1. Field
The present disclosure relates generally to aircraft and, in particular, to inert gas generation systems for aircraft. Still more particularly, the present disclosure relates to a method and apparatus for warming an air separation module in an inert gas generation system for an aircraft.
2. Background
Many aircraft have inert gas generation systems. An inert gas generation system is used to render a fuel tank in an aircraft substantially inert. In these illustrative examples, fuel tanks may have a space above the fuel in which fuel vapors may be present. This space in the fuel tank is referred to as an ullage.
Inert gas generation systems are employed to reduce a possibility of combustion within the ullage in a fuel tank. Inert gas generation systems may be used to reduce the oxygen content in these spaces below a threshold needed for combustion. Without sufficient oxygen, fuel vapors in these locations are unable to ignite.
Inert gas generation systems may introduce an inert gas into these locations. This inert gas may be a gas, such as nitrogen, nitrogen enriched air, carbon dioxide, and other types of inert gases. With the use of an inert gas, the oxygen content may be reduced below a threshold for combustion.
Inert gas generation systems may employ air separation modules to generate inert gas. For example, an air separation module may be configured to generate an inert gas in the form of nitrogen enriched air from the air that is sent into the air separation module. The nitrogen enriched air is air that has higher nitrogen and lower oxygen content than the air that is sent into the air separation module.
Some currently available air separation modules include permeable membranes that separate the oxygen and nitrogen. These permeable membranes produce inert gas more efficiently above a selected temperature or within a selected temperature range. These temperatures are typically elevated temperatures as compared to the temperature when the aircraft is not in use. Material limitations are also present that preclude long-term operation above a certain temperature.
Aircraft inert gas generation systems are typically designed to operate near this temperature limit to reduce the size of the air separation module. For example, a permeable membrane in an air separation module may have a desired operating temperature of about 170 degrees Fahrenheit and a long-term temperature limit of about 190 degrees Fahrenheit. The air input into an air separation module may have these temperatures or even slightly higher temperatures. When an air separation module has not been used for some period of time, the temperature within the air separation module may be lower than the desired temperature.
As a result, a desired temperature for generating nitrogen enriched air at a desired level may not occur until the temperature of the air separation module reaches a desired temperature for providing a desired level of nitrogen in nitrogen enriched air. This warm-up time may take more than about 15 minutes before the air separation module is ready to operate to generate nitrogen enriched air with a desired oxygen level.
This warm-up time may increase the time needed to prepare an aircraft for operation. As a result, the time needed to prepare an aircraft for a flight may be longer than desired with these warm-up times.
Inert gas generation systems may reduce the warm-up time by using a high flow mode. In a high flow mode, the restriction downstream of the air separation module is reduced so that the flow rate of nitrogen enriched air increases. Additionally, the temperature in the air sent into the air separation module also may be increased to reduce the warm-up time. Even with these procedures, the time needed to warm up an air separation module may still be longer than desired.
Therefore, it would be desirable to have a method and apparatus that takes into account at least some of the issues discussed above as well as possibly other issues.