Rules of Thumb in Engineering Practice
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Rules of Thumb in Engineering Practice

Rules of Thumb in Engineering Practice by Donald R. Woods | PDF Free Download.

Engineering Practice Contents

  • Rules of Thumb
  •  Transportation
  •  Energy Exchange
  •  Homogeneous Separation
  • Heterogeneous Separations
  •  Reactors
  • Mixing
  •  Size Reduction 
  • Size Enlargement
  • Process Vessels and Facilities

Preface to Rules of Thumb in Engineering Practice

Brewster’s Dictionary of Phrase and Fable defines a rule of thumb as “a rough guestimate measure, practice or experience, as distinct from theory, in allusion to the use of the thumb for rough measurements. The first joint of the adult thumb measures almost exactly 1 inch (2.5 cm)”. Engineers need such rules of thumb to guide decisions, set goals, check results and to help answer such questions as:

  • When might I use something?
  • How do I obtain an approximate answer?
  • How might I obtain an approximate estimate of the cost?
  • What is reasonable operating know-how?
  • What might I do if something goes wrong?

Some believe that providing a collection of rules of thumb is dangerous – dangerous because engineers might forsake the fundamentals and place too much emphasis on order-of-magnitude estimates.

However, I have found for problem-solving in the industry – for design, for process improvement, and for troubleshooting – rules of thumb are not dangerous; they are essential.

From research on problem-solving, for example, we realize that skilled problem solvers create a rich internal representation of the problem. During the creation of that representation, problem solvers ask many What if? questions.

They solve a simplified version of the problem. They approximate. Rules of thumb are needed to do this well. As we problem solve, research has found that we monitor our thought processes frequently; we check and double-check often.

To do this well requires us to have a rich set of rules of thumb. When we obtain an answer to a problem, skilled problem solvers check that the answer sounds reasonable and that it answers the initial problem. We use rules of thumb to do this.

Rules of thumb are needed by working professionals. This book is unique in its consistency in terminology and units, in its extensive cross-referencing, in the range of process equipment considered, in the depth and breadth of coverage for each piece of equipment,

in the coding of the source of the rule of thumb, in its synthesis of the information into convenient and easy-to-use formats, and because it considers issues not usually considered in books about rules of thumb: career skills, how to function effectively, and the people side of engineering. This book is unique in its consistency in terminology and units.

Tower or column? motionless mixer or static mixer? tray or plate?- these are just some of the terms that are used interchangeably when discussing process equipment. In this book, consistent use of terms has been applied.

The SI units of measurement are used throughout the text. This book is unique in its extensive cross-referencing.

Some pieces of equipment are used for many different purposes. For example, fluidized beds are selected, sized, and operated as heat exchangers, dryers, reactors, coaters, and agglomerators. The details are given for each specific application with cross-referencing to lead to other uses and rules of thumb.

This cross-referencing is included in each pertinent section and in the lengthy index. This book is unique in the range of process equipment considered.

Books on rules of thumb often focus on the familiar equipment: centrifugal pumps, shell and tube exchangers, and distillation columns, few consider solids processing equipment; and solid-solid separators are rarely discussed.

In this book, I have tried to consider an extremely broad range of over 350 types of equipment, especially some of the lesser-known equipment, such as prilling, flakes, electrostatic separators, magnetic separators, foam fractionation, expellers, zone refiners, and multiple hearth furnaces. This book is unique in the depth and breadth of coverage for each piece of equipment.

Wherever possible, for each piece of equipment I have tried to include five dimensions important for the practicing engineer: the area of application (or when to use a particular type of equipment); guidelines for sizing; an approximate capital cost including hard-to-locate installation factors; principles of good practice and approaches for troubleshooting.

More specifically: x Area of Application: how or when to select: when would you use this piece of equipment? What is the usually available size range? x Guidelines: how to size: rules of thumb and short cut sizing for estimating the size of the equipment.

In general, these work within a factor of ten but usually a factor of four. x Capital Cost Guidelines: Costs should be included with any rules of thumb because costs are such vital information to engineering practice.

But these are guidelines – not data! The cost estimates given here are ballpark ideas.

The guideline FOB cost is in US $ for CEPCI = 1000. The L+M* factors are included because few published data are available. Some of these may be shown as a range, for example, 2.3–3.

This means that values have been reported in this range and no recommended value is available at this time.

The L+M* factor includes the FOB cost for carbon steel and excludes taxes, freight, delivery, duties, and instruments unless instruments are part of the package.

The * is added to remind us that the instrumentation material and labor costs have been excluded, whereas most L+M values published in the 60s, 70s, and 80s included the instrumentation material and labor costs.

The alloy corrections are given so that L+M for carbon steel can be reduced appropriately for the alloy used in the equipment.

For some unit operations, the equipment is built of concrete or is a lagoon. For such equipment, the reported cost is the Physical Module, PM cost, or the FOB plus L+M* plus instruments plus taxes and duties.

The cost excludes offsite, home office expense, field expense, and contractor’s fees and contingencies. 

Good Practice: suggestions for good operability and suggestions for sustainability, waste minimization, safety, and environmental concerns. 

Trouble Shooting: the symptom is given “Temperature I design” followed by a prioritized list of possible causes separated by “/”. Sometimes it is convenient to identify a cause and list, in turn, the sub-causes until the root cause is listed.

For example, [ fouling]* might be listed as a “cause” but what causes the fouling? Fouling is not the root cause.

