
experiments, the smallest recirculation rates are for limonene. They are close to
the rates that could be expected from leakage or entrained air.
At the other extreme, it is clear that phenol, hexanal and dichlorohexane
present a strong adsorption, which cannot be completely removed in the
purging sector.
More evidence for adsorption is the dependence on boiling point shown
in Figure 6.16. The transfer ratio for each type of compound increases with
the boiling point of the compound, as can be expected for adsorption or
condensation.
It is often claimed that non-hygroscopic wheels have a lower transfer ratio
than hygroscopic wheels. Alternatively, it is also said among rotating heat
exchanger specialists that non-hygroscopic wheels become hygroscopic with
time (Ruud and Carlsson, 1996). It should be noticed that the recirculation
rates measured in similar conditions (without purging sector) in a unit with a
hygroscopic wheel (EPFL laboratory) and with a non-hygroscopic wheel
(EMPA unit) are clearly correlated (see Figure 6.17). The line in this figure
is a least square fit line, with a correlation coefficient of R ¼ 0:79, and slope
0%
10%
20%
30%
40%
50%
60%
70%
No purging sector
With purging sector
n-Decane
1-Butanol
1-Hexanol
Phenol
1-Hexanal
Benzaldehyde
1,6-Dichlorohexane
Dipropylether
Limonene
m-Xylene
Mesitylene
Figure 6.14 Average VOC recirculation rates measured in the EPFL
auditorium (leaky) unit, with and without purging sector
Source: Roulet et al., 2000.
0%
10%
20%
30%
40%
50%
60%
70%
No purging sector
With purging sector
n-Decane
1-Butanol
1-Hexanol
Phenol
1-Hexanal
Benzaldehyde
1,6-Dichlorohexane
Dipropylether
Limonene
m-Xylene
Mesitylene
Figure 6.15 Average VOC recirculation rates measured in the EPFL
laboratory unit, with and without purging sector
124 Ventilation and Airflow in Buildings
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