Aircraft Pneumatic Systems — Page 218, Lesson 274

Let's pick up with the humidifier, because it's a small unit but it answers a real problem. When an aircraft cruises at high altitude for a long time, the air outside is extremely dry — the ambient air only carries about 1 to 2 percent relative humidity. If we fed that straight into the cabin, the low humidity would cause physical discomfort for the occupants. So the humidifier's job is to raise the moisture content of that conditioning air to a more comfortable level. It does this using the aircraft's drinking water supply — that water is atomized, meaning it's broken into a fine mist, by air taken from the air conditioning supply. So you have a venturi-type device where the airstream itself draws in and atomizes the water. That's the humidifier. Next, the ram air valves. These are the inlet and outlet doors, and they're opened and closed by the pack controller. Their job is to regulate the amount of air entering the ram air duct. This happens automatically as part of the temperature control system, and also during landing and take-off — and the reason for that is to prevent ingestion of foreign matter. So on the ground, you don't want debris, stones, or FOD being sucked into the duct, so the doors close to protect the system. Now the mass flow controller. This component is fitted to ensure that a constant mass flow is supplied to the system regardless of engine rpm. Think about it — engine rpm varies all the time, and bleed air pressure varies with it. The mass flow controller holds the flow steady. There are two versions. When used with blower systems, the mass flow controller spills excess air to atmosphere — it just dumps the surplus. But when fitted to bleed air systems, it uses a variable orifice valve. That valve is calibrated so that the total aerodynamic effect on its internal mechanism automatically adjusts the orifice. In other words, the pressure changes upstream and downstream of the unit physically move the mechanism, which resizes the opening, so that the required mass flow passes to the system irrespective of changes in pressure on either side. So it's a self-adjusting mechanical device, not an electronic one. Finally, temperature control. The temperature of the air entering the cabin is achieved by mixing hot air with cooled air. There are two basic methods: mechanical and electromechanical. The simple, non-automatic manual method uses valves that the crew positions by hand to regulate temperature by mixing hot and cold air before it enters the cabin. Automatic control — for the cabin, flight deck, cargo holds, and so on — works by comparing a pilot-selected temperature with the temperature of the mixed air inlet to the cabin. Sensors in the cabin and in the supply ducts are compared electronically with the selected value. Any difference modulates the hot air bypass valve, allowing more or less air to pass through the cooling components, so you get the correct temperature at the point of mixing. In manual control, the valves move in response to hot/cold or increase/decrease selections from the crew. So the key contrast here: automatic control uses sensors and electronic comparison to modulate the bypass valve; manual control is just the crew moving valves directly. That's the whole temperature control picture.