manufacturers' technical manuals and in the
DEHYDRATORS
Naval Ships' Technical Manual, chapter 551.
Some of these hazards and precautions are as
The removal of moisture from compressed air
follows:
is an important feature of compressed air systems.
Some moisture is removed by the intercoolers and
1. Explosions can be caused by dust-laden air
aftercoolers, as explained earlier in this chapter.
or by oil vapor in the compressor or receiver if
Air flasks and receivers are provided with low-
abnormally high temperatures exist. Leaky or
point drains so any collected moisture may drain
dirty valves, excessive pressurization rates, or
periodically. However, many shipboard uses for
faulty cooling systems may cause these high
compressed air require air with an even smaller
temperatures.
moisture content than is obtained through these
methods. Water vapor in air lines can create other
2. NEVER use distillate fuel or gasoline as a
problems that are potentially hazardous, such as
degreaser to clean compressor intake filters,
the freezing of valves and controls. These
cylinders, or air passages. These oils vaporize
conditions can occur when air at very high
easily and will form a highly explosive mixture
pressure is throttled to a low-pressure area at a
with the air under compression.
high flow rate. The venturi effect of the throttled
air produces very low temperatures, which will
3. Secure a compressor immediately if you
cause any moisture in the air to freeze into ice.
observe that the temperature of the air being
Under these conditions, a valve (especially an
discharged from any stage exceeds the maximum
automatic valve) may become very difficult or
temperature specified.
impossible to operate. Also, moisture in any air
system can cause serious water hammering (a
4. NEVER leave the compressor station after
banging sound) within the system. For these
starting the compressor unless you are sure that
reasons, air dehydrators or dryers are used to
the control and unloading devices are operating
remove most of the water vapor from compressed
properly.
air.
5. Before working on a compressor, be
The Navy uses two basic types of air
sure the compressor is secured and cannot
dehydrators and a combination of the two. These
start automatically or accidentally. Completely
air dehydrators are classified as follows:
blow down the compressor, and then secure
all valves (including the control or unload-
Type I--Refrigeration
ing valves) between the compressor and the
receiver. Follow the appropriate tag-out pro-
Type II--Heater, desiccant
cedures for the compressor control valves
and the isolation valves. When the gauges
Type III--Refrigeration, desiccant
are in place, leave the pressure gauge cut-
out valves open at all times.
Each of these types is designed to meet the
requirements specified for the quality of the
6. Before disconnecting any part of an
compressed air to be used in pneumatic control
air system, be sure the part is not under
systems or for clean, dry air used for shipboard
pressure. Always leave the pressure gauge cut-
out valves open to the sections to which they are
electronic systems. Specific requirements usually
attached.
involve operating pressure, flow rate, dew point,
and purity (percentage of aerosols and size of
7. Avoid rapid operation of manual valves.
particles). We will briefly discuss each of the types
The heat of compression caused by a sudden flow
of air dehydrators.
of high pressure into an empty line or vessel can
cause an explosion if oil or other impurities are
present. Slowly crack open the valves until flow
REFRIGERATION AIR DEHYDRATOR
is noted, and keep the valves in this position
(TYPE I)
until pressure on both sides has equalized.
Keep the rate of pressure rise under 200 psi per
Refrigeration is one method of removing
second.
moisture from compressed air. The dehydrator
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