Sharp JX-9400 Informacje Techniczne Pobierz pdf (Strona 96)

example, using plastic sheeting and adhesive tape. Inﬂated balloons are also

well suited to seal circular ducts.

Tracer gas injection and air sampling tubes are installed at appropriate

points in the main supply or exhaust duc ts, as shown in Figure 4.9, to quantify

any residual ﬂow rate resulting from leakage.

The system fans (or a fan added at one register if required) are used to

pressurize the supply side and depressurize the exhaust side of the network.

Since all inlet and exhaust grilles or ducts are sealed , the ﬂow, q

, through

the fan(s) results from leakage, and is measured as described in Chapter 2,

‘Measurement of the airﬂow rate in a duct’, together with the pressure diﬀer-

ence, p, between the inside and outside of the ducts. The ﬂow rate is the

sum of all leaks downstream of the measurement point in pressurized ducts,

and upstream for depressurized ducts.

A series of measurements is made at diﬀerent fan speeds, and the coeﬃcients

of Equations 4.1 or 4.2 are determined, and the relationship is subsequently

used to calculate the leakage rate at the service pressure diﬀerence.

Flow rate diﬀerence method

If a duct is very leaky, the leakage can be obtained by measuring the diﬀerence

between the ﬂow rates at two locations along the ﬂow. Since additional pressur e

drop should be avoided, tracers are recommended to measure the airﬂow rates.

One tracer should be injected at a point upstream of the ﬁrst location, and a

second tracer injected at the ﬁrst location. The concentrations of each tracer

are measured after the second, downstream location, at a distance where a

good mixing is achieved (see Chapter 2, ‘Sampli ng points for concentration

measurements’). If steady ﬂows can be assumed, two sequential measurements

using a single tracer at each point may be used instead.

For depressurized ducts, only one tracer is necessary since it is diluted by

the air entering the duct through leakage. The tracer gas is injected at the

Tracer A

Sampling

Tracer B

Seal

Sampling

SealSeal

Seal

Figure 4.9 Location of tracer injection and sampling tubes for the

measurement of leakage airﬂow rates in a ventilation system

Source: Roulet and Vandaele, 1991.

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Sharp JX-9400 Informacje Techniczne Strona 96