Chapter 15

Chapter 15Chapter 15

Source of Pneumatic Source of Pneumatic PowerPower

Compressed-Air Unit and Compressed-Air Unit and CompressorCompressor

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

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. Compare the operating characteristics of

positive- and non-positive-displacement air compressors.



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.



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.


Compressed-Air UnitCompressed-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



Very small packages may produce only a fraction of a cubic foot of air per minute (cfm)

DeVilbiss Air Power Company



Large, industrial units may produce thousands of cfm

Badger Iron Works, Inc.



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



A portable unit may be large or small



Portable units allow the compressor to be moved to the work site

Atlas Copco



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



Belt coupling




Mechanical coupling



Basic Compressor Basic Compressor DesignDesign

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


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

Non-positive-displacement compressors use air velocity to increase pressure


Basic Compressor Basic Compressor ClassificationsClassifications

A reciprocating compressor has a positive displacement



Compressor Design and Compressor Design and OperationOperation

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



Large, industrial, reciprocating compressor

Atlas Copco



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



Inline reciprocating compressor




V-type reciprocating compressor




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



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 intermeshing, helical screws

Atlas Copco



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



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



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



Impeller assembly of a 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



Axial-flow dynamic compressor



Pressure is created when high-speed air is slowed by the fixed stator blades



Dynamic compressor designs are used to compress air and other gases for large, industrial applications– Oil refineries– Chemical plants– Steel mills



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

Atlas Copco



Lobe-type compressors are often called blowers They are typically used in applications

requiring air pressure of only 10 to 20 psi



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



Inline, staged, reciprocating compressor



Compressor-Capacity Compressor-Capacity ControlControl

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



Start-stop control: compressor start



Start-stop control: compressor stop



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



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



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


Selecting a Compressor Selecting a Compressor PackagePackage

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


Selecting a Compressor Selecting a Compressor PackagePackage

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


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 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.



The simplest compressor in both design and operating theory is the single-acting, _____ compressor.

reciprocating



Dynamic compressors can also be classified as _____-displacement compressors.

non-positive



The continuous rotating motion of the 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 either side of the piston allow compression and intake during each piston stroke.



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



Compressor-air output and system-air demand are matched by using some type of _____ system.

compressor-capacity control



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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Chapter 15