Aircraft Cabin Ventilation Theory

Ventilation on board of an aircraft is governed by the ventilation equation. In the steady state case, a concentration of any substance depends only on the source strength and the effective air flow rate for ventilation. Not all air for ventilation is effective and helps to lower concentration. Some air leaves the cabin without mixing and rinsing. This is expressed by the ventilation efficiency. The dynamics follows an exponential function and is expressed by a time constant that depends on the air change rate and the ventilation efficiency. The (theoretical) air change rate is the air flow rate divided by the volume of the room. With full mixing (i.e. ventilation efficiency of 1), the concentration is reduced to 36.8% after one air change.