Chapter 15 Source of Pneumatic Power
Transcript of Chapter 15 Source of Pneumatic Power
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Chapter 15Chapter 15
Source of Pneumatic PowerSource of Pneumatic Power
CompressedCompressed--Air Unit and Air Unit and CompressorCompressorpp
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ObjectivesObjectives
Describe the function of a compressed-air unit.
Name and explain the function of each of the components in a compressed-air unit.
Identify the basic designs used in air compressor construction.
C th ti h t i ti f
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Compare the operating characteristics of positive- and non-positive-displacement air compressors.
ObjectivesObjectives
Compare the operating characteristics of rotary d i ti iand reciprocating air compressors.
Describe the general construction characteristics of the various compressor types.
Explain the operation of the various systems used to control the maximum air pressure
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used to control the maximum air pressure available from the compressed-air unit.
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ObjectivesObjectives
Identify the factors that must be considered to ti t th i d t t f testimate the required output of a compressor to
meet the air demands of a pneumatic system.
Interpret performance data supplied by a compressor manufacturer.
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CompressedCompressed--Air UnitAir Unit
The source of compressed air for a pneumatic t i th d i itsystem is the compressed-air unit
– Prime mover
– Compressor
– Other components to condition and store the pressurized air used by the system workstations
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p y y
Compressed air units vary in size
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CompressedCompressed--Air UnitAir Unit
Very small packages may produce only a f ti f bi f t f i i t ( f )fraction of a cubic foot of air per minute (cfm)
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DeVilbiss Air Power Company
CompressedCompressed--Air UnitAir Unit
Large, industrial units may produce thousands f fof cfm
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Badger Iron Works, Inc.
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CompressedCompressed--Air UnitAir Unit
Compressed-air units can be classified as t bl it t l i liportable units or central air supplies
– Physical size is not the only factor in placing a unit in one of these classes
– Easy transport of a unit from one location to another is a more important factor
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– Many portable units have a larger capacity than many stationary central air supplies
CompressedCompressed--Air UnitAir Unit
A portable unit may be large or small
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CompressedCompressed--Air UnitAir Unit
Portable units allow the compressor to be d t th k itmoved to the work site
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Atlas Copco
CompressedCompressed--Air UnitAir Unit
A compressed-air unit consists of:– Prime mover
– Compressor
– Coupling
– Receiver
– Capacity-limiting system
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Capacity limiting system
– Safety valve
– Air filter
– May have a cooler and dryer
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CompressedCompressed--Air UnitAir Unit
The prime mover in a compressed-air unit may bbe:– Electric motor
– Internal combustion engine
– Steam or gas turbine
A coupling connects the prime mover to the
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A coupling connects the prime mover to the compressor
CompressedCompressed--Air UnitAir Unit
Belt coupling
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DeVilbiss Air Power Company
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CompressedCompressed--Air UnitAir Unit
Mechanical coupling
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DeVilbiss Air Power Company
Basic Compressor DesignBasic Compressor Design
A variety of designs are used for air i th d i itcompressors in the compressed-air unit
– Reciprocating piston
– Rotary, sliding vane
– Rotary screw
– Dynamic
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Dynamic
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Basic Compressor DesignBasic Compressor Design
Reciprocating-piston compressors are the most common
Rotary screw compressors are popular in new installations
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Basic Compressor DesignBasic Compressor Design
Inline, reciprocating compressor
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DeVilbiss Air Power Company
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Basic Compressor DesignBasic Compressor Design
The basic operation of any compressor includes th hthree phases– Air intake
– Air compression
– Air discharge
Component parts and physical operation varies
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Component parts and physical operation varies between compressor designs
Basic Compressor Basic Compressor ClassificationsClassifications
Compressors are classified as:– Positive or non-positive displacement
– Reciprocating or rotary
Positive-displacement compressors mechanically reduce the compression chamber size to achieved compression
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size to achieved compression
Non-positive-displacement compressors use air velocity to increase pressure
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Basic Compressor Basic Compressor ClassificationsClassifications
A reciprocating hcompressor has a
positive displacement
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DeVilbiss Air Power Company
Compressor Design and Compressor Design and OperationOperation
Reciprocating compressors use a cylinder and i ti i t t hi ia reciprocating piston to achieve compression
Rotary compressors use continuously rotating vanes, screws, or lobed impellers to move and compress the air
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Compressor Design and Compressor Design and OperationOperation
Reciprocating compressors are commonly used i ti tin pneumatic systems– Very small, single-cylinder, portable compressors
for consumer use
– Large, industrial, stationary units may produce thousands of cubic feet of compressed air per
