Transcript Chapter 15 Source of Pneumatic Power Compressed-Air Unit and Compressor Objectives  Describe the function of a compressed-air unit.  Name and explain the function.

Chapter 15
Source of Pneumatic Power
Compressed-Air Unit and
Compressor
Objectives
 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.
 Compare the operating characteristics of
positive- and non-positive-displacement air
compressors.
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Objectives
 Compare the operating characteristics of rotary
and 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
available from the compressed-air unit.
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Objectives
 Identify the factors that must be considered to
estimate 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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Compressed-Air Unit
 The source of compressed air for a pneumatic
system is the compressed-air unit
– Prime mover
– Compressor
– Other components to condition and store the
pressurized air used by the system workstations
 Compressed air units vary in size
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Compressed-Air Unit
 Very small packages may produce only a
fraction of a cubic foot of air per minute (cfm)
DeVilbiss Air Power Company
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Compressed-Air Unit
 Large, industrial units may produce thousands
of cfm
Badger Iron Works, Inc.
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Compressed-Air Unit
 Compressed-air units can be classified as
portable 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
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Compressed-Air Unit
 A portable unit may be large or small
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Compressed-Air Unit
 Portable units allow the compressor to be
moved to the work site
Atlas Copco
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Compressed-Air Unit
 A compressed-air unit consists of:
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Prime mover
Compressor
Coupling
Receiver
Capacity-limiting system
Safety valve
Air filter
May have a cooler and dryer
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Compressed-Air Unit
 The prime mover in a compressed-air unit may
be:
– Electric motor
– Internal combustion engine
– Steam or gas turbine
 A coupling connects the prime mover to the
compressor
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Compressed-Air Unit
 Belt coupling
DeVilbiss Air Power Company
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Compressed-Air Unit
 Mechanical coupling
DeVilbiss Air Power Company
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Basic Compressor Design
 A variety of designs are used for air
compressors in the compressed-air unit
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Reciprocating piston
Rotary, sliding vane
Rotary screw
Dynamic
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Basic Compressor Design
 Reciprocating-piston compressors are the most
common
 Rotary screw compressors are popular in new
installations
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Basic Compressor Design
 Inline, reciprocating compressor
DeVilbiss Air Power Company
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Basic Compressor Design
 The basic operation of any compressor includes
three phases
– Air intake
– Air compression
– Air discharge
 Component parts and physical operation varies
between compressor designs
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Basic Compressor
Classifications
 Compressors are classified as:
– Positive or non-positive displacement
– Reciprocating or rotary
 Positive-displacement compressors
mechanically reduce the compression chamber
size to achieved compression
 Non-positive-displacement compressors use
air velocity to increase pressure
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Basic Compressor
Classifications
 A reciprocating
compressor has a
positive displacement
DeVilbiss Air Power Company
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Compressor Design and
Operation
 Reciprocating compressors use a cylinder and
a 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
Operation
 Reciprocating compressors are commonly used
in 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
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Compressor Design and
Operation
 Large, industrial, reciprocating compressor
Atlas Copco
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Compressor Design and
Operation
 Reciprocating compressors are available in
single- or multiple-cylinder designs
 Multiple cylinders may be arranged as:
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–
Inline
Opposed
V type
W type
Other cylinder configuration
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Compressor Design and
Operation
 Inline reciprocating
compressor
DeVilbiss Air Power Company
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Compressor Design and
Operation
 V-type reciprocating compressor
DeVilbiss Air Power Company
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Compressor Design and
Operation
 Reciprocating compressors use a single-acting
or 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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Compressor Design and
Operation
 Double-acting
compressor
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Compressor Design and
Operation
 Rotary, sliding-vane compressors use a slotted
rotor 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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Compressor Design and
Operation
 Centrifugal force keeps the vanes in contact
with the walls
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Compressor Design and
Operation
 Rotary screw compressors use intermeshing,
helical 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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Compressor Design and
Operation
 Rotary screw compressors have intermeshing,
helical screws
Atlas Copco
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Compressor Design and
Operation
 Rotary screw compressors have become
popular for larger industrial installations
– Lower initial cost
– Lower maintenance cost
– Adaptable to sophisticated electronic control
systems
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Compressor Design and
Operation
 Sliding vane and screw compressor designs
