Air Conditioning Home and Commercial

P1: GDZ/FFX P2: GDZ/FFX QC: GDZ/FFX T1: GDZ

GB076-Miller-FM GB076-Miller-Sample July 14, 2004 20:16 Char Count= 0

AudelTM

Air Conditioning: Homeand Commercial

All New 5th Edition

i



ii



AudelTM

Air Conditioning: Homeand Commercial

All New 5th Edition

Rex MillerMark Richard Miller

Edwin P. Anderson

iii



Vice President and Executive Group Publisher: Richard SwadleyVice President and Executive Publisher: Robert IpsenVice President and Publisher: Joseph B. WikertExecutive Editor: Carol A. LongEditorial Manager: Kathryn A. MalmDevelopment Editor: Kevin ShaferProduction Editor: Vincent KunkemuellerText Design & Composition: TechBooks

Copyright C 2004 by Wiley Publishing, Inc. All rights reserved.Published simultaneously in Canada

No part of this publication may be reproduced, stored in a retrieval system, or transmit-ted in any form or by any means, electronic, mechanical, photocopying, recording, scan-ning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United StatesCopyright Act, without either the prior written permission of the Publisher, or authorizationthrough payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc.,222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 646-8700. Requests tothe Publisher for permission should be addressed to the Legal Department, Wiley Publishing, Inc.,10475 Crosspoint Blvd., Indianapolis, IN 46256, (317) 572-3447, fax (317) 572-4447, E-mail:[email protected].

Limit of Liability/Disclaimer of Warranty: The publisher and the author make no representa-tions or warranties with respect to the accuracy or completeness of the contents of this workand specifically disclaim all warranties, including without limitation warranties of fitness for aparticular purpose. No warranty may be created or extended by sales or promotional materials.The advice and strategies contained herein may not be suitable for every situation. This workis sold with the understanding that the publisher is not engaged in rendering legal, accounting,or other professional services. If professional assistance is required, the services of a competentprofessional person should be sought. Neither the publisher nor the author shall be liable fordamages arising herefrom. The fact that an organization or web site is referred to in this workas a citation and/or a potential source of further information does not mean that the authoror the publisher endorses the information the organization or web site may provide or recom-mendations it may make. Further, readers should be aware that Internet web sites listed in thiswork may have changed or disappeared between when this work was written and when it isread. For general information on our other products and services please contact our CustomerCare Department within the United States at (800) 762-2974, outside the United States at (317)572-3993 or fax (317) 572-4002.

Trademarks: Wiley, the Wiley Publishing logo, and Audel are trademarks or registered trademarksof John Wiley & Sons, Inc., and/or its affiliates. All other trademarks are the property of theirrespective owners. Wiley Publishing, Inc., is not associated with any product or vendor mentionedin this book.

Wiley also publishes its books in a variety of electronic formats. Some content that appears inprint may not be available in electronic books.

Library of Congress Control Number:

Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

iv

eISBN: 0-7645-7719-0



Contents

Acknowledgments xix

About the Authors xxi

Introduction xxiii

Chapter 1 Air-Conditioning Fundamentals 1Properties of Air 1Air Circulation 2Cleaning and Filtering 2Basic Information 3

Heat Transfer 3Sensible Heat 4Specific Heat 4Latent Heat 4Latent Heat of Fusion 4Latent Heat of Evaporation 5Superheat 5PressureTemperature Relationship 5

Measurements and Measuring Devices 7Thermometer 7Barometer 8Sling Psychrometer 9Pressure Gages 10British Thermal Unit 10Refrigeration Capacity Measurements 10

Summary 12Review Questions 13

Chapter 2 Psychrometry 15Air and Water-Vapor Mixtures 15Humidifying and Dehumidifying 15Relative Humidity 15

Relative Humidity Measurements 16Grains of Moisture 16Specific Volume 17

v



vi Contents

Dew-Point Temperature 17Effective Temperature 17Dry-Bulb Temperature 17Wet-Bulb Temperature 17Wet-Bulb Depression 18Temperature-Humidity Index 18Total Heat Content 18

Psychrometric Charts 18Psychrometric Chart Instructions 19Calculation Examples 19

Comfort Chart 25Summary 27Review Questions 27

Chapter 3 Heat Leakage 29Calculation Examples 30

Example 30Solution 30

Reference Tables 30Summary 52Review Questions 52

Chapter 4 Ventilation Requirements 53Air Leakage 53Natural Ventilation 53

Wind Forces 53Temperature-Difference Forces 54Combined Wind and Temperature Forces 55Roof Ventilators 55Ventilator Capacity 56

Fresh-Air Requirements 57Mechanical Ventilation 58

Volume of Air Required 58Duct-System Resistance 59

Air Filtration 60Effect of Dust on Health 60Various Dust Sources 61



Contents vii

Air-Filter Classification 61Dry Filters 62Wet (or Viscous) Filters 63

Filter Installation 66Humidity-Control Methods 69Humidifiers 69

Air-Washer Method 69Pan Humidifiers 70Electrically Operated Humidifiers 71Air-Operated Humidifiers 71

Dehumidifiers 73Electric Dehumidification 74Controls 75Adsorption-Type Dehumidifiers 76

Air-Duct Systems 77Heat Gains in Ducts 79Resistance Losses in Duct Systems 85

Fans and Blowers 89Air Volume 90Horsepower Requirements 90Drive Methods 91Fan Selection 92Fan Applications 93Fan Operation 94Attic-Fan Installation 95Fan Operation 97


Chapter 5 Room Air Conditioners 101Operation 101Cooling Capacity 103Capacity Requirements 103Installation Methods 104

Double-Hung Window Installation 106Casement Window Installation 111Outside Support Bracket 115



viii Contents

Console-Type Room Air-ConditioningSystems 116

Electrical System 122Electrical Components 124Fan Motors 127Thermostats 128Unit Control Switch 129

Service Operations 129Dismantling Window Air Conditioners 130Electrical Testing 130Compressor and Motor 135Oiling of Motors 137Leaks in System 137Filters 137Interior Cleaning 137Winter Care 137Refrigerant-System Service 138

Troubleshooting Guide(Window-Mounted Units) 143Summary 143Review Questions 145

Chapter 6 Refrigerants 147Desirable Properties 147Chlorofluorocarbon Compounds (CFCs) 147Classifications 148Early Types of Refrigerants 150

Sulfur Dioxide 151Methyl Chloride 151Ethyl Chloride 152Ammonia 153Carbon Dioxide 154Freezol (Isobutane) 154Calcium Chloride (CaCl2) 154

Freon Refrigerants 156Classifications 156Properties of Freons 157



Contents ix

Pressure-Temperature Chart 160Refrigerant Characteristics 160

Critical Temperature 160Hydrofluorocarbons (HFCs) 162Latent Heat of Evaporation 162Specific Heat 163Power Consumption 163Volume of Liquid Circulated 164

Simple Refrigeration System 164Handling Refrigerants 166

Storing and Handling RefrigerantCylinders 166Cylinder Capacity 167First Aid 168


Chapter 7 Compressors for Air Conditioners 171Mechanical Components 171

Compressors 171Condensers 173Receivers 173Evaporators 173Refrigerant Controls 174Expansion Valves 174

Refrigeration Cycle 174Refrigerants 175Compressors 176

Reciprocating Compressors 177Drive Methods 177Compressor Valves 178Pistons and Piston Rings 178Connecting Rods 181Crankshafts 181Shaft Seals 183Lubrication and Cooling 184

Rotary Compressors 184



x Contents

Scroll Compressors 189Scroll Compression Process 189Operation 191Scroll Compressor Models 192

Centrifugal Compressors 192Construction Features 195Lubrication System 196

Installation and Maintenance 198Foundation 198Motor and Belts 199Compressor Flow-Back Protection 199Valve Maintenance 200Shaft-Seal Leaks 201Compressor Knocks 202Stuck or Tight Compressor 202Compressor Removal 202Compressor Replacement 203Compressor Startup 203Winter Shutdown of Water-CooledCompressors 204

Compressor Characteristics 205Compressor Performance 205Compressor Capacity 205Mechanical Efficiency 205Volumetric Efficiency 205Compressor Efficiency Test 205

Troubleshooting Guide 214Summary 214Review Questions 215

Chapter 8 Condensing Equipment for AirConditioners 217Air-Cooled Condensers 217Water-Cooled Condensers 219

Cooling-Water Circuits 220Types of Water-Cooled Condensers 220



Contents xi

Head-Pressure Control Methods 228Water-Cooled Condensers 228Evaporative Condensers 229Air-Cooled Condensers 230

Condenser Maintenance 232Cleaning Precautions 233Charging the System 235Cleaning Time 235Flushing the System 235Cleaning Evaporative Condensers 235Removing Scale Formation 236Gravity and Forced Circulation 236

Outdoor Condensing Units 237Summary 238Review Questions 239

Chapter 9 Evaporators for Air Conditioners 241Types of Evaporators 241

Finned Evaporators 242Plain-Tubing Evaporators 242Evaporator-Coil Construction 243

Refrigerant-Control Methods 245Superheat 245Chilling Units 247Dry-Expansion Coil Arrangements 249Capacity Requirements 251

