The refrigeration cycle has been around since the mid 1800’s and has since become an integral part of our everyday lives. For anyone who doesn’t already know how basic refrigeration works, it is fundamentally important to understand the basics of this cycle before understanding aircraft climate control at 36,000ft, here is a step by step description:
- The cycle begins with the compression of gas (now commonly ammonia) which in turn becomes hot when pressurized.
- This high-pressure gas is run through a condenser unit (series of coils) which can either be water or air cooled; this allows the heat from the gas to dissipate. This loss of heat causes the ammonia gas to condense into a high-pressure liquid.
- The liquid then passes through an expansion valve (high pressure on one end, low pressure on the other).
- The liquid ammonia instantly boils and vaporizes causing its temperature to drop rapidly to -27F, creating the “cold“.
- This cold liquid & vapor combination is passed through another coil which air is blown over to create cool air in buildings/cars etc.
- The compressor then sucks the ammonia back up and repeats the cycle.
This cycle becomes more complex when you have to work in a completely sealed airplane traveling at speeds around 600km/h.
Here are some quick facts about the HVAC/Environmental Control Systems onboard planes which frequent flyers like myself might find interesting:
- In some cases (dependent on the Air Management System configuration) 50% of the supply air onboard is fresh and 50% is HEPA filtered recirculated air.
- On the average flight, each passenger in the economy section gets 7 cubic feet of fresh air per minute, in first class it is about 50 cubic feet.
- The outside air temperature at cruising altitudes is below -35F/-37C.
- The oxygen levels in the air are too low to sustain life at the altitudes planes fly at, so the Environment Control Systems onboard must compress it to a density (accomplished within the engines) which is safe for humans.
- By la, the cabin air pressure cannot be less than the equivalent outside air pressure at 8,000ft.
- The refrigeration unit on a plane is called an “Air Cycle Machine” which uses two air-to-air heat exchangers.
- The Air cycle machine uses what is known as the Reverse Brayton cycle.
- PACK or A/C PACK stands for “Pneumatic Air Cycle Kit”.
Getting Down to the Basics
Now for the basics of how the Environmental Control System of an aircraft generally works (this is not consistent for all planes):
Cold outside air enters through the airplane engines.
Compressors located within the engine compress the low-density air which makes it safe to breathe.
Certain aircraft routes encounter ozone (which is poisonous), so an extra step is added where ozone converters clean the air before traveling to the A/C PACKS.
The now hot compressed air (coined ‘Bleed Air’) travels to the A/C PACKS via a flow control valve to be cooled.
Basic operation of the Air Cycle Machine (ACM): Bleed air enters the first heat exchanger and its temperature is reduced using the outside air (hovers around freezing at cruising altitude). The air then enters the compressor section of the ACM where its temperature & pressure is raised again. The air now enters the second heat exchanger which reduces its temperature again (to cool the bleed air). Finally the air enters the turbine section of the ACM expanding and releasing its energy to rotate the turbine and compressor wheels to continue the process.
As cold air exits the packs it is mixed with warm air via a bypass valve to achieve a desired temperature.
The fresh air is then sent to the mixing manifold where it is mixed with recirculated air from the cabin.
And finally this air is supplied to the cabin through overhead outlets.
Quite simply stated, without this system humans would not be able to survive a regular commercial flight. Hopefully now you have more of an appreciation for all the engineering which makes flying a possibility. If any of the above was unclear, there is an abundance of knowledge available online to clarify. As always feel free to comment with regards to any modernizations to this process or personal knowledge you might have with regards to the topic!
Boeing: Frequently Asked Questions about Cabin Air Systems
Wikipedia: Environmental control system (aircraft)
Decoded Science: Air Conditioning and Pressurization Systems in Modern Aircraft
Disclaimer: Any views or opinions presented in this blog post are solely those of the author and do not necessarily represent those of Aversan Inc.