Unit 14 Air Preparation

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Unit 14 -Air Preparation
In this unit, we will discuss the preparation of
air for use in a pneumatic system. Raw
compressed air should not be used by pneumatic
components until it has been cleaned, had its
pressure adjusted, dehumidified, and in some
cases lubricated. No matter how sophisticated a
pneumatic system may be, if the air in the system
is contaminated, the system will not operate
properly.
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Dirt and its Effect in a Pneumatic System
Dirt, simply put, is any free floating solid or
semi-solid particle in the stream of air flow.
Besides the intake of dust and moisture vapor,
compressed air usually produces many unwanted
compounds which are carried into the receiver
tank. While most will remain in the tank, some
will inevitably make their way into the system
where, if not removed, will cause problems
ranging from sticking valves to total component
failure.
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Dirt is Pollution
All compressors should have an intake filter.
The intake filter will remove a great deal of
contamination but cannot remove fine dusts or
water vapor. An intake of 8 cubic feet of air
will be reduced about 8 times so the humidity
left in the compressed air is multiplied by as
much. The combination of fine dirt particles,
moisture, and compressor discharge temperatures
of about 450 degrees will produce a variety of
contaminant types in such a degree as to
necessitate the cleaning of air at every work
point or piece of equipment.
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Contaminant Type and Source
Contaminants are produced from any one of 3
sources. First, as the system is assembled, they
may built in as would happen in a dirty
environment. Second, dirt can be generated when
moving parts begin to wear. Third, dirt can be
ingested by filthy reassembly practices or
through a seal in an actuator working in a dirty
environment.
There are 3 basic types of contamination. Abrasive
Dirt Dust ,Grinding,Compounds, Sand Soft Dirt
Plastics, Pipe Compounds, Pipe Tape, Paint
Chips Entrained Liquids From water vapor
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Micrometer Scale and Limits of Visibility
As discussed in an earlier lesson, the micrometer
scale is based on units used in the SI system of
measurement. As it relates to contamination, the
human eye can only see a particle if it is larger
than 40 microns. This lower limit of human
visibility would suggest that a lot of
contamination cannot be seen by the naked human
eye.
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Industrial Air
Air is basically a mixture of gases, 78 nitrogen
and 21 oxygen. Besides moisture, air carries a
lot of other things that are not desirable to
have in a pneumatic system. The primary
contaminants are dust particles and water vapor.
In most cases it is best to remove as much of the
dust and moisture as possible before the air
enters the system and this is accomplished
through various devices but mainly by filters and
dryers. The first place to start the removal of
dirt is at the compressor intake.
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Air Intake Filter
The intake filter of a pneumatic system is the
primary defense against contamination. As is
with an internal combustion engine, the intake
filter is critical in relation to proper system
operation and if ignored can reek havoc upon the
system components.
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Compressor Air
Air drawn into the compressor from the outside is
squeezed into a smaller volume which, according
to Boyles Law, will increase its absolute
pressure. This compressed air is under pressure,
hot, and contains a significant amount of water
vapor as well as other contamination and some
lube oil which passes around the piston from the
crankcase . First we will discuss some methods
of moisture removal starting with the air that
comes directly out of the compressor. This
moisture laden air is sent to a device called an
after cooler which is usually mounted somewhere
between the compressor and receiver tank.
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After-Cooler and its Operation
As we have discussed in past lessons, the after
cooler removes moisture simply by cooling the hot
air. As hot air cools it loses its ability to
hold moisture and condensation occurs. The
moisture collects in the after cooler and is then
drained.
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Effects of Moisture in a Pneumatic System
Moisture in a pneumatic system causes a wide
range of problems. In larger quantities,
moisture will wash away lubricants and cause
excessive wear on moving parts. Also, moisture
reacts chemically with heated air and deteriorate
seals. In addition to these problems, water
vapor is a nuisance to the operator and a health
hazard. Consider that a 100 SCFM system can
produce as much as 50 gallons of water in 24
hours and one can appreciate the need to control
moisture and limit its access to the system.
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Moisture Removal
A primary consideration for the removal of
moisture is in design the air delivery system.
When a loop system is not practical, air piping
systems should incorporate a drop in the piping
and take off connections that encourage water to
stay on the bottom while moisture free air is
allowed to continue on to the components it feeds.
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Moisture Removal
Refrigeration Air Drier
There are two main types of moisture removal.
One method is by refrigeration and the other is
by adsorption.
Adsorption Air Drying Unit
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Refrigeration Dryer Operating Principle
To appreciate the operation of the refrigeration
type dryer, recall that heat transfers to any
surface with less heat. Once inside the dryer,
compressor air freely gives up its heat as it
comes into contact with the much cooler walls of
the dryer which are being cooled by refrigerant
gas. As the hot air cools, it loses its ability
to retain moisture and condensation takes place.
The moisture then collects in the bottom of the
dryer unit where it is drained from the system.
The much lighter dry air moves out into the
system.
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Air Line Filters
Controlling pressure and filtering air start at
the compressor. Air also has to be filtered ,
pressure adjusted, and in some cases lubricated
at the beginning of branch line circuits. A
combination of the filter, regulator, and
lubricator is commonly referred to as a FRL.
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Air Line Filters
Inlet
Outlet
Filter
Baffle
Deflector Plate
Air entering a filter is forced to take a path
down across the deflector plate which causes a
swirling action. This swirling action, aided by
inertia, throws out dirt and at the same time
causes water vapor to collect against the walls
of the filter bowl where gravity pulls it down to
the lower level. In the lower level where the
air flow is low, the water and dirt collect where
they can be removed from the system by draining.
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Filter Elements
As with hydraulics, the purpose of a pneumatic
filter is to remove contamination by the action
of retention. There are two basic design types
depth and edge.
The depth type filter element uses a thickness
of material to knock contamination out of the
air. Porous bronze and plastic are common
materials used for this type of filter.
Edge type filter elements trap dirt by
concentrating the flow of air into a tiny area
which causes debris to collect while allowing air
to keep moving. A paper, impregnated with resin,
is most common.
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Air Line Lubricator
There are many different types of air line
lubricators. The one shown uses vacuum to draw
oil into the air stream where it is busted up
or atomized and suspended in the air to be
carried downstream where it can lubricate tools
and equipment.
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FRL
An FRL is a unit consisting of a filter,
regulator, and lubricator. FRLs should be placed
at the delivery of all branch circuits in a
pneumatic system to give the greatest range of
protection and pressure adjustment.
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Maintaining Filters and Lubricators
Filters and lubricators must be checked
periodically or their presence will not make a
difference. A general rule on filters is to
simply replace them after a certain number of
hours in service. Lubricators require constant
inspection to ensure they dont run low on oil.
The best way to keep track of maintenance is to
keep records and schedule maintenance.
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Review
1.        Describe dirt in a pneumatic
system. 2.        The lower limit of visibility
for the human eye is 3.        Name two of the
undesirable things that air carries. 4.       
The primary line of defense for a pneumatic
system is 5.        What is the function of the
after-cooler? 6.        What happens the moisture
in hot air when the air cools? 7.        Why does
air in a receiver cool down? 8.        List four
effects of moisture in a pneumatic
system. 9.        What is the purpose of an air
line filter? 10.     What is the difference
between a depth type filter element and a edge
type element? 11.     How does an air line
lubricator work? 12.     What is a FRL? 13.    
Give an example of how dirt ingresses into a
system. 14.     List two methods of air drying.