2012 ASHRAE Handbook - Heating, Ventilating, and Air-Conditioning Systems and Equipment (SI Edition)

1084 Pages·2012·7.09 MB·English

2012 ASHRAE(cid:2) HANDBOOK
Heating, Ventilating,
and
Air-Conditioning
SYSTEMS AND EQUIPMENT
SI Edition
ASHRAE, 1791 Tullie Circle, N.E., Atlanta, GA 30329
www.ashrae.org
© 2012 ASHRAE. All rights reserved.
DEDICATED TO THE ADVANCEMENT OF
THE PROFESSION AND ITS ALLIED INDUSTRIES
No part of this book may be reproduced without permission in writing from ASHRAE,
except by a reviewer who may quote brief passages or reproduce illustrations in a review
with appropriate credit; nor may any part of this book be reproduced, stored in a retrieval
system, or transmitted in any form or by any means—electronic, photocopying, recording,
or other—without permission in writing from ASHRAE.
Volunteer members of ASHRAE Technical Committees and others compiled the infor-
mation in this handbook, and it is generally reviewed and updated every four years. Com-
ments, criticisms, and suggestions regarding the subject matter are invited. Any errors or
omissions in the data should be brought to the attention of the Editor. Additions and correc-
tions to Handbook volumes in print will be published in the Handbook published the year
following their verification and, as soon as verified, on the ASHRAE Internet Web site.
DISCLAIMER
ASHRAE has compiled this publication with care, but ASHRAE has not investigated,
and ASHRAE expressly disclaims any duty to investigate, any product, service, process,
procedure, design, or the like that may be described herein. The appearance of any technical
data or editorial material in this publication does not constitute endorsement, warranty, or
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entire risk of the use of any information in this publication is assumed by the user.
ISBN 978-1-936504-26-8
ISSN ISSN 1930-7705
The paper for this book was manufactured in an acid- and
elemental-chlorine-free process with pulp obtained from sources
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ASHRAE Research: Improving the Quality of Life
ASHRAE is the world’s foremost technical society in the fields properties and building physics and to promote the application of
of heating, ventilation, air conditioning, and refrigeration. Its mem- innovative technologies.
bers worldwide are individuals who share ideas, identify needs, sup- Chapters in the ASHRAE Handbook are updated through the
port research, and write the industry’s standards for testing and experience of members of ASHRAE Technical Committees and
practice. The result is that engineers are better able to keep indoor through results of ASHRAE Research reported at ASHRAE confer-
environments safe and productive while protecting and preserving ences and published in ASHRAE special publications and in
the outdoors for generations to come. ASHRAE Transactions.
One of the ways that ASHRAE supports its members’ and indus- For information about ASHRAE Research or to become a mem-
try’s need for information is through ASHRAE Research. Thou- ber, contact ASHRAE, 1791 Tullie Circle, Atlanta, GA 30329; tele-
sands of individuals and companies support ASHRAE Research phone: [phone]; www.ashrae.org.
annually, enabling ASHRAE to report new data about material
Preface
The 2012 ASHRAE Handbook—HVAC Systems and Equipment • Chapter 26, Air-to-Air Energy Recovery Equipment, has new per-
discusses various systems and the equipment (components or assem- formance equations, plus new content on capacity control and on
blies) they comprise, and describes features and differences. This multiple-exchanger, liquid-desiccant, thermosiphon, and twin-
information helps system designers and operators in selecting and tower systems.
using equipment. An accompanying CD-ROM contains all the vol- • Chapter 29, Air Cleaners for Particulate Contaminants, was
ume’s chapters in both I-P and SI units. revised to reflect current editions of ASHRAE Standard 52.2 and
This edition includes a new Chapter 18, Variable-Refrigerant- Guideline 26.
Flow Systems, which describes these systems, their components, • Chapter 30, Industrial Gas Cleaning and Air Pollution Control
and applicable standards in detail, and includes a system design Equipment, has been updated to reflect current technology, with
example and important guidance on costs, controls, and safety. added information on Z-flow pack filters.
Some of the volume’s other revisions and additions are as follows: • Chapter 32, Boilers, has an updated section on condensing boilers
and expanded guidance on boiler selection.
• Chapter 5, In-Room Terminal Systems, was extensively revised
• Chapter 37, Solar Energy Equipment, has new content on world-
for usability, and has updates for current practice and technology,
wide solar equipment use, colored collectors, seasonal storage,
including new sections on variable-refrigerant-flow (VRF) units
and photovoltaic (PV) performance and degradation.
and chilled beams.
