Sharp JX-9400 Informacje Techniczne Pobierz pdf (Strona 40)

Assuming that no trac er is lost in between, the mass balance of the tracer gas

is, at steady state:

I ¼ðC  C

ÞQ ð2:3Þ

where:

C is the tracer concentration as obtained by analysis,

the concentration upstream the injection port (if any),

Q the airﬂow rate in the duct, which is then:

Q ¼

ðC  C

ð2:4Þ

This simple method assumes steady state: both airﬂow rate and injection ﬂow

rate are constant and the concentration is recorded only when a constant

concentration is reached. This time is rather short, about ﬁve times the time

period needed for the ﬁrst molecules of tracer gas to reach the analyser if the

air is not recirculated after delivery into a room. If there is recirculation, the

time needed to reach the steady state may be much longer, i.e. ﬁve times

the nominal time constant of the ventilated space. In this case, applying the

method described in ‘Measurement of airﬂow rate in a duct’ (a bove) will

reduce the time needed for measurements.

Note that, in a ﬁrst app roximation, mass balance can be replaced by volume

balance to get volume ﬂow rates instead of mass ﬂow rates. This approximation

remains valid as long as the diﬀerence between indoor and outdoor temperature

is smaller than 18C.

Airﬂow measurements at ventilation grilles

Ventilation inlet grilles should distribute the air in each room or space. In

supply and exhaust units, extract grilles should absorb an equivalent airﬂow

rate. Airﬂows should be balanced so that each room or space receives an

appropriate airﬂow rate. Measurements at each grille are essential to ensure

such a balan ce.

Figure 2.3 Measuring the airﬂow rate in a duct with the tracer gas

dilution method

Source: Awbi, 2007.

Airﬂow Rates in Air Handling Units 19

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Sharp JX-9400 Informacje Techniczne Strona 40