Evaporator Calculations 251Exterior Conditions 253Surface-Area Determination 253

Evaporator Maintenance 255Cycling the Condensing Unit 255Time Clocks 256

Accessory Equipment 257Heat Exchangers 257Surge Drums 260Oil Separators 260Driers 263



xii Contents

Dehydrators 264Strainers 265Mufflers 266Subcoolers 266Check Valves 267


Chapter 10 Water-Cooling Systems 273Atmospheric Cooling Methods 274

Water Towers 274Cooling Ponds 278Spray-Cooling Ponds 279

New Developments 279Cooling-Unit Piping 280

Liquid Lines 281Suction Lines 282Oil Circulation 283Discharge Lines 283Refrigerant-Piping Arrangement 284


Chapter 11 Air-Conditioning Control Methods 289Types of Air-Conditioning Controls 289

Manual Control 289Automatic Control 290Semiautomatic Control 290

Basic Control Types 290Room Thermostats 291Twin-Type Thermostats 294Duct Thermostats 294Immersion Thermostats 295Hygrostats 295Pressure Regulators 297

Limit Controls 297Control Valves 298



Contents xiii

Dampers 299Relays 299Refrigerant Control Devices 300

Automatic Expansion Valves 300Thermostatic Expansion Valves 300Sizing of Thermostatic ExpansionValves 302Low-Side Float Valves 303High-Side Float Valves 305Capillary Tubes 305Crankcase Pressure-Regulating Valve 305Evaporator Pressure-Regulating Valve 306Reversing Valves 306


Chapter 12 Weather Data and Design Conditions 311Indoor Design Conditions 311Outdoor Design Conditions 312

Winter 312Summer 313

Interpolation among Stations 326Weather-Oriented Design Factors 326Summary 328Review Questions 329

Chapter 13 Year-Round Central Air Conditioning 331Central System Features 331

Air-Supply System 332Mixing Chamber 332Heating Coils 333Preheaters 333Cooling Coils 334Air-Supply Fans 334Filters 335Air Washers 335Control Method 335



xiv Contents

Zone Control 336System Selection 337

Absorption-Type Air Conditioning 338Absorption versus Compression 338Cycle of Operation 339

Reverse-Cycle Air Conditioning(Heat Pumps) 341

Reversing Valve Operation 342Control Equipment 345Defrost-Cycle Operation 346Installation and Maintenance 347Indoor Arrangement 347Refrigerant Tubing 348Quick-Connect Couplings 350System Troubles 353

Reversing-Valve Troubleshooting Guide 354Nonportable Air Conditioners 354

Construction Principles 358Plumbing Connections 358Water-Cooling Tower 359Heating Coils 361

Suspended-Type Air Conditioners 361Casings 362Coils 362Filters 363Controls 363

Air-Conditioning TroubleshootingGuide 363Summary 366Review Questions 366

Chapter 14 Automobile Air Conditioning 369Air-Conditioning System Operation 373Component Description 375

Compressors 375Condensers 376Receiver-Dehydrator 377



Contents xv

Thermostatic Expansion Valves 377Evaporators 380Solenoid Control 380Magnetic-Clutch Control 381Suction-Throttle Control 382Air Distribution 384Air-Conditioner Control System 384

Service and Maintenance 384Insufficient or No Cooling 385Air Output Not Normal 385Air Output Normal 385Checking Suction Pressures 386Checking Control Systems 386Air Distribution 386Defective Compressor 386Compressor Clutch 387Condenser 387Thermostatic Expansion Valve 387Evaporator 388Refrigerant Lines 388Suction-Throttle Valve 388Use of Receiver Sight Glass for Diagnosis 389

Installation Procedure 389Troubleshooting Guide 395Summary 395Review Questions 396

Chapter 15 Motors and Motor Controls 399Motor Voltage 399Polyphase Motors 399

Squirrel Cage Motors 400Wound-Rotor Motors 401Synchronous Motors 403

Single-Phase Motors 406Split-Phase Motors 406Capacitor-Start Motors 407Two-Value Capacitor Motors 408



xvi Contents

Permanent Split-Capacitor Motors 408Repulsion Induction Motors 409Repulsion-Start Induction Motors 409Capacitors 411

AC Motor Controls 413Squirrel Cage Motors 413Wound-Rotor Motors 414Synchronous Motors 416

Single-Phase Motor Controls 418Overload Protectors 418Start Relays 419

Compressor Motor Controls 420Temperature-Motor Control 420Pressure-Motor Control 421Solenoid Valves 421Condensing-Water Controls 421

Belt Drives 422Motor Maintenance 422Horsepower Calculations 423

Motor Horsepower Calculated fromMeter Readings 423Horsepower Calculated from Load(Mechanics Data) 424Horsepower Calculations for SpecificApplications (Approximate) 424Power-Conversion Factors 425

Troubleshooting Guide 425Summary 425Review Questions 428

Chapter 16 Maintenance 431General Procedure 431Head Pressures 432Suction Pressure 433Back Pressure 434Refrigerant Charge 434Compressor Service Valves 435



Contents xvii

Gages 436Combination Gages 437Adding Refrigerant 437Removing Refrigerant 439Adding Oil 439Removing Oil 439System Pump-Down 440Purging System of Noncondensables 441Leak Test 441

Halide-Torch Method (For FreonRefrigerants) 441Electronic Leak Detector 442

Evacuating the System 443Checking Compressor Valves 444Troubleshooting Guide 444Summary 450Review Questions 451

Appendix A Professional Organizations 453

Glossary 459

Index 481



xviii



Acknowledgments

No book can be written without the aid of many people. It takes agreat number of individuals to put together the information avail-able about any particular technical field into a book. The field of airconditioning is no exception. Many firms have contributed to theinformation, illustrations, and analysis of the book.

The authors would like to thank every person involved for hisor her contributions. Following are some of the firms that suppliedtechnical information and illustrations.

Acme Industrial, Inc.American Society of Heating and Ventilating EngineersAmerican Standard ControlsArmstrong Machine WorksASHRAEAutomatic Products, Inc.Barneby Cheney CorporationBuffalo Forge CompanyCargoaire Engineering CorporationCarrier CorporationControls Company of AmericaCopeland Refrigeration CorporationCrocker-Wheeler CorporationDunham-Bush, Inc.Frick CompanyGeneral Electric CompanyGeneral Motors CorporationGibson Refrigerator Sales CorporationHoneywell, Inc.Hotpoint Co.LennoxMarsh Instruments Co. Inc.Mueller Brass Co.Paragoan Electric CompanyPenn Controls, Inc.

xix



xx Acknowledgments

Ranco InternationalRefrigeration Research, Inc.Rockwell-Standard CorporationRudd Manufacturing CompanySporlan Valve CorporationTaylor Instrument CompanyTecumsehTrane CompanyTurner CorporationWestinghouse CorporationWorthington Corporation



About the Authors

Rex Miller was a Professor of Industrial Technology at The StateUniversity of New York, College at Buffalo for more than 35 years.He has taught on the technical school, high school, and college levelfor more than 40 years. He is the author or co-author of more than100 textbooks ranging from electronics through carpentry and sheetmetal work. He has contributed more than 50 magazine articles overthe years to technical publications. He is also the author of sevencivil war regimental histories.

Mark Richard Miller finished his BS degree in New York and movedon to Ball State University, where he obtained the masters and wentto work in San Antonio. He taught in high school and went tograduate school in College Station, Texas, finishing the doctorate.He took a position at Texas A&M University in Kingsville, Texaswhere he now teaches in the Industrial Technology Department asa Professor and Department Chairman. He has co-authored sevenbooks and contributed many articles to technical magazines. Hishobbies include refinishing a 1970 Plymouth Super Bird and a 1971Road-runner.

Edwin P. Anderson was a professional engineer and the author ofnumerous books for the trades, including air conditioning, sheetmetal, home appliance, and electrical.

xxi



xxii



Introduction

This book is designed to fill the need for a concise, basic book forthose who are in need of acquiring practical information on theinstallation, operation, and servicing of air-conditioning equipmentin the home, as well as in the commercial and industrial applications.The subject is presented in a simple non-technical language, yet thetreatment is sufficiently complete to enable the reader to diagnoseand correct troubles that may occur from time to time.

The material in this book has been carefully organized to pro-vide a practical understanding of the construction, operation, andfundamentals so important to diagnosing operating faults in an air-conditioning system. A complete description of the purpose andfunction of each operating component, along with a full coverageof refrigerants used in various applications, is included.

Each chapter is fully illustrated. Numerous troubleshootingcharts will assist the operator or service technician in the properdiagnosis and prompt repair procedure to use when a malfunctionoccurs.

xxiii



xxiv


GB076-01 GB076-Miller-Sample July 14, 2004 22:26 Char Count= 0

Chapter 1Air-Conditioning FundamentalsAir conditioning may be defined as the simultaneous control of all(or at least the first three) of those factors affecting both the physicaland chemical conditions of the atmosphere within any structure.These factors include temperature, humidity, motion, distribution,dust, bacteria, odors, and toxic gases, most of which affect humanhealth and comfort to a greater or lesser degree.

Air that has been properly conditioned has had one (or a combi-nation) of the foregoing processes performed on it. For example, ithas been heated or cooled; it has had moisture removed from it (de-humidified); it has been placed in motion by means of fans or otherapparatus; and it has been filtered and cleaned. These processes maybe placed in the following order for ready reference:

1. Heating.2. Cooling.3. Humidifying.4. Dehumidifying.5. Circulating.6. Cleaning and filtering.

The impression prevailing in many instances is that refrigeratingor heating equipment cools or heats a room. This is only partly truesince all the work performed by the equipment is on the air withinthe room, not on the room itself. In this connection, it is good toremember that air is only a vehicle or conveyance used to transportheat and moisture from one point to another. Air is a tangible item,and every cubic foot of air surrounding a person has a certain weight,depending on its temperature, the amount of moisture it is carrying,and its altitude above sea level.

