HONEYWELL E M AUTOMATIC CONTROL for · tremendous advances in equipment, system design, and application. In this edition, microprocessor controls are shown in most of the control

ENGINEERING MANUAL OF AUTOMATIC CONTROL i

HONEYWELL

ENGINEERING MANUAL of

AUTOMATICCONTROL forCOMMERCIAL BUILDINGS

ENGINEERING MANUAL OF AUTOMATIC CONTROLii

Copyright 1934, 1940, 1953, 1988, 1991 and 1997 by Honeywell Inc.

All rights reserved. This manual or portions thereof may not be reporducedin any form without permission of Honeywell Inc.

Library of Congress Catalog Card Number: 97-72971

Honeywell Europe S.A.3 Avenue du Bourget1140 BrusselsBelgium

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Home and Building ControlHoneywell Limited-Honeywell Limitée155 Gordon Baker RoadNorth York, OntarioM2H 3N7

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Home and Building ControlHoneywell Inc.Honeywell PlazaP.O. Box 524Minneapolis MN 55408-0524

Printed in USA

ENGINEERING MANUAL OF AUTOMATIC CONTROL iii

FOREWORD

The Minneapolis Honeywell Regulator Company published the first edition of the Engineering Manual ofAutomatic Control in l934. The manual quickly became the standard textbook for the commercial buildingcontrols industry. Subsequent editions have enjoyed even greater success in colleges, universities, and contractorand consulting engineering offices throughout the world.

Since the original 1934 edition, the building control industry has experienced dramatic change and madetremendous advances in equipment, system design, and application. In this edition, microprocessor controls areshown in most of the control applications rather than pneumatic, electric, or electronic to reflect the trends inindustry today. Consideration of configuration, functionality, and integration plays a significant role in thedesign of building control systems.

Through the years Honeywell has been dedicated to assisting consulting engineers and architects in theapplication of automatic controls to heating, ventilating, and air conditioning systems. This manual is an outgrowthof that dedication. Our end user customers, the building owners and operators, will ultimately benefit from theefficiently designed systems resulting from the contents of this manual.

All of this manual’s original sections have been updated and enhanced to include the latest developments incontrol technology. A new section has been added on indoor air quality and information on district heating hasbeen added to the Chiller, Boiler, and Distribution System Control Applications Section.

This twenty-first edition of the Engineering Manual of Automatic Control is our contribution to ensure thatwe continue to satisfy our customer’s requirements. The contributions and encouragement received from previoususers are gratefully acknowledged. Further suggestions will be most welcome.

Minneapolis, MinnesotaOctober, 1997

KEVIN GILLIGANPresident, H&BC Solutions and Services

ENGINEERING MANUAL OF AUTOMATIC CONTROLiv

ENGINEERING MANUAL OF AUTOMATIC CONTROL v

PREFACE

The purpose of this manual is to provide the reader with a fundamental understanding of controls and howthey are applied to the many parts of heating, ventilating, and air conditioning systems in commercial buildings.

Many aspects of control are presented including air handling units, terminal units, chillers, boilers, buildingairflow, water and steam distribution systems, smoke management, and indoor air quality. Control fundamentals,theory, and types of controls provide background for application of controls to heating, ventilating, and airconditioning systems. Discussions of pneumatic, electric, electronic, and digital controls illustrate that applicationsmay use one or more of several different control methods. Engineering data such as equipment sizing, use ofpsychrometric charts, and conversion formulas supplement and support the control information. To enhanceunderstanding, definitions of terms are provided within individual sections. For maximum usability, each sectionof this manual is available as a separate, self-contained document.

Building management systems have evolved into a major consideration for the control engineer when evaluatinga total heating, ventilating, and air conditioning system design. In response to this consideration, the basics ofbuilding management systems configuration are presented.

The control recommendations in this manual are general in nature and are not the basis for any specific job orinstallation. Control systems are furnished according to the plans and specifications prepared by the controlengineer. In many instances there is more than one control solution. Professional expertise and judgment arerequired for the design of a control system. This manual is not a substitute for such expertise and judgment.Always consult a licensed engineer for advice on designing control systems.

It is hoped that the scope of information in this manual will provide the readers with the tools to expand theirknowledge base and help develop sound approaches to automatic control.

ENGINEERING MANUAL OF AUTOMATIC CONTROLvi

ENGINEERING MANUAL OF AUTOMATIC CONTROL vii

CONTENTS

Foreward ............................................................................................................................................................. iii

Preface ................................................................................................................................................................ v

Control System Fundamentals ............................................................................................ 1

Control Fundamentals ....................................................................................................................................... 3Introduction .......................................................................................... 5Definitions ............................................................................................ 5HVAC System Characteristics ............................................................. 8Control System Characteristics ........................................................... 15Control System Components .............................................................. 30Characteristics And Attributes Of Control Methods ............................. 35

Psychrometric Chart Fundamentals ................................................................................................................ 37Introduction .......................................................................................... 38Definitions ............................................................................................ 38Description of the Psychrometric Chart ............................................... 39The Abridged Psychrometric Chart ..................................................... 40Examples of Air Mixing Process .......................................................... 42Air Conditioning Processes ................................................................. 43Humidifying Process............................................................................ 44ASHRAE Psychrometric Chart ............................................................ 53

Pneumatic Control Fundamentals .................................................................................................................... 57Introduction .......................................................................................... 59Definitions ............................................................................................ 59Abbreviations ....................................................................................... 60Symbols ............................................................................................... 61Basic Pneumatic Control System ........................................................ 61Air Supply Equipment .......................................................................... 65Thermostats ........................................................................................ 69Controllers ........................................................................................... 70Sensor-Controller Systems ................................................................. 72Actuators and Final Control Elements ................................................. 74Relays and Switches ........................................................................... 77Pneumatic Control Combinations ........................................................ 84Pneumatic Centeralization .................................................................. 89Pneumatic Control System Example ................................................... 90

Electric Control Fundamentals ......................................................................................................................... 95Introduction .......................................................................................... 97Definitions ............................................................................................ 97How Electric Control Circuits Classified .............................................. 99Series 40 Control Circuits.................................................................... 100Series 80 Control Circuits.................................................................... 102Series 60 Two-Position Control Circuits ............................................... 103Series 60 Floating Control Circuits ...................................................... 106Series 90 Control Circuits.................................................................... 107Motor Control Circuits .......................................................................... 114

ENGINEERING MANUAL of

AUTOMATICCONTROL

ENGINEERING MANUAL OF AUTOMATIC CONTROLviii

Electronic Control Fundamentals ..................................................................................................................... 119Introduction .......................................................................................... 120Definitions............................................................................................ 120Typical System .................................................................................... 122Components ........................................................................................ 122Electtonic Controller Fundamentals .................................................... 129Typical System Application .................................................................. 130

Microprocessor-Based/DDC Fundamentals .................................................................................................... 131Introduction .......................................................................................... 133Definitions............................................................................................ 133Background ......................................................................................... 134Advantages ......................................................................................... 134Controller Configuration ...................................................................... 135Types of Controllers ............................................................................. 136Controller Software .............................................................................. 137Controller Programming ...................................................................... 142Typical Applications ............................................................................. 145

Indoor Air Quality Fundamentals ..................................................................................................................... 149Introduction .......................................................................................... 151Definitions............................................................................................ 151Abbreviations ....................................................................................... 153Indoor Air Quality Concerns ................................................................ 154Indoor Air Quality Control Applications ................................................ 164Bibliography ......................................................................................... 170

Smoke Management Fundamentals ................................................................................................................. 171Introduction .......................................................................................... 172Definitions............................................................................................ 172Objectives ............................................................................................ 173Design Considerations ........................................................................ 173Design Principles ................................................................................ 175Control Applications ............................................................................ 178Acceptance Testing ............................................................................. 181Leakage Rated Dampers .................................................................... 181Bibliography ......................................................................................... 182

Building Management System Fundamentals ................................................................................................. 183Introduction .......................................................................................... 184Definitions............................................................................................ 184Background ......................................................................................... 185System Configurations ........................................................................ 186System Functions ................................................................................ 189Integration of Other Systems............................................................... 197

ENGINEERING MANUAL OF AUTOMATIC CONTROL ix

Control System Applications ............................................................................................... 199

Air Handling System Control Applications ...................................................................................................... 201Introduction .......................................................................................... 203Abbreviations ....................................................................................... 203Requirements For Effective Control .................................................... 204Applications-General ........................................................................... 206Valve and Damper Selection ............................................................... 207Symbols ............................................................................................... 208Ventilation Control Processes ............................................................. 209Fixed Quantity of Outdoor Air Control ................................................. 211Heating Control Processes.................................................................. 223Preheat Control Processes ................................................................. 228Humidification Control Process ........................................................... 235Cooling Control Processes .................................................................. 236Dehumidification Control Processes ................................................... 243Heating System Control process ......................................................... 246Year-Round System Control processes .............................................. 248ASHRAE Psychrometric Charts .......................................................... 261

Building Airflow System Control Applications ............................................................................................... 263Introduction .......................................................................................... 265Definitions ............................................................................................ 265Airflow Control Fundamentals ............................................................. 267Airflow Control Applications ................................................................. 281References .......................................................................................... 292

Chiller, Boiler, and Distribution System Control Applications ....................................................................... 293Introduction .......................................................................................... 297Abbreviations....................................................................................... 297Definitions............................................................................................ 297Symbols ............................................................................................... 298Chiller System Control ......................................................................... 299Boiler System Control .......................................................................... 329Hot And Chilled Water Distribution Systems Control ........................... 337High Temperature Water Heating System Control .............................. 376District Heating Applications ................................................................ 382

Individual Room Control Applications ............................................................................................................ 399Introduction .......................................................................................... 401Unitary Equipment Control .................................................................. 412Hot Water Plant Considerations .......................................................... 428

ENGINEERING MANUAL OF AUTOMATIC CONTROLx

Engineering Information ....................................................................................................... 429

Valve Selection and Sizing ................................................................................................................................ 431Introduction .......................................................................................... 432Definitions............................................................................................ 432Valve Selection .................................................................................... 436Valve Sizing ......................................................................................... 441

Damper Selection and Sizing ............................................................................................................................ 451Introduction .......................................................................................... 453Definitions............................................................................................ 453Damper Selection ................................................................................ 454Damper Sizing ..................................................................................... 463Damper Pressure Drop ....................................................................... 468Damper Applications ........................................................................... 469

General Engineering Data ................................................................................................................................. 471Introduction .......................................................................................... 472Weather Data ...................................................................................... 472Conversion Formulas And Tables ........................................................ 475Electrical Data ..................................................................................... 482Properties Of Saturated Steam Data................................................... 488Airflow Data ......................................................................................... 489Moisture Content Of Air Data .............................................................. 491

Index ....................................................................................................................................... 494

ENGINEERING MANUAL OF AUTOMATIC CONTROL

CONTROL FUNDAMENTALS

1

CONTROLSYSTEMS

FUNDMENTALS

Contents

Introduction ............................................................................................................ 5

Definitions ............................................................................................................ 5

HVAC System Characteristics ............................................................................................................ 8General ................................................................................................ 8Heating ................................................................................................ 9General ................................................................................................ 9Heating Equipment .............................................................................. 10Cooling ................................................................................................ 11General ................................................................................................ 11Cooling Equipment .............................................................................. 12Dehumidification .................................................................................. 12Humidification ...................................................................................... 13Ventilation ............................................................................................ 13Filtration............................................................................................... 14

Control System Characteristics ............................................................................................................ 15Controlled Variables ............................................................................ 15Control Loop ........................................................................................ 15Control Methods .................................................................................. 16

General ........................................................................................... 16Analog And Digital Control .............................................................. 16

Control Modes ..................................................................................... 17Two-Position Control ....................................................................... 17

General ....................................................................................... 17Basic Two-Position Control ......................................................... 17Timed Two-Position Control ........................................................ 18

Step Control .................................................................................... 19Floating Control ............................................................................... 20Proportional Control ........................................................................ 21

General ....................................................................................... 21Compensation Control ................................................................ 22

Proportional-Integral (Pi) Control .................................................... 23Proportional-Integral-Derivative (Pid) Control ................................. 25Enhanced Proportional-Integral-Derivative (epid) Control .............. 25Adaptive Control ............................................................................. 26

Process Characteristics ....................................................................... 26

ControlFundamentals



4

Load ................................................................................................ 26Lag .................................................................................................. 27General ........................................................................................... 27Measurement Lag ........................................................................... 27Capacitance .................................................................................... 28Resistance ...................................................................................... 29Dead Time ....................................................................................... 29

Control Application Guidelines ............................................................ 29

Control System Components ............................................................................................................ 30Sensing Elements ............................................................................... 30

Temperature Sensing Elements ...................................................... 30Pressure Sensing Elements ............................................................ 31Moisture Sensing Elements ............................................................ 32Flow Sensors .................................................................................. 32Proof-Of-Operation Sensors ........................................................... 33

Transducers ........................................................................................ 33Controllers ........................................................................................... 33Actuators ............................................................................................. 33Auxiliary Equipment ............................................................................. 34

Characteristics And Attributes Of Control Methods .............................................................................................. 35



5

INTRODUCTION

This section describes heating, ventilating, and airconditioning (HVAC) systems and discusses characteristics andcomponents of automatic control systems. Cross-referencesare made to sections that provide more detailed information.

A correctly designed HVAC control system can provide acomfortable environment for occupants, optimize energy costand consumption, improve employee productivity, facilitateefficient manufacturing, control smoke in the event of a fire,and support the operation of computer and telecommunicationsequipment. Controls are essential to the proper operation ofthe system and should be considered as early in the designprocess as possible.

