0831131632_HVAC_book

THIS HANDBOOK PROVIDES COMPREHENSIVETECHNICAL INFORMATION TO HEATING,VENTILATING, AND AIR CONDITIONING

ENGINEERS, DESIGNERS AND PRACTITIONERS

HVAC:Handbook of Heating,

Ventilation andAir Conditioning for

Design and Implementation

BY

ALI VEDAVARZ, PH.D., PEDeputy Director of Engineering, New York City Capital Projects,

New York City Housing Authority andIndustry Professor, Polytechnic University, Brooklyn, NY

SUNIL KUMAR, PH.D.Professor of Mechanical Engineering and Dean of Graduate School

Polytechnic University, Brooklyn, NY

MUHAMMED IQBAL HUSSAIN, PEMechanical Engineer, Department of Citywide Administrative Services

New York City, NY

2007

INDUSTRIAL PRESS INC.

NEW YORK

COPYRIGHT 2007 by Industrial Press Inc., New York, NY.

Library of Congress Cataloging-in-Publication Data

Vedavarz, Ali.

HVAC: handbook of heating ventilation and air conditioning / Ali Vedavarz, Sunil Kumar, MuhammedHussain.

p. cm.

ISBN 0-8311-3163-2

ISBN13 978-0-8311-3163-0

I. Heating--Handbooks, manuals, etc. 2. Ventilation--Handbooks, manuals, etc. 3. Air conditioning--Handbooks, manuals, etc. 4. Buildings--Environmental engineering--Handbooks, manuals, etc. I. Kumar, Sunil.II. Hussain, Muhammed Iqbal. III. Title.

Printed and bound in the United States of America

All rights reserved. This book or parts thereof may not be reproduced, stored in a retrieval system,or transmitted in any form without permission of the publishers.

TH7011.V46 2006697--dc22

2006041837

Cover Photo: Image published with kind permission of CVRD and Bluhm Engineering.

INDUSTRIAL PRESS, INC.

989 Avenue of the Americas

New York, New York 10018 -5410

1st Edition

First Printing

10 9 8 7 6 5 4 3 2 1

PREFACE ix

This Handbook provides comprehensive technical information in a modular form to heating, venti-lating, and air conditioning (HVAC) designers and practitioners, namely engineers, architects, con-tractors, and plant engineers. It is also a handy reference for students mastering the intricacies of the HVAC rudiments. Each chapter is self-contained to the extent possible and emphasis is placed on graphical and tabular presentations of data that are useful for easy understanding of fundamentals and solving problems of design, installation, and operation.

This Handbook draws upon the material presented in the Handbook of Air Conditioning, Heating,and Ventilating, Third Edition, Industrial Press, which forms the basis of the presentation. New top-ics and chapters have been introduced and previous information updated or rewritten. Examplesusing software solution tools have been added alongside traditional solutions using formulae fromthe handbook. The organization, however, remains, in the literal sense, a handbook.

We gratefully acknowledge the contributors and editors of the aforementioned Handbook of AirConditioning, Heating, and Ventilating, whose knowledge is embedded throughout the presentbook. We did not have the opportunity to meet any of them, but their written legacy has left an indel-ible imprint on the present work.

An important source of information is the ASHRAE (American Society of Heating, Refrigerating,and Air-Conditioning Engineers) repertoire of publications. ASHRAE serves as the authoritative,and occasionally the sole, source of up-to-date HVAC related data and analysis. We acknowledgetheir permission to use material from various publications, especially the latest ASHRAE Handbookseries.

ASHRAE Publications1791 Tullie Circle, NEAtlanta, GA 30329Web Site: www.ashrae.org

We also acknowledge three corporations for supplying us with material for inclusion in the Hand-book. We profusely thank Mr. Michael White of Bell & Gossett (an ITT Division), Mr. Kent Silveriaand Mr. Thomas Gorman of Trane Corporation, and Mr. Steven Boediarto of Preferred Utilities, forfacilitating the acquisition of these materials.

The Bell & Gossett corporation has graciously provided the ESP-PLUS software package toaccompany the Handbook. This software, a $100 value, permits users to select components based ondesign or operating conditions.

Bell & Gossett (ITT Fluid Handling)8200 N. Austin AveMorton Grove, IL 60053 Web Site: www.bellgossett.com

The Trane corporation has generously allowed us to include their Trace Load 700 load calculationlimited capability demonstration version software with the Handbook.

Trane C.D.S. Department3600 Pammel Creek RoadLa Crosse, WI 54601Web Site: www.trane.com

PREFACEx

We are also grateful to the Preferred Utilities corporation for making available their publication onthe topic of combustion analysis, and consenting to let us base our combustion chapter on it.

Preferred Utilities Mfg. Corp31-35 South StreetDanbury, CT 06810Web Site: www.preferred-mfg.com

We acknowledge the input of our good friend, colleague, and HVAC critic, Mr. Naji Raad, whoseexperience in the profession provided a critical review of the manuscript. We thank our editors atIndustrial Press, Mr. Christopher McCauley and Mr. Riccardo Heald, for their editorial input andsuggestions, for reading the manuscript as it developed, and keeping the project on track; and JanetRomano for her cover design and production assistance. We acknowledge the effort of the many stu-dents at Polytechnic University who helped in researching for material, proofreading the manu-script, checking examples, and drawing figures. Those who deserve special recognition are Mr.Saurabh Shah and Mr. Christopher Bodenmiller for the graphics, Mr. Nayan Patel, Mr. Pranav Patel,and Mr. Prabodh Panindre for research, calculations, and proofing. Finally, we thank KathleenMcKenzie, freelance book editor, for her considerable contribution to this Handbooks style, formatand readability.

Every effort has been made to prevent errors, but in a work of this scope it is inevitable that somemay creep in. We request your forgiveness and will be grateful if you call any such errors to ourattention by emailing them to [email protected].

Ali Vedavarz, Sunil Kumar, Muhammed Iqbal Hussain

New York CityDecember 2006

TABLE OF CONTENTS

iii

1. FUNDAMENTALS

11 Fundamentals of Thermodynamics13 Conservation of Mass13 First Law of Thermodynamics14 Second Law, Reversibility, and Possible Processes14 Thermodynamic Cycles16 Fundamentals of Fluid Flow16 Flow in Pipes and Ducts111 Noise from Fluid Flow111 Fundamentals of Heat Transfer114 Overall Heat Transfer115 Fins and Extended Surfaces118 Some Details of Heat Exchange119 Augmentation of Heat Transfer

2. PSYCHROMETRY

21 Psychrometrics21 Ideal Gas Approximation21 Equation of State22 Humidity Ratio22 Relative Humidity 22 Degree of Saturation22 Wet Bulb Temperature23 Partial Pressure of Water Vapor24 Dew Point Temperature24 Saturation24 Enthalpy25 Wet Bulb Temperature26 Properties of Moist Air27 Psychrometric Chart Presentation212 Thermodynamic Properties of Water at Saturation218 Thermodynamic Properties of Moist Air

3. AIR CONDITIONING PROCESSES

31 Introduction31 Heating and Cooling Process32 Cooling with Dehumidification33 Heating with Humidification33 Adiabatic Mixing of Two Air Streams35 Evaporative Cooling35 Heating and Air Conditioning System Cycles

4. INDOOR AIR QUALITY AND VENTILATION

41 Indoor Air Quality41 Ventilation Procedure45 Concentration of Air Pollutants46 Indoor Air Quality Procedure48 Filters410 Hepa Filters410 Carbon Media Filters410 Fiber and Foam Filters410 Ozone410 Ultraviolet Light

5. LOAD ESTIMATING FUNDAMENTALS

51 Conduction51 Thermal Conductivities of Materials52 Convection54 Thermal Radiation54 Emissivities of Some Materials56 Overall Heat Transfer Coefficient58 Parallel Arrangement511 Coefficient of Transmission

(Continued)5. LOAD ESTIMATING FUNDAMENTALS

517 Relative Thermal Resistances of Building Materials518 Surface Conductances and Resistances518 Emittance Values of Various Surafces519 Thermal Resistances of Plane Airspaces521 Thermal Properties of Building and Insulating Materials527 Coefficients of Heat Transmission of Various Fenestrations528 Transmission Coefficients for Wood and Steel Doors529 Outdoor Air Load Components

6. HEATING LOAD CALCULATIONS

61 Introduction 61 Calculating Design Heating Loads62 Heat loss Through Walls, Roofs, and Glass Area62 Heat Loss from Walls below Grade63 Below-Grade Wall U-Factors63 Heat Loss from Basement Floor Below Grade64 Heat Loss Coefficients 64 Heat Loss from Floor Slab On Grade66 Ventilation and Infiltration Heat Loss

7. COOLING LOAD CALCULATIONS

71 Transfer Function Method (TFM)71 Heat Source in Conditioned Space72 Heat Gain from Occupants73 Heat Gain from Cooking Appliances76 Heat Gain from Medical Equipments76 Heat Gain from Computer 76 Heat Gain from Office Equipments76 CLTD/SCL/CLF Calculation Procedure77 Cooling Load by CLTD/SCL/CLF Method78 Roof Numbers79 CLTD for Roofs711 CLTD for Walls727 Code Number for Wall and Roof728 Wall Types731 CLTD for Glass731 Zone Types for CLF Tables731 Zone Types for SCL and CLF Tables735 Residential Cooling Load Procedure736 SCL for Glass744 CLF for People and Unhooded Equipments746 CLF for Hooded Equipments747 Window GLF for Residences749 CLTD for Residences750 SC for Windows750 SLF for Windows750 Air Exchange Rates

8. DUCT DESIGN

81 Introduction81 Pressure Head and Energy Equation82 Friction Loss Analysis87 Dynamic Losses87 Ductwork Sectional Losses88 Fan System Interface88 Pressure Changes System89 Duct System Design89 Design Considerations812 Duct Design Methods813 Duct Design Procedures813 Automated Duct Design814 Duct Fitting Friction Loss Example814 Equal Friction Method Example815 Resistance in Low Pressure Duct System Example815 Static Regain Method Example817 Fitting Loss Coefficients