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minute
Compressor Design and Compressor Design and OperationOperation
Large, industrial, reciprocating compressor
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Atlas Copco
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Compressor Design and Compressor Design and OperationOperation
Reciprocating compressors are available in i l lti l li d d isingle- or multiple-cylinder designs
Multiple cylinders may be arranged as:– Inline
– Opposed
– V type
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– V type
– W type
– Other cylinder configuration
Compressor Design and Compressor Design and OperationOperation
Inline reciprocating compressor
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DeVilbiss Air Power Company
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Compressor Design and Compressor Design and OperationOperation
V-type reciprocating compressor
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DeVilbiss Air Power Company
Compressor Design and Compressor Design and OperationOperation
Reciprocating compressors use a single-acting d bl ti i tor double-acting compression arrangement
– Single-acting compressors compress air during one direction of piston travel
– Double-acting compressors have two compression chambers, allowing compression on both extension
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and retraction of the piston
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Compressor Design and Compressor Design and OperationOperation
Double-acting compressor
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Compressor Design and Compressor Design and OperationOperation
Rotary, sliding-vane compressors use a slotted t t i i bl t irotor containing movable vanes to compress air
– Rotor is placed off center in a circular compression chamber, allowing the chamber volume to change during rotation
– These volume changes allow the intake,
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compression, and discharge of air during compressor rotation
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Compressor Design and Compressor Design and OperationOperation
Centrifugal force keeps the vanes in contact ith th llwith the walls
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Compressor Design and Compressor Design and OperationOperation
Rotary screw compressors use intermeshing, h li l t f h b th t ihelical screws to form chambers that move air from the atmosphere into the system on a continuous basis
This produces a nonpulsating flow of air at the desired pressure level
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desired pressure level
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Compressor Design and Compressor Design and OperationOperation
Rotary screw compressors have intermeshing, h li lhelical screws
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Atlas Copco
Compressor Design and Compressor Design and OperationOperation
Rotary screw compressors have become l f l i d t i l i t ll tipopular for larger industrial installations
– Lower initial cost
– Lower maintenance cost
– Adaptable to sophisticated electronic control systems
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y
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Compressor Design and Compressor Design and OperationOperation
Sliding vane and screw compressor designs ft i j t il i t th i t ioften inject oil into the airstream moving
through the compressors– Reduces wear on vane and screw contact surfaces
– Improves the seal between the surfaces
Oil is removed by a separator to provide near-
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Oil is removed by a separator to provide nearoilless compressed air for the pneumatic system
Compressor Design and Compressor Design and OperationOperation
The basic operating theory of dynamic i ti th ki ti fcompressors is converting the kinetic energy of
high-speed air into pressure
Dynamic compressor designs are either:– Centrifugal
– Axial
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Axial
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Compressor Design and Compressor Design and OperationOperation
Centrifugal dynamic compressor:– An impeller increases airspeed
– Prime mover energy is converted into kinetic energy as airspeed rapidly increases through the impeller
– Kinetic energy is converted to air pressure as air
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movement slows in the volute collector
Compressor Design and Compressor Design and OperationOperation
Centrifugal dynamic compressor
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Compressor Design and Compressor Design and OperationOperation
Impeller assembly of t if l d ia centrifugal dynamic
compressor
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Compressor Design and Compressor Design and OperationOperation
Axial-flow dynamic compressor:– Rotating rotor blades increase airspeed
– Fixed stator blades decrease airspeed
– Kinetic energy is converted to air pressure
– Series of rotor and stator sections are staged to form the axial-flow compressor
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p
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Compressor Design and Compressor Design and OperationOperation
Axial-flow dynamic compressor
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Compressor Design and Compressor Design and OperationOperation
Pressure is created h hi h d iwhen high-speed air
is slowed by the fixed stator blades
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Compressor Design and Compressor Design and OperationOperation
Dynamic compressor designs are used to i d th f lcompress air and other gases for large,
industrial applications– Oil refineries
– Chemical plants
– Steel mills
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Compressor Design and Compressor Design and OperationOperation
Lobe-type compressors consist of two impellers ith t th l b th t t iwith two or three lobes that operate in an
elongated chamber in the compressor body– Spinning impellers trap air in chambers that form
between the lobes
– As the impellers turn, this trapped air is swept from
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the inlet port to the outlet port to increase system pressure
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Compressor Design and Compressor Design and OperationOperation
