Cleaning and maintaining the required pressure: how airplane air systems work

When you're flying at 35,000 feet, you might be surprised at how fresh, clean air circulates in a pressurized cabin. No wonder manufacturers have spent decades mastering this complex process.

Airplanes fly in conditions that are unfavorable for human life. When the atmospheric pressure of the air decreases, the density of oxygen also decreases, which makes it difficult to transfer this vital element from the human lungs to the blood," writes Travel+Leisure.

While on the ground the atmospheric pressure is approximately 14.7 pounds per square inch (PSIA), at cruising altitude it is only 3.47 PSIA. Under these conditions, most people lose consciousness within 60 seconds and die shortly afterward. And this is without taking into account the air temperature, which can range from -45 to -85 degrees Fahrenheit.

Environmental control systems (ECS) are designed to maintain cabin pressure at a level acceptable to humans, while ensuring that the air we breathe is fresh and at a comfortable temperature. These processes are interconnected.

The ECS cyclically moves a 50/50 mixture of indoor and outdoor air around the cabin to create the pressure we need.

"The outside air goes into the compressors and is compressed to a density that makes the oxygen safe to breathe," says David Space, former associate technician for Boeing's Commercial Airplane Environmental Control Systems group. - This process makes the air hot, so it needs to be cooled. The air then enters a large chamber called a mixing manifold, where it is mixed with recirculated air from the cabin, after which the cabin air is passed through a specialized HEPA (High Efficiency Particulate Air) filter. The mixed air then moves through the air ducts of the aircraft from back to front and to the connecting ducts where the ceiling connects to the sideboard. At the same time, air exits the cabin through the return air grilles where the cabin meets the floor. Half of the air from the cabin is discharged overboard, maintaining a continuous flow of air."

This continuous flow helps maintain a constant pressure. The air exchange rate in the cab is higher than in our homes, offices, or other public spaces. "The air changes 12 to 15 times in an hour, and the HEPA filter goes through 25 to 30 cycles," he says.

HEPA filters used to purify air in airplanes are of the same quality as those used in operating rooms. They can capture very small particles, including most viruses and bacteria. Atmospheric conditions make the outside air naturally sterile.

Cabin air management systems are even more sophisticated on newer aircraft, such as the Boeing 787 Dreamliner, where the pressure has been reduced to a much more comfortable 11.78 PSIA. This means there is more oxygen in the air, which research shows can help alleviate the discomfort (read: fatigue, dizziness, and dehydration) that some people experience during a flight.

When designing the Dreamliner, Boeing chose a filtration system that removes pollutants from the cabin air without creating new particles that can irritate the eyes and respiratory system.

Air management systems also control the temperature and humidity in the cabin, which are largely influenced by the number of people on the plane. External factors, such as the amount of light coming in through the windows or whether the aircraft was parked in the sun before takeoff, can also affect the initial cabin temperature.

The crew calibrates the aircraft according to the number of passengers on board each flight, ensuring that the airspace management system can accurately calculate the temperature requirements for each area of the aircraft based on the number of passengers on board.

Pilots can also control these systems in flight, but Space noted that they rarely do so. This is because changing the overall temperature settings can make some on board less comfortable. Cabin air management systems are programmed to balance the temperature in the cabin.

These systems are also designed to address a problem that is unique to aircraft. Most passengers are sedentary during a flight, which means they have a lower metabolic rate and are more prone to feeling cold. Flight attendants, on the other hand, are constantly moving - pushing carts back and forth, which likely increases their body temperature during the flight.

To resolve this contradiction, the air is first blown through the aisles, providing flight attendants with cool air while avoiding drafts in the cabin and keeping passengers comfortable.

Of course, some people may be more prone to feeling warm or cold, so they created these comfortable overhead nozzles. These tiny vents distribute the same purified air that flows throughout the aircraft, but the nozzles have special accelerators that increase the individual airflow. In essence, they are mini-fans that play an important role in ensuring a pleasant flight for every passenger.