Environmental Engineering Reference
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Figure 3.16 Cooling power and coefficient of performance of mechanical night ventilation system
and 155W for the exhaust fan, closely matching the design value. The specific total
consumption of 0 . 17Wm 3 h is excellent. During night ventilation with a measured
supply air volume flow of 4021m 3 h 1 at a pressure drop of 395 Pa, the supply air fan
consumed 1072W, while the exhaust air fan at a slightly lower flow and pressure drop
(3600 m 3 h 1 and 356 Pa) required 850W. The specific total consumption related to
the supply air flow is 0 . 48Wm 3 h, still a good value for ventilation systems, but 75%
higher than the design value.
Further measurements in several rooms were taken in 2006 during a very hot period
in July. Internal loads averaged over 24 hours were only 4Wm 2 in the top floor and
5Wm 2 in the bottom floor offices. External loads including sun shading systems
were 7 . 5Wm 2 on average on the ground floor and 4 . 6Wm 2 on the top floor. The
ground floor has a significant heat transfer to the ground due to the rather low insulation
thickness of the floor ( U -value of 0 . 35Wm 2 K 1 ). The measured heat flux via the
floor to the earth was between 2 and 3Wm 2 on average. This is the same amount that
the measured heat flux removed from the ceiling during night ventilation (2 . 7Wm 2
on average). The measured effective discharging time of the ceiling during mechanical
night ventilation was about 13 hours with a ceiling temperature decrease of 1 . 5 K. The
main limitation on a more effective night discharging of storage masses is the total air
exchange rate, which is limited here to 2 air exchanges per hour. The air outlet injection
to the ceilingworked effectively, as can be seen from the infraredmeasurements during
the day and after a period of night ventilation (Figure 3.17). Ambient air temperatures
during night ventilation were 4 . 2 K lower than room temperatures and a mean cooling
power of 7 . 6Wm 2 was achieved.
There is a clear temperature stratification in the building of about 1 K for rooms with
the same orientation. This can be attributed to a heat flux of 2-3Wm 2 via the rather
badly insulated ground floor, the radiative exchange with the cooler ground floor and
higher storage masses of the ground floor ceiling. Strong drops in room temperature
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