• Chapter 39, Condensers, has added research results on the effects
• Chapter 8, Combustion Turbine Inlet Cooling, has a new section
of liquid inundation and oil on heat transfer.
on indirect evaporative cooling, plus new figures showing psy-
• Chapter 40, Cooling Towers, has added content on hybrid cool-
chrometric processes and capacity enhancement and costs.
ing towers, variable-frequency drive (VFD) operation, and free
• Chapter 9, Applied Heat Pump and Heat Recovery Systems, has
cooling.
a new section and figure on applying lead-chiller heat pump water
• Chapter 41, Evaporative Air-Cooling Equipment, revised through-
heating.
out, has a new section on sound attenuation.
• Chapter 10, Small Forced-Air Heating and Cooling Systems, has
• Chapter 44, Centrifugal Pumps, has been updated, with several
been revised for current practice, with new content and references
new figures and a new section on differential pressure control
on duct system efficiency.
with predefined control curves.
• Chapter 12, District Heating and Cooling, updated throughout,
• Chapter 45, Motors, Motor Controls, and Variable-Speed Drives,
has an extensive new section on economic comparisons, plus
has new sections on running motors above base speed and on
several new detailed examples.
VFD-induced bearing currents.
• Chapter 14, Condenser Water Systems, has added guidance on
• Chapter 51, Thermal Storage, has new content on unitary thermal
using lake or river water and on freeze protection and winter oper-
storage systems (UTSSs), two new detailed sizing examples, sev-
ation.
eral new figures, and extensive new guidance on equipment selec-
• Chapter 15, Medium- and High-Temperature Water Heating, has
tion and operation.
added information on pump seal cooling and water treatment.
• Chapter 17, Ultraviolet Lamp Systems, has new results from This volume is published, both as a bound print volume and in
ASHRAE research project RP-1509 on degradation of materials electronic format on a CD-ROM, in two editions: one using inch-
irradiated by UV-C energy. pound (I-P) units of measurement, the other using the International
• Chapter 19, Duct Construction, has a rewritten section on duct System of Units (SI).
leakage, and new information on air dispersion systems and Corrections to the 2009, 2010, and 2011 Handbook volumes can
factory-built grease duct systems. be found on the ASHRAE Web site at http://www.ashrae.org and in
• Chapter 20, Room Air Distribution Equipment, has revised and the Additions and Corrections section of this volume. Corrections
expanded sections on chilled beams and fan-coils. for this volume will be listed in subsequent volumes and on the
• Chapter 21, Fans, has added descriptions of types of fans and ASHRAE Web site.
their applications; many upgraded figures; vibration categories, Reader comments are enthusiastically invited. To suggest im-
grades and limits; and a complete rewrite of the controls section. provements for a chapter, please comment using the form on the
• Chapter 22, Humidifiers, has updated figures, plus new content on ASHRAE Web site or, using the cutout page(s) at the end of this vol-
management systems, environmental conditions, and residential ume’s index, write to Handbook Editor, ASHRAE, 1791 Tullie Cir-
steam and industrial high-pressure and gas-fired equipment. cle, Atlanta, GA 30329, or fax [phone], or e-mail to mowen
• Chapter 25, Mechanical Dehumidifiers and Related Components, @ashrae.org.
has new content on whole-house residential, natatorium, and Mark S. Owen
industrial equipment. Editor
CONTENTS
Contributors
ASHRAE Technical Committees, Task Groups, and Technical Resource Groups
ASHRAE Research: Improving the Quality of Life
Preface
AIR-CONDITIONING AND HEATING SYSTEMS
Chapter 1. HVAC System Analysis and Selection (TC 9.1, Large Building Air-Conditioning Systems)
2. Decentralized Cooling and Heating (TC 9.1)
3. Central Cooling and Heating (TC 9.1)
4. Air Handling and Distribution (TC 9.1)
5. In-Room Terminal Systems (TC 9.1)
6. Panel Heating and Cooling (TC 6.5, Radiant Heating and Cooling)
7. Combined Heat and Power Systems (TC 1.10, Cogeneration Systems)
8. Combustion Turbine Inlet Cooling (TC 1.10)
9. Applied Heat Pump and Heat Recovery Systems
(TC 6.8, Geothermal Heat Pump and Energy Recovery Applications)
10. Small Forced-Air Heating and Cooling Systems
(TC 6.3, Central Forced Air Heating and Cooling Systems)
11. Steam Systems (TC 6.1, Hydronic and Steam Equipment and Systems)
