Physics
Heat Transfer Efficiency
Heat transfer efficiency refers to the effectiveness of transferring thermal energy from one system to another. It is a measure of how much of the input energy is successfully transferred as useful heat. Factors such as insulation, material conductivity, and design play a crucial role in determining the efficiency of heat transfer processes.
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10 Key excerpts on "Heat Transfer Efficiency"
eBook - PDFSolar Energy
Renewable Energy and the Environment
- Robert Foster, Majid Ghassemi, Alma Cota(Authors)
- 2009(Publication Date)
- CRC Press(Publisher)
55 3 Fundamentals of Engineering Thermodynamics and Heat Transfer 3.1 INTRODUCTION This chapter provides an introduction to heat transfer and engineering thermodynamics. The sci-ence of thermodynamics deals with energy interaction between a system and its surroundings. These interactions are called heat transfer and work. Thermodynamics deals with the amount of heat transfer between two equilibrium states and makes no reference to how long the process will take. However, in heat transfer, we are often interested in rate of heat transfer. Heat transfer processes set limits to the performance of environmental components and systems. The content of this chapter is intended to extend the thermodynamics analysis by describing the different modes of heat transfer. It also provides basic tools to enable the readers to estimate the magnitude of heat transfer rates and rate of entropy destruction in realistic environmental applications, such as solar energy systems. The transfer of heat is always from the higher temperature medium to the lower temperature medium. Therefore, a temperature difference is required for heat transfer to take place. Heat trans-fer processes are classified into three types: conduction, convection, and radiation. Conduction heat transfer is the transfer of heat through matter (i.e., solids, liquids, or gases) with-out bulk motion of the matter. In other words, conduction is the transfer of energy from the more energetic to less energetic particles of a substance due to interaction between them. This type of heat conduction can occur, for example, through the wall of a boiler in a power plant. The inside surface, which is exposed to gases or water, is at a higher temperature than the outside surface, which has cooling air next to it. The level of the wall temperature is critical for a boiler. Convection heat transfer is due to a moving fluid. The fluid can be a gas or a liquid; both have applications in an environmental process.
eBook - PDF- Ronald K. McLaughlin, R. Craig McLean, W. John Bonthron(Authors)
- 2016(Publication Date)
- Butterworth-Heinemann(Publisher)
The Fundamentals of Heat and Mass Transfer and Thermodynamics 2.1 INTRODUCTION To the services engineer dealing with thermal environmental problems a thorough understanding of the basic principles of heat and mass transfer and elementary thermodynamics is of vital importance. This chapter is intended as a suitable introduction to the fundamental theory of these subjects. The application of this theory to practical design problems is covered extensively in later chapters. 2.2 CONDUCTION 2.2.1 Nature of conduction Conduction is the term given to the process within a medium whereby heat is transmitted from a region of higher temperature to a region of lower temperature, without appreciable displace-ment of the particles of the medium. Conduction involves the direct transfer of kinetic energy at a molecular level and may have several distinct operating mechanisms associated with it, e.g. the elastic collision of molecules in a fluid medium or the motion of free electrons in metals. Regardless of the exact mech-anism, a knowledge of which is unimportant anyway when con-duction is considered as an engineering problem, the observable effect of the transfer between neighbouring regions will be the elimination of the temperature difference between them. Alter-natively, if the temperatures of these regions are maintained constant by the addition and removal of heat at appropriate points, the result will be a continuous flow of heat from the higher temperature region to the lower temperature one. Conduction is the only mode of heat transfer which occurs in opaque solids. It is also an important process in fluids but in these 39 2 40 The Fundamentals of Heat and Mass Transfer and Thermodynamics media it is usually combined with convection, and in some cases with radiation also. Conduction is the dominant heat transfer mechanism in fluid regions where laminar flow conditions exist, e.g. in the laminar sub-layer immediately adjacent to a solid boundary.
