Hvac Controls: Operation & Maintenance

330 Pages·2001·17.05 MB·English

by  Guy W., Jr Gupton

HVAC Controls
Operation & Maintenance
Third Edition
Guy W. Gupton, Jr.
Published by
THE FAIRMONT PRESS, INC.
700 Indian Trail
Lilburn, GA 30047
Library of Congress Cataloging-in-PublicationD ata
Gupton, Guy W., 1926-
HVAC controls: operation & maintenance/Guy W. Gupton, Jr.-- 3rd ed.
p. cm.
Includes index.
ISBN 0-8817 3-341-5
1. Heating--Control. 2. Ventilation--Control.3 . Air conditioning--Control.
I. Title.
TH7466.5 .G872 001
697--dc21
00-066249
HVAC controls operation 6 maintenance/Guy W. Gupton, Jr.
02002 by The Fairmont Press. All rights reserved. No part of this
publication may be reproduced or transmitted in any form or by any
means, electronic or mechanical, including photocopy, recording, or
any information storage and retrieval system, without permission in
writing from the publisher.
Published by The Fairmont Press, Inc.
700 Indian Trail
Lilbum, GA 30047
Printed in the United States of America
1 0 9 8 7 6 5 4 3 2 1
O-L3-0bL037-7 PH
0-BBL73-341-5 FP
While every effort is made to provide dependable information, the publisher, authors,
and editors cannot be held responsible for any errors or omissions.
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iv
Preface to the Third Edition
In the four years since the publication of the second edition of this
book, there have been continuing changes in the automatic temperature
control industry due to the widespread use of direct digital control
(DDC) systems.
This book is intended to provide guidance in the operation and
maintenance of all types of ATC systems. At the time of writing the first
edition, the majority of systems in use were of the electric/electronic and
pneumatic types. With the rapid increase in installations of DDC sys-
tems, it became necessary to include material in the second edition that
will provide basic coverage of DDC systems.
This book includes basic procedures in the operation and mainte-
nance of DDC systems, particularly in the initial checkout and operator
training on newly installed systems. Those procedures are also appli-
cable to the recommissioning of existing DDC systems and in recurrent
training of DDC system operators and maintenance technicians.
The complexity of DDC system programming and the major differ-
ences in program language between system manufacturers, limits the
discussion in this book of the actual programming of DDC systems. That
type of information is application specific and must be obtained from
the system manufacturer’s literature and training seminars.
ix
Table of Contents
Chapter 1 Basic Functions of HVAC
...............................................
Systems and Control Systems 1
........................
Chapter 2 HVAC Equipment-to-Control Interactions 23
Chapter 3 Operating and Maintaining HVAC Control Systems .....7 3
Chapter 4 The Mathematics of
..............................
Control Systems: Controller Equations 85
Chapter 5 Performance Prediction in HVAC Control Systems ......1 15
..........................................
Chapter 6 HVAC Control System Set-Up 131
Chapter 7 Maintaining Electric and Electronic Control Systems .. 155
........................
Chapter 8 Maintaining Pneumatic Control Systems 159
Chapter 9 Maintaining Local Loop to BAS Interfaces ..................... 177
................
Chapter 10 HVAC Control System Checkout Procedures 199
....
Chapter 11 Fine Tuning Program for Pneumatic Control Systems 209
...........................................
Chapter 12 Troubleshooting ATC Systems 233
Chapter 13 Tools & Fixtures for
.......................
ATC System Operation and Maintenance 241
Chapter 14 Training Control System
...........................
Operating and Maintenance Personnel 257
Chapter 15 Installing Hybrid Pneumatic and
..........................................
Direct Digital Control Systems 267
V
......................
Chapter 16 Operating Direct Digital Control Systems 275
Chapter 17 Testing Direct Digital Control Systems ............................ 281
Chapter 18 A Short Course in Psychrometrics ................................... 287
Glossary of Terms ........................................................................................ 299
............................................................................................................
