Heat Transfer Applications for the Practicing Engineer E-Book

Louis Theodore

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Essential Engineering Calculations Series
• Sprache: Englisch

Seitenzahl: 777

Veröffentlichungsjahr: 2011

Beschreibung

This book serves as a training tool for individuals in industry and academia involved with heat transfer applications. Although the literature is inundated with texts emphasizing theory and theoretical derivations, the goal of this book is to present the subject of heat transfer from a strictly pragmatic point of view. The book is divided into four Parts: Introduction, Principles, Equipment Design Procedures and Applications, and ABET-related Topics. The first Part provides a series of chapters concerned with introductory topics that are required when solving most engineering problems, including those in heat transfer. The second Part of the book is concerned with heat transfer principles. Topics that receive treatment include Steady-state Heat Conduction, Unsteady-state Heat Conduction, Forced Convection, Free Convection, Radiation, Boiling and Condensation, and Cryogenics. Part three (considered the heart of the book) addresses heat transfer equipment design procedures and applications. In addition to providing a detailed treatment of the various types of heat exchangers, this part also examines the impact of entropy calculations on exchanger design, and operation, maintenance and inspection (OM&I), plus refractory and insulation effects. The concluding Part of the text examines ABET (Accreditation Board for Engineering and Technology) related topics of concern, including economies and finance, numerical methods, open-ended problems, ethics, environmental management, and safety and accident management.

Leseprobe

Contents

Cover

Half Title page

Title page

Copyright page

Dedication

Preface

Introductory Comments

Part One: Introduction

Chapter 1: History of Heat Transfer

Introduction

Peripheral Equipment

Recent History

References

Chapter 2: History of Chemical Engineering: Transport Phenomena vs Unit Operations

Introduction

History of Chemical Engineering

Transport Phenomena vs Unit Operations

What is Engineering?

References

Chapter 3: Process Variables

Introduction

Units and Dimensional Consistency

Key Terms and Definitions

References

Chapter 4: Conservation Laws

Introduction

The Conservation Laws

The Conservation Law for Momentum

The Conservation Law for Mass

The Conservation Law for Energy

References

Chapter 5: Gas Laws

Introduction

Boyle’s and Charles’ laws

The Ideal Gas Law

Standard Conditions

Partial Pressure and Partial Volume

Non-Ideal Gas Behavior

References

Chapter 6: Heat Exchanger Pipes and Tubes

Introduction

PIPES

TUBES

Valves and Fittings

Noncircular Conduits

Flow Considerations

References

Part Two: Principles

Chapter 7: Steady-State Heat Conduction

Introduction

Fourier’s Law

Conductivity Resistances

Microscopic Approach

Applications

References

Chapter 8: Unsteady-State Heat Conduction

Introduction

Classification of Unsteady-State Heat Conduction Processes

Microscopic Equations

Applications

References

Chapter 9: Forced Convection

Introduction

Convective Resistances

Heat Transfer Coefficients: Qualitative Information

Heat Transfer Coefficients: Quantitative Information

Microscopic Approach

References

Chapter 10: Free Convection

Introduction

Key Dimensionless Numbers

Describing Equations

Environmental Applications(2)

References

Chapter 11: Radiation

Introduction

Energy and Intensity

Radiant Exchange

Kirchoff’s Law

Emissivity Factors

View Factors

References

Chapter 12: Condensation and Boiling

Introduction

Condensation Fundamentals

Condensation Principles

Boiling Fundamentals

Boiling Principles

References

Chapter 13: Refrigeration and Cryogenics

Introduction

Background Material

Equipment

Materials of Construction

Insulation and Heat Loss

Storage and Transportation

Hazards, Risks, and Safety

Basic Principles and Applications

References

Part Three: Part Three: Heat Transfer Equipment Design Procedures and Applications

Chapter 14: Introduction to Heat Exchangers

Introduction

Energy Relationships

Heat Exchange Equipment Classification

The Log Mean Temperature Difference (LMTD) Driving Force

Overall Heat Transfer Coefficients

The Heat Transfer Equation

References

Chapter 15: Double Pipe Heat Exchangers

Introduction

Equipment Description

Describing Equations

Calculation of Exit Temperatures(7)

