Use of Partial Recirculation to Limit Build-Up of Cabin Carbon Dioxide Concentrations to Safe Limits per ASHRAE Standard-62 2020-01-1245

Carbon dioxide exhaled by occupants remains within the cabin during operation of HVAC unit in recirculation mode. The CO₂ inhaled by the occupants goes into their blood stream that negatively affects occupant’s health. ASHRAE Standard-62 (1999) specifies the safe levels of carbon dioxide in conditioned space for humans. The CO₂ concentration limit per ASHRAE is 700 ppm over ambient conditions on a continuous basis. Based on the test data, at worst case scenario (idle condition where body leakage will be a minimal) results in CO₂ concentrations of 1601, 2846, 4845 and 6587 ppm respective for 1 to 4 occupants in 30 minutes.

Author has also conducted test by imposing ASHRAE standard-62. A controller was programmed for operating the blower unit’s intake door to go from recirculation to OSA mode when the measured carbon dioxide ppm level goes above 1100 ppm. The door stays in OSA mode until the cabin carbon dioxide falls to approximately 500ppm. By imposing these limits, the blower unit’s intake door cycles between 3 minutes to 6.5 minutes with four to one occupants in the cabin. Hence, cycle time for two and three occupants in cabin will fall between 3 to 6.5 minutes.

ASHRAE Standard-62 has an additional requirement for maintaining good air quality within the ambient condition. Per this requirement, we need to introduce a min of 15 cfm/person (0.43 m³/min) in the conditioned space to maintain good air quality. Even though this requirement is primarily for elimination the body odors, the author has investigated if this airflow requirement is also sufficient to maintain cabin CO₂ concentrations as stated above. For the vehicle investigated, the above data represents a partial recirculation factor (PRF) of 19.2%. Based on the simulation and experimentation, it was determined that this PRF is not sufficient to maintain cabin carbon dioxide concentrations below 1100ppm. A PRF of approximately 40% was required to maintain the cabin carbon dioxide concentrations per ASHRAE standard (i.e., ppm level <=1100ppm).

Finally, partial pressures of CO2 and O₂ have also been presented along with O₂ concentrations within the cabin. Based on the idle condition, partial pressure for CO₂ increased by 660 Pa in 30 minutes of test drive with 4 occupants. Detailed results have been presented in the paper.