Transcription
UnderstandingPsychrometricsThird EditionDonald P. Gatley
ISBN 978-1-936504-31-2 2002, 2005, 2013 ASHRAE. All rights reserved.1791 Tullie Circle, NE · Atlanta, GA 30329 · www.ashrae.orgASHRAE is a registered trademark of the American Society of Heating,Refrigerating and Air-Conditioning Engineers, Inc.Printed in the United States of AmericaCover design by Tracy BeckerASHRAE has compiled this publication with care, but ASHRAE has not investigated, andASHRAE 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 dataor editorial material in this publication does not constitute endorsement, warranty, or guaranty by ASHRAE of any product, service, process, procedure, design, or the like. ASHRAEdoes not warrant that the information in the publication is free of errors, and ASHRAE doesnot necessarily agree with any statement or opinion in this publication. The entire risk of theuse of any information in this publication is assumed by the user.No part of this publication may be reproduced without permission in writing fromASHRAE, except by a reviewer who may quote brief passages or reproduce illustrations ina review with appropriate credit, nor may any part of this publication be reproduced, storedin a retrieval system, or transmitted in any way or by any means—electronic, photocopying,recording, or other—without permission in writing fromASHRAE. Requests for permissionshould be submitted at www.ashrae.org/permissions.Library of Congress Cataloging-in-Publication DataGatley, D. P.Understanding psychrometrics / Donald P. Gatley.—Third edition.pages cmIncludes bibliographical references and index.Summary: “Updates the second edition to provide readers a reference that agrees with the latestinternational standards. The third edition also includes a revised equation for the adiabatic saturationprocess, an summary of the 2009 RP-1485 ASHRAE research, as well as minor edits to the text”—Provided by publisher.ISBN 978-1-936504-31-2 (hardcover)1. Hygrometry. 2. Humidity. I. American Society of Heating, Refrigerating and Air-ConditioningEngineers. II. Title.QC915.G37 2012551.57'10287--dc232012042195ASHRAE STAFFSPECIAL PUBLICATIONSMark Owen, Editor/Group Manager of Handbook and Special PublicationsCindy Sheffield Michaels, Managing EditorMatt Walker, Associate EditorRoberta Hirschbuehler, Assistant EditorSarah Boyle, Editorial AssistantMichshell Phillips, Editorial CoordinatorPUBLISHING SERVICESDavid Soltis, Group Manager of Publishing Services and Electronic CommunicationsTracy Becker, Graphics SpecialistJayne Jackson, Publication Traffic AdministratorPUBLISHERW. Stephen Comstock
Psychrometrics—the physics of moist air:an applied science dealing with the properties and processes of moist air.PrefacePsychrometrics is a subscience of physics dealing with the propertiesand 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 troposphere is treated as a non-varying mixture of nitrogen (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 mixtures of the gas of one substance (any dry gas component) and the condensable vapour of a second substance.The clock in the margin of this paragraph and elsewhere in this bookindicates text that should be read by a student or new user who wants toacquire most of the basics of psychrometrics in four to six hours of reading.Psychrometrics is a basic science that underlies agricultural and aeronautical engineering; air conditioning; drying of crops, grains, and pharmaceuticals; 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 shortcut formulae, tables, 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 readers are frustrated in their pursuit of fundamental psychrometric knowledge because existing texts are incomplete, overly complicated, not well organized into learning elements, or contain obsolete termsand calculations that have little relevance in today's world of fast personalcomputers, psychrometric software, and computer-generated psychrometric charts tailored to a site-specific barometric pressure or altitude.xi
Understanding Psychrometrics, Third Edition Psychrometrics can be simply explained and is solidly based on: (1)the ideal gas equation, (2) Dalton’s model of partial pressures, (3) conservation of energy, and (4) conservation of mass. In the author’s 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. Such knowledge may not be required in typical designs, but it can prevent costly mistakes when altitude or barometric pressure differ fromstandard sea level values, when subfreezing temperatures exist, or whenthe specific volume of the moist air differs substantially from that of Standard Temperature and Pressure (STP) air (arbitrarily fixed at 15 C and101,325 Pa).What is different about this psychrometric text? It is devoted solelyto psychrometrics. Psychrometrics is usually covered in one or two chapters of a thermodynamics textbook. This text is written for ease of understanding and not with brevity in mind, but at the same time the informationis presented in such a way that the reader can choose not 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 calculations and plotting on printed psychrometric charts to software-based psychrometric calculations and graphics, (2) to present in one text a thoroughcoverage of psychrometric fundamentals, and (3) to assist designers andpractitioners in the transition to the Système International system of unitsand calculations. Therefore, the objectives of this publication are to: xii 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 complete psychrometrics learning module. Put to rest the impression that psychrometric calculations based onideal gas formulations (at normal air-conditioning temperaturesand pressures) are imprecise, inaccurate, or based largely on empirical formulae. The text includes comparative data for ideal gas calculations and the ultimate-in-accuracy real moist air formulationsby Herrmann-Kretzschmar-Gatley (ASHRAE RP-1485). In contrast to statements in some texts, ideal-gas-based psychrometriccalculations are far more accurate than heat gain, duct pressure loss,and other calculations used by air-conditioning engineers andmeteorologists.
