Thus, this edition uses IP units and equations as primary, with SI units and equations as secondary, in accordance with the SI Guide for HVAC&R, American Society of Heating, Refrigerati
Trang 2Heating, Ventilating, and Air-Conditioning Systems
Trang 4Heating, Ventilating, and Air-Conditioning Systems
Second Edition
Herbert W Stanford III and
Adam F Spach
Trang 5© 2019 by Taylor & Francis Group, LLC
CRC Press is an imprint of Taylor & Francis Group, an Informa business
No claim to original U.S Government works
Printed on acid-free paper
International Standard Book Number-13: 978-1-138-60241-0 (Hardback)
This book contains information obtained from authentic and highly regarded sources Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained If any copyright material has not been acknowledged, please write and let
us know so we may rectify in any future reprint.
Except as permitted under U.S Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers.
For permission to photocopy or use material electronically from this work, please access www.copyright.com (http:// www.copyright.com/) or contact the Copyright Clearance Center, Inc (CCC), 222 Rosewood Drive, Danvers, MA
01923, 978-750-8400 CCC is a not-for-profit organization that provides licenses and registration for a variety of users For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged.
Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for
identification and explanation without intent to infringe.
Visit the Taylor & Francis Web site at
http://www.taylorandfrancis.com
and the CRC Press Web site at
http://www.crcpress.com
Trang 6Preface xiii
Authors xv
Section I Basic Concepts 1 HVAC Basics 3
1.1 Overview of HVAC 3
1.1.1 Definition of Air-Conditioning 3
1.1.2 Energy Impact of HVAC Systems 4
1.2 Buildings as Thermodynamic Systems 5
1.2.1 Systems Concepts 5
1.2.2 Energy Gains and Losses in Building Spaces 6
1.2.3 Building Envelope Design 7
1.3 Common HVAC Elements 9
1.3.1 Structural Support for HVAC Components 9
1.3.2 Electrical Aspects of HVAC Systems 12
1.4 Definitions of HVAC Terms 25
Bibliography 31
2 The Air-Conditioning Process 33
2.1 Introduction 33
2.2 Air and Its Properties 34
2.2.1 Atmospheric Air 34
2.2.2 Water Vapor 35
2.2.3 Terminology 35
2.2.4 Basic Psychrometrics 36
2.3 Methods of Measurement and Analysis 36
2.3.1 Thermodynamic Wet-Bulb Temperature 36
2.3.2 The Psychrometer 37
2.3.3 The Psychrometric Chart 38
2.4 Typical Air-Conditioning Processes 41
2.4.1 Sensible Heating or Cooling 41
2.4.2 Combined Sensible and Latent Cooling (Dehumidification) 42
2.4.3 Heating and Humidification 43
2.4.4 Evaporative Cooling 44
2.4.5 Air Mixing 45
2.5 Psychrometric Analysis of Complete Systems 46
2.5.1 Space Heating and Cooling Loads 46
2.5.2 Sensible Cooling Load 47
2.5.3 Latent Cooling Loads 48
2.5.4 Psychrometric Chart Representation of Space Conditions 49
2.5.5 The Coil Line 50
2.5.6 Coil Contact and Bypass Factors 51
Trang 72.5.7 Psychrometric Analysis of Complete Systems 52
Bibliography 52
3 HVAC Systems Concepts 53
3.1 Introduction 53
3.2 All-Air Systems 53
3.2.1 Single-Zone Systems 56
3.2.2 VAV Systems 58
3.2.3 Dual–Duct Systems 59
3.2.4 Multizone Systems 62
3.2.5 VAV/Variable Temperature Systems 62
3.3 Air–Water Secondary Systems 65
3.3.1 Active and Passive Chilled Beam Systems 65
3.3.2 FCUs and Unit Ventilators 67
3.4 Unitary Systems 68
3.4.1 Incremental Units 68
3.4.2 Packaged Units 69
3.4.3 Split Systems 69
3.4.4 Variable Refrigerant Flow Systems 70
3.5 Heat Pumps 72
3.5.1 The Heat Pump Concept 72
3.5.2 Air-Source Heat Pumps 74
3.5.3 Water-Source Heat Pumps 76
3.5.3.1 Closed Circuit Heat Pump Systems 76
3.5.3.2 Closed Circuit Geothermal Heat Pump Systems 77
3.5.3.3 Open Circuit Geothermal Heat Pump Systems 80
3.5.3.4 Gas-Fired Engine-Driven Heat Pumps 80
3.5.3.5 Heat Recovery Chiller/Heat Pump System 80
3.5.4 Advanced Technology Heat Pumps 81
Bibliography 82
Section II The Design Method 4 HVAC Systems Design 85
4.1 Introduction 85
4.2 Criteria for HVAC Design 87
4.3 Compliance with Building Codes and Standards 88
4.3.1 International Mechanical Code 88
4.3.2 International Energy Conservation Code 88
4.3.3 NFPA Standards and Guidelines 90
4.3.4 ASHRAE Standards and Guidelines 90
4.4 Designing for Maintainability 91
4.4.1 HVAC Equipment Location and Access 92
4.4.2 Mechanical Equipment Rooms for Air-Handling Equipment 93
4.4.3 Mechanical Equipment Rooms for Boilers and Chillers 93
4.4.4 Ceiling Cavity Space 94
4.5 Designing for Energy Conservation 95
Trang 84.5.1 Introduction to ASHRAE Standard 90.1 95
4.5.2 Simplified Energy Analysis 96
4.5.3 Building Energy Modeling 97
4.5.4 Exhaust Air Heat Recovery 99
4.5.5 Internal Source Heat Recovery 102
4.5.6 Thermal Energy Storage 103
4.6 Designing for Sustainability 105
4.6.1 High-Performance Building Elements 105
4.6.2 Introduction to ASHRAE Standard 189.1 108
4.6.3 Net Zero/Sum Zero-Energy Buildings 108
4.7 Construction Documents Quality Control 110
Bibliography 112
5 Heating/Cooling Load Calculation 113
5.1 Introduction 113
5.1.1 Building Thermal Balance 113
5.1.2 Purpose of Load Calculations 113
5.2 Mechanics of Building Heat Loss and Gain 113
5.2.1 Design Conditions 114
5.2.2 Heat Transfer through Walls and Roofs 115
5.2.3 Heat Transfer through Fenestration 118
5.2.4 Heat Loss through Floors and Basements 122
5.2.5 Internal Heat Gains 124
5.2.6 Infiltration Loads 125
5.3 Load Computation Methodologies 128
5.3.1 Heat Balance and Radiant Time Series 128
5.3.2 Total Equivalent Temperature Difference with Time Averaging 130
5.3.3 Transfer Function 131
5.3.4 Computer-Based Calculation Tools 131
5.5 Benchmark Heating/Cooling Loads 132
Bibliography 134
6 Air Tempering and Distribution 135
6.1 Heating and Cooling Coils 135
6.1.1 Direct Expansion Refrigerant Cooling Coils 138
6.1.2 Chilled Water Cooling Coils 140
6.1.3 Hot Water Heating Coils 141
6.1.4 Steam Heating Coils 141
6.1.5 Electric Resistance Heating Coils 141
6.1.6 Preheating and Precooling Coils 143
6.1.7 Desiccant Cooling and Dehumidification 146
6.2 Air Distribution Design 147
6.2.1 Room Air Diffusion 147
6.2.2 Typical Air Distribution Patterns 148
6.2.3 Large Space Air Distribution 154
6.2.4 Displacement Ventilation Air Distribution 155
6.3 Terminal Units 157
6.3.1 VAV Single-Duct TUs 157
Trang 96.3.2 VAV Single-Duct Fan-Powered TUs 160
6.3.3 VAV Dual Duct TUs 160
Bibliography 161
7 Duct Design 163
7.1 Introduction 163
7.1.1 Steps in Duct System Design 163
7.1.2 Duct Air Balancing Rationale 164
7.2 Energy and Pressure Relationships 164
7.3 Friction Losses 166
7.3.1 Calculation of Friction Losses in Ducts 166
7.3.2 Calculation of Friction Losses in Fittings 170
7.4 Methods of Duct Design 171
7.4.1 Constant Velocity Method 171
7.4.2 Velocity Reduction Method 171
7.4.3 Static Regain Method 171
7.4.4 Equal Friction Method 172
7.4.5 Duct Construction Criteria 173
7.4.6 Duct Design Guidelines 174
7.5 Special Exhaust Systems Design 176
7.5.1 Laboratory Exhaust Systems 176
7.5.2 Industrial Exhaust Systems 179
7.6 Duct Insulation 181
7.7 Fire and Smoke Control in Duct Systems 182
Bibliography 183
8 Piping Design 185
8.1 Introduction 185
8.1.1 Viscosity 185
8.1.2 Weight Density, Specific Volume, and Specific Gravity 185
8.1.3 Mean Velocity of Flow 186