Effectiveness Factor and Number of Transfer Units

Wilson’s Method

References

Chapter 16: Shell and Tube Heat Exchangers

Introduction

Equipment Description

Describing Equations

The “F” Factor

Effectiveness Factor and Number Of Transfer Units

References

Chapter 17: Fins and Extended Surfaces

Introduction

Fin Types

Describing Equations

Fin Effectiveness and Performance

Fin Considerations

References

Chapter 18: Other Heat Exchange Equipment

Introduction

Evaporators

Waste Heat Boilers

Condensers

Quenchers

References

Chapter 19: Insulation and Refractory

Introduction

Describing Equations

Insulation

Refractory

References

Chapter 20: Operation, Maintenance, and Inspection (OM&I)

Introduction

Installation Procedures

Operation

Maintenance and Inspection

Testing

Improving Operation and Performance

References

Chapter 21: Entropy Considerations and Analysis

Introduction

Qualitative Review of the Second Law

Describing Equations

The Heat Exchanger Dilemma

Applications

References

Chapter 22: Design Principles and Industrial Applications

Introduction

General Design Procedures

Process Schematics

Purchasing A Heat Exchanger

Applications

References

Chapter 23: Environmental Management

Introduction

Environmental Management History

Environmental Management Topics

Applications

References

Chapter 24: Accident and Emergency Management

Introduction

Legislation

USEPA’s Risk Management Program

Hazard Risk Assessment

Applications

References

Chapter 25: Ethics

Introduction

Teaching Ethics

The Case Study Approach

Applications

References

Chapter 26: Numerical Methods

Introduction

History

Partial Differential Equations

Regresion Analysis

Optimization

References

Chapter 27: Economics and Finance

Introduction

The Need for Economic Analyses

Definitions

Principles of Accounting

Applications

References

Chapter 28: Open-Ended Problems

Introduction

Developing Students’ Power Of Critical Thinking

Creativity

Brainstorming

Inquiring Minds

Applications

References

Appendix A: Units

A.1 The Metric System

A.2 The SI System

A.3 Seven Base Units

A.4 Two Supplementary Units

A.5 SI Multiples and Prefixes

A.6 Conversion Constants

A.7 Selected Common Abbreviations

Appendix B: Tables

Appendix C: Figures

Appendix D: Steam Tables

Index

Heat Transfer Applications for the Practicing Engineer

Copyright © 2011 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New JerseyPublished simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

Theodore, Louis.

Heat transfer applications for the practicing engineer/Louis Theodore.p. cm. – (Essential engineering calculations series; 4)Includes index.ISBN 978-0-470-64372-3 (hardback) 1. Heat exchangers. 2. Heat–Transmission. I. Title. TJ263.T46 2011 621.402’2–dc23

2011016265

oBook ISBN: 9780470937228 ePDF ISBN: 9780470937211 ePub ISBN: 9781118002100

ToJack Powers

My friend,a future basketball Hall-of-Famer,anda true quality individual

Preface

We should be careful to get out of an experience only the wisdom that is in it—and stop there; lest we be like the cat that sits down on a hot stove-lid. She will never sit down on a hot stove-lid again—and that is well; but also she will never sit down on a cold one anymore.

Mark Twain (Samuel Langhorne Clemens 1835–1910),Pudd’nhead Wilson, Chapter 19

This project was a rather unique undertaking. Heat transfer is one of the three basic tenants of chemical engineering and engineering science, and contains many basic and practical concepts that are utilized in countless industrial applications. The author therefore considered writing a practical text. The text would hopefully serve as a training tool for those individuals in industry and academia involved directly, or indirectly, with heat transfer applications. Although the literature is inundated with texts emphasizing theory and theoretical derivations, the goal of this text is to present the subject of heat transfer from a strictly pragmatic point-of-view.

The book is divided into four Parts: Introduction, Principles, Equipment Design Procedures and Applications, and ABET-related Topics. The first Part provides a series of chapters concerned with introductory topics that are required when solving most engineering problems, including those in heat transfer. The second Part of the book is concerned with heat transfer principles. Topics that receive treatment include steady-state heat conduction, unsteady-state heat conduction, forced convection, free convection, radiation, boiling and condensation, and cryogenics. Part Three—considered by the author to be the “meat” of the book—addresses heat transfer equipment design procedures and applications. In addition to providing a detailed treatment of the various types of heat exchangers, this part also examines the impact of entropy calculations on exchanger design, operation, maintenance and inspection (OM&I), plus refractory and insulation effects. The concluding Part of the text examines ABET (Accreditation Board for Engineering and Technology)-related topics of concern, including environmental management, safety and accident management, ethics, numerical methods, economics and finance, and open-ended problems. An appendix is also included. An outline of the topics covered can be found in the Table of Contents.

