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with

 ¼



ð1  RÞ

1  R þ 

ð2:63Þ

The theoretical exponential can be ﬁtted to the experimental points, as shown

in Figure 2.11 depicting an actual experiment. The concentrati on increase in

the supply duct, C

 C

, quickly reaches its steady-state value, while a good

ﬁt of the exponential can be obtained within two time constants, allowi ng the

determination of the steady-state concentration and time constant without

waiting for equilibrium that is reached after at least ﬁve time constants.

Appropriate method for assessing the

recirculation ratio

In Equations 2.33, 2.34 and 2.41 the concentration diﬀerence C

 C

is at the

denominator, and these two concentrations are close to each other at steady

state when the recirculation ratio is high. This leads to a large conﬁdence

interval of the calculated airﬂow rates. In units with large recirculation

ratios, it is better to inject the tracer gas at location 3 instead of location 2.

The supply airﬂow rate can then be determined with better accuracy, using:

 C

ð2:64Þ

Assuming that the conﬁdence interval is the same for both concentrations, the

conﬁdence interval is:

Q

ﬃ TðP ; 1Þ

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ



I



þ 2



C

 C



ð2:65Þ

Tracer gas concentration (mg/m

)

0 2 4 6 8 10 12 14 16

Concentration difference [mg/m

]

C3¢

C3¢–C3

Tracer injection

Time (h)

Figure 2.11 Tracer gas concentrations in the supply duct, upstream (3) and

downstream (3

) of the tracer gas injection port

Note: Points are measured concentrations, while lines are exponential ﬁts.

Source: Roulet and Zuraimi, 2003.

36 Ventilation and Airﬂow in Buildings

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