8.1.4 Piping Basics 187
8.2 Hydronic Piping 191
8.2.1 Pipe Routing and Sizing 192
8.2.2 Pressure Loss Calculation 196
8.2.3 Expansion and Air Removal 198
8.2.4 Freeze Protection 201
8.3 Steam and Condensate Piping 202
8.3.1 Pipe Routing and Sizing 203
8.3.2 Steam Trap Application and Sizing 206
8.3.3 Control of Steam Piping Expansion 209
8.4 Refrigerant Piping 210
8.4.1 Design Considerations 210
8.4.2 Suction Lines 213
8.4.3 Liquid Lines 215
8.5 Piping Insulation 215
8.5.1 Hot Piping Insulation 215
8.5.2 Cold Piping Insulation 215
Bibliography 218
Trang 10Section III Systems and Components
9 Pumps and Fans 221
9.1 Introduction 221
9.2 Pumps and Applications 221
9.2.1 Pump Types and Characteristics 221
9.2.2 Pump Arrangements 225
9.2.3 Water Flow Volume Control 230
9.3 Fans 231
9.3.1 Fan Energy Relations 231
9.3.2 Fan Types and Characteristics 233
9.3.3 Fan and System Characteristics 237
9.3.4 System Effects 238
9.3.5 Variable Air Volume Fan Control 242
9.4 Pump and Fan Affinity Laws 243
Bibliography 243
10 Terminal Systems and Components 245
10.1 Systems Types and Applications 245
10.1.1 Selecting the Right System for the Application 245
10.1.2 Space Needs and Other Requirements for Terminal Systems 245
10.2 AHUs and Components 249
10.2.1 AHUs 249
10.2.2 Particulate Air Filtration 253
10.2.3 Gas-Phase Filtration 255
10.2.4 Ultraviolet Air and Surface Treatment 256
10.2.5 Airside Economizer Cycle 257
10.2.6 Cooling Coils Condensate Drainage 261
10.3 Humidification 262
10.4 Ventilation and Building Pressurization 264
10.4.1 Outdoor Air Intakes 265
10.4.2 Understanding and Applying ASHRAE Standard 62.1 267
10.4.3 Demand Control Ventilation 271
10.4.4 Dedicated Outdoor Air Systems 274
Bibliography 275
11 Refrigeration Systems and Components 277
11.1 Refrigeration Basics 277
11.1.1 Refrigerant Issues 277
11.1.2 Vapor Compression Refrigeration Cycle 278
11.1.3 Vapor Absorption Refrigeration Cycle 282
11.2 Direct Expansion Refrigeration Systems 285
11.3 Chilled Water Refrigeration Systems 288
11.3.1 Determining Chilled Water Supply Temperature 289
11.3.2 Establishing Chilled Water Temperature Range 289
11.3.3 Vapor Compression Cycle Water Chillers 290
11.3.4 Scroll Compressor Water Chillers 292
11.3.5 Rotary Screw Compressor Water Chillers 297
11.3.6 Centrifugal Compressor Water Chillers 297
Trang 1111.3.7 Absorption Water Chillers 299
11.3.8 Chilled Water System Configurations 300
11.3.9 Chilled Water Buffer Tanks 307
11.4 Condensers and Cooling Towers 308
11.4.1 Air-Cooled Condensers 308
11.4.2 Cooling Tower Fundamentals 309
11.4.3 Tower Configuration and Application 314
11.4.4 Closed Circuit Evaporative Cooler 319
11.5 Evaporative Air Cooling 320
11.6 Waterside Economizer Cycle 320
Bibliography 323
12 Heating Systems and Components 325
12.1 Firing Fossil Fuels 325
12.1.1 Oil-Fired Systems 325
12.1.2 Gas-Fired Systems 328
12.2 Furnaces 329
12.3 Boilers 330
12.3.1 Boiler Types, Ratings, and Efficiency 330
12.3.2 Application Considerations 331
12.3.3 Boiler/Furnace Venting 332
12.4 Hydronic Heating Systems 333
12.5 Steam Heating Systems 335
12.5.1 Steam Quality 335
12.5.2 Steam Heat Transfer 338
12.5.3 Steam-to-Water Heat Exchangers 338
12.5.4 Feedwater System 339
12.5.5 Steam Pressure Reducing Valves 339
12.5.6 Steam Condensate Receivers and Pumps 343
Bibliography 344
13 HVAC Controls 345
13.1 DDC Fundamentals 346
13.2 DDC Input/Output Points 351
13.2.1 Input/Output Basics 351
13.2.2 DI Devices 352
13.2.3 AI Devices 353
13.2.4 DO Devices 363
13.2.5 AO Devices 364
13.3 Final Control Elements 366
13.3.1 Control Valves 366
13.3.2 Control Dampers 368
13.4 Direct Digital Controllers and Systems 370
13.4.1 DDC System Architecture 370
13.4.2 Controllers and Control Loops 371
13.4.3 Networks and Communication 372
13.5 HVAC Sequences of Operation 375
13.6 DDC System Security 377
Trang 1213.7 DDC System Design Checklist 379
Bibliography 380
Section IV Special Considerations 14 Special HVAC Design Considerations 383
14.1 Indoor Air Quality 383
14.2 Antiterrorism Design for HVAC Systems 385
14.2.1 Introduction 385
14.2.2 Terror Threats against Buildings 386
14.2.3 HVAC Design to Minimize Explosive Threats 387
14.2.4 HVAC Design to Minimize CBR Threats 388
14.3 HVAC Water Treatment 390
14.3.1 Hot and Chilled Water Systems Chemical Treatment 390
14.3.2 Evaporative Cooling Water Systems Chemical Treatment 390
14.3.3 Evaporative Cooling Water Systems Nonchemical Treatment 394
14.3.4 Steam and Condensate Systems Chemical Treatment 396
14.3.5 Legionella Risk Management in HVAC Water Systems 399
14.4 Vibration and Noise Mitigation 400
14.4.1 Noise Definition and Design Criteria 400
14.4.2 Indoor Noise Control in HVAC Systems 401
14.4.3 Outdoor Noise Issues in HVAC Systems 404
14.5 HVAC Systems Start-Up and Commissioning 407
14.5.1 HVAC Systems Operation Prior to Commissioning 407
14.5.2 The Commissioning Process 408
14.6 Correcting HVAC Performance for Altitude and Temperature 410
Bibliography 411
15 Engineering Economics and Design Decision-Making 413
15.1 Introduction 413
15.2 Defining HVAC Design Alternatives 415
15.3 Estimating Capital Requirements 417
15.3.1 Capital Cost Estimating 417
15.3.2 Time/Money Relationships 418
15.4 Recurring Costs Determination 420
15.4.1 Utilities Costs 420
15.4.2 Maintenance Costs 421
15.5 Component Service Life and Replacement 422
15.6 Comparing Alternatives and Dealing with Uncertainty 424
15.6.1 Alternatives with Different Economic Lives 427
15.6.2 Sensitivity Analysis 427
15.6.3 Selecting Alternatives within the Project Budget 428
15.7 Overview of LCCA 429
15.7.1 Basic Practices for LCCA 429
15.7.2 Calculations and Analysis Tools 429
Bibliography 430
Trang 1316 Building Information Modeling 431
16.1 Introduction 431
16.2 BIM Advantages 432
16.3 Applying BIM 434
16.3.1 BIM Use by HVAC Designers 435
16.3.2 BIM Use by Contractors/Subcontractors 436
16.3.3 BIM Use by Owners 436
16.4 AIA Contracts 436
Bibliography 437
17 Construction Contract Administration 439
17.1 Introduction 439
17.2 Design Period Considerations 440
17.3 Preconstruction Procedures and Practices 442
17.4 Construction Period Procedures and Practices 444
17.5 Completion and Closeout Procedures and Practices 448
17.6 Avoiding Litigation Pitfalls 449
17.7 Construction Administration Checklist 452
17.7.1 Construction Administration Practices and Procedures Checklist 453
Index 457
Trang 14energy consumption ramifications of design decisions
indus-trial buildings
Air-Conditioning Engineers (ASHRAE) Handbook as the primary reference source.
These objectives continue to apply in this second edition
Another major impetus to the writing of this book was the fact that for many years
I made a very good living investigating and solving HVAC system operating problems in buildings, many of which were the result of inadequate or incorrect design Even now, a recent survey conducted by one major industry magazine indicates that the majority of all building owners queried responded that there were problems and ongoing dissatisfaction with the HVAC systems in their buildings These experiences indicate that HVAC design engineers are still too often doing a less-than-adequate job in the delivery of the final product: an HVAC system that “satisfies.”
