Control Systems
`AND
`pplications
`ror AVAC/R
`
`Thomas Horan
`
`
`
`ECOBEE 1007
`
`001
`
`

`

`Control Systems and
`Applicationsfor
`HVAC/R
`
`002
`
`

`

`
`
`Control Systems and
`Applications for
`HVAC/R
`
`Thomas J. Horan
`
`PRENTICE HALL
`
`Upper Saddle River, NewJersey
`
`Colttimbus, Olio
`
`003
`
`

`

`Library of Congress Cataloging-in-Publication Data
`Horan, Thomas J.
`Control systems and applications for WVAC/R/ ThomasJ. Horan,
`. cam,
`nehies index.
`ISBN 0-13-125196-1
`
`1. Commercial buildings—Heating and ventilation—Control.
`2. Commercial buildings—Air conditioning-—Control. 3, Automatic
`1. Title.
`TH7392.C65H67=1997
`
`Editor: Ed Francis
`Production Editor: Christine M. Harrington
`Production Coordinator: Ruttle Graphics, Inc.
`Cover Designer: Proof Positive /Farrowlyne Assoc,, Ine,
`Text Designer: Ruttle Graphics, ine.
`Production Manager: Deidra M, Schwartz
`Marketing Manager: Danny Hoyt
`
`This book was set in Palatino and was printed and bound by Quebecor
`Printing/ Book Press. The cover was printed by Phoenix Color Corp.
`
`©1997 by Prentice-Hall, Inc,
`Upper Saddle River, New Jersey 07458
`
`All rights reserved. No part of this book may be reproduced, in any form or by any means, without
`permission in writing from the publisher.
`
`Printed in the Linited States of America
`
`Web 7FOS432
`
`ISBN: 0-13-085179-5
`
`PRENTICE-HALL INTERNATIONAL (UK) Limirep, LONDON
`PRENTICE-HALL OF AUSTRALIA Pry. LIMITED, SYDNEY
`PRENTICE-HALL CANADA INC, TORONTO
`PRENTICE-HALL HISPANGAMERICANA, S.A,, Mextoo
`Prennce-HAl or lwoia Penare Linrren, New DeLut
`PRENTICE-HALL OF JAPAN, Inc,, Tokyo
`PEARSON EDUCATION ASIA Pre. LTb., SINGAPORE
`EDITORA PRENTICE-HALL DO BRASIL, LTDA, RIO DE JANEIRO
`
`To my wife Kathy,
`my best friend whose love
`brings immeasurable joy and
`contentment into mylife.
`
`004
`
`

