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3.4.1 Governing equation
Figure 3.13 Chamber air heat balance
Figure 3.13 shows the chamber air heat balance. Thermal energy added into chamber air
comes in by means of inlet, heat convection and evaporation from the water below it.
Such acquired thermal energy is transferred to enthalpy by means of increasing the
chamber air temperature i.e. change of heat storage in chamber air. The thermal energy
is carried out by the outlet and dissipation to ambient through the chamber walls
surrounding the air space. For simplification, the upper part of the vertical chamber wall
(wall 2) and the top part of the chamber (wall 3) are assumed to have the same
temperature (see Figure 3.5). It is further assumed that the outlet from the chamber has
the same thermal properties as the chamber air. Note that when the flow is from ADU
toward the patient, the inlet is from ADU via the connecting duct and outlet goes into
HADT. However, during reverse flow, the inlet is the air re-entering the chamber from
HADT and outlet is the air pushing back into the connecting duct from the chamber.
The governing equation of the chamber-air thermal balance may be expressed as:
Q Q Q Q Q Q
(3.50)
Cai wa va Cao CaSt C23i Qwa is the same as it is in Eq. (3.7) for heat balance in the water. Equations for the
remaining terms in Eq. (3.50) will be analysed and derived in following subsections.
3.4.2 Heat carried into chamber air by evaporated molecules
The heat brought into chamber air by the evaporated water molecules is the same as that
analysed in the water heat balance analysis in section 3.3. However, as long as this
vapour does not condense, the latent heat will be kept in these water vapour molecules
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