1、UnderstandingPsychrometricsThird EditionAbout the AuthorDonald P. Gatley, PE, Fellow ASHRAE, is a 1954 mechanical engineering graduate ofVanderbilt University. His career includes 13 years with the Trane Company, 8 years indesign/build mechanical contracting, and 30+ years in the practice of consult
2、ing engineer-ing. Outside activities include service as chair of Georgias building energy code committeeandaspresidentoftheSouthfaceEnergyInstitute.Heisknownforhisworkinthepreventionand correction of building moisture problems, energy conservation in buildings, and withhotels. He is a recipient of t
3、he ASHRAE Distinguished Service Award and two ASHRAEInternational Energy Awards.Gatley also authored the EPRI Cool Storage Ethylene Glycol Design Guide (1992), theEPRI Cool Storage Open Hydronic Systems Design Guide (1995) and two chapters of theMold DaveKnebel,P.E.,forintroducingmetoSandyKleinsEESS
4、oftware,whicheliminatedthedrudgeryofcalculatingpsychromet-ric properties; Doug Reindl, Ph.D., P.E., for demonstrating the calculusbehind the NACA pressure vs. elevation equation; Rex Raiza, DallasHVAC designer and software author, for sharing his knowledge on part-load psychrometrics; and Jason Lero
5、y, Roy Crawford, Dick Cawley, EdKeuper, Lew Harriman, and Manuel Conde-Petit for their constructivereviews.Thanks to Micki Geshwiler for her suggestions and attempts toimprove my grammar. Thanks to Matt Walker, Cindy Michaels, ErinHoward, and all of the ASHRAE Special Publications and graphics staff
6、for dealingwith theidiosyncrasies ofconvertingtheword-processeddoc-ument to a publishing format.Thanks also to Fred Nelson of the University of Missouri-Rolla forproviding high-resolution digital files for 16 new psychrometric charts.Understanding Psychrometrics, Third EditionxMy wife, Jane, deserve
7、s the most thanks for putting up with endlessbreakfast and lunch discussions about fan temperature rise, dew-pointtemperature,saturation, andotherequally thrillingtopics, andforher rolein grammar checking.For any factual or grammatical errors that have crept into the textdespite the efforts of those
8、 mentioned above, I am solely responsible.The cover illustration for this edition is used with permission of theartist, Tim Padfield, Ph.D., who is an expert in the preservation of art andthe microclimates of museums and historic buildings, the author of Phys-ics in Conservation, and an artist of bu
9、ilding-science-related paintingsthat include both message and humor.After career assignments atVicto-ria and Albert Museum, University of Leeds, Smithsonian Institution,NationalMuseumofDenmark,andtheTechnicalUniversityofDenmark,he is now a freelance consultant in preventive conservation living in Ha
10、r-bertonford,Totnes,UK.Visithiswebsiteformoreofhisartworkandlinksto pages on conservation physics.The illustration portrays Garry Thomson, author of the influentialbook The Museum Environment, as a figure in a Romanesque wall paint-ing.Hewieldsaslingpsychrometerandalightmeterwhilekeepingafootdelicat
11、ely poised on the head of the air-conditioning engineer in the base-ment. He repels the agents of decayweather, radiation, and saltbyintoning the recommended protective museum environment: 50% RH,20C, and 100 lux.xiPsychrometricsthe physics of moist air:an applied science dealing with the properties
12、 and processes of moist air.PrefacePsychrometricsisasubscienceofphysicsdealingwiththepropertiesand processes of moist air. Moist air is defined as a mixture of two gases:dry air and water vapour (the gas phase of H2O). Dry air within the tro-posphere is treated as a non-varying mixture of nitrogen (
13、78.1%), oxygen(20.9%), argon (0.9%), and other trace gases, including carbon dioxide(0.04%). Some broaden the definition of psychrometrics to cover mix-tures of the gas of one substance (any dry gas component) and the con-densable vapour of a second substance.The clock in the margin of this paragrap
14、h and elsewhere in this bookindicates text that should be read by a student or new user who wants toacquiremostofthebasicsofpsychrometricsinfourtosixhoursofreading.Psychrometricsisabasicsciencethatunderliesagriculturalandaero-nautical engineering; air conditioning; drying of crops, grains, and phar-
15、maceuticals; dehydration; dehumidification; humidification; moisturecontrol; meteorology; weather reporting; food science engineering; andpiloting of aircraft. It is possible to work in these fields without a goodunderstanding of fundamental-level psychrometrics by the use of short-cut formulae, tab
16、les, and charts.While it is true that designers can survivein these fields with minimal and incomplete psychrometric skills withoutcreating many problems for themselves, their employers, and clients,there is no justification for this lack of knowledge when psychrometricscan be quickly learned.Many r
17、eaders are frustrated in their pursuit of fundamental psychro-metric knowledge because existing texts are incomplete, overly compli-cated,notwellorganizedintolearningelements,orcontainobsoletetermsand calculations that have little relevance in todays world of fast personalcomputers, psychrometric so
