HYDRAULICS & PNEUMATICS Presented by: Dr. Abootorabi Compressed Air Source of Pneumatic Power 1

HYDRAULICS & PNEUMATICS

Presented by: Dr. Abootorabi

Compressed AirSource of Pneumatic

Power

1

Basic Source of System Air

The source of air used in pneumatic systems is the

atmosphere.

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The gases in atmospheric air are:

Nitrogen (79%)

Oxygen (20%)

Other gases (1%)


Composition of atmospheric air

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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.

4

The weight of the gases in the atmosphere exerts

pressure.

Atmospheric pressure is 14.7 pounds per square inch at

sea level.


Atmospheric pressure varies by elevation

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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.6


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

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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

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Water legs are used to collect

and remove liquid water from

pneumatic lines.

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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.

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Dry compressed air contains water vapor, but the relative

humidity is sufficiently low to prevent the formation of liquid

water at the ambient temperature of the workstation.

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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:

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Compression and Expansion of Air

In an operating pneumatic system, the continuous

interaction of temperature, pressure, and volume

changes make calculations complex.

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Engineering data are available from component

manufacturers and data handbooks that can be used to

estimate performance from compressors and other

system components.

Reaction of Air to Temperature, Pressure, and Volume

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

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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).

1 ft3 ≈ 0.028 m3

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Compressed-Air Unit

Large, industrial units may produce thousands of cfm.

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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 supplies18

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.

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Compressed-Air Unit

A compressed-air unit consists of:

Prime mover

Compressor

Coupling

Receiver

Capacity-limiting system

Safety valve

Air filter

May have a cooler and dryer21

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:

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DeVilbiss Air Power Company

Compressed-Air Unit

Mechanical coupling:

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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

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Reciprocating-piston compressors are the most common.

Rotary screw compressors are popular in new installations.

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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.

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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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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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Large, industrial, reciprocating compressor:

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Atlas Copco


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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


Double-acting compressor

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Multiple cylinders may be arranged as:

Inline

Opposed

V type

W type

Other cylinder configuration

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Inline reciprocating

compressor:

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V-type reciprocating compressor:

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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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Centrifugal force keeps the vanes in contact with the walls

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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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Rotary screw compressors have intermeshing, helical screws:

41


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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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.43


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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Impellers from a lobe-type compressor.

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Lobe-type compressors are often called blowers.

They are typically used in applications requiring air

pressure of only 10 to 20 psi.

46


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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Centrifugal dynamic compressor:

An impeller increases airspeed

Prime mover energy is converted into kinetic energy

as air speed rapidly increases through the impeller

Kinetic energy is converted to air pressure as air

movement slows in the volute collector

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Centrifugal dynamic

compressor:

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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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Pressure is created when

high-speed air is slowed

by the fixed stator blades.

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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 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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Inline, staged, reciprocating compressor:

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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 systems include:

Bypass

Start-stop

Inlet valve unloading

Speed variation

Inlet size variation

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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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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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Start-stop control uses a

pressure-sensitive switch

to start and stop the

compressor to maintain a

preselected pressure

range.

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Start-stop control: compressor start

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Start-stop control: compressor stop

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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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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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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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The end.

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Documents

HYDRAULICS & PNEUMATICS Presented by: Dr. Abootorabi Compressed Air Source of Pneumatic Power 1