`

`vit
`Contents
`
`Contents
`
`Introduction to Control Theory
`
`1
`
`Introduction to Control Systems
`4
`1]
`What is the Meaning of Control?
`V2
`What is the Purpose of a Control System?
`la
`Control 5ystems Classifications
`12
`1
`Summary
`1
`1.5
`Exercises
`l7
`
`3
`
`7
`
`19
`Control System Terminology
`2,1
`Air Handling System Terminology
`22
`Control Loop terminology
`2h
`ee
`Surenvary
`aa
`24
`Exercises
`44
`
`2p).
`
`Basic Control Loop Components
`4.1
`Control Loops
`36
`3,2
`Controllers
`39
`23
`Final Controlled Devices
`1.4
`The Contral Agent
`44
`45
`Basic Control Loop Construction
`3.6
`Summary
`45
`47
`Exercises
`46
`
`40.
`
`35
`
`4s
`
`48
`Control Loop Configurations
`4,]
`Open anc Closed Loop Configurations
`4.0
`Closed Loop Configurations
`Suh
`43
`Transfer Functions
`54
`44
`Block Diagrams
`AG
`4.5
`Summary
`Gl
`4b
`Exercises
`6]
`
`49
`
`Part 2
`
`Chapter 7
`
`Chapter 8
`
`Chapter 9
`
`Chapter 10
`
`63
`
`04
`
`69
`
`
`
`Operational Details of Control Devices
`Sensors
`105
`71
`Measuring the Control Paint
`72
`Sensor Operating Characteristics
`Pod
`Types of HVAC Sensers
`W2
`74
`Locating Sensors
`120
`7 Summary
`122
`7.
`Exercises
`122
`
`106
`
`105
`
`103
`
`124
`Controllers
`a1
`Operational Characteristics of Controllers
`4.2
`Controller Modes
`126
`a3
`Timed Two-Position Conteller Made
`64
`Floating Controller Mode
`144
`6.5
`138
`Proportional Controller Mode
`8.6
`Proportional + Integral Controller Mode (PI)
`rs
`142
`Proportional + Integral + Derivative Mode (PID)
`O68
`Analyzing the Response of Modulating Controller Modes
`69
`Summary
`147
`a.10
`Exercises
`146
`
`125
`
`131
`
`140
`
`143
`
`151
`
`Modulating Final Controlled Devices
`a1
`Valves and Dampers
`152
`9.2
`Valve Construction
`152
`O38
`How Fluids React to Changes in the Flow Area of the Valve Port
`94
`Valve Flow Characteristics
`16]
`O85
`Valve Flow Terminology
`165
`9.6
`Damper Characteristics
`168
`a7
`Summary
`71
`9.6
`Exercises
`172
`
`158
`
`Sizing and Selecting Control Valves and Dampers
`10.1
`Modulating Flow Characteristics of Valves
`10.2
`Flow Coefficient Sizing Formulas
`V7?
`1.3
`Calculating Required Flowrate for a Process
`10.4
`Sizing Valves for Steam Flow
`182
`10.5
`185
`Calculating the Process Steam Load
`10.6
`186
`Other Valve Selection Considerations
`10.7
`Selecting a Valve Using a Manufacturer's Catalog Table
`10.8
`DamperSizing Procedures
`190
`1.9
`Actuator Selection
`193
`TOTO
`Exercises
`195
`
`173
`
`\74
`
`179
`
`188
`
`Elements of a Transfer Function
`ba
`531
`Conttol Device Tansler Functions
`5a
`The Proportional Gain of a Transier Function
`7
`54
`The Phase Flement of a Transfer Function
`54
`The Timing Element of a Controller Transfer Function
`5,5
`Summary
`74
`
`5.6—Exercises 75 Chapter 11
`Selecting a Controller Mode based on the Process
`Characteristics
`197
`V1.1
`Thermal Process Charactéristies
`W2
`Process Timing Characteristics
`14
`Time Constant
`205
`11.4
`Determining a Process Controller Mode Based on the Process
`Characteristics
`206
`Summary
`Exercises
`
`a7
`Applying Transfer Functions
`6.1
`74
`Linear Control Systems
`62
`The General Form af the Linear Transter Function Equation
`64
`Developing the Transfer Function for a Sensar
`#1
`bat
`The Controller Transfer Function
`oF
`6.45
`Transfer Functions for Final Controlled Devices
`56
`Ob
`Summary
`a8
`67
`Exercises
`bt)
`
`7
`
`198
`202
`
`212
`ay
`
`005
`
`