TABLE OF CONTENTSiv

9. PIPE SIZING

91 Pressure Drop Equations91 Valve and Fitting Losses93 Water Piping93 Flow Rate Limitations93 Noise Generation93 Erosion93 Allowances for Aging94 Water Hammer94 Hydronic System Piping96 Valve and Fitting Pressure Drop928 Service Water Piping929 Plastic Pipe929 Cold Water Pipe Sizing931 Steam Flow in Pipes931 Steam Flow Formulas932 Vertical Pipes932 Steam Piping959 Gas Piping For Buildings959 Residential Piping961 Commercial-Industrial Piping972 Compressed Air Systems972 Compressed Air978 Viscosity of Liquids980 Piping980 Types of Materials991 Plastics Pipe991 Joining Techniques993 Standards for Specification and Identification993 Design Parameters996 Installation997 Codes and Regulations997 Pipe Fittings997 Taper Pipe Thread9132 Laying Lengths of Pipe with Screwed Fittings9134 Allowable Spaces for Pipes9134 Expansion of Pipe9136 Corrosion Resistance9136 Pipe Support Spacing9139 Gate, Globe, and Check Valves9139 Operation9141 Maintenance Methods9142 Formulas for Sizing Control Valves9142 To Determine Valve Size9142 To Determine Valve Capacity9142 For Vapors Other Than Steam9143 Identification of Piping Systems9143 Dangerous Materials9143 Fire Protection Materials and Equipment9144 Safe Materials9144 Protective Materials9144 Method of Identification9144 Heat Losses in Piping9144 Heat Losses from Bare Pipe9145 Heat Losses from Steam Piping9157 Heat Loss from Insulated Pipe9158 Cold Surface Temperature

10. HYDRONIC HEATING AND COOLING SYSTEM

101 Basic System104 Temperature Classifications104 Closed Hydronic System Components Design104 Convectors or Terminal Units104 Boiler104 Air Eliminations Methods106 Pressure Increase Due to Change in Temperature106 Expansion Tank107 Expansion Tank Sizing

(Continued)10. HYDRONIC HEATING AND COOLING SYSTEM

108 Characteristics of Centrifugal Pumps108 Operating Characteristics109 Pump Laws109 Change of Performance1010 Centrifugal Pump Selection1010 Total Dynamic Head1011 Net Positive Suction Head (NPSH)1011 Pumping System1016 Parallel Pumping1017 Series Pumping1018 Design Procedures1018 Preliminary Equipment Layout1019 Final Pipe Sizing and Pressure Drop Determination1019 Final Pressure Drop1019 Final Pump Selection1019 Freeze Prevention

11. ENERGY CALCULATION

111 Degree Day111 65F as the Base112 Application of Degree Days114 Predicting Fuel Consumption115 Predicting Future Needs117 Empirical Constants117 Load Factor and Operating Hours117 Limitations118 Degree-Days Abroad119 Degree Days for Various US Locations

12. COMBUSTION

121 Combustion Basics123 Efficiency Calculations127 Saving Fuel with Combustion Controls1211 Combustion Considerations1211 Pressure and Flow Basic Principles1212 Atomizing Media Considerations1212 Combustion Air Considerations1213 Flue Gas Considerations1214 Gas Fuel Firing Considerations1214 Fuel Oil Firing Considerations1215 Operational Rules of Thumb1216 Common Application1220 Combustion Control Strategies1220 Control System Errors1220 Combustion Control Strategies1221 Parallel Positioning Systems1222 Fully Metered Control1223 Feedwater Control Systems1224 Draft Control1226 Oxygen Trim1227 Combustion Air Flow Control Techniques1228 Flue Gas Recirculation (FGR)1233 Fuel Oil Handling System Design1233 Determination of Required Flow Rate1234 Stand by Generator Loop Systems1234 Multiple Pumps1234 Burner Loop Systems1236 Maximum Inlet Suction1237 Pump Discharge Pressure1237 Piping System Design1237 Pump Set Control System Strategies

TABLE OF CONTENTS v

13. AIR CONDITIONING SYSTEMS

131 Air Conditioning Systems131 Single Package Units135 Single Package Installations137 Installation of Split Systems138 Zoning Unitary Installations1310 Selection Procedure1314 Evaporative Air Conditioning1314 Permissible Air Motion1317 Variable Volume AC System1318 Initial Costs1320 Cooling Considerations1321 Overlapping1322 Heat Recovery1322 Heating Cooling Systems1323 Air Systems1326 Controls1327 Air Water Systems1330 Sources of Internal Heat1331 Heat from Service Refrigeration1331 Exhaust Air Heat Recovery Systems1336 Heat Pumps1336 Reverse-Cycle Principle1336 Coefficient of Performance1337 Heating Season Performance Factor1337 Types of Heat Pumps1338 Air-to-Air Heat Pumps1339 Water-to-Water Heat Pumps1340 Water-to-Air Heat Pumps1340 Air-to-Water Heat Pumps1341 Ground Source Heat Pumps1341 Special Heat Sources1342 Operating and Installation Factors1342 Outdoor Temperature Effects1343 Thermostats1343 Heat Anticipators1344 Equipment Arrangement1344 Electrohydronic Heat Recovery1345 Cooling Cycle1347 System Design1347 Supplementary Heat1347 Optimized Data for Heat Pump1348 Development of Equations1348 Development of Tables1349 Selecting Air Handling Units1354 Well Water Air Conditioning1354 Heat Pump/Solar Energy Application1354 System Description and Operation1360 High Velocity Dual Duct Systems1360 Advantages and Disadvantages1360 Dual Duct Cycles1365 Duct Sizing Technique1365 Large vs. Small Ducts1366 Design Velocity1366 Maximum Velocity1367 Sizing High Pressure Ducts1368 Return Air Ducts1368 Low Pressure Ductwork1369 Basic Arrangement1369 Zoning1370 Ceiling Plenum1373 Modular Type Office Buildings1376 Constant Volume Mixing Units1377 Apparatus Floor Area1380 Construction Details1381 Automatic Control Applications1381 Rooftop Multizone Units1384 Multizone Unit Control1388 Damper Control1388 Economizer Control Cycle1388 Unit Ventilator Control

(Continued)13. AIR CONDITIONING SYSTEMS

1391 Hot Water System Control1394 Mixing Box Control1395 Rotary Air-to-Air Heat Exchanger Control1395 Automatic Control for Dual Duct System1397 Winterizing Chilled Water System1397 Water Circulation to Prevent Freeze-Up1399 Mechanical Draft Cooling Towers13102 Atmospheric Cooling Towers13104 Quantity of Cooling Water Required13105 Roof is a Location for AC Equipment13105 Advantages of Roof13106 Disadvantages of Roof13107 Servicing Cooling Plant13107 Servicing Cooling Plant for Summer Use13107 Water System13107 Air Handling System13107 Compressor Oil13107 Condenser13108 Refrigeration Unit13108 Check Oil13108 Compressor 13108 Air Conditioning Equipment Maintenance13108 Air Handling Equipment13108 Air Distribution Equipment13108 Water-Using Equipment13108 Cooling Equipment13110 Air Conditioning Maintenance Schedule13111 Unit Air Conditioners13111 Central Systems13111 Condensing Water Circuit13112 Cooling Water System13112 Filters and Ducts13112 Air Conditioning Maintenance Procedure13112 Refrigerant Circuit and Controls13113 Condensing Water Circuit13113 Cooling Water System13113 Filters and Ducts13114 Rotating Apparatus13114 Unit Air Conditioners13114 Checklist for Air Conditioning Surveys

14. AIR HANDLING AND VENTILATION

141 Terminology, Abbreviations, and Definitions143 Fan Laws1411 Fan Performance Curves1416 Class Limits for Fans1421 Fan Selection1426 Fan Inlet Connections1427 Fan Discharge Conditions1431 Useful Fan Formulas1432 Nomographs for Fan Horsepower1432 Monographs for Fan Horsepower and Actual Capacity1434 Fan Selection Questionnaire1437 Air Flow in Ducts1440 Pitot Traverse1440 Friction Losses1440 Correction for Roughness1440 Rectangular Duct1452 Air Balancing and Air Turning Hardware1456 Air Distribution1456 Fire Dampers and Fire Protection1456 Duct System Design1459 High Velocity System Design1468 Step by Step Design1468 Main Duct1470 Branch Trunk Ducts1471 Single Branch Lines1472 Duct Design by Computer1473 Fibrous Glass Duct Construction

TABLE OF CONTENTSvi

(Continued)14. AIR HANDLING AND VENTILATION

1475 Determining Required Air Volume1475 Estimating Weight of Metal 1477 Apparatus Casing Construction1477 Condensate Drains for Air Conditioning Units1478 Air Filters and Dust Collectors1478 Air Filters1479 Dust Collectors1482 Dry Centrifugal Collectors1482 Wet Collectors1482 Fabric Collectors1483 Electrostatic Precipitators1483 Breeching Design and Construction1483 Expansion1484 Aerodynamics1485 Access1485 Round Breeching Construction1485 Rectangular Breeching Construction1490 Chimney Draft and Velocities1492 Forced Draft and Draft Control1494 Sizing of Large Chimneys1495 Chimney Design and Construction1496 Balancing Small Air Conditioning Systems1497 Balancing Medium and Large Systems1498 Balancing Duct Distribution1498 Balancing Systems Using Booster Fans1499 Air Balancing by Balancing and Testing Engineers

15. STEAM HEATING SYSTEM DESIGN

151 Large Systems151 Equivalent Direct Radiation151 Piping Connections to Boilers153 Direct Return Connection153 Common Return Header153 Two Boilers with Common Return Header and Hartford

Connection154 Two Boilers with Separate Direct Return Connections from

Below154 Separate Direct Return Connections154 Connections to Steam Using Equipment1526 Piping Application1530 Industrial and Commercial Steam Requirements1539 Flash Steam Calculations1540 Sizing of Vertical Flash Tanks1540 To Size Flash Tank1541 To Size Float Trap1541 Airbinding1546 Estimating Friction in Hot Water Piping1549 Hot Water Heating Systems1549 Service Water Heating1549 Operating Water Temperature1549 Air Removal from System1549 Water Flow Velocity1549 Prevention of Freezing1549 Water Circulation below Mains1549 Limitation of Pressure1550 System Adaptability1550 Use of Waste Steam Heat1550 Heat from District Steam System1550 Summer Cooling1550 Types of Water Heating Systems1552 Design Recommendations for Hot Water Systems1552 Water Velocity1552 Pump Location1552 Air Venting1553 Balancing Circuits1553 Filling Pressure1553 Preventing Backflow1553 Connecting Returns to Boiler1553 Locating the Circulating Pump