The “science” of HVAC design is well developed One has only to read the ASHRAE Handbook and other texts currently in use in the academic environment to realize that the basics of psychometrics, heat gain/loss calculations, fluid flow, fan and pump operations, and so on are well defined and readily available to the practicing engineer Thus, while this text also addresses these fundamental topics, it is, rather, the “art” of successful HVAC design, including proper system and component application, which was incorporated into the first edition and significantly expanded in this second edition
Design goes beyond load calculations and duct and pipe sizing In fact, these are the smallest parts of the design effort It is more important for the design engineer to have a proper grasp of systems applications, controls, and the basics of “fitting the right solution to solve the problem.” In this text, significant emphasis is placed on “systems” considerations
of each basic topic
A word about units: In 1981, ASHRAE published their Handbook-Fundamentals in which the primary unit designations were in accordance with the Système International (SI) The
result was a handbook that was useless to the practicing professional in the United States, and there subsequently was a “revolt” within the membership of ASHRAE between the academic and practicing sides of the organization The compromise was the publishing of
Trang 15the 1985 Handbook-Fundamentals (and all subsequent volumes) in two separate editions, one
using the inch-pound (IP) system and the other using SI
The first edition of this book, published in 1988, utilized only IP units Since then, though, the HVAC market has become more global in nature and now ASHRAE requires dual unit publications Thus, this edition uses IP units and equations as primary, with SI units and
equations as secondary, in accordance with the SI Guide for HVAC&R, American Society of
Heating, Refrigerating, and Air-Conditioning Engineers, Inc., Atlanta, Georgia, 2013.Now, 30 years since the first edition of this book was published, a number of HVAC topics have changed: heating/cooling load calculation methodologies, indoor air- quality considerations, control systems and sequences of operation, increased emphasis on energy efficiency and sustainability, etc Since my semiretirement in 1998, I have written and taught on a number of these developing issues in HVAC design and this new text incorporates much of that material
However, the HVAC design business has also changed over the last 30 years… production methods (the transition from hand drafting to computer-aided drafting to building information modeling), the general reduction in contractor skills and capabilities ( resulting in a need for the detail and coordination within design documents), the impact
of greatly expanded building codes and industry standards, etc., are all markedly different that they were in 1988
This created the need for a coauthor who had the expertise and experience to address these areas in this edition Therefore, I asked Adam F Spach, P.E., a friend and colleague with whom I’ve worked over the last 15 years and consider to be one of the brightest younger engineers I’ve ever met, to join me in the writing of this new edition
Finally, I rededicate this book to Guy Hammer Cheek, P.E (1927–1987), my friend, my mentor, and the man who introduced me to the idea of “HVAC systems concepts” and who diligently encouraged me to pick the right one(s) for each application
Herbert W Stanford III, P.E.
North Carolina
2019
Trang 16Herbert W Stanford III, P.E, is a North Carolina native and 1966 graduate of North Carolina State University with a B.S in mechanical engineering He is a registered professional engineer in North Carolina In 1977, he founded Stanford White, Inc., an engineering consulting firm located in Raleigh, North Carolina and semiretired in 1998 Currently, Mr Stanford is actively engaged in investigative and forensic engineering, teaching, and writing within a broad range of topics relative to buildings, especially HVAC systems, indoor environmental quality, and building operations and maintenance
Mr Stanford developed the Facilities Condition Assessment Program for the State of North Carolina and the Life Cycle Cost Methodology used by North Carolina to evaluate building design decisions He has taught a series of short courses on current building top-ics at the University of Toledo (Ohio), North Carolina State University, and the University
of North Carolina at Charlotte and is a life member of the ASHRAE Mr Stanford is the author of HVAC Water Chillers and Cooling Towers: Fundamentals, Application, and Operation (Second Edition, CRC Press, 2011); The Health Care HVAC Technician (MGI Systems, Inc., 2008), a program and training manual for hospital HVAC maintenance per-sonnel; and Effective Building Maintenance (Fairmont Press, 2010)
Adam F Spach was born in upstate New York and graduated from Alfred University with
a B.S in mechanical engineering in 2000 After graduation, he relocated to North Carolina and has called it his home since Mr Spach has worked as an HVAC consulting engineer since 2001 and is licensed as a professional engineer in North Carolina and several other states He joined Stanford White, Inc., Raleigh, NC in 2005 and is now an associate with that firm He currently focuses on providing engineering services for educational (K-12 and university), commercial, recreational, research, and health care facilities Mr Spach has a passion for sustainable design in the built environment He was appointed to the Wake County Citizen’s Energy Advisory Committee, serving as the vice chair in 2010–2014, and currently co-chairs the High Performance Building Task Force sponsored by North Carolina Chapter of the American Institute of Architects He has made presentations at the annual N.C State Construction Conference on three occasions and at the annual North Carolina Sustainable Energy Conference on five occasions Mr Spach routinely lectures at the North Carolina State Design School
Trang 18Basic Concepts
Trang 20HVAC Basics
1.1 Overview of HVAC
1.1.1 Definition of Air-Conditioning
Modern-day air-conditioning was created in the early 20th century based on the vision and
works of Hermann Rietschel, Alfred Wolff, Stuart Cramer, and Willis Carrier Cramer,
a textile engineer in Charlotte, North Carolina, is credited with coining the phrase conditioning” in 1906 Willis Carrier, though he did not actually invent air-conditioning
“air-or take the first documented scientific approach to applying it, is credited with integrating the scientific method, engineering, and business of this developing technology and creat-
ing the industry we know today as heating, ventilating, and air-conditioning (HVAC).
By 1929, the American Society of Heating and Ventilating Engineers defined comfort conditioning as “… the process of treating the air so as to control simultaneously its tem-perature, humidity, cleanliness, and distribution.” Today the organization incorporates that term in its current name: The American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc (ASHRAE) The same definition of air-conditioning applies today, but, of course, there have been major refinements in the art since 1929
air-Comfort air-conditioning means the maintenance of those indoor atmospheric factors affecting comfort:
1 The desired range of air temperatures
2 An acceptable humidity
3 Minimal atmospheric particulates, including pollens and bacteria
4 An acceptable odor level
5 An acceptable degree and pattern of air motion
Air-conditioning, in its broadest sense, is the engineering science of designing means of controlling the air in the human environment for comfort and health This broad area includes control of humidity and heating, as well as cooling It is distinguished from
refrigeration, in that refrigeration is one of the processes typically involved in an
air-conditioning system (i.e., refrigeration systems cool the air, either directly or indirectly) However, it is still true that the term air-conditioning is taken by the lay public to refer only to cooling and dehumidification of air For this reason, the acronym HVAC is still the most common term used when referring to the multipurpose systems found in modern buildings
Trang 21This book is concerned with the engineering design involved in application, sizing, ponent selection, layout, and control of HVAC systems in buildings These tasks are gener-ally performed by consulting engineers and designers, working with the building owner (and, as applicable, the architect for the building) These designers are not concerned with the detailed design of HVAC components themselves (fans, pumps, chillers, and so on), although HVAC engineers are expected to know the general operating characteristics and respective advantages of, for example, centrifugal-electric refrigeration machines versus steam absorption water chillers, they need not necessarily be familiar with the detailed heat transfer characteristics or the internal control mechanisms of the machines HVAC designers are best referred to as “assemblers” who bring together multiple components under common control to satisfy the performance requirements listed previously.
com-But, long gone are the days when an HVAC designer had to know only about HVAC systems and components. Today, there are a myriad of issues that impact HVAC design and have to be included as part of the design process, including general construction elements, energy efficiency and consumption, indoor air quality, anti-terrorism, and others addressed by this book
1.1.2 Energy Impact of HVAC Systems
Before any building is constructed, the space that the building will occupy consumes no energy However, the moment the building is created, there arises a need to provide an internal environment that is different from the surrounding environment (i.e., the build-ing must be either warmer or cooler and more or less humid than outdoors as outdoor con-ditions change over time) And, since walls and roofs block natural light, artificial lighting must usually be provided even during daylight hours Thus, an energy-use burden is cre-ated by almost every building in satisfying these environmental requirements
Although there were Roman hot air heating systems and ice-based cooling was applied
by the Egyptians, HVAC is basically a modern technology dating, realistically, only from the early 20th century Significant scientific and practical development did not occur until after World War I and the widespread use of building cooling did not take hold until after World War II
The design and application of HVAC systems can be divided into five historical periods:
1 Pre-1960. Prior to the late 1960s, HVAC systems tended to have noncritical design criteria and were basically simple in scope Heating systems were designed for 65°F [18°C] to 70°F [21°C] indoor temperature, while the indoor temperature for cooling was generally selected to be 15°F [8°C] below the outside peak tempera-ture (typically, about 80°F [27°C]) Systems tended to be single zone, multi-zone,
or, in very large buildings, dual-duct or induction types Emphasis in this period was on low first cost, and since energy was cheap, little or no attention was paid to operating efficiency
2 1960–1975. This period was the “heyday” of complex and very inefficient HVAC systems No longer was 80°F [27°C] interior design and multi-zone air distribu-tion acceptable Summer indoor design temperatures fell as low as 72°F [22°C]
and every space had to have a thermostat to maintain temperature control within
±1.5°F [0.8°C] Ventilation air quantities equivalent to 25% of the system supply airflow were very common To satisfy these more critical design criteria while keeping first costs reasonable, reheat systems became very popular
Trang 223 1975–1984. The energy cost upheavals of 1973–1974 and 1977–1978 forced designers
to reevaluate design criteria and system concepts to reduce energy consumption and cost During this period, design temperatures were 76°F–78°F [24°C–26°C] for cooling and 68°F–72°F [20°C–22°C] for heating Design ventilation airflow rates were reduced to about 20% of the 1960–1975 values Non-reheat systems, particu-larly those incorporating variable air volume concepts, gained wide acceptance The first energy codes came into being during this period
4 1984–1998. In the early 1980s, the “energy crisis” had disappeared But, many of the lessons learned worked to help offset the rapid inflation of energy costs during that period Variable air volume systems became more common However, design indoor temperatures returned to values that resulted in real comfort conditions and, by 1989, ventilation rates had returned to about 75% of their pre-1975 values due to widespread complaints about indoor air quality and the common occur-rence of “sick building syndrome.”
