空気再循環 (Air Recirculation)

空気再循環とは？

Air recirculation is the process by which a portion of the cabin air is collected from the cabin floor, filtered, and reintroduced into the cabin supply air stream alongside fresh outside air. On a typical commercial aircraft, approximately 50% of cabin air is recirculated and 50% is fresh outside air (bleed air or electrically compressed air) — a ratio that provides a far higher ventilation rate than most buildings. The complete cabin air volume is replaced every 2–3 minutes, giving commercial aircraft among the best indoor air quality of any occupied space.

システムの仕組み

The Environmental Control System (ECS) manages both cabin pressure and air quality simultaneously. The recirculation loop operates as follows:

• Mixed conditioned air (fresh bleed air + recirculated air) enters the cabin through outlets at the base of the overhead bin panels or through ceiling nozzles.
• Air flows downward through the cabin space, picking up heat, CO₂, and humidity from passengers.
• Return air exits through floor-level grilles and is routed to the underfloor distribution system.
• Approximately half the return air passes through HEPA (High Efficiency Particulate Air) filters that remove 99.97% of particles ≥ 0.3 microns — including bacteria, mold spores, and most virus-laden aerosols.
• The filtered air is mixed with conditioned fresh air and re-supplied to the cabin.
• The remaining return air is discharged overboard through outflow valves, maintaining cabin pressure.

HEPAフィルタとCOVID-19の背景

HEPA filters used in aircraft air recirculation systems are the same grade used in hospital operating rooms and pharmaceutical clean rooms. During the COVID-19 pandemic, extensive studies confirmed that in-flight transmission risk was lower than in many ground-based settings, in large part due to the combination of HEPA filtration and the high air exchange rate. The downward airflow pattern in most commercial cabins also tends to limit fore-aft aerosol spread, containing contaminated air within a few rows of the source.

The Boeing 787 and Airbus A350 use electrically powered cabin fans and conditioning packs that recirculate cabin air more efficiently than bleed-air systems, with finer temperature zoning control and lower noise levels from the ECS ducting.

主な事例

The Airbus A380 ECS manages cabin conditioning across two full passenger decks using eight air conditioning packs — four per deck — with HEPA filtration on all recirculated air streams. The result is a 3-minute total cabin air replacement cycle that handles over 500 passengers' worth of CO₂, moisture, and contaminants simultaneously. The Boeing 787 ECS, free from bleed-air constraints, allows airlines to increase cabin humidity to 15–20% compared to the typical 5–8% on older aircraft — a significant factor in reducing passenger dehydration on long-haul sectors.

関連コンポーネント

Air recirculation is tightly coupled with cabin pressure management — both functions are controlled by the same ECS controller. The recirculation fans draw power from the aircraft's main electrical bus, and on the Boeing 787 this electrical load is supplied directly by the engine generators rather than via the traditional bleed air pathway. The galley and lavatory areas have independent exhaust systems that vent cooking smells and waste gases overboard without passing through the cabin recirculation loop.