Aircraft Life Support Systems Part 1: Oxygen System

INTRODUCTION

In the aircraft industry, several systems are designed to provide safety and comfort for crew and passengers while traveling. Oxygen gets rarified with altitude, so life support is a very important system

The cabin is pressurized in order to provide breathable air, but reaching a sea level pressure is not advisable since it would lead to a significant pressure differential between the aircraft exterior and the cabin interior. This difference could damage the aircraft structure.

Additionally, the cabin altitude is different from the flight altitude. In fact, the cabin altitude corresponds to the one reached according to the cabin pressure. Usually a commercial flight cruises at an altitude of 35,000 ft, but thanks to the pressurization system, the cabin altitude is around 6,000-8,000 ft.  Indeed, the oxygen system provides breathable oxygen to the crew and passengers if any problem were to occur during the flight.

AIRCRAFT EMERGENCY OXYGEN SYSTEM:

In a normal situation, a bleed air system is used to provide fresh air throughout the flight duration. The air is hot and must be cooled and pressurized to make it breathable.  In the event of an emergency, the plane is already equipped with oxygen systems which are linked to passengers and cabin crew through masks. In fact, there are two oxygen systems on board. One designed for the crew, and the second for the passengers.

If the cabin pressure drops making cabin altitude about 14,000 ft, the emergency system are be triggered. The emergency system provides oxygen to passengers for 15 to 20 minutes, and for the crew members for around 30 minutes. This is enough time for the aircraft to descend to a lower altitude and being the cabin altitude to a safe breathable level.

Here, the crew oxygen system schematic of the Boeing 737 class is shown in Figure 1.

Figure 1-Crew oxygen system
Figure 1-Crew oxygen system

The main challenges of oxygen equipment are:

  • Fitting the dimensions of the plane
  • Secure (no leakage for example)
  • Responsive (to cabin pressure and cabin altitude)
  • Easy for passengers to use the oxygen system through the deployed masks quickly, before the effects of altitude are felt:
  • At 25,000 ft: a person has 3 minutes of consciousness
  • At 41,000 ft: a person has 30 seconds of consciousness

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FLIGHT CREW OXYGEN

The flight crew oxygen should be designed and made with a lot of care, because if any trouble occurs during the flight, the crew must be able to handle the situation and take the airplane and its passengers down safely.

The oxygen system includes:

  • Oxygen tank
  • Cylinder assembly:
    • Pressure regulator
    • Pressure transducer which sends an electrical signal received by a high-pressure indicator in the Aft overhead panel
    • System shutoff valve
    • Relief valve (set to relieve at approximately 100psi)
    • Filler valve
  • Distribution manifold
  • Individual breathing devices (that include mask/regulators, mask storage boxes, oxygen control panels, mask-mounted regulators)

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For additional safety, the system includes a pressure relief device to prevent excessive pressure.

This blog will discuss two different systems. In the first one, oxygen is stored in a liquid state (LOX). The main advantages are that LOX is less bulky and less costly than storing the equivalent capacity of high-pressure gaseous oxygen. In the second, oxygen is stored in a gaseous state. Here, oxygen doesn’t need to be vaporized and can therefore be used more readily.

GASEOUS OXYGEN

In this state, oxygen is kept in either a 76 ft3 or 114 ft3 (2.15 m3 or 3.22 m3) tank installed on the forward right sidewall of the forward cargo compartment at 1850 psi and 70°F. Once the system is triggered, oxygen goes through the cylinder assembly. When oxygen gets into the pressure regulator, its pressure decreases to around 60-85psi. The fluid is then relieved by the relief valve at 100 psi in order to provide all outlet ports with oxygen, numbered from 1 to 4 in Figure 2.

How many ports does an aircraft need and how quickly in the event of an emergency will the oxygen be available?

Engineers can try different system configurations and conditions (state of oxygen source, pipes materials, etc.) to answer these questions but should always keep safety, economical and physical aspects in mind.

Software like AxSTREAM NET™ allows us to estimate the time required for the oxygen to be provided from the moment the cabin pressure drops below the trigger value to the oxygen flowing into the masks. By performing this type of analysis on different setups engineers can figure out which system is the most efficient from multiple points of view depending on the desired priorities.

Thanks to the different elements available in AxSTREAM NET™ , we can reliably model the flight crew oxygen network through a series of fluid elements (and heat transfer components as well, if relevant) as we can see in the following figure. For simulation sake, we set an ambient pressure around 14.5 psi for the systems presented below.

Figure 2 Flight Crew Oxygen Network
Figure 2 Flight Crew Oxygen Network

Here, after performing the steady-state simulation one can review the pressure contour of the flight crew oxygen system. Analysis and optimization of such time dependent system could also be done through time (unsteady state simulation) with AxSTREAM NET™ in order to have a better approach and a better comprehension of the designed system.

Figure 3-Pressure contour
Figure 3-Pressure contour

LIQUID OXYGEN

Liquid oxygen (LOX) is kept in a tank at the following conditions: -297°F and 450psi (liquid phase). In order to have LOX vaporization, fluid passes through warming coils where it turns to gas. Before the coils network, the fluid should be released at a low pressure (around 20psi) which is achieved through a pressure regulator.

A liquid oxygen model created in AxSTREAM NET™ is presented below.

Figure 4 – Liquid oxygen system and warming coils
Figure 4 – Liquid oxygen system and warming coils

The software allows us to perform all kinds of calculation for these systems. Numerous post-processing capabilities exist including the contour tool presented in the figure below which helps users see the temperature (or any other selected parameter) for each component through the whole system.

Figure 5-Temperature contour of the liquid oxygen system
Figure 5-Temperature contour of the liquid oxygen system

Engineers can then easily try different configurations and optimize the warmings coils modeling for example.

To conclude, AxSTREAM NET™  allows us to model and simulate different kinds of systems, including application utilizing heat transfer like the liquid oxygen example and its boiler coils. Analysis of the fluid flow behavior through time is also possible in an unsteady (transient) simulation to, for example, determine the time it takes for the oxygen to be provided to the crew upon triggering of the depressurization alert. Engineers can test several configurations in order to get the most efficient design.

Sources:

  1. B737 GENERAL FAMILIARIZATION COURSE
  2. https://cgs.aero/aircraft-oxygen-systems-and-components/
  3. http://www.b737.org.uk/aircraftsystems.htm
  4. https://www.aeronewstv.com/en/lifestyle/how-it-works/2808-altitude-versus-cabin-altitude.html
  5. https://www.youtube.com/watch?v=jba8AcEah1Q
  6. http://www.airproducts.com/~/media/Files/PDF/company/safetygram-6.pdf