Transcript Document

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.
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.
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.
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
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
Air Line Filters
Inlet
Outlet
Filter
Deflector
Baffle
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.
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.
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.
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.
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 don’t run low on oil. The best way to keep track of
maintenance is to keep records and schedule maintenance.