Sharp JX-9400 Informações Técnicas descarregar pdf (Página 97)

upstream end of the duct and its concentration is measured at both ends to give

the ﬂow rate at each.

The leakage of the whole supply or exhaust network may be determined by

measuring the diﬀerence between the airﬂow rate in the main duct (close to the

fan) and the sum of all the ﬂow rates at the individual inlet or extract terminals.

For this purpose, the main airﬂow rate through the fan may be measured with a

tracer, and the ﬂow rates at the terminals may be determined by one of the

methods described in Chapter 2, ‘Airﬂow measurements in air han dling

units’, the most appropriate being the compensated ﬂowmeter (see Chapter 2,

‘Compensated ﬂowmeter’).

With this method, the leakage ﬂow rate is the diﬀerence of two large

numbers. Therefore, it is not the best one for tight or only slightly leaky

ducts. Because of its ease of use, it can nevertheless be used for diagnosis

purposes, to detect if the ductwork is very le aky or not.

Diﬀerential building pressurization

The methods described above measure the leakage of the air duct system. From

the point of view of energy saving, however, it may be useful to measure the air

leakage to or from outside only, and not leakages between the system and the

interior of the building.

For this purpose, the duct system is assumed to be a part of the envelope and

the duct leakage is obtained by diﬀerence. In a ﬁrst test, the closed building is

pressurized after sealing the outdoor air intake and exhaust of the building,

with all the registers and returns open (see Figure 4.10, left). In a second

test, all registers and returns are sealed (see Figure 4.10, right). The diﬀerence

in airﬂow rate between the two tests, for each pressure diﬀerence, results from

duct leakage to the outside.

The major advantage of this method is that it needs little equipment, in

addition to that required for envelope leakage measurements. However, it is

prone to inaccuracy since the duct leakage is again obtained as the di ﬀerence

between the measurements of two large airﬂow rates.

Conditioned space

SealSeal

Conditioned space

Seal

Figure 4.10 Two measurements providing, by diﬀerence, the duct leakage

to outside of the conditioned space

76 Ventilation and Airﬂow in Buildings

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