5 1999-Today In addition to ever more stringent limits on building energy tion imposed by building codes, in the mid-1990s, the term “sustainability” began
consump-to enter the HVAC designer’s lexicon Some building owners wanted their ings and systems to be not only energy efficient but also to reduce their negative imprint on the environment in general While the definition of what constitutes
build-“sustainable building design” is still evolving, modern HVAC designers are often required by their clients to focus their designs on sustainability, without the func-tion and performance of HVAC systems being impaired
Today, while the primary goal for any HVAC system remains its satisfactory function and performance, there are two other important secondary goals for the HVAC systems designer, as discussed in Chapter 14
1 Minimize the energy burden imposed by the system ASHRAE standards, energy conservation building codes, and simply good design stewardship are all imposed
to meet this goal
2 Minimize the system’s overall negative impact on the environment, not limited to just energy consumption, making it more “sustainable.”
1.2 Buildings as Thermodynamic Systems
1.2.1 Systems Concepts
Buildings may be viewed as thermodynamic systems The advantage of such an approach
is that it allows an overall view of the processes of heat gain and loss that are operating continually to change the environmental conditions felt by the occupants Figure 1-1 shows the general concept of a system as applied to a building The system boundary is a real or imaginary separation between the area “inside” and the external environment
We are concerned with maintaining some desired conditions of temperature and ity within the boundary However, the energy inputs and outputs act to change these con-ditions unless we compensate for them According to the first law of thermodynamics, all
Trang 23humid-of the energy entering the system boundaries can be accounted for … it is either stored in the system or it is returned back through the system boundaries.
Virtually all of the energy that enters the boundaries of a building is transformed tually into thermal energy, and it is typically felt as an increase in indoor temperature (internal energy) of the building Some energy enters or leaves the building directly in the form of thermal energy The energy that enters or leaves as a result of a differing inside and outside temperature is referred to as “heat” (although heat is also loosely used to refer
even-to any form of thermal energy)
1.2.2 Energy Gains and Losses in Building Spaces
Figure 1-2 shows the typical methods by which energy enters and leaves building spaces.From this figure, it is quite clear that energy typically enters in one form and leaves in another For example, on a hot summer day, the building is heated by the solar radiation
on the walls (which results in conduction through the walls) and by direct solar radiation through glass (which results in heating of the interior surface) Conduction through the walls and glass due to indoor/outdoor temperature differences also occurs The electrical energy that enters the building is eventually converted to heat, either mechanically by fans
or other motor-operated devices, or indirectly, such as by lighting
The important concept is that all of this energy must eventually begin to leave the building at roughly the same daily rate that it enters or else the building will just become increasingly warmer The primary mechanism for relieving the buildup of thermal energy
is conduction (and convection) through the envelope, which will occur when the outside temperature falls below the inside temperature (such as during the evening) This is, in the
Chemical energy input (fuel combustion)
mha Energy in exfiltration and/or exhaust air
energy input
FIGURE 1-1
Building viewed as a thermodynamic system.
Trang 24absence of mechanical cooling, a self-limiting process; the building would lose heat until only the indoor temperature becomes nearly equal to the outside temperature, at which point the conduction heat transfer would cease If there were significant internal heat gen-eration The temperature inside would continue to climb and conduction from inside to outside would resume At some point, however, the temperature may be just high enough
to maintain a conduction rate that would balance the internal heat generation rate and a steady-state condition would have been achieved This sort of dynamic temperature fluc-tuation happens continuously in all buildings, although it may be diminished by mechani-cal heating or cooling
This is a simplified look at the process, and there are several complicating factors For example, the walls of the building have mass, which has a thermal storage capacity The effect of thermal mass is to retard the flow of thermal energy (since a portion of it must be stored in the walls along the way) That is, the wall materials are heated or cooled as the
“wave” of energy makes its way from the hotter to the cooler side of the wall
1.2.3 Building Envelope Design
Every building must have both a thermal and a moisture envelope, and it is an important part of the HVAC designer’s role to ensure that the envelope is correctly designed and con-structed, even though that is not normally part of his or her direct design responsibility.The role of the thermal envelope is to reduce or eliminate heat transfer across envelope boundaries driven by temperature differences and solar radiation The role of the moisture
Transmission through roof and/or ceiling
Internal heat gains from lights, people, and appliances
Infiltration/
exfiltration through cracks
at doors and windows
Solar heat gain
FIGURE 1-2
Basic elements of heat transfer to and from a building space.
Trang 25envelope is to prevent the migration of water, in liquid or vapor form, into the building The moisture envelope always consists of the rain barrier to stop liquid water from entering the building, the air barrier to stop water vapor introduced by infiltration, and the vapor bar-rier (roofs) or retarder (walls) to stop moisture vapor migration due to humidity differences.Figure 1-3 defines the various climate zones in the United States and can be used for establishing envelope design criteria.
Note, however, that ASHRAE and the U.S Department of Energy may revise the aries of climate zones every 3–5 years in response to climate change that is underway Designers should always review climate zones as defined in the most current edition of
bound-ASHRAE Standard 90.1.
In all climate zones, the thermal envelope should be located at or near the inside of walls, roofs, and floors The thermal envelope performance depends on selection of appropriate U-factors for walls, roofs, and floors and solar transmission and shading requirements for
building fenestration The HVAC engineer’s role is to advise the architect to ensure high thermal performance and reduced heat loss and heat gain due to each of these factors.
The moisture envelope design is equally important This envelope element consists of a rain barrier, an air barrier, and a vapor barrier or retarder The configuration of the mois-
ture envelope depends on the outdoor climate and it is a primary requirement for the HVAC designer to ensure that the wall design(s) correctly define the correct configuration.
In all climate zones, the rain barrier and the air barrier must be installed at or diately inboard of the wall cladding The role of these two barriers is to prevent liquid
imme-FIGURE 1-3
Climate zones in the United States (Public Domain (U.S Department of Energy) https://basc.pnnl.gov/system/ files_force/images/IECCmap_Revised.jpg?download=1)
Trang 26moisture from entering the exterior wall and, ultimately, into the air-conditioned ing But, the location of the vapor barrier or retarder, which is designed to prevent water vapor from entering the exterior wall, is more complex and varies by the type of climate
build-as defined in the following:
1 In hot, humid climate zones 1, 2A, 2B, and 3A, the vapor retarder must never be placed
on the indoor wall since the outdoor moisture will be trapped in the wall Vinyl wallcovering is an effective vapor barrier that should never be used in hot, humid climates Rather, the vapor retarder must be located immediately inboard (or as part of) the air barrier, near the outer boundary of the wall construction
2 In a mixed, humid climate such as zone 4A and in a mixed, dry climate such as zone
4B, the placement of the vapor retarder requires more analysis by the designer In this type of climate, the vapor retarder should be installed roughly in the “thermal middle” of the wall assembly
An interior vapor barrier would be detrimental, as it would prevent the wall assembly from drying toward the interior during cooling periods The wall assem-bly is more forgiving without the interior vapor barrier than if one were installed.For hospitals, laboratories, and so on, located in mixed, humid climates, for which humidification is provided during the winter and the indoor dew point temperature remains above 40°F [4°C], detailed analysis of the vapor retarder placement is required to establish the correct vapor retarder location
3 In cold climate zones 5, 6, and 7, indoor moisture levels tend to be higher than
out-door levels during winter, and thus, the vapor retarder must be located at or near the inside surface of the wall
4 In hot, dry climates, such as zone 3B, no vapor retarder is required since there is
little moisture vapor outdoors or indoors to negatively impact wall construction
1.3 Common HVAC Elements
There certain elements that apply to almost all HVAC systems that are generally referred
to as “common work” or “common elements.” Again, while these elements may not be part of the HVAC designer’s direct design responsibility, he or she is charged with insuring that they are addressed correctly.
