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condensation severity but cannot calculate the actual amount of condensate built-up in a
period of time. This is because once condensate starts to build up, the thermal
conditions on the surface changes and the condensation rate will be no longer the same
[42].
3.6.2 Condensation within the HADT
When there is no patient’s breathing, the CPAP machine is working in a steady state
and provides a steady flow. The condensation in the HADT can be predicted by
comparing the dew point temperature of the airflow to the local HADT wall (the local
lump) temperature. If the wall temperature is lower than the airflow dew point,
condensation will occur on the inner surface of the wall. By data regression between
5°C ~60°C [61], dew point can be expressed as below (see Appendix V for regression
details):
T 17.222ln d 93.343( C)
(3.82)
DewPt
Where d is specific humidity defined in chapter 3.
The condensation can also be predicted by comparing relative humidity of the airflow in
the HADT to the saturated relative humidity at the local HADT wall temperature. If the
airflow relative humidity is higher than the wall temperature saturated relative humidity,
there will be water vapour spared out from the airflow onto the HADT wall.
With the patient’s breathing, the humidity of airflow in the HADT is fluctuating. The
surface temperature may be higher or lower than the dew point. The condensation under
such fluctuation can only be calculated by humidity comparison. Net condensation
within a breath cycle can be used to predict the condensation build-up. The evaporation
rate in a lump of the HADT may be expressed as:
m k A ( C C )
(3.83)
ev ev Tli Tsv Tav
Where CTsv is the saturated absolute humidity level at the local HADT wall temperature
and CTav is the absolute humidity of airflow through the lump.
Analogous to heat convection, the mass convection coefficient k ev may be expressed as:
67