Properties of AirAir is a mixture made up primarily of two gases, being approx-imately 23 parts oxygen and 77 parts nitrogen by weight. Othergases in air include carbon dioxide, carbon monoxide, ozone, neon(in small quantities), and certain gases that are of no particular in-terest in the field of air conditioning.

Ozone is produced by sparks around electrical equipment andby the discharge of atmospheric electricity or lightning. Neon isa gas in its normal form and is used in signs for advertising.

1



2 Chapter 1

Carbon monoxide is not present in the atmosphere except in con-gested motor traffic. It is dangerous, being produced by the incom-plete combustion of carbon. It is also given off by stoves, furnaces,and cigarettes. Air containing carbon monoxide in excess of 1/10 of1 percent is fatal to human beings.

Oxygen, the most important constituent of air, constitutes aboutone-fourth of the air by weight and one-fifth of the air by volume,and on it depends the existence of all animal life. Nitrogen is arelatively inert gas whose principal function is to dilute oxygen.

Air CirculationAir-conditioning equipment must circulate a sufficient volume of airat all times for two main reasons:

The air must be constantly moving to carry away the mois-ture and heat immediately surrounding the body. If this is notdone, the occupants soon become uncomfortable, even thoughthe relative humidity of the room as a whole is comparativelylow.

The air must be constantly drawn into the conditioner andpassed out over the cool evaporator so that the moisture itabsorbs from the room may be condensed and eliminatedthrough the drain.

Although the movement of a sufficient volume of air at all times ismost essential, direct drafts must be prevented. The condensation ofmoisture on the evaporator surface during summer operation pro-duces a measure of cleaning because this moisture absorbs a con-siderable amount of impurities from the air passing over the moistevaporator surfaces. As the condensation of moisture continues, itdrips off the evaporator and is carried off to the drain, taking theimpurities with it. For very dirty air, special provision must be madeby installing air filters.

Cleaning and FilteringThere are numerous air-cleaning and filtering devices on the market.Such devices serve to eliminate particles carried in the air that aredetrimental to health and comfort and that cause property damage.These may be classified as dust, fumes, and smoke. Dust and fumeswill settle in still air, whereas smoke is actuated by motion ratherthan by gravity and, if not removed, will remain in motion in the air.

Air washing is effective in removing dust and fumes such as smokebecause they are soluble in water, but carbons, soot, and similar



Air-Conditioning Fundamentals 3

substances are not removed by this method of cleaning. Dry andviscous filters have been developed to make it possible to cleanse airof these substances. To fulfill the essential requirements of clean air,an air cleaner should have the following qualifications:

It should be efficient in the removal of harmful and objection-able impurities in the air (such as dust, dirt, pollens, bacteria,and so on).

It should be efficient over a considerable range of air velocities. It should have a large dust-holding capacity without excessive

increase of resistance. It should be easy to clean and handle, or be able to clean itself

automatically. It should leave the air passage through the filter or cleaner

free from entrained moisture or charging liquids used in thecleaner.

Basic InformationTo obtain a clear concept of the functioning of air-conditioning sys-tems, you must understand the physical and thermal properties un-derlying the production of artificial cold. Since air conditioning dealslargely with the problem of removal of heat from a room or space,the following definitions should be understood.

Heat TransferFirst, it should be noted that heat is an active form of energy, muchthe same as mechanical and electrical energy. Heat may be trans-ferred by three methods: conduction, convection, and radiation.

Conduction is heat transfer that takes place chiefly in solids,wherein the heat is passed from one molecule to another with-out any noticeable movement of the molecules.

Convection is heat transfer that takes place in liquids andgases, where the molecules carry the heat from one point toanother.

Radiation is heat transfer in wave motion (such as light andradio waves) that takes place through a transparent mediumwithout affecting that mediums temperature. An illustrationof this is the suns rays passing through air. The air temperatureis noticeably affected. Radiant heat is not apparent until itstrikes an opaque surface, where it is absorbed and manifestsitself in a temperature rise.



4 Chapter 1

Sensible HeatSensible heat is that form of heat that causes a change in the temper-ature of a substance. This heat can be measured by a thermometer.For example, when the temperature of water is raised from 32F to212F, an increase in the sensible-heat content is taking place.

Specific HeatThe British thermal unit (Btu) required to raise the temperature ofone pound of a substance 1F is termed its specific heat. (The Btuis discussed later in this chapter.) By definition, the specific heat ofwater is 1.00, but the amount of heat required to raise the temper-ature of various substances through a given temperature range willvary. Since water has a very large heat capacity, it has been takenas a standard; and since 1 pound of water requires 1 Btu to raiseits temperature 1F, its rating on the specific heat scale is 1.00. Ironhas a lower specific heatits average rating is 0.130; ice is 0.504,and air is 0.238. The more water an object contains (as in the caseof fresh food or air), the higher the specific heat.

Latent HeatLatent heat (meaning hidden heat) is a form of heat that causes asubstance to change its physical state from a solid to liquid, a liquidto vapor, or vice versa. For example, when a liquid is evaporated toa gas, the change of physical state is always accompanied by the ab-sorption of heat. Evaporation has a cooling effect on the surround-ings of the liquid since the liquid obtains from its surroundings thenecessary heat to change its molecular structure. This action takesplace in the evaporator of an air-conditioning system. Any liquidtends to saturate the surrounding space with its vapor. This prop-erty of liquids is an important element in all air-conditioning work.

On the other hand, when a gas is condensed to a liquid, thechange of physical state is always accompanied by the giving upof heat. This action takes place in the condensing unit of an air-conditioning system because of the mechanical work exerted on thegas by the compressor.

Latent Heat of FusionThe change of a substance from a solid to a liquid, or from a liquidto a solid, involves the latent heat of fusion. One pound of water ata temperature of 32F requires the extraction of 144 Btu to cause itto freeze into solid ice at 32F. Every solid substance has a latent-heat value in varying degrees, and that amount required to convertit, or bring about a change of state, is termed the latent heat offusion. This heat, assimilated or extracted, as the case may be, is not




measurable with a thermometer because the heat units are absorbedor expanded in intermolecular work, separating the molecules fromtheir attractive forces so that a change of state is effected.

Latent Heat of EvaporationThe change of a substance from a liquid to a vapor or from a vaporback to a liquid involves the latent heat of evaporation. Carefulmeasurements have determined that the conversion of 1 pound ofpure water at 212F to steam at 212F requires 970 Btu when carriedout at the normal pressure of the atmosphere encountered at sealevel. If heat is added and a count is kept of the Btu expended, it willbe found that when all the water has been changed to steam, 970heat units will have been used. The further addition of heat wouldserve only to heat the steam, such as would be possible if it had beentrapped or the experiment performed in a closed vessel so that heatcould be applied to it.

SuperheatSuperheat is a term used frequently, especially for refrigerant controladjustment. Superheat is sensible heat absorbed by a vapor or gasnot in contact with its liquid, and consequently it does not follow thetemperaturepressure relationship. Therefore, superheat is sensibleheat absorbed by the vapor raising the temperature of the vapor orgas without an appreciable change in pressure.

A gas is usually considered a vapor in a highly superheated state,or a vapor not near its condensing point. Water in the air that is closeto the condensing point is termed water vapor. Inasmuch as super-heat is sensible heat, its effect can be measured with a thermometerand is merely the temperature rise in degrees Fahrenheit. Therefore,a 10F superheat means a vapor that has absorbed sufficient heatto raise the vapor temperature 10F above the temperature of thevaporizing liquid.

PressureTemperature RelationshipExtensive investigation of gases and their behavior has shown that agiven weight expands or contracts uniformly 1/459 of its original vol-ume for each degree it is raised or lowered in temperature above orbelow 0F, provided the pressure on the gas remains constant. Thisfact is known as the law of Charles. Following this same reasoning,assume 459F as absolute zero.

Actually, this temperature or condition has never been attained.The law of conservation of matter states that matter can be neithercreated nor destroyed, although it can be changed from one forminto another. Temperature within a few degrees of absolute zero



6 Chapter 1

has been reached by liquefying oxygen, nitrogen, and hydrogen, butthese (like other gases) change their physical state from gas intoliquid and fail to disappear entirely at these low temperatures. Thefact that absolute zero has never been reached is also explained byanother law, known as the law of conservation of energy. It hasalready been explained that heat is a form of energy. This law statesthat energy can be neither created nor destroyed, although it can bechanged from one form into another.

Having considered the effect of temperature on a gas, the nextstep is the effect of pressure on gases to aid the study of refrigeration.In 1662, Robert Boyle announced a simple relation existing betweenthe volume of a gas and the pressure applied to it, which has sincebecome known to scientists as Boyles law:

At constant temperature, the volume of a given weight of gasvaries inversely as the pressure to which it is subjected. Themore pressure applied to a gas, the smaller its volume becomesif the temperature remains the same; likewise, if the pressureis released or reduced, the volume of the gas increases.

Mathematically, this might be expressed:

P V = p vwhere P is the pressure on the gas at volume V , and p is the pressureon the same weight of gas at volume v.