eBook - PDF- Stephan A. Mayer, Daniel I. Sessler, Stephan A. Mayer, Daniel I. Sessler(Authors)
- 2004(Publication Date)
- CRC Press(Publisher)
9 Thermodynamics and Heat Transfer THOMAS L. MERRILL Biomedical Engineering Department, Wyeth Research, Princeton, New Jersey, U.S.A. BACKGROUND Thermodynamics and heat transfer provide the language and the concepts to understand the fundamental nature of temper- ature and how it can be controlled (1–5). For scientists and engineers, these concepts are necessary tools to analyze and develop therapeutic temperature modulation technologies. Examples of temperature control are ubiquitous, from the ice cream freezers in your grocery store to the comfortable pas- senger cabins inside jet airliners. Stripped to its core, thermodynamics studies energy from four perspectives: generation, storage, transfer, and dissipa- tion. More specifically, the first law of thermodynamics, a 265 natural law, provides the starting point for all thermal prob- lems. From a first law perspective, the problem of jet airliner cabin temperature control is no different than patient temper- ature control. Thermodynamics, however, inherently deals with equi- librium states—states without temperature gradients. For example, although the first law can tell you the rate or the amount of heat transfer needed to maintain comfortable living conditions in a building, it is silent on what insulation should be selected. The science of heat transfer provides an essential missing link—linking the rate of heat transfer to temperature gra- dients and material properties. For example, consider the heating load for a home. Two important temperatures are needed: the desired indoor temperature and the range of outdoor temperatures. Taking these temperatures and the building design and materials into consideration, heat transfer concepts enable us to calculate the expected heat losses and the resultant internal heat generation needed for adequate tem- perature control. These loss calculations are based on four different modes of heat transfer: conduction, convection, radi- ation, and advection.
eBook - PDF- Ethirajan Rathakrishnan(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
That is, heat transfer can predict the temperature of both the metal bar and the water as a function of time. In other words, unlike ther-modynamics, heat transfer can answer the transient energy transfer questions such as the following: • Can heat be supplied to a system without employing high temperature difference? • How long would it take to supply a certain amount of heat energy to the 1 2 Basic Concepts and Definitions system? • How much heat energy is transferred between two specified instances of time during a process? • What sort of temperature distribution exists in the system? • How large an area is necessary to transfer the desired heat energy? Heat transfer finds application in all processes involving energy transfer. In this introductory text we will discuss heat transfer briefly, highlighting the basic principles of the subject. Basically there are three modes of heat transfer: conduction, convection and radiation. • Conduction is an energy transfer process from more energetic particles of a substance to the adjacent, less energetic ones as a result of the interaction between the particles. • Convection is the mode of heat transfer between a solid surface and the adjacent liquid or gas that is in motion. • Radiation is a heat transfer mode in which the energy is emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules, dictated by their temperature.
- Arun Datta(Author)
- 2013(Publication Date)
- CRC Press(Publisher)
141 © 2010 Taylor & Francis Group, LLC chapter four Heat transfer Introduction Heat transfer is an important unit operation that is used in almost all chemical industries. In industries, heat is either gained or lost by a typical fluid stream. Once a particular fluid loses heat, there will be a stream that will gain heat. There are different types of heat-transfer equipment or heat exchangers, starting from a simple double-pipe exchanger to the highly complex multi-pass shell and tube exchanger. Conductive heat transfer Heat transfer per unit area through conduction is proportional to the tem-perature gradient and mathematically defined as [1]: Q kA dt dx = - (4.1) The temperature gradient − dt / dx is the change in temperature in the x direction. Consider a heat conduction element in Figure 4.1 that receives Q x heat through the left face and rejects Q x+dx heat through the right face. Heat received and rejected by the element can be defined as Q k dz dy dT dx x = - (4.2a) Q k dz dy dT dx d T dx dx x dx + = -- 2 2 (4.2b) Heat gained by the element through the x direction will be Q Q k dz dy d T dx dx x x dx -= + 2 2 (4.3) 142 Process engineering and design using visual basic ® © 2010 Taylor & Francis Group, LLC Now, heat gained by the element can also be defined as Q Q c dx dy dz dT d x x dx -= + 1 3 6 . ρ θ (4.4) From Equations 4.3 and 4.4 dT d k c d T dx θ ρ = 3 6 2 2 . (4.5) Equation 4.5 is Fourier’s general equation. The term 3.6 k / c ρ is called the thermal diffusivity, m 2 /h. Multiplication factor 3.6 is used to balance the unit. Equation 4.5 is the heat flow through one direction and if all three directions are considered, the general equation of heat flow will be dT d k c d T dx d T dy d T dz θ ρ = + + 3 6 2 2 2 2 2 2 . (4.6) Heat conduction through a composite wall Heat conduction through a composite wall is presented in Figure 4.2.
