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Chapter 5 | Major Aircraft Systems
seat row may be equipped with a separate self-contained generating unit. There are several important disadvantages to chemical generator systems, the first being that once generators are activated there is no way to turn off the flow of oxygen. Units continue to supply oxygen until all of the chemicals are expended. Another disadvantage is that the chemical reaction required to produce oxygen generates heat, posing the risk of passengers burning themselves if the generating unit is touched.
Environmental Systems Environmental systems on large turbine-powered aircraft are complex, compared with the light aircraft systems that you may have experienced in the past. An airline aircraft is so large that balancing temperatures throughout the vehicle can be challenging. Besides, these aircraft operate through such a huge range of temperatures that their systems must be quite versatile. A given airplane might sit for two hours on a 105°F ramp, then experience temperatures of –40°F in flight. In piston aircraft, cabin heat is generated through combustion heaters or engine exhaust heat. Turbine environmental systems, however, get their heat from
engine bleed air. Modified bleed air is introduced into the aircraft for pressurization and heating, and in many cases, for cooling. Proper operation of environmental systems is rather critical in pressurized turbine aircraft. Since pressurization air is high-pressure bleed air (read “hot”), all environmental air must be cooled, to some degree, before it’s put into the cabin. You may be used to thinking of air conditioning as a luxury in aircraft, but in many turbine aircraft out-of-service environmental cooling equipment means “no-go.” A combination of heat exchangers, air cycle machines, and/or vapor cycle machines are used to modify and control cabin temperatures in turbine airplanes.
Heat Exchangers Heat exchangers are simple, passive devices that transfer heat between two different fluids (Figure 5.13). Your car radiator is an excellent example of a heat exchanger. Air passing through the radiator from the grill absorbs heat from the engine coolant pumped through the radiator core. In aircraft, heat exchangers are used to absorb and remove heat in a variety of applications within the environmental system and elsewhere.
cool cool
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hot
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heat is transferred from hotter fluid (gas) to cooler one
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warm Heat exchangers: When two fluids (including gases) of different temperatures come in contact, heat is transferred from the hotter fluid to the cooler one. Heat exchangers use this principle for temperature control in many turbine aircraft applications, including bleed and environmental systems, oil coolers, and fuel heaters.
FIGURE 5.13 | Heat exchangers.