Preface Provide practitioners and those new to the field with multiple definitions of basic terms to help in their understanding and communication.Provide those fluent in inch-pound or metric psychrometric calculations with an easy transition to Systéme International psychrometrics.Provide all of the algorithms required for psychrometric calculations; 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 calculators and personal computers to aid engineers and meteorologists in their work.Motivate manufacturers of cooling coils and desiccant dehumidifiers to utilize proven algorithms in their selection and psychrometric software to eliminate inconsistency.EndnoteEach reader will determine the degree to which this book meets theauthor’s objectives. Once the basic principles are understood, the psychrometric practitioner will undoubtedly find progressively easier andmore efficient ways to apply them in the solution of meteorological, airconditioning, drying, dehumidification, humidification, and other problems. For the air-conditioning and drying industries, this may lead to thedevelopment of compound or hybrid cycles that are more efficient and lesscostly.You, the reader, will be the ultimate test of my four-hour learning conviction. Hopefully, that conviction will prove true for the majority. For all,it is hoped that this book will be the resource that (1) takes the mystery outof psychrometric calculations, (2) makes a convincing case for the accuracy 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 the public better.Please e-mail suggested corrections, comments, and criticism to theauthor at [email protected] P. Gatley, P.E.January 2002 (1st edition)xiii
Preface to the Third EditionIn the late 1990s, when the first edition of this book was written, psychrometrics was a mature science, 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 CO2 in Earth’s atmosphere, IAPWS has issued new models for the calculation 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 airconditioning and meteorological psychrometric calculations, but studentsand practitioners should have a reference that agrees with the latest international 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, as well as minor edits to the text.xv
Understanding Psychrometrics, Third EditionSupporting FilesThis publication is accompanied by a limited demonstration version oftheASHRAE LibHuAirProp add-in.Also included are the hw.exe programfrom the second edition and PDF files of 13 ultra-high-pressure and 12existing ASHRAE psychrometric charts, plus 3 new 0 C to 400 C, 0–1.0humidity ratio charts for 5.53, 101.325, and 2000 kPa.The LibHuAirProp add-in allows for duplication of portions of the realmoist-air psychrometric tables in ASHRAE Handbook—Fundamentals forboth standard sea-level atmosphere pressure and pressures from 5 to10,000 kPa. The hw.exe program is included to enable users to compare the2009 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 publisher.xvi
ContentsAcknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ixPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xiPreface to the Third Edition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvIntroduction1—How to Use this Book . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32—Etymology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93—Moist Air—The Psychrometric Substance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .134—Water Vapour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .195—Basics of the Psychrometric Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .316—Underlying Theory and Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377—Ideal Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 438—Psychrometric Pioneers and Charts from the First 100 Years . . . . . . . . . . . . . . . . . 49Part I: Psychrometric Pioneers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49Part II: A Selection of Psychrometric Charts from the First 100 Years . . . . . . . . . 79Psychrometric Properties9—Psychrometric Properties and Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10510—Dry-Bulb Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10911—Wet-Bulb Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11312—Dew-Point Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12113—Relative Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .127vii
