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Where d Ca is the specific humidity of the chamber air and m C the mass flow rate in the
chamber and may be considered as the average of connecting duct mass flow rate and
HADT mass flow rate. m ev can be calculated by Eq. (3.40).
3.5 HADT heat balance
After the airflow is heated and humidified in the chamber, it continues flowing toward
patient through the HADT. When there is reverse flow, the airflow may firstly be
stopped and then reverse toward the chamber from the mask. No matter which direction
the air flows, if there is no heat added in, the air cools down because of heat transfer
into the ambient through the HADT wall.
When the air temperature drops down below its dew point, the specific humidity will
drop because of condensation. If this occurs, the built-up condensate in the HADT can
increase pressure drop in it thus reduce the pressure in the mask [36]. Also cooler air
with lower specific humidity continuously blows into patient’s upper airway may make
the patient uncomfortable. To avoid these, the HADT has embedded copper wire to
provide heat to the tube wall so to heat up or maintain airflow temperature. The tube
heating will be turned on when it is needed and its heating level is adjustable. Figure
3.14 shows the HADT wall.
Figure 3.14 Heated air delivery tube wall [19]
HADT thermal balance analysis is to determine the temperature change of the air and
the potentiality for condensation along the HADT under various combinations of
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