Properly applied automatic controls ensure that a correctlydesigned HVAC system will maintain a comfortableenvironment and perform economically under a wide range ofoperating conditions. Automatic controls regulate HVACsystem output in response to varying indoor and outdoorconditions to maintain general comfort conditions in officeareas and provide narrow temperature and humidity limitswhere required in production areas for product quality.

Automatic controls can optimize HVAC system operation.They can adjust temperatures and pressures automatically toreduce demand when spaces are unoccupied and regulateheating and cooling to provide comfort conditions whilelimiting energy usage. Limit controls ensure safe operation ofHVAC system equipment and prevent injury to personnel anddamage to the system. Examples of limit controls are low-limit temperature controllers which help prevent water coilsor heat exchangers from freezing and flow sensors for safeoperation of some equipment (e.g., chillers). In the event of afire, controlled air distribution can provide smoke-freeevacuation passages, and smoke detection in ducts can closedampers to prevent the spread of smoke and toxic gases.

HVAC control systems can also be integrated with securityaccess control systems, fire alarm systems, lighting controlsystems, and building and facility management systems tofurther optimize building comfort, safety, and efficiency.

DEFINITIONS

The following terms are used in this manual. Figure 1 at theend of this list illustrates a typical control loop with thecomponents identified using terms from this list.

Analog: Continuously variable (e.g., a faucet controlling waterfrom off to full flow).

Automatic control system: A system that reacts to a changeor imbalance in the variable it controls by adjustingother variables to restore the system to the desiredbalance.

Algorithm: A calculation method that produces a controloutput by operating on an error signal or a time seriesof error signals.

Compensation control: A process of automatically adjustingthe setpoint of a given controller to compensate forchanges in a second measured variable (e.g., outdoorair temperature). For example, the hot deck setpointis normally reset upward as the outdoor airtemperature decreases. Also called “reset control”.

Control agent: The medium in which the manipulated variableexists. In a steam heating system, the control agent isthe steam and the manipulated variable is the flow ofthe steam.

Control point: The actual value of the controlled variable(setpoint plus or minus offset).

Controlled medium: The medium in which the controlledvariable exists. In a space temperature control system,the controlled variable is the space temperature andthe controlled medium is the air within the space.

Controlled Variable: The quantity or condition that ismeasured and controlled.

Controller: A device that senses changes in the controlledvariable (or receives input from a remote sensor) andderives the proper correction output.

Corrective action: Control action that results in a change ofthe manipulated variable. Initiated when thecontrolled variable deviates from setpoint.

Cycle: One complete execution of a repeatable process. Inbasic heating operation, a cycle comprises one onperiod and one off period in a two-position controlsystem.

Cycling: A periodic change in the controlled variable fromone value to another. Out-of-control analog cyclingis called “hunting”. Too frequent on-off cycling iscalled “short cycling”. Short cycling can harm electricmotors, fans, and compressors.

Cycling rate: The number of cycles completed per time unit,typically cycles per hour for a heating or coolingsystem. The inverse of the length of the period of thecycle.



6

Deadband: A range of the controlled variable in which nocorrective action is taken by the controlled systemand no energy is used. See also “zero energy band”.

Deviation: The difference between the setpoint and the valueof the controlled variable at any moment. Also called“offset”.

DDC: Direct Digital Control. See also Digital and Digitalcontrol.

Digital: A series of on and off pulses arranged to conveyinformation. Morse code is an early example.Processors (computers) operate using digitallanguage.

Digital control: A control loop in which a microprocessor-based controller directly controls equipment basedon sensor inputs and setpoint parameters. Theprogrammed control sequence determines the outputto the equipment.

Droop: A sustained deviation between the control point andthe setpoint in a two-position control system causedby a change in the heating or cooling load.

Enhanced proportional-integral-derivative (EPID) control:A control algorithm that enhances the standard PIDalgorithm by allowing the designer to enter a startupoutput value and error ramp duration in addition tothe gains and setpoints. These additional parametersare configured so that at startup the PID output variessmoothly to the control point with negligibleovershoot or undershoot.

Electric control: A control circuit that operates on line or lowvoltage and uses a mechanical means, such as atemperature-sensitive bimetal or bellows, to performcontrol functions, such as actuating a switch orpositioning a potentiometer. The controller signalusually operates or positions an electric actuator ormay switch an electrical load directly or through arelay.

Electronic control: A control circuit that operates on lowvoltage and uses solid-state components to amplifyinput signals and perform control functions, such asoperating a relay or providing an output signal toposition an actuator. The controller usually furnishesfixed control routines based on the logic of the solid-state components.

Final control element: A device such as a valve or damperthat acts to change the value of the manipulatedvariable. Positioned by an actuator.

Hunting: See Cycling.

Lag: A delay in the effect of a changed condition at one pointin the system, or some other condition to which it is

related. Also, the delay in response of the sensingelement of a control due to the time required for thesensing element to sense a change in the sensedvariable.

Load: In a heating or cooling system, the heat transfer thatthe system will be called upon to provide. Also, thework that the system must perform.

Manipulated variable: The quantity or condition regulatedby the automatic control system to cause the desiredchange in the controlled variable.

Measured variable: A variable that is measured and may becontrolled (e.g., discharge air is measured andcontrolled, outdoor air is only measured).

Microprocessor-based control: A control circuit that operateson low voltage and uses a microprocessor to performlogic and control functions, such as operating a relayor providing an output signal to position an actuator.Electronic devices are primarily used as sensors. Thecontroller often furnishes flexible DDC and energymanagement control routines.

Modulating: An action that adjusts by minute increments anddecrements.

Offset: A sustained deviation between the control point andthe setpoint of a proportional control system understable operating conditions.

On/off control: A simple two-position control system in whichthe device being controlled is either full on or full offwith no intermediate operating positions available.Also called “two-position control”.

Pneumatic control: A control circuit that operates on airpressure and uses a mechanical means, such as atemperature-sensitive bimetal or bellows, to performcontrol functions, such as actuating a nozzle andflapper or a switching relay. The controller outputusually operates or positions a pneumatic actuator,although relays and switches are often in the circuit.

Process: A general term that describes a change in a measurablevariable (e.g., the mixing of return and outdoor airstreams in a mixed-air control loop and heat transferbetween cold water and hot air in a cooling coil).Usually considered separately from the sensingelement, control element, and controller.

Proportional band: In a proportional controller, the controlpoint range through which the controlled variablemust pass to move the final control element throughits full operating range. Expressed in percent ofprimary sensor span. Commonly used equivalents are“throttling range” and “modulating range”, usuallyexpressed in a quantity of engineering units (degreesof temperature).



7

SETPOINT

60

0

130

190

RESET SCHEDULE

HWSETPOINT

OA TEMPERATURE

160

159

148

AUTO

41

INPUT

OUTPUT

30

PERCENTOPEN

VALVE

STEAM

FLOW

OUTDOORAIR

OUTDOORAIR

CONTROLPOINT

HOT WATERRETURN

HOT WATERSUPPLY

HOT WATERSUPPLY

TEMPERATURECONTROLLED

MEDIUM

CONTROLLEDVARIABLE

MEASUREDVARIABLE

MEASUREDVARIABLE

SETPOINT

ALGORITHM INCONTROLLER

FINAL CONTROLELEMENT

CONTROLAGENT

MANIPULATEDVARIABLE

M10510

Proportional control: A control algorithm or method in whichthe final control element moves to a positionproportional to the deviation of the value of thecontrolled variable from the setpoint.

Proportional-Integral (PI) control: A control algorithm thatcombines the proportional (proportional response)and integral (reset response) control algorithms. Resetresponse tends to correct the offset resulting fromproportional control. Also called “proportional-plus-reset” or “two-mode” control.

Proportional-Integral-Derivative (PID) control: A controlalgorithm that enhances the PI control algorithm byadding a component that is proportional to the rate ofchange (derivative) of the deviation of the controlledvariable. Compensates for system dynamics andallows faster control response. Also called “three-mode” or “rate-reset” control.

Reset Control: See Compensation control.

Sensing element: A device or component that measures thevalue of a variable.

Setpoint: The value at which the controller is set (e.g., thedesired room temperature set on a thermostat). Thedesired control point.

Short cycling: See Cycling.

Step control: Control method in which a multiple-switchassembly sequentially switches equipment (e.g.,electric heat, multiple chillers) as the controller inputvaries through the proportional band. Step controllersmay be actuator driven, electronic, or directlyactivated by the sensed medium (e.g., pressure,temperature).

Throttling range: In a proportional controller, the control pointrange through which the controlled variable must passto move the final control element through its fulloperating range. Expressed in values of the controlledvariable (e.g., degrees Fahrenheit, percent relativehumidity, pounds per square inch). Also called“proportional band”. In a proportional roomthermostat, the temperature change required to drivethe manipulated variable from full off to full on.

Time constant: The time required for a dynamic component,such as a sensor, or a control system to reach 63.2percent of the total response to an instantaneous (or“step”) change to its input. Typically used to judgethe responsiveness of the component or system.

Two-position control: See on/off control.

Zero energy band: An energy conservation technique thatallows temperatures to float between selected settings,thereby preventing the consumption of heating orcooling energy while the temperature is in this range.

Zoning: The practice of dividing a building into sections forheating and cooling control so that one controller issufficient to determine the heating and cooling

requirements for the section.

Fig. 1. Typical Control Loop.



8

HVAC SYSTEM CHARACTERISTICS

Figure 2 shows how an HVAC system may be distributed ina small commercial building. The system control panel, boilers,motors, pumps, and chillers are often located on the lower level.The cooling tower is typically located on the roof. Throughoutthe building are ductwork, fans, dampers, coils, air filters,heating units, and variable air volume (VAV) units anddiffusers. Larger buildings often have separate systems forgroups of floors or areas of the building.

Fig. 2. Typical HVAC System in a Small Building.

The control system for a commercial building comprisesmany control loops and can be divided into central system andlocal- or zone-control loops. For maximum comfort andefficiency, all control loops should be tied together to shareinformation and system commands using a buildingmanagement system. Refer to the Building ManagementSystem Fundamentals section of this manual.

The basic control loops in a central air handling system canbe classified as shown in Table 1.

Depending on the system, other controls may be requiredfor optimum performance. Local or zone controls depend onthe type of terminal units used.

DAMPER

AIR FILTER

COOLING COIL

FAN

CHILLER

PUMP

COOLINGTOWER HEATING

UNIT

DUCTWORK

VAV BOXDIFFUSER

BOILERCONTROLPANEL

M10506

GENERAL

An HVAC system is designed according to capacityrequirements, an acceptable combination of first cost andoperating costs, system reliability, and available equipmentspace.



9

ControlLoop Classification Description

Ventilation Basic Coordinates operation of the outdoor, return, and exhaust air dampers to maintainthe proper amount of ventilation air. Low-temperature protection is often required.

Better Measures and controls the volume of outdoor air to provide the proper mix ofoutdoor and return air under varying indoor conditions (essential in variable airvolume systems). Low-temperature protection may be required.

Cooling Chiller control Maintains chiller discharge water at preset temperature or resets temperatureaccording to demand.

Cooling towercontrol

Controls cooling tower fans to provide the coolest water practical under existingwet bulb temperature conditions.

Water coil control Adjusts chilled water flow to maintain temperature.

Direct expansion(DX) systemcontrol

Cycles compressor or DX coil solenoid valves to maintain temperature. Ifcompressor is unloading type, cylinders are unloaded as required to maintaintemperature.

Fan Basic Turns on supply and return fans during occupied periods and cycles them asrequired during unoccupied periods.

Better Adjusts fan volumes to maintain proper duct and space pressures. Reduces systemoperating cost and improves performance (essential for variable air volumesystems).

Heating Coil control Adjusts water or steam flow or electric heat to maintain temperature.

Boiler control Operates burner to maintain proper discharge steam pressure or water temperature.For maximum efficiency in a hot water system, water temperature should be resetas a function of demand or outdoor temperature.

Table 1. Functions of Central HVAC Control Loops.

HEATING

GENERAL

Building heat loss occurs mainly through transmission,infiltration/exfiltration, and ventilation (Fig. 3).

ROOF20°FTRANSMISSION

VENTILATION DUCT

EXFILTRATION

DOORWINDOW

PREVAILINGWINDS

INFILTRATION

70°F

C2701

Fig. 3. Heat Loss from a Building.

The heating capacity required for a building depends on thedesign temperature, the quantity of outdoor air used, and thephysical activity of the occupants. Prevailing winds affect therate of heat loss and the degree of infiltration. The heatingsystem must be sized to heat the building at the coldest outdoortemperature the building is likely to experience (outdoor designtemperature).

Transmission is the process by which energy enters or leavesa space through exterior surfaces. The rate of energytransmission is calculated by subtracting the outdoortemperature from the indoor temperature and multiplying theresult by the heat transfer coefficient of the surface materials.The rate of transmission varies with the thickness andconstruction of the exterior surfaces but is calculated the sameway for all exterior surfaces:

Energy Transmission per

Unit Area and Unit Time = (TIN - T

OUT) x HTC

Where:T

IN= indoor temperature

TOUT

= outdoor temperatureHTC = heat transfer coefficient

= Btu

Unit Time x Unit Area x Unit TemperaturHTC



10

Infiltration is the process by which outdoor air enters abuilding through walls, cracks around doors and windows, andopen doors due to the difference between indoor and outdoorair pressures. The pressure differential is the result oftemperature difference and air intake or exhaust caused by fanoperation. Heat loss due to infiltration is a function oftemperature difference and volume of air moved. Exfiltrationis the process by which air leaves a building (e.g., throughwalls and cracks around doors and windows) and carries heatwith it. Infiltration and exfiltration can occur at the same time.