The author cannot claim sole authorship to all of the problems and material in this text. The present book has evolved from a host of sources, including: notes, homework problems and exam problems prepared by several faculty for a required one-semester, three-credit, “Principles II: Heat Transfer” undergraduate course offered at Manhattan College; I. Farag and J. Reynolds, “Heat Transfer”, A Theodore Tutorial, East Williston, N.Y., 1994; J. Reynolds, J. Jeris, and L. Theodore, “Handbook of Chemical and Environmental Engineering Calculations”, John Wiley & Sons 2004, and J. Santoleri, J. Reynolds, and L. Theodore’s “Introduction to Hazardous Waste Incineration”, 2nd edition, John Wiley & Sons, 2000. Although the bulk of the problems are original and/or taken from sources that the author has been directly involved with, every effort has been made to acknowledge material drawn from other sources.

It is hoped that this writing will place in the hands of industrial, academic, and government personnel, a book covering the principles and applications of heat transfer in a thorough and clear manner. Upon completion of the text, the reader should have acquired not only a working knowledge of the principles of heat transfer operations, but also experience in their application; and, the reader should find himself/herself approaching advanced texts, engineering literature, and industrial applications (even unique ones) with more confidence. The author strongly believes that, while understanding the basic concepts is of paramount importance, this knowledge may be rendered virtually useless to an engineer if he/she cannot apply these concepts to real-world situations. This is the essence of engineering.

Last, but not least, I believe that this modest work will help the majority of individuals working and/or studying in the field of engineering to obtain a more complete understanding of heat transfer. If you have come this far, and read through most of the Preface, you have more than just a passing interest in this subject. I strongly suggest that you try this text; I think you will like it.

My sincere thanks are extended to Dr. Jerry Maffia and Karen Tschinkel at Manhattan College for their help in solving some of the problems and proofing the manuscript, and to the ever reliable Shannon O’Brien for her valuable assistance.

LOUIS THEODORE

Introductory Comments

Prior to undertaking the writing of this text, the author (recently) co-authored a text entitled “Thermodynamics for the Practicing Engineer”. It soon became apparent that some overlap existed between thermodynamic and heat transfer (the subject of this text). Even though the former topic is broadly viewed as a science, heat transfer is one of the unit operations and can justifiably be classified as an engineering subject. But what are the similarities and what are the differences?

The similarities that exist between thermodynamics and heat transfer are grounded in the three conservation laws: mass, energy, and momentum. Both are primarily concerned with energy-related subject matter and both, in a very real sense, supplement each other. However, thermodynamics deals with the transfer of energy and the conversion of energy into other forms of energy (e.g., heat into work), with consideration generally limited to systems in equilibrium. The topic of heat transfer deals with the transfer of energy in the form of heat; the applications almost exclusively occur with heat exchangers that are employed in the chemical, petrochemical, petroleum (refinery), and engineering processes.

The aforementioned transfer of heat occurs between a hot and a cold body, normally referred to as the source and receiver, respectively. (The only exception is in cryogenic applications.) When this transfer occurs in a heat exchanger, some or all of the following 10 topics/subjects can come into play:

1. The class of heat exchanger

2. The physical surface arrangement of the exchanger

3. The quantity or rate of heat transferred

4. The quantity or rate of heat “lost” in the application

5. The temperature difference between the source and receiver

6. The prime mover(s) required in the application (e.g., pump, fan, etc.)

7. The entropy gain (i.e., the quality energy lost in the application)

8. The cost to design, construct, and start up a new application

9. The cost to operate the exchanger

10. The cost to maintain the exchanger

Each of the above topics receive treatment once or several times in this text.