1.3.1 Structural Support for HVAC Components
HVAC systems typically consist of many different components that must be installed, ported, and anchored individually
sup-HVAC equipment must be supported in accordance with the equipment manufacturer’s requirements, coordinated with the equipment location within the building and building structural system
1 Floor and Housekeeping Pad Installation Floor-mounted equipment must always be installed on concrete housekeeping pad (or equivalent) The pads for air-handling units should be 6+″ [150+ mm] high to allow for the installation of drain pan traps,
Trang 27while pads for all other equipment should be 4″ [100 mm] high The equipment should be anchored and grouted to housekeeping pads.
2 Structural Slab, Pier, or Foundation Installation. Cooling towers, tanks, chillers, and (sometimes) boilers are typically heavy enough to require a structural slab, pier(s),
or foundations for support Review each component with the project structural designer
to determine requirements
3 Suspended Installation. For suspended equipment, structural steel framing to tribute the imposed operating loads without stressing building structural ele-ments or causing damage to the building substrate is normally required This frame, which is then suspended from the building structure with vibration isola-tors, an appropriately sized all-thread rod, serves an equipment platform
4 Rooftop Installation. The equipment may be mounted on structural steel supports (the preferred method) or on continuous roof curb:
a Structural steel supports must be as detailed on the drawings, and their design requires the input of a structural engineer The supports must allow clear space under the equipment for roof maintenance and replacement (at least 36″ [1,000 mm] under equipment up to 60″ [1,500 mm] wide and at least 48″ [1,200 mm] under equipment over 60″ [1,500 mm] wide) and include appropriate service walkways and handrails, steps, ladders, and so on
b Continuous roof curb tops are required to be at least 17″ [430 mm] above the roof surface to prevent leakage into the building in the event of roof flooding due to blocked roof drains The top of all roof curbs must be level, with pitch
built into curb when deck slopes are 2% or greater Often, structural steel forcement below the roof is required to avoid stressing building structural elements and must be reviewed with the project structural designer.
rein-Ductwork support requirements are well defined in HVAC Duct Construction Standards— Metal and Flexible, Sheet Metal and Contractor’s National Association’s (SMACNA), with which every HVAC designer should become familiar
1 Horizontal Suspended Installation. Ductwork hangers may be fabricated with sheet
metal straps or all-thread rod SMACNA HVAC Duct Construction Standards
pro-vides numerous details for individual duct hanger methods Where multiple ducts are routed together, they may be supported by a common “trapeze” hanger, essentially a single cross member with two or more vertical hangers
2 Vertical Installation Vertical ducts must be anchored and supported at each floor
3 Rooftop Installation. Horizontal ductwork installed above a roof must be supported
by roof support rails integrated with the building roof and structural members to both support the weight of the duct and to provide vertical and horizontal anchor-
ing against wind forces Consult with the project structural designer to determine requirements and provide details as necessary.
Piping systems are generally classified on the basis of their service temperature, which impacts strength, potential thermal expansion/contraction, and insulation requirements (see Chapter 6), as summarized in Table 1-1
Based on these classifications, hangers and supports for piping systems are typically
selected in accordance with the requirements of Standard Practice-58, Pipe Hangers and Supports, Manufacturers Standardization Society (MSS)
Trang 281 Suspended Horizontal Installation Horizontal piping is typically supported by factory-fabricated horizontal-piping hangers complying with MSS 58 Where multiple horizontal pipes are routed together, “trapeze” hangers may be field fab-ricated from structural steel members or from preformed channel members and suspended by two or more all-thread hanger rods Each pipe on a trapeze hanger must be individually supported, and if the piping is insulated, the insulation must
be protected by using MSS Type 39 pipe saddles for Classification Type 1B piping and MSS Type 40 insulation shields for Classification Types 1A, 2, and 3 piping at each pipe support Install supports with maximum spacing and all-thread hanger
rods sized in accordance with Table 1-2, where NPS defines nominal pipe size.
2 Vertical Installation Vertical piping requires the use of factory-fabricated riser clamps complying with MSS Type 8 for support Vertical piping must be anchored and supported at each floor In tall, concrete structure buildings, it is typically necessary to provide expansion devices on every other floor or so to account for long-term “creep” or shrinkage of the structure
3 Rooftop Installation Horizontal rooftop piping requires the use of factory-fabricated roof pipe rails with individual pipe supports (as for trapeze hangers)
TABLE 1-1
Piping Systems Classification
Classification Temperature Range Typical HVAC Applications
Type 1: Hot systems Type 1A: 120°F–250°F
[50–120°C] Hot water, low pressure steam (≤15 psig [100 kPa]), steam condensate, low pressure boiler feedwater Type 1B: >250°F [120°C] High pressure steam (>15 psig [100 kPa]), high
pressure boiler feedwater, high temperature hot water Type 2: Ambient systems 71°F–120°F [22–50°C] Condenser water (indoor), oil, fuel gas
Type 3: Cold systems Type 3A: 32°F–70°F
[0°C–49°C] Chilled water, cooling coil condensate, condenser water (outdoor), condenser water (all) with waterside
economizer cycle Type 3B: <32°F [0°C] Liquid and cold gas refrigerant, chilled water with ice
thermal storage system
Trang 29The HVAC designer must review seismic requirements with the project architect/ structural engineer at the beginning of the design process The responsibility for the design of manufactured equipment and/or field-fabricated components installed to with-stand seismic loading, including comprehensive engineering analysis by a qualified pro-fessional engineer using performance requirements and design criteria specified by the
HVAC designer, should be delegated to the contractor by the project specifications This is
a very specialized design process and is typically not within the skill set of most HVAC designers Note that seismic design criteria may trigger building code requirements for special inspec- tions by a third party for this portion of the work during the construction period
As with any design, there is certain basic information that is required before seismic restraints can be selected and placed The building owner, architect, and structural engi-neer must make the decisions that form the basis for the information required to select the seismic restraints for the building This is the information that is included in the proj-ect specifications The following parameters must be defined by the design professionals having responsibility for HVAC systems in a building and should be determined by the structural engineer of record
1 Occupancy Category or Seismic Use Group This is defined by the applicable building code on the basis of building use and specifies which buildings are required for emergency response or disaster recovery
2 Seismic Design Category. This determines whether or not seismic restraint is required
3 Short Period Design Response Acceleration Parameter. This value is used to compute the horizontal seismic force used to design and/or select the seismic restraints required
The responsibility for the design of manufactured equipment and/or field-fabricated ponents installed outdoors to withstand wind loading, including comprehensive engi-neering analysis by a qualified professional engineer, using performance requirements and design criteria specified by the HVAC designer, should be delegated to the equipment suppliers and/or the project structural designer by the project specifications Again, this
com-is a very specialized design process and com-is typically not within the skill set of most HVAC designers
Equipment and/or field-fabricated components installed outdoors must be fabricated and anchored to the ground or building structure, as applicable, to withstand a wind load imposed on the largest vertical or projected surface area at the maximum wind speed dictated by the applicable building code based on the project location Anchoring by using structural frames, straps, or other hold-down devices attached to foundations, structural supports, or roof curbs, as applicable, is required
1.3.2 Electrical Aspects of HVAC Systems
For most HVAC designers, electrical engineering, in general, and Article 430 of the National Electrical Code (NEC), “Motors, Motor Circuits, and Controllers,” in particular, are not his
or her strong suit But, every HVAC designer must have a working knowledge of basic electrical terms and relationships and the electrical requirements of various HVAC com-ponents, particularly motors, and be aware of the information required by the project elec-trical designer to ensure that electrical service to each component is adequate
Trang 30Building power systems utilize alternating current (AC) voltage levels, roughly defined
as LV (low voltage, ≤600 V) and HV (high voltage, >600 V), with a frequency of 50 or 60 Hz
An electrical circuit has the following three basic components:
1 Voltage (V) is defined as the electrical potential difference that causes electrons to
flow
2 Current (I) is defined as the flow of electrons and is measured in amperes (amps).
3 Resistance (R) is defined as the opposition to the flow of electrons and is measured
in ohms
These components are defined by Ohm’s law in accordance with Eq (1-1)
In order to flow, electricity must have a continuous, closed path The word circuit refers to
the entire course an electric current travels, from the source of power, through an cal device, and back to the source Every circuit is comprised of three major components:
1 A conductive “medium,” such as a wire
2 A “source” of electrical power
3 A “load” that needs electrical power to operate
The current flows to the loads through a “hot” wire and returns via a “neutral” wire, under normal conditions, maintained at zero volts
There are also two optional components that can be included in any electrical circuit: control devices and protective devices Control and protective devices, however, are not required for a circuit to function A power circuit is defined as any circuit that carries power to electrical loads A control circuit is a special type of circuit that uses control devices to determine when loads are energized or de-energized by controlling the cur-rent flow
In direct current (DC) circuits, power (P) is simply a product of voltage and current (amps),
as shown by Eq (1-2)
where
For AC circuits, power factor (PF) must be considered PF is defined as the ratio of the “real”
power flowing to the load to the “apparent” power in the circuit A PF of less than one means that the voltage and current waveforms are not in phase, and real power is the capacity of the circuit for performing work under specific loading, while apparent power
is the product of the current and voltage of the circuit
For resistive loads, such as electric heating coils, and so on, PF = 1.0 For inductive or capacitive loads, such as motors, the PF can be estimated from Table 1-3
For single-phase power circuits, power is defined by Eq (1-3)
PF
Trang 31For three-phase power circuits, there are three conductors supplying the load rather than only two as for single-phase loads The current in one conductor supplying the three-phase load is 120° shifted in phase from the current flowing in each of the other wires A factor that takes all of this into account is the number 1.73, the square root of three Thus, polyphase power is defined by Eq (1-4).
