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The model is analysed based on sea level conditions. The specific heat is assumed
constant due to the low range of temperature between 5 ~ 65°C.
Based on the geometry and measured highest flow rate of about 53 L/min. through the
bias vent holes at the highset pressure setting, the airflow velocity through the holes is
calculated as no more than 60 m/s which is the highest velocity in this system. Thus the
air flow velocities in this model are all below 1/3 of the speed of sound for considering
air compressibility [39]. Furthermore, comparing with the atmospheric pressure, the
working pressure is also very low. Therefore an incompressible fluid is assumed.
2.2 System overview
Figure 2.2 CPAP system
Figure 2.2 shows the schematic diagram of a CPAP system in use. The components of
the CPAP machine include an air delivery unit (ADU), a connecting duct, a humidifier,
a heated air delivery tube (HADT), an elbow with bias vent holes and a mask.
ADU provides and maintains a proper positive pressure to the mask and then to
patient’s upper airway. The connecting duct guides the air flow into the humidifier
chamber. The humidifier which has a heating plate underneath receives the air and adds
water vapour to the air by heating and evaporating the water in the chamber. The HADT
connects the humidifier and delivers humidified air to the mask. To prevent
condensation in the tube, the HADT is heated by embedded wire winding along inside
the tube wall. There is an elbow with bias vent holes locating between the HADT and
the mask. These holes are for purging exhaled air to provide patient fresh air. The mask
is an interface between the machine and the patient.
This project will provide two models. One is for analysing the re-breath of exhaled air
with higher CO 2 concentration level while the other is for thermodynamic analysis.
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