12. District Heating and Cooling (TC 6.2, District Energy)
13. Hydronic Heating and Cooling (TC 6.1)
14. Condenser Water Systems (TC 6.1)
15. Medium- and High-Temperature Water Heating (TC 6.1)
16. Infrared Radiant Heating (TC 6.5)
17. Ultraviolet Lamp Systems (TC 2.9, Ultraviolet Air and Surface Treatment)
18. Variable-Refrigerant-Flow Systems [TC 8.7, Variable Refrigerant Flow (VRF)]
AIR-HANDLING EQUIPMENT AND COMPONENTS
Chapter 19. Duct Construction (TC 5.2, Duct Design)
20. Room Air Distribution Equipment (TC 5.3, Room Air Distribution)
21. Fans (TC 5.1, Fans)
22. Humidifiers (TC 5.11, Humidifying Equipment)
23. Air-Cooling and Dehumidifying Coils (TC 8.4, Air-to-Refrigerant Heat Transfer Equipment)
24. Desiccant Dehumidification and Pressure-Drying Equipment
(TC 8.12, Desiccant Dehumidification Equipment and Components)
25. Mechanical Dehumidifiers and Related Components
(TC 8.10, Mechanical Dehumidification Equipment and Heat Pipes)
26. Air-to-Air Energy Recovery Equipment (TC 5.5, Air-to-Air Energy Recovery)
27. Air-Heating Coils (TC 8.4)
28. Unit Ventilators, Unit Heaters, and Makeup Air Units
(TC 6.1 and TC 5.8, Industrial Ventilation)
29. Air Cleaners for Particulate Contaminants (TC 2.4, Particulate Air Contaminants and
Particulate Contaminant Removal Equipment)
30. Industrial Gas Cleaning and Air Pollution Control Equipment
[TC 5.4, Industrial Process Air Cleaning (Air Pollution Control)]
HEATING EQUIPMENT AND COMPONENTS
Chapter 31. Automatic Fuel-Burning Systems (TC 6.10, Fuels and Combustion)
32. Boilers (TC 6.1)
33. Furnaces (TC 6.3)
34. Residential In-Space Heating Equipment (TC 6.5)
35. Chimney, Vent, and Fireplace Systems (TC 6.10)
36. Hydronic Heat-Distributing Units and Radiators (TC 6.1)
37. Solar Energy Equipment (TC 6.7, Solar Energy Utilization)
COOLING EQUIPMENT AND COMPONENTS
Chapter 38. Compressors (TC 8.1, Positive Displacement Compressors, and TC 8.2, Centrifugal Machines)
39. Condensers (TC 8.4, TC 8.5, Liquid-to-Refrigerant Heat Exchangers, and
TC 8.6, Cooling Towers and Evaporative Condensers)
40. Cooling Towers (TC 8.6)
41. Evaporative Air-Cooling Equipment (TC 5.7, Evaporative Cooling)
42. Liquid Coolers (TC 8.5)
43. Liquid-Chilling Systems (TC 8.1 and TC 8.2)
GENERAL COMPONENTS
44. Centrifugal Pumps (TC 6.1)
45. Motors, Motor Controls, and Variable-Speed Drives
(TC 1.11, Electric Motors and Motor Control)
46. Pipes, Tubes, and Fittings (TC 6.1)
47. Valves (TC 6.1)
48. Heat Exchangers (TC 6.1)
PACKAGED, UNITARY, AND SPLIT-SYSTEM EQUIPMENT
Chapter 49. Unitary Air Conditioners and Heat Pumps (TC 8.11, Unitary and Room Air Conditioners
and Heat Pumps)
50. Room Air Conditioners and Packaged Terminal Air Conditioners (TC 8.11)
GENERAL
51. Thermal Storage (TC 6.9, Thermal Storage)
52. Codes and Standards
Additions and Corrections
Index
Composite index to the 2009 Fundamentals, 2010 Refrigeration, 2011 HVAC Applications, and
2012 HVAC Systems and Equipment volumes
Comment Pages
CHAPTER 1
HVAC SYSTEM ANALYSIS AND SELECTION
Selecting a System...................................................................... 1.1 Security....................................................................................... 1.8
HVAC Systems and Equipment.................................................. 1.4 Automatic Controls and
Space Requirements................................................................... 1.6 Building Management System................................................ 1.9
Air Distribution.......................................................................... 1.7 Maintenance Management System............................................. 1.9
Pipe Distribution........................................................................ 1.8 Building System Commissioning................................................ 1.9
AN HVAC system maintains desired environmental conditions
in a space. In almost every application, many options are
available to the design engineer to satisfy a client’s building pro-
gram and design intent. In the analysis, selection, and implementa-
tion of these options, the design engineer should consider the
criteria defined here, as well as project-specific parameters to
achieve the functional requirements associated with the project
design intent. In addition to the design, equipment, and system
aspects of the proposed design, the design engineer should consider
sustainability as it pertains to responsible energy and environmental
design, as well as constructability of the design.