`

`
`
` Vili Contents
`
`
`
` Contents ix
`
`219
`
`ay
`
`239
`
`254
`
`260
`264
`
`215
`
`Chapter 18
`
`47
`
`439
`343
`444
`
`444
`
`Chapter 19
`
`353
`DuctStatic Pressure Control
`19.1
`Fan Pressure
`453
`458
`19.2
`Static Pressure Sensors
`14.3
`Ilustration of a Supply Air Static Pressure Control Strategy
`19,4
`Illustration of a Dual Fan Static Pressure Control Strategy
`19.5
`Exercises
`36
`
`359
`$63
`
`The Calibration and Response of Modulating Control Loops
`337
`Humidity Control Strategies
`18.1
`Overview of Humiclifications Strategies
`12.1
`Calibrating an Open Loop Controller
`216
`18.2 Wustration of a Humidilying Control Strategy
`12.2 Operational Objectives for a Closed Loop Response
`18.3
`Overview of the Dehumidification Processes
`12.3
`The Five Possible Loop Responses
`226
`18.4
`[Ilusteation of a Dehumidifying Control Strategy
`12.4
`Criteria for Evaluating Control Loop Responses
`18.5 Maintaining a Constant Humidity Set Point
`12.5
`Stinamary
`234
`18.6
`Chemical Dehumidification
`350
`12.6
`Exercises
`236
`
`18.7—Exercises 35]
`Microprocessor-Based Control Systems
`238
`13,1) What are Micropracessor-Based Control Syslemsé
`132)
`DDC Hardware Points
`246
`13.3
`Electrical Magnetic Interference
`13.4
`DDC Programs
`256
`13.5
`Energy Management Strategies
`13.6
`Software Analysis Functions
`13.7
`Summary
`264
`13.8
`Exercises
`265
`
`Chapter 20
`
`370
`Zone Terminal Device Control
`370
`20.1
`Single Duct Variable Air Volume Boxes
`
`20.2—|Nlustration of a Cooling Only VAV Terminal Linit 374
`HVACApplications
`267
`20.3 Whustration of a VAV Terminal Unit with a Reheat Coil
`376
`204
`Fan Powered VAV Terminal Units
`479
`20.5
` (lustration of an Intermittent Fan Powered
`Documenting Control Processes
`VAV Terminal Unit with a Reheat Coil
`14.1
`System Drawings
`270
`14.2
`Sequence of Operation
`Dual Duct Boxes
`382
`20.6
`14.3
`Flowcharts
`279
`Variable Volume Dual Duct Terminal Unit
`20.7
`14.4
`Summary
`286
`Constant Volume Terminal Units
`38h
`20.8
`14.5
`Exercises
`286
`Fan Coil Units
`386
`20.9
`20.10 Unit Ventilators
`389
`20.11 Mlustration of an ASHRAE Cycle 1
`Unit Ventilator Control Strategy
`20,12 Illustration of an ASHRAE Cycle 2
`392
`Unit Ventilater Control Strategy
`20,13 ASHRAE Cycle 3 Unit Ventilator Control Strategy
`20,14 Exercises
`395
`
`269
`
`272
`
`286
`Mixing Damper ControlStrategies
`288
`15.1 Mixed Air Damper Strategies
`290
`15.20) Mixing Damper Minimum Position
`294
`15.3) Mixing Damper Economizer Strategy
`294
`15.4
`Commanding Dampers to their Normal Position
`15.5
` Enthalpy-Based Economizer Loop Design and Operation
`15.6
`Summary
`4035
`15.7
`Exercises
`304
`
`294
`
`380
`
`384
`
`390
`
`395
`
`397
`Application Response and Analysis
`307
`Heating Control Strategies
`399
`Single Zone Air Handling Unit Configurations
`16.1
`Overview of the Different Heating Control Strategies
`400
`21.1
`Single Zone, Constant Volume Air Handling Units
`16.2
`Preheat Control Strategies
`308
`21.2
`Ilustration of a Single Zone Constant Volume Dual Fan AHU with
`311
`16,3
`Hlustration of a Steam Preheat Control Strategy
`Humidification
`402
`16.4—Illustration of a Hot Water Preheat Control Strategy 16
`
`21.3
`Process Responses to Load Changes
`406
`16.5
`Illustration of a Hot Water Heating Control Strategy
`149
`21.4
`Troubleshooting Systems
`408
`[6.6
`Zone Heating Applications
`323
`lo?
`Ewercises
`323
`21.5
`Analysis of a Single Zone, Constant Volume Air Handling Unit
`21.6
`Exercises
`411
`
`407
`
`Part 4
`Chapter 21
`
`404
`
`325
`Cooling Control Strategies
`325
`V7.1
`Overview of the Different Cooling Strategies
`527
`17.2
`Illustration of a Modulating Chilled Water Contral Strategy
`17.4 Mlustration of a Discharge Air Temperature Control Strategy Incarporat-
`ing Sequenced Hat and Chilled Water Valve Actuator Springs
`332
`Zone Cooling Applications Overview
`334
`Exercises
`345
`
`17.4
`17.5
`
`Chapter 22
`
`Single Path, Constant Volume, Muliple Zone Air
`Handling Units with Reheat Coils
`ais
`22.1
`Single Path Air Handling Units with Reheat Coils -
`Multiple Zone Applications
`a3
`Control Strategy Differences Between Single Path,
`Single Zone Air Handling Units andSingle Path,
`Multiple Zane Air Handling Unit Applications
`
`22.2
`
`aid
`
`006
`
`