Understanding Psychrometrics, Third Edition14—Barometric Pressure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13315—Specific Enthalpy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13716—Specific Volume. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14317—Humidity Ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14718—Water Vapour Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .153Basic Psychrometric Processes19—Psychrometric Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15920—Process Calculations and Definitions of Sensible and Latent Enthalpy Change . . . .18321—Why Do Air-Conditioning Engineers Need Psychrometrics? . . . . . . . . . . . . . . . . . .197Processes in Meteorology22—Adiabatic Expansion and Adiabatic Compression. . . . . . . . . . . . . . . . . . . . . . . . 207Additional Information23—Fan Temperature Rise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21324—Frequently Asked Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22125—Altitude Effects on Psychrometrics and HVAC . . . . . . . . . . . . . . . . . . . . . . . . . . 23526—Psychrometric Program Listing and Comparison Table . . . . . . . . . . . . . . . . . . . . 245AppendicesI—ICAO Equation Relating Barometric Pressure and Altitude . . . . . . . . . . . . . . . . . . 255II—Water—Facts and Trivialities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .261III—Other Psychrometric Substances. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269IV—Charts, Tables, and Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .271V—Real Gas Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .371Conversion Equalities: I-P to SI Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .381viii
Introduction
Give a man a fish, provide food for a day.Teach a man to fish, provide food for a lifetime.—Chinese proverb1How 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 reference for those with prior knowledge of psychrometrics.The book also serves as a four- to six-hour introduction to the scienceof 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 should be read in a four- tosix-hour learning session. Since this book was written as both a learning module and a referencebook, many of the definitions, physical phenomena, and equations areexplained in more detail than found in a basic book. The reader shouldhave a general background knowledge equivalent to a high school physicsor general science course. For those somewhat familiar with psychrometrics in inch-pound (I-P) units, this text will not only provide an easy transition to psychrometrics in Système International (SI) units but also aquick refresher on the underlying principles of psychrometrics. Learningpsychrometrics today is easier because of the SI coherent system of units,which eliminates duplicate units and conversions, and because of the personal computer and software that has all but eliminated not only manualcomputations using approximate formulae but also graphical solutionsusing the psychrometric 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 thisbook, definitions and new concepts are introduced as needed and eachpsychrometric property is treated in a separate chapter. In general, equations that interrelate the properties are introduced with the property.3
Understanding Psychrometrics, Third Edition An exception to the as needed introduction of terms and concepts isChapter 5, “Basics of the Psychrometric Chart,” which precedes the individual chapters on psychrometric properties. The psychrometric chart isa convenient and useful tool for determining psychrometric properties andvisualizing the changes to moist air properties in psychrometric processes. 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 Pioneers and Charts from the First 100 Years,” is left to the reader. It maypique the reader’s interest. It may help to explain the evolution of this science 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 properties defines and discusses the property at several levels of detail. Thosenew to psychrometrics and those interested in a quick refresher or overview may want to read only the definitions and the first level of detail andthen move on to the next chapter. In most cases, multiple definitions areprovided. The first definition is suitable for general lay-person discussion.Subsequent definitions are more complete and technically more accurate.A definition serves no purpose unless it provides meaning and understanding for the reader. Multiple definitions allow the reader to choose thedefinitions that have the most meaning. Readers are encouraged to customize definitions.A skeleton psychrometric chart highlighting only the isolines of theproperty under discussion is included in each property chapter.Chapter 25 on altitude effects for locations other than sea level isrequired reading for those readers who live in or design for deep underground applications or applications for projects at high elevations; but, formost users concerned with psychrometrics within 500 metres of sealevel, this chapter can be scanned for future reference or skipped altogether. This chapter also illustrates the change in altitude (or local barometric pressure) effect on the appearance of the psychrometric chart.JARGONThe two quotations below are from a paper presented by Frank C.Quinn at the 1985 International Symposium on Moisture and HumidityControl. The first quotation suggests why psychrometrics has multiple andsometimes confusing terms. The second, as a consideration for students4