Boyles law has been found to be only approximately true, espe-cially for the refrigerant gases, which are more easily liquefied. Thevariations from the law are greater as the point of liquefaction orcondensing of any gas is reached, although the material movementof air is determined by this law. It will be found that if the temper-ature is held constant and sufficient pressure is applied to a givenweight of gas, it will change from the gaseous state into the liquidstate. The point at which this change of state takes place is knownas the point of liquefaction or condensing.

There is a definite relationship existing between the pressure, tem-perature, and volume at which a given weight of gas may exist. Therelationship is used extensively in scientific work and is known asthe combined law of Boyle and Charles. It may be expressed math-ematically as follows:

P VT

= p vt

where P and p are expressed in the absolute pressure scale in poundsper square foot; V and v are expressed in cubic feet; and T andt are expressed in degrees on the absolute-temperature scale.




When the pressure, temperature, or volume of a gas is varied,a new set of conditions is created under which a given weight ofgas exists in accordance with the preceding mathematical equation.If a gas is raised to a certain temperature (which varies with eachindividual gas), no matter how much pressure is applied to it, itwill be found impossible to condense. This temperature is known asthe critical temperature. The pressure corresponding to the criticaltemperature is termed the critical pressure. Above the critical points,it is impossible to vaporize or condense a substance.

Measurements and Measuring DevicesThe instrument commonly used for measuring temperature is knownas the thermometer, which operates on the principle of the expan-sion and contraction of liquids (and solids) under varying intensitiesof heat. The ordinary thermometer charged with mercury operateswith a fair degree of accuracy over a wide range. It becomes use-less, however, where temperatures below 38F (38 degrees belowzero Fahrenheit) are to be indicated, because mercury freezes at thispoint; and another liquid, such as alcohol (usually colored for easyobservation), must be substituted. The upper range for mercurialthermometers is quite high (about 900F) so it is at once apparentthat for ordinary service and general use the mercury thermometeris usually applicable.

ThermometerIn operation, the thermometer depends on the effect of heat on themain body of mercury or alcohol that expands or contracts in a bulbor reservoir. This action raises or lowers the height of the liquid in thecapillary tube forming the thermometer stem. Several thermometerscales are in existence and are used in various countries. The English(or Fahrenheit) scale is commonly used in the United States, theCelsius in France, and the Reamur in Germany. Since the Celsiusscale is so widely used in scientific work in all countries, an illustra-tion of the comparison of thermometers is shown in Figure 1-1 sothat any one scale may be converted to another. The freezing pointon the Fahrenheit scale is fixed at 32, and on both the Celsius andReamur scales it is placed at 0. On the Fahrenheit scale, the boilingpoint of pure water under the normal pressure encountered at sealevel is 212; on the Celsius scale, it is 100; and in the case of theReamur, it is 60.

If it is desired to convert 50 Celsius to Fahrenheit, the methodwould be in accordance with the following formula:

F = 9/5C + 32



8 Chapter 1

212210200190180170160150140130120110100908070605040322520100

100

90

80

70

60

50

40

30

20

10

0

180

DIVI

SION

S

100

DIVI

SION

S

BOILING POINT OF WATER

FREEZING POINT OF WATER

FAHRENHEIT CELSIUS

TO CONVERT DEGREECELSIUS TO DEGREE

FAHRENHEIT USE FORMULA:F = 9/5C + 32

TO CONVERT DEGREEFAHRENHEIT TO DEGREECELSIUS USE FORMULA:

C = 5/9 (F 32)

Figure 1-1 Relationship between Fahrenheit and Celsius temperaturescales.

Therefore,

50 9/5 + 32 = 122FOr, using a calculator, it becomes

50 1.8 + 32 = 122F (since 9/5 = 1.8)To convert 50F to Celsius:

F = 5/9 (C 32), or 0.5555555 C 32Therefore,

0.5555555 50 32 = 10F (since 5/9 = 0.5555555)BarometerThe barometer is an instrument for the measurement of atmosphericpressure. In its earliest form, it consisted simply of a glass tube some-what in excess of 30 inches in length filled with mercury. This tube




was inverted in a cup partially filled with mercury, as shown inFigure 1-2. The height of the mercury column in the tube is a mea-sure of the existing atmospheric pressure.

MERCURY TUBE

760 MM29.92 IN

ATMOSPHERIC PRESSURE

MERCURY BOWL

Figure 1-2 Method of obtaining barometric pressure.

Standard atmospheric pressure at sea level is 29.921 inches ofmercury. Most of the pressure gages used in engineering calculationsindicate gage pressure, or pounds per square inch (psi). Barometerreadings may be converted into gage pressure by multiplying inchesof mercury by 0.49116. Thus, if the barometer reads 29.921 inches,the corresponding gage pressure equals 0.49116 29.921, or14.696 psi. Table 1-1 is a convenient conversion table based on thestandard atmosphere, which, by definition, equals 29.921 inches ofmercury (in Hg), or 14.696 psi.

Sling PsychrometerRelative humidity is measured by an instrument known as the slingpsychrometer, which uses two mercury thermometers mounted sideby side (see Figure 1-3). One of the thermometers is the sametype used to measure the temperature of the air; it is called the



10 Chapter 1

Table 1-1 Atmospheric Pressure for VariousBarometer Readings

Barometer (in Hg) Pressure (psi) Barometer (in Hg) Pressure (psi)

28.00 13.75 29.75 14.6128.25 13.88 29.921 14.69628.50 14.00 30.00 14.7428.75 14.12 30.25 14.8629.00 14.24 30.50 14.9829.25 14.37 30.75 15.1029.50 14.49 31.00 15.23

dry-bulb thermometer. The other, called the wet-bulb thermometer,has a small wick saturated with water attached to its bulb. If the airis dry (less humid), the evaporation from the saturated wick will befaster and consequently the temperature will be lower.

Pressure GagesPressure gages, as the name implies, are used for pressure measure-ments on refrigeration systems as a means of checking performance.Gages for the high-pressure side of the system have scales readingfrom 0 to 300 pounds per square inch gage (psig), or for higher pres-sures, from 0 to 500 psig (Figure 1-4). Gages for the low-pressurepart of the system are termed compound gages since their scalesare graduated for pressures above atmospheric pressure in poundsper square inch gage and for pressures below atmospheric pressure(vacuum) in inches of mercury. The compound gage is calibratedfrom 30 inches of vacuum to pressures ranging from 60 to 150 psig,depending on gage design.

British Thermal UnitThe unit of measuring the quantity of heat is the British thermal unit(Btu) and is the heat required to raise the temperature of 1 poundof pure water (at its greatest density) 1F.

Refrigeration Capacity MeasurementsThe cooling effect is measured by a unit known as a ton of refrig-eration. A ton of refrigeration is obtained when 1 ton (2000 lb) ofice at 32F is melted to water at 32F in 24 hours. If it is remem-bered that the latent heat of fusion is 144 Btu/lb, it follows thatthe ton represents a unit cooling effect of 144 2000, or 288,000Btu/24 hr, 12,000 Btu/hr, or 200 Btu/min. Thus, for air-conditioning




10

1040

5060

7080

9010

100

4050

6070

8090

100

10

PSYC

HROM

ETER

SLI

DE R

ULE

7060

5040

3020

010

2030

4050

6070

8090

7060

3040

10

Figure 1-3 Sling psychrometer. Note the thermometers plus a sliderule for obtaining relative humidity.

120 110 500450

400

350

300250

120110

160

100

90

80

200

130140

150

200

150

100

50

0

100

90

80

706050

40

30

20

10

010 20

9590

80

70

6050

40

30

20

10010

2040

5030

10

010

20 3040

60

90

150140130

120

110100

8060

4020

VAC 250

MARSH MARSH

R-28

R-12

R-28

R-12

Figure 1-4 Compound (left) and high-pressure (right) gages.(Courtesy Marsh Instrument Co., Inc.)



12 Chapter 1

calculation, the size of the required condensing unit (expressed intons) can be obtained by dividing the heat gain of the structure(expressed in Btu/hr) by 12,000. The foregoing may be written asfollows:

Refrigeration (in tons) = Btu/hr heat gain12,000

SummaryFactors affecting both physical and chemical conditions of the atmo-sphere are temperature, humidity, motion, distribution, dust, bac-teria, odor, and toxic gases. Air is a mixture of two gases: oxygenand nitrogen. There are still other gases in the air, such as carbondioxide, carbon monoxide, ozone, and neon in small quantities. Theair must be constantly moving to carry away the moisture and heat.

Air filters are used to eliminate particles of dirt or dust carried inthe air that are detrimental to health and comfort. Water filters areeffective in removing dust and some fumes and smoke, but carbonand soot are not removed by this method. Dry filters and electronicfilters make it possible to clean the air of most harmful dust andpollen.

Heat is an active form of energy much the same as mechanicaland electrical energy. Heat is transferred by three methods: con-duction, convection, and radiation. Conduction means the flow ofheat through a solid substance (such as iron). The transfer of heat byconvection means the carrying of heat by air rising from a heated sur-face. Radiation takes place in the absence of matter, as in the passageof heat through the vacuum inside the bulb of an incandescent lamp.

Sensible heat is a form of heat that causes a change in the tem-perature of a substance. Specific heat is the Btu required to raisethe temperature of one pound of substance one degree Fahrenheit.Latent heat is the form of heat that changes the physical state of asubstance, such as solid to liquid or liquid to a vapor. Latent heatof fusion is the point at which a substance changes from a solid toa liquid or from a liquid to a solid.

