CHAPTER 3
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
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