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Fluid Power Systems (ME353)
Fall 2012
Lecture 12
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Conditioning and Distributionof Compressed Air
Controlling Dirt, Moisture,Temperature, and Pressure
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Maximum pneumatic system operating efficiency is achieved when system
compressed air is:
– Consistently clean
– Free from moisture
– At a relatively uniform temperature
Pneumatic systems
need to carefully filterthe air taken into the
compressor intake to
extend the service life
of
–
Compressor – Other system
components
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The temperature of both the intake and compressed air is important
– Temperature changes are reflected in air pressure and volume per the
general gas law
– Temperature influences the ability of air to retain water vapor
Air in a pneumatic system may be cooled before, during, or after
compression
– Intake air temperature usually depends on the location of the compressor
air intake
– Intercoolers and aftercoolers are used to remove heat of compression
Intercoolers cool compressed air between the stages of a multiple-stage
compressor
Aftercoolers cool the air after the air has been compressed
Either air or water can be used as the cooling medium in these devices
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The receiver is the storage unit for compressed air
Typically, the receiver is a metal, cylindrical tank with domed ends
In addition to air storage, the receiver:
– Dampens system pressure pulsations
– Removes water vapor from system air
– In smaller systems, serves as the mount for the prime mover and
compressor
Formulas are available for calculating the volume needed for a receiver
These formulas consider:
– Cubic feet of free atmospheric air needed per minute
– Desired cycle time
– Atmospheric, initial receiver, and final receiver air pressures
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The air distribution system delivers high-pressure, conditioned air from the
receiver to workstations with a minimum of pressure drop
The type of distribution system depends on the size of the facility and the
level of demand for compressed air
Four general categories of air distribution systems are used with pneumatic
systems
– Centralized grid with fixed piping
– Decentralized grid with fixed piping
– Loop system with fixed piping
– Flexible hoses for portable compressor systems
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Centralized grid has one centralized compressor station and one line network
for a facility
Decentralized grid has individual compressors in several locations providing
air to smaller distribution networks
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Loop system has a main line that forms a continuous loop with compressors
located at one or more locations
This design provides maximum airflow with a minimum of pressure drop
between the compressors and the individual workstations
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© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.11
Air-Distribution System
Loop system
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Special attention must given to the setup of a hose air distribution in order to
minimize pressure drop
– Minimize hose length
– Reduce the number of couplings
– Eliminate kinks in the hose
Care must be taken to protect the hoses from abrasion in the work
environment
Typical hose distribution system
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Proper sizing of pipe for a fixed air distribution system is difficult because of
variety of work and loads conditions:
The pipe in air distribution lines should be installed with a pitch of 1 per
10 of line
– Allows liquid water to drain to water traps
– Water can be remove from traps either manually or with automatic
drain devices
Drop lines
lead from the main air distribution line to the workstations – Should be attached to the top side of the distribution line
– This prevents water in the distribution lines from entering the
workstation lines
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Final preparation of air at a workstation is accomplished by an FRL unit
– Air filter
– Pressure regulator
– Lubricator
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FRL air filter removes:
– Airborne dirt remaining in the atmospheric air compressed in the system
– Rust and scale from the interior of the distribution lines
– Liquid water that has condensed in the drop line
– Atomized oil from the operating compressor
Typical air filter uses
centrifugal force and porous
filter material to remove
unwanted materials from
system air
– Inlet passageway swirls
the incoming air,
creating a centrifugalforce that separates air
and contaminants
– Porous filter material
traps other undesirable
materials
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The pressure regulator in an FRL unit reduces system distribution line
pressure to the level needed by workstation tools and circuit actuators
Unit is also necessary as air pressure in the distribution line fluctuates due to
varying air demands and the characteristics of compressor-capacity control
Factors to consider when
selecting a system regulator are:
– Regulator style
– Pressure range
– Airflow range
– Conductor connection size
Several regulator designs are available for use in a pneumatic system
– Direct-operated regulator
– Basic, diaphragm-chamber regulator
–
Relieving-type regulator – Balanced-poppet valve regulator
– Pilot-operated regulator
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The lubricator in an FRL unit meters oil into pressurized system air at the
workstation
This provides lubrication for system valves, actuators, and air-powered tools
