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bringing additional tracer into the analyser, thus biasing the concentration

measurement. To avoid this, use diﬀerent colours for injection and sampling

tubes.

Principle of the interpretation procedure

The ductwork is modelled by nodes con nected by ducts. In principle, the same

equations as those used in Chapter 1, ‘Application to buildings, multi-zone’, to

assess airﬂow rates measurements in multi-zone buildings could be used, the

nodes being considered as zones. There is, however, an impo rtant diﬀerence

since, in many cases, the directions of inter-zone airﬂows are known, leading

to important simpliﬁcations in the system of equations.

In addition, tracer gas and air mass conservation equations can be written

for each node in the duct network, and this provides, in most cases, a

number of equations much larger than the number of unknown airﬂow rates

(see fo r example ‘Building the system of equations’, below). There are several

ways to use this peculiarity, which are described below for information. We

have nevertheless found that in practice the most robust system of equations

(the system that is the least sensitive to measurement uncertainties) can easily

be purpose-built for each type of air handling unit, as shown in ‘Simplest

way’, below.

Node by node method

The method presented in Chapter 1, ‘Zone by zone systems of equations’,

can also be applied to ductwork and air handling units. Airﬂow and tracer

Figure 2.7 Example of multiple injection devices

Airﬂow Rates in Air Handling Units 23

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Sharp JX-9400 Informazioni Techniche Pagina 44