eBook - PDFHeat Transfer Basics
A Concise Approach to Problem Solving
- Jamil Ghojel(Author)
- 2023(Publication Date)
- Wiley(Publisher)
1 Heat Transfer Basics: A Concise Approach to Problem Solving, First Edition. Jamil Ghojel. © 2024 John Wiley & Sons, Inc. Published 2024 by John Wiley & Sons, Inc. Companion website: www.wiley.com/go/ghojel/heat_transfer 1 Basic Concepts of Heat and Mass Transfer 1.1 Heat Transfer and Its Relationship With Thermodynamics Heat transfer can be looked at as a branch of classical thermodynamics with specific distinguish- ing features. This can best be explained by considering the processes taking place in the cylinder of the internal combustion engine schematically shown in Figure 1.1a. The thermodynamic processes taking place in the cylinder are elegantly and succinctly stated by the first law of thermodynamics (principle of conservation of energy) in the form − = ∆ Q W U (1.1) where ● Q is the heat generated by the combustion of the injected fuel in the cylinder at the end of the compression process, resulting in high-pressure, high temperature gaseous products ● W is the work done by the piston as the combustion products expand with the piston moving downwards ● ∆U is the balance of the heat left from the combustion process to raise the internal energy and temperature of the gases inside the cylinder. The first law of thermodynamics states that energy can be converted (transformed) from one form (heat) to another (work) but cannot be created or destroyed and that the process takes place between two states at equilibrium. The first law does not provide information on the amount of energy that can be transformed, energy transformation direction, and its effectiveness. This gap is filled by the second law of thermodynamics which stipulates that only part of the heat will be con- verted to work, heat will flow spontaneously from a high temperature source to a lower tempera- ture sink, and the flow will be accompanied by irreversible changes that degrade the resultant energy making it difficult to be utilized further.
eBook - PDFThermodynamics
Concepts and Applications
- Stephen R. Turns(Author)
- 2006(Publication Date)
- Cambridge University Press(Publisher)
Semantics In spite of our attempts here to be very precise about the definition of heat, some semantic problems arise out of traditions and nomenclature developed prior to the advent of modern thermodynamic principles. Specifically, in the common term heat transfer, the word transfer is redundant. The science of heat transfer was developed in the early 1800s [8], prior to our understanding that heat is not possessed by a body and to the development of energy conservation. 1 Rates of heat transfer and rates per unit area are also of importance and need to be distinguished. We adopt the following symbols to denote these heat interactions: We also use the word adiabatic to describe a process in which there is no heat transfer. Later in this chapter we will elaborate on the principles of heat transfer. [] W/m 2 . Q # – heat flux [] J/s or W, Q # rate of heat transfer [] J, Q heat (or heat transfer) 224 Thermodynamics Heat transfer is important in food preparation and body temperature regulation. 1 When Fourier published his theory of heat transfer (1811–1822), the common wisdom was that caloric (or heat) was a material substance, despite the fact that Benjamin Thompson in 1798 had shown that friction produces an inexhaustible supply of heat. Regardless of the fundamental nature of heat, Fourier’s mathematical analyses describing temperature distributions in solids are accurate descriptions and stand yet today as the foundation of heat-transfer theory. 4.3b Work Definition Another fundamental transfer of energy across a system boundary is work. All forms of work, regardless of their origin, are fundamentally expressions of a force acting through a distance, (4.5) that is, work is the scalar product of a vector force and the displacement vector where , and are the unit vectors in a Cartesian coordinate system. We adopt the notation to indicate the incremental quantity of work done along the differential path associated with the tangent of ds.
eBook - PDF- Carey Merritt(Author)
- 2022(Publication Date)
- Wiley(Publisher)
Process Steam Systems: A Practical Guide for Operators, Maintainers, Designers, and Educators, Second Edition. Carey Merritt. © 2023 John Wiley & Sons, Inc. Published 2023 by John Wiley & Sons, Inc. 3 UNDERSTANDING HEAT TRANSFER It is nearly impossible to fully grasp how a steam system works without a good understanding of heat transfer. A boiler in very simple terms converts fuel energy into steam. It does this by combustion and heat transfer. Overall system efficiency is directly related to heat transfer within the system. This chapter will show the reader what types of heat transfer take place within the steam system equipment and the importance of each. The reader should review this entire section before trying to apply any one heat transfer-type calculations as system heat balances should take into consideration the interrelationship of the three types of heat transfer. Once combustion takes place in the boiler furnace, the thermal energy must be transferred to the water to make steam. Similarly, the energy in the steam must be transferred to a product to complete the conversion of fuel energy to product energy. We know that whenever a temperature gradient exists, transfer of heat energy will occur. This may take the form of either conduction-, convection-, or radiation-type heat transfer. The three types are shown in Figure 3.1. To be able to fully appreciate the efficiency of a steam system we must understand where, when, and what affects these three types of heat transfer mechanisms. The efficiency of all steam systems is optimized by optimizing desirable heat transfer and preventing unwanted heat transfer. The Figure 3.2 below shows where heat energy transfer occurs in a typical steam system. RADIATION-TYPE HEAT TRANSFER The heat transfer due to the emission of energy from surfaces in the form of electromagnetic waves is known as thermal radiation.