`

`
`
`ha Pa ie
`
`2.
`2:
`
`on
`Nh
`22.6
`
`IHustration of a Single Path, Constant Volume, Multiple Zone AHU with
`418
`Reheat
`422
`Process Responses to Load Changes
`Analysis of a Single Path, Multiple Zone Air Handling Unit with Zone
`424
`Reheat
`428
`Exercises
`
`Single Path, Variable Volume, Multiple Zone Air Handling Units
`430
`with Reheal Coils
`23.)
`430
`Variable Air Volume Systerns
`23,2
`Illustration af a Single Path, Variable Volume,
`432
`Multiple Zone AHU with Reheat
`4356
`Process Responses to Load Changes
`Analysis of a Single Path, Variable Volume Air Handling Linit
`440
`with Reheat
`Exercises
`444
`
`Dual Path, Constant Volume Air Handling Units
`24.1
`4as
`Dual Path Air Plandling Units
`24.2
`446
`Illustration of a Mullizone AHL
`453
`24.3
`Process Responses to Load Changes
`M44
`Analysis of a Multizane Air Handling Unit
`24.5
`459
`Exercises
`
`456
`
`445
`
`43
`
`461
`Make-Up Air Handling Units
`461
`25.1
`Make-up Air Handling Units
`25.2
`IIlustration of a Constant Volume, Make-up Air Unit
`466
`Process Responses to Load Changes
`460
`Analysis of a Make-up Air Unit
`Exercises
`470
`
`5.4
`5.5
`
`Pohope 53
`
`Steam to Hot Water Converters
`26.1
`Steam Converters
`474
`26,2
`(lustration of a Hot Water Converter
`26.3
`475
`Process Responses to Load Changes
`26.4
`Analysis.of a Steam to Hot Water Converter
`26,5
`Exercises
`478
`
`471
`
`476
`
`479
`
`Fume Hood Control Systems
`avid
`Fume Honel Systems
`27.2
`Ilustration of a Fume Hood Control System
`487
`27.3
`Process Responses to Load Changes
`274
`Analysis of a Fume Hood Control System
`275
`493
`Exercises
`
`485
`
`497
`
`495
`
`505
`
`
`
`Preface
`
`Of all the mechanical and design tasks performed by HVAC technicians, tech-
`nologists, and engineers, those dealing with control system design, calibration,
`and analysis tend to be the least understood and mostdifficult to master. To work
`effectively with control systems, a person must become fluent in a variety ofdisci-
`plines that go beyond the boundaries found in a typical HVACprogramcurricu-
`lum. Today’s HVAC technician must be conversant
`in mechanical, electrical,
`electronic, computer, and thermal technologies. This requirement makes the con-
`trol field one of the more difficult occupations in which to excel. These require-
`ments also make control system design, installation, and analysis one ofthe more
`challenging, exciting, and rewarding Aelds in which a mechanically inclined per-
`son can specialize.
`The first control devices were mechanical in nature, easily understood by per-
`sons closely related to the HVAC/R field, Electromechanical relays, contactors,
`thermostats, and a variety of other components were easily calibrated and ana-
`lyzed because their response or reaction to stimuli were easily seen. Modulating
`pneumatic devices used to control the flow of mass and energy wereslightly more
`challenging to understand because their internal components consisted of small
`obscure springs, bleed orifices, diaphragms, levers, tubes, and adjustable dials,
`Although more complex, they were still based on mechanical force and balance
`principles that could be intuitively analyzed by the mechanically minded person.
`Once electronic components evolved into a cost-effective alternative to pneu-
`matic components, the age of “black box” control systems began. The devices
`operated on low voltage and current signals generated in tiny plastic coated
`deviees withlittle wires protruding from their ends. There was no easy way intu-
`itively to reference these devices back to their mechanical equivalent. An elec-
`tronic control device could be wpened up on the bench, its schematic reviewed,
`and the technician could walk away from it without knowing howit performedits
`function, Control companies were required to hire people with electronies back-
`grounds to design control systems for mechanical equipment and thermal
`processes for which they had little training. Mechanically based control techm-
`cians began to back away from the electronic systems, leaving them to the few
`technicians that had a desire to work with the newtechnology.
`
`xi
`
`007
`
`