(Continued)15. STEAM HEATING SYSTEM DESIGN

1553 Sizing the Expansion Tank1554 Compressed Air to Reduce Tank Size1554 Piping Details1555 Design of Piping Systems1558 Design of Two Pipe Reversed Return System1558 Final Check of Pipe Sizes1558 Design of Two Pipe Direct Return System1559 Piping for One-Pipe Diversion System1559 Sizing Piping for Main1559 Sizing Piping for Branches1559 Pipe Size Check1560 Piping for One-pipe Series System1560 Combination of Piping Systems1560 Sizing Hot Water Expansion Tanks1560 Conditions Affecting Design1561 Sizing Hot Water Expansion Tanks1561 High Temperature Water Systems1563 High Temperature Drop1563 Heat Storage1563 Limitation of Corrosion1563 Pressurization of HTW System1563 Steam Pressurization1564 Gas Pressurization1564 Air Pressurization1564 Nitrogen Pressurization1565 Expansion Tanks1565 Expansion Conditions1565 Determining Expansion Tank Size1565 Location of Steam Pressurizing Tank1566 Nitrogen Pressurizing Tanks1566 Application of HTW for Process Steam1566 Circulating Pumps1567 Pumps for HTW Systems1567 Manufacturers Information1567 Pump Specifications1568 Net Positive Suction Head1568 Effect of Cavitation Within Pump1568 Pump Construction for HTW Systems1568 Circulating Pump Seals1569 Boiler Recirculating Pump1569 Boilers for HTW Systems1569 Boiler Emergency Protection1569 Pipe, Valves, and Fittings for HTW Systems1569 Valve Installation1570 Welded Joints1570 Venting of Piping1570 Effect of Load Variation on Operation1571 Pipe Sizing for HTW Systems1573 Ratings of Steel Boilers1573 Ratings1574 Ratings for Steel Boilers1576 Stack Dimensions1581 Heating and Cooling Media1581 Brine1581 Glycerine1581 Glycol1581 Other Media1582 Warm Air Heating1582 Early Types1583 Current Types1585 Furnace Performance1585 Testing and Rating of Furnaces1586 Acceptable Limits1587 Selection of Furnace1587 Rule for Selection1587 Blower Characteristics1588 Blower Sizes1588 Duct System Characteristics1588 Trends1589 Warm Air Registers1590 Return Air Intakes

TABLE OF CONTENTS vii

(Continued)15. STEAM HEATING SYSTEM DESIGN

1591 Arrangement of Furnace and Ducts1594 Basic Thermostatic Controls1594 Continuous Air Circulation1595 Continuous Blower Operation1595 Intermittent Blower Operation15109 Steam Supplied Unit Heater15109 Gas Fired Radiant Heaters15112 Sizing of Steam Traps15117 Unit Heaters15124 Checklist for Heating System Servicing

16. NOISE AND VIBRATION CONTROL

161 Noise and Vibration161 Definitions and Terminology162 Noise Criteria162 Speech Interference Criteria162 Sound Levels of Sources167 Ratings and Standards167 Airborne Sound Transmission167 Vibration Isolation169 Isolation Mount Selection1613 Airborne Noise Through Ducts1613 Regenerated Noise1613 Other Mechanical Noise Sources1614 Calculation of Sound Levels from HVAC Systems1614 Description of Decibels1614 Addition of Decibels1615 The Sabin1617 Determination of Sound Pressure Level1620 Noise in Ducted Systems1623 Fan Noise Generation1623 Estimating Fan Noise1624 Distribution of Sound Power at Branch Takeoffs1624 Attenuation of Untreated Duct1624 Duct Lining Attenuation1625 Sound Attenuation of Plenums1626 Duct Lining and Elbows1627 Open End Reflection Loss1627 Air Flow Noise1631 Flow Noise Generation of Silencers1631 Sound Transmission Through Duct Walls1632 Calculation of Sound Levels in Ducted Systems1636 Control of Cooling Tower Noise1636 Fan Noise1637 Water Noise1637 Drive Components1637 External Noise Sources1638 Configuration Factors1639 Location1639 Reducing Sound Generated1639 Half-Speed Operation1639 Oversizing of the Tower1639 Changing Leaving Conditions1640 Sound Absorbers1640 Obtaining Desired Sound Levels1640 Acoustical Problems in High Velocity Air Distribution1640 System Noise 1642 Air Handling Apparatus Rooms1642 Selection of Fan Isolation Bases1642 Apparatus Casings1642 Dampers and Air Valves1643 Flexible Connectors1643 Air Distributing Systems1643 Duct Velocities1643 Choice of Duct Design Method1643 Ductwork Adjacent to Apparatus Room1644 Duct Connections to Apparatus Casings1644 Type Duct Construction1644 Fittings for High Velocity Ductwork

(Continued)16. NOISE AND VIBRATION CONTROL

1645 Take-off Fittings1645 Dual Duct Area Ratio1646 Dampers as a Noise Generating Source1646 Sound Barrier for High Velocity Ductwork1646 Sound Traps1646 Cross Over of Horizontal Dual Duct Mains1647 Testing of High Pressure Ductwork1647 Terminal Devices1648 Radiation Protection at Wall Openings for Duct or Pipe1649 Medical Installations

17. MOTORS AND STARTERS

171 NEMA Motor Classifications171 Locked Rotor Torque172 Classification of Single-Phase, Induction Motors by Design

Letter172 Torque, Speed, and Horsepower Ratings for Single-Phase

Induction Motors173 Classification by Environmental Protection and Method of

Cooling173 Standard Voltages and Frequencies for Motors176 The National Electrical Code176 Grounding178 Motor and Load Dynamics, and Motor Heating178 Torque Speed Relationships179 Torque, Inertia, and Acceleration Time1710 Dynamics of the Motor and the Load1711 Motor Heating and Motor Life1712 Rotor Heating During Starting1712 Single Phase Motors1712 Types of Motors1715 Repulsion-Induction1715 Large Single-Phase Motors1716 Application1717 Loading1718 Motor Protection1718 Motor Selection1718 Analysis of Application1719 Polyphase Motors1719 Enclosure1720 Bearings1720 Quietness1720 Polyphase, Squirrel Cage Induction Motors1721 Speed Control1721 Two-Speed Polyphase, Squirrel Cage Induction Motors1721 Two Speed Motors Come in Two Types1723 Wound-Rotor Polyphase Induction Motors1724 Variable Speed1724 Synchronous Motors1725 Hermetic Type Motor Compressors1725 Hermetic Compressors to 5 hp1729 Starters1729 Motor Controllers1729 Overcurrent Protection1730 Overload Protection1731 Starters for Large AC Motors1733 Winding and Reduced-voltage Starting1733 Electric Utility Limitations1733 Minimizing Mechanical Shocks1733 Application1736 Types of Starters1736 Open Circuit Transition1737 Advantages and Disadvantages1739 Useful Formulas1739 Electric Motor Maintenance

TABLE OF CONTENTSviii

18. DESIGN PROCEDURE, ABBREVIATIONS, SYMBOLS

181 Design Procedure181 Contract and Mechanical Drawings181 HVAC Drawings181 Floor Plans185 Valve Symbols186 Piping Symbols187 Pipe Fittings Symbols188 Abbreviations for Scientific and Engineering Terms189 Lists of Abbreviations and Symbols

19. CLIMATIC DESIGN INFORMATION

191 Climatic Design Conditions191 Applicability and Characteristics of the Design Conditions1927 Dry Bulb and Wet Bulb Temperature for US Locations

20. UNITS AND CONVERSIONS

201 U.S. Customary Unit System201 Linear Measures201 Surveyor's Measure201 Nautical Measure201 Square Measure201 Cubic Measure201 Shipping Measure202 Dry Measure202 Liquid Measure202 Old Liquid Measure202 Apothecaries' Fluid Measure202 Avoirdupois or Commercial Weight202 Troy Weight, Used for Weighing Gold and Silver202 Apothecaries' Weight202 Measures of Pressure203 Miscellaneous

(Continued)20. UNITS AND CONVERSIONS

204 U.S. System And Metric System Conversion204 Length and Area204 Mass and Density205 Volume and Flow206 Force, Energy, Work, Torque and Power Conversion207 Velocity and Acceleration208 Metric Systems Of Measurement208 Measures of Length208 Square Measure208 Surveyors Square Measure208 Cubic Measure208 Dry and Liquid Measure208 Measures of Weight2010 Binary Multiples2010 Terminology of Sheet Metal

21. INDEX

FUNDAMENTALS OF THERMODYNAMICS

11

FUNDAMENTALS

This chapter covers the fundamentals of thermody-namics, fluid mechanics, and heat transfer as related to thetheory and practice of air conditioning, heating and venti-lation. Basic concepts needed for the HVAC professionalare presented, while advanced topics are not considered.Excellent resources, such as the ASHRAE Handbookseries and many great textbooks in the areas of thermalfluid sciences, treat the details and advanced topics thathave been omitted from this focused chapter.

Fundamentals of Thermodynamics

Thermodynamics is the study of energy, its transfor-mations, and its relation to states of matter or substance.Thermodynamics deals with equilibrium conditions thatare typical of steady state, and any changes are consideredto be quasi-equilibrium processes where change occursslowly and incrementally so as to allow each incrementalintermediate state to reach equilibrium before it advancesfurther.

A pure substance has a homogeneous and invariablechemical composition. It can exist in more than onephase, but chemical composition is the same in all phases.If a substance exists as liquid at saturation temperatureand pressure, it is called a saturated liquid. If the tempera-ture of the liquid is lower than the saturation temperaturefor the existing pressure, it is called either a subcooled liq-uid (the temperature is lower than the saturation tempera-ture for the given pressure) or a compressed liquid (thepressure is greater than the saturation pressure for thegiven temperature). When a substance exists as part liquidand part vapor at the saturation temperature, its quality isdefined as the ratio of the mass of vapor to the total mass.Quality has meaning only when the substance is in a satu-rated state; i.e., at saturation pressure and temperature. Ifa substance exists as vapor at the saturation temperature,it is called saturated vapor. The term dry saturated vaporis used to emphasize that the quality of the substance is100%. When the vapor is at a temperature greater than thesaturation temperature, it is superheated vapor. The pres-sure and temperature of superheated vapor are indepen-dent properties, since the temperature can increase whilethe pressure remains constant. Gases are highly super-heated vapors.