Part One

Introduction

Part One serves as the introductory section to this book. It reviews engineering and science fundamentals that are an integral part of the field of heat transfer. It consists of six chapters, as noted below:

1. History of Heat Transfer

2. History of Chemical Engineering: Transport Phenomena vs Unit Operations

3. Process Variables

4. Conservation Laws

5. Gas Laws

6. Heat Exchanger Pipes and Tubes

Those individuals with a strong background in the above area(s) may choose to bypass all or some of this Part.

Chapter 1

History of Heat Transfer*

INTRODUCTION

After a review of the literature, the author has concluded that the concept of heat transfer was first introduced by the English scientist Sir Isaac Newton in his 1701 paper entitled “Scala Graduum Caloris.”(1) The specific ideas of heat convection and Newton’s Law of Cooling were developed from that paper.

Before the development of kinetic theory in the middle of the 19th century, the transfer of heat was explained by the “caloric” theory. This theory was introduced by the French chemist Antoine Lavoisier (1743–1794) in 1789. In his paper, Lavoisier proposed that caloric was a tasteless, odorless, massless, and colorless substance that could be transferred from one body to another and that the transfer of caloric to a body increased the temperature, and the loss of calorics correspondingly decreased the temperature. Lavoisier also stated that if a body cannot absorb/accept any additional caloric, then it should be considered saturated and, hence, the idea of a saturated liquid and vapor was developed.(2)

Lavoisier’s caloric theory was never fully accepted because the theory essentially stated that heat could not be created or destroyed, even though it was well known that heat could be generated by the simple act of rubbing hands together. In 1798, an American physicist, Benjamin Thompson (1753–1814), reported in his paper that heat was generated by friction, a form of motion, and not by caloric flow. Although his idea was also not readily accepted, it did help establish the law of conservation of energy in the 19th century.(3)

In 1843, the caloric theory was proven wrong by the English physicist James P. Joule (1818–1889). His experiments provided the relationship between mechanical work and the nature of heat, and led to the development of the first law of thermodynamics of the conservation of energy.(4)

The development of kinetic theory in the 19th century put to rest all other theories. Kinetic theory states that energy or heat is created by the random motion of atoms and molecules. The introduction of kinetic theory helped to develop the concept of the conduction of heat.(5)

The earlier developments in heat transfer helped set the stage for the French mathematician and physicist Joseph Fourier (1768–1830) to reconcile Newton’s Law of Cooling, which in turn led to the development of Fourier’s Law of Conduction. Newton’s Law of Cooling suggested that there was a relationship between the temperature difference and the amount of heat transferred. Fourier took Newton’s Law of Cooling and arrived at a convection heat equation.(6) Fourier also developed the concepts of heat flux and temperature gradient. Using the same process as he used to develop the equation of heat convection, Fourier subsequently developed the classic equation for heat conduction that has come to be defined as Fourier’s law.(7)

Two additional sections complement the historical contents of this chapter. These are:

Peripheral Equipment

Recent History

PERIPHERAL EQUIPMENT

With respect to heat transfer equipment, the bulk of early equipment involved the transfer of heat across pipes. The history of pipes dates back to the Roman Empire. The ingenious “engineers” of that time came up with a solution to supply the never-ending demand of a city for fresh water and then for disposing of the wastewater produced. Their system was based on pipes made out of wood and stone, and the driving force of the water was gravity.(8) Over time, many improvements have been made to the piping system. These improvements include material choice, shape, and size of the pipes: pipes are now made from different metals, plastic, and even glass, with different diameters and wall thicknesses. The next challenge was the connection of the pipes and that was accomplished with fittings. Changes in piping design ultimately resulted from the evolving industrial demands for specific heat transfer requirements and the properties of fluids that needed to be heated or cooled.(9)

