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Fluid Power
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Fluid Power Systems (ME353)
Fall 2012
Lecture 11
Compressed Air
The Energy Transmitting Medium
Basic Source of System Air
The source of air used in pneumatic systems is the atmosphere
The gases in atmospheric air are:
– Nitrogen (79%)
– Oxygen (20%)
– Other gases (1%)
In addition to gases, the atmosphere contains water vapor and entrapped dirt
Both of these influence air compression and the final quality of the system air
Atmospheric pressure varies by elevation
Pneumatic System Compressed Air
Atmospheric air is typically referred to as free air
Free air must be conditioned before it can be used in a pneumatic system
Certain locations require considerable preparation of free air to make it
usable in a pneumatic system
Free air at construction sites often
requires extra filtration
The conditioning of compressed air for use in pneumatic systems involves:
– Removal of entrapped dirt
– Removal of water vapor
– Removal of heat
– Incorporation of lubricants
The amount of water vapor air can hold depends
on the temperature of the air
– The higher the temperature, the greater the
amount of water that can be retained by the
air
– Saturation is reached when air holds the
maximum amount of water for the given
temperature
Water legs are used to collect and
remove liquid water from
pneumatic lines
Relative humidity expresses the percentage of water in the air compared to
the maximum amount that can be held at the specified temperature
Dew point is the temperature at which water vapor in the saturated air begins
to be released in liquid form
At the dew point, any increase in humidity is released as liquid water, as on a
fogged mirror
A lubricant is added to dry compressed air distributed by the pneumatic
system workstation
This is for protection of system components
A lubricator for a pneumatic workstation
Compression and Expansion of Air
In an operating pneumatic system, the continuous interaction of temperature,
pressure, and volume changes make calculations complex
Two compression models are used to express air compression
– Isothermal compression
– Adiabatic compression
These models are used for expansion as well
Isothermal compression assumes
that all heat is removed, resulting in
a constant temperature
Adiabatic compression assumes all
heat is retained, resulting in both
increased temperature and pressure
Actual compression is somewhere
between isothermal and adiabatic
compression
When air is compressed, there are changes in temperature, pressure, and
volume that follow the relationships expressed by the general gas law
– (P1 V1) T1 = (P2 V2) T2
– Specific system pressure, temperature, and volume changes may be
difficult to verify
Any change in air pressure results
in temperature or volume changes
Changes in the volume of air
result in pressure or temperature
changes
Increases or decreases in air
temperature result in pressure or
volume changes
Source of Pneumatic Power
Compressed-Air Unit and Compressor
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
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
Portable units allow the compressor to be moved to the work site
A compressed-air unit consists of:
– Prime mover
– Compressor
– Coupling
– Receiver
– Capacity-limiting system
– Safety valve
– Air filter
– May have a cooler and dryer
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
Basic Compressor Design
A variety of designs are used for air compressors in the compressed-air unit
– Reciprocating piston
– Rotary, sliding vane
– Rotary screw
– Dynamic
Reciprocating-piston compressors
are the most common
Rotary screw compressors are
popular in new installations
Inline, reciprocating compressor
The basic operation of any compressor includes three phases
– Air intake
– Air compression
– Air discharge
Component parts and physical operation varies between compressor designs
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
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
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
Reciprocating compressors are available in single- or multiple-cylinder designs
Multiple cylinders may be arranged as:
– Inline
– Opposed
– V type
– W type
– Other cylinder configuration
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
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
Centrifugal force
keeps the vanes in
contact with the
walls
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
Rotary screw compressors have
become popular for larger
industrial installations
– Lower initial cost
– Lower maintenance cost
– Adaptable to sophisticated
electronic control systems
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 near-oilless compressed air for the
pneumatic system
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
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
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
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
Impellers from a lobe-type compressor
Lobe-type compressors are often called blowers
They are typically used in applications requiring air pressure of only 10 to 20
psi
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
Compressor-capacity control systems include:
– Bypass
– Start-stop
– Inlet valve unloading
– Speed variation
– Inlet size variation
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
Start-stop capacity control is commonly used with small,
electric motor-driven compressor packages that operate
pneumatic systems consuming air on an intermittent basis
Start-stop control uses a
pressure-sensitive switch to
start and stop the compressor to
maintain a preselected pressure
range
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