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evaporator and returned to the cabin as colder air. The liquid refrigerant inside the evaporator’s coils absorbs heat and turns to vapor again before returning to the compressor to continue the cycle.
Of course, that’s a very simplified description of how the system works. I don’t want to go into too much detail because unless you have the proper certification(s), there isn’t a lot you can do yourself to maintain the system.
Air cycle systems have the advantage of not having to contain a pressurized gas to condition air, but the tradeoff is that these systems utilize more moving parts to make everything work. These systems use the same thermo- dynamic principles as vapor cycle systems, but instead of compressing and expanding a refrigerant to remove heat from the air, they compress and expand the air itself. Gay-Lussac’s Law states, “The pressure of a given mass of gas varies directly with the absolute temperature of the gas when the volume is kept constant.” The pressure and temperature of a gas mass are directly proportional, so when we compress the air inside an air cycle machine, it heats proportionally to the amount we compress it, and vice versa when we expand it.
Air cycle systems typically use a combination of outside air and bleed air tapped from the engines and/or the APU. If you drilled a hole in your leaf blower and stuck a tube through the hole so it blew air over your face while you used it, you’d be using the same concept. When bleed air is pulled from an engine or the APU, it’s at high pressure and high temperature, so the hot bleed air is plumbed through a “precooler,” which looks and works like a car’s radiator. The hot air passes through the inside of the precooler, and cold ram air is forced over its cooling fins, pulling a lot of heat from the bleed air. Bleed air leaving the engines can be over four hundred degrees Celsius and may lose a hundred degrees as it goes through the precooler. So how do we turn three-hundred-degree air
Even a thin layer of dust on a cabin temperature sensor can cause the environmental system to operate erratically.
into super-cooled air to dump into the cabin? We run it through an air cycle machine, which is nothing more than a big heat exchanger.
Each manufacturer calls this piece of magical equip- ment something different. Air Cycle Machine (ACM) and Environmental Control Unit (ECU) seem to be the leading nomenclature in my experience. For simplicity’s sake, I’ll use “ACM” from here on.
The slightly cooled bleed air comes out of the pre- cooler, and some of it is routed directly into the cabin via an emergency pressurization valve, while most of it is piped into the ACM. Allowing the capability to pump unconditioned hot air into the cabin might sound coun- terintuitive, especially if you’ve ever operated out of South Florida in the summer. However, if the ACM fails and won’t allow any conditioned air into the cabin, you still have a positive air source for pressurization. It’ll get hotter than South Florida in the summer inside the cabin, but everyone aboard will be conscious while the crew gets the airplane on the ground as soon as possible.
Some pilots use empty legs to check the function of the emergency pressurization. It can be alarming for the co- pilot, especially when he or she is staring out the window and not expecting a loud rush of air. I’m not condoning it – I’m just saying it happens, in theory.
Now that hot air is coming into the ACM, we must decide what to do with it. To control the rate of air released from the ACM, we use temperature control valves which are modulating valves, meaning they can be in any position
A ram air scoop feeds the ACM on this Falcon 900LX
May 2023 / TWIN & TURBINE • 19