Latent heat of evaporation is the point at which a substancechanges from a liquid to a vapor or from a vapor back to a liq-uid. It takes 970 Btu to change 1 lb of pure water at 212F to steamwhen atmospheric pressure is at sea level. Superheat is sensible heatabsorbed by a vapor or gas not in contact with its liquid and con-sequently does not follow the temperaturepressure relationship.

A thermometer is an instrument used for determining the temper-ature of a body or space. The barometer is an instrument used for




measuring the atmospheric pressure. Standard atmospheric pressureat sea level is 29.921 in Hg, or 14.696 psi.

Review Questions1. Name the three most important factors that affect human

health and comfort.2. What factors besides heating and cooling are necessary in an

air-conditioning system?3. What are the chemical constituents of air?4. Why is air movement a necessary part of air conditioning?5. What are the devices used for cleaning and filtering air?6. What are the three principal methods by which heat may be

transferred through space?7. If the specific heat of water is taken as 1.00, what is the specific

heat of iron?8. How does latent heat affect the change of state in various

substances?9. What is meant by the latent heat of evaporation?

10. What is superheat?11. State the relations between pressure, temperature, and volume

for a given weight of gas.12. How many Btus are required to convert 1 pound of water at

32F to 1 pound of ice at the same temperature?13. Define the law of conservation of matter.14. Define Boyles law.15. State the relations between the various temperature scales.16. What instrument is commonly used for measurement of

atmospheric pressure?17. State the relations between absolute and gage pressure.18. What is a British thermal unit?19. What is a ton of refrigeration, and what is the Btu equivalent?20. What is the old name used for degrees Celsius?



14



Chapter 2PsychrometryPsychrometry is that branch of physics relating to the measurementor determination of atmospheric conditions, particularly regardingthe moisture mixed with air. In calculations of all air-conditioningsituations, you should understand that the dry air and water vaporcomposing the atmosphere are separate entities, each with its owncharacteristics. This water vapor is not dissolved in the air in thesense that it loses its own individuality, but merely serves to moistenthe air.

Air and Water-Vapor MixturesWater vapor is a gaseous form of water at a temperature belowthe boiling point of water. It is the most variable constituent of theatmosphere. At certain temperatures and barometric pressures, it isextremely unstable in either gaseous or liquid form. This is evidentby the formation and disappearance of clouds and fog. Water vaporconstitutes about 3 percent of the total air by volume in hot, humidweather, and about 1/5 of 1 percent of total air by volume in dry,cold weather. Water vapor is actually steam at very low pressure.Hence, its properties are those of steam at low temperatures, and itsactions are comparable to steam.

Humidifying and DehumidifyingThe air becomes humidified when moisture is added to it and is de-humidified when moisture is removed. Perhaps it may seem strangethat the addition of moisture to air and the removal of moisture fromair are two of the six important essentials of proper air conditioning.It does seem odd that the amount of moisture in the air of a roomshould have any effect on the personal comfort of the occupant. Anexcess or deficiency of moisture, however, does have a very notice-able effect that will immediately become apparent once the meaningof the somewhat mysterious term relative humidity is understood.

Relative HumidityRelative humidity may be defined as the ratio of the quantity ofvapor actually present in the air to the greatest amount possible atany given temperature. It follows that the relative humidity of airat any given temperature can be obtained merely by dividing theamount of moisture actually in the air by the amount of moisturethat the air can hold at that temperature, and then multiplying theresult by 100 to get the percentage factor.

15



16 Chapter 2

Relative Humidity MeasurementsRelative humidity is measured by an instrument known as the slingpsychrometer (Figure 2-1), which consists simply of two ordinarythermometers securely fastened in a frame to which a chain is at-tached. By means of this chain, the instrument can be rapidly whirledaround so the thermometer bulbs contact the maximum amount ofair. Around the bulb of one thermometer is a small piece of cloth,which is dampened with water before taking a reading. The theoryof the instrument is simply that the evaporation of moisture fromthe bulb of the wet thermometer causes it to read lower than the onethat is dry. The rate of evaporation depends directly on the amountof moisture in the air at the time the test is made. The differencebetween the readings of the two thermometers enables one to findthe relative humidity.

Figure 2-1 Typical slingpsychrometer.(Courtesy Taylor Instrument Company.)

Grains of MoistureGrains of moisture represent the weight of water vapor present in1 cubic foot of air. The grain is a unit of weight. It is the basic unitof the English weight system, derived from the weight of a grain ofwheat. There are 7000 grains in a pound, so 1 grain of moistureweighs 1/7000 (0.000142) pound.



Psychrometry 17

Specific VolumeSpecific volume is the number of cubic feet of moist air requiredto contain 1 pound of dry air molecules. Specific volume is the re-ciprocal of the specific gravity. It is equal to the number of cubiccentimeters occupied by 1 gram of a substance when the specificgravity is referred to water at 40C (39.2F) as a standard.

Dew-Point TemperatureThe saturation temperature for any given quantity of water vaporin the atmosphere is known as the dew point. By definition, fora given atmospheric pressure, the dew point is the temperature ofsaturation at which moisture begins to change into the form of tinywater droplets or dew.

Effective TemperatureAs applied to air conditioning, the effective temperature is an empir-ically determined index of the degree of warmth or cold as apparentto the human body. It takes into account the temperature, moisturecontent, and motion of the surrounding air. Effective temperaturesare not strictly a degree of heat in the same sense that dry-bulb tem-peratures are. For example, the effective temperature can be loweredby increasing the rate of airflow even though wet- and dry-bulb tem-peratures remain constant. For space cooling and heating, however,the air-movement factor is considered a constant at approximately20 ft/min. Under this condition, effective temperature is determinedby the wet- and dry-bulb thermometer readings only.

Dry-Bulb TemperatureAn ordinary thermometer is used to measure dry-bulb temperature.Two types of liquid are used in thermometers: colored alcohol andmercury. The alcohol thermometer is more common because it isless expensive and can measure the normal range of air temperature.Since mercury freezes at approximately 38F, mercury thermome-ters are not practical for measuring extremely low temperatures.

Wet-Bulb TemperatureWet-bulb temperature is the temperature at which the air becomessaturated if moisture is added to it without the addition or sub-traction of heat. Thus, if the bulb of an ordinary thermometer issurrounded with a moistened wick, placed in a current of air, andsuperheated with water vapor, the reading obtained will be at somepoint below the dry-bulb temperature. The minimum reading thusobtained is the wet-bulb temperature of the air.



18 Chapter 2

Wet-Bulb DepressionSince outdoor summer air is rarely fully saturated, there is normally aconsiderable difference between its dry- and wet-bulb temperatures.The difference between the two temperatures is called the wet-bulbdepression. For example, if the dry-bulb temperature reading is 80and the corresponding wet-bulb temperature is 65, the wet-bulbdepression is 80 65, or 15.

Temperature-Humidity IndexThe temperature-humidity index numerically describes the humandiscomfort resulting from temperature and moisture. It is computedby adding dry- and wet-bulb temperature readings, multiplyingthe sum by 0.4, and adding 15. Summer estimates indicate about10 percent of the populace are uncomfortable before the indexpasses 70, more than 50 percent are uncomfortable after it passes 75,and almost all are uncomfortable at 80 or above.

Total Heat ContentTotal heat content is the amount of heat energy stored in the gaseousair and water vapor, measured in British thermal units (Btu). All thedifferent methods of treating air can be pictured on the psychromet-ric chart. The magnitude of the changes required can be determinedby plotting the conditions of the air entering and leaving the appa-ratus, and then connecting the two points by a straight line.

Psychrometric ChartsA psychrometric chart is a graphical representation of the funda-mental mathematical relationship dealing with the thermodynamicproperties of moist air. The various charts shown in the figures on thefollowing pages will be found valuable in making air-conditioningcalculations. The chart readings can be obtained for dry-bulb,wet-bulb, and dew-point temperatures and other heat-treatmentproperties.

Three basic formulas can be utilized to determine (1) the totalheat to be removed from both the gases and water vapor; (2) thesensible heat removed from the air only; and (3) the latent heatremoved in condensing the water vapor:

Total heat = ft3/min 4.5 (h1 h2) = Btu/hrSensible heat = ft3/min 1.08 (DB1 DB2) = Btu/hrLatent heat = ft3/min 68 (WB1 WB2) = Btu/hr



Psychrometry 19

where the following is true:

h1 = total heat for entering air (Btu/lb)h2 = total heat for leaving air (Btu/lb)DB1 = entering air for dry-bulb temperatureDB2 = leaving air for dry-bulb temperatureWB1 = entering air for wet-bulb temperatureWB2 = leaving air for wet-bulb temperature

Most air-conditioning calculations are made assuming normalsea-level pressure and standard temperatures, but occasionally thealtitude or high-temperature industrial applications will requireconverting from standard conditions to those actually present. UseTable 1-1 (Chapter 1) to obtain corrected atmospheric pressure.

