75 psig, T1 is 800, P1 is 237.8 psig, and T2 is
300°F:
Solution:
Substituting:
PNEUMATIC GASES
In chapter 1, you learned that many factors
are considered in determining whether to use
hydraulics or pneumatics as a power source in a
fluid power system. Once it is determined that
pneumatics will be used as the source of power,
some of the same factors are considered in
selecting the pneumatic gas.
QUALITIES
The ideal fluid medium for a pneumatic
system is a readily available gas that is
nonpoisonous (nontoxic), chemically stable, free
from any acids that cause corrosion of system
components, and nonflammable. It also will not
support combustion of other elements.
Gases that have these desired qualities may not
have the required lubricating power. Therefore,
lubrication of the components of some pneumatic
systems must be arranged by other means. For
example, some air compressors are provided with
a lubricating system, some components are
lubricated upon installation or, in some cases,
lubrication is introduced into the air supply line.
Two gases meeting these qualities and most
commonly used in pneumatic systems are com-
pressed air and nitrogen.
COMPRESSED AIR
Compressed air is a mixture of all gases
contained in the atmosphere. In this manual,
compressed air is referred to as a gas when it is
used as a fluid medium.
The unlimited supply of air and the ease of
compression make compressed air the most widely
used fluid for pneumatic systems. Although
moisture and solid particles must be removed
from the air, it does not require the extensive
distillation or separation process required in the
production of other gases.
Compressed air has most of the desired
properties and characteristics of a gas for
pneumatic systems. It is nonpoisonous and
nonflammable but does contain oxygen, which
supports combustion. One of the most undesirable
qualities of compressed air as a fluid medium for
pneumatic systems is moisture content. The
atmosphere contains varying amounts of moisture
in vapor form. Changes in the temperature of
compressed air will cause condensation of
moisture in the pneumatic system. This condensed
moisture can be very harmful to the system, as
it increases corrosion, dilutes lubricants, and may
freeze in lines and components during cold
weather. Moisture separators and air driers
(dehydrators) are installed in the compressed air
lines to minimize or eliminate moisture in
systems where moisture would deteriorate system
performance.
The supply of compressed air at the required
volume and pressure is provided by an air
compressor. (For information on air compressors,
refer to Naval Ships’ Technical Manual, chapter
551.) In most systems the compressor is part of
the system with distribution lines leading from the
compressor to the devices to be operated. In these
systems a receiver is installed in-line between the
compressor and the device to be operated to help
eliminate pulsations in the compressor discharge
line, to act as a storage tank during intervals when
the demand for air exceeds the compressor’s
capacity, and to enable the compressor to shut
down during periods of light load. Other systems
receive their supply from cylinders which must be
filled at a centrally located air compressor and
then connected to the system.
Compressed air systems are categorized by
their operating pressures as follows: high-pressure
(HP) air, medium-pressure (MP) air, and low-
pressure (LP) air.
High-Pressure Air Systems
HP air systems provide compressed air at a
nominal operating pressure of 3000 psi or 5000
psi and are installed whenever pressure in excess
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