Ventilation brings in fresh outdoor air that may requireheating. As with heat loss from infiltration and exfiltration,heat loss from ventilation is a function of the temperaturedifference and the volume of air brought into the building orexhausted.

HEATING EQUIPMENT

Selecting the proper heating equipment depends on manyfactors, including cost and availability of fuels, building sizeand use, climate, and initial and operating cost trade-offs.Primary sources of heat include gas, oil, wood, coal, electrical,and solar energy. Sometimes a combination of sources is mosteconomical. Boilers are typically fueled by gas and may havethe option of switching to oil during periods of high demand.Solar heat can be used as an alternate or supplementary sourcewith any type of fuel.

Figure 4 shows an air handling system with a hot water coil.A similar control scheme would apply to a steam coil. If steamor hot water is chosen to distribute the heat energy, high-efficiency boilers may be used to reduce life-cycle cost. Watergenerally is used more often than steam to transmit heat energyfrom the boiler to the coils or terminal units, because waterrequires fewer safety measures and is typically more efficient,especially in mild climates.

THERMOSTAT

HOT WATERSUPPLY

VALVE

DISCHARGEAIR

FAN

HOT WATERRETURN C2702

Fig. 4. System Using Heating Coil.

An air handling system provides heat by moving an airstream across a coil containing a heating medium, across anelectric heating coil, or through a furnace. Unit heaters (Fig.5) are typically used in shops, storage areas, stairwells, anddocks. Panel heaters (Fig. 6) are typically used for heatingfloors and are usually installed in a slab or floor structure, butmay be installed in a wall or ceiling.

C2703

UNIT HEATER

COIL

FAN

STEAM ORHOT WATERSUPPLY

CONDENSATEOR HOT WATERRETURN

STEAM TRAP(IF STEAM SUPPLY)

Fig. 5. Typical Unit Heater.

C3035

DISCHARGE AIR

WALL

OUTDOORAIR

MIXINGDAMPERS

RETURNAIR

COOLING COIL

DRAIN PAN

HEATING COIL

FAN

Fig. 6. Panel Heaters.

Unit ventilators (Fig. 7) are used in classrooms and mayinclude both a heating and a cooling coil. Convection heaters(Fig. 8) are used for perimeter heating and in entries andcorridors. Infrared heaters (Fig. 9) are typically used for spotheating in large areas (e.g., aircraft hangers, stadiums).

HOT WATERSUPPLY

HOT WATERRETURN

GRID PANEL

HOT WATERSUPPLY

HOT WATERRETURN

SERPENTINE PANEL

C2704

Fig. 7. Unit Ventilator.



11

Fig. 8. Convection Heater.

WARM AIR

FINNED TUBE

RETURN AIR

FLOORSUPPLY

RETURN

TO OTHERHEATING UNITS

FROM OTHERHEATING UNITS

C2705

REFLECTOR

INFRAREDSOURCE

C2706

RADIANT HEAT

Fig. 9. Infrared Heater.

In mild climates, heat can be provided by a coil in the centralair handling system or by a heat pump. Heat pumps have theadvantage of switching between heating and cooling modesas required. Rooftop units provide packaged heating andcooling. Heating in a rooftop unit is usually by a gas- or oil-fired furnace or an electric heat coil. Steam and hot water coilsare available as well. Perimeter heat is often required in colderclimates, particularly under large windows.

A heat pump uses standard refrigeration components and areversing valve to provide both heating and cooling within thesame unit. In the heating mode, the flow of refrigerant throughthe coils is reversed to deliver heat from a heat source to theconditioned space. When a heat pump is used to exchange heatfrom the interior of a building to the perimeter, no additionalheat source is needed.

A heat-recovery system is often used in buildings where asignificant quantity of outdoor air is used. Several types ofheat-recovery systems are available including heat pumps,runaround systems, rotary heat exchangers, and heat pipes.

In a runaround system, coils are installed in the outdoor airsupply duct and the exhaust air duct. A pump circulates themedium (water or glycol) between the coils so that mediumheated by the exhaust air preheats the outdoor air entering thesystem.

A rotary heat exchanger is a large wheel filled with metalmesh. One half of the wheel is in the outdoor air intake andthe other half, in the exhaust air duct. As the wheel rotates, themetal mesh absorbs heat from the exhaust air and dissipates itin the intake air.

A heat pipe is a long, sealed, finned tube charged with arefrigerant. The tube is tilted slightly with one end in theoutdoor air intake and the other end in the exhaust air. In aheating application, the refrigerant vaporizes at the lower end

in the warm exhaust air, and the vapor rises toward the higherend in the cool outdoor air, where it gives up the heat ofvaporization and condenses. A wick carries the liquidrefrigerant back to the warm end, where the cycle repeats. Aheat pipe requires no energy input. For cooling, the process isreversed by tilting the pipe the other way.

Controls may be pneumatic, electric, electronic, digital, ora combination. Satisfactory control can be achieved usingindependent control loops on each system. Maximum operatingefficiency and comfort levels can be achieved with a controlsystem which adjusts the central system operation to thedemands of the zones. Such a system can save enough inoperating costs to pay for itself in a short time.

Controls for the air handling system and zones arespecifically designed for a building by the architect, engineer,or team who designs the building. The controls are usuallyinstalled at the job site. Terminal unit controls are typicallyfactory installed. Boilers, heat pumps, and rooftop units areusually sold with a factory-installed control packagespecifically designed for that unit.

COOLING

GENERAL

Both sensible and latent heat contribute to the cooling loadof a building. Heat gain is sensible when heat is added to theconditioned space. Heat gain is latent when moisture is addedto the space (e.g., by vapor emitted by occupants and othersources). To maintain a constant humidity ratio in the space,water vapor must be removed at a rate equal to its rate ofaddition into the space.

Conduction is the process by which heat moves betweenadjoining spaces with unequal space temperatures. Heat maymove through exterior walls and the roof, or through floors,walls, or ceilings. Solar radiation heats surfaces which thentransfer the heat to the surrounding air. Internal heat gain isgenerated by occupants, lighting, and equipment. Warm airentering a building by infiltration and through ventilation alsocontributes to heat gain.

Building orientation, interior and exterior shading, the angleof the sun, and prevailing winds affect the amount of solarheat gain, which can be a major source of heat. Solar heatreceived through windows causes immediate heat gain. Areaswith large windows may experience more solar gain in winterthan in summer. Building surfaces absorb solar energy, becomeheated, and transfer the heat to interior air. The amount ofchange in temperature through each layer of a compositesurface depends on the resistance to heat flow and thicknessof each material.

Occupants, lighting, equipment, and outdoor air ventilationand infiltration requirements contribute to internal heat gain.For example, an adult sitting at a desk produces about 400 Btuper hour. Incandescent lighting produces more heat thanfluorescent lighting. Copiers, computers, and other officemachines also contribute significantly to internal heat gain.



12

COOLING EQUIPMENT

An air handling system cools by moving air across a coilcontaining a cooling medium (e.g., chilled water or arefrigerant). Figures 10 and 11 show air handling systems thatuse a chilled water coil and a refrigeration evaporator (directexpansion) coil, respectively. Chilled water control is usuallyproportional, whereas control of an evaporator coil is two-position. In direct expansion systems having more than onecoil, a thermostat controls a solenoid valve for each coil andthe compressor is cycled by a refrigerant pressure control. Thistype of system is called a “pump down” system. Pump downmay be used for systems having only one coil, but more oftenthe compressor is controlled directly by the thermostat.

TEMPERATURECONTROLLER

SENSOR

CONTROLVALVE

CHILLED WATERSUPPLY

CHILLEDWATERCOIL

COOL AIR

CHILLED WATER RETURN

C2707-2

Fig. 10. System Using Cooling Coil.

D

X

TEMPERATURECONTROLLER SENSOR

COOL AIR

C2708-1

EVAPORATORCOIL

SOLENOIDVALVE

REFRIGERANTLIQUID

REFRIGERANTGAS

Fig. 11. System Using Evaporator(Direct Expansion) Coil.

Two basic types of cooling systems are available: chillers,typically used in larger systems, and direct expansion (DX)coils, typically used in smaller systems. In a chiller, therefrigeration system cools water which is then pumped to coilsin the central air handling system or to the coils of fan coilunits, a zone system, or other type of cooling system. In a DXsystem, the DX coil of the refrigeration system is located inthe duct of the air handling system. Condenser cooling forchillers may be air or water (using a cooling tower), while DXsystems are typically air cooled. Because water cooling is moreefficient than air cooling, large chillers are always water cooled.

Compressors for chilled water systems are usuallycentrifugal, reciprocating, or screw type. The capacities ofcentrifugal and screw-type compressors can be controlled byvarying the volume of refrigerant or controlling the compressorspeed. DX system compressors are usually reciprocating and,in some systems, capacity can be controlled by unloadingcylinders. Absorption refrigeration systems, which use heatenergy directly to produce chilled water, are sometimes usedfor large chilled water systems.

While heat pumps are usually direct expansion, a large heatpump may be in the form of a chiller. Air is typically the heatsource and heat sink unless a large water reservoir (e.g., groundwater) is available.

Initial and operating costs are prime factors in selectingcooling equipment. DX systems can be less expensive thanchillers. However, because a DX system is inherently two-position (on/off), it cannot control temperature with theaccuracy of a chilled water system. Low-temperature controlis essential in a DX system used with a variable air volumesystem.

For more information control of various system equipment,refer to the following sections of this manual:

— Chiller, Boiler, and Distribution SystemControl Application.

— Air Handling System Control Applications.— Individual Room Control Applications.

DEHUMIDIFICATION

Air that is too humid can cause problems such ascondensation and physical discomfort. Dehumidificationmethods circulate moist air through cooling coils or sorptionunits. Dehumidification is required only during the coolingseason. In those applications, the cooling system can bedesigned to provide dehumidification as well as cooling.

For dehumidification, a cooling coil must have a capacityand surface temperature sufficient to cool the air below its dewpoint. Cooling the air condenses water, which is then collectedand drained away. When humidity is critical and the coolingsystem is used for dehumidification, the dehumidified air maybe reheated to maintain the desired space temperature.

When cooling coils cannot reduce moisture contentsufficiently, sorption units are installed. A sorption unit useseither a rotating granular bed of silica gel, activated aluminaor hygroscopic salts (Fig. 12), or a spray of lithium chloridebrine or glycol solution. In both types, the sorbent materialabsorbs moisture from the air and then the saturated sorbentmaterial passes through a separate section of the unit thatapplies heat to remove moisture. The sorbent material givesup moisture to a stream of “scavenger” air, which is thenexhausted. Scavenger air is often exhaust air or could beoutdoor air.



13

Fig. 12. Granular Bed Sorption Unit.

Sprayed cooling coils (Fig. 13) are often used for spacehumidity control to increase the dehumidifier efficiency andto provide year-round humidity control (winter humidificationalso).

DRY AIR

HUMIDAIR

ROTATINGGRANULARBED

SORPTIONUNIT

SCAVENGERAIR

HEATINGCOIL

HUMID AIREXHAUST

C2709

MOISTUREELIMINATORS

SPRAYPUMP M10511

COOLINGCOIL

Fig. 13. Sprayed Coil Dehumidifier.

For more information on dehumidification, refer to thefollowing sections of this manual:

— Psychrometric Chart Fundamentals.— Air Handling System Control Applications.

HUMIDIFICATION

Low humidity can cause problems such as respiratorydiscomfort and static electricity. Humidifiers can humidify aspace either directly or through an air handling system. Forsatisfactory environmental conditions, the relative humidityof the air should be 30 to 60 percent. In critical areas whereexplosive gases are present, 50 percent minimum isrecommended. Humidification is usually required only duringthe heating season except in extremely dry climates.

Humidifiers in air handling systems typically inject steamdirectly into the air stream (steam injection), spray atomizedwater into the air stream (atomizing), or evaporate heated waterfrom a pan in the duct into the air stream passing through theduct (pan humidification). Other types of humidifiers are awater spray and sprayed coil. In spray systems, the water canbe heated for better vaporization or cooled fordehumidification.

For more information on humidification, refer to thefollowing sections of this manual:

— Psychrometric Chart Fundamentals.— Air Handling System Control Applications.

VENTILATION

Ventilation introduces outdoor air to replenish the oxygensupply and rid building spaces of odors and toxic gases.Ventilation can also be used to pressurize a building to reduceinfiltration. While ventilation is required in nearly all buildings,the design of a ventilation system must consider the cost ofheating and cooling the ventilation air. Ventilation air must bekept at the minimum required level except when used for freecooling (refer to ASHRAE Standard 62, Ventilation forAcceptable Indoor Air Quality).

To ensure high-quality ventilation air and minimize theamount required, the outdoor air intakes must be located toavoid building exhausts, vehicle emissions, and other sourcesof pollutants. Indoor exhaust systems should collect odors orcontaminants at their source. The amount of ventilation abuilding requires may be reduced with air washers, highefficiency filters, absorption chemicals (e.g., activatedcharcoal), or odor modification systems.

Ventilation requirements vary according to the number ofoccupants and the intended use of the space. For a breakdownof types of spaces, occupancy levels, and required ventilation,refer to ASHRAE Standard 62.