The integrated design process (IPD) includes members of the
entire project team (e.g., owner, architect, construction team) in the
decision process. In this American Institute of Architects (AIA)-
supported process, all team members take part in the overall build-
ing design process and, in most situations, share in project profits
and risks. For more information, refer to the AIA’s Center for
Integrated Practice (CIP) at http://network.aia.org/AIA/Centerfor
IntegratedPractice/Home/, or see Chapter 58 of the 2011 ASHRAE
Handbook—HVAC Applications.
HVAC systems are categorized by the method used to produce,
deliver, and control heating, ventilating, and air conditioning in the
conditioned area. This chapter addresses procedures for selecting an
appropriate system for a given application while taking into account
pertinent issues associated with designing, building, commission-
ing, operating, and maintaining the system. It also describes and
defines the design concepts and characteristics of basic HVAC sys-
tems. Chapters 2 to 5 describe specific systems and their attributes,
based on their heating and cooling medium, the commonly used
variations, constructability, commissioning, operation, and mainte-
nance.
This chapter is intended as a guide for the design engineer,
builder, facility manager, and student needing to know or reference
the analysis and selection process that leads to recommending the
optimum system for the job. The approach applies to HVAC con-
version, building system upgrades, system retrofits, building reno-
vations and expansion, and new construction for any building:
small, medium, large, below grade, at grade, low-rise, and high-
rise. This system analysis and selection process (Figure 1) helps
determine the optimum system(s) for any building, regardless of
facility type. Analysis examines objective, subjective, short-term,
and long-term goals.
SELECTING A SYSTEM
The design engineer is responsible for considering various sys-
tems and equipment and recommending one or more system options
that will meet the project goals and perform as desired. It is imper-
ative that the design engineer and owner collaborate to identify and
The preparation of this chapter is assigned to TC 9.1, Large Building Air- Fig. 1 Process Flow Diagram
Conditioning Systems. (Courtesy RDK Engineers)
1.1
1.2 2012 ASHRAE Handbook—HVAC Systems and Equipment (SI)
prioritize criteria associated with the design goal. In addition, if the • Code requirements
project includes preconstruction services, the designer and operator • Available capacity
should consult with the construction manager to take advantage of • Available space
a constructability analysis as well as the consideration of value- • Available utility source
engineered options. Occupant comfort (as defined by ASHRAE • Available infrastructure
Standard 55), process heating, space heating, cooling, and ventila- • Building architecture
tion criteria should be considered and should include the following: • System efficiency versus energy budget
• Temperature The design engineer should closely coordinate the system con-
• Humidity straints with the rest of the design team, as well as the owner, to
• Air motion overcome design obstacles associated with the HVAC systems
• Air purity or quality under consideration for the project.
• Air changes per hour
• Air and/or water velocity requirements Constructability Constraints
• Local climate The design engineer should take into account HVAC system con-
• Space pressure requirements tructability issues before the project reaches the construction docu-
• Capacity requirements, from a load calculation analysis ment phase. Some of these constraints may significantly affect the
• Redundancy success of the design and cannot be overlooked in the design phase.
• Spatial requirements Some issues and concerns associated with constructability are
• Security concerns
• Existing conditions
• First cost
• Maintaining existing building occupancy and operation
• Operating cost, including energy and power costs
• Construction budget
• Maintenance cost
• Construction schedule
• Reliability
• Ability to phase HVAC system installation
• Flexibility
• Equipment availability (i.e., delivery lead times)
• Controllability
• Equipment ingress into designated space
• Life-cycle analysis
• Equipment maintainability
• Sustainability of design
• Acoustics and vibration Few projects allow detailed quantitative evaluation of all alterna-
• Mold and mildew prevention tives. Common sense, historical data, and subjective experience can
be used to narrow choices to one or two potential systems.
Because these factors are interrelated, the owner, design engi-
Heating and air-conditioning loads often contribute to con-
neer, and operator must consider how these criteria affect each
straints, narrowing the choice to systems that fit in available space
other. The relative importance of factors such as these varies with
and are compatible with building architecture. Chapters 17 and 18
different owners, and often changes from one project to another for
of the 2009 ASHRAE Handbook—Fundamentals describe meth-
the same owner. For example, typical owner concerns include first
ods to determine the size and characteristics of heating and air-
cost compared to operating cost, extent and frequency of mainte-
conditioning loads. By establishing the capacity requirement,
nance and whether that maintenance requires entering the occupied
equipment size can be determined, and the choice may be narrowed
space, expected frequency of system failure, effect of failure, and
to those systems that work well on projects within the required size
time required to correct the failure. Each concern has a different pri-
range.
ority, depending on the owner’s goals.