1 · How to Use this Bookand occasional users of psychrometrics, may provide the motivation forothers to use consistent terms and eliminate improper terminology.There are many ways to measure and express moisture and humidity,and often the term, or value (units) used is a carryover from the early daysof a particular industry.The measurements and consequently appreciation for the influencesof water are unnecessarily complicated by improper terminology (andsymbols) and usage and the proliferation of measurement techniqueswhich have led to ambiguity in the rationale of many moisture/humidityinstruments.Since psychrometrics evolved from many fields, it should come as nosurprise that different or substitute words are often used to describe thesame psychrometric property. This is unfortunately true in air-conditioning and meteorology psychrometrics. Much of the jargon is the result ofpioneers approaching psychrometrics from the different fields of meteorology, agricultural engineering, air-conditioning, drying, aeronomy, andaeronautical engineering. Other jargon was introduced by theoretical scientists who borrowed and truncated technical terms from the allied sciences of chemistry and physics.Some substitute words do not qualify as synonyms as defined byWebster’s International Dictionary, ASHRAE Terminology of HVAC&R,meteorology books, or scientific dictionaries. This text uses only theauthor’s preferred word, which is most often the meteorologist’s andASHRAE’s preferred word. The goal is not to limit or restrict the readeror to redefine the technology; rather, it is felt that consistent word usagewill eliminate possible reader confusion. Table 1-1 lists some preferredwords and substitutes.Table 1-1—Psychrometric VocabularyPreferred Wordair, the mixture of twocomponents: watervapour and dry air. Nosubscript is used formixture properties.dry air, DA, one of thetwo components of themixture air.Words Sometimes Used as Substitutesmoist air (Consideration was given to the term moistair, which was not chosen because the abbreviationMA might be confused with mixed air, the blend ofoutside and return air.)air (Since this could be confused with the previousterm, air, this text will always use the term dry air.)Note that dry air in the troposphere has nearly constant constituents of 78% nitrogen, 21% oxygen, and1% other gases. Also called “bone dry air.”water vapour, WV, one vapour, gas phase of water, humidity, moisture, moisof the two components ture vapour, low pressure, or ultra low pressure steamof the mixture air.5
Understanding Psychrometrics, Third EditionTable 1-1—Psychrometric Vocabulary (Continued)Preferred Wordbarometric pressure,pBAR, units Pa (pascal)Words Sometimes Used as Substitutestotal pressure, atmospheric pressure. (Note: if a process operates at significantly higher or lower pressurethan the location average barometric pressure, thenthat process should be analyzed at actual pressure.)humidity ratio, W,(a combination and contraction of water vapourwhich equals mWV/(humidity) and mixing ratio), moisture content, mixmDA, units kgWV/kgDA ing ratio, water vapour to dry air mixing ratio, and,incorrectly, specific humidity or absolute humiditywet-bulb temperathermodynamic wet-bulb temperature, temperature ofture, tWB, units C or K adiabatic saturation (equals psychrometer wet-bulbtemperature within 1% at 3 to 5 m/s air velocity acrossthe sensing element)water vapour prespartial pressure or partial pressure of water vapoursure, pWV, units Paspecific enthalpy, h,with units of energy/mDA, which in SI unitsis kJ/kgDAspecific volume, vX,units of m3/kgDA(Note: The x of vx canbe DA for dry air, WVfor water vapour, ornone for air.)adiabatic processenthalpy, total heat, and heat content (Heat is a formof energy crossing a boundary due to temperaturedifference; enthalpy is a composite of three properties: h u p · v where u is internal energy, p is pressure, v is specific volume, and the product “p · v” isflow work. Enthalpy difference and heat transfer happen to be equal in a constant pressure steady flowprocess with no shaft work.)the reciprocal of density, U; i.e., vDA 1/UDA(Note: When dealing with a gas or a vapour, specificvolume is generally used in preference to density. Toavoid confusion, do not use both. Important: If density is used in psychrometrics, its units are mass of dryair per unit volume of the moist air.)a process with no transfer of heat (due to temperaturedifference in either direction) across the systemboundary. An adiabatic process may include shaftwork or “p · v” flow work. For an open system, anadiabatic process may include mass transfer of substance(s) across the system boundary.Obsolete Words andWords That Should Not Be Used Because of Dual MeaningsIn the last 100 years, authors, scientists, researchers, and manufacturers have used various words to describe the many psychrometric properties. Some of these words have dual meanings and their use is6