Psychrometric Chart InstructionsIn Figure 2-2, note the point encircled in the center of the chart,which represents a condition of 80F dry-bulb temperature and50.9 percent relative humidity. The lines on the chart sloping up-ward to the left represent constant wet-bulb temperature, which,in this case, is 67F. The moisture content of the encircled point is78 grains per pound of dry air. The total heat in Btu per pound, asnoted, is scaled at the extreme right. The heat removed in the coolingof air can be obtained by finding the difference in the total heat cor-responding to the wet-bulb temperature at the beginning and thefinal conditions. For example, if air is to be cooled from a conditionof 65F dry-bulb temperature and 60 percent relative humidity, thetotal heat (from Figure 2-2) is 24.5 Btu/lb of dry air.

When calculating the amount of refrigeration required for a par-ticular job, it is generally necessary to make allowances for the var-ious conditions, such as heat losses caused by leakage (infiltration),the heating effect caused by electrical apparatus, and body heat frompersons, which exists in theaters and auditoriums where a large num-ber of persons are gathered (Figure 2-3).

Calculation ExamplesIt is now possible to proceed with illustrative examples of a generalnature so that the solutions can be used on other problems if otherdetails are known.



27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49

19 20 21 22 23 2425 26

12 13 14 15 16 17 18

30

35

40

45

50

55

60

65

70

75

80

85

90

95

100

105

110

12.5 CU. FT

13.0 CU. FT

13.5 CU. FT

14.0 CU. FT

90% RELATIVE. HUMIDITY

80%70%

60%50%

40%

30%

20%10%

14.5 CU. FT

CONSTANT

VOLUME C

U. FT PER

LB OF DRY

AIR

CONS

TANT

TEM

PERA

TURE

WET

-BUL

B F

CONSTAN

T RELATIVE; HUMIDITY

85

90

95

100

105

110W

ET-B

ULB

& D

EW-P

OIN

T TE

MPE

RATU

RES

DRY-

BULB

TEM

PERA

TURE

F

ENTHALPY BTU. PER LB OF DRY AIR

DEW POINT 20

0 10 20 30 40 50 60 70 80 90 100 110

.95SENSIBLE HEAT RATIOGRAINS OF MOISTURE PER LB. OF DRY AIR

.90 .85 .80 .75 .70 .65 .60 .55 .50 .45 .40

120 130 140 150 160 170 180

25 30 35 40 45 50 55 60 65 70 75 80

35

40

45

50

55

60

65

70

75

80

Figure 2-2 Psychrometric charts. (Courtesy Acme Industrial, Inc.)

80F DB

65F WB

45% RH

57 DP68 GR

Figure 2-3 Simplifiedpsychrometric chart showingthe method of obtainingreadable values from chart inFigure 2-2.

20



Psychrometry 21

Example 1Given 80F dry-bulb and 65F wet-bulb temperature, find the dewpoint, relative humidity, and grains of moisture per pound of dry air.

SolutionFigure 2-4 shows the solution schematically. The intersection onthe chart of the two temperature conditions shows a dew point of57.0 and a relative humidity of 45 percent. There are 68 grains ofmoisture/lb of dry air.

DRY-BULBTEMPERATURE

WET

-BUL

B

TEM

PERA

TURE GRAIN

SDEW

POINT

HUMIDITY

VOLUME

TOTAL HEAT PER

POUND OF DRY AIR

SENSIBLE HEAT RATIO

SATURATIONLINE

RELATIVE

Figure 2-4 Schematic solutionfor Example 1.

Example 2Find the total heat load for cooling 6000 ft3/min of air entering at83F dry-bulb and 70F wet-bulb, and leaving at 60F dry-bulb and58F wet-bulb.

SolutionFigure 2-5 shows the schematic solution. Locate the entering andleaving conditions on chart. Read the total heat of 34.1 Btu/lb forentering air and 25.2 Btu/lb for leaving air. The total heat may beobtained by substitution of values:

Total Heat = ft3/min 4.5(h1 h2)= 6000 4.5(34.1 25.2)= 240,300 Btu/hr (approx.)



22 Chapter 2

60F DB

WB

WB58F

70F

83F DB

25.2 h 2

34.1 h 1

Figure 2-5 Schematicsolution for Example 2.

Example 3Find the apparatus dew point for air entering at 82F dry-bulb and68F wet-bulb, and leaving at 59F dry-bulb and 57F wet-bulb.

SolutionFigure 2-6 shows the schematic solution. By extending the line toconnect the entering and leaving conditions of air to the saturationcurve, the apparatus dew point or average surface temperature ofthe coil can be found. Following this procedure, the apparatus dewpoint is 54.5.

57F WB

54.5 A DB

59F DB 82F DB

68F WB


Example 4Assume that air is entering at 81F dry-bulb and 69F wet-bulb, andleaving at 56F dry-bulb and 54F wet-bulb. If the total heat loadis 460,000 Btu/hr, what is the sensible heat factor?



Psychrometry 23

SolutionFigure 2-7 shows the schematic solution. By using the sensible-heatfactor (as noted on the chart), the ratio of the sensible heat to thetotal load can easily be found. First, locate the entering and leav-ing conditions and draw a straight line connecting the two pointson the chart. Next, locate the sensible-heat ratio reference pointon the chart for 81F dry-bulb and 69F wet-bulb. Draw a linethrough the reference point parallel with the line representing theentering and leaving conditions. Extend the line and read 0.54sensible-heat factor. The sensible heat load equals 0.54 460,000,or 248,400 Btu/hr.

54F WB

56F DB 81F DB

69F WB

0.54

SEN

SIBL

E HE

AT F

ACTO

R Figure 2-7 Schematicsolution for Example 4.

Example 5Find the dry-bulb and wet-bulb temperatures for an air mixture of7500 cubic feet per minute (ft3/min) of inside air at 76F dry-bulband 63F wet-bulb, mixed with 2500 ft3/min of outside air enteringat 95F dry-bulb and 75F wet-bulb.

SolutionWith reference to the schematic solution in Figure 2-8, the followingchart readings will indicate the procedure. Locate the conditions forinside and outside air on the chart. Draw a line connecting the twopoints. The ratio of inside to outside air is 2:1. Divide the line intofour equal parts. Count up one equal part of the line from the insideair condition. This point represents the conditions of the air mixture,which are 80.9F dry-bulb and 66.4F wet-bulb.



24 Chapter 2

66.4F WB63F WB

76.6

F

DB

80.9

F

DB

1

3


Example 6It is desired to install a central air-conditioning system in a 3000-seat theater for outside conditions of 90F dry-bulb and 60 percentrelative humidity, and inside conditions of 80F dry-bulb and 63Fdew-point temperature. The heating effect caused by the electriclights is 1500 Btu/min, and the heat leakage through the walls, ceil-ing, and so on, is 3000 Btu/min. The sensible heat is assumed to be7.0 Btu/min per person, and the amount of water vapor emitted is18.9 grains. To summarize, the heating values are as follows:

Sensible heating effect of audience = 7 3000 = 21,000 BtuHeat leakage = 3000 BtuHeating effect due to illumination = 1500 BtuHeat equivalent due to fan (est. 45 hp) = 1900 BtuTotal heating effect per minute = 27,400 Btu

SolutionIf the specific heat of air at 64.7F and saturated with moisture istaken as 0.2472 and the temperature rise of the air is 15.3F, theheat capacity of the air will be 15.3 0.2472 = 3.78 Btu. The totalamount of air to be circulated by the fan to absorb 27,400 Btu/minis 27,400 3.78, or 7250 lb, approximately. The volume of 1 lbof air saturated with moisture at 64.7F is 13.5 ft3. Thus, the totalcapacity of the fan per minute is 7250 13.5, or 97,875 ft3, andthe amount supplied per person is 32.6 ft3/min.

From the preceding discussion, it will be noted that the amountof water vapor emitted is 18.9 3000 7000, or 8.1 lb/min. Therefrigeration required to condense this amount will be 8.1 1061,or 8600 Btu, making a total of 36,000 Btu/min to be removed from



Psychrometry 25

the air if all of it is to be recirculated. If only one-half is to be recir-culated and the other half is to be exhausted into the atmosphere,there will be 18,000 Btu/min of refrigeration required for the air tobe recirculated. The fresh air will enter the spray chamber at 90Fand 60 percent relative humidity, which corresponds to a dew-pointtemperature of 74.2 and a moisture content of 127 grains/lb of dryair. The refrigeration required to cool this air to the temperature ofthe spray chamber, taken from the total heat curve, is 41.3 29.3,or 12.0 Btu/lb, and the total refrigeration is 7250 2 12.0, or43,500 Btu. The total demand on the refrigeration equipment be-comes 43,500 + 18,000, or 61,500 Btu. Adding 5 percent for safety,the amount becomes 64,600 Btu/min, or 323.0 tons of refrigeration.

Comfort ChartBecause of the possibility of shock to the human system, it is notdesirable to have too great a difference between indoor and outdoortemperatures, particularly during the summer season. Some of theindoor temperatures given in Table 2-1 may seem high at first, butthey have been selected as the result of many tests and can be consid-ered reasonably correct. Deviations from this table must be madefor climatic characteristics of the various sections of the countryand the length of time spent in the conditioned area. Where severalhours are spent in the conditioned area, the temperatures shown inthe table are in order.

Table 2-1 Recommended Air Velocities in Ducts

Outside Desirable Indoor Desirable TemperatureTemperature Temperature Reduction

100 82 18

95 80 15

90 78 12

85 76.5 8.5

80 75 5

75 73.5 1.5

70 72

It should be remembered that, from the standpoint of physicalcomfort, low humidity with high temperature is comparable to highhumidity with lower temperatures. From this, it is apparent thatto obtain the greatest physical comfort in cold weather withouthigh temperatures, it is necessary to increase the humidity of the



26 Chapter 2

room conditioned. It also explains why persons in some parts of thecountry do not suffer unduly even though the outside air tempera-tures are 120F or more but with low relative humidity.