often 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 nearoilless compressed air for the pneumatic system
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Compressor Design and
Operation
 The basic operating theory of dynamic
compressors is converting the kinetic energy of
high-speed air into pressure
 Dynamic compressor designs are either:
– Centrifugal
– Axial
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Compressor Design and
Operation
 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
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Compressor Design and
Operation
 Centrifugal dynamic
compressor
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Compressor Design and
Operation
 Impeller assembly of
a centrifugal dynamic
compressor
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Compressor Design and
Operation
 Axial-flow dynamic compressor:
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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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Compressor Design and
Operation
 Axial-flow dynamic compressor
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Compressor Design and
Operation
 Pressure is created
when high-speed air
is slowed by the fixed
stator blades
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Compressor Design and
Operation
 Dynamic compressor designs are used to
compress air and other gases for large,
industrial applications
– Oil refineries
– Chemical plants
– Steel mills
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Compressor Design and
Operation
 Lobe-type compressors consist of two impellers
with 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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Compressor Design and
Operation
 Impellers from a lobe-type compressor
Atlas Copco
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Compressor Design and
Operation
 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
Operation
 Compressor staging involves connecting a
number 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 Design and
Operation
 Inline, staged, reciprocating compressor
DeVilbiss Air Power Company
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Compressor-Capacity
Control
 Compressor-capacity control refers to the
system 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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Compressor-Capacity
Control
 Compressor-capacity control systems include:
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Bypass
Start-stop
Inlet valve unloading
Speed variation
Inlet size variation
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Compressor-Capacity
Control
 Bypass control uses a relief-type valve to
exhaust 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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Compressor-Capacity
Control
 Start-stop capacity control is commonly used
with small, electric motor-driven compressor
packages that operate pneumatic systems
consuming air on an intermittent basis
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Compressor-Capacity
Control
 Start-stop control uses
a pressure-sensitive
switch to start and stop
the compressor to
maintain a preselected
pressure range
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Compressor-Capacity
Control
 Start-stop control: compressor start
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Compressor-Capacity
Control
 Start-stop control: compressor stop
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Compressor-Capacity
Control
 Inlet valve unloading controls compressor
output 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
engines or electric motors as the prime mover
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Compressor-Capacity
Control
 Varying compressor speed can control
compressor capacity
– Can be used with reciprocating and rotary
compressor designs
– Primarily used on large, industrial installations
– Sensors monitor pressure and send a signal to
control compressor speed
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Compressor-Capacity
Control
 Varying the size of the compressor inlet can
control 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
– Primarily used on dynamic compressors
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Selecting a Compressor
Package
 Establishing the level of system air
consumption 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
– Percentage of time each functions
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Selecting a Compressor
Package
 Other factors must be considered during system
compressor 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 Question
Compressed-air units may be classified as a(n)
_____ or _____.
portable unit; central air supply
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Review Question
List the components found in a typical
compressed-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
maximum 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 Question
The simplest compressor in both design and
operating theory is the single-acting, _____
compressor.
reciprocating
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Review Question
Dynamic compressors can also be classified
as _____-displacement compressors.
non-positive
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Review Question
The continuous rotating motion of the
compression elements identifies a(n) _____
compressor design.
rotary
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Review 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
either side of the piston allow compression and
intake during each piston stroke.
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Review Question
In a two-stage, reciprocating compressor, the
outlet port of the first compression chamber is
connected to the _____ port of a second
compression chamber.
inlet
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Review Question
Compressor-air output and system-air demand
are matched by using some type of _____
system.
compressor-capacity control
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Review Question
Name four factors that make selecting a
compressor difficult.
A. The variety of compressor designs, B. load
variations in a pneumatic system, C. the variety of
auxiliary equipment available, and D. demands of
future growth of the system.
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