Oxygen systems will be one of 3 types: demand, diluter-demand or continuous. System can be a combination of combination of these 3 types and this is often the case. The type of system is determined by the oxygen mask(s) used in the system. For commercial airplanes, crew masks will be able to operate in either the diluter-demand or full demand mode. Passenger masks will generally be continuous flow. Crew masks used in military and space will be demand type of mask. Military masks may also be capable of operating in a demand-diluter mode.
This module discusses continuous flow masks to describe how a continuous flow type of system operates.
Continuous Flow Mask
A continuous flow mask always flows oxygen. Continuous flow masks are much simpler than crew masks in design and operation. In commercial aircraft, continuous flow masks are used in a passenger cabin. They are sufficient for passengers since in a decompression scenario, passengers do not have perform strenuous duties (unlike the crew). Continuous masks are light weight and inexpensive. A continuous flow mask is shown in Figure 1.
Figure 1 Continuous Flow Oxygen Mask
Continuous flow masks will contain a mask, which connects to a plastic bag and then a tube that connects to the oxygen supply. Often the tube connecting to the oxygen supply will contain an inline flow indicator. The bag is sized for a certain volume based on TSO C64A and subsequently SAE AS 8025 specifications. The last item installed in the mask is an orifice. This orifice will usually be in the tube that connects the bag with the oxygen supply. The orifice is sized to provide the required minimum mass flow of oxygen at the maximum rated altitude for the mask (approximately 25,000 feet).
In the continuous flow mask, oxygen will flow through the orifice, into the tube and inline flow indicator, then to the bag and finally into the mask (see Figure 1). The orifice (not shown in Figure 1) may be installed in the flow tube or in the box where the oxygen mask is stowed. The orifice is used to control the flow rate through the mask. The upstream pressure to the orifice is typically in the 60-70 psig range. At this pressure, the flow is choked (sonic) and the orifice flow is independent of downstream pressure. Hence flow rate is a function of orifice upstream pressure and orifice flow area. Note that the orifice downstream pressure is the pressure in the bag, which is slightly above ambient pressure.
Between each inhalation by the mask user, oxygen flows into the bag. The bag never fully inflates between inhalations, however, oxygen is still gathering in the bag. The mask (sometimes called a cup) also has an inlet valve (to pull in atmospheric air) and an outlet valve. When the user takes a breath, air is pulled in through the inlet valve as well as from the oxygen bag. When the user exhales, air is pushed out through the exhalation check valve (see Figure 2). For a mask properly designed to meet the TSO C64A and SAE AS 8025 requirements, the amount of oxygen in the ambient air plus the additional oxygen pulled in from the bag will meet the minimum required amount of oxygen as specified in 14CFR 25.1443(d).
Figure 2 Continuous Flow Oxygen Mask Valves
Since the mask is always flowing, at some conditions more oxygen will be provided than is actually required. Continuous flow masks are designed to meet the worse case flow requirements, which will be at the highest altitude recommended by the mask manufacturer. For a typical commercial airline passenger mask, this rating will normally be 25,000 feet. At lower altitudes, the mask oxygen flow will be about the same as at 25,000 feet but less oxygen would be required to meet the minimum oxygen flow requirements (since oxygen in air naturally increases as altitude decreases). Hence continuous flow masks are not efficient and “waste” a portion of the oxygen supply. To partially reduce the wasted flow, some systems (which use a pressurized bottle) will use an altitude compensating pressure regulator. An altitude compensating pressure regulator varies the inlet pressure to the mask (technically it controls the inlet pressure to the orifice installed in the mask assembly). At higher altitudes the inlet pressure will be higher and at lower altitudes the inlet pressure will be lower. Since mask (orifice) flow is approximately proportional to inlet pressure, flow will be reduced at lower altitudes when less oxygen is required. This improves oxygen flow efficiency allowing a given oxygen supply to last longer (which saves bottle weight and volume).
Some oxygen systems may be continuous flow, but have an adjustable regulator valve that can be used to regulate oxygen flow. Since flow is proportional to inlet pressure, controlling inlet pressure will control oxygen flow. In this case, there will normally be a placard that provides a recommended pressure setting for a given altitude.
The altitude limit for continuous flow masks will be around 25,000 feet. Operation above 25,000 feet should be limited to very brief time intervals (2-3 minutes). Above 25,000 feet, continuous flow masks are unable to provide sufficient oxygen.
In passenger oxygen systems that are supplied by a pressurized bottle, mask(s) are installed (stowed) in a box that is contained in the cabin headliner. More than one mask may be installed in the box (3 is usually the maximum). A shut off valve isolates the passenger masks from bottle pressure. When the shut off valve is opened, there is an initial pressure surge in the oxygen lines (50-100 psi) which pushes the masks out of the box and causes the masks to “fall” out of the stowage box. Attached to the mask is a lanyard with a pin that is installed in a small shut off valve in the box. When the pin is pulled oxygen will flow to the mask. In commercial aircraft, the lanyard must be pulled to don the mask. The reason for the lanyard is to prevent oxygen flowing to unused masks, thereby preserving oxygen to only the masks that are donned by passengers.