The movement of the fluids to be heated or cooled was accomplished with prime movers, particularly pumps. The first pump can be traced back to 3000 B.C., in Mesopotamia, where it was used to supply water to the crops in the Nile River Valley.(10) The pump was a long lever with a weight on one side and a bucket on the other. The use of this first pump became popular in the Middle East and was used for the next 2000 years. At times, a series of pumps would be put in place to provide a constant flow of water to crops far from the source. The most famous of these early pumps is the Archimedean screw. The pump was invented by the famous Greek mathematician and inventor Archimedes (287–212 B.C.). The pump was made of a metal pipe in which a helix-shaped screw was used to draw water upward as the screw turned. Modern force pumps were adapted from an ancient pump that featured a cylinder with a piston “at the top that create[d] a vacuum and [drew] water upward.”(10) The first force pump was designed by Ctesibus (285–222 B.C.) of Alexandria, Egypt. Leonardo Da Vinci (1452–1519) was the first to come up with the idea of lifting water by means of centrifugal force; however, the operation of the centrifugal pump was first described scientifically by the French physicist Denis Papin (1647–1714) in 1687.(11) In 1754, Leonhard Euler further developed the principles on which centrifugal pumps operated; today, the ideal pump performance term, “Euler head,” is named after him.(12)

RECENT HISTORY

Heat transfer, as an engineering practice, grew out of thermodynamics at around the turn of the 20th century. This arose because of the need to deal with the design of heat transfer equipment required by emerging and growing industries. Early applications included steam generators for locomotives and ships, and condensers for power generation plants. Later, the rapidly developing petroleum and petrochemical industries began to require rugged, large-scale heat exchangers for a variety of processes. Between 1920 and 1950, the basic forms of the many heat exchangers used today were developed and refined, as documented by Kern.(13) These heat exchangers still remain the choice for most process applications. Relatively speaking, there has been little since in terms of “new” designs. However, there has been a significant amount of activity and development regarding peripheral equipment. For example, the 1930s saw the development of a line of open-bucket steam traps, which today are simply referred to as steam traps. (Note: Steam traps are used to remove condensate from live steam in heat exchangers. The trap is usually attached at the bottom of the exchanger. When condensate enters the steam trap, the liquid fills the entire body of the trap. A small hole in the top of the trap permits trapped air to escape. As long as live steam remains, the outlet remains closed. As soon as sufficient condensate enters the trap, liquid is discharged. Thus, the trap discharges intermittently during the entire time it is in use.)

Starting in the late 1950s, at least three unrelated developments rapidly changed the heat exchanger industry.

1. With respect to heat-exchanger design and sizing, the general availability of computers permitted the use of complex calculational procedures that were not possible before.

2. The development of nuclear energy introduced the need for precise design methods, especially in boiling heat transfer (see Chapter 12).

3. The energy crisis of the 1970s severely increased the cost of energy, triggering a demand for more-efficient heat utilization (see Chapter 21).(14)

As a result, heat-transfer technology suddenly became a prime recipient of large research funds, especially during the 1960s and 1980s. This elevated the knowledge of heat-exchanger design principles to where it is today.(15)

REFERENCES

1. E. LAYTON, History of Heat Transfer: Essays in the Honor of the 50th Anniversary of the ASME Heat Transfer Division, date and location unknown.

2. Y. CENGEL, Heat Transfer, 2nd edition, McGraw-Hill, New York City, NY, 2003.

3. http://en.wikipedia.org/wiki/Benjamin_Thompson#Experiments_on_heat

4. http://en.wikipedia.org/wiki/James_Prescott_Joule

5. http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thercond.html

6. J. P. HOLMAN, Heat Transfer, 7th edition, McGraw-Hill, New York City, NY, 1990.

7. J. B. FOURIER, Théorie Analytique de la Chaleur, Gauthier-Villars, Paris, 1822; German translation by Weinstein, Springer, Berlin, 1884; Ann. Chim. Phys., 37(2), 291 (1828); Pogg. Ann., 13, 327 (1828).

8. http://www.unrv.com/culture/roman-aqueducts.php,2004.

9. P. ABULENCIA and L. THEODORE, Fluid Flow for the Practicing Engineer, John Wiley & Sons, Hoboken, NJ, 2009.

10. http://www.bookrags.com/sciences/sciencehistory/water-pump-woi.html

11. A. H. CHURCH and J. LAL, Centrifugal Pumps and Blowers, John Wiley & Sons Inc., Hoboken, NJ, 1973.

12. R. D. FLACK, Fundamentals of Jet Propulsion with Applications, Cambridge University Press, New York City, NY, 2005.

13. D. KERN, Process Heat Transfer, McGraw-Hill, New York City, NY, 1950.

14. L. THEODORE, F. RICCI, and T. VAN VLIET, Thermodynamics for the Practicing Engineer, John Wiley & Sons, Hoboken, NJ, 2009.