For motors in the United States, power is typically defined by the actual load imposed on
the motor in terms of brake horsepower (bHP), while the motor nameplate rating is given as horsepower (HP) In either case, the conversion from W to bHP or HP is defined by Eq (1-5)
= ×
Where EFF is the motor efficiency at the given load percentage
The total amount of energy consumed by the load in a power circuit is computed by
multiplying the P by the length of time the load is “on” (hours) This is most commonly
expressed in “Kilowatt Hours” (or kWh), where a kW is equal to 1,000 W
While the design of the electrical service to HVAC equipment is typically performed by the project electrical engineer, the HVAC designer must carefully coordinate HVAC elec-trical loads, voltage requirements, and safety and control devices required by the NEC to ensure that (1) electrical power is provided as needed, (2) in each case it is the right type
of power (voltage and phase), and (3) procedures of connection of electrical power to each HVAC component is clearly defined in the project specifications
The general term conductor applies to anything that conducts the flow of electricity In
the United States, electrical wires are conductors that are sized using two different
sys-tems: the American Wire Gauge System (AWG) and the Thousand Circular Mill System Both
systems designate wire size based on diameter or cross-sectional areas In the AWG
sys-tem, every six-gauge decrease corresponds to a doubling of the wire diameter and every three-gauge decrease doubles the wire’s cross-sectional area In the SI system, wire gauge
is defined as 10 times the wire diameter in millimeters, so a 50-gauge metric wire would
be 5 mm in diameter
Note that in AWG, the diameter goes up as the gauge goes down, but for SI gauges, it is the opposite To avoid confusion, in SI units, wire size is more commonly specified in mil-limeters rather than in gauges
The current carrying capacity of a particular wire is dictated by its “capacity,” it can be allowed to conduct, as limited to the maximum permitted by the NEC, based on the type
of wire (copper or aluminum) and the maximum temperature rating of the wire’s tion (60°C, 75°C, or 90°C)
insula-TABLE 1-3
Typical Motor PF
Motor Nameplate (HP) Speed (rpm)
PF 50% Load 75% Load 100% Load
Trang 32As electrical energy is generated, it is transformed and transported through a network
of wires, substations, and transformers to the consumer Typically, electric energy leaves the utility distribution substation and is distributed via three-phase distribution lines to switchboards within the building Switchboards take a large block of power and break
it down into separate circuits, each of which is controlled and protected by the fuses or switchgear of the switchboard
A panel board is an enclosed assembly with circuit breakers Branch circuits feed power to receptacles, switches, and equipment in the building Likewise, motor control centers, with
integral switches and/or motor starters, may be supplied as part of the electrical tion system ready to connect field wiring to the electrical motors served
distribu-Overcurrent protection is installed to provide automatic means for interrupting ing”) a circuit in which the current rises above their rating due to a fault or short circuit Two types of over-current devices are in common use: circuit breakers and fuses, both rated in amperes
(“open-A circuit breaker is a switching device capable of protecting the distribution line or feeder
connected to it from overloads and faults If a circuit is overloaded, the mechanism inside the breaker trips the switch and breaks the circuit The circuit breaker may be reset by simply flipping the switch A circuit breaker is capable of ignoring short-period overloads (such as the initial current required in the starting of a motor) without “tripping” but pro-tects against prolonged overloads
A fuse is a thermal device used for protecting, typically, switchgear equipment and
cables against over-currents When a fuse element overheats due to an over-current tion and “blows,” the circuit breaks
condi-A ground fault interrupter is a device that detects small current leaks and disconnects the
hot wire to the circuit It can also be part of a circuit breaker or part of an electrical outlet
Relays are small, very fast-acting automatic switches designed to protect an electrical tem from faults and overloads It is usually an electromagnetic device which has a coil When this coil is supplied with power, a magnetic field is created which will operate a mechani-cal switch When a relay senses a problem, it quickly sends a signal to one or many circuit breakers to open, or trip, thus protecting it from damage as well as human life from injury
sys-A contactor is an electrically operated switch, a large relay in effect, which can be made to
switch a motor, heater bank, capacitor bank, and so on, on and off directly or by a remote controller, such as a thermostat, humidistat, timer, pilot devices, or any other protective devices Although it is a switch, a contactor is designed to interrupt an electric current
repeatedly and frequently When a contactor breaks the current, an electrical arc is
estab-lished across the contacts and a good amount of heat energy is generated This increases when the frequency of breaking the current increases and may result in the “welding” or fusing of the contacts and contactor failure
The simplest and most common starting device for HVAC electrical motors is the the-line (ACL) starter, consisting of a main contactor and a thermal or electronic overload relay The disadvantage of the direct-on-line method is very high starting current (6–10 times the rated motor currents) and high starting torque, causing slipping belts, heavy wear on bearings, and gear boxes, and so on ACL starting is typically applied to motors only 40 HP [30 kW] or less
across-For larger motors, reduced voltage starters are typically applied:
1 Wye-delta starting device consists normally of three contactors, an overload relay, and a timer for setting the time in the star position (starting position) The start-ing current is about 30% of the direct-on-line starting device The starting torque
Trang 33is about 25% of the direct-on-line starting torque The stress on an application is reduced compared to the direct-on-line starting method.
2 Part-winding starting uses only a portion (usually one-half but sometimes thirds) of the motor winding, increasing the impedance seen by the power system
two-It is to be used only for voltage recovery and must not be left on the start tion for more than 2–3 s The motor is not expected to accelerate on the start con-nection The advantages of part-winding starting include the following:
connec-a Starting current is 60%–75% of normal, depending on the specific winding connection
b Starting torque is very low (may not even turn the shaft)
c Winding heating is very high on start connection
3 An autotransformer starter is connected so the motor is on the secondary of an
auto-transformer during starting The autoauto-transformer has taps, to limit the voltage, applied to the motor at 50%, 65%, or 80% of full voltage Because the line current varies as the square of the impressed voltage, these same taps equate to 25%, 42%, and 64% of the full-voltage value of line current The autotransformer starter was historically the most flexible of reduced voltage starters until the advent of the solid-state starter
4 Solid-state starters use back-to-back thermistors for each line to the motor These
six thermistors control power to the motor The power adjusts by not completely turning on the thermistors during starting In other words, only a portion of the three-phase sinusoidal wave is supplied to the motor during start
Because of these control features, the big advantage of the solid-state starter is the large number of starting characteristics The standard soft-start mode simply ramps the voltage from a preset initial torque value to 100% during a user-selected time of 0–30 s
Another available control mode is a start based on current limitation In this mode, the current is limited (between 50% and 600%), as is the duration (between
0 and 30 s) Other available operating modes include kick start, soft stop, and pump control options The last option starts a pump motor on a curve rather than
a straight line ramp This causes the hydraulic system to react as if there were a closed discharge valve behind the pump, opening during starting
5 A variable frequency drive (VFD) is a type of motor controller that includes an
advanced solid-state power controller Instead of simply opening and closing the power circuit such as an ACL starter or ramping the motor voltage up or down such as a soft start to turn the motor on and off, a VFD controls motor speed and, thus, power
The distance between the VFD and the motor it controls can create an opportunity for motor winding failure due to reflected wave HVs caused by locating the motor too distant from the VFD Several drive manufacturers have Internet-based calculators that will tell designers if the distance between the motor and the drive is too far, but the common rec-ommendation is to keep the VFD within 50 ft [15 m] and within the line of sight
VFDs utilized for most HVAC applications are the integrated gate bipolar transistor pulse width modulation type in accordance with National Electrical Manufacturers Association
(NEMA) Publication ICS 2, listed and labeled as a complete unit and arranged to provide
vari-able speed of an inverter duty induction motor by adjusting output voltage and frequency
Trang 34As building electrical systems have incorporated more and more electronic devices that are “nonlinear load,” including electronic lighting ballasts, computers, and VFDs, prob-lems with harmonic interferences have arisen Consequently, some designers require the
VFD manufacturer to demonstrate compliance with IEEE Standard 519, which is written
to manage harmonic contributions from facilities into the primary electrical distribution system operated by an electric utility The context of the standard is to provide a basis for determining if a customer reflects an excessive amount of harmonic content, with respect
to the electrical demand load and the size of electrical service infrastructure, into the ity distribution system
util-However, applying IEEE Standard 519 to a VFD makes no sense! This is the equivalent of
an HVAC designer requiring an air handler manufacturer to demonstrate that his ucts do not compromise the ability to achieve a 30% energy reduction in the building
prod-Thus, project specifications for VFDs should not include a reference to IEEE Standard 519.
