Sharp JX-9400 Informacje Techniczne Pobierz pdf (Strona 60)

Age of Air and

Ventilation Eﬃciency

The airﬂow patterns should, in principle, be organized in order that new air

is brought to the head of the occupants, so that they get fresh, clean air, and

that contaminants be evacuated as quickly as possible, before being mixed

with indoor air. However, air, as any other ﬂuid, always follows the easiest

path. T his means that the airﬂow does not necessarily follow expected

patterns. Since air is transparent, unexpected airﬂow patterns are noticed

only when things go wrong. Depending on the airﬂow distribution in rooms

and for a given airﬂow rate, the concentration of contaminants in the occu-

pied space may vary by a fact or of two or more. Therefore, measurements

may be useful to check if the airﬂ ow patterns are as expected. Such measure-

ments allow checks to ensure that:

the air change eﬃciency is as large as possible,

clean air is supplied to the right places,

air contaminants are quickly removed.

The measurement of the age of the air allows for the detection of possible

shortcuts and dead zones and for the checking of the general airﬂow pattern

in the room or in a building.

Deﬁnitions

The quantities deﬁned below are explained in greater detail elsewhere

(Sandberg, 1984; Sutcliﬀe, 1990) and are only brieﬂy described here.

Age of the air

Let us assume that the molecules of outdoor air start their indoor life when

entering the building or the ventilation system. These arrive at a give n loca-

tion, r, in a room after a time, 

, that varies from one particle to the other. 

is called the re sidence time of the particle in the room, or its age. Note that

the air elements themselves, i.e. oxygen and nitrogen molecules, do not age.

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Sharp JX-9400 Informacje Techniczne Strona 60