Loads vary over time based on occupied and unoccupied periods,
Additional Goals and changes in weather, type of occupancy, activities, internal loads,
and solar exposure. Each space with a different use and/or exposure
In addition to the primary goal of providing the desired environ-
may require its own control zone to maintain space comfort. Some
ment, the design engineer should be aware of and account for other
areas with special requirements (e.g., ventilation requirements) may
goals the owner may require. These goals may include the following:
need individual systems. The extent of zoning, degree of control
• Supporting a process, such as operation of computer equipment required in each zone, and space required for individual zones also
• Promoting a germ-free environment narrow system choices.
• Increasing marketability of rental spaces No matter how efficiently a particular system operates or how
• Increasing net rental income economical it is to install, it can only be considered if it (1) main-
• Increasing property salability tains the desired building space environment within an acceptable
• Public image of the property tolerance under expected conditions and occupant activities and (2)
physically fits into, on, or adjacent to the building without causing
The owner can only make appropriate value judgments if the
objectionable occupancy conditions.
design engineer provides complete information on the advantages
Cooling and humidity control are often the basis of sizing
and disadvantages of each option. Just as the owner does not usually
HVAC components and subsystems, but ventilation requirements
know the relative advantages and disadvantages of different HVAC
may also significantly impact system sizing. For example, if large
systems, the design engineer rarely knows all the owner’s financial
quantities of outdoor air are required for ventilation or to replace air
and functional goals. Hence, the owner must be involved in system
exhausted from the building, the design engineer may only need to
selection in the conceptual phase of the job. The same can be said for
consider systems that transport and effectively condition those large
operator participation so that the final design is sustainable.
outdoor air volumes.
System Constraints Effective heat delivery to an area may be equally important in
selection. A distribution system that offers high efficiency and com-
Once the goal criteria and additional goal options are listed, fort for cooling may be a poor choice for heating. The cooling,
many system constraints must be determined and documented.
humidity, and/or heat delivery performance compromises may be
These constraints may include the following:
small for one application in one climate, but may be unacceptable in
• Performance limitations (e.g., temperature, humidity, space pressure) another that has more stringent requirements.
HVAC System Analysis and Selection 1.3
HVAC systems and associated distribution systems often occupy • Chapter 10 of the 2009 ASHRAE Handbook—Fundamentals cov-
a significant amount of space. Major components may also require ers physiological principles, comfort, and health.
special support from the structure. The size and appearance of ter- • Chapter 19 of the 2009 ASHRAE Handbook—Fundamentals cov-
minal devices (e.g., grilles, registers, diffusers, fan-coil units, radi- ers methods for estimating annual energy costs.
ant panels, chilled beams) affect architectural design because they • Chapter 36 of the 2011 ASHRAE Handbook—HVAC Applications
are visible in the occupied space. covers methods for energy management.
Construction budget constraints can also influence the choice • Chapter 37 of the 2011 ASHRAE Handbook—HVAC Applications
of HVAC systems. Based on historical data, some systems may not covers owning and operating costs.
be economically viable within the budget limitations of an owner’s • Chapter 39 of the 2011 ASHRAE Handbook—HVAC Applications
building program. In addition, annual maintenance and operating covers mechanical maintenance.
budget (utilities, labor, and materials) should be an integral part of • Chapter 48 of the 2011 ASHRAE Handbook—HVAC Applications
any system analysis and selection process. This is particularly impor- covers noise and vibration control.
tant for building owners who will retain the building for a substantial
Other documents and guidelines that should be consulted are
number of years. Value-engineered solutions can offer (1) cost-
ASHRAE standards; local, state, and federal guidelines; and special
driven performance, which may provide a better solution for lower
agency requirements [e.g., U.S. General Services Administration
first cost; (2) a more sustainable solution over the life of the equip-
(GSA), Food and Drug Administration (FDA), Joint Commission
ment; or (3) best value based on a reasonable return on investment.
on Accreditation of Healthcare Organizations (JCAHO), Facility
Sustainable energy consumption can be compromised and
Guidelines Institute (FGI), Leadership in Energy and Environmen-
long-term project success can be lost if building operators are not
tal Design (LEED™)].
trained to efficiently and effectively operate and maintain the build-
ing systems. For projects in which the design engineer used some
Selection Report
form of energy software simulation, the resultant data should be
As the last step, the design engineer should prepare a summary
passed on to the building owner so that goals and expectations can be
report that addresses the following:
measured and benchmarked against actual system performance.
