T his book has been written with the double purpose of being used as a textbook for university courses in unit operations and as a consulting book in the professional field of process engineering. Thus every attempt has been made to maintain a difficult equilibrium in the weights assigned to theory and practice. It is assumed that the reader has taken courses in thermodynamics and is familiar with energy balances and the calculation of physical properties in simple systems with and without change of phase. Thus only a few basic concepts considered convenient for the fluidity of the redaction were included. It is also assumed that the reader have some basic knowledge of fluid mechanics and can write mechanical energy balances and calculate pressure drops in piping systems.
A difficult decision was to choose the approach to Chap. 4 on convection. In the chemical engineering curricula in many universities, unit operations courses are preceded by a course in transport phenomena. In other universities, they are not. Some knowledge of the theory of transport phenomena helps in an understanding of the principles of convective heat transfer and allows a more elegant treatment of this subject, but a treatment based on transport equations also increases the mathematical complexity and is not particularly useful in daily engineering practice. Therefore, after rewriting Chap. 4 a few times, it was finally decided to avoid presenting a theoretical approach based on transport equations so as not to drive away readers who
are not friends of mathematics.
Other chapters present subjects associated with the thermal design of different kinds of heat transfer equipment. Nowadays, in the professional field, this task is performed almost exclusively by commercial software. There are many programs on the market, some of them developed by important companies that have well-known heat transfer researchers working for them. Additionally, since these programs are used worldwide, many users supply important feedback that allows error corrections and fine-tuning of the correlations.
Thus it is difficult to conceive of heat transfer equipment design without the help of these programs. However, most of these programs are presented as a “black box,” and the suppliers offer very little information about their content and the correlations used. This makes their use difficult. It is necessary for users to know how the program will use its input data to evaluate the importance of each input data field.
For example, to calculate the boiling heat transfer coefficient, some programs use correlations based on critical properties. Other programs do not use the critical properties and calculate boiling heat transfer coefficients based on properties that are difficult to predict, such as surface tension of the boiling liquid. This means that some variables that are important in some programs are not important in others. Boiling heat transfer is probably the most complicated subject. There are limits to the maximum heatflux density that sometimes define equipment design. Prediction of these limits is ambiguous and varies according to the approach of different authors. Frequently, when comparing designs created with different software packages, one discovers important differences in the calculated heat transfer areas due to different criteria in the adoption of these limits. In some cases, the limits are incorporated as simple “rules of thumb” without any theoretical background and can be changed by the user if desired, which obviously changes the program results. Thus it is important that users have the necessary knowledge to be capable of investigating the calculation path on which the design is performed
In heat transfer equipment design, many independent variables must be adopted. To simplify the use of this software by nonexperienced users, the programs usually have default values for many of the variables. There is a natural tendency to accept the first results offered by the program without investigating the effect of modifications in some of these variables. Sometimes, however, the modification of the default values allows a significant improvement in the design.
For example, a change in the number of tube rows or the air face velocity in an air cooler can be translated in a substantial decrease in the required heat transfer area. This is why it is very important that users know the theoretical background behind the programs to make an efficient use of them. On the other hand, the complexity and diversity of situations that may exist in the design of heat transfer equipment make it impossible to cover all the possible situations with general and simple correlations such as those presented in this book.
This book includes design methods that, based on the author’s experience, allow readers to obtain reasonable results in most cases. However, it is not possible to guarantee that in certain specific situations the use of these general methods do not result in appreciable deviations in the heat transfer coefficients. Therefore, the recommendation is to use the commercial software, but doing so with enough knowledge
of the subject to be able to evaluate and analyze the results. To that end, having a grasp of the simple tools presented in this book will allow process engineers to perform their own calculations and to detect the critical aspects of the design.
Eduardo Cao, "Heat Transfer in Process Engineering"
McGraw-Hill Professional | 2009-08-12 | ISBN: 0071624082 | 576 pages | PDF | 8,1 MB
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