PRESSURIZATION AND AIR-CONDITIONING SYSTEMS

Terminal Objective: Upon completion of this chapter, you will be able to recognize the operational and component differences between air cycle and refrigerant cycle air-conditioning systems (ACS).

Transferring a human being from his natural environment on the earth's surface to the environment existing at 40,000 feet places him in surroundings in which he cannot survive without artificial aids. Even at half that altitude, breathing becomes very rapid; and above 25,000 feet un-consciousness occurs, quickly followed by death. A brief study of the earth's atmosphere tells us why this condition exists.

STRUCTURE OF THE ATMOSPHERE

Learning Objective: Recognize the affect high altitude flight could have on flight personnel because of decreased atmos-pheric pressure.

The envelope of atmosphere surrounding the earth is a gaseous mixture consisting chiefly of nitrogen and oxygen. There are traces of other gases, but they have no significance as far as body functions are concerned. Chemical analysis has shown that the proportions of nitrogen and oxygen are constant throughout the thickness of the atmosphere, up through 200,000 feet or more.

ATMOSPHERIC PRESSURE

Although the chemical content of the atmosphere remains fairly constant, the density (mass per unit volume) of the atmosphere varies with altitude. At 18,000 feet the density is about one-half of the density at sea level, and at 36,000 feet it is only about one-fourth of the density at sea level. The atmospheric pressure also varies with the altitude. The pressure exerted by the atmosphere may be compared to the pressure of a column of water. If holes are made in the container of the column, the force with which the water spurts out of the upper holes will be considerably less than that at the bottom of the column. Similarly, the pressure exerted by the atmosphere is much greater near the surface of the earth than it is at high altitudes. For example, the pressure of the atmosphere at sea level is 14.7 psi, while the pressure at 40,000 feet above sea level is 2.72 psi, and at 60,000 feet is 1 psi. As an aircraft ascends to higher altitude, the resulting decrease in atmospheric pressure may affect flight personnel in several ways. The most noticeable effect is in breathing. Breathing is a mechanical process that depends heavily on atmospheric pressure. When a person inhales, he automatically raises his ribs and depresses his diaphragm so that the chest cavity is enlarged. This reduces the air pressure within the cavity below that of the atmosphere outside. Air is thus pushed into the lungs. When he exhales, he reduces the chest cavity, increasing the pressure within it. This pushes the air out of the lungs. When low atmospheric pressures are en-countered, the lungs are not filled so completely when inhaling. With lower density, a person gets fewer molecules of air in each breath. If he gets fewer molecules of air in each breath, he also gets fewer molecules of oxygen, and no person can live unless he gets a sufficient amount of oxygen. This problem may be solved up to certain altitudes by the proper use of oxygen equipment; however, at extremely high altitudes (above 35,000 feet), the atmospheric pressure is so low that the pressure of the blood and other liquids in the body are no longer balanced. The human body then tends to burst. In some cases, blood vessels near the surface may burst, causing hemorrhages in the ears, eyes, and breathing passages. The outside air temperature also changes with altitude. For example, at approximately 18,000 feet the outside air temperature will be - 4 F (- 20 C), and at approximately 37,000 feet the outside air temperature will be - 67 F ( - 55 C). Above 37,000 feet the air continues to thin, but the air temperature will remain constant for several miles and then begin to rise slowly. Thus, the lowest outside air temperature to be encountered by an aircraft would occur at a height of about 7 miles. NOTE: The conversion formula for con-verting Fahrenheit to Celsius (centigrade) is (F-32).

For example, - 4 F is converted as

Conversion of a Celsius temperature to a Fahrenheit reading is accomplished using the following formula:

For example, - 55 C is converted as

Remember not to drop the + and - signs when converting. These variations in outside air temperature and atmospheric pressure are considered by the air-craft manufacturer when designing the aircraft.