Impellers from a lobe-type compressor
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Atlas Copco
Compressor Design and Compressor Design and OperationOperation
Lobe-type compressors are often called blowers
They are typically used in applications requiring air pressure of only 10 to 20 psi
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Compressor Design and Compressor Design and OperationOperation
Compressor staging involves connecting a b f b i it i i tnumber of basic compressor units in series to
raise air pressure in small increments
This method permits easier control of air temperature, which results in more-efficient compressor package operation
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compressor package operation
Compressor Design and Compressor Design and OperationOperation
Inline, staged, reciprocating compressor
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DeVilbiss Air Power Company
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CompressorCompressor--Capacity Capacity ControlControl
Compressor-capacity control refers to the t th t t h th d i t tsystem that matches the compressed-air output
to the system-air demand
The better the air output of the compressor matches system consumption, the more cost effective the operation of the system
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effective the operation of the system
CompressorCompressor--Capacity Capacity ControlControl
Compressor-capacity control systems include:– Bypass
– Start-stop
– Inlet valve unloading
– Speed variation
– Inlet size variation
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Inlet size variation
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CompressorCompressor--Capacity Capacity ControlControl
Bypass control uses a relief-type valve to h t iexhaust excess air
Air is continuously delivered to the system at the compressor’s maximum flow rate
This type of control is not considered desirable as it is inefficient
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as it is inefficient
CompressorCompressor--Capacity Capacity ControlControl
Start-stop capacity control is commonly used ith ll l t i t d iwith small, electric motor-driven compressor
packages that operate pneumatic systems consuming air on an intermittent basis
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CompressorCompressor--Capacity Capacity ControlControl
Start-stop control uses itia pressure-sensitive
switch to start and stop the compressor to maintain a preselected pressure range
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CompressorCompressor--Capacity Capacity ControlControl
Start-stop control: compressor start
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CompressorCompressor--Capacity Capacity ControlControl
Start-stop control: compressor stop
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CompressorCompressor--Capacity Capacity ControlControl
Inlet valve unloading controls compressor t t b h ldi th i l t loutput by holding the inlet valve open
whenever maximum system pressure is achieved– Allows the prime mover to operate continuously
– Can be used in systems having internal combustion
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y gengines or electric motors as the prime mover
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CompressorCompressor--Capacity Capacity ControlControl
Varying compressor speed can control itcompressor capacity
– Can be used with reciprocating and rotary compressor designs
– Primarily used on large, industrial installations
– Sensors monitor pressure and send a signal to
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p gcontrol compressor speed
CompressorCompressor--Capacity Capacity ControlControl
Varying the size of the compressor inlet can t l itcontrol compressor capacity
– Compressor operates at a constant speed
– The volume of air that can enter the compressor is restricted
– Output varies with the size of the inlet
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p
– Primarily used on dynamic compressors
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Selecting a Compressor Selecting a Compressor PackagePackage
Establishing the level of system air ti i k f t h l ticonsumption is a key factor when selecting a
compressor
This can be accomplished by identifying:– Actuators used in the system
– Compressed-air needs of each item
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Compressed air needs of each item
– Percentage of time each functions
Selecting a Compressor Selecting a Compressor PackagePackage
Other factors must be considered during system l ticompressor selection
– Compressor and prime mover type
– Method of compressor-capacity control
– Auxiliary controls such as coolers, separators, and driers
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System instrumentation
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Review QuestionReview Question
Compressed-air units may be classified as a(n) _____ or _____.
portable unit; central air supply
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Review QuestionReview Question
List the components found in a typical d i it d d ib th i f ticompressed-air unit and describe their function.
A. Prime mover to supply system energy; B. coupling to mechanically connect prime mover and compressor; C. compressor to pressurize atmospheric air; D. receiver to store conditioned air; E. capacity-limiting switch to limit the
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; p y gmaximum pressure produced by the compressor; F. safety valve to vent pressure if the capacity-limiting switch fails; and G. may also include filters, coolers, and dryers.
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Review QuestionReview Question
The simplest compressor in both design and ti th i th i l tioperating theory is the single-acting, _____
compressor.
reciprocating
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Review QuestionReview Question
Dynamic compressors can also be classified di l tas _____-displacement compressors.
non-positive
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Review QuestionReview Question
The continuous rotating motion of the i l t id tifi ( )compression elements identifies a(n) _____
compressor design.
rotary
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Review QuestionReview Question
Describe the double-acting compressor design.