Figure 14 shows a ventilation system that supplies 100percent outdoor air. This type of ventilation system is typicallyused where odors or contaminants originate in the conditionedspace (e.g., a laboratory where exhaust hoods and fans removefumes). Such applications require make-up air that isconditioned to provide an acceptable environment.

EXHAUST

TOOUTDOORS

EXHAUSTFAN

RETURNAIR

SPACE

MAKE-UPAIR

SUPPLY FAN

COILFILTER

OUTDOORAIR

SUPPLY

C2711

Fig. 14. Ventilation System Using 100 PercentOutdoor Air.

In many applications, energy costs make 100 percent outdoorair constant volume systems uneconomical. For that reason,other means of controlling internal contaminants are available,such as variable volume fume hood controls, spacepressurization controls, and air cleaning systems.

A ventilation system that uses return air (Fig. 15) is morecommon than the 100 percent outdoor air system. The return-air ventilation system recirculates most of the return air fromthe system and adds outdoor air for ventilation. The return-airsystem may have a separate fan to overcome duct pressure



14

losses. The exhaust-air system may be incorporated into theair conditioning unit, or it may be a separate remote exhaust.Supply air is heated or cooled, humidified or dehumidified,and discharged into the space.

DAMPER RETURN FAN

RETURNAIR

EXHAUSTAIR

DAMPERS

OUTDOORAIR

MIXEDAIR

FILTER COIL SUPPLY FAN

SUPPLYAIR

C2712

Fig. 15. Ventilation System Using Return Air.

Ventilation systems as shown in Figures 14 and 15 shouldprovide an acceptable indoor air quality, utilize outdoor airfor cooling (or to supplement cooling) when possible, andmaintain proper building pressurization.

For more information on ventilation, refer to the followingsections of this manual:

— Indoor Air Quality Fundamentals.— Air Handling System Control Applications.— Building Airflow System Control Applications.

FILTRATION

Air filtration is an important part of the central air handlingsystem and is usually considered part of the ventilation system.Two basic types of filters are available: mechanical filters andelectrostatic precipitation filters (also called electronic aircleaners). Mechanical filters are subdivided into standard andhigh efficiency.

Filters are selected according to the degree of cleanlinessrequired, the amount and size of particles to be removed, andacceptable maintenance requirements. High-efficiencyparticulate air (HEPA) mechanical filters (Fig. 16) do notrelease the collected particles and therefore can be used forclean rooms and areas where toxic particles are released. HEPAfilters significantly increase system pressure drop, which mustbe considered when selecting the fan. Figure 17 shows othermechanical filters.

C2713

CELL

PLEATED PAPER

AIR FLOW

Fig. 16. HEPA Filter.

PLEATED FILTER

BAG FILTER

Fig. 17. Mechanical Filters.

Other types of mechanical filters include strainers, viscouscoated filters, and diffusion filters. Straining removes particlesthat are larger than the spaces in the mesh of a metal filter andare often used as prefilters for electrostatic filters. In viscouscoated filters, the particles passing through the filter fiberscollide with the fibers and are held on the fiber surface.Diffusion removes fine particles by using the turbulence presentin the air stream to drive particles to the fibers of the filtersurface.

An electrostatic filter (Fig. 18) provides a low pressure dropbut often requires a mechanical prefilter to collect largeparticles and a mechanical after-filter to collect agglomeratedparticles that may be blown off the electrostatic filter. Anelectrostatic filter electrically charges particles passing throughan ionizing field and collects the charged particles on plateswith an opposite electrical charge. The plates may be coatedwith an adhesive.



15

Fig. 18. Electrostatic Filter.

The sensor can be separate from or part of the controllerand is located in the controlled medium. The sensor measuresthe value of the controlled variable and sends the resultingsignal to the controller. The controller receives the sensorsignal, compares it to the desired value, or setpoint, andgenerates a correction signal to direct the operation of thecontrolled device. The controlled device varies the controlagent to regulate the output of the control equipment thatproduces the desired condition.

HVAC applications use two types of control loops: openand closed. An open-loop system assumes a fixed relationshipbetween a controlled condition and an external condition. Anexample of open-loop control would be the control of perimeterradiation heating based on an input from an outdoor airtemperature sensor. A circulating pump and boiler are energizedwhen an outdoor air temperature drops to a specified setting,and the water temperature or flow is proportionally controlledas a function of the outdoor temperature. An open-loop systemdoes not take into account changing space conditions frominternal heat gains, infiltration/exfiltration, solar gain, or otherchanging variables in the building. Open-loop control alonedoes not provide close control and may result in underheatingor overheating. For this reason, open-loop systems are notcommon in residential or commercial applications.

A closed-loop system relies on measurement of thecontrolled variable to vary the controller output. Figure 19shows a block diagram of a closed-loop system. An exampleof closed-loop control would be the temperature of dischargeair in a duct determining the flow of hot water to the heatingcoils to maintain the discharge temperature at a controllersetpoint.

AIRFLOW

AIRFLOW

ALTERNATEPLATESGROUNDED

INTERMEDIATEPLATESCHARGEDTO HIGHPOSITIVEPOTENTIAL

THEORETICALPATHS OFCHARGES DUSTPARTICLESPOSITIVELY CHARGED

PARTICLES

SOURCE: 1996 ASHRAE SYSTEMS AND EQUIPMENT HANDBOOK

PATH OFIONS

WIRES AT HIGHPOSITIVEPOTENTIAL

C2714

–

+

–

–

–

–

+

+

+

CONTROL SYSTEM CHARACTERISTICS

Automatic controls are used wherever a variable conditionmust be controlled. In HVAC systems, the most commonlycontrolled conditions are pressure, temperature, humidity, andrate of flow. Applications of automatic control systems rangefrom simple residential temperature regulation to precisioncontrol of industrial processes.

CONTROLLED VARIABLES

Automatic control requires a system in which a controllablevariable exists. An automatic control system controls thevariable by manipulating a second variable. The secondvariable, called the manipulated variable, causes the necessarychanges in the controlled variable.

In a room heated by air moving through a hot water coil, forexample, the thermostat measures the temperature (controlledvariable) of the room air (controlled medium) at a specifiedlocation. As the room cools, the thermostat operates a valvethat regulates the flow (manipulated variable) of hot water(control agent) through the coil. In this way, the coil furnishesheat to warm the room air.

CONTROL LOOP

In an air conditioning system, the controlled variable ismaintained by varying the output of the mechanical equipmentby means of an automatic control loop. A control loop consistsof an input sensing element, such as a temperature sensor; acontroller that processes the input signal and produces an outputsignal; and a final control element, such as a valve, that operatesaccording to the output signal.



16

Fig. 19. Feedback in a Closed-Loop System.

In this example, the sensing element measures the dischargeair temperature and sends a feedback signal to the controller.The controller compares the feedback signal to the setpoint.Based on the difference, or deviation, the controller issues acorrective signal to a valve, which regulates the flow of hotwater to meet the process demand. Changes in the controlledvariable thus reflect the demand. The sensing element continuesto measure changes in the discharge air temperature and feedsthe new condition back into the controller for continuouscomparison and correction.

Automatic control systems use feedback to reduce themagnitude of the deviation and produce system stability asdescribed above. A secondary input, such as the input from anoutdoor air compensation sensor, can provide informationabout disturbances that affect the controlled variable. Usingan input in addition to the controlled variable enables thecontroller to anticipate the effect of the disturbance andcompensate for it, thus reducing the impact of disturbances onthe controlled variable.

CONTROL METHODS

GENERAL

An automatic control system is classified by the type ofenergy transmission and the type of control signal (analog ordigital) it uses to perform its functions.

The most common forms of energy for automatic controlsystems are electricity and compressed air. Systems maycomprise one or both forms of energy.

Systems that use electrical energy are electromechanical,electronic, or microprocessor controlled. Pneumatic controlsystems use varying air pressure from the sensor as input to acontroller, which in turn produces a pneumatic output signalto a final control element. Pneumatic, electromechanical, andelectronic systems perform limited, predetermined controlfunctions and sequences. Microprocessor-based controllers usedigital control for a wide variety of control sequences.

Self-powered systems are a comparatively minor but stillimportant type of control. These systems use the power of themeasured variable to induce the necessary corrective action.For example, temperature changes at a sensor cause pressureor volume changes that are applied directly to the diaphragmor bellows in the valve or damper actuator.

Many complete control systems use a combination of theabove categories. An example of a combined system is thecontrol system for an air handler that includes electric on/offcontrol of the fan and pneumatic control for the heating andcooling coils.

Various control methods are described in the followingsections of this manual:

— Pneumatic Control Fundamentals.— Electric Control Fundamentals.— Electronic Control Fundamentals.— Microprocessor-Based/DDC Fundamental.

See CHARACTERISTICS AND ATTRIBUTES OFCONTROL METHODS.

ANALOG AND DIGITAL CONTROL

Traditionally, analog devices have performed HVAC control.A typical analog HVAC controller is the pneumatic type whichreceives and acts upon data continuously. In a pneumaticcontroller, the sensor sends the controller a continuouspneumatic signal, the pressure of which is proportional to thevalue of the variable being measured. The controller comparesthe air pressure sent by the sensor to the desired value of airpressure as determined by the setpoint and sends out a controlsignal based on the comparison.

The digital controller receives electronic signals fromsensors, converts the electronic signals to digital pulses(values), and performs mathematical operations on thesevalues. The controller reconverts the output value to a signalto operate an actuator. The controller samples digital data atset time intervals, rather than reading it continually. Thesampling method is called discrete control signaling. If thesampling interval for the digital controller is chosen properly,discrete output changes provide even and uninterrupted controlperformance.

Figure 20 compares analog and digital control signals. Thedigital controller periodically updates the process as a functionof a set of measured control variables and a given set of controlalgorithms. The controller works out the entire computation,including the control algorithm, and sends a signal to anactuator. In many of the larger commercial control systems,an electronic-pneumatic transducer converts the electric outputto a variable pressure output for pneumatic actuation of thefinal control element.

SETPOINT

FEEDBACKCONTROLLER

SECONDARYINPUT

CORRECTIVESIGNAL

FINAL CONTROLELEMENT

PROCESS DISTURBANCES

CONTROLLEDVARIABLESENSING

ELEMENT

MANIPULATEDVARIABLE

C2072



17

Fig. 20. Comparison of Analog and Digital Control Signals.

CONTROL MODES

Control systems use different control modes to accomplishtheir purposes. Control modes in commercial applicationsinclude two-position, step, and floating control; proportional,proportional-integral, and proportional-integral-derivativecontrol; and adaptive control.

TWO-POSITION CONTROL

GENERAL

In two-position control, the final control element occupiesone of two possible positions except for the brief period whenit is passing from one position to the other. Two-position controlis used in simple HVAC systems to start and stop electricmotors on unit heaters, fan coil units, and refrigerationmachines, to open water sprays for humidification, and toenergize and deenergize electric strip heaters.

In two-position control, two values of the controlled variable(usually equated with on and off) determine the position ofthe final control element. Between these values is a zone calledthe “differential gap” or “differential” in which the controllercannot initiate an action of the final control element. As thecontrolled variable reaches one of the two values, the finalcontrol element assumes the position that corresponds to thedemands of the controller, and remains there until the controlledvariable changes to the other value. The final control elementmoves to the other position and remains there until thecontrolled variable returns to the other limit.

An example of differential gap would be in a cooling systemin which the controller is set to open a cooling valve when thespace temperature reaches 78F, and to close the valve whenthe temperature drops to 76F. The difference between the twotemperatures (2 degrees F) is the differential gap. Thecontrolled variable fluctuates between the two temperatures.

Basic two-position control works well for many applications.For close temperature control, however, the cycling must beaccelerated or timed.

BASIC TWO-POSITION CONTROL

In basic two-position control, the controller and the finalcontrol element interact without modification from amechanical or thermal source. The result is cyclical operationof the controlled equipment and a condition in which thecontrolled variable cycles back and forth between two values(the on and off points) and is influenced by the lag in thesystem. The controller cannot change the position of the finalcontrol element until the controlled variable reaches one orthe other of the two limits of the differential. For that reason,the differential is the minimum possible swing of the controlledvariable. Figure 21 shows a typical heating system cyclingpattern.

TEMPERATURE(°F)

OFF

ON

75

74

73

72

71

70

69

68

TIME

UNDERSHOOTCONDTION

DIFFERENTIAL

DIAL SETTING

OVERSHOOT CONDTION

C2088

Fig. 21. Typical Operation of Basic Two-Position Control.

The overshoot and undershoot conditions shown in Figure21 are caused by the lag in the system. When the heating systemis energized, it builds up heat which moves into the space towarm the air, the contents of the space, and the thermostat. Bythe time the thermostat temperature reaches the off point (e.g.,72F), the room air is already warmer than that temperature.When the thermostat shuts off the heat, the heating systemdissipates its stored heat to heat the space even more, causingovershoot. Undershoot is the same process in reverse.

In basic two-position control, the presence of lag causes thecontroller to correct a condition that has already passed ratherthan one that is taking place or is about to take place.Consequently, basic two-position control is best used insystems with minimal total system lag (including transfer,measuring, and final control element lags) and where closecontrol is not required.

ANALOG CONTROL SIGNAL

DIGITAL CONTROL SIGNAL

OPEN

FINALCONTROLELEMENTPOSITION

CLOSED

OPEN

FINALCONTROLELEMENTPOSITION

CLOSED

TIME

TIME C2080



18

Figure 22 shows a sample control loop for basic two-positioncontrol: a thermostat turning a furnace burner on or off inresponse to space temperature. Because the thermostat cannotcatch up with fluctuations in temperature, overshoot andundershoot enable the temperature to vary, sometimesconsiderably. Certain industrial processes and auxiliaryprocesses in air conditioning have small system lags and canuse two-position control satisfactorily.