A property of a substance is any observable character-istic of the material. An intrinsic property of the materialis one that does not depend on the shape or volume ormass of the material. Such properties are pressure andtemperature, or properties that are usually expressed inthe units of per unit mass, such as specific volume. This isin contrast with extrinsic properties which depend on the

actual quantity of the material under consideration, suchas volume. The thermodynamic state of a substance isdefined by listing its intrinsic properties. The most com-mon intrinsic thermodynamic properties are temperatureT, pressure p, specific volume v (which is the inverse ofdensity ), specific entropy s, specific enthalpy h, andspecific internal energy u. It has been established that athermodynamic state of a substance can be uniquely iden-tified by two independent intrinsic properties. For exam-ple, temperature and pressure are such properties, exceptin the saturation region where the same temperature andpressure can have an infinite number of states corre-sponding to quality from 0 to 100%. Enthalpy (h) isdefined by

(1)

where u =internal energy per unit mass.

Each property in a given state has only one definitevalue, and any property always has the same value for agiven state, regardless of how the substance arrived at thatstate. The thermodynamic property entropy s measuresthe molecular disorder of a system. The more mixed a sys-tem, the greater its entropy; conversely, an orderly orunmixed configuration is one of low entropy. Figs. 1-1aand 1-1b schematically show the liquid and vapor statesof water using two properties to uniquely identify thestates. Fig. 1-1a shows the states in (p, h) coordinates anda line showing the states along a constant temperature isalso presented. Fig. 1-1b shows the same information in(T, s) coordinates and a constant pressure line is shown.Within the dome where the saturated states exist it is pos-sible to increase the specific volume (and other intrinsic,per unit mass based properties) even when keeping thepressure and temperature constant (for example line B-A-C in Figs. 1-1a and 1-1b).

Fig 1-1a. Thermodynamic states of water in p-h coordinates

h u pv+=

FUNDAMENTALS OF THERMODYNAMICS12

Fig 1-1b. Thermodynamic states of water in T-s coordinates

The specific heats at constant pressure Cp and volumeCv are defined as the heat added to a unit mass at constantpressure or volume, respectively, to cause temperature ofthe mass to increase by one unit. Mathematically, byusing the first law for closed systems and the definition ofwork (described later in the chapter), these are defined as

(2)

For ideal gases the following holds true rigorously

(3)

and for real gases this is a good engineering approxima-tion. For incompressible substances the two specific heatsare the same and the above expressions also holds. Thus,liquids and solids, which are treated as incompressible,are described by Equation (3).

The basic entity analyzed by thermodynamics istermed a thermodynamic system. A thermodynamic sys-tem is an identifiable or specific region in space or anidentifiable or specific quantity of matter. It is bounded bythe system surface or the system boundaries. The sur-roundings include everything external to the system, andthe system is separated from the surroundings by the sys-tem boundaries. These boundaries can be movable orfixed, real or imaginary. There are two types of systems:closed and open. In a closed system, no mass enters orleaves the system. A closed system is thus a fixed masssystem. In an open system, mass may enter or leave thesystem and therefore the boundaries of such a system areimaginary in some parts where the mass is exchangedwith the surroundings. Energy can always cross the sys-tem boundaries, whether open or closed, except for an iso-lated closed system, which is one that does not allowenergy to cross its boundaries. Almost all HVAC systemsthat will be studied here fall in the open system category.

Energy has the capacity for producing an effect on thesystem and is described differently depending on whetherit is stored or is crossing the system boundary. Thermal(internal) energy is the energy possessed by the matter ina system caused by the internal motion of the molecules,the intermolecular forces, and other microscopic mecha-nisms of energy storage within the material. It is impossi-ble to find the absolute (total) value of internal energy of amaterial, and it is therefore measured with reference to astandard state in which the value of the internal energy isarbitrarily set to zero. The internal energy is the sum of allthese microscopic energies such that when the internalenergy of an isolated closed system (fixed mass) changes,the result is a change in the temperature of the system.Internal energy is an intrinsic property of material and itsvalue per unit mass can be used to uniquely define thestate of the material. The specific internal energy u is theinternal energy per unit mass. Potential energy is theenergy possessed by a system due to the elevation of thesystem:

(4)

where m =mass;g =local acceleration of gravity, and;z =elevation above horizontal reference plane.

Potential energy is not an intrinsic property of material.Kinetic energy is the energy possessed due to the bulkvelocity of the flowing material and is expressed as

(5)

where V =velocity of a fluid stream.Kinetic energy is not an intrinsic property of material.

The types of energy that are defined only when theycross the system boundaries are heat and work. Heat (Q)and work (W) cannot be stored and they only exist whenenergy is being transferred across a system boundary.Heat is the mechanism that transfers energy across theboundary of systems solely through temperature differ-ence. Heat always flows from higher to lower tempera-tures. Heat is considered positive by convention whenenergy is added to a system (see Fig. 1-2). Work is themechanism that transfers energy across the boundary ofsystems through forces and movement. (Here it has thetraditional characterization: it is the product of force anddistance.) If the total effect produced in the system can bereduced to the raising of a weight, then nothing but workhas crossed the boundary. By convention work is consid-ered positive when energy is removed from a system, i.e.,the system does work on its surroundings (see Fig. 1-2).For example, when a system comprising a cylinder andmovable piston expands so that the piston is moving out-wards, work is done by the system on the surroundingsand is therefore considered positive in any analysis of sys-tem. Mechanical or shaft work is the work associated witha rotating shaft such as a turbine, air compressor, or inter-

CvuT------

v= Cp

hT------

p=

du CvdT= dh CpdT=

PE mgz=

KE12---mV

2=

FUNDAMENTALS OF THERMODYNAMICS 13

nal combustion engine. Flow work is energy associatedwith the movement of fluid in conjunction with forces offluid pressure as it crosses the boundary of an open sys-tem. It can be more easily understood as the work done bythe fluid to push itself against the other fluid particles as itforces itself to enter or exit the system. Work done byentering fluid streams is negative (pushing into the sys-tem), while work done by fluid streams exiting the systemis positive (pushing the surroundings). The magnitude offlow work per unit mass is determined by the expression

(6)where p =pressure and;

v =specific volume, or the inverse of density.

Fig 1-2. Energy flow in general thermodynamic system

A property of a system is any observable characteristicof the system. A process is a change in state that can bedefined as any change in the properties of a system. A pro-cess is described by its initial and final equilibrium states,its path, and the interactions that take place across systemboundaries as it goes forth. A cycle is a process or a seriesof processes wherein the initial and final states of the sys-tem are identical. Therefore, at the conclusion of a cycle,all properties have the same value they had at the begin-ning.

Conservation of Mass.Conservation of mass in aclosed system is the default since no mass leaves or entersthe system. For an open system the conservation of massindicates that the difference between the mass enteringand leaving is equal to the increase of mass in the system.For the general case of multiple flow streams the conser-vation of mass is written as

(7)

For steady flow processes the above can be modified as

(8)

For flow through a pipe or duct the mass flow rate isrelated to the velocity by

(9)

where =density of the flowing fluid; A =cross-sectional area through which the fluid

is flowing;V =velocity;v =specific volume, or the inverse of the density.i = indicate initial states; andf = indicate final states.

It is assumed that the velocity is uniform across the cross-section. If it is not, then V in the above mass flow rate def-inition is average velocity. A is normal to the direction ofthe fluid flow.

First Law of Thermodynamics.The first law ofthermodynamics is often called the law of the conserva-tion of energy. After a system is defined, the conservationof energy states that

(10)

Fig. 1-2 illustrates energy flows into and out of a thermo-dynamic system. For a closed system this is written as

(11)

where m =mass of the system;U =internal energy of systems; andm =internal energy per unit mass;

For the general case of an open system with multiplemass flows in and out of the system, the energy balancecan be written

(12)

The steady flow process is important in engineeringapplications. Steady flow signifies that no quantitiesassociated with the system vary with time. Consequently,

(13)

Here the addition of the dot on top of the variables m, Q,and W indicates the time derivative (d/dt) which yields therate of mass flow, the rate of heat flow, and the rate ofwork done, respectively. In the above two equations theflow work of the entering and exiting streams is indicatedby the pv terms, and, therefore, the W in the equations isthe work done by moving system boundaries plus shaftwork, and any other work not described by these catego-ries. In the above energy conservation (Equations (12)

Flow work pv=

min m out mf mi[ ]system=

min mout=

m VA VAv

-------= =

energy in energy out increase in energy=

Q W Uf Ui m uf ui( )= =

min u pvV

2

2------ gz+ + +

in

m out u pvV

2

2------ gz+ + +

outQ W+ =

mf uV

2

2------ gz+ +

fmi u

V2

2------ gz+ +

i

system

m in u pvV

2

2------ gz+ + +

in

m out u pvV

2

2------ gz+ + +

outQ W+ 0=

FUNDAMENTALS OF THERMODYNAMICS14

and (13)) assume that velocity is uniform across the crosssection. However, in real fluid systems it is not uniformdue to viscosity of the fluid. The change to the kineticenergy term is discussed in the section Fundamentals ofFluid Flow.

Second Law, Reversibility, and Possible Pro-cesses.The second law of thermodynamics can beexpressed in many ways. Here it is being introduced todistinguish and quantify processes that can only proceedin one direction (irreversible) from those that are revers-ible. It also indicates which processes cannot exist.

The second law for a closed system is written as

(14)

where the subscripts f and i indicate final and initial state,respectively, and the system entropy S = ms, where m isthe mass of the system and s is specific entropy. The tem-perature T must be in absolute units (Kelvin or Rankine).The equality sign holds for reversible processes and theinequality for irreversible processes. The above form ofthe second law shows that the entropy of the system eitherincreases or remains same, assuming the heat flow is pos-itive. For adiabatic processes, that is where Q = 0, theright side of either Equation (14) vanishes, clearly indi-cating that entropy increases if the process is not revers-ible. For constant temperature processes

(15)

where the equality holds for reversible processes. For anopen system the above can be modified as

(16)

It is assumed in Equation (16) that the inlet and outletproperties remain invariant with time. For steady flowsystems with invariant inlet and outlet properties the sec-ond law can be rewritten in the form

(17)

The open system of Equation (17) can give insightsabout reversible and irreversible processes if we considera single input, single output stream process. If the processis adiabatic, then the inlet and outlet entropies will be thesame for reversible processes; otherwise the outletentropy will be greater than the inlet. If the process is iso-thermal, then the difference between the rate of entropyflowing out and in will be equal to the heat addition ratedivided by temperature only if the process is reversible.Adiabatic reversible processes are also termed isentropicbecause entropy remains same. On a thermodynamicchart where entropy is the horizontal axis, the adiabaticreversible processes will be vertical lines, but the irrevers-ible processes will always veer right (towards increasingentropy) from their starting point. Thus, for any given

starting point, the only thermodynamically possible adia-batic processes are those that end in the half plane to theright of the vertical line drawn from the starting point inthe thermodynamic chart where specific entropy is on thehorizontal axis. Thus, the second law can show processesthat are reversible, irreversible and possible, or impossi-ble.