50

50

55

60

65

65

60

55

75

80

85

90%

1009080

7060

4030

2010

1020

3040

5060

7080

90100

80%

70%

60%

50%

40%

30%

20%

10%

70

70

75

40

50

60

70

80

90

60 70 80 90 100

DRY-BULB TEMPERATURE F

WET

-BUL

B TE

MPE

RATU

RE

F

WINTER

COMFOR

T

ZONE

SUMM

ER

COMF

ORT

ZONE

AVERAGE WINTER COMFORT ZONE

AVERAGE SUMMER COMFORT ZONEOPTIMUM WINTER COMFORT LINE

OPTIMUM SUMMER COMFORT LINE

RELA

TIVE H

UMIDI

TY

PERCENT OFSUBJECTS FEELING

COMFORTABLE

55 E

FFEC

T. TE

MP.

LIN

E

PERCENT OFSUBJECTSFEELINGCOMFORTABLE

AIR MOVEMENT OR TURBULENCE 15 TO 25 FT PER MIN.

50

Figure 2-9 Comfort charts. (Courtesy American Society of Heating & Ventilating Engineers.)

With reference to the comfort chart in Figure 2-9, it should benoted that both summer and winter comfort zones apply to inhab-itants of the United States only. Application of the winter comfortline is further limited to rooms heated by central systems of theconvection type. The line does not apply to rooms heated by radi-ant methods. Application of the summer comfort line is limited tohomes, offices, and so on, where the occupants become fully adaptedto the artificial air conditions. The line does not apply to theaters,department stores, and so on, where the exposure is less than three



Psychrometry 27

hours. The optimum comfort line shown pertains to Pittsburgh andother cities in the northern portion of the United States and South-ern Canada at elevations not in excess of 1000 feet above sea level.An increase of approximately 1F should be made for every 5 re-duction in north latitude.

SummaryWater vapor is a gaseous form of water at a temperature belowits boiling point. At certain temperatures and barometric pressures,water is extremely unstable in either gaseous or liquid form. Theair becomes humidified when moisture is added and is dehumidifiedwhen moisture is removed. An excess or deficiency of moisture hasa very noticeable effect on comfort.

Relative humidity may be defined as the ratio of the quantity ofmoisture actually present in the air to the greatest amount possibleat any given temperature. The relative humidity of the air at anygiven temperature can be obtained merely by dividing the amountof moisture actually in the air by the amount of moisture the aircan hold at that temperature. Relative humidity is measured by aninstrument called the sling psychrometer.

Grains of moisture is the weight of water vapor present in 1 cubicfoot of air. A grain is the basic unit of the English weight system, and7000 grains = 1 lb. One grain of moisture weighs 1/7000 (0.000142)pound.

The saturation temperature for any given quantity of water va-por in the atmosphere is known as the dew point. For a given at-mospheric pressure, it is the temperature at which moisture beginsto change into the form of tiny water droplets or dew. Total heatcontent is the amount of heat energy stored in the gaseous air andwater vapor, and is measured in British thermal units (Btu).

When calculating the amount of refrigeration required for a par-ticular job, it is generally necessary to make allowances for variousconditions, such as heat loss from infiltration, heat from electricalapparatus, and body heat. Most air-conditioning calculations aremade assuming sea-level pressure and standard temperature.

Review Questions1. What is meant by the term psychrometry?2. State the amount of water vapor present in the air during hot,

humid weather.3. What is the purpose of humidification and dehumidification

of air?



28 Chapter 2

4. What is meant by the term relative humidity?5. How is relative humidity measured?6. Define dew point, effective temperature, and dry- and wet-bulb

temperature.7. What is meant by wet-bulb depression?8. What is the function of the psychrometric chart in making

air-conditioning calculations?9. Describe the construction and use of the sling psychrometer.

10. If it is assumed that the dry- and wet-bulb temperatures register80F and 65F, respectively, what is the temperature humidityindex?



Chapter 3Heat LeakageThe thermal properties of building materials affect the design of theair-conditioning and heating systems. The rate of heat flow throughwalls, floors, and ceilings is usually the basis for calculating the heatand/or cooling load required for a particular building or space.

In addition, consideration must be given to air leakage that takesplace through various apertures, which must be properly evaluated.This air leakage takes place through cracks around windows anddoors, through walls, fireplaces, and chimneys. Although the leak-age or air infiltration through fireplaces and chimneys may be con-siderable, it is usually neglected since the dampers should be closedduring periods of extremely hot weather.

In this connection, you should note that, because of the numer-ous variables affecting air-conditioning loads, it is often difficult tocalculate precisely the required size of an air-conditioning unit for aparticular building or space. This will easily be realized when con-sideration is given to the fact that most of the components of thecooling load vary greatly during a 24-hour period. Economic con-sideration should thus be the determining factor in the selection ofequipment for cooling-season operation in comfort air conditioning,since available weather and other necessary data vary for differentlocations.

Heat leakage is always given in Btu per hour per degree Fahren-heit temperature difference per square foot of exposed surface(Btu/ft2/hr/F). Prior to designing an air-conditioning system, an es-timate must be made of the maximum probable heat loss for eachroom or space to be cooled. Therefore, before attempting to placeeven a small room cooler into service, check the walls and determinejust what size unit will do a good job of cooling and dehumidifying.

Heat losses may be divided into two groups: Losses through confining walls, floors, ceilings, glass, or other

surfaces Infiltration losses caused by leaks through cracks and crevices

around doors and windows

The heat leakage through walls, floors, and ceilings can be deter-mined by means of a formula and depends on the type and thicknessof the insulating material used. The formula for heat leakage is asfollows:

H = K A (t1 t2)

29



30 Chapter 3

where the following is true:

H = the heat requiredK = heat-transfer coefficient, Btu/ft2/hr/FA = area, ft2t1 t2 = temperature gradient through wall, F

Calculation ExamplesA real-world example shows how this formula works.

ExampleCalculate the heat leakage through an 8-inch brick wall having anarea of 200 ft2 if the inside temperature is 70F and the outsidetemperature is 10F.

SolutionIf it is assumed that the heat-transfer coefficient of a plain brick wallis 0.50, substitution of values in the foregoing equation will be asfollows:

H = 0.50 200 (70 10) = 6000 BtuThe heat leakage through floors, ceilings, and roofs may be

estimated in the same manner. The K (heat-transfer coefficient)depends on the construction and particular insulating materialsused.

Reference TablesTables 3-1 to 3-20 cover almost every type and combination of walls,floors, and ceilings encountered in the field. Partition walls are alsoincluded so that these may be estimated for heat leakage. Althoughthe K values given in the tables do not agree entirely with the datagiven in various handbooks, they will give sufficiently close valuesfor adaptation since, in most instances, only approximate values canbe obtained. In making use of the data, remember that the outsidearea is used as a basis for estimating. Ceiling and floor constructionmust also be determined, and, in some situations, the four walls ofthe room may not be of the same construction (since one or moreof the walls may be partitions).



Heat Leakage 31

X

YK = X + Y

X = WALL

Y = INTERIOR CONSTRUCTION

Table 3-1 Concrete Wall (No Exterior Finish)Valuesof K in Btu/ ft2/hr/1F

Thickness (X)

6 8 10 12 16Wall Construction (Y) inches inches inches inches inches

Plain wallno interior finish 0.58 0.51 0.46 0.41 0.341/2-inch plasterdirect on concrete 0.52 0.46 0.42 0.38 0.321/2-inch plaster on wood lath,

furred0.31 0.29 0.26 0.24 0.22

3/4-inch plaster on metal lath,furred

0.34 0.32 0.29 0.27 0.24

1/2-inch plaster on 3/8-inchplasterboard, furred

0.32 0.30 0.27 0.25 0.22

1/2-inch plaster on 1/2-inch boardinsulation, furred

0.21 0.20 0.19 0.18 0.17

1/2-inch plaster on 1-inchcorkboard, set in 1/2-inch cement

0.16 0.15 0.14 0.14 0.13

1/2-inch plaster on 11/2-inchcorkboard, set in 1/2-inch cement

0.15 0.14 0.14 0.13 0.12

1/2-inch plaster on 2-inchcorkboard, set in 1/2-inch cement

0.13 0.12 0.11 0.10 0.09

1/2-inch plaster on wood lath on2-inch fur, 15/8-inch gypsum fill

0.20 0.19 0.18 0.17 0.16



32 Chapter 3

EXTERIOR 1 STUCCO ON WIRE MESH

ZX Y

X = CONCRETE

Y = INSULATION

Z = EXTERIOR STUCCO 1

K = X + Z + Y

Table 3-2 Concrete Wall (Exterior Stucco Finish)Values ofK in Btu/ ft2/hr/1F

Thickness of Concrete (X)