Even though the HVAC designer’s work may be complete after sys- • The originally established goals
tem commissioning and turnover to the owner, continuous accept- • Criteria for selection
able performance is expected. Refer to ASHRAE Guideline 0 and to
• Important factors, including advantages and disadvantages
ASHRAE’s Building Energy Quotient (bEQ) program (http://www.
• System integration with other building systems
buildingeq.com/).
• Other goals
System operability should be a consideration in the system
• Security concerns
selection. Constructing a highly sophisticated, complex HVAC sys-
• Basis of design
tem in a building where maintenance personnel lack the required
• HVAC system analysis and selection matrix
skills can be a recipe for disaster at worst, and at best requires the
• System narratives
use of costly outside maintenance contractors to achieve successful
• Budget costs
system operation.
• Final recommendation(s)
Narrowing the Choices
A brief outline of each of the final selections should be provided.
The following chapters in this volume present information to In addition, HVAC systems deemed inappropriate should be noted
help the design engineer narrow the choices of HVAC systems: as having been considered but not found applicable to meet the
owner’s primary HVAC goal.
• Chapter 2 focuses on a distributed approach to HVAC.
The report should include an HVAC system selection matrix that
• Chapter 3 provides guidance for large equipment centrally located
identifies the one or two suggested HVAC system selections (pri-
in or adjacent to a building.
mary and secondary, when applicable), system constraints, and
• Chapter 4 addresses all-air systems.
other constraints and considerations. In completing this matrix
• Chapter 5 covers building piping distribution, including in-room
assessment, the design engineer should have, and identify in the
terminal systems.
report, the owner’s input to the analysis. This input can also be
Each chapter summarizes positive and negative features of vari- applied as weighted multipliers, because not all criteria carry the
ous systems. Comparing the criteria, other factors and constraints, same weighted value.
and their relative importance usually identifies one or two systems Many grading methods are available to complete an analytical
that best satisfy project goals. In making choices, notes should be matrix analysis. Probably the simplest is to rate each item excellent,
kept on all systems considered and the reasons for eliminating those very good, good, fair, or poor. A numerical rating system such as 0
that are unacceptable. to 10, with 10 equal to excellent and 0 equal to poor or not applica-
Each selection may require combining a primary system with a ble, can provide a quantitative result. The HVAC system with the
secondary (or distribution) system. The primary system converts highest numerical value then becomes the recommended HVAC
energy derived from fuel or electricity to produce a heating and/or system to accomplish the goal.
cooling medium. The secondary system delivers heating, ventila- The system selection report should include a summary followed
tion, and/or cooling to the occupied space. The systems are inde- by a more detailed account of the HVAC system analysis and system
pendent to a great extent, so several secondary systems may work selection. This summary should highlight key points and findings that
with a particular primary system. In some cases, however, only one led to the recommendation(s). The analysis should refer to the system
secondary system may be suitable for a particular primary system. selection matrix (such as in Table 1) and the reasons for scoring.
Once subjective analysis has identified one or more HVAC sys- With each HVAC system considered, the design engineer should
tems (sometimes only one choice remains), detailed quantitative note the criteria associated with each selection. Issues such as
evaluations must be made. All systems considered should provide close-tolerance temperature and humidity control may eliminate
satisfactory performance to meet the owner’s essential goals. The some HVAC systems from consideration. System constraints,
design engineer should provide the owner with specific data on each noted with each analysis, should continue to eliminate potential
system to make an informed choice. Consult the following chapters HVAC systems. Advantages and disadvantages of each system
to help narrow the choices: should be noted with the scoring from the HVAC system selection
1.4 2012 ASHRAE Handbook—HVAC Systems and Equipment (SI)
Table 1 Sample HVAC System Analysis and Selection Matrix (0 to 10 Score)
Goal: Furnish and install an HVAC system that provides moderate space temperature
control with minimum humidity control at an operating budget of 220 kW/m2 per year
Categories System #1 System #2 System #3 Remarks
1. Criteria for Selection:
• 25.6°C space temperature with ±1.7 K control during occupied cycle, with
40%rh and ±5% rh control during cooling.
• 20°C space temperature with ±1 K, with 20% rh and ±5% rh control during
heating season.