The second law for cycles is described next. It furtherrefines the concept of possible and impossible series ofprocesses occurring in a cycle.

Thermodynamic Cycles.Thermodynamic cyclesthat make it possible to remove heat from cold spaces anddump the heat in hot ambient spaces are discussed here.These cycles make it possible for air conditioning andrefrigeration systems to exist and they form the basis ofHVAC engineering.

The performance of a refrigeration or air conditioningthermodynamic cycle is usually described by a coeffi-cient of performance. COP is defined as the benefit of thecycle (amount of heat removed) divided by the requiredenergy input to operate the cycle, or

(18)

The first law for cycles indicates that

(19)

where Q =heat added and;W =work done.

For the individual processes that make up the cycle, thefirst law for open systems is used. Since cyclic deviceshave only one working fluid and the mass flow rate of thefluid is the same through each process due to conservationof mass, the open system Equation (13) can be modifiedas below for each process in the cycle:

(20)

The kinetic and potential energy terms are usuallyneglected in comparison to enthalpy in refrigeration andair conditioning thermodynamic cycles because theirmagnitudes are usually several orders smaller thanenthalpy.

The Carnot cycle is an ideal cycle that is made up ofcompletely reversible processes and operates betweentwo fixed temperatures. This cycle is useful since it is athermodynamic ideal for refrigeration cycles that need tooperate between two temperatures: the temperature of theconditioned space and the temperature of the external(hot) ambient. Also heat is transferred from the cold spaceto the ambient. The properties of the Carnot cycle are thatit can be used as a refrigerator or a heat pump, as well as a

dSQT

------- Sf SiQT

-------i

f

T Sf Si( ) Q

Sf Si( ) system ms( )out ms( )in+QT

-------i

f

m s( )out m s( )in

Q

T----

COP useful refrigerating effectenergy from external source-------------------------------------------------------------------=

Q W=Qnet Wnet=

Q net W

net=

m hout hin( ) Q W+ 0=

FUNDAMENTALS OF THERMODYNAMICS 15

work-producing device, depending on the input quanti-ties. Fig. 1-3 shows the Carnot cycle on temperatureentropy coordinates. Heat is withdrawn at the constanttemperature TR from the region to be refrigerated. Heat isrejected at the constant ambient temperature To. The cycleis completed by two isentropic processes that connect thehigh and low temperatures at the two extremes of entropyvalues. From Equation (17) the energy transfers are givenby

(21)

(22)

Since the cycle is being run to remove the heat QR, theCOP of the Carnot cycle will be

(23)

where Wnet work that would be supplied through isen-tropic processes of the cycle.

Equation (23) also shows that the COP of the ideal revers-ible Carnot cycle is a function only of the two absolutetemperatures between which it operates.

Fig 1-3. Carnot cycle

Fig 1-4. Possible thermodynamic realization of a Carnot cycle using a pure substance

The second law for cycles states that (1) no refrigerat-ing cycle may have a COP higher than that for a reversible

cycle operated between the same temperature limits, and(2) all reversible cycles, when operated between the sametemperature limits, have the same COP. Proofs of thesestatements are not presented here. They can be found instandard thermodynamics textbooks. Although the firstlaw states that net heat flow into the cycle is equal to thenet work done by the cycle (see Equation (19)), con-straints are placed by the second law. For example, it isimpossible to put heat into a system and convert it com-pletely to work, a perfect conversion. The second lawforces heat to be rejected (which is wasted), and the heatrejection should be to a different temperature reservoirthan the temperature of the reservoir from which the heatis added for the cycle to exist. For refrigerating cycles italso establishes the only possible cycles are the ones inwhich the COP is less than the ideal COP, which is basedonly on the two extreme temperatures of the cycle.

The Carnot cycle can be created by exploiting the con-stant temperature and constant pressure characteristics ofthe phase change saturation region of pure substances forconstant temperature heat addition and rejection pro-cesses. The isentropic processes that link the statesbetween the high and low temperatures can now be pro-cesses between high and low pressures within the satura-tion dome. Movement from a low to high pressure is easysince pumps and compressors are available, and fromhigh to low pressure can be achieved via turbines. This isshown in Fig. 1-4.

However, in a real device the Carnot cycle shown inFig. 1-4 is difficult to obtain because of practical consid-erations. It is difficult to stop process heat addi-tion/removal precisely at state 2. The compression frompressure of 2 to the higher pressure at 3 involves a sub-stance that is mixed liquid and vapor at entry for whichreliable compression devices are difficult to make (it iseasy to either pump liquid alone or compress vapor). Sim-ilar problems occur at the expansion from 4 to 1, perhapsvia a turbine, where the inlet is saturated liquid and theoutlet is a wet saturated mixture of liquid and vapor. Toovercome these problems the standard practical refrigera-tion cycle is shown in Figs. 1-5a and 1-5b. In this the con-stant temperature (and constant pressure) heat additionprocess ends when the entire saturated mixture has turnedto saturated vapor at state 2 and a vapor compressor isused from pressure 2 to 3. From 3 to 4 the heat rejection isisobaric, and is only isothermal when in the saturationdome region. The isentropic expansion process from 4 to1 of the Carnot cycle is replaced by a simple expansionvalve that does not preserve entropy but preservesenthalpy. The advantage is that a moving device isavoided and a passive valve is replaced instead.

QR TR S2 S1( )= Q

R m TR s2 s1( )=

Qo To S3 S4( )= Q

R m To s2 s1( )=

COPQRWR--------

QRQo QR--------------------

TRTo TR------------------= = =

FUNDAMENTALS OF FLUID FLOW16

Fig 1-5a. Practical refrigeration cycle in (T,s) coordinates

Fig 1-5b. Practical refrigeration cycle in P-h coordinates

A pure refrigerant or an azeotropic mixture can be usedas the working fluid in the practical refrigerator to main-tain constant temperature during phase changes throughmaintenance at constant pressure. Because of such con-cerns as high initial cost and increased maintenancerequirements, the practical machine has one compressor,and the expander (engine or turbine) is replaced by a sim-ple expansion valve. The valve throttles the refrigerantfrom high pressure to low pressure. The highest tempera-ture in the cycle is at state 3. Thus, for ideal COP the tem-perature of 3 has to be used, and not that of state 4. Thecomponents of the refrigerator are sketched in Fig. 1-6.Because of the use of a compressor to go from the lowtemperature state 2 to high temperature state 3, the cyclesare called vapor compression cycles.

Fig 1-6. Components of the refrigerator or air conditioner

Fundamentals of Fluid Flow

In addition to density discussed in the previous section,the other property that is of importance in fluid flow is vis-cosity. The viscosity of the fluid links the shear stress on afluid layer to the local velocity gradient across the layer.In the terminology of Fig. 1-7, the following relationdefines viscosity

(24)

where =shear stress (tangential force per unit area);and

=viscosity.

Fig 1-7. Velocity profiles and gradients in viscous flows

The density of air and water at standard conditions of68F and 14.696 psi (sea level atmospheric pressure) are0.075 lbm/ft3 and 62.3 lbm/ft3, respectively. All fluids arecompressible to some degree; fluid density depends onpressure. Steady liquid flow may ordinarily be treated asincompressible, and incompressible flow analysis is sat-isfactory for gases and vapors at velocities below about4000 to 8000 fpm, except in very long conduits. The unitsof viscosity are (force time)/length2. At standard condi-tions the viscosities of air and water are 1.2105 lbm/ft-s(= 3.7107 lbf-s/ft2) and 6.7104 lbm/ft-s (= 2.1105lbf-s/ft2), respectively. Kinematic viscosity is viscositydivided by density. The values of kinematic viscosity atstandard conditions for air and water are 1.7104 ft2/sand 1.08105 ft2/s, respectively. In general if the Machnumber of the fluid is less than 0.30 the fluid can be con-sidered incompressible.

In this chapter the fluid mechanics of pipe and ductflows are emphasized. This is because in HVAC systemsflows in pipes and ducts are dominant methods of fluidtransport, both airflow to the conditioned space and liq-uids that are the working fluids in the systems. Externalflows, such as flows of large fluid volumes over sub-merged objects, compressible flows such as in turbinesand compressors, and the like are not discussed for thesake of brevity and relevance (or lack thereof).

Flow in Pipes and Ducts.The conservation of mass(or continuity equation) follows the same formulation asdescribed before in the thermodynamics section, Equa-tions (8) and (9). For conservation of energy some simpli-fications and modifications can be made. For fluidmechanics analyses only conservation of mechanicalenergy is considered. If steady flow is considered along a

dVdy-------=

FUNDAMENTALS OF FLUID FLOW 17

single path system then the conservation of energy foropen systems, Equation (13), can be rewritten as

(25)

Here it is assumed that no work producing shafts orwork producing moving boundaries are present and thatthe conservation of mass then permits the inlet and outletmass flow rates to be the same. Pressure energy, kineticenergy, and potential energy are the mechanical energiesof the flow. Since it was shown in the previous thermody-namics section that for most fluids, especially away fromthe saturation dome, the internal energy u is primarily afunction of temperature, u is not considered a mechanicalenergy. The conservation of mechanical energy is thenstated as

(26a)

(26b)

This is also the Bernoullis equation, which is the con-servation of mechanical energy in an ideal fluid with nolosses, no heat additions/removal, and no work. Ber-noullis equation shows that the sum of pressure energy,kinetic energy, and potential energy is constant along astreamline of an ideal fluid. Even when temperature vari-ations are present or the internal energies are not constantor heat transfer is present, the above mechanical energyconservation (and modifications presented below in caseof losses) can be separately considered before the entireconservation of energy Equation (13) is eventuallyapplied if needed.