Wall 6 8 10 12 16 18Construction (Y) inches inches inches inches inches inches

Plain wallsno interiorfinish

0.54 0.48 0.43 0.39 0.33 0.28

1/2-inch plaster direct onconcrete

0.49 0.44 0.40 0.36 0.31 0.27

1/2-inch plaster on woodlath, furred

0.31 0.29 0.27 0.25 0.23 0.22

3/4-inch plaster on metallath, furred

0.32 0.30 0.28 0.26 0.24 0.23

1/2-inch plaster on3/8-inch plasterboard,furred

0.31 0.29 0.27 0.25 0.23 0.22

1/2-inch plaster on woodlath on 2-inch furringstrips with 15/8-inchcellular gypsum fill

0.20 0.19 0.18 0.17 0.16 0.15

1/2-inch plaster on1/2-inch boardinsulation, furred

0.22 0.21 0.20 0.19 0.18 0.17

1-inch 0.17 0.16 0.15 0.145 0.140 0.131/2-inch plaster on

11/2-inch sheet corkset in cement

0.145 0.140 0.135 0.130 0.12 0.11

2-inch 0.12 0.118 0.115 0.110 0.105 0.100



Heat Leakage 33

1/2 CEMENT MORTAR

X

Y

4

X = CONCRETE

Y = INTERIOR FINISH

K = X + 1/2 CEMENT + 4 BRICK + Y

Table 3-3 Concrete Wall (Brick Veneer)Valuesof K in Btu/ ft2/hr/1F

Concrete (X)

6 8 10 12Wall Construction (Y) inches inches inches inches

Plain wallno interior finish 0.39 0.36 0.33 0.301/2-inch plaster direct on concrete 0.37 0.34 0.30 0.291/2-inch plaster on wood lath, furred 0.24 0.23 0.21 0.203/4-inch plaster on metal lath, furred 0.27 0.25 0.23 0.221/2-inch plaster on wood lath, on

2-inch furring strips, with 15/8-inchcellular gypsum fill

0.18 0.17 0.16 0.15


0.25 0.24 0.22 0.21


0.17 0.16 0.14 0.12

1/2-inch plaster on 1-inch boardinsulation, furred

0.15 0.14 0.12 0.11


0.13 0.12 0.11 0.10


0.11 0.10 0.09 0.08



34 Chapter 3

Z

Z = 4 CUT STONE

XY

Y = INSULATION

K = X + Y + ZX = CONCRETE

1/2 MORTAR

Table 3-4 Concrete Wall (4-in Cut Stone)Values of K inBtu/ ft2/hr/1F

Thickness of Concrete (X)


Plain wallsno interior finish 0.46 0.42 0.38 0.34 0.301/2-inch plaster direct on concrete 0.42 0.38 0.34 0.32 0.283/4-inch plaster on metal lath, furred 0.29 0.27 0.26 0.24 0.221/2-inch plaster on wood lath,

furred0.28 0.26 0.25 0.23 0.21


0.28 0.26 0.25 0.23 0.21

1/2-inch plaster on wood lath on2-inch furring strips, filled with15/8-inch gypsum

0.18 0.175 0.17 0.16 0.15


0.21 0.20 0.19 0.18 0.17


0.16 0.155 0.15 0.14 0.13

1/2-inch plaster on 1-inch sheetcork, set in 1/2-inch cement ormortar

0.15 0.145 0.140 0.135 0.130

11/2-inch 0.138 0.135 0.130 0.125 0.122-inch 0.115 0.110 0.108 0.105 0.100



Heat Leakage 35

K = X + YX = WALLY = INTERIOR FINISH

Y

X

Table 3-5 Cinder and Concrete Block WallValues of K inBtu/ ft2/hr/1F

Thickness (X) and Kind of Blocks

Concrete Cinder


Plain wallno interior finish 0.46 0.34 0.31 0.231/2-inch plasterdirect on blocks 0.42 0.32 0.29 0.211/2-inch plaster on wood lath, furred 0.28 0.23 0.22 0.173/4-inch plaster on metal lath, furred 0.29 0.24 0.23 0.171/2-inch plaster on 3/8-inch

plasterboard, furred0.28 0.23 0.22 0.17


0.21 0.18 0.17 0.14


0.16 0.14 0.14 0.11

1/2-inch plaster on 11/2-inch sheetcork set in 1/2-inch cement

0.13 0.12 0.12 0.10

1/2-inch plaster on 2-inch sheet corkset in 1/2-inch cement

0.11 0.10 0.10 0.09

1/2-inch plaster on wood lath, on2-inch furring strips, 15/8-inchgypsum fill

0.18 0.16 0.16 0.13



36 Chapter 3

1/2 MORTAR

4

X = WALL & EXTERIORY = INTERIOR FINISH

K FACTORS GIVEN FOR TOTAL WALL THICKNESS(X + 1/2 MORTAR + 4 BRICK + Y )

XY

Table 3-6 Cinder and Concrete Block Wall(Brick Veneer)Values of K in Btu/ ft2/hr/1F

Thickness (X) and Kind of Blocks

Concrete Cinder


Plain wallno interior finish 0.33 0.26 0.25 0.181/2-inch plasterdirect on blocks 0.31 0.25 0.23 0.181/2-inch plaster on wood lath, furred 0.23 0.19 0.18 0.153/4-inch plaster on metal lath, furred 0.24 0.20 0.19 0.151/2-inch plaster on wood lath, on

2-inch furring strips, 15/8-inchgypsum fill

0.16 0.14 0.14 0.12


0.18 0.16 0.15 0.13


0.14 0.13 0.12 0.10


0.23 0.19 0.18 0.15

1/2-inch plaster on 11/2-inch sheetcork, set in 1/2-inch cement mortar

0.12 0.11 0.11 0.09

1/2-inch plaster on 2-inch sheet cork,set in 1/2-inch cement mortar

0.10 0.09 0.09 0.08



Heat Leakage 37

YX

4

1/2 CEMENT MORTAR

K VALUES GIVEN FOR TOTAL WALL THICKNESS:X + 1/2 MORTAR + 4 BRICK + Y

X = WALL & EXTERIORY = INTERIOR FINISH

OUTSIDE

Table 3-7 Brick Veneer on Hollow Tile Wall(Brick Veneer)Values of K in Btu/ ft2/hr/1F

Tile Thickness (X)


Plain wallno interior finish 0.29 0.27 0.25 0.22 0.181/2-inch plaster on hollow tile 0.28 0.25 0.24 0.21 0.171/2-inch plaster on wood lath,

furred0.20 0.19 0.18 0.17 0.15


0.21 0.20 0.19 0.18 0.16


0.20 0.19 0.18 0.17 0.15

1/2-inch plaster on wood lath,on 2-inch furring strips,with 15/8-inch cellulargypsum fill

0.14 0.13 0.12 0.11 0.10

1/2-inch plaster on 1/2-inchboard insulation, furred

0.16 0.15 0.14 0.13 0.12

1/2-inch plaster on 1-inchboard insulation, furred

0.14 0.13 0.12 0.11 0.10

1/2-inch plaster on 11/2-inchboard insulation, furred

0.13 0.12 0.11 0.10 0.09

1/2-inch plaster on 2-inchboard insulation, furred

0.12 0.11 0.10 0.09 0.08



38 Chapter 3

1/2 CEMENT

Z

X Y

K = X + Y + Z X = TILE

Y = INSULATION

Z = 4 CUT STONE

Table 3-8 Hollow Tile Wall (4-in Cut-Stone Veneer)Valuesof K in Btu/ ft2/hr/1F

Thickness of Tile (X)


Plain wallsno interior finish 0.30 0.29 0.28 0.221/2-inch plasterdirect on hollow tile 0.28 0.27 0.26 0.213/4-inch plaster on metal lath, furred 0.22 0.21 0.20 0.181/2-inch plaster on wood lath, furred 0.21 0.20 0.19 0.171/2-inch plaster on 3/8-inch plasterboard,

furred0.21 0.20 0.19 0.17

1/2-inch plaster on 1/2-inch cork boardset in 1/2-inch cement

0.17 0.16 0.15 0.14

1/2-inch plaster on 1-inch corkboard setin 1/2-inch cement

0.14 0.13 0.13 0.12

1/2-inch plaster on 11/2-inch corkboardset in 1/2-inch cement

0.13 0.12 0.11 0.10

1/2-inch plaster on 2-inch corkboard setin 1/2-inch cement

0.11 0.10 0.09 0.08

1/2-inch plaster on wood lath, 2-inchfurring, with 15/8 gypsum fill

0.15 0.14 0.13 0.12

1/2-inch plaster on 1/2-inch boardinsulation

0.18 0.17 0.16 0.15



Heat Leakage 39

YX

K VALUES GIVEN FOR TOTAL WALL THICKNESSX + Y + STUCCO FINISH

X = TILE THICKNESSY = INTERIOR FINISH

EXTERIOR STUCCO

Table 3-9 Hollow Tile Wall (Stucco Exterior)Values of K inBtu/ ft2/hr/1F

Tile Thickness (X)


Plain wall with stucconointerior finish

0.32 0.30 0.28 0.22 0.18

1/2-inch plasterdirect onhollow tile

0.31 0.29 0.27 0.21 0.17

1/2-inch plaster on wood lath,furred

0.22 0.20 0.18 0.16 0.12


0.23 0.21 0.19 0.17 0.13


0.21 0.20 0.18 0.16 0.12

1/2-inch plaster on wood lath, on2-inch furring strips, with15/8-inch cellular gypsum fill

0.16 0.15 0.14 0.13 0.12


0.15 0.14 0.13 0.12 0.11

1/2-inch plaster on 1-inch bo

Documents

Air Conditioning Home and Commercial