A connecting rod and crosshead are used to convert the rotary motion of the crankshaft to the reciprocating motion. Compression chambers on ith id f th i t ll i d
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either side of the piston allow compression and intake during each piston stroke.
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Review QuestionReview Question
In a two-stage, reciprocating compressor, the tl t t f th fi t i h b ioutlet port of the first compression chamber is
connected to the _____ port of a second compression chamber.
inlet
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inlet
Review QuestionReview Question
Compressor-air output and system-air demand t h d b i t fare matched by using some type of _____
system.
compressor-capacity control
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Review QuestionReview Question
Name four factors that make selecting a diffi ltcompressor difficult.
A. The variety of compressor designs, B. load variations in a pneumatic system, C. the variety of
ili i t il bl d D d d f
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auxiliary equipment available, and D. demands of future growth of the system.
GlossaryGlossary
Air filterA component designed to remove solid particles– A component designed to remove solid particles, moisture, and/or lubricant from pneumatic system air.
Capacity-limiting system– A system used to control the maximum air pressure
produced by the pneumatic system compressor. Compressor start-stop inlet valve unloading and
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Compressor start-stop, inlet valve unloading, and other methods can control capacity.
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GlossaryGlossary
Compressed air unit– Commonly used to designate a pneumatic
compressor station that includes a prime mover, compressor, reservoir, and pressure control components.
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GlossaryGlossary
Cooler– A pneumatic system dryer that uses a refrigeration
element to lower the temperature of system air for the purpose of removing moisture.
Coupling– A general term used for devices that connect
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gsystem components such as fluid conductors and power transmission shafts.
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GlossaryGlossary
Double-acting compressor– A compressor design in which air intake and
compression are completed in chambers located on both the top and underside of the compressor pistons. These compressors were common in larger industrial installations, but are being replaced by current rotary designs
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current rotary designs.
GlossaryGlossary
Dryer – The pneumatic system component designed to
remove water vapor from the compressed air. The unit is usually located in the compressor station area and may use refrigeration, chemical, or mechanical means to reduce the water content of the air.
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GlossaryGlossary
Dynamic compressor– A device that compresses air or other gas using
rotating vanes or impellers. These moving components increase pressure by converting the energy in the high velocity air to pressure.
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GlossaryGlossary
Lobe-type compressor U ll id l l i C t ti– Usually provide only low-pressure air. Construction involves specially-designed elements with lobes that provide a sealed pumping chamber. Air is swept from the compressor inlet to the outlet as the lobes rotate. Often referred to as blowers.
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GlossaryGlossary
Non-positive-displacement compressor– A compressor that does not have a variable-volume
pumping chamber. An impeller or other device is used to move the fluid. The inertia of that fluid movement produces pressure when flow is resisted.
Positive displacement
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– A pump or rotary actuator design with a positive seal between the inlet and outlet, which produces a constant volume of fluid for each revolution.
GlossaryGlossary
Prime mover– The source of energy for any fluid power system.
Commonly used prime movers are electric motors and internal combustion engines.
Receiver– A tank in a pneumatic system, located close to the
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p ycompressor, that stores and assists in conditioning compressed air.
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GlossaryGlossary
Reciprocating compressor– A common compressor design using a cylinder,
piston, crankshaft, and valves similar to an internal combustion engine. The reciprocating action of the piston brings air into the cylinder, where it is compressed and then moved into the system.
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GlossaryGlossary
Rotary compressor– A compressor design that compresses air using a
continuous process rather than the stop-and-go action of a reciprocating unit. Examples include positive-displacement screw and vane units and non-positive-displacement centrifugal and axial-flow dynamic compressors
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flow dynamic compressors.
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GlossaryGlossary
Rotary screw compressor– A compressor unit that uses intermeshing screws to
form chambers that linearly move air through the compressor. The design provides a continuous, positive displacement of the air.
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GlossaryGlossary
Rotary sliding-vane compressor – A compressor unit using a rotor with chambers
separated by sliding vanes. Turning the unit produces an almost pulsation-free stream of compressed air.
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GlossaryGlossary
Single-acting compressor– A reciprocating-piston compressor in which intake
and compression occur in the cylinder space above the piston during one rotation of the compressor crankshaft. The design may contain multiple cylinders, but the cylinders are not staged.
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GlossaryGlossary
Staging– A process commonly used in the design of
pneumatic compressors where the outlet of one compressor cylinder is connected to the intake of the next cylinder to obtain higher system pressures.
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