Download Estimation of air flow rates in large buildings based on measurements

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