`

`Preface
`
`rT
`
`Preface
`
`xii
`
`of its operation. Part Four also lists troubleshooting techniques and solutions to
`common problems in HVACprocesses.
`The text can be used along with a control vendor’s parts catalog to instruct new
`students on the different options available fora given application and how to look
`up items in a catalog. Laboratory assignments can be developed that analyze
`actual operating systemsinstalled in the school's buildings. The catalog and asso-
`ciated specification sheets can be used to give specific operating parameters of the
`componentsin the system and their calibration procedures.
`The book is based upon howand why a process responds in a particular way
`and how to adjust the parameters of the loop to improveits response.lf 1s applic-
`able to electrornechanical, pneumatic, analog electronic, and digital electronic con-
`trol systems because ofits focus on the system’s response, not its construction, In
`any control system, the adjustment of a particular control loop parameter will
`zenerate the same response.It is my hope that the booksets a firm foundation of
`the basics of controls so the reader may develop into a control system specialist—
`able to design, specify, and calibrate control systems.
`
`Current digital control systems have brought another technology discipline—
`computer science—into the FTVAC control field. These new systems require com-
`puter hardware technicians to design and diagnose solid state circuit boards.
`Computer programmers are needed to develop the programming code that oper-
`ates the control system hardware and its communication networks. Additional
`software must be written to operate the connected mechanical and electrical
`equipment that maintains a comfortable environment in the building, Conse-
`quently, operational problems are no longer confined to hardware. A bug in the
`software can create an intermittent problem that will be very time consuming to
`find. This may force the building operatorto close the evolutionary path of control
`systems by operating the affected equipment by hand.
`Today's HVAC control systems requirement for electronic and computer tech-
`nology people to work on systems maintained by mechanically trained [TVAC
`technicians created a newrift in the HVAC field. Two differently tramed groups of
`people with different perceptions of the HVAC process must work together to
`maintain the operational efficiency of systems and buildings. Many IIVAC techni-
`cians shy away from control-related problems that incorporate computersin favor
`of calling a person trained in computer science to service the system, Unfortu-
`nately, that is usually not the best alternative. The HVAC technician knows the
`operation and responseof the building and its systemsbetter than an outside ser-
`vice person and, therefore, is better qualified to tie the control system into the
`mechanical systems. This is the driving force behind the development ofthis book.
`I wrote this book to compile control system information from the electronic,
`computer, mechanical, and thermal disciplines into one text, This book can be
`used by any systems-oriented person wha wants or needs to know how and why
`a process responds the way it does to changes in a measured variable. It intro-
`duces the reader to the fundamental characteristics of all control systems and
`processes using HVAC systemsas a basis for the applications and examples, The
`book is written in a manner that develops the reader's analytical skills in diagnos-
`ing and rectifying problems in any system.
`Part One of this book begins with the basics.It describes the purpose, terminol-
`ogy, and configurations of basic control loops. The section ends with a description
`and applications of the basic relationship of all control systems called the linear
`transfer function. The transfer function is the formula used by all technicians to
`calibrate devices from a simple sensor to complex PID controller modes,
`Part Two describes the individual components found in a control loop, their cal-
`ibration parameters, and selection criteria. It also describes the construction, oper-
`ation, and sizing procedures for valves and dampers. Other chapters describe how
`to select a controller mode based upon the characteristics of the process and how
`to calibrate the control loop to maximize efficiency. The last chapter gives a com~
`prehensive overviewof digital control systems and programming.
`Parts Three and Four of the text describe the control strategies most often used
`in commercial HVAC systems. Examples include a schematic representation of the
`process or system, a sequence of operation, programming flowchart, and analysis
`
`008
`
`