1 · How to Use this BookTable 1-2WordReason That Use of the Word Is Discouragedspecific humidity, units Dual meaning. ASHRAE and the World Meteorologiof kgWV/kg(DA WV)cal Organization define specific humidity as the ratioof the mass of water vapour in moist air to the totalmass of the dry-air and water-vapour components. Itis also used incorrectly as the ratio of water vapour inmoist air to mass of dry air, i.e., humidity ratio.absolute humidity,Possible dual meaning. Absolute humidity is thewater vapour density expressed as the mass of waterunits of kgWV/m3vapour per unit volume. Today it is a rarely used term.In some cases it has also been used synonymouslywith humidity ratio.degree of saturation, Now rarely used. It was an important property used inP mWV/mWVS,interpolation of tabular statepoint properties. Degreeof saturation is the ratio of the humidity ratio at aunits nonedefined statepoint to the humidity ratio at saturation at(Also called percentage humidity and satu- the same dry-bulb temperature. It is equal to relativeration ratio; a decimal humidity only at 100% and 0%. It differs from relative humidity by several percent in the 50% RH range.ratio.)Many people and some texts incorrectly use the definition of degree of saturation as the definition forrelative humidity.Sigma functionNow rarely used. This term was an aid in the days ofmanual psychrometric calculations. Sigma function isthe enthalpy at the saturated wet-bulb temperature lessthe enthalpy of liquid water at the same temperature.Its need has been superseded by psychrometric chartswith enthalpy edge scales or enthalpy deviation curvesand by software algorithms and personal computers.discouraged. Some are still accurate terms; however, they are no longerneeded in instruction or calculations. These were valuable to the user inthe pre-handheld calculator, pre-personal computer era when a printedpsychrometric chart and a table of property values were indispensable.This text is confined to terms that are in common everyday psychrometricuse today. The terms in Table 1-2 are not used in this text.SubscriptsMultiple letter subscripts are used in many equations. The intent isnot to influence the science of psychrometrics or naming conventions butrather to aid in faster recognition for the first-time reader.7
Understanding Psychrometrics, Third EditionPSYCHROMETRIC ALGORITHMS Chapter 26, “Psychrometric Program Listing and ComparisonTable,” includes a listing of SI algorithms for calculation of statepointproperties along with SI to IP conversion equations. The algorithms canbe solved by equation solver software such as EES (Engineering EquationSolver from F-Chart Software) or they can be reconfigured into FORTRAN or some other language. They can also be modified and used inspreadsheet software. Looping or iteration routines are required for thesolution of some properties. Tables of selected accurate property valuesare included so that the user can verify the accuracy of these and otheralgorithms or psychrometric software.PHYSICAL LAWS, MODELS, ASSUMPTIONS, ANDTHE ACCURACY OF PSYCHROMETRIC CALCULATIONS Equations in this text are based on ideal (perfect) gas behavior for (1)the dry air component, (2) the water-vapour component, and (3) the moistair mixture of the two components. This requires that the behavior of eachconstituent of the mixture can be modeled by the perfect gas equation:pX vX RX T X The equations use Dalton’s model (also called Dalton's law or rule) ofan ideal gas mixture, which states that the gas and water-vapour moleculesshare the same volume and that the total pressure of the mixture is equalto the sum of the pressures of the individual components. Each of the constituent gases exerts a pressure (called partial pressure), which is the pressure that the constituent gas would exert if it alone occupied the volume.Air-conditioning psychrometric processes are almost always modeled as steady flow, constant pressure processes.Dry air, water vapour, and the combined moist air mixture closelyfollow perfect gas behavior in the temperature range of –40qC to 65qC andat total pressures up to 300 kPa.Additional accuracy is built int
Understanding Psychrometrics, Third Edition xii Psychrometrics can be simply explained and is solidly based on: (1) the ideal gas equation, (2) Dalton's model of partial pressures, (3) con-
