Sharp JX-9400 Informations techniques télécharger pdf (Page 42)

In principle, the method described above in ‘Tracer gas dilution’ can be

applied to each branch of a duct network. However, this requires as many

tracer gas injections and air sampling measurements as there are airﬂow

rates. Experience has shown that the experiment can be made simpler, as

shown in Figure 2.5, where most practical and eﬃcient injection locations are

indicated by arrows. In this ﬁgure, air samp ling points for the required

tracer gas concentration measurements are also shown.

If several tracer gases are needed but not available, it is possible to use the

same tracer gas in several experiments, injecting the tracer successively at

diﬀerent locations. In this case, care should be taken to ensure constant airﬂow

rates in the system. In particular, frequency controllers of the fans should be

blocked at a constant frequency. It is also recommended to start with injection

at location 2, then 3 and ﬁnally 1. This strategy shortens the time required

between two experiments to reduce the tracer gas concentration in the

system to a negligible level.

Two tracer gases or two successive measurements with one tracer gas allow

in most cases assessment of all primary and most seconda ry airﬂow rates:

tracer one injected in the main return air duct;

tracer two (or a second run with tracer one) injected in the outside air duct.

Additional injection ports may be useful to increase the accuracy. These are:

tracer three (or a third run with tracer one) injected in the main supply air

duct, allowing the direct and more accurate determination of supply airﬂow

rate;

tracer four (or a fo urth run with tracer one) injected in the control room at

constant concentration to det ermine leakage from the control room into

the air handling unit.

The optimal tracer gas injection rate depends on the design airﬂow rate Q

the duct and on the required concentration, C, itself depending on the sensi-

tivity of the tracer gas analyser. A good method is to adjust the tracer gas

injection ﬂow rate on the basis of the outdoor airﬂow rate Q

.IfC

is the

C1'

C2 C3

C3'

C4'

Figure 2.5 Locations of tracer gas injection (arrows), and sampling points for

concentration measurements (C

) in a typical supply and exhaust air handling unit.

Source: Roulet et al., 1999.

Airﬂow Rates in Air Handling Units 21

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Sharp JX-9400 Informations techniques Page 42