Fig. 22. Basic Two-Position Control Loop.

TIMED TWO-POSITION CONTROL

GENERAL

The ideal method of controlling the temperature in a spaceis to replace lost heat or displace gained heat in exactly theamount needed. With basic two-position control, such exactoperation is impossible because the heating or cooling systemis either full on or full off and the delivery at any specific instantis either too much or too little. Timed two-position control,however, anticipates requirements and delivers measuredquantities of heating or cooling on a percentage on-time basisto reduce control point fluctuations. The timing is accomplishedby a heat anticipator in electric controls and by a timer inelectronic and digital controls.

In timed two-position control, the basic interaction betweenthe controller and the final control element is the same as forbasic two-position control. However, the controller respondsto gradual changes in the average value of the controlledvariable rather than to cyclical fluctuations.

Overshoot and undershoot are reduced or eliminated becausethe heat anticipation or time proportioning feature results in afaster cycling rate of the mechanical equipment. The result iscloser control of the variable than is possible in basic two-position control (Fig. 23).

Fig. 23. Comparison of Basic Two-Position and TimedTwo-Position Control.

HEAT ANTICIPATION

In electromechanical control, timed two-position control canbe achieved by adding a heat anticipator to a bimetal sensingelement. In a heating system, the heat anticipator is connectedso that it energizes whenever the bimetal element calls for heat.On a drop in temperature, the sensing element acts to turn onboth the heating system and the heat anticipator. The heatanticipator heats the bimetal element to its off point early anddeenergizes the heating system and the heat anticipator. Asthe ambient temperature falls, the time required for the bimetalelement to heat to the off point increases, and the cooling timedecreases. Thus, the heat anticipator automatically changesthe ratio of on time to off time as a function of ambienttemperature.

Because the heat is supplied to the sensor only, the heatanticipation feature lowers the control point as the heatrequirement increases. The lowered control point, called“droop”, maintains a lower temperature at design conditionsand is discussed more thoroughly in the following paragraphs.Energizing the heater during thermostat off periodsaccomplishes anticipating action in cooling thermostats. Ineither case, the percentage on-time varies in proportion to thesystem load.

THERMOSTAT

FURNACE

SOLENOIDGAS VALVE

C2715

72

71

73

70

69

74

75

68

CONTROLPOINT

TIME C2089

TIMED TWO-POSITION CONTROL

72

71

73

70

69

74

75

68

OFF

ON

TEMPERATURE(°F)

DIFFERENTIAL

DIAL SETTING

UNDERSHOOTCONDITION

TIME

OVERSHOOTCONDITION

BASIC TWO-POSITION CONTROL

TEMPERATURE(°F)

OFF

ON



19

TIME PROPORTIONING

Time proportioning control provides more effective two-position control than heat anticipation control and is availablewith some electromechanical thermostats and in electronic andmicroprocessor-based controllers. Heat is introduced into thespace using on/off cycles based on the actual heat load on thebuilding and programmable time cycle settings. This methodreduces large temperature swings caused by a large total lagand achieves a more even flow of heat.

In electromechanical thermostats, the cycle rate is adjustableby adjusting the heater. In electronic and digital systems, thetotal cycle time and the minimum on and off times of thecontroller are programmable. The total cycle time setting isdetermined primarily by the lag of the system under control.If the total cycle time setting is changed (e.g., from 10 minutesto 20 minutes), the resulting on/off times change accordingly(e.g., from 7.5 minutes on/2.5 minutes off to 15 minutes on/5minutes off), but their ratio stays the same for a given load.

The cycle time in Figure 24 is set at ten minutes. At a 50percent load condition, the controller, operating at setpoint,produces a 5 minute on/5 minute off cycle. At a 75 percentload condition, the on time increases to 7.5 minutes, the offtime decreases to 2.5 minutes, and the opposite cycle ratiooccurs at 25 percent load. All load conditions maintain thepreset 10-minute total cycle.10

7.5

5

2.5

0

SELECTEDCYCLE TIME(MINUTES)

100 75 50 25 0

LOAD (%)

ON

OFF

C2090

73-1/4

72

70-3/4

0

1-1/4

2-1/2

DR

OO

P (°F

)

CO

NT

RO

L P

OIN

T (°F

)

DESIGNTEMPERATURE

OUTDOOR AIRTEMPERATURE

0% 100%LOADC2091-1

NO LOADTEMPERATURE

Fig. 24. Time Proportioning Control.

Because the controller responds to average temperature orhumidity, it does not wait for a cyclic change in the controlledvariable before signaling corrective action. Thus control systemlags have no significant effect.

Droop in heating control is a lowering of the control pointas the load on the system increases. In cooling control, droopis a raising of the control point. In digital control systems,droop is adjustable and can be set as low as one degree or evenless. Figure 25 shows the relationship of droop to load.

Fig. 25. Relationship between Control Point, Droop,and Load (Heating Control).

Time proportioning control of two-position loads isrecommended for applications such as single-zone systems thatrequire two-position control of heating and/or cooling (e.g., agas-fired rooftop unit with direct-expansion cooling). Timeproportioning control is also recommended for electric heatcontrol, particularly for baseboard electric heat. With timeproportioning control, care must be used to avoid cycling thecontrolled equipment more frequently than recommended bythe equipment manufacturer.

STEP CONTROL

Step controllers operate switches or relays in sequence toenable or disable multiple outputs, or stages, of two-positiondevices such as electric heaters or reciprocating refrigerationcompressors. Step control uses an analog signal to attempt toobtain an analog output from equipment that is typically eitheron or off. Figures 26 and 27 show that the stages may bearranged to operate with or without overlap of the operating(on/off) differentials. In either case, the typical two-positiondifferentials still exist but the total output is proportioned.

74

ONOFF

ONOFF

ONOFF

ONOFF

ONOFF

5

4

3

2

1

DIFFERENTIAL

THROTTLING RANGE

72SPACE TEMPERATURE (°F)

100%0% LOADC2092-1

STAGES

Fig. 26. Electric Heat Stages.



20

Fig. 27. Staged Reciprocating Chiller Control.

Figure 28 shows step control of sequenced DX coils andelectric heat. On a rise in temperature through the throttlingrange at the thermostat, the heating stages sequence off. On afurther rise after a deadband, the cooling stages turn on insequence.

ACTUATOR

AIRFLOW

DAMPER

REFERENCEPRESSURE

PICK-UP

STATICPRESSURE

PICK-UP

FLOATINGSTATIC

PRESSURECONTROLLER

C2717

zero, and the sequence repeats until all stages required to meetthe load condition are on. On a decrease in load, the processreverses.

With microprocessor controls, step control is usually donewith multiple, digital, on-off outputs since software allowseasily adjustable on-to-off per stage and interstage differentialsas well as no-load and time delayed startup and minimum onand off adjustments.

FLOATING CONTROL

Floating control is a variation of two-position control and isoften called “three-position control”. Floating control is not acommon control mode, but is available in most microprocessor-based control systems.

Floating control requires a slow-moving actuator and a fast-responding sensor selected according to the rate of responsein the controlled system. If the actuator should move too slowly,the controlled system would not be able to keep pace withsudden changes; if the actuator should move too quickly, two-position control would result.

Floating control keeps the control point near the setpoint atany load level, and can only be used on systems with minimallag between the controlled medium and the control sensor.Floating control is used primarily for discharge control systemswhere the sensor is immediately downstream from the coil,damper, or device that it controls. An example of floatingcontrol is the regulation of static pressure in a duct (Fig. 29).

ONOFF

ONOFF

ONOFF

ONOFF

4

3

2

1

DIFFERENTIAL

THROTTLING RANGE

76

SPACE TEMPERATURE (°F)

72

100%0% LOADC2093

STAGES

74SETPOINT

SPACE ORRETURN AIR

THERMOSTATACTUATOR

SOLENOIDVALVES

FAN

DISCHARGEAIR

DIRECT EXPANSIONCOILS

MULTISTAGEELECTRIC HEAT

STEPCONTROLLER

STAGE NUMBERS6

5

4

3

2

1

C2716

D

X

D

X

Fig. 28. Step Control with Sequenced DX Coils andElectric Heat.

A variation of step control used to control electric heat isstep-plus-proportional control, which provides a smoothtransition between stages. This control mode requires one ofthe stages to be a proportional modulating output and the others,two-position. For most efficient operation, the proportionalmodulating stage should have at least the same capacity asone two-position stage.

Starting from no load, as the load on the equipment increases,the modulating stage proportions its load until it reaches fulloutput. Then, the first two-position stage comes full on andthe modulating stage drops to zero output and begins toproportion its output again to match the increasing load. Whenthe modulating stage again reaches full output, the second two-position stage comes full on, the modulating stage returns to

Fig. 29. Floating Static Pressure Control.

In a typical application, the control point moves in and outof the deadband, crossing the switch differential (Fig. 30). Adrop in static pressure below the controller setpoint causes theactuator to drive the damper toward open. The narrowdifferential of the controller stops the actuator after it has moveda short distance. The damper remains in this position until thestatic pressure further decreases, causing the actuator to drivethe damper further open. On a rise in static pressure above thesetpoint, the reverse occurs. Thus, the control point can floatbetween open and closed limits and the actuator does not move.When the control point moves out of the deadband, thecontroller moves the actuator toward open or closed until thecontrol point moves into the deadband again.



21

Fig. 30. Floating Control.

VALVE

CONTROLLER

SENSOR

CHILLEDWATER

RETURNAIR

DISCHARGEAIR

COIL C2718

PROPORTIONAL CONTROL

GENERAL

Proportional control proportions the output capacity of theequipment (e.g., the percent a valve is open or closed) to matchthe heating or cooling load on the building, unlike two-positioncontrol in which the mechanical equipment is either full on orfull off. In this way, proportional control achieves the desiredheat replacement or displacement rate.

In a chilled water cooling system, for example (Fig. 31), thesensor is placed in the discharge air. The sensor measures theair temperature and sends a signal to the controller. If acorrection is required, the controller calculates the change andsends a new signal to the valve actuator. The actuatorrepositions the valve to change the water flow in the coil, andthus the discharge temperature.

73 74 75 76 77

100%OPEN

50%OPEN

CLOSED

POSITION OF FINALCONTROL ELEMENT

ACTUATORPOSITION

CONTROL POINT (°F)

THROTTLING RANGE

C2095

Fig. 31. Proportional Control Loop.

In proportional control, the final control element moves to aposition proportional to the deviation of the value of thecontrolled variable from the setpoint. The position of the finalcontrol element is a linear function of the value of the controlledvariable (Fig. 32).

T1 T2 T3 T4 T5 T6

TIME

NO LOAD

FULL LOAD

CLOSED

OPEN

DAMPERPOSITION

DEADBAND

C2094

LOAD

OFF

OFF

ON

ON

“CLOSE”SWITCH

DIFFERENTIAL

“OPEN”SWITCH

DIFFERENTIAL

T7

CONTROLLER

CONTROL POINT

SETPOINT

Fig. 32. Final Control Element Position as a Function ofthe Control Point (Cooling System).

The final control element is seldom in the middle of its rangebecause of the linear relationship between the position of thefinal control element and the value of the controlled variable.In proportional control systems, the setpoint is typically themiddle of the throttling range, so there is usually an offsetbetween control point and setpoint.



22

Fig. 33. Relationship of Offset to Load(Heating Application).

The throttling range is the amount of change in the controlledvariable required for the controller to move the controlleddevice through its full operating range. The amount of changeis expressed in degrees Fahrenheit for temperature, inpercentages for relative humidity, and in pounds per squareinch or inches of water for pressure. For some controllers,throttling range is referred to as “proportional band”.Proportional band is throttling range expressed as a percentageof the controller sensor span:

“Gain” is a term often used in industrial control systems forthe change in the controlled variable. Gain is the reciprocal ofproportional band:

The output of the controller is proportional to the deviationof the control point from setpoint. A proportional controllercan be mathematically described by:

V = KE + M

An example of offset would be the proportional control of achilled water coil used to cool a space. When the cooling loadis 50 percent, the controller is in the middle of its throttlingrange, the properly sized coil valve is half-open, and there isno offset. As the outdoor temperature increases, the roomtemperature rises and more cooling is required to maintain thespace temperature. The coil valve must open wider to deliverthe required cooling and remain in that position as long as theincreased requirement exists. Because the position of the finalcontrol element is proportional to the amount of deviation, thetemperature must deviate from the setpoint and sustain thatdeviation to open the coil valve as far as required.

Figure 33 shows that when proportional control is used in aheating application, as the load condition increases from 50percent, offset increases toward cooler. As the load conditiondecreases, offset increases toward warmer. The opposite occursin a cooling application.

Outdoor Air Discharge AirTemperature Temperature

Condition (F) (F)

Outdoor designtemperature

0 100

Light load 70 70

100

70

0(FULL

RESET)

70(RESETSTART)

OUTDOOR AIR TEMPERATURE (°F)

DIS

CH

AR

GE

AIR

TE

MP

ER

AT

UR

E S

ET

PO

INT

(°F

)

C2719

(FULL RESET)

Where:V = output signalK = proportionality constant (gain)E = deviation (control point - setpoint)

M = value of the output when the deviation iszero (Usually the output value at 50 percentor the middle of the output range. Thegenerated control signal correction is addedto or subtracted from this value. Also called“bias” or “manual reset”.)