In the case of real fluids, where viscosity introducesdrag, loss in mechanical energy occurs. This loss ofmechanical energy is usually converted to increase ininternal energy or is dissipated as heat, as per general con-servation of energy as described in the thermodynamicssection. In such a case of losses Equation (26a) of conser-vation of mechanical energy (presented in the form ofenergy per unit mass) is rewritten as

(27)

where Eloss = mechanical energy loss per unit mass, or rateof mechanical energy loss per unit mass flow rate.

If mechanical energy is added to the flow throughappropriate forms of work such as shaft work, the abovecan be modified to include a source term for mechanicalenergy. This is shown in

(28)

where Emech = mechanical energy added into the pipe orduct flow via a device such as fan, blower, or pump.

Another consideration in the case of real fluids is thevelocity V in Equation (28). The velocity V in the above isthe average velocity through the pipe and is obtained fromthe mass flow rate. Because of the viscosity for real fluids,the velocity varies from zero at the duct or pipe walls to amaximum along the centerline. Since it is not mathemati-cally true that the square of the average velocity is equal tothe average of squared velocity unless the velocity profileis flat, this introduces an additional kinetic energy factor in Equation (27) as follows:

(29)

The kinetic energy factor is the ratio of the true kineticenergy of the flow to the kinetic energy represented by themean or average velocity. It is shown that the value of thiskinetic energy factor is 2.0 for laminar flow in circularpipes and 1.54 for laminar flow in wide rectangular chan-nels. For turbulent flow the value is close to 1.0. Thus, formost HVAC applications, where the flow is turbulent, theoriginal Bernoulli equation Equation (27), with losses issufficient.

Reynolds number is an important unitless parameter influid mechanics. It is defined for flow in pipes or ducts as

(30)

where D = diameter for circular pipes or a characteristiclength for non-circular pipe.For other cross-sections it can be the hydraulic diameterDh which is defined as

(31)

where A = cross-sectional area, and P = perimeter.For external flows, characteristic length is usually thelength of the object along the direction of the flow, or thevalue of the lengths local coordinate depending on theapplication. For pipe and duct flows the flow is laminar ifthe Reynolds number is less than 2300 and turbulent forgreater values. This value of the critical Reynolds numberis observed via experiments. For external flows parallel toa surface the flow remains laminar until the local Rey-nolds number reaches 5 105. Thus, the critical Reynoldsnumber for internal flow is 2300 and for the external flowis 5 105. If the Reynolds number exceeds the criticalvalue, the flow transitions to turbulence; below the criti-cal value, the flow is laminar.

u pv V2

2------ gz+ + +

inu pv V

2

2------ gz+ + +

out

Q

m----+ 0=

pv V2

2------ gz+ +

inpv V

2

2------ gz+ +

out 0=

p--- V

2

2------ gz+ + constant=

pv V2

2------ gz+ +

inpv V

2

2------ gz+ +

out Eloss=

pv V2

2------ gz+ +

inpv V

2

2------ gz+ +

out+

+Emech Eloss=

pv V2

2------ gz+ +

inpv V

2

2------ gz+ +

out Eloss=

Re VD

------------=

Dh4AP

-------=

FUNDAMENTALS OF FLUID FLOW18

Laminar flows are those where viscous effects domi-nate and the flow is orderly and layered. Fluid particlestravel in smooth trajectories, without fluctuations. Insteady internal flows, such as in pipes and ducts, thevelocity profile is of a form that the forces due to pressureare balanced by viscous shear forces introduced by thepresence of a wall (the velocity has to be zero at the wall).This gives rise to parabolic velocity profiles in the pipe orduct, where the velocity vanishes at the walls and is amaximum at the centerline.

Turbulent flows which typically have higher velocitiesthan laminar flows, involve random perturbations or fluc-tuations of the velocity and pressure, characterized by anextensive hierarchy of scales or frequencies. Only flowsinvolving random perturbations without order or period-icity are turbulent; the velocity in such a flow varies withtime or locale of measurement (Fig. 1-8). Turbulence canbe quantified by statistical factors. The velocity mostoften used in velocity profiles is the temporal averagevelocity, and the strength of the turbulence is character-ized by the root mean square of the instantaneous varia-tion in velocity about the temporal average velocity. Theeffects of turbulence cause fluid to diffuse momentum,heat, and mass very rapidly across the flow. Because ofthe rapid fluctuations, fluid particles do not travel insmooth trajectories. The fluctuation allows particles tocross layers and thus cause a greater uniformity of flowproperties and characteristics than in the laminar case.Because of this the turbulent velocity profiles in pipes orducts are flatter throughout the core of the flow around thecenterline and only fall off to zero velocity at the walls ina small region near the walls. It is because of the flattervelocity profile that the kinetic energy factor in Equation(29) is near unity, usually ranging from 1.01 to 1.10.

Fig 1-8. Velocity fluctuations with respect to time in turbulent flows

Time average velocity is defined by the Equation (32)

(32)

The laminar and turbulent velocity profiles in a pipeflow are schematically compared in Fig. 1-9. Turbulentflow profiles are flat compared to the more pointed pro-files of laminar flow. Near the wall, velocities of the tur-

bulent profile must drop to zero more rapidly than those ofthe laminar profile, so the shear stress and friction aremuch greater in the turbulent flow case.

The velocity profiles shown in Fig. 1-9 correspond tofully developed flows. Fully developed flow regions arefar away from inlets, from sections of sudden changes incross-section, and from sources of mechanical energyinput. The entrance flow region is the region from an inletto the location where the fully developed region starts. Itcorresponds to the length Le of the pipe or duct needed forthe flow to gradually change from its inlet profile to thenew conditions.

Fig 1-9. Laminar and turbulent velocity profiles in a circular duct

Note that if the cross-sectional area remains constantthen by the conservation of mass the average velocitythroughout the pipe, including the inlet and fully devel-oped regions, remains the same although the shape of theprofile changes.

With laminar flow following a rounded entrance, theentrance length Le depends on the Reynolds number:

(33)

At Re = 2000, a length of 120 diameters is needed toestablish the fully developed parabolic velocity profile.However, the pressure gradient reaches the developedvalue of much sooner. With turbulent flow, a length of 80to 100 diameters following the rounded entrance areneeded for the velocity profile to become fully developed,but the friction loss per unit length reaches a value close tothat of the fully developed flow value more quickly. Aftersix diameters, the loss rate at a Reynolds number of 105 isonly 14% above that of fully developed flow in the samelength, while at 107 it is only 10% higher. For a sharpentrance, the flow separation causes a greater distur-bance, but fully developed flow is achieved in about halfthe length required for a rounded entrance. With suddenexpansion, the pressure change settles out in about eighttimes the diameter change (D2D1), while the velocityprofile takes at least a 50% greater distance to return tofully developed pipe flow.

The mechanical energy loss in a duct or pipe of con-stant cross-section due to friction is given by the follow-ing:

V1T--- V td

0

T

=

LeD----- 0.06Re

FUNDAMENTALS OF FLUID FLOW 19

(34)where f =friction factor.

Fig 1-10. Friction factor (Moodys chart)

Losses due to friction are traditionally termed majorlosses in a pipe or duct system because for long pipes thisloss dominates. However, for HVAC applications thismay not be the case, and so the terminology of majorlosses may not be as appropriate as simple friction loss.Here L is the length of the pipe and D is the diameter. Fornoncircular ducts, the diameter may be replaced by thehydraulic diameter Dh. Friction factor is a function of theReynolds number and the relative roughness of the pipeor duct walls. For large Reynolds numbers its value isfairly constant and is only a function of the roughness /D,where is the average height of roughness. This region iscalled the fully rough region. The value of friction factoris obtained via experiments and the values are given inFig. 1-10 and in Equations (35) to (38).

In the laminar region, where the Reynolds number isless than 2300, the friction factor is independent of theroughness. This is because the dominant viscous effectssuppress any fluctuations introduced by roughness. Thevalue of f for this region is given as

(35)

for circular cross-section pipes. In non circular pipesexact values for f can be derived for laminar flow, and theuse of hydraulic diameter in Equation (35) may lead tosignificant errors.

In turbulent flows for smooth pipes the friction factor isempirically correlated as

(36)

(37)

Another correlation for smooth pipes is

(38)

Emajor-loss Eloss-friction fLD----V

2

2------= =

f 64Re------=

f 0.3164

Re0.25

----------------= for Re 105

m ah1 m

ah2 q m whw+ +=

m ah1 m

ah2 q m a W1 W2( )hw+ +=

q m a h1 h2( ) m a W1 W2( )hw=

IN

OUT1 2

m

Wh1

1

a

q

Condensate drainm w

m

Wh

2

a2

W1

2W

1

x3

m acfm

--------- 5000 60

13.833------------------------ 21687 lbm/hr = = =

AIR CONDITIONING PROCESSES 33

The enthalpy of air at entering h1= 31.4 Btu/lbm, W1 =0.0112 lbv/lbda, h2 = 22.2 Btu/lbm, W2 = 0.0082 lbv/lbda,and the enthalpy of condensation hw = 23.0 Btu/lbv.Applying the Equation (15)

Heating with Humidification.In most commercialfacilities such as large office spaces, hospitals, and mod-ern schools where central heating and cooling HVAC sys-tems are used, it is desirable to humidify the suppliedheated air to various room and spaces in order to maintaincomfortable relative humidity, especially in the locationswhere the outdoor relative humidity during winter seasonis very low. In the heating with humidification process,air first is heated by the heating coil or gas furnace andthen is humidified by adding moisture before it is sup-plied to the space.

Fig 3-8. Schematic of heating with humidification process

Fig 3-9. Psychrometric diagram of heating with humidification process

The schematic of this process is shown in Fig. 3-8. Theair conditioning system on psychrometric chart represen-tation of this process is shown in Fig. 3-9.

