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Estimation of air flow rates in large buildings based on measurements Hannes Konder, Mr, research assistent; [email protected], www.bph.tuwien.ac.at Thomas Bednar, Prof., assoc. Professor; [email protected], www.bph.tuwien.ac.at KEYWORDS: air flow rate, large building, measurement, CO2, tracer gas, atrium SUMMARY: The estimation of large wind and buoyancy driven air flow rates in large buildings is essential for the calculation of the annual energy use for cooling and heating. In this paper a new method is presented, how to measure these large air flow rates. The design of a stable CO2 emitting source with large rates is shown. This source has been applied to measure the time-dependent air flow rate in a multi-storey office building, where the five storeys are connected one to another by an atrium. The air-flow rate at the inlet opening calculated from the measurement of the CO2 concentration is compared with the air-flow rate calculated from the mean air velocity at the inlet opening. Both time-dependend rates show a good agreement. Furthermore the source is powerful enough so that the tracer gas can be detected in the upper floors even if the air flow rate rises due to increasing air velocity. 1. Introduction The estimation of large wind and buoyancy driven air flow rates in large buildings is essential for the calculation of the annual energy use for cooling and heating. Researchers in the past have carried out measurements of buoyancy-driven air flows in the staircase of a residential building (see Peppes et.al. 2001 and Peppes et. al. 2002). In this paper the measurement of these transient air flow rates in a multi-storey office building is presented. The measured data in this case was used, to estimate the efficiency of natural cooling during the night in the summer. The measurements had to give the answer to the following questions: How large is the air flow rate into the office building and are offices located far away from the main air flow reached by the natural ventilation. 2. The building The governmental office building in which the measurements took place, is part of a series of identic buildings located, one next to the other in a row, in Austria. For a description of the buildings see Dreyer et. al. 2006. The five storeys of the building are connected one to another by an atrium, which is situated next to the staircase. The offices are accessible by gangways on both sides of the atrium and of the staircase. In FIG. 1 a wire frame model of the building is illustrated, where only one office and one gangway on one side of the atrium is shwon. The natural ventilation during the night can be activated by three openings, connected in series, in the ground floor and one single opening on the roof at the top of the atrium (see FIG. 1). 3. Measurement-setup 3.1 The source Tracer gases can be used to measure air flow rates within buildings. Since both, CO2 as a tracer gas and CO2 – sensors, like they are used by the building service departurements, are not very expensive, CO2 has been used for these measurements. Another reason for using CO2 as a tracer gas was that the measurements where carried out whilest the building was ordinary used by the owner and therefor a gas had to be used, which is detectable even far below the maximum allowable concentration (MAC). But as in this case large air flow rates where expected, a conventional CO2-source like a gas bottle with a rather low production rate could not be used. Earlier measurements in single offices in this building have shown that such a conventional source has a maximum production rate up to 0.5 g/s. This upper limit in the production rate is due to icing of the blow-off valve, caused by the decompression of the gas. FIG. 1: wire frame model of the office building The solution of the icing problem was found in a simple, new source that instead of compressed CO2 uses dry ice (see FIG. 2). Within the isolating walls of the box-source made of extruded polystyrene, an electric heater is covered on all sides by dry ice. The power of the heater has to be equal to the energy demand of the dry ice to perform the production rate required. In this case an electric power of 2.4 kW was installed to run a production rate of approximate 3.2 g/s. (see FIG. 2). As due to local sublimation next to the bars of the heater, the heater sometimes was not completely covered by dry ice and the production rate varies therefor from 3.1 to 3.4 g/s. Some peaks in FIG. 2 are due to the crashing of a small dry ice dome above the electric heater. The gas produced is blown off the source from a hole with a diameter of 10 cm. As the gas coming out of the source was rather cold in comparison to the temperature of the air driven into the building by buoyancy, a small fan was placed next to the hole of the box to achive a well mixing of the gas with the air. The installed power inside the source had no influence on the natural buoyancy, since the whole power installed is consumed by the dry ice. The box used is 1.4 m high with a volume of approximate 0.5 m³. With that volume, the source can be run for about 5 hours without refilling. That way a rather stable source could be build for the further measurements. Refilling with dry ice FIG. 2: section of the new CO2 source with a production rate > 3 g/s 3.2 Location of the sensors and of the source As determining the air flow rate with the tracer gas at the inlet opening of the building was one of the aims of the measurements, various CO2 detectors have been set in the foyer of the office building. FIG. 3: ground floor of the foyer and section of the atrium and of the staircase of the building including the positions of the CO2 sensors and of the hot-wire anemometers In FIG. 3 the ground floor of the foyer is illustrated where most of the CO2 sensors were installed. Next to the ground floor a section of the staircase and of the atrium with all the vertical positions of the sensors are shown. Since the reliability of the measured data is not known, for validation hot-wire anemometers were used in the measurement setup at the opening no 2 (see FIG. 3). The source was located in the corner of the room between opening no 2 and opening no 3. The room was provided with two fans to garantee a sufficient air mixing. This is one of the fundamental conditions to meet when measuring with tracer gases. 