eBook - PDF- Richard W. Hartel, D.B. Hyslop, D.B. Hyslop, T.A. Howell Jr.(Authors)
- 2008(Publication Date)
- CRC Press(Publisher)
153 chapter ten Heat transfer Heat transfer is arguably the most important transport process in the food industry, since processing of most foods requires either an increase or a decrease in temperature. Thermal processing, or heating to destroy unde-sirable microorganisms, is one of the main techniques a food technologist utilizes to ensure safety of the food supply. Sterilization and pasteurization require that a food be heated to a certain temperature and held there for suf-ficient time to destroy microorganisms before being cooled again. Both the heating and cooling steps require an understanding of heat-transfer prin-ciples. There are numerous other examples of heat transfer in the food indus-try, from baking a food in an oven, to solidification of chocolate in a cooling tunnel. Nearly every food processing system utilizes heat transfer in some form or another. Thus, knowledge of the principles of heat transfer is critical for the food technologist. Furthermore, using the principles of heat transfer together with enthalpy balances provides a powerful tool for the food tech-nologist to solve complex food-processing problems. Heat transfer occurs in one of three modes: conduction, convection, or radiation. Sometimes all three modes of heat transfer may occur at the same time. Conduction heat transfer involves transfer of thermal energy from one molecule to another. A metal rod placed in proximity to a heat source will begin initially to heat up on the end near the heat source. However, after some time, the other end will also be hot, as the molecules of the metal trans-fer the heat along the length of the rod. Materials that are good heat conduc-tors have high thermal conductivity, k ; whereas insulators, materials with low k , do not transfer heat very well by conduction. Convection heat transfer occurs when a fluid carries thermal energy from one place to another.
eBook - PDF- Alina Adriana Minea(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
249 7.4.2 . Presenting.the.Benefits.of.Efficiency. ........................................... 250 7.4.3 . Relating.Efficiency.to.Priorities. ................................................... 251 7.5 . . Applications.on.Heat.Transfer.Enhancement.in.Process.Heating. ..... 251 7.5.1 . Theoretical.Methods.of.Intensifying.Transfer.Processes. ......... 251 7.5.2 . Particularities.on.Furnaces.with.Forced.Convection. ............... 252 7.5.2.1 . Laminar.Convection,.Impulse.and.Heat. Transfer.in.One-Dimensional.Flows. ............................ 252 7.5.2.2 . Turbulent.Convection,.Impulse.and.Heat. Transfer.at.Turbulent.Flow. ............................................ 257 230 Advances in Industrial Heat Transfer 7.1 Introduction This.chapter.contains.some.basic.issues.about.the.relation.between.productiv-ity.and.technology.and.a.few.techniques.related.to.industrial.energy.savings. and.is.structured.on.six.sections . .The.first.one.is.an.introduction.followed. by.a.section.that.contains.information.about.general.aspects.on.process.heat-ing.and.specific.equipments . .Further.on,.a.few.performance.improvement. opportunities.in.industrial.systems.will.be.discussed,.emphasising.specific. aspects.on.fuel-based.systems.as.well.as.electric-based.ones . .Section.7 .4 .is.a. short.study.on.process.heating.system.economics . .Basic.applications.on.heat. transfer.enhancement.in.process.heating.are.discussed.in.Section.7 .5, .under-lying.the.most.important.equations . .Last.section.contains.some.important. conclusions.and.recommendations . Energy. efficiency. is. generally. the. largest,. least. expensive,. most. quickly. deployable,.least.visible,.least.understood,.and.most.neglected.way.to.pro-vide.energy.services . .The.largest.energy.user.in.most.countries.is.industry:. approximately.half.of.all.industrial.energy.use.is.used.in.specific.processes. in.the.energy-intensive.industries,.like.heating . .The.heat.transfer.in.furnace.
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