18、ftware, and computer-generated psychro-metric charts tailored to a site-specific barometric pressure or altitude.Understanding Psychrometrics, Third EditionxiiPsychrometrics can be simply explained and is solidly based on: (1)the ideal gas equation, (2) Daltons model of partial pressures, (3) con-se
19、rvation of energy, and (4) conservation of mass. In the authors opinion,there is every reason to pursue a solid foundation in psychrometrics whenit takes less than four hours to acquire this background for a lifetime ofuse.Suchknowledgemaynotberequiredintypicaldesigns,butitcanpre-vent costly mistake
20、s when altitude or barometric pressure differ fromstandard sea level values, when subfreezing temperatures exist, or whenthespecificvolumeofthemoistairdifferssubstantiallyfromthatofStan-dard Temperature and Pressure (STP) air (arbitrarily fixed at 15C and101,325 Pa).What is different about this psyc
21、hrometric text? It is devoted solelyto psychrometrics. Psychrometrics is usually covered in one or two chap-ters of a thermodynamics textbook. This text is written for ease of under-standingandnotwithbrevityinmind,butatthesametimetheinformationis presented in such a way that the reader can choose no
22、t only the chaptersto explore but also the level of detail within each chapter. It also includesan extensive listing of the pioneers of psychrometrics.The underlying reasons for this new psychrometric publication are:(1) to address the changes brought on by the shift from manual calcula-tions and pl
23、otting on printed psychrometric charts to software-based psy-chrometriccalculations andgraphics, (2)topresent inone texta thoroughcoverage of psychrometric fundamentals, and (3) to assist designers andpractitioners in the transition to the Systme International system of unitsand calculations. Theref
24、ore, the objectives of this publication are to:Provide a resource equivalent to a reference book as well as abasic refresher course for those who use psychrometrics on arecurring basis.Provide students and air-conditioning designers, meteorologists,process engineers, and other users with a four-hour
25、 complete psy-chrometrics learning module.Put to rest the impression that psychrometric calculations based onideal gas formulations (at normal air-conditioning temperaturesandpressures)areimprecise,inaccurate,orbasedlargelyonempir-ical formulae. The text includes comparative data for ideal gas cal-c
26、ulations and the ultimate-in-accuracy real moist air formulationsby Herrmann-Kretzschmar-Gatley (ASHRAE RP-1485). In con-trast to statements in some texts, ideal-gas-based psychrometriccalculationsarefarmoreaccuratethanheatgain,ductpressureloss,and other calculations used by air-conditioning enginee
27、rs andmeteorologists.PrefacexiiiProvide practitioners and those new to the field with multiple def-initions of basic terms to help in their understanding and commu-nication.Provide those fluent in inch-pound or metric psychrometric calcu-lations with an easy transition to Systme International psychr
28、o-metrics.Provide all of the algorithms required for psychrometric calcula-tions; e.g., few texts provide the correlation between barometricpressure and altitude, nor do they provide formulas for wet-bulband dew-point temperatures below freezing. Algorithms can beinput into hand-held programmable ca
29、lculators and personal com-puters to aid engineers and meteorologists in their work.Motivate manufacturers of cooling coils and desiccant dehumidi-fierstoutilizeprovenalgorithmsintheirselectionandpsychromet-ric software to eliminate inconsistency.EndnoteEach reader will determine the degree to which
30、 this book meets theauthors objectives. Once the basic principles are understood, the psy-chrometric practitioner will undoubtedly find progressively easier andmore efficient ways to apply them in the solution of meteorological, air-conditioning, drying, dehumidification, humidification, and other p
31、rob-lems. For the air-conditioning and drying industries, this may lead to thedevelopmentofcompoundorhybridcyclesthataremoreefficientandlesscostly.You,thereader,willbetheultimatetestofmyfour-hourlearningcon-viction.Hopefully,thatconvictionwillprovetrueforthemajority.Forall,itishopedthatthisbookwillb
32、etheresourcethat(1)takesthemysteryoutof psychrometric calculations, (2) makes a convincing case for the accu-racy of calculations based on the ideal gas equation, (3) gives a far bettergrasp of dew-point, moisture, and psychrometric processes, and (4)allows practitioners to serve their clients and t
33、he public better.Please e-mail suggested corrections, comments, and criticism to theauthor at don.gatleymail.ashrae.org.Donald P. Gatley, P.E.January 2002 (1st edition)xvPreface to the Third EditionIn the late 1990s, when the first edition of this book was written, psy-chrometrics was a mature scien
