Sharp JX-9400 Technické informace stáhnout pdf (Strana 159)

maintained under pressure. It is noted, however, that these two conditions

complicate the experimental arrangement since control leads must extend to

the end of the injection tube, and the system, under pressure, will be more

sensitive to leaks.

In buildings with a large internal volume it may be necessary to discharge

large amounts of tracer. If this is the case, then the following method may be

used: the operator works out a zigzag or circular path through the area that

will give good coverage of the building. The time taken to walk along the

path is noted. The amount of gas required to dose the area is evaluated (from

knowledge of the building volume and the re quired initial concentration),

and the gas ﬂow rate needed to discharge that volume of gas in the time

taken to walk along the path is calculated. The gas cylinder is set to discharge

the gas at the required rate and the operator walks along the path carrying the

discharging cylinder. Some mixing of air and tracer will occur as a consequence

of the movement of the operator through the building.

Mixing of tracer gas in ducts and air handling units is described in Chapter

2, ‘Tracer gas injection ports’.

Tracer density problem

If the tracer gas density diﬀers signiﬁcantly from that of the air, that is, if

its molecular weight diﬀers much from 29g/mole, the concentrated tracer

rises or falls (depending on its density) directly as it leaves the injection duc t.

Injection jets help to avoid this, while diﬀusers make the phenomenon worse,

since they lower the injection speed of the concentrated tracer.

Since there is no non-toxic tracer gas with the density of air (apart from

ethane, which is explosive, the closest are highly toxic carbon monoxide,

hydrogen cyanide and nitrous oxide!), a simple approach is to dilute the

tracer in air at about 1 :10 or more, and to use this diluted mixture. This not

only adjusts the density, but also increases the injection ﬂow rate – thus helping

mixing – and makes the problem of ﬂow control easier in relatively small rooms

or for small concentrations.

In any case, the tracer concentration should not exceed a value that signiﬁ-

cantly changes the density of air. A proposed limit is:

lim

 3  10

4



air



tracer



2

tracer

ð7:10Þ

where M is the mo lecular mass of the tracer gas, in grams per mole. The factor

3  10

4

corresponds to a change in density for a temperature variation of 0.1 K

in pure air. Such changes are very unlikely to have a signiﬁcant eﬀect on the

airﬂows in a space. With common tracer gases, this limit concentration is

much larger than the concentrations comm only used.

Several experiments have shown that if the tracer is properly injected, the

errors caused by the tracer density are negligible in relation to other sources

of error (Sandberg and Blomqvist, 1985).

138 Ventilation and Airﬂow in Buildings

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