Although the control point in a proportional control systemis rarely at setpoint, the offset may be acceptable.Compensation, which is the resetting of the setpoint tocompensate for varying load conditions, may also reduce theeffect of proportional offset for more accurate control. Anexample of compensation is resetting boiler water temperaturebased on outdoor air temperature. Compensation is also called“reset control” or “cascade control”.

COMPENSATION CONTROL

GENERAL

Compensation is a control technique available in proportionalcontrol in which a secondary, or compensation, sensor resetsthe setpoint of the primary sensor. An example of compensationwould be the outdoor temperature resetting the dischargetemperature of a fan system so that the discharge temperatureincreases as the outdoor temperature decreases. The samplereset schedule in Table 2 is shown graphically in Figure 34.Figure 35 shows a control diagram for the sample reset system.

Table 2. Sample Reset Schedule.Fig. 34. Typical Reset Schedule for Discharge Air

OFFSET

OFFSET50%LOAD

COOLER

WARMER

SETPOINT0%LOAD

CONTROL POINT

C2096

100%LOAD

Proportional Band = Throttling Range

Sensor Span x 100

Gain = 100

Proportional Band

Control.



23

In an application requiring negative reset, a change in outdoorair temperature at the reset sensor from 0 to 60F resets the hotwater supply temperature (primary sensor) setpoint from 200to 100F. Assuming a throttling range of 15 degrees F, therequired authority is calculated as follows:

Authority = 192%

The previous example assumes that the spans of the twosensors are equal. If sensors with unequal spans are used, acorrection factor is added to the formula:

Assuming the same conditions as in the previous example,a supply water temperature sensor range of 40 to 240F (spanof 200 degrees F), an outdoor air temperature (compensation)sensor range of -20 to 80F (span of 100 degrees F), and athrottling range of 10 degrees F, the calculation for negativereset would be as follows:

Authority = 92%

The effects of throttling range may be disregarded with PI resetcontrols.

PROPORTIONAL-INTEGRAL (PI) CONTROL

In the proportional-integral (PI) control mode, reset of thecontrol point is automatic. PI control, also called “proportional-plus-reset” control, virtually eliminates offset and makes theproportional band nearly invisible. As soon as the controlledvariable deviates above or below the setpoint and offsetdevelops, the proportional band gradually and automaticallyshifts, and the variable is brought back to the setpoint. Themajor difference between proportional and PI control is thatproportional control is limited to a single final control elementposition for each value of the controlled variable. PI controlchanges the final control element position to accommodateload changes while keeping the control point at or very nearthe setpoint.

Authority = Change in setpoint + TR

Change in compensation input x 100

Authority = Change in setpoint + TR


Fig. 35. Discharge Air Control Loop with Reset.

Compensation can either increase or decrease the setpointas the compensation input increases. Increasing the setpointby adding compensation on an increase in the compensationvariable is often referred to as positive or summercompensation. Increasing the setpoint by adding compensationon a decrease in the compensation variable is often referred toas negative or winter compensation. Compensation is mostcommonly used for temperature control, but can also be usedwith a humidity or other control system.

Some controllers provide compensation start point capability.Compensation start point is the value of the compensationsensor at which it starts resetting the controller primary sensorsetpoint.

COMPENSATION AUTHORITY

Compensation authority is the ratio of the effect of thecompensation sensor relative to the effect of the primary sensor.Authority is stated in percent.

The basic equation for compensation authority is:

For proportional controllers, the throttling range (TR) isincluded in the equation. Two equations are required when thethrottling range is included. For direct-acting or positivecompensation, in which the setpoint increases as thecompensation input increases, the equation is:

Direct-acting compensation is commonly used to preventcondensation on windows by resetting the relative humiditysetpoint downward as the outdoor temperature decreases.

For reverse-acting or negative compensation, in which thesetpoint decreases as the compensation input increases, theequation is:

Authority = Change in setpoint – TR


Authority = Change in setpoint


Authority = 100200

x 200 – 100 + 10

60 – 0 x 100

TEMPERATURECONTROLLER

SENSOR

FAN

RETURN

SUPPLY

OUTDOOR AIRTEMPERATURE

SENSOR

DISCHARGEAIR

C2720 = 200 – 100 + 15

60 – 0 x 100

Authority =

Compensation sensor spanPrimary sensor span x

Change in setpoint ± TRChange in compensation input x 100

Correction Factor



24

Reset error correction time is proportional to the deviationof the controlled variable. For example, a four-percent deviationfrom the setpoint causes a continuous shift of the proportionalband at twice the rate of shift for a two-percent deviation. Resetis also proportional to the duration of the deviation. Resetaccumulates as long as there is offset, but ceases as soon asthe controlled variable returns to the setpoint.

With the PI controller, therefore, the position of the finalcontrol element depends not only upon the location of thecontrolled variable within the proportional band (proportionalband adjustment) but also upon the duration and magnitude ofthe deviation of the controlled variable from the setpoint (resettime adjustment). Under steady state conditions, the controlpoint and setpoint are the same for any load conditions, asshown in Figure 37.

PI control adds a component to the proportional controlalgorithm and is described mathematically by:

V = KE + ∫ Edt + M


T1

= reset timeK/T

1= reset gain

dt = differential of time (increment in time)M = value of the output when the deviation

is zero

Integral windup, or an excessive overshoot condition, canoccur in PI control. Integral windup is caused by the integralfunction making a continued correction while waiting forfeedback on the effects of its correction. While integral actionkeeps the control point at setpoint during steady stateconditions, large overshoots are possible at start-up or duringsystem upsets (e.g., setpoint changes or large load changes).On many systems, short reset times also cause overshoot.

Integral windup may occur with one of the following:— When the system is off.— When the heating or cooling medium fails or is not

available.— When one control loop overrides or limits another.

Integral windup can be avoided and its effects diminished.At start-up, some systems disable integral action until measuredvariables are within their respective proportional bands.Systems often provide integral limits to reduce windup due toload changes. The integral limits define the extent to whichintegral action can adjust a device (the percent of full travel).The limit is typically set at 50 percent.

The reset action of the integral component shifts theproportional band as necessary around the setpoint as the loadon the system changes. The graph in Figure 36 shows the shiftof the proportional band of a PI controller controlling anormally open heating valve. The shifting of the proportionalband keeps the control point at setpoint by making furthercorrections in the control signal. Because offset is eliminated,the proportional band is usually set fairly wide to ensure systemstability under all operating conditions.

Integral

KT

1

90 95 100 105 110

CLOSED

50% OPEN

100% OPEN

SETPOINT (°F)

PROPORTIONAL BANDFOR SEPARATE LOAD

CONDITIONS

HEATINGVALVE

POSITION

0%LOAD

50%LOAD

100%LOAD

= CONTROL POINTTHROTTLING RANGE = 10 DEGREES F C2097-1

T1 T2 T3 T4

CLOSED

VALVEPOSITION

OPEN

SETPOINT

TIMEC2098

DEVIATIONFROM

SETPOINT

INTEGRAL ACTION

CONTROL POINT (LOAD CHANGES)

PROPORTIONAL CORRECTION

Fig. 36. Proportional Band Shift Due to Offset.

Reset of the control point is not instantaneous. Wheneverthe load changes, the controlled variable changes, producingan offset. The proportional control makes an immediatecorrection, which usually still leaves an offset. The integralfunction of the controller then makes control corrections overtime to bring the control point back to setpoint (Fig. 37). Inaddition to a proportional band adjustment, the PI controlleralso has a reset time adjustment that determines the rate atwhich the proportional band shifts when the controlled variabledeviates any given amount from the setpoint.

Fig. 37. Proportional-Integral Control Response toLoad Changes.



25

PROPORTIONAL-INTEGRAL-DERIVATIVE (PID)CONTROL

Proportional-integral-derivative (PID) control adds thederivative function to PI control. The derivative functionopposes any change and is proportional to the rate of change.The more quickly the control point changes, the more correctiveaction the derivative function provides.

If the control point moves away from the setpoint, thederivative function outputs a corrective action to bring thecontrol point back more quickly than through integral actionalone. If the control point moves toward the setpoint, thederivative function reduces the corrective action to slow downthe approach to setpoint, which reduces the possibility ofovershoot.

The rate time setting determines the effect of derivativeaction. The proper setting depends on the time constants ofthe system being controlled.

The derivative portion of PID control is expressed in thefollowing formula. Note that only a change in the magnitudeof the deviation can affect the output signal.

V = KTD

Where:V = output signalK = proportionality constant (gain)

TD

= rate time (time interval by which thederivative advances the effect ofproportional action)

KTD

= rate gain constantdE/dt = derivative of the deviation with respect to

time (error signal rate of change)

The complete mathematical expression for PID controlbecomes:

V = KE + ∫Edt + KTD + M


T1

= reset timeK/T

1= reset gain

dt = differential of time (increment in time)T

D= rate time (time interval by which the

derivative advances the effect ofproportional action)

KTD

= rate gain constantdE/dt = derivative of the deviation with respect to

time (error signal rate of change)M = value of the output when the deviation

is zero

SETPOINT

T1 T2 T3 T4 T5 T6

C2099

CONTROL POINT OFFSET

TIME

SETPOINT

T1 T2 T3 T4 T5 T6

C2100

CONTROL POINT OFFSET

TIME

SETPOINT

T1 T2 T3 T4 T5 T6

C2501TIME

OFFSET

The graphs in Figures 38, 39, and 40 show the effects of allthree modes on the controlled variable at system start-up. Withproportional control (Fig. 38), the output is a function of thedeviation of the controlled variable from the setpoint. As thecontrol point stabilizes, offset occurs. With the addition ofintegral control (Fig. 39), the control point returns to setpointover a period of time with some degree of overshoot. Thesignificant difference is the elimination of offset after thesystem has stabilized. Figure 40 shows that adding thederivative element reduces overshoot and decreases responsetime.

dt

dEdt

dEKT

1

Proportional Integral Derivative

Fig. 38. Proportional Control.

Fig. 39. Proportional-Integral Control.

Fig. 40. Proportional-Integral-Derivative Control.

ENHANCED PROPORTIONAL-INTEGRAL-DERIVATIVE (EPID) CONTROL

The startup overshoot, or undershoot in some applications,noted in Figures 38, 39, and 40 is attributable to the verylarge error often present at system startup. Microprocessor-based PID startup performance may be greatly enhanced byexterior error management appendages available withenhanced proportional-integral-derivative (EPID) control.Two basic EPID functions are start value and error ramp time.



26

The start value EPID setpoint sets the output to a fixed valueat startup. For a VAV air handling system supply fan, a suitablevalue might be twenty percent, a value high enough to get thefan moving to prove operation to any monitoring system andto allow the motor to self cool. For a heating, cooling, andventilating air handling unit sequence, a suitable start valuewould be thirty-three percent, the point at which the heating,ventilating (economizer), and mechanical cooling demands areall zero. Additional information is available in the Air HandlingSystem Control Applications section.

The error ramp time determines the time duration duringwhich the PID error (setpoint minus input) is slowly ramped,linear to the ramp time, into the PID controller. The controllerthus arrives at setpoint in a tangential manner withoutovershoot, undershoot, or cycling. See Figure 41.

Adaptive control is also used in energy managementprograms such as optimum start. The optimum start programenables an HVAC system to start as late as possible in themorning and still reach the comfort range by the time thebuilding is occupied for the lease energy cost. To determinethe amount of time required to heat or cool the building, theoptimum start program uses factors based on previous buildingresponse, HVAC system characteristics, and current weatherconditions. The algorithm monitors controller performance bycomparing the actual and calculated time required to bring thebuilding into the comfort range and tries to improve thisperformance by calculating new factors.

PROCESS CHARACTERISTICS

As pumps and fans distribute the control agent throughoutthe building, an HVAC system exhibits several characteristicsthat must be understood in order to apply the proper controlmode to a particular building system.

LOAD

Process load is the condition that determines the amount ofcontrol agent the process requires to maintain the controlledvariable at the desired level. Any change in load requires achange in the amount of control agent to maintain the samelevel of the controlled variable.

Load changes or disturbances are changes to the controlledvariable caused by altered conditions in the process or itssurroundings. The size, rate, frequency, and duration ofdisturbances change the balance between input and output.

Four major types of disturbances can affect the quality ofcontrol:

— Supply disturbances— Demand disturbances— Setpoint changes— Ambient (environmental) variable changes

Supply disturbances are changes in the manipulated variableinput into the process to control the controlled variable. Anexample of a supply disturbance would be a decrease in thetemperature of hot water being supplied to a heating coil. Moreflow is required to maintain the temperature of the air leavingthe coil.

Demand disturbances are changes in the controlled mediumthat require changes in the demand for the control agent. Inthe case of a steam-to-water converter, the hot water supplytemperature is the controlled variable and the water is thecontrolled medium (Fig. 42). Changes in the flow ortemperature of the water returning to the converter indicate ademand load change. An increased flow of water requires anincrease in the flow of the control agent (steam) to maintainthe water temperature. An increase in the returning watertemperature, however, requires a decrease in steam to maintainthe supply water temperature.

SETPOINT

T1 T2 T3 T4 T5 T6

M13038ELAPSED TIME

OFFSET

T7 T8

CONTROL POINT

0

100

STARTVALUE

ERRORRAMPTIME

AC

TU

AT

OR

PO

SIT

ION

PE

RC

EN

T O

PE

N

Fig. 41. Enhanced Proportional-Integral-Derivative(EPID) Control.