15. J. TABOREK, Process Heat Transfer, Chem. Eng., New York City, NY, August 2000.

* Part of this chapter was adapted from a report submitted by S. Avais to L. Theodore in 2007.

Chapter 2

History of Chemical Engineering: Transport Phenomena vs Unit Operations

INTRODUCTION

Although the chemical engineering profession is usually thought to have originated shortly before 1900, many of the processes associated with this discipline were developed in antiquity. For example, filtration operations were carried out 5000 years ago by the Egyptians. During this period, chemical engineering evolved from a mixture of craft, mysticism, incorrect theories, and empirical guesses.

In a very real sense, the chemical industry dates back to prehistoric times when people first attempted to control and modify their environment. The chemical industry developed as any other trade or craft. With little knowledge of chemical science and no means of chemical analysis, the earliest “chemical engineers” had to rely on previous art and superstition. As one would imagine, progress was slow. This changed with time. The chemical industry in the world today is a sprawling complex of raw-material sources, manufacturing plants, and distribution facilities which supplies society with thousands of chemical products, most of which were unknown over a century ago. In the latter half of the 19th century, an increased demand arose for engineers trained in the fundamentals of chemical processes. This demand was ultimately met by chemical engineers.

Three sections complement the presentation for this chapter. They are:

History of Chemical Engineering

Transport Phenomena vs Unit Operations

What is Engineering?

HISTORY OF CHEMICAL ENGINEERING

The first attempt to organize the principles of chemical processing and to clarify the professional area of chemical engineering was made in England by George E. Davis. In 1880, he organized a Society of Chemical Engineers and gave a series of lectures in 1887 which were later expanded and published in 1901 as “A Handbook of Chemical Engineering.” In 1888, the first course in chemical engineering in the United States was organized at the Massachusetts Institute of Technology (MIT) by Lewis M. Norton, a professor of industrial chemistry. The course applied aspects of chemistry and mechanical engineering to chemical processes.(1)

Chemical engineering began to gain professional acceptance in the early years of the 20th century. The American Chemical Society was founded in 1876 and, in 1908, organized a Division of Industrial Chemists and Chemical Engineers while authorizing the publication of the Journal of Industrial and Engineering Chemistry. Also in 1908, a group of prominent chemical engineers met in Philadelphia and founded the American Institute of Chemical Engineers.(1)

The mold for what is now called chemical engineering was fashioned at the 1922 meeting of the American Institute of Chemical Engineers when A. D. Little’s committee presented its report on chemical engineering education. The 1922 meeting marked the official endorsement of the unit operations concept and saw the approval of a “declaration of independence” for the profession.(1) A key component of this report included the following:

Any chemical process, on whatever scale conducted, may be resolved into a coordinated series of what may be termed “unit operations,” as pulverizing, mixing, heating, roasting, absorbing, precipitation, crystallizing, filtering, dissolving, and so on. The number of these basic unit operations is not very large and relatively few of them are involved in any particular process … An ability to cope broadly and adequately with the demands of this (the chemical engineer’s) profession can be attained only through the analysis of processes into the unit actions as they are carried out on the commercial scale under the conditions imposed by practice.

The key unit operations were ultimately reduced to three: Fluid Flow,(2) Heat Transfer (the subject title of this text), and Mass Transfer.(3) The Little report also went on to state that:

Chemical Engineering, as distinguished from the aggregate number of subjects comprised in courses of that name, is not a composite of chemistry and mechanical and civil engineering, but is itself a branch of engineering, …

A time line diagram of the history of chemical engineering between the profession’s founding to the present day is shown in Figure 2.1. As can be seen from the time line, the profession has reached a crossroads regarding the future education/curriculum for chemical engineers. This is highlighted by the differences of Transport Phenomena and Unit Operations, a topic that is discussed in the next section.

Figure 2.1 Chemical engineering time-line.

TRANSPORT PHENOMENA VS UNIT OPERATIONS

As indicated in the previous section, chemical engineering courses were originally based on the study of unit processes and/or industrial technologies. It soon became apparent that the changes produced in equipment from different industries were similar in nature (i.e., there was a commonality in the fluid flow operations in the petroleum industry as with the utility industry). These similar operations became known as the aforementioned Unit Operations. This approach to chemical engineering was promulgated in the Little report, as discussed earlier in the previous section, and to varying degrees and emphasis, has dominated the profession to this day.