`

`Acknowledgments
`
`PART 1
`
`
`Introduction to
`Control Theory
`
`I take this opportunity to thank the many people who have made this textbook
`possible. My wife, Kathy, who supported me when I went back to college and
`encouraged meto complete this project although it took considerable time away
`frora our family; my children, Holly, Weather, Michelle, and Kelly, who couldn't
`understand what possessed their dad to devote countless hours in front of the
`computer; my in-laws, Hazel and Mike Santelli, along with Gram, who helped us
`out when | went back to college; and Frank and Gina, who have always made me
`feel special.
`If it weren't for the love of God and these people, [ would not have
`been able to write this book to share with others the enthusiasm that I experience
`tuning and troubleshooting control systems,
`| would also like to acknowledge those wha sparked my interest in control sys-
`tems: the faculty at Rochester Institute of Technology, in particular, Ron Amberger,
`who taught me how to analyze systems using the fundamental relationships that
`exist in science and technology; Robert Bateson, whose textbook sparked within
`me an interest in control systems that has developedinto a rewarding career; and
`Dick Shaw and L. J. Bishop who have given me the opportunities to apply the
`knowledge foundin this text.
`A special thanks to members of my family who have influenced the book
`through their enthusiastic interest in the project and my lite—my parents, Marge
`and Tom; George and Cindy; Kevin and Jennie; Tom and Sue; Steve and Debbie;
`Pat; Kathy; Matt; Jeff; Leah; Michael Jr; Megan; Christine; Ginnette; Brittany;
`Katie:
`the rest of our clan; and Michael, Jim, Susie, and Marie who are always
`remembered in our thoughts.
`Finally, thanks to my friends, especially Paul and Carol Lewis, Tom Page, Bob
`Lumsden, Ron Salik, Rob and Sue Hirsch, Bruce Weber, Richard Orlando, Larry
`and Denise Edmonson, and you,
`
`009
`
`

`

`CHAPTER1
`
`
`Introduction to
`Control Systems
`
`Working with control system technology is one of the most exciting and challeng-
`ing aspects of the heating, ventilating, and air-conditioning field. Mastering the
`abilities needed to operate and analyze mechanical and computer-based control
`systems offers dedicated technicians the greatest potential for recognition and
`success in their careers. The information presented in this textbook will assist the
`reader in developing the skills required to analyze, modify, and calibrate any type
`of HVAC control system,
`Before technicians can develop the skills needed to analyze the operation and
`response of a control system, they need to understand the meaning and purpose of
`a control system. Chapter | introduces the reader to the basic nature and purpose
`of any type of control system.
`
`iE
`
`OBJECTIVES
`
`Upon completion of this chapter, the reader will be able Lo:
`
`1. Define the term control as it relates to HVAC systems.
`2. Explain why control systems are required.
`3. Describe the characteristics of automatic control systems.
`.
`4. Describe the different types of control systems and their advantages.
`
`010
`
`