The conservation of the mass and energy equations areas follows:

Conservation of mass:

(16)

(17)

(18)

Conservation of energy:

(19)

(20)

(21)

Equation (21) can be written in the following usefulform:

(22)

Adiabatic Mixing of Two Air Streams. Many airconditioning applications require the mixing of two airstreams. This is particularly true for large buildings, andmost process plants, office spaces, and hospitals, in whichthe space return air must be mixed with a certain requiredoutdoor fresh air for proper ventilation before it enters theair conditioning unit. In this process, the heat transfer tothe surrounding space is usually small and can be ignored.The schematic of this process is shown in Fig. 3-10. Thepsychrometric representation of this process is shown inFig. 3-11. The mass and energy conservation equationsfor this process are as follows:


(23)

(24)

(25)


(26)

Combining Equations (23) to (26) gives:

(27)

q m a h1 h2( ) W1 W2( )hw( )=

21687 31.4 22.2( ) 0.0112 0.0082( ) 23( )=198024 Btu=

16.5 ton=

Heating medium

m ah

hmw

w

21 x

qW11

m ah

W22

1

2

X

W1

2W

m a1 m

a2 m

a= =

m v1 m

w+ m

v2=

m w m

a W2 W1( )= where W2 W1

AIR CONDITIONING PROCESSES34

Fig 3-10. Adiabatic mixing of two streams process

Solving Equations (23) to (27) for h3 and W3 gives:

(28)

and

(29)

Fig 3-11. Psychrometric diagram of adiabatic mixing process

Example 3:Find the condition of mixed air in which1500 cfm of outside air 90F at 30% relative humidity ismixed with 4500 cfm return air of 75F at 60% relativehumidity.Solution: First we will find out the outside air and return

air properties. We are given these data:

By applying the psychrometry chart (Fig. 2-5)

The condition of the mixed air is

Example 4:Find the heat transfer rate and mass flow rateof a heating and adiabatic humidification process where2000 cfm air enters at 40F and 40% relative humidity andleaves at 110F and a relative humidity of 30%.

Solution: First we will find out the outside air and returnair properties. Given

Mass flow rate of dry air

The specific volume of air at 40F and 40% is 12.62from the psychrometry chart Fig. 2-5.

By applying the psychrometry chart (Fig. 2-5)

1

2

m

wh1

1

a m

wh

a

3

3

3

2whm

2w

h3

m a1m a2---------h1 h2+

1m a1m a2---------+

---------------------------=

W3

m a1m a2---------W1 W2+

1m a1m a2---------+

-------------------------------=

1

23

WW

W1

2

3

cfmoa 1500= cfmra 4500=

Toa 90= Tra 75=

oa 30= ra 60=

oa 14.04= ra 13.70=

Woa 0.009= Wra 0.0111=

hoa 31.54= hra 30.20=

m oa1500 60

14.04------------------------=

6410=

m ra4500 60

13.70------------------------=

19708=

m m m

oa m

ra+ 6410 19708+ 26118= = =

hmhoa m

oa hra m ra+

m m------------------------------------------------------=

31.54 6410 30.20 19708+26118

-------------------------------------------------------------------------=

30.52 Btu/lbm=

WmWoa m

oa Wra m ra+

m m----------------------------------------------------------=

0.009 6410 0.0111 19708+26118

----------------------------------------------------------------------------=

0.0105 lbv lbv=

cfm 2000=

T1 40= T2 110=

1 40%= 2 30%=

m1cfm 60

--------------------- 2000 60

12.62------------------------ 9508 lb m/hr= = =

W1 0.002= W2 0.016=

h1 11.83= h2 44.93=

m 2 m

a= hw 1135=


Steam flow rate,

Applying the energy balance equation for heating andhumidifying equation

Psychrometric diagram of Example 4

Evaporative Cooling.Conventional cooling sys-tems such as rooftop and system air conditioning systemsand heat pump systems operate on a refrigeration cyclethat has high initial and operating and maintenance cost.The high operating cost is associated with the high elec-tricity consumption of the compressor. The conventionalrefrigerant system can be used in any part of the world.However, in hot and dry climates, we can avoid the highcost of cooling by using the evaporative coolers. Theevaporative cooler is based on a simple principle that aswater evaporates, the latent heat of vaporization isabsorbed from the water and the surrounding air. As aresult, both water and the air are cooled during this pro-cess. The schematic process of evaporative cooling isshown in Fig. 3-12. The psychrometric representation ofthis process is shown in Fig. 3-13. During the humidifica-tion process the enthalpy of moist air and the wet-bulbtemperature of the air remain approximately constant.


(30)

(31)

(32)


(33)

Fig 3-12. Evaporative cooling system

Fig 3-13. Psychrometric diagram for evaporative cooling system

Heating and Air Conditioning System Cycles.Fig.3-14 shows a schematic flow diagram of a simple air con-ditioning cycle. The psychrometric chart representationof a typical cooling and heating systems based on Fig. 3-14 are shown in Figs. 3-15 and 3-16.

Fig 3-14. Air conditioning system

m 1W1 m

w+ m

2W2=

m w m

1 W2 W1( )=

9508 0.016 0.002( )=133 lbm/hr=

m 1h1 q+ m 2h2 m

whw=

q m 2h2 m

1h1 m

whw=

m 1 h2 h1( ) mwhw=

9508 44.93 11.83( )= 133 1135163760 Btu/hr=

A

B

C

m a0 m

a1 m

a= =

m v0 m

w+ m

v1=

m w m

a W1 W0( )=

m ah0 m

ah1=

h0 h1=

or

Twb0 Twb1=

Conditioned01

space

Make-upwater

0

1

2 W

W0

2


Fig 3-15. Psychrometric diagram of heating/humidifying process

Fig 3-16. Air conditioning cooling system

The following examples will provide good practice andan approach to the analysis of HVAC cycles.

Example 5:Determine the sensible and latent heat load,if 5000 cfm conditioned air is supplied to a room at 55Fand 90% relative humidity. The space is to be maintainedat 75F at sensible heat factor (SHF) 0.80?

Solution: The total cooling load for the room is

Applying the sensible heat factor equation

where =total heat loss, Btu/hr

=sensible heat loss, Btu/hr

=latent heat loss, Btu/hr

Then latent heat is

Example 6:A room is to be maintained at 75F and 50%relative humidity. The outside air condition is 95F and60% relative humidity. The outdoor air requirements forthe occupants is 500 cfm. The total heat gain to the spaceis 60,000 Btu/hr with a 0.80 SHF. Determine the quantityand the state of the air supplied to the space and therequired capacity of cooling and dehumidifying equip-ment.

Solution: Assume that the conditions of air after thecooling coil is 55F and 90% relative humidity. Nowmake a schematic diagram to locate the points on the psy-chrometric chart.

Applying the energy balance equation around the room

o

m

r s

mix

ing

room

heating

Dry-bulb temperature

Hum

idity

Rat

io

o

m

rs

Dry-Bulb Temperature

Hum

idity

Rat

io

q t 1.10 cfm T=

1.10 5000 75 55( )=110000 Btu/hr=

SHFq s

q s ql+

----------------=

SHFq sq t-----=

q s qt SHF=

110000 0.80=88000 Btu/hr=

q t

q s

q l

q s ql+ q

t=

q l qt q

s=

110000 88000=

22000 Btu/hr=

T0 95= 0 60=

T2 55= 2 90=

T3 75= 3 50=

h0 46.4= W0 0.021= v0 14.45=

h2 22.2= W2 0.008= v2 13.13=

h3 28.1= W3 0.009= v3 13.66=

m 2h2 q+ m 2h3=

m 2q

h3 h2( )----------------------=

6000028.1 22.2( )

-------------------------------=

10170 lb/hr=


The flow rate of dry air is

The flow rate of outside air is

The return air quantity will be (101702076) or 8094lbm/hr. Assume return air condition and room air condi-tion are same.

Now we find the mixed air condition by the mixing ofreturn air and outside air.

Applying the energy balance equation around the cool-ing coil:

Example 7:A room is to be maintained at 75F and 50%relative humidity. The outside air is 30F and 50% rela-tive humidity. The outdoor air requirements for the occu-pants is 500 cfm. Sensible and latent heat losses from thespaces are 120,000 Btu/hr and 30,000 Btu/hr. Determinethe quantity of air supplied at 120F to the space and therequired capacity of heating and humidifying equipment.

Solution: The figure below is the schematic for the prob-lem.

Draw a line at point 3 parallel to SHF= 0.80, whichintersect 120F at point 2.

Applying the energy balance equation around the room

The flow rate of dry air is

0

132

cfmra m

2310170 13.66

60---------------------------------- 2315 cfm= = =

m 4cfmoa

-------------- 500 60

14.45--------------------- 2076 lbm hr= = =

m 1 m

0 m

4+ 8094 2076+ 10170 lb= = =

h1h0 m

0 h4 m 4+m 1

--------------------------------------------=

46.4 2076 28.1 8094+10170

----------------------------------------------------------------=

31.84 Btu/lb=

W1W0 m

0 W4 m 4+m 1

------------------------------------------------=

0.021 2076 0.009 8094+10170

----------------------------------------------------------------------=

0.0115=

m 1h1 qc m

2h2+=

q c m

2 h1 h2( )=

10170 31.84 22.2( )=98038 Btu/hr=

8.17 ton=

SHFq s

q s ql+

----------------=

120000120000 30000+---------------------------------------=

0.80=

0

1

3

2

x

m 2h2 m

2h3 qt+=

m 2q t

h3 h2( )----------------------=

15000046.2 28.2( )

-------------------------------=

8333 lb/hr=

cfmra m

238333 13.66

60------------------------------- 1898 cfm= = =


The flow rate of outside air is

The return air quantity will be (83332427) or 5906lbm/hr. Assume return air condition and room air condi-tion are the same. Neglecting the return fan effect.

Now we find the mixed air condition by the mixing ofreturn air and outside air.

