Chapter 15
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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.

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Chapter 15

Chapter 15

Source of Pneumatic Power

Compressed-Air Unit and Compressor


Objectives

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.

Permission granted to reproduce for educational use only.


Objectives1

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.

Permission granted to reproduce for educational use only.


Objectives2

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.

Permission granted to reproduce for educational use only.


Compressed air unit

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

Permission granted to reproduce for educational use only.


Compressed air unit1

Compressed-Air Unit

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

DeVilbiss Air Power Company

Permission granted to reproduce for educational use only.


Compressed air unit2

Compressed-Air Unit

  • Large, industrial units may produce thousands of cfm

Badger Iron Works, Inc.

Permission granted to reproduce for educational use only.


Compressed air unit3

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

Permission granted to reproduce for educational use only.


Compressed air unit4

Compressed-Air Unit

  • A portable unit may be large or small

Permission granted to reproduce for educational use only.


Compressed air unit5

Compressed-Air Unit

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

Atlas Copco

Permission granted to reproduce for educational use only.


Compressed air unit6

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 dryer

Permission granted to reproduce for educational use only.


Compressed air unit7

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

Permission granted to reproduce for educational use only.


Compressed air unit8

Compressed-Air Unit

  • Belt coupling

DeVilbiss Air Power Company

Permission granted to reproduce for educational use only.


Compressed air unit9

Compressed-Air Unit

  • Mechanical coupling

DeVilbiss Air Power Company

Permission granted to reproduce for educational use only.


Basic compressor design

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

Permission granted to reproduce for educational use only.


Basic compressor design1

Basic Compressor Design

  • Reciprocating-piston compressors are the most common

  • Rotary screw compressors are popular in new installations

Permission granted to reproduce for educational use only.


Basic compressor design2

Basic Compressor Design

  • Inline, reciprocating compressor

DeVilbiss Air Power Company

Permission granted to reproduce for educational use only.


Basic compressor design3

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

Permission granted to reproduce for educational use only.


Basic compressor classifications

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

Permission granted to reproduce for educational use only.


Basic compressor classifications1

A reciprocating compressor has a positive displacement

Basic Compressor Classifications

DeVilbiss Air Power Company

Permission granted to reproduce for educational use only.


Compressor design and operation

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

Permission granted to reproduce for educational use only.


Compressor design and operation1

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

Permission granted to reproduce for educational use only.


Compressor design and operation2

Compressor Design and Operation

  • Large, industrial, reciprocating compressor

Atlas Copco

Permission granted to reproduce for educational use only.


Compressor design and operation3

Compressor Design and Operation

  • 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

Permission granted to reproduce for educational use only.


Compressor design and operation4

Inline reciprocating compressor

Compressor Design and Operation

DeVilbiss Air Power Company

Permission granted to reproduce for educational use only.


Compressor design and operation5

Compressor Design and Operation

  • V-type reciprocating compressor

DeVilbiss Air Power Company

Permission granted to reproduce for educational use only.


Compressor design and operation6

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

Permission granted to reproduce for educational use only.


Compressor design and operation7

Double-acting compressor

Compressor Design and Operation

Permission granted to reproduce for educational use only.


Compressor design and operation8

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

Permission granted to reproduce for educational use only.


Compressor design and operation9

Compressor Design and Operation

  • Centrifugal force keeps the vanes in contact with the walls

Permission granted to reproduce for educational use only.


Compressor design and operation10

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

Permission granted to reproduce for educational use only.


Compressor design and operation11

Compressor Design and Operation

  • Rotary screw compressors have intermeshing, helical screws

Atlas Copco

Permission granted to reproduce for educational use only.


Compressor design and operation12

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

Permission granted to reproduce for educational use only.


Compressor design and operation13

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 near-oilless compressed air for the pneumatic system

Permission granted to reproduce for educational use only.


Compressor design and operation14

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

Permission granted to reproduce for educational use only.


Compressor design and operation15

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

Permission granted to reproduce for educational use only.


Compressor design and operation16

Centrifugal dynamic compressor

Compressor Design and Operation

Permission granted to reproduce for educational use only.


Compressor design and operation17

Impeller assembly of a centrifugal dynamic compressor

Compressor Design and Operation

Permission granted to reproduce for educational use only.


Compressor design and operation18

Compressor Design and Operation

  • 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

Permission granted to reproduce for educational use only.


Compressor design and operation19

Compressor Design and Operation

  • Axial-flow dynamic compressor

Permission granted to reproduce for educational use only.


Compressor design and operation20

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

Compressor Design and Operation

Permission granted to reproduce for educational use only.


Compressor design and operation21

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

Permission granted to reproduce for educational use only.


Compressor design and operation22

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

Permission granted to reproduce for educational use only.


Compressor design and operation23

Compressor Design and Operation

  • Impellers from a lobe-type compressor

Atlas Copco

Permission granted to reproduce for educational use only.


Compressor design and operation24

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

Permission granted to reproduce for educational use only.


Compressor design and operation25

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

Permission granted to reproduce for educational use only.


Compressor design and operation26

Compressor Design and Operation

  • Inline, staged, reciprocating compressor

DeVilbiss Air Power Company

Permission granted to reproduce for educational use only.


Compressor capacity control

Compressor-Capacity Control

  • Compressor-capacity controlrefers 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

Permission granted to reproduce for educational use only.


Compressor capacity control1

Compressor-Capacity Control

  • Compressor-capacity control systems include:

    • Bypass

    • Start-stop

    • Inlet valve unloading

    • Speed variation

    • Inlet size variation

Permission granted to reproduce for educational use only.


Compressor capacity control2

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

Permission granted to reproduce for educational use only.


Compressor capacity control3

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

Permission granted to reproduce for educational use only.


Compressor capacity control4

Start-stop control uses a pressure-sensitive switch to start and stop the compressor to maintain a preselected pressure range

Compressor-Capacity Control

Permission granted to reproduce for educational use only.


Compressor capacity control5

Compressor-Capacity Control

  • Start-stop control: compressor start

Permission granted to reproduce for educational use only.


Compressor capacity control6

Compressor-Capacity Control

  • Start-stop control: compressor stop

Permission granted to reproduce for educational use only.


Compressor capacity control7

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

Permission granted to reproduce for educational use only.


Compressor capacity control8

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

Permission granted to reproduce for educational use only.


Compressor capacity control9

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

Permission granted to reproduce for educational use only.


Selecting a compressor package

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

Permission granted to reproduce for educational use only.


Selecting a compressor package1

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

Permission granted to reproduce for educational use only.


Review question

Review Question

Compressed-air units may be classified as a(n) _____ or _____.

portable unit; central air supply

Permission granted to reproduce for educational use only.


Review question1

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.

Permission granted to reproduce for educational use only.


Review question2

Review Question

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

reciprocating

Permission granted to reproduce for educational use only.


Review question3

Review Question

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

non-positive

Permission granted to reproduce for educational use only.


Review question4

Review Question

The continuous rotating motion of the compression elements identifies a(n) _____ compressor design.

rotary

Permission granted to reproduce for educational use only.


Review question5

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.

Permission granted to reproduce for educational use only.


Review question6

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

Permission granted to reproduce for educational use only.


Review question7

Review Question

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

compressor-capacity control

Permission granted to reproduce for educational use only.


Review question8

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.

Permission granted to reproduce for educational use only.


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