4. Results and discussion The measurements took place in a late September night. Therefore, the buoyancy driven natural ventilation started rather late that day, because the temperatures within the building where not so high as they usually are during a hot period in the summer. Nevertheless between 6:00 pm and midnight, when the indoor – outdoor temperature difference increased, also the measured mean air velocity at opening no 2 continuosly increased and reached a final value around 0.6 m/s (FIG. 4). At the same time, with an increasing air velocity and therefore an increasing air flow rate, the measured concentrations of the tracer gas decreased in the foyer (sensors CO2 5000-1, CO2 5000-2, CO2 5000-3) and increased in the upper part of the atrium (sensors CO2 2000-3 and CO2 2000-5). Also the sensor located in the gangway on the 2nd floor, about 15 m far away from the atrium (sensor CO2 2000-2), was reached by the tracer gas and a concentration comparable with that one in the atrium at the fifth floor was reached. The high concentration of CO2 in the foyer, measured before 6:00 pm is due to the early start of the measurement. Since the CO2 source has already been activated at 12:00 am, a high amount of the tracer was accumulated in the foyer due to the lack of a distributing air flow. These measurement results show that the production rate of the source was high enough to supply the whole building with a detectable concentration of the tracer. All graphs shown in FIG. 4 include the background content c0 of c0 = 350 ppm CO2 in the air. As measuring the air flow rate into the building by using the tracer was one of the main aims of that work, the air flow rate was deduced out of the measured production rate of the source divided by the measured concentration in the foyer. This was done with the following equation V = p c − c0 where V is the air flow rate into the building in m³/h, p is the measured production rate of the source in g/h, c is the mean measured concentration of the tracer in the foyer using the sonsors CO2 5000-1, CO2 5000-2 and CO2 5000-3 in g/m³ and c0 is the background content of the tracer in the outdoor air in g/m³. The time dependent air flow rate for this measurement is shown in FIG. 6, named as CO2 measurement. For validation of this calculated data, the result was compared with the air flow rate calculated from the air velocity measurement. This second air flow rate was obtained by multiplying the mean air velocity at opening no 2 by its area. Since opening no 2 due to security reasons was covered by a fence (see FIG. 3) not the whole area but only 71% of it could be taken into account. FIG. 4: measured data: air velocity at the inlet at opening no 2 and concentrations of the tracer gas FIG. 3 shows opening no 2 with the position of the hot-wire anemometers used to measure the air velocities. As only 3 sensors where available, the air velocity v4 in the fourth quadrant of the opening had to be calculated. To do this, the ratios of the air velocities v1/v2 and v3/v2 were built (see FIG. 5). As theses ratios can asumed to be linear, the velocity v4 in the fourth quadrant was calculated with the following assumption: v4 = v1 ⋅ v3 v2 With this assumption the mean air velocity (FIG. 4) was calculated. FIG. 5: ratio v2/v1 and v2/v3 against v2 at opening no 2 FIG. 6: comparison of the two calculated air flow rates at the inlet of the office building As one can see, the two air flow rates are well comparable and only at rather small air flow rates the measuremet with the tracer gas underestimates the air flow rate measured by the air velocity. 5. Conclusions Using a new, stable source of the tracer gas CO2, air flow rates in large building can be measured. The measured data is validated with simultaneous measurements of the air velocity at the inlet opening of the large building. Furthermore the source is powerful enough to elevate the measured concentration of the tracer gas above the background content at sensors far away from the inlet even if the air flow rate at the inlet opening increases up to 8000 m³/h. 6. References Peppes A.A., Santamouris M., Asimakopoulos D. N. (2001). Buoyancy-driven flow through a stairwell, Building and Environment, Vol. 36, p. 167-180 Peppes A.A., Santamouris M., Asimakopoulos D. N. (2002). Experimental and numercal study of buoyancydriven stairwell flow in a three storey building, Building and Environment, Vol. 37, p. 497-506 Dreyer J., Bednar T., Konder H., Sofic M. (2006). Kurzbericht über die messtechnische Erfassung der Effektivität der Fenster-, Gebäude- und mechanischen Lüftung unter verschiedenen Betriebszuständen und Randbedingungen, Vienna University of Technology, Institut für Hochbau und Technologie, Fachbereich für Bauphysik und Bauakustik