34、ce, and I thought that little if anythingwould change over the following century. Since then, the universal gasconstant has been revised by CODATA, the molar mass of dry air hasincreased by 0.0001 kg/kmol every four to five years due to the increaseof CO2in Earths atmosphere, IAPWS has issued new mo
35、dels for the cal-culation of H2O properties, and in 2009 ASHRAE replaced the 1983Hyland-Wexler real-moist-air numerical model with the research projectRP-1485 LibHuAirProp model. These changes have little effect on air-conditioningandmeteorologicalpsychrometriccalculations,butstudentsand practitione
36、rs should have a reference that agrees with the latest inter-national standards.The third edition includes the above changes, a straightforward andmore elegant equation for the adiabatic saturation process in the wet-bulbtemperature chapter, an appendix summarizing the 2009 RP-1485ASHRAE research, a
37、s well as minor edits to the text.Understanding Psychrometrics, Third EditionxviSupporting FilesThispublicationisaccompaniedbyalimiteddemonstrationversionoftheASHRAELibHuAirPropadd-in.Alsoincludedarethehw.exeprogramfrom the second edition and PDF files of 13 ultra-high-pressure and 12existing ASHRAE
38、 psychrometric charts, plus 3 new 0C to 400C, 01.0humidity ratio charts for 5.53, 101.325, and 2000 kPa.TheLibHuAirPropadd-inallowsforduplicationofportionsoftherealmoist-airpsychrometrictablesinASHRAEHandbookFundamentalsforboth standard sea-level atmosphere pressure and pressures from 5 to10,000kPa.
39、Thehw.exeprogramisincludedtoenableuserstocomparethe2009 ASHRAE numerical model real moist-air psychrometric propertieswith the 1983 ASHRAE-Hyland-Wexler properties.These files can be downloaded at www.ashrae.org/UP3. If the filesor information at the link are not accessible, please contact the publi
40、sher.Introduction3Give a man a fish, provide food for a day.Teach a man to fish, provide food for a lifetime.Chinese proverb1 How to Use this BookvMIn general, each chapter is written as a stand-alone section so thatit is possible to pick and choose chapters and, as such, the text serves asa referen
41、ce for those with prior knowledge of psychrometrics.Thebookalsoservesasafour-tosix-hourintroductiontothescienceof psychrometrics for those new to the subject, and this influenced thesequencing of chapters. The clock in the margin of this paragraph andelsewhere in this book indicates the text that sh
42、ould be read in a four- tosix-hour learning session.Sincethisbookwaswrittenasbothalearningmoduleandareferencebook, many of the definitions, physical phenomena, and equations areexplained in more detail than found in a basic book. The reader shouldhaveageneralbackgroundknowledgeequivalenttoahighschoo
43、lphysicsor general science course. For those somewhat familiar with psychromet-rics in inch-pound (I-P) units, this text will not only provide an easy tran-sition to psychrometrics in Systme International (SI) units but also aquick refresher on the underlying principles of psychrometrics. Learningps
44、ychrometrics today is easier because of the SI coherent system of units,which eliminates duplicate units and conversions, and because of the per-sonal computer and software that has all but eliminated not only manualcomputations using approximate formulae but also graphical solutionsusing the psychr
45、ometric chart.Psychrometrics brings with it many new terms and concepts. Sometexts introduce all of the terms and concepts in a single chapter. For many,including the author, this can produce brain overload and confusion andis an obstacle to the learning process, which is, after all, the goal. In th
46、isbook, definitions and new concepts are introduced as needed and eachpsychrometric property is treated in a separate chapter. In general, equa-tions that interrelate the properties are introduced with the property.Understanding Psychrometrics, Third Edition4An exception to the as needed introductio
47、n of terms and concepts isChapter 5, “Basics of the Psychrometric Chart,” which precedes the indi-vidual chapters on psychrometric properties. The psychrometric chart isaconvenientandusefultoolfordeterminingpsychrometricpropertiesandvisualizing the changes to moist air properties in psychrometric pr
48、o-cesses.This early chapter is intended to give the reader an overview of thechart to help in overall understanding and to add clarity and meaning aseach psychrometric property is developed and discussed in subsequentchapters.The choice of whether or not to read Chapter 8, “Psychrometric Pio-neers a
49、nd Charts from the First 100 Years,” is left to the reader. It maypique the readers interest. It may help to explain the evolution of this sci-ence and the plethora of sometimes overlapping and confusing termsintroduced into psychrometrics from the fields of meteorology, drying ofmaterials, air conditioning, thermodynamics, chemical engineering, andagricultural engineering.Each of the chapters dealing with individual psychrometric proper-ties defines and discusses the property at several levels of detail. Thosenew to psychrometrics and those interested in a quic
