mixture Supply Air Temperature (°C) Underfloor Air Temperature (°C) Return Air Temperature (°C) Differential Air Temperature – ∆T (°C) Indoor Relative Air Humidity (%) Mixture Supply Relative Air Humidity (%) Return Relative Air Humidity (%)
21 21.9 62 4.50 15.4 17.9 23.2 7.8 55 70 39
22 23.7 58 5.07 17.2 19.4 24.4 7.2 55 66 38
23 27.1 46 4.01 17.3 19.4 23.6 6.3 56 67 43
24 18.6 74 3.01 18.5 19.6 23.2 4.7 63 71 50
25 28.9 47 5.87 19.0 22.5 27.7 8.7 53 70 40
26 30.6 47 3.69 20.1 22.3 27.4 7.3 57 67 42
To illustrate the air temperature and air velocity profiles obtained by the measures the results for the setpoints of 21°C, 25°C and 26°C are shown in Figures 2 and 3. The air temperature profiles show that from 0.1 to 0.6m in height the temperature variation was not significant. The temperature stratification, typical phenomena for this kind of air conditioning system, has presented a gain around 2°C from 0.10m to 1.70m in height. For Occupied Classroom, at 0.60m in height, the setpoints for air temperature were reached for all of them (including 22°C), considering a tolerance of ±0.5°C. For 23°C and 24°C, the thermal conditions were not reached because the system operational setpoints were not adequate. The air velocity profiles show that in most of the points in the occupied zone, values were around 0.10m/s. At levels 0.60m and 2.00m, greater air movements were verified, probably due to the gain of thermal load until 0.60m and the effects of the exhaust air fan, respectively.
Figure 2. Air Temperature Profile: Occupied and Unoccupied Classroom
Figure 3. Air Velocity Profile: Occupied and Unoccupied Classroom The contaminant removal effectiveness index CRE = TSP2.60/TSP1.10, where TSP is the mean value of the Total Suspension Particle in the return (2.60m) and in the breathing zone for seated people (1.10m) respectively. This index was calculated in each different air temperature setpoint, with and without people in the classroom (Occupied/Unoccupied). The measured TSP values were significantly low. The resultant CRE, shown in Figure 4, were often greater for occupied classroom. The typical CRE index for the underfloor air conditioning system, that is 1, was reached in three situations: at the setpoint of 23°C
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occupied, and at 25°C occupied and unoccupied. It can be observed that for the setpoint of 25°C, the CRE was greater than 1 either for occupied or unoccupied classroom.
Figure 4. Contaminant Removal Effectiveness Index X Air Temperature Setpoint CONCLUSIONS At this stage of the research, it was observed an apparent relation of the CRE index with the presence of people. It is also possible a relation of the CRE index with the return airflow that is related to the exhaust fan frequency, i.e., the higher the frequency the greater the return airflow. Nevertheless, these relations will be better verified in the second part of this study, when this procedure will be repeated. Due to the low air velocities and the observation of low absolute values of the TSP measured during the occupied period (lower than 0.035mg/m3), it is reasonable to infer that this kind of air conditioning system does not disperse the air contaminants in the ambient. ACKNOWLEDGEMENT The researcher was supported by the Coordination of Improvement of Higher Education (CAPES). REFERENCES Abe, V. C., Inatomi, T. A. H. and Leite, B. C. C. 2006. Air Quality in UFAD Systems: literature overview. In: Proceedings of Healthy Buildings: Creating A Healthy Indoor Environment For People, 2006, Lisbon. Vol. 4. pp. 425-430. ASHRAE. 2005. ANSI/ASHRAE Standard 55-2005, Thermal Environmental Conditions for Human Occupancy, Atlanta: American Society of Heating, Refrigerating, and Air Conditioning Engineers, Inc. Cermak, R., Melikov, A. K. 2006. Air quality and thermal comfort in an office with underfloor, mixing and displacement ventilation. International Journal of Ventilation. Vol.5 (3), pp 323-332. Ikeda, J. 2008. Determination of the adjustment index of the cooling ceiling air conditioning system control. São Paulo – Brazil: University of São Paulo (Masters Degree Thesis), 190p (in Portuguese). ISO. 1998. International Standards Organization. ISO 7726 - 1998: Thermal environments – Instruments and methods for measuring physical quantities. Geneva. Leite, B. C. C. 2003. Underfloor air supply system applied to office buildings: thermal comfort and operational conditions evaluation. São Paulo – Brazil: University of São Paulo (PhD Thesis), 162p (in Portuguese). Mundt, E. 2001. Non-buoyant pollutant sources and particles in displacement ventilation. Building and Environment, Vol. 36 (7), pp 829-836. Mundt, E., Mathisen, H. M., Nielsen, P. V., Moser, A. 2004. Ventilation Effectiveness. Guidebook n. 2. Belgium: Rehva, 74 p.