The Unit Operations approach was adopted by the profession soon after its inception. During the many years since 1880 that the profession has been in existence as a branch of engineering, society’s needs have changed tremendously and, in turn, so has chemical engineering.

The teaching of Unit Operations at the undergraduate level has remained relatively static since the publication of several early-to-mid 1900 texts. Prominent among these was one developed as a result of the recommendation of an advisory committee of more than a dozen educators and practicing engineers who recognized the need for a chemical engineering handbook. Dr. John H. Perry of Grasselli Chemical Co. was persuaded to undertake this tremendous compilation. The first edition of this classic work was published in 1934; the latest edition (eighth) was published in 2008. (The author of this text has served as an editor and author of the section on Environment Management for the past three editions). However, by the middle of the 20th century, there was a slow movement from the unit operation concept to a more theoretical treatment called transport phenomena. The focal point of this science was the rigorous mathematical description of all physical rate processes in terms of mass, heat, or momentum crossing boundaries. This approach took hold of the education/curriculum of the profession with the publication of the first edition of the Bird et al.(5) book. Some, including the author of this text, feel that this concept set the profession back several decades since graduating chemical engineers, in terms of training, were more applied physicists than traditional chemical engineers.

There has fortunately been a return to the traditional approach of chemical engineering in recent years, primarily due to the efforts of the Accreditation Board for Engineering and Technology (ABET). Detractors to this approach argue that this type of practical education experience provides the answers to ‘what’ and ‘how’ but not ‘why’ (i.e., a greater understanding of both physical and chemical processes). However, the reality is that nearly all practicing engineers are in no way presently involved with the ‘why’ questions; material normally covered here has been replaced, in part, with a new emphasis on solving design and open-ended problems. This approach is emphasized in this text.

One can qualitatively describe the differences between the two approaches discussed above. Both deal with the transfer of certain quantities (momentum, energy, and mass) from one point in a system to another. Momentum, energy, and mass are all conserved (see Chapter 4). As such, each quantity obeys the conservation law within a system:

(2.1)

This equation may also be written on a time rate basis:

(2.2)

The conservation law may be applied at the macroscopic, microscopic, or molecular level. One can best illustrate the differences in these methods with an example. Consider a system in which a fluid is flowing through a cylindrical tube (see Figure 2.2), and define the system as the fluid contained within the tube between points 1 and 2 at any time.

Figure 2.2 Flow through a cylinder.

If one is interested in determining changes occurring at the inlet and outlet of the system, the conservation law is applied on a “macroscopic” level to the entire system. The resultant equation describes the overall changes occurring to the system (or equipment). This approach is usually applied in the Unit Operation (or its equivalent) courses, an approach which is highlighted in this text. The resulting equations are almost always algebraic.

In the microscopic approach, detailed information concerning the behavior within a system is required and this is occasionally requested of or by the engineer. The conservation law is then applied to a differential element within the system which is large compared to an individual molecule, but small compared to the entire system. The resulting equation is usually differential, and is then expanded via an integration to describe the behavior of the entire system. This has been defined as the transport phenomena approach.

The molecular approach involves the application of the conservation laws to individual molecules. This leads to a study of statistical and quantum mechanics—both of which are beyond the scope of this text. In any case, the description of individual particles at the molecular level is of little value to the practicing engineer. However, the statistical averaging of molecular quantities in either a differential or finite element within a system can lead to a more meaningful description of the behavior of a system.

Both the microscopic and molecular approaches shed light on the physical reasons for the observed macroscopic phenomena. Ultimately, however, for the practicing engineer, these approaches may be valid but are akin to killing a fly with a machine gun. Developing and solving these equations (in spite of the advent of computer software packages) is typically not worth the trouble.

Traditionally, the applied mathematician has developed the differential equations describing the detailed behavior of systems by applying the appropriate conservation law to a differential element or shell within the system. Equations were derived with each new application. The engineer later removed the need for these tedious and error-prone derivations by developing a general set of equations that could be used to describe systems. These are referred to as the transport equations. In recent years, the trend toward expressing these equations in vector form has also gained momentum (no pun intended). However, the shell-balance approach has been retained in most texts, where the equations are presented in componential form—in three particular coordinate systems—rectangular, cylindrical, and spherical. The componential terms can be “lumped” together to produce a more concise equation in vector form. The vector equation can in turn, be re-expanded into other coordinate systems. This information is available in the literature.(5,6)

WHAT IS ENGINEERING?

