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

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.3

Compare the operating characteristics of positive- and non-positive-displacement air compressors.


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


used to control the maximum air pressure available from the compressed-air unit.

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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.


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


p y y

Compressed air units vary in size

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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)


DeVilbiss Air Power Company


Large, industrial units may produce thousands f fof cfm


Badger Iron Works, Inc.

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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


– Many portable units have a larger capacity than many stationary central air supplies


A portable unit may be large or small


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Portable units allow the compressor to be d t th k itmoved to the work site


Atlas Copco


A compressed-air unit consists of:– Prime mover

– Compressor

– Coupling

– Receiver

– Capacity-limiting system


Capacity limiting system

– Safety valve

– Air filter

– May have a cooler and dryer

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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


A coupling connects the prime mover to the compressor


Belt coupling



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Mechanical coupling



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


Dynamic

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Reciprocating-piston compressors are the most common

Rotary screw compressors are popular in new installations



Inline, reciprocating compressor



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The basic operation of any compressor includes th hthree phases– Air intake

– Air compression

– Air discharge

Component parts and physical operation varies


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


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



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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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


minute


Large, industrial, reciprocating compressor


Atlas Copco

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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


– V type

– W type

– Other cylinder configuration


Inline reciprocating compressor



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V-type reciprocating compressor




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


and retraction of the piston

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Double-acting compressor



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,


compression, and discharge of air during compressor rotation

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Centrifugal force keeps the vanes in contact ith th llwith the walls



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


desired pressure level

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Rotary screw compressors have intermeshing, h li lhelical screws


Atlas Copco


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


y

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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-


Oil is removed by a separator to provide nearoilless compressed air for the pneumatic system


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


Axial

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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


movement slows in the volute collector


Centrifugal dynamic compressor


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Impeller assembly of t if l d ia centrifugal dynamic

compressor



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


p

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Axial-flow dynamic compressor



Pressure is created h hi h d iwhen high-speed air

is slowed by the fixed stator blades


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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



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


the inlet port to the outlet port to increase system pressure

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Impellers from a lobe-type compressor


Atlas Copco


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 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


compressor package operation


Inline, staged, reciprocating compressor



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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


effective the operation of the system


Compressor-capacity control systems include:– Bypass

– Start-stop

– Inlet valve unloading

– Speed variation

– Inlet size variation


Inlet size variation

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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


as it is inefficient


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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Start-stop control uses itia pressure-sensitive

switch to start and stop the compressor to maintain a preselected pressure range



Start-stop control: compressor start


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Start-stop control: compressor stop



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


y gengines or electric motors as the prime mover

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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


p gcontrol compressor speed


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


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


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


System instrumentation

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Review QuestionReview Question

Compressed-air units may be classified as a(n) _____ or _____.

portable unit; central air supply



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


; 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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The simplest compressor in both design and ti th i th i l tioperating theory is the single-acting, _____

compressor.

reciprocating



Dynamic compressors can also be classified di l tas _____-displacement compressors.

non-positive


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The continuous rotating motion of the i l t id tifi ( )compression elements identifies a(n) _____

compressor design.

rotary



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


either side of the piston allow compression and intake during each piston stroke.

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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


inlet


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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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


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


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.


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


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


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.


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


– 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


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.


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


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.


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.


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.


chapter 15 source of pneumatic power

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