Typical HVAC VFDs are six pulse drives that are adequate for most applications if the trical designer is competent in configuring the electrical distribution system for the imposed HVAC equipment loads and potential harmonics. For electrical motors exceeding 75 HP [56 kW] (such
elec-as for large fans, pumps, or chillers), 12 or even 18 pulse drives may be specified elec-as needed.Basic VFD requirements for HVAC applications include the following:
1 Input AC Voltage Tolerance. Plus or minus 10%
2 Input Frequency Tolerance. Plus or minus 2 Hz
3 Minimum Efficiency. Ninety-six percent at design frequency and full load
4 Minimum Harmonics. VFDs should be equipped with internal 5% impedance DC link reactors to minimize power line harmonics and to provide near unity PF VFDs without DC link reactors should include 5% impedance line side reactors.Total harmonic distortion should not exceed 10% for motors <10 HP [7.5 kW], 8% for motors 10 ≤ HP <25 [7.5 ≤ kW ≤ 18.6], and 5% for motors ≥25 HP [18.6 kW] Maximum individual allowable distortion at any individual harmonic shall not exceed 5% for motors <10 HP [7.5 kW], 5% for motors 10 ≤ HP < 25 [7.5 ≤ kW ≤ 18.6], and 3% for motors ≥25 HP [18.6 kW] Testing and documentation of total harmonic distortion at the VFD-circuit breaker terminals is required
5 Short Circuit Interrupt Capability. ≥65,000 A
6 Input Transient Protection. Integral surge suppressors
7 Phase Protection Loss-of-phase, reverse-phase, and phase imbalance
8 Motor Overload Relay. Adjustable and capable of NEMA 250, Class 20 performance, with notch filter to prevent operation of the controller-motor-load combination at
a natural frequency of the combination
9 Temperature Compensation. VFD should automatically adjust current fallback based
on output frequency for temperature protection of self-cooled fan-ventilated motors at slow speeds
10 PF VFD should automatically boost PF at lower speeds by including a full-wave
diode bridge rectifier to maintain displacement PF at near unity regardless of speed or load
11 Manual Bypass VFD should include magnetic contactor to safely transfer
motor between controller output and bypass controller circuit when motor is
at zero speed The motor shall operate at full speed when in bypass control
Trang 35Controller-off-bypass selector switch sets the mode, and indicator lights give indication of the mode selected The unit must be capable of stable operation (starting, stopping, and running), with motor completely disconnected from con-troller (no load).
12 Isolating Switch A non-load-break switch is required for service/isolation of
the VFD and permits safe troubleshooting and testing, both energized and de- energized, while the motor is operating in bypass mode
13 Integral Disconnecting Means Each VFD must include factory-assembled,
molded-case circuit breakers with permanent instantaneous magnetic trips in each pole, with fault current interrupting the rating suitable for application and ampere rat-ings as indicated
14 Unit Controls Manufacturer-furnished, factory-installed VFD controls, including
final control elements, must interface seamlessly with the HVAC system direct digital control system
For fractional HP [kW] motors and smaller packaged HVAC equipment (≤5 tons [18 kW]), single-phase power is typically sufficient Single-phase circuits may also be required to provide power for control systems and “interlock wiring” (see Chapter 13) For motors larger than 1 HP [0.75 kW] and for larger HVAC packaged equipment, three-phase (typi-cally referred to as “polyphase”) electrical service is normally required
The NEMA defines 20 types of motor enclosures, which fall into two broad categories:
“open” and “totally enclosed.” Open motors have ventilation openings allowing for air cooling of the motor windings, while totally enclosed motors do not
The most common open motor is the open drip-proof (ODP) type in which ventilation
openings are positioned to keep particles and water from falling into the motor Most motors found in commercial buildings are ODP motors The water resistance of ODP motors can be improved by requiring “splash-proof” motors that add protection from material that may enter the motor from below, while “guarded” motors use screens or baffles to protect the motor from particle entry
Totally enclosed motors are designed to prevent free exchange of air between the inside
and the outside of the motor The most common totally enclosed motor is the totally enclosed fan-cooled (TEFC) in which a fan on the opposite end of the motor from the load draws air over the case to provide cooling
Pumps, fans, cooling towers, and so on utilize general purpose motors But, for some applications, a “definite purpose motor” may be provided by equipment manufacturers A definite purpose motor is any motor designed, listed, and offered in standard ratings with standard operating characteristics and mechanical construction, for use under service conditions other than usual or for a particular application The definite purpose family of motors includes those used for refrigeration compressors, oil burners, unit heaters, pack-aged terminal air-conditioning units and heat pumps, computer room air conditioning units, steam boiler feedwater pumps, and many more
All electrical motors must be rated for continuous duty at ambient temperature of 40°C and at an altitude of 3,300 ft [1,000 m] above sea level Motor capacity and torque char-acteristics must be sufficient for the motor to start, accelerate, and operate its connected load at designated speeds, at installed altitude and environment, with indicated operating sequence, and without exceeding nameplate ratings or service factor (Note: The need for starting torque when large fans that are powered by low horsepower motors, common for return air applications, can be a critical factor in motor selection.)
Trang 36Table 1-4 summarizes the typical motor standard (“nameplate”) horsepower (HP) ings utilized for HVAC applications.
rat-Designers should select motors on the following basis to minimize oversizing and loss
of efficiency, while maintaining overall motor reliability:
1 If the imposed load brake horsepower is <6.0 [4.5 kW], select the first available
motor with a nameplate rating exceeding the required brake horsepower If the
first available motor has a nameplate horsepower that exceeds the required brake
horsepower by less than 50%, select the next available motor For example, a 5.9 bHP
[4.4 kW] load should be met with a 10 HP [7.5 kW] motor since a 7.5 HP [6 kW] motor provides only 27% more horsepower than the load But, a 5.0 bHP [3.7 kW] load may be met with a 7.5 HP [5.6 kW] motor
2 If the required brake horsepower is ≥6.0 [4.5 kW], select the first available motor with a nameplate rating exceeding the required brake horsepower [load kW] If the first available motor has a nameplate horsepower that exceeds the required
brake horsepower by less than 30%, select the next available motor.
Synchronous motor speed is its speed under “no load” conditions, and the actual speed under normal load conditions will typically be approximately 50 rpm lower The synchro-nous speed of an induction motor depends on the frequency of the power supply and on the number of poles for which the motor is wound The higher the frequency, the faster
a motor runs The more poles the motor has, the slower it runs Generally, power supply frequency in the western hemisphere is 60 Hz, while Europe and the rest of the eastern hemisphere utilize 50 Hz power Thus, motor synchronous speeds can be summarized as shown in Table 1-5
Motors for HVAC applications are normally selected for a 1,800 rpm rotational speed at
60 Hz [1,500 rpm at 50 Hz] Motors rated at higher rpm may be required for pumps with high pressure requirements, and motors rated at lower rpm may occasionally be needed for pumps or fans with low pressure requirements
TABLE 1-4
Standard Electrical Motor Ratings (HP [kW])
Trang 37Single-phase, constant speed motors may be any of the following types: permanent-split capacitor ; split phase; capacitor start, inductor run; or capacitor start, capacitor run Constant speed motors 1/12 HP [0.07 kW] and smaller shall be shaded-pole type Two-speed motors should be variable-torque, permanent-split-capacitor type.