Rapidly moving system air passing
through a lubricator breaks up droplets
of oil, forming a mist or fog
This mist is transported through the
workstation lines to system components
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Effectively moving compressed air through a distribution system requires
appropriate conductors and connectors
Conductors can be classified as
- Rigid (Pipes) - Flexible (Hoses)
Conductors and the associated fittings must be properly sized and
selected to minimize the pressure drop (minimum length and minimum
no. of fittings and bends)
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Work Performers ofPneumatic Systems
Cylinders, Motors, and
Other Devices
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Pneumatic systems convert the potential energy of compressed air into force
and movement using:
– Cylinders
• Often called linear actuators
• Force generated is controlled by system pressure
• Speed of movement is determined by the volume of air allowed to enter the
unit
– Motors
• Often called rotary actuators
• Torque depends on air pressure and the internal structure of the motor
• Operating speed is determined by the internal displacement of the motor per
revolution and the volume of compressed air passing through the motor
– Variety of other specially designed actuators and processes to assist in or
complete a task
• Reciprocating movement
• Process assistance
• Nozzles
• Impact tools
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The basic structure of pneumatic cylinders is very similar to those used in
hydraulic systems
– Lower system operating pressures allow the use of lighter materials in
pneumatic-system components
– Water vapor present in compressed air requires the use of corrosion-
resistant materials or coatings for component parts
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Resilient seals prevent both internal and external leaks
Some manufacturers produce nonlubricated pneumatic cylinders
– Do not require the addition of oil to the system compressed air
– Special coatings on the surface of the cylinder bore and other bearing
surfaces provide lubrication – Coatings are not scraped off during the operation of the actuator
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Pneumatic cylinders may be single or double acting
Two factors are basic to determining required actuator size
– Cylinder force output
– Absolute air consumption required to produce desired system
performance The force output of a cylinder is determined by system air pressure and the
effective area of the cylinder piston F = P × A
Air consumption of a cylinder can be estimated
– Calculate the volume of air displaced during one cycle of the cylinder
– Multiplying the displacement volume by the number of cycles per
minute and the absolute pressure ratio CFM = V × Pr × N
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Pneumatic motors are used to power:
– Many large stationary machines
– A large variety of portable hand tools
These motors range in size from fractional-horsepower units to motors
producing over 50 horsepower
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Air motors are available in many designs
-Vane - Piston - Turbine -Other, specialized designs
The vane air motor is the most common design which can be found in hand
tools and large, stationary installations
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Piston air motors are most often found in installations requiring higher
horsepower output
They are available in both axial and radial piston designs
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Turbine motors incorporate air nozzles to direct air onto a turbine
– Nozzle produces high-speed air, which results in very high output-shaft
speeds
– In specialized applications, the speed of turbine motors can reach
100,000 rpm
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Pneumatic reciprocating motors use percussive or non-percussive techniques
to transfer energy from compressed air to a workpiece
– Percussive tools provide multiple, physical impacts to overcome
resistance – Non-percussive devices generally repeat a cycle to provide linear motion
that is used to directly operate a machine
Total input force of a percussion-type reciprocating motor is determined by:
- System air pressure - Area of the piston
The paving breaker is a very
common application of a
reciprocating motor
– Often called a jackhammer
– Used in a number ofapplications
– Models available ranging in
size from hand tools to large
units mounted on mobile
support equipment
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Reciprocating motors are used to power a variety of tools found in the
foundry, construction, and general metal fabrication industries
– Scaling hammers
–
Chipping hammers – Riveting hammers
– Tampers
– Rammers
Reciprocating motors commonly operate from under 1000 to over 3000movements per minute of operation
The operating rate depends on the task performed and the type of material
being cleaned, formed, or trimmed
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Compressed air is often used to assist functions without applying force to
linear or rotary actuators
– Spraying
– Drying
– Material agitation
– Material transfer
Compressed air is used to agitate material
– Assure proper mixing of liquids
– Prevent the settling of solid materials in a suspension – Provide oxygenation
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A nozzle is a very important device in many systems using compressed air
– In turbine motors, nozzles produce rapid airflow to assure the high speed
rotation of the turbine output shaft
–
Nozzles may be convergent or convergent-divergent
Blowguns are simple tools, but
must be very carefully handled
