Sharp JX-9400 Informacje Techniczne Strona 139

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of Figure 6.10. The concentration of these VOCs in the air is analysed at the
four locations shown in Figure 6.10. Locations C
6
and C
3
should be far
enough from the rotating heat exchanger to ensure a good mixing after
sorption and desorption.
If there is no transfer at all, C
3
¼ C
1
¼ C
0
, the outdoor concentration of
each compound, and C
6
¼ C
4
. If some VOCs are transferred, there should
be, at steady state, equilibrium between the amount of VOC taken in extract
air and the amount transferred to supply air:
Q
e
ðC
4
C
6
Þ¼Q
s
ðC
3
C
1
Þð6:2Þ
The transfer rate can be calculated as the ratio of the mass flow rate of VOC
delivered into supply air, and the mass flow rate of VOC in extract air:
R ¼
Q
s
ðC
3
C
1
Þ
Q
e
C
4
¼
Q
s
ðC
3
C
1
Þ
Q
e
C
6
Q
s
ðC
3
C
1
Þ
ð6:3Þ
Injection technique
Injection could be performed in two ways:
1 At a constant flow rate for example, by evaporating the mix placed in an
open pan. Interpretation is performed as described in Chapter 1, ‘Constant
injection rate’, or Chapter 2, ‘Tracer gas dilution’. The problem is that the
injection rate, I, is rather difficult to control for each component in the mix.
2 Pulse injection sampling is started first. Soon after the sampling starts, a
known mass of VOC mix is injected, either with a spray or, better, by flash
evaporation (see Figure 6.11). This injection does not need to be very short,
but it should be shorter than the sampling time. Sampling is continued after
the end of the injection for more than two air changes, ensuring that VOC
concentration is returned down to background level. Interpretation is
performed as described in Chapter 1, ‘Pulse injection’.
The amount injected is such that the resulting concentration values are clearly
above the concentration outdoors, but below the saturation limit of the
samplers.
C
6
C
4
C
1
C
3
Exhaust air
Outdoor air
Supply air
Extract air
- +
Figure 6.10 Schematics of an air handling unit showing location of VOC
injection and sampling points, C
i
, for concentration analysis
Source: Roulet et al., 2000.
118 Ventilation and Airflow in Buildings
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