`

` 4 Chapter 1 Introduction to Control Systems 5
`
`
`
`
`
`
`
`WHATIS THE MEANING OF CONTROL?
`
`Control systems are found within all aspects of our environment. The human
`body and other organisms contained the first control systems that were used to
`maintain their core temperatures, rate of food metabolism, balance, and move-
`ment. Each of these life-sustaining processes is regulated by signals generated in
`the organism's brain based upon input cata from its environment.
`Control
`technologies were first developed during the industrial revolution
`when mechanical and electrical equipment incorporated systems to automatically
`regulate their operation. Automating the operation of equipment improved oper-
`ating, efficiencies and allowed for the manufacture of standardized parts, Any
`mechanical or electrical system that maintains a temperature, moisture content,
`position, level, size, quantity, velocity, intensity, or other characteristic of a process
`incorporates control devices that operate collectively to maintain the process.
`The first step in developing an understanding of the need for control systemsis
`to review what it means to control a piece of equipment. A good definition of con-
`tral that applies to heating, ventilating, and air-conditioning systems is “to apply
`a regulating influence upona device to make it performas required,” where the reg-
`lating influence is a force applied by a person, electric circuit, mechanical mecha-
`nism, or machine and the device is a mechanical or electric component (valve,
`damper, relay, etc.) that changes the amount of mass (quantity of water, air, fuel,
`material, etc.) or energy (electric current, heat, etc.) that is delivered into a process.
`Control systems vary the strength and direction of the regulating lorce applied to
`the control device to vary the mass or energy flowing into a process in response to
`changes in a measured condition. By varying the regulating force, the process is
`maintained at a value that falls within the permissible operating range that was
`established whenthe system was designed.
`Four examples of commonly used mechanical equipment that incorporate con-
`trol devices are a lawn mower,radio, toilet, and sink. These are all non-TTVAC sys-
`tems that are used by operators who have no understanding of control systems.
`The throttle lever on a lawn mower, volume control of the radio,float in the toilet
`tank, and the faucet valves on a sink represent control devices to which a regulat-
`ing force is applied to maintain oneparticular process condition. The lawn mower
`throttle lever allows the user to manually regulate the speed of (he cutting blade
`from zero revolutions per minute (rpm) to a maximumvalue of 3500 rpm. This
`maximum speed limit is determined by the engine's design and is based on the
`mechanical limitations of the engine and the operator's safely. The volume control
`ona radiv allowsthe listener to vary the sound intensity of the music from quiet
`to the maximumoutput capability of the unit's audio transformer, The float found
`inside of the toilet tank opens and closes the supply water valve to maintain the
`proper level within the tank. Finally, a sink's faucet valves allow the user to mod-
`ulate the flow of hot and cold water from 0%to 100% to provide the desired flaw
`rate or temperature of the water leaving the spout. Fach of these examples relies
`
`upon an input signal from the user toinitiate a change in the process. Control sys-
`tems that require a person to apply the regulating force to the control device are
`called munual control systems.
`Commercial and industrial HVACapplicationsuse electric and mechanical con-
`trol systems to position valves, dampers, and other mechanisms fo maintain the
`desired temperature, humidity level, and static pressure within a given area or
`yone. These systems regulate the amount of mass (water, air, fuel) or energy (heat,
`electricity) transferred between a source and the process to maintain a desired
`condition, A windowair conditioner uses a simple control system to maintain the
`temperature within a room at the desired level, The control system cycles the com-
`pressor Gn and off in response to changes in the room's temperature. When the
`temperature rises above a permissible level, the control system closes a relay that
`allows energy to flow into the compressor motor, allowing refrigerant to be circu-
`lated between the evaporator and condenser. This transfer of heal energy from the
`interior room to the outside prohibits the room temperature from rising above a
`user-defined comfortable level. As the compressor continues to operate, the room
`temperature decreases. As it drops belowa predetermined comfort level, the con-
`trol system stops the flow of energy into the compressor motor, stopping, the
`refrigerating, effect. These functions occurred without the need of a person being
`present to provide the force to close the control relay. Control systems that do not
`need humanintervention to maintain a process conditionare called artomatic con-
`trol systems. HVAC control systems can be designed to operate manually or auto-
`matically. The choice is based upon the equipment, process, and user
`requirements. Keep in mind, whether the system responds automatically or man-
`ually, its purpose—te apply a regulating influence upon a device to make it operate as
`required—remains the same-
`In addition to maintaining comfort and other process conditions, mechanical
`and electric systems incorporate control devices that ensure that the equipment
`operates safely. These safety control devices enable and disable (turn on or off)
`equipment based upon a monitored equipment or process condition. When the
`condition exceeds a safe operating limil, the control system automatically changes
`the operating state of the equipment to minimize safety hazards. The compressors
`used in the air-conditioning and refrigeration applications incorporate safety con-
`trols that will disable (turn off) the system whenever an abnormal pressure, tem-
`perature, or electric current flow exists. Safety-oriented control systems are used
`to automatically protect equipment, vecupants, and buildings fromfires or other