Index 339
vi
Chapter 1
Basic Functions of HVAC
Svstems and Control Svstems
J J
he purpose of a Heating, Ventilating, and Air Conditioning
(HVAC) system is to provide and maintain a comfortable en-
vironment within a building for the occupants or a suitable
environment for the process being conducted.
This book covers basic HVAC systems of the all-air type, where all
functions of heating, ventilating, and air conditioning are performed by
an air handling system. Some functions for central hydronic cooling and
heating distribution are included which also apply to air-water and all-
water types of systems.
The principal functions of HVAC systems and control systems are:
To maintain comfortable conditions in the space by providing the
desired cooling and heating outputs, while factors which affect the
cooling and heating outputs vary.
To maintain comfortable conditions while using the least amount
of energy.
To operate the HVAC system so as to provide a healthy environ-
ment for occupants and safe conditions for equipment.
The ability of a system operator to diagnose and correct automatic
temperature control (ATC) system operating problems requires a work-
ing knowledge of HVAC system types, the components of HVAC sys-
tems, the intended function of those components in the HVAC system,
and the HVAC equipment-to-control interactions. Many problems which
are considered to be HVAC system design problems are found to be ATC
system operating problems.
In order to determine whether the ATC system is functioning prop-
erly, it is necessary to determine how each control sequence is intended
to function.
2 HVAC CONTROLAPERAT&I OMNAI NTENANCE
Although HVAC systems must be designed to satisfy the maxi-
mum cooling and heating loads at design conditions, HVAC systems do
not operate at full capacity very often. Systems seem to operate for most
hours of the year at near half capacity, with variations due to changes in
outside conditions for time-of-day and time-of-year and to changes in
internal heat releases. The ATC system must be designed, set up, and
operated to recognize changes and to maintain the space temperature at
partial load.
HVAC SYSTEM CONTROL FUNCTIONS
Controlled Parameters
An HVAC system functions to provide a controlled environment in
which these parameters are maintained within desired ranges:
Temperature
Humidity
Air Distribution
Indoor Air Quality
In order to accomplish this task, the ATC control system must be
designed so as to directly control the first three parameters. The fourth
parameter, indoor air quality, is influenced by the first three but may
require separate control methods which are beyond the scope of this
book.
Approaches to Temperature Control
Temperature control in an air conditioning system that uses air as
a delivery medium may use one of the following approaches:
Vary the temperature of air supplied to the space while keeping the
airflow rate constant. This is the basic constant volume, variable
temperature approach.
Vary the airflow rate while keeping the temperature constant for
air supplied to the space. This is the variable volume, constant
temperature approach.
BASICF UNCTIONOSF HVAC SYSTEMANSD CONTROSLY STEMS 3
Vary the airflow rate and change the temperature for air supplied
to the space. This is the variable volume and temperature ap-
proach.
Vary both the supply air temperature and flow rate where the air-
flow rate is varied down to a minimum value, then energy input
to reheat the coil is controlled to vary the supply air temperature.
This is the variable volume reheat approach.
Approaches to Humidity Control
Humidity control in a conditioned space is done by controlling the
amount of water vapor present in the air in the space. When relative
humidity at the desired temperature setpoint is too high, dehumidifica-
tion is required to reduce the amount of water vapor in the air for hu-
midity control. Similarly, when relative humidity at the desired tempera-
ture setpoint is too low, humidification is required to increase the
amount of water vapor in the air for humidity control.
Because relative humidity varies significantly with dry bulb tem-
perature, it is important to state dry bulb temperature and relative hu-
midity together, such as 70°F and 50% RH. For example, at a room air
condition of 70°F dry bulb and 50% RH, the moisture content, or specific
humidity, is 54.5 grains of water per pound of dry air. Air with the same
specific humidity at 60°F will have about 71% RH and when read at 80°F
will have about 36% RH.
Commonly used dehumidification methods include:
Surface dehumidification on cooling coils simultaneous with sen-
sible cooling.
Sprayed coil dehumidifier with indirect cooling coils.
Direct dehumidification with desiccant-based dehumidifiers.
Humidification is not always required in an HVAC system but,
when required, it is provided by a humidifier.
Commonly used humidification methods include:
Water spray humidifier.
