Sharp JX-9400 Informacje Techniczne Pobierz pdf (Strona 64)

assumes that the tracer gas behaves the same as the air: no adsorption and

same buoyancy, which is the case if the tracer concentration is small. It can

be readily understood that if the air is marked at the in let by a short pulse of

tracer gas, and if the tracer molecules follow the air molecules, they will

arrive at a given location at the same time as the air molecules. The time

spent between injection and the detection of most tracer gas molecules by the

analyser is the age of the tracer, hence the age of the air at the air sampling

location. The pulse technique is, however, not the only one and the following

three strategies can be used:

Step-down – a uniform concentration of trac er is achieved at the beginning

of the test, when the injection is stopped;

Step-up – the tracer is injected at the air inlet at a constan t rate from the

starting time throughout the test;

Pulse – a shor t pulse of tracer is released in the air inlet at the starting

time.

The probability functions and the local mean ages at any point, r, can be

calculated from the time history of the net tracer concentration, C

(t), which

is the measured concentration minus the background concentration. It was

shown, however, that for rooms with a sing le air inlet and a single air outlet,

the step-up method should be preferred, since it is the easiest to perform and

gives the best accuracy (Roulet and Cretton, 1992).

For the step-up technique, the tracer gas is injected at a constant rate into

the supply air in the outside air duct, starting at a known time t

. It should

be ensured that the tracer and the air are fully mixed in the supply duct to

produce a steady concentration, C

, at the air inlet. If C

cannot be meas-

ured, the equilibrium concentration within the enclosure, C

, may be used

instead. The notations for air sampling refer to Figure 2.5.

Tracer gas concentration at the locations where the age of air is looked for

is recorded. The sampling time interval should be short enough to record the

transient evolution of the concentration. It should then be much shorter than

the expected age of air. A good value is one tenth of the nominal time

constant, which can be estimated by dividing the ventilated volume by the

design airﬂow rate, or better, by the actual airﬂow rate if it is known.

One important location is in the exhaust duct, where C

¼ C

is meas-

ured. Recording the evolution of the tracer gas concentration at this location

provides both the actual nominal time constan t and the mean age of air in the

ventilated space. Injection rate is maintained constant until a steady state is

obtained. Depending on the air change eﬃciency, this may take up to four

time constants. An example of such a record is given in Figure 3.4.

When the concentration stabilizes at a value noted, C

, the step-up

experiment is ended. However, it is recommended to continue recording the

concentration after having stopped the tracer gas injection, since this will

provide a second measurement of the age of the air, using the decay method.

For this purpose, the time when injection is closed should be noted, since

this time is the starting time of the decay experiment.

Age of Air and Ventilation Eﬃciency 43

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