– Produce high air velocity
– Can cause serious injury
– Designs are available with
a bleed-off to increase
safety
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Pneumatic-powered impact wrenches are commonly used in service and
manufacturing industries
These tools are used to tighten or remove bolts and nuts
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Nail drivers are used to install staples, nails, or other fasteners in wood
and other materials
Pneumatic models have become popular for use by general consumers, as
well as by the construction industry
Pneumatic powered nail drivers use high-impact force from an air-driven
piston and piston rod to quickly and easily install fasteners
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Pneumatic rock drills are commonly used in quarries, mining, and road
construction
The holes bored by this equipment are used for the placement of explosive
charges that break the rock along a line connecting the holes
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Pneumatic-powered gripping tools are used by robotic equipment to handle
materials
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Controlling a PneumaticSystem
Pressure, Direction, and Flow
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The operation of a control valve depends on a series of control elements in
the valve
Control elements include:
– Fixed orifices
– Spools
– Pistons
– Diaphragms
– Precision-machined sliding plates
– Needle and poppet valves
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Varying the position of control elements within a control valve determines:
– Setting of the valve
– Accuracy of system control
The force used to position the internal elements of a control valve is provided by:
– Internal springs
– Internal air pressure
– External pilot pressure
– Electromagnets
– Manually applied force
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Pressure control is provided in three specific areas of a pneumatic system:
– Air distribution system
– Workstation areas
– Portions of circuits operating within the workstation areas
The pressure in the distribution lines of a pneumatic system is controlled by:
- Bypass - Compressor-capacity control system
Bypass pressure control limits maximum system pressure by bleeding excess
air to the atmosphere through a relief-type valve
Compressor-capacity control limits maximum pressure using one of several
different control methods
– Compressor start-stop
– Inlet valve unloading
– Compressor speed control
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Control of air pressure at the workstation is provided by a pressure regulator
It is often a part of the filter/pressure regulator/lubricator (FRL) unit
commonly used at the workstation
Pressure regulator is required at the workstation
due to:
– Air from the distribution line is at a higher
pressure than desired
– Varying distribution line pressure caused by
the compressor-capacity control system
When sections of a circuit require maximum air
pressures lower than that provided by the FRL unit,
additional pressure regulators may be inserted inthe circuit to provide the needed lower pressures
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Directional control valves can be grouped into four categories:
- Shut-off valves - Check valves
- Three-way valves - Four-way valves
Shut-off valves used in pneumatic systems include:
-Globe - Gate - Ball - Spool - Needle
These valves are used to allow or block airflow in circuit lines
Check valves automatically allow the free flow of air in one direction and
block airflow in the opposite direction
They are commonly used to block or allow air to flow around control valves
during different phases of circuit operation
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Three-way directional control valves:
– Pressurize a single-acting cylinder during extension
– Vent air during retraction to allow the cylinder to be returned to the
initial position by external forces
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Four-way directional control valves direct compressed air to power a
cylinder during both extension and retraction
They also power air motors during both directions of rotation
Four-way directional control valves are available in four-port and five-port
configurations
– A common exhaust is used in the four-port design
– The five-port design uses separate exhaust ports for the extension and
retraction positions of the valve
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In most directional control valves, resilient materials are used to seal the
space between the bore and the spool
– In packed-spool designs, sealing materials are attached to the valve spool
lands
– In packed-bore designs, sealing materials are attached to the bore in the
valve body
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Three center position configurations are commonly available for pneumatic
three-position, five-port valves
-Blocked center - Open center - Pressure center
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Flow Control Valves use tow common methods of controlling pressurized
airflow in pneumatic circuits which are:
– Fixed-size orifices
– Needle valves
Preferred method of flow control in pneumatic
circuits is the meter-out design
– Involves the placement of the flow control
valve in the outlet line of the actuator
– Favored due to the compressibility of air
Sh l l i ll l h hi h f
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Shuttle valves automatically select the higher pressure of two sources
Allows a circuit to always be connected to the highest pressure
Exhaust mufflers:
– Reduce the level of air exhaust
noise
– Prevent the entrance of dirt and
liquids into the exhaust ports of
directional control valves and
actuators