Steam grid humidifier.
Steam pan humidifier.
4 HVAC CONTROLS-~PERATIO&N MAINTENANCE
Methods of Temperature Control
Temperature control in a space is done by a temperature controller,
commonly called a thermostat, which is set to the desired temperature
value or setpoint. A temperature deviation, or offset, from the setpoint
causes a control signal to be sent to the controlled device at the HVAC
system component which is being controlled. In this book, the term
"temperature controller" means thermostats and temperature sensor/
controller devices, as well as remote bulb type temperature controllers.
When the temperature in a conditioned space is to be controlled by
heat exchange to supply air from a heating or cooling coil, the tempera-
ture control signal will cause a change in the flow of the cooling or
heating medium through the coil. With a chilled water or heated water
coil, the temperature controller may position a water valve to vary the
flow rate of heated or chilled medium through the coil or may position
face and bypass dampers at the coil to vary the proportion of air passing
through the air side of the coil to that which bypasses the coil and is not
conditioned,
Automatic control valves used to control water flow through a
water coil may be either two-way or three-way pattern and may be
positioned in either two-position or modulating sequence. Valves used
to control steam flow through a coil are two-way type and may be
positioned in either two-position or modulating sequence.
Methods of Humidity Control
Dehumidification is usually done at the same time as the sensible
cooling by a surface dehumidification process on the system cooling
coils, either indirect cooling using chilled water or other heat transfer
medium or direct expansion refrigerant evaporator coils. Dehumidifica-
tion in low dew point process systems may be done in a separate dehu-
midification unit.
Air leaving the cooling coil during surface dehumidification is
often near a saturated condition. When cooling in a process area is con-
trolled from the relative humidity in order to remove water vapor, the
supply air will often be cooled more than is required for sensible or dry
cooling of the space and may require reheating to prevent overcooling
of the space. When the supply air is reheated to the temperature re-
quired to maintain the space temperature at the desired level, and the
required air volume is supplied to the space, that air volume will also
maintain humidity at the desired level.
BASICF UNCTIONOSF HVAC SYSTEMANSD CONTROSLY STEMS 5
Humidity relationships in HVAC systems are expressed in percent
relative humidity and noted as % RH.
The system humidity controller, commonly called a humidistat, is
located in the conditioned area, preferably adjacent to the thermostat, to
ensure that the ambient temperature is that which the humidity is to be
based upon. The space humidity controller is set at the desired relative
humidity setpoint. A change in relative humidity from that setpoint
causes a control signal to be sent to the controlled component.
For example, to control a duct-mounted, steam grid humidifier,
when the space relative humidity drops below the humidity controller
setpoint, a control signal is generated to open the steam valve at the inlet
to the duct-mounted humidifier unit. When the steam valve is posi-
tioned open, steam flows through the humidifier in the supply air
stream to the space, which raises the space relative humidity. A second
humidity controller located in ductwork downstream from the humidi-
fier acts as a high-limit safety controller. When the relative humidity of
the airstream approaches the saturation point, the high-limit controller
overcalls the space humidity controller to reposition the steam valve and
decrease the steam flow. This will prevent condensation and water
carryover downstream from the humidifier. The control of an electrically
heated steam humidifier is similar to valve-controlled, with electric con-
tactors being the controlled devices.
Methods of Air Volume Control
When variations of supply air volume are used to control the space
temperature, the temperature controller may cycle the fan motor in on-
off sequence, may modulate the fan motor speed, or damper the airflow,
such as through volume control dampers in air terminal units.
For example, in a fan-coil unit system, the space temperature can
be regulated by regulating the airflow rather than the water flow
through the coil. When the space temperature rises or drops from the
desired level, the temperature controller will either vary the fan speed
through a solid state speed controller or cycle the fan "on" and operate
the fan until the space temperature changes in response to load gener-
ated and capacity applied, then cycle the fan "off."
In a Variable Air Volume (VAV) system, the supply air volume
delivered to the space will vary as the temperature controllers on in-
dividual terminal units position each of the modulating dampers on in-
dividual terminal units. The central station air handling unit fan will