Sharp JX-9400 Información técnica descargar pdf (Pagina 39)

from 0.05 to 5 m/s, and are well suited for speeds of 1–5 m/s, which are typical in

ventilation ducts.

Helix anemometers measure the rotation speed of a small helix that is placed

perpendicular to the airﬂow. The pressure diﬀerence between the front and the

side of the Pitot tube is proportional to the square of the air speed. The helix

anemometer and the Pitot tube are most accurate for air speeds abo ve 10 m/s,

and therefore may not be the best for measurement in ducts, where speeds as

low as 1 m/s can be measured.

Test procedure

The location of the measurement in the duct should be at least 8 diameters

downstream and 3 diameters upstream of any disturbances in the ﬂow, such

as a bend in the duct or a change in cross-se ction. Flow-straightening vanes

located at 1.5 diameters upstrea m will improve the measurement accuracy.

Several measurements across the duct should be taken to enable integration.

It is advisable to notionally divide the duc t section into sub-sections of equal

area and to take measurements at their centres. The ASHRAE Handbook of

Fundamentals (ASHRAE, 2001) proposes a division of 16 to 64 rectangular

sub-sections for a rectangular duct, and 20 annuli for cylindrical ducts (see

Figure 2.2). In the latter, any asymmetry in the ﬂow may be taken into account

by taking measurements along two orthogonal directions.

Tracer gas dilution

Measurement of airﬂow rate in a duct is the simplest application of the tracer

gas dilution technique. It is illustrated in Figure 2.3. The tracer is injected at

a known constant ﬂow rate, I. The air is analysed downstream, far enough

from the injection port to have a good mixing of the tracer into the air (see

‘Sampling points for concentration measurements’, below).

0.316 R

0.548 R

0.949 R

0.707 R

0.837 R

Figure 2.2 Location of the measurement points in circular and rectangular ducts

Source: ASHRAE, 2001.

18 Ventilation and Airﬂow in Buildings

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Sharp JX-9400 Información técnica Pagina 39