Applying the energy balance equation around the heat-ing coil:

Applying the mass balance equation around the heatingcoil:

Example 8:An existing building space will be an officespace for 200 people. The space design loads are as fol-lows:

Summer: 300,000 Btu/hr sensible (gain), 75,000Btu/hr latent (gain)

Winter: 600,000 Btu/hr sensible (loss), negligiblelatent

Fan: 4 inch of water pressure drop with 80% efficiency

Find the

A) Summer air flow, cfm

B) Winter air flow, cfm

C) Cooling coil rating, tons

D) Sensible cooling coil rate, Btu/hr

E) Latent cooling coil rate, Btu/hr

F) Heating coil rating, MBH

G) Humidifier rating, gal/hr

Solution:

Summer cooling load:

m 4cfmoa

-------------- 500 60

12.36--------------------- 2427 lb/hr= = =

m 1 m

0 m

4+ 5906 2427+ 8333 lb= = =

h1h0 m

0 h4 m 4+m 1

--------------------------------------------=

9.07 2427 28.1 5906+8333

----------------------------------------------------------------=

22.55 Btu/lb=

W1W0 m

0 W4 m 4+m 1

------------------------------------------------=

0.0017 2427 0.009 5906+8333

-------------------------------------------------------------------------=

0.0068 lb/lb=

m 1h1 qh+ m

2h2=

q h m

2 h2 h1( )=

8330 46.2 22.55( )=197005 Btu/hr=

m 1W1 m

w+ m

2W2=

m w m

1 W2 W1( )=

8330 0.012 0.0068( )=43.3 lb/hr=

Location

Dry Bulb Tempera-ture, Tdb

Wet Bulb Tempera-ture, Twb

RelativeHumidity Enthalpy

HumidityRatio

Summer

OA 95 74 37.5 37.50 0.0133

RA 75 55.67 29.31 0.0103

SA 55 100 23.30 0.0092

MA

Winter

OA 7 100 2.883 0.0011

RA 72 50 26.42 0.0084

SA 135 7.65 41.77 0.0084

MA

FANCOOLINGCOIL(SUMMER)

HEATINGCOIL WITHHUMIDIFIER

SPACE

winter : 135 deg. Fsummer : 55 deg. F

SA

RA

RAEA

OAMA

q s 1.10 cfm Tra Tsa( )=

cfmq s

1.10 Tra Tsa( )--------------------------------------------=

3000001.10 75 55( )---------------------------------------=

13636=


Mass of air:

Mass of water:

Humidity ratio of room air:

At humidity ratio 0.10338 and 75F, hra = 29.31Btu/hr.

Outside air requirement as per ASHRAE Code is 20cfm/person. So the total outside air requirement = 200 20 = 4000 cfm.

Mass of air

The exhaust air will be 4000 cfm. So the return air willbe 13636 4000 = 9636 cfm and in mass 59894 16830 =43064 lb/h.

Mixed air condition:

Fan power:

Cooling coil capacity:

Winter load:

Mass of air:

Outside air requirement as per ASHRAE Code is 20cfm/person. So the total outside air requirement = 200 20 = 4000 cfm.

Mass of air:

m acfm 60

---------------------=

13636 6013.66

---------------------------=

59894 lb/h=

m lq l

1100------------=

750001100

---------------=

68.18 lb/h=

Wra Wsam lm a------+=

0.0092 68.1859894---------------+=

0.010338=

m oa4000 60

------------------------=

4000 6014.26

------------------------=

16830 lb/h=

hmm oa hoa m ra hra+

m oa m

ra+------------------------------------------------------=

16830 37.5 43064 29.31+59894

-------------------------------------------------------------------------=

31.61 Btu/lb=

Wmm oa Woa m ra Wra+

m oa m

ra+----------------------------------------------------------=

16830 0.0133 43064 0.0103+59894

----------------------------------------------------------------------------------=

0.0111=

Pcfm pt6350 f-----------------------=

13636 46350 0.80----------------------------=

10.737 hp=

8 kw=

q coil m

a hm hs Wm Ws( )hc( )=

59894 31.61 23.30 0.0111 0.0092( )32.0( )=494078 Btu/hr=

41.2Ton=


cfmq s

1.10 Tra Tsa( )--------------------------------------------=

6000001.10 135 72( )------------------------------------------=

8568=

m acfm 60

---------------------=

8568 6013.56

------------------------=

37911 lb/h=

m oa4000 60

------------------------=

4000 6011.77

------------------------=

20390 lb/h=


The exhaust air will be 4000 cfm. so the return air willbe 8568 4000 = 4568 cfm and in mass 37911 20390 =17521 lb/h.


Fan power:

where =total pressure loss, inches of water

=efficiency of fan

Heating coil capacity:

Humidifier rating:

Example 9:A small space requires two zones. Maxi-mum heat loss and heat gain for the zones are givenbelow:

Space design condition:

Summer: 75F

Winter: 75F, 50% relative humidity

Find:

A) The required fan capacity and cooling coil and heat-ing coil capacity for double duct multi-zone system (ductloss = 2.0 inches of water).

B) The required fan capacity and cooling coil and heat-ing coil capacity for variable volume with reheat (turndown 50% and cooling coil discharge 55F (duct loss =3.0 inches of water).

C) The required fan capacity and cooling coil and heat-ing coil capacity for Four pipe induction (1:1 inductionratio) with primary air provided at 55F and 130F (ductloss = 3.0 inches of water).

D) Four pipe induction (1:1 induction ratio) with pri-mary air provided at 55F and 130F.

Solution 4 (4-pipe fan coil units):

Winter Summer Ventila-tion

Require-mentSensible Latent Sensible Latent

Room A 48000 2000 60000 2000 600

Room B 36000 2400 48000 2400 500

hmm oa hoa m ra hra+

m oa m

ra+------------------------------------------------------=

20390 2.88 17521 26.42+37911

-------------------------------------------------------------------------=

13.75 Btu/lb=

Wmm oa Woa m ra Wra+

m oa m

ra+----------------------------------------------------------=

20390 0.0011 17521 0.0084+37911

----------------------------------------------------------------------------------=

0.0044 =

Pcfm pt6350 f-----------------------=

8568 46350 0.80----------------------------=

6.74 hp=

5.03 kw=

pt

f

q coil m

a hs hm Ws Wm( )hc+( )=

37911 41.77 13.75 0.0084 0.0044( )32.0+( )=1067 MBH=

m humidifier m

a Ws Wm( )=

37911 0.0084 0.0044( )=152 lb/hr=

Location

Dry Bulb Tempera-ture, Tdb

Wet Bulb Tempera-ture, Twb

RelativeHumidity Enthalpy

HumidityRatio

Summer

OA 95 74 37.5 37.50 0.0133

RA 75 50

SA 55 100 23.20 0.0m,m092

MA

Winter

OA 7 100 3.057 0.0013

RA 75 50 26.42 0.0093

SA 135

MA

VARIABLE VOLUMESUPPLY FAN

winter : 135 deg. Fsummer : 55 deg. F

RA

RAEA

OA MA

ZONE A ZONE B

PR

EH

EA

T C

OIL

CO

OL

ING

CO

IL

VAV BOX WITHREHEAT COIL

VAV BOX WITHREHEAT COIL


Room-A:

Summer cooling load:

Mass of air:

Mass of water:


Room temperature 75F and humidity ratio 0.009352,relative humidity 50% and enthalpy hra= 28.15 Btu/h.

Outside air requirement = 600 cfm.

Mass of air:

The exhaust air will be 600 cfm. So the return air will be2727 600 = 2127 cfm and in mass 11978 2524 =9454 lb/hr.


Fan power:

Enthalpy after fan (due to fan power):

Cooling coil capacity (condensate water at 64F hc =32.0 Btu/lb):

Winter supply temperature (same fan is applied, givessame cfm)


cfmq s

1.10 Tra Tsa( )--------------------------------------------=

600001.10 75 55( )---------------------------------------=

2727=

m acfm 60

---------------------=

2727 6013.66

------------------------=

11978 lb/h=

m lq l

1100------------=

20001100------------=

1.82 lb/h=


0.0092 1.8211978---------------+=

0.009352=

m oacfmoa 60

---------------------------=

600 6014.26

---------------------=

2524 lb/h=

hmam oa hoa m ra hra+

m oa m

ra+------------------------------------------------------=

2524 37.5 9454 28.15+11978

-------------------------------------------------------------------=

30.12 Btu/lb=

Wmam oa Woa m ra Wra+

m oa m

ra+----------------------------------------------------------=

2524 0.0133 9454 0.009352+11978

----------------------------------------------------------------------------------=

0.01018 lb of water/lb of air=

Pcfm pt6350 f-----------------------=

2727 26350 0.80----------------------------=

1.07 hp=

0.80 kw=

hf hmP 2545

m a---------------------+=

30.12 0.80 254511978

----------------------------+=

30.30 Btu/h=

Wf Wm=

0.01018=

q coil m

a hf hs Wf Ws( )hc( )=

11978 30.30 23.30 0.01018 0.0092( )32.0( )=83468 Btu/hr=

6.96 ton=

q s 1.10 cfm Tsa Tra( )=

Tsa Traq s

1.10 cfm--------------------------+=

75 480001.10 2727----------------------------+=

91F=


Mass of water:

Supply air humidity ratio:

Corresponding temperature and humidity ratio of supplyair the relative humidity & enthalpy of supply air29.5% and 31.94 Btu/hr.


Fan power:


Heating coil capacity:

Humidifier rating

Room-BSummer cooling load:

Mass of air:

Mass of water:

m lq l

1100------------=

20001100------------=

1.82=

Wsa Wram lm a------=

0.0093 1.8211978---------------=

0.009178=

hmam oa hoa m ra hra+

m oa m

ra+------------------------------------------------------=

2524 3.057 9454 26.42+11978

----------------------------------------------------------------------=

21.50 Btu/lb=

Wmam oa Woa m ra Wra+

m oa m

ra+----------------------------------------------------------=

2524 0.0013 9454 0.009352+11978

----------------------------------------------------------------------------------=

0.0076=

Pcfm pt6350 f-----------------------=

2727 26350 0.80----------------------------=

1.07 hp=

0.80 kw=

hfa hmaP 2545

ma---------------------+=

21.50 0.80 254511978

----------------------------+=

21.66 Btu/h=

Wfa Wma=

0.0076 lb of water/lb of air=

qcoil m

a hsa hfa Wsa Wfa( )hc+( )=

11978 31.94 21.66 0.0092 0.0076( )32.0+( )=124 MBH=

m humidifier m

a Wsa Wfa( )=

11978 0.0092 0.0076( )=19.17 lb/hr=


cfmq s

1.10 Tra Tsa( )--------------------------------------------=

480001.10 75 55( )---------------------------------------=

2182=

m acfm 60

---------------------=

2182 6013.66

------------------------=

9584 lb/h=

m lql

1100------------=

24001100------------=

2.18 lb/h=



Room temperature 75F and humidity ratio 0.00943, rel-ative humidity 50.85% and enthalpy hra= 28.31

Outside air requirement = 500 cfm.

Mass of air:

The exhaust air will be 500 cfm. So the return air will be2182 500 = 1682 cfm and in mass 9584 2104 = 7480lb/h.


Fan power:


Cooling coil capacity (condensate water at 64F hc =32.0 Btu/h):

Winter supply temperature (same fan is applied, givessame cfm):

Mass of water:

Supply air humidity ratio:

Corresponding temperature and humidity ratio of supplyair the relative humidity & enthalpy of supply air30.12% and 31.58 Btu/hr.


0.0092 2.189584------------+=

0.00943=

m oacfmoa 60

---------------------------

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