compressed with the air. When air is compressed,
Before describing the various types of air
it becomes very hot. As we mentioned earlier, hot
compressors found on gas turbine ships, we will
discuss the composition of air and some of the
air is capable of holding great amounts of water.
The water is removed as the compressed air is
things air may contain. This discussion should
routed through coolers. The coolers remove the
help you understand why air compressors have
special features that prevent water, dirt, and oil
heat from the airstream and cause some of the
water vapor to condense into liquid (condensate).
vapor from getting into compressed air piping
The condensate must be periodically drained from
systems.
the compressor.
Air is composed mainly of nitrogen and
Although the coolers will remove some of the
oxygen. At atmospheric pressure (within the range
water from the air, simple cooling between the
of temperatures for the earth's atmosphere), air
stages of compression (intercooling) and cooling
is in a gaseous form. The earth's atmosphere also
contains varying amounts of water. Depending
of the airstream after it leaves the compressor
(aftercooling) will not make the air dry. When
on weather conditions, water will appear in a
variety of forms, such as rain (liquid water), snow
clean dry air suitable for pneumatic control and
crystals, ice (solid water), and vapor. Vapor is
other shipboard systems is required, air from the
compressor is routed through air-drying units.
composed of tiny drops of water that are light
Many air-drying units are capable of removing
enough to stay airborne. Clouds are an example
enough water vapor from the airstream to cause
of water vapor.
the dew point to be as low as 60F. Some of
Since air is a gas, it will expand when it is
heated. Consequently, the heating of air will cause
the more common devices used to remove water
a given amount of air to expand, take up more
vapor from the airstream, such as dehydrators,
will be explained later in this chapter.
space (volume), and hold more water vapor.
When a given amount of air at a given temper-
CLASSIFICATION OF
ature and pressure is no longer able to soak up
water vapor, the air is saturated, and the
AIR COMPRESSORS
humidity is 100 percent.
An air compressor may be classified according
When air cools, its density increases; however,
to pressure (low, medium, or high), type of
its volume and ability to hold water decrease.
compressing element, and whether the discharged
When temperature and pressure conditions cause
air is oil-free.
air to cool and to reach the dew point, any water
Because of our increasing need for oil-free air
vapor in the air will condense into a liquid state
aboard ship, the oil-free air compressor is
(water). In other words, one method of drying air
gradually replacing most of the standard low-
out is to cool it until it reaches the dew point.
pressure and high-pressure air compressors. For
Besides nitrogen, oxygen, and water vapor, air
this reason, we will focus most of our discussion
contains particles of dust and dirt that are so tiny
on the features of oil-free air compressors.
and lightweight that they remain suspended in the
According to the Naval Ships' Technical
air. You may wonder how the composition of air
Manual, chapter 55 1, compressors are classified
directly affects the work of an air compressor.
as low, medium, or high pressure. Low-pressure
Although one cubic foot of air will not hold a
air compressors (LPACs) have a discharge
tremendous amount of water or dirt, you should
pressure of 150 psi or less. Medium-pressure
realize that air compressors have capacities that
compressors have a discharge pressure of 151 psi
are rated in hundreds of standard cubic feet per
to 1,000 psi. Compressors that have a discharge
minute (cfm). This is a very high rate of flow.
pressure above 1,000 psi are classified as high-
When a high flow rate of dirty, moisture-laden
pressure air compressors (HPACs).
air is allowed to enter and pass through an air
compressor, the result is rapid wear of the seals
Low-Pressure or Ship's
and load-bearing parts, internal corrosion (rust),
Service Air Compressors
and early failure of the unit. The reliability and
useful life of any air compressor is extended by
Only two types of LPACs are used on gas
the installation of filters. Filters will remove most
turbine ships, the screw type and the reciprocating
of the dirt and dust from the air before it enters
type.
the equipment. On the other hand, most of the
water vapor in the air at the intake will pass
SCREW TYPE.--The helical-screw type of
compressor is a relatively new design of oil-free air
directly through the filter material and will be
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