A discussion on chemical engineering is again warranted before proceeding to the heat transfer material presented in this text. A reasonable question to ask is: What is Chemical Engineering? An outdated but once official definition provided by the American Institute of Chemical Engineers (AIChE) is:

Chemical Engineering is that branch of engineering concerned with the development and application of manufacturing processes in which chemical or certain physical changes are involved. These processes may usually be resolved into a coordinated series of unit physical operation and chemical processes. The work of the chemical engineer is concerned primarily with the design, construction, and operation of equipment and plants in which these unit operations and processes are applied. Chemistry, physics, and mathematics are the underlying sciences of chemical engineering, and economics is its guide in practice.

The above definition has been appropriate up until a few decades ago since the profession grew out of the chemical industry. Today, that definition has changed. Although it is still based on chemical fundamentals and physical principles, these principles have been de-emphasized in order to allow the expansion of the profession to other areas (biotechnology, semiconductors, fuel cells, environment, etc.). These areas include environmental management, health and safety, computer applications, and economics and finance. This has led to many new definitions of chemical engineering, several of which are either too specific or too vague. A definition proposed by the author is simply “chemical engineers solve problems.” This definition can be extended to all engineers and thus “engineers solve problems.”

Obviously, the direction of the engineering profession, and chemical engineering in particular, has been a moving target over the past 75 years. For example, a distinguished AIChE panel in 1952 gave answers to the question: “Whither, chemical engineering as a science?” The panel concluded that the profession must avoid freezing concepts into a rigid discipline that leaves no room for growth and development. The very fluidity of chemical engineering must continue to be one of its most distinguishing aspects. In 1964, J. Hedrick of Cornell University (at an AIChE Tri-Section Symposium in Newark, NJ) posed the question “Will there still be a distinct profession of chemical engineering twenty years from now?” The dilemma has surfaced repeatedly in the past 50 years. More recently Theodore(7) addressed the issue; here is part of his comments:

One of my goals is to keep in touch with students following graduation. What I have learned from graduates in the workforce is surprising—approximately 75% of them use little to nothing of what was taught in class. Stoichiometry? Sometimes. Unit operations? Sometimes. Kinetics? Not often. Thermodynamics? Rarely. Transport Phenomena? Forget about it. It is hard to deny that the chemical engineering curriculum is due for an overhaul.

The traditional chemical engineers who can design a heat exchanger, predict the performance of an adsorber, specify a pump, etc., have become a dying breed. What really hurts is that I consider myself in this category. Fortunately (or perhaps unfortunately), I’m in the twilight of my career.

Change won’t come easy. Although several universities in the U.S. are pioneering new programs and course changes aimed at the chemical engineer of the furture, approval by the academic community is not unanimous. Rest assured that most educators will do everything in their power to protect their “turf”.

But change really does need to come. Our profession owes it to the students.

The main thrust of these comments can be applied to other engineering and science disciples.

REFERENCES

1. N. SERINO, 2005 Chemical Engineering 125th Year Anniversary Calendar, term project, submitted to L. Theodore, 2004.

2. P. ABULENCIA and L. THEODORE, Fluid Flow for the Practicing Engineer, John Wiley & Sons, Hoboken, NJ, 2009.

3. L. THEODORE and F. RICCI, Mass Transfer for the Practicing Engineer, John Wiley & Sons, Hoboken, NJ, 2010.

4. D. GREEN and R. PERRY (editors), Perry’s Chemical Engineers’ Handbook, 8th edition, McGraw-Hill, New York City, NY, 2008.

5. R. N. BIRD, W. STEWART, and E. LIGHTFOOT, Transport Phenomena, John Wiley & Sons, Hoboken, NJ, 1960.

6. L. THEODORE, Transport Phenomena for Engineers, International Textbook Company, Scranton, PA, 1971 (with permission).

7. L. THEODORE, The Challenge of Change, CEP, New York City, NY, January 2007.