Variable speed single-phase motors should be electronic commutation motors (ECMs), i.e.,
brushless DC motors using a built-in inverter to convert AC power supplied to DC power and a magnet rotor needed to operate the motor This type of motor is speed-controllable via internal circuitry down to about 20% of full speed (i.e., 80% or 5:1 turndown) via a controller output signal ECMs are able to achieve greater efficiency in airflow systems
than conventional permanent split capacitor (PSC) motors used in the past Initially, silicon
rectifier controllers were added to PSC motors to provide variable speed control But, as the PSC motor speed was reduced, efficiency suffered, falling from 65% to 70% to as low as 12% ECMs, on the other hand, maintain a high level (65%–75%) of efficiency over almost its full speed range Additionally, unlike PSC motors, ECMs are not prone to overheating and do not require additional measures to offset the generation of heat
Polyphase motors, located indoors or otherwise weather protected, should be in
accor-dance with NEMA Publication MG 1, Design B, medium duty induction open ODP type
with 1.15 service factor and Class F insulation, rated as “Premium” efficiency motors, meeting the requirements of DOE in the United States or the requirements of IEC 60034-30 IE3 ratings promulgated by the European Union The use of premium efficiency motors reduces energy consumption by an average 6% (2%–10% depending on motor rating) over standard efficiency motors
Multispeed polyphase motors with a 2:1 speed ratio, often used in cooling tower
applica-tions, may be consequent pole, single winding type in order minimize first cost Multispeed
poly-phase motors with other than a 2:1 speed ratio should have separate winding for each speed.Polyphase motors used with VFDs must comply with additional requirement(s):
1 Early VFDs produced a great deal of electrical “noise” that resulted in transient currents overheating motor windings and causing insulation failure Additionally, the step frequency pattern produced by a VFD to emulate the AC power sine wave can impose HVs for short periods that can also result in breakdown of the motor insulation To address these issues, motor windings must be copper magnet wire with moisture-resistant insulation varnish and be designed and tested to resist transient spikes, high frequencies, and short-time rise pulses produced by pulse-width modulation inverters Motors controlled by VFDs must comply with NEMA
Publication MG 1, Part 31, to provide windings capable of withstanding up to 1,600
2 Harmonics produced by VFDs may produce transient currents along the motor shaft that ultimately may result in motor bearing failure A relatively low cost
measure to address this problem is to install a shaft grounding ring (SGR) to “bleed”
current from the motor shaft to the motor casing, which is grounded In effect, the SGR creates a low-resistance electrical path from the motor shaft to a ground that bypasses the bearings It is recommended that the designer specify SGRs on motors that are 10 HP [7.5 kW] or larger
For large motors, replacement of bearings becomes hugely expensive, sometimes exceeding the cost of the motor Therefore, NEMA recommends that, for motors
100 HP [75 kW] rating and larger ceramic bearings or electrical insulated ings, to provide minimum 50 MΩ resistance at up to 1,000 VDC be incorporated
Trang 38bear-With either of these measures, the bearings cease to be part of an electrical path to ground and pitting will not occur.
When using insulated bearings, it is also necessary to use shaft brushes If lated bushings are used without the brushes, there is no mechanism to remove volt-ages that accumulate on the motor rotor/shaft assembly Eventually, the voltage will increase to the point where an electrical arc will occur around the insulated bear-ing or, even worse, to the motor windings This particular problem is much more prevalent where the motor mechanically drives a load through an insulated belt;
insu-in this case, the insu-insulated belt allows a static charge to accumulate on the insu-insulated rotor similar to the operation of the old hand-cranked static electricity generators
3 Polyphase motors located outdoors, including motors within cooling towers or evaporative coolers, should comply with the following:
a To provide the minimum level of weather protection, the motor must be a TEFC
type with weather-resistant motor junctions, terminal box, and motor coating
b To provide the maximum level of weather protection, the motor should be
TEFC, “Cast Iron Construction Only” complying with the requirements of
IEEE Standard 45, Recommended Practice for Electric Installations on Shipboard, and
be classified as a “Marine Motor.”
c For any motor installed outdoors that does not run for significant periods of time, condensation within the motor windings must be prevented by equip-ping the motor with internal cartridge or strip heaters that are energized dur-ing nonoperating periods to maintain a temperature inside the motor 5°F–10°F [3°C–6°C] warmer than the ambient air
V-belt drives, consisting of belt and pulley assemblies, are used in HVAC applications (most commonly for fans) to perform two functions: transfer motor power to fans and
to reduce the fan speed from motor speed to the required load speed The power fer requirement dictates the belt(s)’ required load rating, while the sheaves (one on the motor and one on the load) dictate the speed change in direct ratio to their diameters For example, a motor sheave half the diameter of the fan sheave will reduce the fan’s speed to half the motor’s speed
trans-V-belt drives should be specified to utilize American National Standards Institute (ANSI)/Rubber Manufacturers Association (RMA) “cogged” V-belts with properly selected motor pulley and drive sheaves Generally, drives should be selected for the motor name-
plate horsepower, plus the recommended ANSI/RMA service factor (but, not less than 20%) to account for drive mechanical losses and potential load fluctuations, in addition to meeting the ANSI/RMA required allowances for pitch diameter, center distance, and arc of contact
1 V-belt drives rated as 1 HP [0.75 kW] and less may be provided with single standard
V-belt
2 Drives 1.5 HP [1.1 kW] and larger should be provided with raw edge cogged V belts,
the number of belts necessary to transmit the required power with 95% minimum efficiency but in no case less than 2
3 Belt drives for fans utilized as part of smoke control and/or smoke venting system
must be rated for the motor nameplate horsepower, plus 50% additional service
and arc of contact and must have at least two belts
Trang 39Sheaves and pulleys should be fixed pitch type, statically and dynamically balanced While adjustable pulleys are available, they are expensive, prone to early failure, accelerate belt wear, and are not recommended.
For personnel safety, belt drive guards are required for any exposed belt drive Belt drive
guards are also required for any belt drive installed within an air-handling unit that is designed to be personnel-accessible when in operation Belt drive guards must comply with U.S Occupational Health and Safety Administration (OSHA) and SMACNA require-ments Each guard must be designed with provisions to allow routine adjustment of belt tension, lubrication, and use of tachometer to measure the load rotational speed with the guard in place
The following should be considered with regard to belt drives:
1 Drives should be installed with adequate provisions for center distance ment to accommodate belt stretch
2 Centers should not exceed 2-1/2 to 3 times the sum of the sheave diameters nor be less than the diameter of the larger sheave
3 The angle of wrap (arc) created by belt contact on the smaller sheave should not be
less than 120°, computed by Eqs (1-6) and (1-7)
where
α1 = Angle of wrap for the small pulley (degrees)
α2 = Angle of wrap for the large pulley (degrees)
4 Sheave ratios should not exceed 8:1 To determine pulley diameters to drive a load, use Eq (1-8)
where
5 The belt speed should not exceed 5,000 ft/min [1,500 m/min] nor be less than 1,000 ft/min [300 m/min] A speed of 4,000 ft/min [1,200 m/min] is considered best practice Belt speed can be calculated by Eq (1-9)
Trang 40Sb = belt speed (ft/min) [m/min]
6 Sheaves should be dynamically balanced when rim speeds in excess of 5,000 ft/min [25 m/s] occur
7 A service factor of 1.2 is normal for belts operating between 16 and 24 h/day
For pumps and fans using variable speed motors, direct drive motor connections are
preferred In this case, the pump or fan shaft is aligned with the motor shaft and the two
are connected by use of a shaft coupling There are several advantages to direct drive over
the use of belt drives:
1 Lower first cost
2 Reduced maintenance cost by eliminating the need for periodic belt and sheave replacement
3 Reduced energy consumption due to eliminating the 3%–5% mechanical losses associated with belts and sheaves
Shaft couplings directly connect a motor shaft to the shaft of a fan or pump so that both the motor and the driven load rotate at the same speed Couplings connecting to polyphase motors should be a flexible type capable of absorbing vibration and be rated for motor name-plate horsepower [kW] plus an additional 50% service factor Couplings should be “drop-out” type to allow disassembly and removal without removing equipment shaft or motor
Shaft coupling guards are required for each directed connected motor assembly and must
comply with ANSI B15.1 and OSHA 1910.219 Guards should be easily removable for access and service and provided with openings for speed checks, and so on without removal.Enclosures for disconnect switches, starters, VFDs, control panels, and any other panel enclosures housing electrical equipment needed by HVAC components are rated based on NEMA standard ratings Panel enclosures must be suitable for the environment in which they will be installed, and HVAC designers should require NEMA-rated enclosures based
on Table 1-6
Finally, HVAC designers must be aware that coordination between them and electrical designers is required to prevent interference with electrical equipment by HVAC pipes, ducts, and so on, as dictated by code Figure 1-4 provides a graphical summary of the separations required in installations
TABLE 1-6
NEMA Enclosure Ratings for Electrical Equipment
1 Indoors only, dry, low dust, and noncorrosive environment
3R Outdoors, weatherproof, and rainproof
4 Outdoors, watertight, and rainproof
4X Same as 4 plus corrosion resistant (recommended for coastal environments)
7 Hazardous locations Class I, Groups A, B, C, or D
9 Hazardous locations Class II, Groups E, F, or G
12 Indoors subject to circulating nonhazardous dust or dripping noncorrosive liquids