• First cost
• Equipment life cycle
2. Important Factors:
• First-class office space stature
• Individual tenant utility metering
3. Other Goals:
• Engineered smoke control system
• ASHRAE Standard 62.1 ventilation rates
• Direct digital control building automation
4. System Constraints:
• No equipment on first floor
• No equipment on ground adjacent to building
5. Energy use as predicted by use of an industry-acceptable computerized energy
model
6. Other Constraints:
• No perimeter finned-tube radiation or other type of in-room equipment
TOTAL SCORE
matrix. This process should reduce HVAC selection to one or two Spatial Requirements. A decentralized system may or may not
optimum choices for presentation to the owner. Examples of simi- require equipment rooms. Because of space restrictions imposed on
lar installations for other owners should be included with this the design engineer or architect, equipment may be located on the
report to support the final recommendation. Identifying a third roof and/or the ground adjacent to the building. Depending on sys-
party for an endorsement allows the owner to inquire about the suc- tem components, additional space may be required in the building
cess of other HVAC installations. for chillers and boilers. Likewise, a decentralized system may or
may not require duct and pipe shafts throughout the building.
HVAC SYSTEMS AND EQUIPMENT First Cost. A decentralized system probably has the best first-
cost benefit. This feature can be enhanced by phasing in the pur-
Many built, expanded, and/or renovated buildings may be ideally
chase of decentralized equipment as needed (i.e., buying equipment
suited for decentralized HVAC systems, with equipment located in,
as the building is being leased/occupied).
throughout, adjacent to, or on top of the building. The alternative to
Operating Cost. A decentralized system can save operating cost
this decentralized approach is to use primary equipment located in
by strategically starting and stopping multiple pieces of equipment.
a central plant (either inside or outside the building) with water and/
When comparing energy consumption based on peak energy draw,
or air required for HVAC needs distributed from this plant.
decentralized equipment may not be as attractive as larger, more
energy-efficient centralized equipment.
Decentralized System Characteristics
Maintenance Cost. A decentralized system can save mainte-
The various types of decentralized systems are described in nance cost when equipment is conveniently located and equipment
Chapter 2. The common element is that the required cooling is dis- size and associated components (e.g., filters) are standardized.
tributed throughout the building, with direct-expansion cooling of When equipment is located outdoors, maintenance may be difficult
air systems. during bad weather.
Temperature, Humidity, and Space Pressure Requirements. Reliability. A decentralized system usually has reliable equip-
A decentralized system may be able to fulfill any or all of these ment, although the estimated equipment service life may be less
design parameters, but typically not as efficiently or as accurately as than that of centralized equipment. Decentralized system equipment
a central system. may, however, require maintenance in the occupied space.
Capacity Requirements. A decentralized system usually re- Flexibility. A decentralized system may be very flexible because
quires each piece of equipment to be sized for zone peak capacity, it may be placed in numerous locations.
unless the systems are variable-volume. Depending on equipment Level of Control. Decentralized systems often use direct
type and location, decentralized systems do not benefit as much refrigerant expansion (DX) for cooling, and on/off or staged heat.
from equipment sizing diversity as centralized systems do. This step control results in greater variation in space temperature
Redundancy. A decentralized system may not have the benefit and humidity, where close control is not desired or necessary. As
of back-up or standby equipment. This limitation may need review. a caution, oversizing DX or stepped cooling can allow high indoor
Facility Management. A decentralized system can allow the humidity levels and mold or mildew problems.
building manager to maximize performance using good business/ Noise and Vibration. Decentralized systems often locate noisy
facility management techniques in operating and maintaining the machinery close to building occupants, although equipment noise
HVAC equipment and systems. may be less than that produced by large central systems.
HVAC System Analysis and Selection 1.5
Constructability. Decentralized systems frequently consist of Sound and Vibration. Centralized systems often locate noisy
multiple and similar-in-size equipment that makes standardiza- machinery sufficiently remote from building occupants or noise-
tion a construction feature, as well as purchasing units in large sensitive processes.
quantities. Constructability. Centralized systems usually require more coor-
dinated installation than decentralized systems. However, consolida-
Centralized System Characteristics
tion of the primary equipment in a central location also has benefits.
These systems are characterized by central refrigeration systems
Among the largest centralized systems are HVAC plants serving
and chilled-water distribution. This distribution can be to one or
groups of large buildings. These plants improve diversity and gen-
more major fan rooms, depending on building size, or to floor-by-
erally operate more efficiently, with lower maintenance costs, than
floor chilled-water air-handling units throughout the building.
individual central plants. Economic considerations of larger cen-
Details of these systems are covered in Chapter 3.
tralized systems require extensive analysis. The utility analysis
Temperature, Humidity, and Space Pressure Requirements.
may consider multiple fuels and may also include gas and steam
A central system may be able to fulfill any or all of these design
turbine-driven equipment. Multiple types of primary equipment
parameters, and typically with greater precision and efficiency than
using multiple fuels and types of HVAC-generating equipment
a decentralized system.
