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