ADAPTIVE CONTROL

Adaptive control is available in some microprocessor-basedcontrollers. Adaptive control algorithms enable a controller toadjust its response for optimum control under all loadconditions. A controller that has been tuned to controlaccurately under one set of conditions cannot always respondwell when the conditions change, such as a significant loadchange or changeover from heating to cooling or a change inthe velocity of a controlled medium.

An adaptive control algorithm monitors the performance ofa system and attempts to improve the performance by adjustingcontroller gains or parameters. One measurement ofperformance is the amount of time the system requires to reactto a disturbance: usually the shorter the time, the better theperformance. The methods used to modify the gains orparameters are determined by the type of adaptive algorithm.Neural networks are used in some adaptive algorithms.

An example of a good application of adaptive control isdischarge temperature control of the central system coolingcoil for a VAV system. The time constant of a sensor varies asa function of the velocity of the air (or other fluid). Thus thetime constant of the discharge air sensor in a VAV system isconstantly changing. The change in sensor response affectsthe system control so the adaptive control algorithm adjustssystem parameters such as the reset and rate settings to maintainoptimum system performance.



27

HEAT LOSS

COLDAIR

VALVE

THERMOSTAT

C2074

SPACE

Fig. 42. Steam-to-Water Converter.

A setpoint change can be disruptive because it is a suddenchange in the system and causes a disturbance to the hot watersupply. The resulting change passes through the entire processbefore being measured and corrected.

Ambient (environmental) variables are the conditionssurrounding a process, such as temperature, pressure, andhumidity. As these conditions change, they appear to the controlsystem as changes in load.

LAG

GENERAL

Time delays, or lag, can prevent a control system fromproviding an immediate and complete response to a change inthe controlled variable. Process lag is the time delay betweenthe introduction of a disturbance and the point at which thecontrolled variable begins to respond. Capacitance, resistance,and/or dead time of the process contribute to process lag andare discussed later in this section.

One reason for lag in a temperature control system is that achange in the controlled variable (e.g., space temperature) doesnot transfer instantly. Figure 43 shows a thermostat controllingthe temperature of a space. As the air in the space loses heat,the space temperature drops. The thermostat sensing elementcannot measure the temperature drop immediately becausethere is a lag before the air around the thermostat loses heat.The sensing element also requires a measurable time to cool.The result is a lag between the time the space begins to loseheat and the time corrective action is initiated.

Fig. 43. Heat Loss in a Space Controlled by aThermostat.

Lag also occurs between the release of heat into the space,the space warming, and the thermostat sensing the increasedtemperature. In addition, the final control element requires timeto react, the heat needs time to transfer to the controlledmedium, and the added energy needs time to move into thespace. Total process lag is the sum of the individual lagsencountered in the control process.

MEASUREMENT LAG

Dynamic error, static error, reproducibility, and dead zoneall contribute to measurement lag. Because a sensing elementcannot measure changes in the controlled variable instantly,dynamic error occurs and is an important factor in control.Dynamic error is the difference between the true and themeasured value of a variable and is always present when thecontrolled variable changes. The variable usually fluctuatesaround the control point because system operating conditionsare rarely static. The difference is caused by the mass of thesensing element and is most pronounced in temperature andhumidity control systems. The greater the mass, the greaterthe difference when conditions are changing. Pressure sensinginvolves little dynamic error.

Static error is the deviation between a measured value andthe true value of the static variable. Static error can be causedby sensor calibration error. Static error is undesirable but notalways detrimental to control.

Repeatability is the ability of a sensor or controller to outputthe same signal when it measures the same value of a variableor load at different times. Precise control requires a high degreeof reproducibility.

CONTROLLER

VALVE

STEAM(CONTROL AGENT)

FLOW (MANIPULATED VARIABLE)

WATERTEMPERATURE(CONTROLLED

VARIABLE)

HOT WATER SUPPLY(CONTROLLED

MEDIUM)

HOT WATERRETURN

CONVERTER

STEAM TRAP

CONDENSATE RETURN

C2073

LOAD



28

The difference between repeatability and static error is thatrepeatability is the ability to return to a specific condition,whereas static error is a constant deviation from that condition.Static error (e.g., sensor error) does not interfere with the abilityto control, but requires that the control point be shifted tocompensate and maintain a desired value.

The dead zone is a range through which the controlledvariable changes without the controller initiating a correction.The dead zone effect creates an offset or a delay in providingthe initial signal to the controller. The more slowly the variablechanges, the more critical the dead zone becomes.

CAPACITANCE

Capacitance differs from capacity. Capacity is determinedby the energy output the system is capable of producing;capacitance relates to the mass of the system. For example,for a given heat input, it takes longer to raise the temperatureof a cubic foot of water one degree than a cubic foot of air.When the heat source is removed, the air cools off morequickly than the water. Thus the capacitance of the water ismuch greater than the capacitance of air.

A capacitance that is large relative to the control agent tendsto keep the controlled variable constant despite load changes.However, the large capacitance makes changing the variableto a new value more difficult. Although a large capacitancegenerally improves control, it introduces lag between the timea change is made in the control agent and the time the controlledvariable reflects the change.

Figure 44 shows heat applied to a storage tank containing alarge volume of liquid. The process in Figure 44 has a largethermal capacitance. The mass of the liquid in the tank exertsa stabilizing effect and does not immediately react to changessuch as variations in the rate of the flow of steam or liquid,minor variations in the heat input, and sudden changes in theambient temperature.

LIQUID IN

HEATINGMEDIUM IN

LIQUID OUTHEATING

MEDIUM OUT C2076

Figure 45 shows a high-velocity heat exchanger, whichrepresents a process with a small thermal capacitance. Therate of flow for the liquid in Figure 45 is the same as for theliquid in Figure 44. However, in Figure 45 the volume andmass of the liquid in the tube at any one time is small comparedto the tank shown in Figure 44. In addition, the total volumeof liquid in the exchanger at any time is small compared to therate of flow, the heat transfer area, and the heat supply. Slightvariations in the rate of feed or rate of heat supply show upimmediately as fluctuations in the temperature of the liquidleaving the exchanger. Consequently, the process in Figure 45does not have a stabilizing influence but can respond quicklyto load changes.

Fig. 45. Typical Process with Small ThermalCapacitance.

Figure 46 shows supply capacitance in a steam-to-waterconverter. When the load on the system (in Figure 44, coldair) increases, air leaving the heating coil is cooler. Thecontroller senses the drop in temperature and calls for moresteam to the converter. If the water side of the converter islarge, it takes longer for the temperature of the supply water torise than if the converter is small because a load change in aprocess with a large supply capacitance requires more time tochange the variable to a new value.

LIQUIDIN

STEAMIN

TANK

LIQUIDOUT

CONDENSATE RETURN

C2075

Fig. 44. Typical Process with Large ThermalCapacitance.

CONVERTER

STEAM

VALVE

CONTROLLER

HOT WATER SUPPLY(CONSTANT FLOW,

VARYINGTEMPERATURE)

HOT WATER RETURN

COLD AIR(LOAD)

CONDENSATERETURN

STEAM TRAP

HEATING COIL

HOT AIR(CONTROLLED

VARIABLE)

PUMP

C2077

Fig. 46. Supply Capacitance (Heating Application).



29

In terms of heating and air conditioning, a large office areacontaining desks, file cabinets, and office machinery has morecapacitance than the same area without furnishings. When thetemperature is lowered in an office area over a weekend, thefurniture loses heat. It takes longer to heat the space to thecomfort level on Monday morning than it does on othermornings when the furniture has not had time to lose as muchheat. If the area had no furnishings, it would heat up muchmore quickly.

The time effect of capacitance determines the processreaction rate, which influences the corrective action that thecontroller takes to maintain process balance.

RESISTANCE

Resistance applies to the parts of the process that resist theenergy (or material) transfer. Many processes, especially thoseinvolving temperature control, have more than one capacitance.The flow of energy (heat) passing from one capacitance througha resistance to another capacitance causes a transfer lag(Fig. 47).

Fig. 47. Schematic of Heat Flow Resistance.

A transfer lag delays the initial reaction of the process. Intemperature control, transfer lag limits the rate at which theheat input affects the controlled temperature. The controllertends to overshoot the setpoint because the effect of the addedheat is not felt immediately and the controller calls for stillmore heat.

The office described in the previous example is comfortableby Monday afternoon and appears to be at control point.However, the paper in the middle of a full file drawer wouldstill be cold because paper has a high thermal resistance. As aresult, if the heat is turned down 14 hours a day and is at comfortlevel 10 hours a day, the paper in the file drawer will neverreach room temperature.

An increase in thermal resistance increases the temperaturedifference and/or flow required to maintain heat transfer. Ifthe fins on a coil become dirty or corroded, the resistance tothe transfer of heat from one medium to the other mediumincreases.

DEAD TIME

Dead time, which is also called “transportation lag”, is thedelay between two related actions in a continuous processwhere flow over a distance at a certain velocity is associatedwith energy transfer. Dead time occurs when the control valveor sensor is installed at a distance from the process (Fig. 48).

VALVE

HWS

PROCESSCONTROLLED

MEDIUM IN

HWR

24 FT

2 FT

CONTROLLEDMEDIUM OUT

SENSOR ATLOCATION 1

SENSOR ATLOCATION 2

CONTROLLER

VELOCITY OF CONTROLLED MEDIUM: 12 FT/S

DEAD TIME FOR SENSOR AT LOCATION 1: = 0.166 SEC

DEAD TIME FOR SENSOR AT LOCATION 2: = 2.0 SEC

2 FT12 FT/S

24 FT12 FT/S

C2079STEAM

IN HEAT CAPACITYOF STEAMIN COILS RESISTANCE TO

HEAT FLOW(E.G., PIPES, TANK WALLS)

COLD WATERIN

HEAT CAPACITYOF WATER

IN TANK

HOT WATER

OUT

C2078

Fig. 48. Effect of Location on Dead Time.

Dead time does not change the process reactioncharacteristics, but instead delays the process reaction. Thedelay affects the system dynamic behavior and controllability,because the controller cannot initiate corrective action until itsees a deviation. Figure 48 shows that if a sensor is 24 feetaway from a process, the controller that changes the positionof the valve requires two seconds to see the effect of thatchange, even assuming negligible capacitance, transfer, andmeasurement lag. Because dead time has a significant effecton system control, careful selection and placement of sensorsand valves is required to maintain system equilibrium.

CONTROL APPLICATION GUIDELINES

The following are considerations when determining controlrequirements:

— The degree of accuracy required and the amount of offset,if any, that is acceptable.

— The type of load changes expected, including their size,rate, frequency, and duration.

— The system process characteristics, such as timeconstants, number of time lag elements, andreaction rate.



30

Each control mode is applicable to processes having certaincombinations of the basic characteristics. The simplest modeof control that meets application requirements is the best modeto use, both for economy and for best results. Using a control

Table 3. Control Applications and Recommended Control Modes.

CONTROL SYSTEM COMPONENTS

M10518

Control Application Recommended Control Modea

Space Temperature P, PID

Mixed Air Temperature PI, EPID

Coil Discharge Temperature PI, EPID

Chiller Discharge Temperature PI, EPID

Hot Water Converter Discharge Temperature PI, EPID

Airflow PI Use a wide proportional band and a fast reset rate. For someapplications, PID may be required.

Fan Static Pressure PI , EPID

Humidity P, or if very tight control is required, PI

Dewpoint Temperature P, or if very tight control is required, PIa PID, EPID control is used in digital systems.

mode that is too complicated for the application may result inpoor rather than good control. Conversely, using a controlmode that is too basic for requirements can make adequatecontrol impossible. Table 3 lists typical control applicationsand recommended control modes.

Control system components consist of sensing elements,controllers, actuators, and auxiliary equipment.

SENSING ELEMENTS

A sensing element measures the value of the controlledvariable. Controlled variables most often sensed in HVACsystems are temperature, pressure, relative humidity, and flow.

TEMPERATURE SENSING ELEMENTS

The sensing element in a temperature sensor can be a bimetalstrip, a rod-and-tube element, a sealed bellows, a sealed bellowsattached to a capillary or bulb, a resistive wire, or a thermistor.Refer to the Electronic Control Fundamentals section of thismanual for Electronic Sensors for Microprocessor BasedSystems.

A bimetal element is a thin metallic strip composed of twolayers of different kinds of metal. Because the two metals havedifferent rates of heat expansion, the curvature of the bimetalchanges with changes in temperature. The resulting movementof the bimetal can be used to open or close circuits in electriccontrol systems or regulate airflow through nozzles inpneumatic control systems. Winding the bimetal in a coil (Fig. 49) enables a greater length of the bimetal to be used ina limited space.

Fig. 49. Coiled Bimetal Element.

The rod-and-tube element (Fig. 50) also uses the principleof expansion of metals. It is used primarily for insertion directlyinto a controlled medium, such as water or air. In a typicalpneumatic device, a brass tube contains an Invar rod which isfastened at one end to the tube and at the other end to a springand flapper. Brass has the higher expansion coefficient and isplaced outside to be in direct contact with the measuredmedium. Invar does not expand noticeably with temperaturechanges. As the brass tube expands lengthwise, it pulls theInvar rod with it and changes the force on the flapper. Theflapper is used to generate a pneumatic signal. When the flapperposition changes, the signal changes correspondingly.



31

Fig. 50. Rod-and-Tube Element.

