My prior duo of posts about airplane turbine and reciprocating engines was popular, so I’ll expand the series to talk about the mysterious inner workings of other airplane parts. This week, let’s talk cabin cooling.
Smaller reciprocating engine aircraft usually have very simple cooling for the passengers – ram air; that is the air that is forced into the cabin by the forward motion of the airplane. Kind of like the “air conditioning” on your first car.
Some fancier reciprocating-engine aircraft have coolant/compressor air conditioning systems, known as vapor-cycle air conditioning. These run on a principle similar to what operates your home or car AC system.
To whit: imagine a long, long continuous tube. The tube contains the refrigerant and sends it through several stages. First, vaporized refrigerant passes through a compressor, where it’s squeezed, which heats it up. Still in vaporous form, it then it hits a condenser. The condenser is a series of coils that have cool(er than the vapor) air flowing past them. When the hot vapor goes into the coils it condenses into liquid and loses its heat.
The heated air goes overboard as a waste product. The liquefied coolant goes into an expansion valve. Attentive readers may remember from the turbine post that fluids lose pressure when they’re sent through a divergent opening. They also lose heat.
In this case the drop in pressure is very sudden and causes an extreme drop in temperature. The liquid then goes through the evaporator coil where the warm ambient air blows over it and gets cooled down while at the same time turning the liquid inside the coil into vapor, which goes to the compressor to begin another cycle.
This cool air is sent out to the cabin where passengers appreciate the ability to fly without schvitzing all over their spiffy outfits.
Turbine engines have a completely different and surprisingly simple system. In a turbine engine, hot air is bled off from the turbine compressor and sent through something called an air-cycle machine.
Here’s how the air-cycle machine works: bleed air goes into a primary heat exchanger. A heat exchanger is just what it sounds like – it exchanges one temperature of air for another – in this case hot bleed air for cooler ambient air. This doesn’t cool the bleed air to ambient temperature, but it cools it quite a bit, and vents the heat overboard.
From here the air goes through a compressor and gets squeezed, which also heats it up. From the compressor it goes to yet another heat exchanger (still compressed). By this time, the air is quite a bit hotter and denser than it was when it started, because even though it was cooled by the initial heat exchanger, after that it’s been through the compressor.
So then it goes through a turbine (a spinny thing that’s driven by moving air or liquid), which both expands it (and, as we already know, expansion causes it to cool) and absorbs heat energy to perform the work of spinning the turbine. Voila – cool, cool air for your cabin comfort.
Both types of system have water as a by-product, and there are various ways of dealing with the water, but that’s a topic for another post, or maybe not because it’s not as cool (heh) as air-cycle machines.