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:
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:
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!
Aversan provided software engineering services for the development of safety-critical control system software (such as power and air management systems) on the Airbus A350 XWB for Honeywell.
Spencer Johnstone holds a B.Eng in Software Engineering from the University of Ontario Institute of Technology. He has been with Aversan Inc. for almost a year and a half and has worked in the E-Health department on Requirements Verification Testing, Cross Application & Integration Testing, and more recently has transitioned to a Defect Management role. When not in the office, Spencer can likely be found trying to squeeze in a quick 18 holes before the sun sets or snapping pictures with his DSLR.
Boeing: Frequently Asked Questions about Cabin Air Systems
http://www.boeing.com/boeing/commercial/cabinair/facts.page
Wikipedia: Environmental control system (aircraft)
http://en.wikipedia.org/wiki/Environmental_control_system_(aircraft)
Decoded Science: Air Conditioning and Pressurization Systems in Modern Aircraft
http://www.decodedscience.com/air-conditioning-and-pressurization-systems-in-modern-aircraft/10938
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.