(e.g., centrifugal and absorption chillers) may be combined in one
Capacity Requirements. A central system usually allows the plant. Chapters 13 to 15 provide design details for central plants.
design engineer to consider HVAC diversity factors that reduce
installed equipment capacity. As a result, this offers some attractive Primary Equipment
first-cost and operating-cost benefits.
Redundancy. A central system can accommodate standby The type of decentralized and centralized equipment selected for
equipment that decentralized configurations may have trouble buildings depends on a well-organized HVAC analysis and selection
accommodating. report. The choice of primary equipment and components depends
on factors presented in the selection report (see the section on
Facility Management. A central system usually allows the
Selecting a System). Primary HVAC equipment includes refrigera-
building manager to maximize performance using good business/
tion equipment; heating equipment; and air, water, and steam deliv-
facility management techniques in operating and maintaining the
ery equipment.
HVAC equipment and systems.
Spatial Requirements. The equipment room for a central sys- Many HVAC designs recover internal heat from lights, people,
tem is normally located outside the conditioned area: in a basement, and equipment to reduce the size of the heating plant. In buildings
penthouse, service area, or adjacent to or remote from the building. with core areas that require cooling while perimeter areas require
A disadvantage of this approach may be the additional cost to fur- heating, one of several heat reclaim systems can heat the perimeter to
nish and install secondary equipment for the air and/or water dis- save energy. Sustainable design is also important when considering
tribution. Other considerations are the access requirements and recovery and reuse of materials and energy. Chapter 9 describes heat
physical constraints that exist throughout the building to the instal- pumps and some heat recovery arrangements, Chapter 37 describes
lation of the secondary distribution network of ducts and/or pipes solar energy equipment, and Chapter 26 introduces air-to-air energy
and for equipment replacement. recovery. In the 2011 ASHRAE Handbook—HVAC Applications,
First Cost. Even with HVAC diversity, a central system may not Chapter 36 covers energy management and Chapter 41 covers build-
be less costly than decentralized HVAC systems. Historically, cen- ing energy monitoring. Chapter 35 of the 2009 ASHRAE Hand-
tral system equipment has a longer equipment service life to com- book—Fundamentals provides information on sustainable design.
pensate for this shortcoming. Thus, a life-cycle cost analysis is very The search for energy savings has extended to cogeneration or
important when evaluating central versus decentralized systems. total energy [combined heat and power (CHP)] systems, in which
Operating Cost. A central system usually has the advantage of on-site power generation is added to the HVAC project. The eco-
larger, more energy-efficient primary equipment compared to nomic viability of this function is determined by the difference
decentralized system equipment. In addition, the availability of between gas and electric rates and by the ratio of electricity to heating
multiple pieces of HVAC equipment allows staging of this equip- demands for the project. In these systems, waste heat from generators
ment operation to match building loads while maximizing opera- can be transferred to the HVAC systems (e.g., to drive turbines of cen-
tional efficiency. trifugal compressors, serve an absorption chiller, provide heating or
Maintenance Cost. The equipment room for a central system process steam). Chapter 7 covers cogeneration or total energy sys-
provides the benefit of being able to maintain HVAC equipment tems. Alternative fuel sources, such as waste heat boilers, are now
away from occupants in an appropriate service work environment. being included in fuel evaluation and selection for HVAC applica-
Access to occupant workspace is not required, thus eliminating dis- tions.
ruption to the space environment, product, or process. Because of Thermal storage is another cost-saving concept, which provides
the typically larger capacity of central equipment, there are usually the possibility of off-peak generation of chilled water or ice. Ther-
fewer pieces of HVAC equipment to service. mal storage can also be used for storing hot water for heating. Many
Reliability. Centralized system equipment generally has a lon- electric utilities impose severe charges for peak summer power use
ger service life. or offer incentives for off-peak use. Storage capacity installed to
Flexibility. Flexibility can be a benefit when selecting equip- level the summer load may also be available for use in winter, thus
ment that provides an alternative or back-up source of HVAC. making heat reclaim a viable option. Chapter 51 has more informa-
tion on thermal storage.
Air Distribution Systems
With ice storage, colder supply air can be provided than that
The various air distribution systems, including dedicated outdoor available from a conventional chilled-water system. This colder air
air systems (DOAS), are detailed in Chapter 4. Any of the preceding allows use of smaller fans and ducts, which reduces first cost and (in
system types discussed can be used in conjunction with DOAS. some locations) operating cost. Additional pipe and duct insulation
Level of Control. Centralized systems generally use chilled is often required, however, contributing to a higher first cost. These
water for cooling, and steam or hydronic heat. This usually allows life-cycle savings can offset the first cost for storage provisions and
for close control of space temperature and humidity where desired the energy cost required to make ice. Similarly, thermal storage of
or necessary. hot water can be used for heating.