In a remote-bulb controller (Fig. 51), a remote capsule, orbulb, is attached to a bellows housing by a capillary. The remotebulb is placed in the controlled medium where changes intemperature cause changes in pressure of the fill. The capillarytransmits changes in fill pressure to the bellows housing andthe bellows expands or contracts to operate the mechanicaloutput to the controller. The bellows and capillary also sensetemperature, but because of their small volume compared tothe bulb, the bulb provides the control.

Fig. 51. Typical Remote-Bulb Element.

Two specialized versions of the remote bulb controller areavailable. They both have no bulb and use a long capillary (15to 28 feet) as the sensor. One uses an averaging sensor that isliquid filled and averages the temperature over the full lengthof the capillary. The other uses a cold spot or low temperaturesensor and is vapor filled and senses the coldest spot (12 inchesor more) along its length.

Electronic temperature controllers use low-mass sensingelements that respond quickly to changes in the controlledcondition. A signal sent by the sensor is relatively weak, but isamplified to a usable strength by an electronic circuit.

The temperature sensor for an electronic controller may bea length of wire or a thin metallic film (called a resistancetemperature device or RTD) or a thermistor. Both types ofresistance elements change electrical resistance as temperaturechanges. The wire increases resistance as its temperatureincreases. The thermistor is a semiconductor that decreases inresistance as the temperature increases.

Because electronic sensors use extremely low mass, theyrespond to temperature changes more rapidly than bimetal orsealed-fluid sensors. The resistance change is detected by abridge circuit. Nickel “A”, BALCO, and platinum are typicalmaterials used for this type of sensor.

In thermocouple temperature-sensing elements, twodissimilar metals (e.g., iron and nickel, copper and constantan,iron and constantan) are welded together. The junction of thetwo metals produces a small voltage when exposed to heat.Connecting two such junctions in series doubles the generatedvoltage. Thermocouples are used primarily for high-temperature applications.

Many special application sensors are available, includingcarbon dioxide sensors and photoelectric sensors used insecurity, lighting control, and boiler flame safeguardcontrollers.

PRESSURE SENSING ELEMENTS

Pressure sensing elements respond to pressure relative to aperfect vacuum (absolute pressure sensors), atmosphericpressure (gage pressure sensors), or a second system pressure(differential pressure sensors), such as across a coil or filter.Pressure sensors measure pressure in a gas or liquid in poundsper square inch (psi). Low pressures are typically measured ininches of water. Pressure can be generated by a fan, a pump orcompressor, a boiler, or other means.

Pressure controllers use bellows, diaphragms, and a numberof other electronic pressure sensitive devices. The mediumunder pressure is transmitted directly to the device, and themovement of the pressure sensitive device operates themechanism of a pneumatic or electric switching controller.Variations of the pressure control sensors measure rate of flow,quantity of flow, liquid level, and static pressure. Solid statesensors may use the piezoresistive effect in which increasedpressure on silicon crystals causes resistive changes in thecrystals.

FLAPPERSPRING

SIGNAL PORT

BRASS TUBE

INVAR ROD

EXTENSION SPRING

SENSOR BODYC2081

MECHANICAL OUTPUTTO CONTROLLER

BELLOWS

LIQUIDFILL

CAPILLARYCONTROLLED

MEDIUM(E.G., WATER)

BULB

C2083



32

FLOW SENSORS

Flow sensors sense the rate of liquid and gas flow in volumeper unit of time. Flow is difficult to sense accurately under allconditions. Selecting the best flow-sensing technique for anapplication requires considering many aspects, especially thelevel of accuracy required, the medium being measured, andthe degree of variation in the measured flow.

A simple flow sensor is a vane or paddle inserted into themedium (Fig. 53) and generally called a flow switch. Thepaddle is deflected as the medium flows and indicates that themedium is in motion and is flowing in a certain direction. Vaneor paddle flow sensors are used for flow indication andinterlock purposes (e.g., a system requires an indication thatwater is flowing before the system starts the chiller).

MOISTURE SENSING ELEMENTS

Elements that sense relative humidity fall generally into twoclasses: mechanical and electronic. Mechanical elementsexpand and contract as the moisture level changes and arecalled “hygroscopic” elements. Several hygroscopic elementscan be used to produce mechanical output, but nylon is themost commonly used element (Fig. 52). As the moisturecontent of the surrounding air changes, the nylon elementabsorbs or releases moisture, expanding or contracting,respectively. The movement of the element operates thecontroller mechanism.

NYLON ELEMENT

RELATIVE HUMIDITY SCALEC2084

LOW HIGH

ON/OFF SIGNALTO CONTROLLER

SENSOR

PIVOT

FLOW

PADDLE (PERPENDICULAR TO FLOW)C2085

Fig. 52. Typical Nylon Humidity Sensing Element.

Electronic sensing of relative humidity is fast and accurate.An electronic relative humidity sensor responds to a changein humidity by a change in either the resistance or capacitanceof the element.

If the moisture content of the air remains constant, therelative humidity of the air increases as temperature decreasesand decreases as temperature increases. Humidity sensors alsorespond to changes in temperature. If the relative humidity isheld constant, the sensor reading can be affected by temperaturechanges. Because of this characteristic, humidity sensorsshould not be used in atmospheres that experience widetemperature variations unless temperature compensation isprovided. Temperature compensation is usually provided withnylon elements and can be factored into electronic sensorvalues, if required.

Dew point is the temperature at which vapor condenses. Adew point sensor senses dew point directly. A typical sensoruses a heated, permeable membrane to establish an equilibriumcondition in which the dry-bulb temperature of a cavity in thesensor is proportional to the dew point temperature of theambient air. Another type of sensor senses condensation on acooled surface. If the ambient dry-bulb and dew pointtemperature are known, the relative humidity, total heat, andspecific humidity can be calculated. Refer to the PsychrometricChart Fundamentals section of this manual.

Fig. 53. Paddle Flow Sensor.

Flow meters measure the rate of fluid flow. Principle typesof flow meters use orifice plates or vortex nozzles whichgenerate pressure drops proportional to the square of fluidvelocity. Other types of flow meters sense both total and staticpressure, the difference of which is velocity pressure, thusproviding a differential pressure measurement. Paddle wheelsand turbines respond directly to fluid velocity and are usefulover wide ranges of velocity.

In a commercial building or industrial process, flow meterscan measure the flow of steam, water, air, or fuel to enablecalculation of energy usage needs.

Airflow pickups, such as a pitot tube or flow measuringstation (an array of pitot tubes), measure static and totalpressures in a duct. Subtracting static pressure from totalpressure yields velocity pressure, from which velocity can becalculated. Multiplying the velocity by the duct area yieldsflow. For additional information, refer to the Building AirflowSystem Control Applications section of this manual.

Applying the fluid jet principle allows the measurement ofvery small changes in air velocity that a differential pressuresensor cannot detect. A jet of air is emitted from a small tubeperpendicular to the flow of the air stream to be measured.



33

The impact of the jet on a collector tube a short distance awaycauses a positive pressure in the collector. An increase invelocity of the air stream perpendicular to the jet deflects thejet and decreases pressure in the collector. The change inpressure is linearly proportional to the change in air streamvelocity.

Another form of air velocity sensor uses a microelectroniccircuit with a heated resistance element on a microchip as theprimary velocity sensing element. Comparing the resistanceof this element to the resistance of an unheated elementindicates the velocity of the air flowing across it.

PROOF-OF-OPERATION SENSORS

Proof-of-operation sensors are often required for equipmentsafety interlocks, to verify command execution, or to monitorfan and pump operation status when a central monitoring andmanagement system is provided. Current-sensing relays,provided with current transformers around the power lines tothe fan or pump motor, are frequently used for proof-of-operation inputs. The contact closure threshold should be sethigh enough for the relay to drop out if the load is lost (brokenbelt or coupling) but not so low that it drops out on a lowoperational load.

Current-sensing relays are reliable, require less maintenance,and cost less to install than mechanical duct and pipe devices.

TRANSDUCERS

Transducers convert (change) sensor inputs and controlleroutputs from one analog form to another, more usable, analogform. A voltage-to-pneumatic transducer, for example, convertsa controller variable voltage input, such as 2 to 10 volts, to alinear variable pneumatic output, such as 3 to 15 psi. Thepneumatic output can be used to position devices such as apneumatic valve or damper actuator. A pressure-to-voltagetransducer converts a pneumatic sensor value, such as 2 to 15psi, to a voltage value, such as 2 to 10 volts, that is acceptableto an electronic or digital controller.

CONTROLLERS

Controllers receive inputs from sensors. The controllercompares the input signal with the desired condition, orsetpoint, and generates an output signal to operate a controlleddevice. A sensor may be integral to the controller (e.g., athermostat) or some distance from the controller.

Controllers may be electric/electronic, microprocessor, orpneumatic. An electric/electronic controller provides two-position, floating, or modulating control and may use amechanical sensor input such as a bimetal or an electric inputsuch as a resistance element or thermocouple. Amicroprocessor controller uses digital logic to compare inputsignals with the desired result and computes an output signalusing equations or algorithms programmed into the controller.Microprocessor controller inputs can be analog or on/offsignals representing sensed variables. Output signals may beon/off, analog, or pulsed. A pneumatic controller receives inputsignals from a pneumatic sensor and outputs a modulatingpneumatic signal.

ACTUATORS

An actuator is a device that converts electric or pneumaticenergy into a rotary or linear action. An actuator creates achange in the controlled variable by operating a variety of finalcontrol devices such as valves and dampers.

In general, pneumatic actuators provide proportioning ormodulating action, which means they can hold any position intheir stroke as a function of the pressure of the air delivered tothem. Two-position or on/off action requires relays to switchfrom zero air pressure to full air pressure to the actuator.

Electric control actuators are two-position, floating, orproportional (refer to CONTROL MODES). Electronicactuators are proportional electric control actuators that requirean electronic input. Electric actuators are bidirectional, whichmeans they rotate one way to open the valve or damper, andthe other way to close the valve or damper. Some electricactuators require power for each direction of travel. Pneumaticand some electric actuators are powered in one direction andstore energy in a spring for return travel.

Figure 54 shows a pneumatic actuator controlling a valve.As air pressure in the actuator chamber increases, thedownward force (F1) increases, overcoming the springcompression force (F2), and forcing the diaphragm downward.The downward movement of the diaphragm starts to close thevalve. The valve thus reduces the flow in some proportion tothe air pressure applied by the actuator. The valve in Figure 54is fully open with zero air pressure and the assembly is thereforenormally open.



34

Fig. 54. Typical Pneumatic Valve Actuator.

A pneumatic actuator similarly controls a damper. Figure55 shows pneumatic actuators controlling normally open andnormally closed dampers.

DIAPHRAGM

AIRPRESSURE

SPRING

FLOW

VALVE

ACTUATORCHAMBER

F1

F2

C2086

NORMALLY OPEN DAMPER

ACTUATOR ACTUATOR

SPRING

PISTONROLLING DIAPHRAGM

AIRPRESSURE

AIRPRESSURE

NORMALLYCLOSED DAMPER

C2087

PUSH ROD

CRANK ARM

ACTUATOR

DAMPER

C2721

Fig. 55. Typical Pneumatic Damper Actuator.

Electric actuators are inherently positive positioning. Somepneumatic control applications require accurate positioningof the valve or damper. For pneumatic actuators, a positivepositioning relay is connected to the actuator and ensures thatthe actuator position is proportional to the control signal. Thepositive positioning relay receives the controller output signal,reads the actuator position, and repositions the actuatoraccording to the controller signal, regardless of external loadson the actuator.

Electric actuators can provide proportional or two-positioncontrol action. Figure 56 shows a typical electric damperactuator. Spring-return actuators return the damper to eitherthe closed or the open position, depending on the linkage, on apower interruption.

Fig. 56. Typical Electric Damper Actuator.

AUXILIARY EQUIPMENT

Many control systems can be designed using only a sensor,controller, and actuator. In practice, however, one or moreauxiliary devices are often necessary.

Auxiliary equipment includes transducers to convert signalsfrom one type to another (e.g., from pneumatic to electric),relays and switches to manipulate signals, electric power andcompressed air supplies to power the control system, andindicating devices to facilitate monitoring of control systemactivity.



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CHARACTERISTICS AND ATTRIBUTES OF CONTROL METHODS

Review the columns of Table 4 to determine the characteristics and attributes of pneumatic, electric, electronic, andmicroprocessor control methods.

Table 4. Characteristics and Attributes of Control Methods.

Pneumatic Electric Electronic Microprocessor

Naturallyproportional

Requires cleandry air

Air lines maycause troublebelow freezing

Explosion proof

Simple, powerful,low cost, andreliable actuatorsfor large valvesand dampers

Simplestmodulatingcontrol

Most common forsimple on-offcontrol

Integral sensor/controller

Simple sequenceof control

Broadenvironmentallimits

Complexmodulatingactuators,especially whenspring-return

Precise control

Solid staterepeatability andreliability

Sensor may beup to 300 feetfrom controller

Simple, remote,rotary knobsetpoint

High per-loopcost

Complexactuators andcontrollers

Precise control

Inherent energy management

Inherent high order (proportional plus integral)control, no undesirable offset

Compatible with building management system.Inherent database for remote monitoring,adjusting, and alarming.

Easily performs a complex sequence of control

Global (inter-loop), hierarchial control via communications bus (e.g., optimize chillers basedupon demand of connected systems)

Simple remote setpoint and display (absolutenumber, e.g., 74.4)

Can use pneumatic actuators

Documents

HONEYWELL E M AUTOMATIC CONTROL for · tremendous advances in equipment, system design, and application. In this edition, microprocessor controls are shown in most of the control