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constant. In rooms with complete mixing, the room mean age of air equals

the nominal time constant, and the air exchange eﬃciency is 50 per cent.

Short-circuiting of air will also leave dead zones in the room, giving rise to

an eﬃciency that is lower than 0.5 (see Figure 3.1).

Table 3.1 shows the air exchange eﬃciency and the corresponding room

mean age of the air, assuming that the nominal time constant or the mean age

at exhaust is one hour.

Some typical probability density curves of the age of the air at the exhaust

are illustrated in Figure 3.2, the air exchange eﬃciency being used as

parameter. If the air is displaced like a piston, all air particles reach a given

location in the room at the same time, and they reach the exhaust at a time

Figure 3.1 Ventilation modes with typical airﬂow patterns and air

change eﬃciencies

Source: Roulet, 2004.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

00.511.522.53

f(τ)

25%

50%

66%

80%

90%

99%

Figure 3.2 Typical probability density curves for the age of the air

Note: The parameter is the air exchange eﬃciency.

Table 3.1 Nominal time constant and room mean age of the air corresponding

to the probability curves shown in Figures 3.2 and 3.3

Air exchange eﬃciency 

25% 50% 66% 80% 90% 99%

Room mean age of the air hi 1.44 0.99 0.76 0.62 0.55 0.50

Age of Air and Ventilation Eﬃciency 41

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Sharp JX-9400 Technical Information Seite 62