`hazards that may result from equipment operating outside the limits of their
`design parameters.
`The following applications describe how some typical HVAC control systems
`apply regulating forces upon devices to control mechanical equipment so it oper-
`ates in accordance withits intended design. Application 1,1 describes the response
`of a control system that maintains a specified static air pressure within an HVAC
`duct. Application 1.2 describes a contral system that maintains the temperature of
`the air leaving a hot water coil at a desired temperature. Each applicationis out-
`lined in three sections labeled Process, Objective, and Response. The Process section
`
`011
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`Chapter 1
`
`Introduction to Control Systems
`
`“I
`
`states the commonly used description of the condition that the control loop is
`required to maintain ala desired value. The Objective section describes the specific
`requirements of the process. The Response section details howthe controls operate
`to meet the process objective. This format is used throughout the text to introduce
`common HVAC processes and their typical response to changes in a measured
`variable.
`
`APPLICATION 1.1
`
`Process: Variable Air Volume SystemStatic Pressure Control Loop
`Variable Air Volume systems maintainthe static pressure ofthe air in the duct that
`supplies conditioned air into the zones within a building.
`
`Objective: Maintainthestatic pressure in the supply duct at 1.2 inches of water
`gauged by varying the amount of air (mass) entering the fan blades through the
`fan's inlet vane dampers.
`
`Response: Whenthe amountofair entering the system fanis less than the quan-
`tity of air leaving the diffusers in the zones, the static pressure within the ductwill
`decrease. When more air enters the fan than leaves the diffusers, the duct pressure
`increases.
`
`The control system measures the static pressure within the duct. As the pressure
`decreases, the control system opens the fan inlet vane dampers toincreasethe air
`flow into the duct and, consequently, increases the static pressure. The signal to
`open the dampers comes from a control device called a controller. As the static
`pressure increases above the design value, the controller modulates the dampers
`toward their closed position, reducing air flow into the duct. The controller is a
`contral device that determines how to position the inlet vane dampers based upon
`the measured static pressure,
`
`
`
`Controller-—=co n
`_4)
` Measure
`>SHH:
`
`
`Modulate
`Airilow
`
`Application 1.7
`
`Static Pressure Control Syster Sehernatic
`
`WHATIS THE PURPOSE OF A CONTROL SYSTEM?
`
`The purpose of a control system is to balance the flow of mass or energy trans-
`ferred into the process withils present load, where;
`
`A. The process is the physical condition (temperature, humidity, level, flowrate,
`etc.) that is maintained by the control system at a desired value.
`6. A load is the amount of mass (air, water, fuel, etc.) or energy (heat, electricity)
`needed to maintain the process at its desired condition.
`C. Balancing the flowof mass or energy transferred into the process withthe load
`indicates that the amount of energy entering the process equals the amount of
`energy leaving the process.
`
`The key word in the definition of the purpose of a control systemis the word
`halance, Maintaining a balance between the quantity transferred and the load in
`HVAC processes preserves occupant comfort and maximizes the operating effi-
`ciency of the equipment. When the mass or energy transferred into the process is
`greater than its present load, the efficiency of the process decreases as the process
`drifts outside of the limits of its desired range, producing comfort or operational
`problems.
`A descriptive example of fhe definition of the purpose of a control loop is a
`summer temperature control application for an office space. The process of this
`application is to maintain the space temperature at a desired value of 74°F. The
`load is the quantity of heat that must be removed from the space ty maintainthe
`temperature al 74°F The room's cooling load is comprised of a combination of
`solar gain, wall conductance gain, infiltration air, internal equipment heat, and
`occupant loads, When the quantity of energy Mowing outof the conditioned space
`equals the total room load, its temperature remains constant. When the control
`system maintains the room at 74°, the process is maintained al the maximum pos-
`sible efficiency for those conditions. Figure 1-1 shows a pictorial definition of the
`purposeof control systems, balancing the energy transfer with the load.
`All control systems maintain their process condition by altering the flow of
`mass or energy being transferred until a balance is reached. In ITVAC processes,
`the flow of heat, air, and water make up a majority of the mass or energy flows
`regulated by control systems.
`Hot and chilled water are used to transfer heat, volumesofair are regulated to
`maintain room temperatures and movement. Moisture is transferred to affect
`changesin the relative humidity in zones and water flow is regulated to maintain
`condenser head pressures. Depending on what level a technician classifies the
`transfer, it will always be a mass or energy process, Whichone it may beis notcrit-
`ical to the controls technician. Therefore, the text will use the term Heat to classity
`most energy transfers that maintain temperatures and the term mass to describe
`those processes that maintain humidity levels, liquid levels and pressures.
`
`012
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`Chapter 1
`
`Introduction to Control Systems
`
`4
`
`Balance
`oo
`
`Mass or
`
`_v
`a
`
`“t
`Ye
`
`1
`r
`=comtort > Energy
`
`Efficiencyws ~pee
`
`
`
`Figure 1.1 Control Loops Maintain a Balance Between the Load and the Mass or Energy
`Transterred into the Process
`
`In actual processes, the load is dynamic. Dynamic means the environmentis con-
`stantly changing, and consequently, the load alsois varying. To maximize comfort
`and operating efficiency of real systems, control systems are designed and cali-
`brated (tuned) (o balance the HVA