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column) and the airﬂow rate through the fan is measur ed using any of the

following methods:

The airﬂow rate through a fan depends on the pressure diﬀerential and its

rotation speed. Measuring these two quant ities allows assessment of the

airﬂow rate from the characteristic curve of the fan. Blower doors use this

method.

A suit able airﬂow meter such a nozzle or a sharp-edged oriﬁce is installed in

the airﬂow circuit (see Figure 4.5).

The tracer gas dilution technique, as described in Chapter 2, ‘Tracer gas

dilution’.

These measurements are repeated for several pressure diﬀerentials, ranging

from a few Pascals to about 60 Pa, or even more for some cases.

The minimum pressure is limited by the noise of the pressure diﬀerential,

for example, the random pressure variations resulting from wind and stack

eﬀect. Therefore, measurements should be performed when there is no wind

and the minimum pressure diﬀerential is in practice twice the natural

pressure diﬀerential. The maximum pressure is limited by the resistance of

the object by practical limits such as fan airﬂow rate combined with the

object’s leakage. Note that 100 Pa is a pressure that can result from 40 km/h

wind velocity.

Since fan pressurization is subject to the disturbing inﬂuence of natural

pressure ﬂuctuations created by the wind, most measurements are made at

pressure diﬀerentials far above those created by natural forces. This may

lead to inaccuracy if the results are extrapolated to lower pressure diﬀerentials.

Two general models are used to characterize air permeability. The power

law, fully empirical, reﬂects the fact that leakage is a combination of various

cracks and openings that may be arranged in parallel and series network:

q ¼ Cp

ð4:1Þ

where:

q is the volume airﬂow rate through the leakage site (m

/s);

p is the pressure diﬀerence across the leakage site (Pa);

n is the ﬂow exponent (0:5 < n < 1);

C is the airﬂow coeﬃcient (m

1

n

Since the airﬂow may be either laminar or turbulent, and the airﬂow rate is

proportional to the pressure diﬀerential in laminar ﬂows and to its square

root in turbulent ﬂows, Etheridge propo ses a quadratic law, that expresses

that the ﬂow is a mix of laminar and turbulent ﬂow arranged in parallel (Ether-

idge and Sandberg, 1996):

p ¼ aq

þ bq ð4:2Þ

where:

a and b are coeﬃcients representing respectively the turbulent and lam inar

parts of the quadratic law (Pa s/m

and Pa s

60 Ventilation and Airﬂow in Buildings

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