Possible root causes are given in a separate listing under [Fouling]*. Such documentation is required because (i) we need to keep looking for causes until we find one that we can measure or change, and (ii) sometimes,

with extruders, for example, the cause is because the temperature is too hot or too cold. We need guidance as to what is really the cause when the temperature is too hot and what is really the root cause when the temperature is too cold. 

This book is unique because it attempts to code the source of the rule of thumb. This is important because not all rules of thumb arise from the same source. Some are a generalization of fundamentals, for example, the friction factor for turbulent flow is about 0.005; 1 kg of steam evaporates about 5 kg of organic.

Such rules of thumb will not change over time. Some are based on safety considerations; these may change as we learn more about hazards and safe operation.

Some are developed from economic analysis, but as the relative costs change then the rule of thumb will change.

For example, the “economic velocity” for pumping liquids is about 1 m s–1; but this will change as the relative costs of power, labor and materials change. Some rules of thumb are based on insurance policies or the law.

Such heuristics will change as policies and laws change. For example, in 1961 in the UK the insurance costs for a polymerizer to which live steam was attached were much, much higher than if hot water was attached directly to the vessel.

So rules of thumb about the good practice were developed in that industry to account for this. Coding is needed to remind us of the basis of the rule of thumb.

This book is unique in its synthesis of the information. This is not a convenient reproduction of material taken from different sources.

Information from different sources is compared and contrasted, gaps are filled, and the information is presented in forms useful to the sizing, selecting, and operation of the equipment. For example, sizing maps, given for pumps, heat exchangers, columns, and reactors, illustrate pictorially how different factors affect the decisions.

For filtering centrifuges, articles often describe which filtering centrifuge to select to handle slurries described by such qualitative terms as “fast filtering”, “medium filtering” and so on.

But the quantitative design parameters for filtering centrifuges are the cake build-up rate, the intrinsic permeability, and the particle size. Table 5.2, for example, relates “qualitative terms” to the “quantitative design parameters”.

A similar synthesis and clarification are done, in Table 5.3, relating “qualitative” measures of filtration to such quantitative design parameters as the rate of cake formation, filtrate rate, and cake resistance. The density-weighted velocity is important in sizing tray columns, KO pots, absorbers, and demisters.

Table 4.1 synthesizes and summarizes the values for these different applications. Corrosion, foam formation, stable emulsion formation, and fouling are concerns that affect the successful operation of many different types of equipment.

These concerns are addressed consistently, where needed, throughout the book. Interfacial and surface engineering aspects are included in the descriptions of pertinent unit operations. The selection criteria for reactors are synthesized in the series of tables in Chapter 6.

Data are given for estimating the residence times in different reactors and for different types of reactions. This book is unique because it considers issues not usually considered in books about rules of thumb.

Most books and articles about rules of thumb focus on processing equipment but effective engineers also need to communicate, work effectively in teams, solve problems, and lead.

Rules of thumb are summarized for “systems” thinking and for “career skills” such as problem-solving, creativity, leadership, entrepreneurship, and e-business.

This book uses a format similar to my other books: Process Design and Engineering Practice and Successful Trouble Shooting for Process Engineers and the section on Design that I coauthored in Marcel Dekker’s “Handbook for Chemical Engineers” Another uniqueness is the information on residence times for reactions.

Data from industrial reactors were used to create Figs. 6.5 to 6.7. These show how the residence time for reaction varies with temperature, phase, and the heat of reaction.

To my knowledge, this is the first time such an analysis has been presented in the open literature. Other detailed information is given on how the type of reaction can be used to estimate the residence time.

Unique also is the extensive analysis of dispersed phase systems, whether these are gas bubbles in liquids, sprays, a liquid-liquid dispersion, or a liquid-solid system.

Characteristics of different contactors for such systems are summarized in Tables 1.1 to 1.3 and Figs. 1.1 and 1.2. To aid in retrieving information, an extensive index is given.

Chapter 1 gives rules of thumb for physical and thermal properties, corrosion and process control; for engineering decisions related to batch versus continuous processing, the characteristics of heterogeneous phase contacting, economics, problem-solving, goal setting, decision making, thermal pinch, systems thinking, process design, process improvement, troubleshooting and environmental and safety issues.

Rules of thumb are included in the communication, listening, interpersonal skills, teamwork, performance review, leadership, entrepreneurship, entrepreneurship, e-business, and self-management.

The section on heterogeneous phase contacting includes figures and tables that compare area/volume, oxygen transfer rates, and other key characteristics of a wide range of contacting devices for gas-liquid, liquid-liquid, and particulate systems.

Chapter 2 considers equipment for transportation: gases, liquids, fluid mixtures, and solids Chapter 3 addresses energy exchange equipment.

This includes drives, motors, and turbines as well as equipment for the thermal energy exchange.

Chapter 4 describes equipment to separate homogeneous phases; these range from evaporators and distillation to membrane processes.

Chapter 5 focuses on equipment for the separation of heterogeneous phases. The chapter starts with a convenient general selection guide. More specific guides are given for liquid-solid separations, Section 5.5, and for solid-solid separations, Section 5.18.

Chapter 6 starts with the criteria for the selection of a reactor configuration. Data are given for sizing reactors. Then the details are given for over 30 reactor configurations. Chapter 7 considers mixing (of gases, liquids, mixtures, and solids).

Chapter 8 explores size reduction operations, such as foams, sprayers, emulsification, crushing and grinding, and cell disintegration.

Chapter 9 summarizes key information about equipment to increase or change the size of drops, bubbles, and particles: demisters, coalescers, flocculation, spray dryers, fluidized beds, agglomeration, pelletizing, extrusion, flakes, prilling, and coating.

Chapter 10 considers process and storage vessels, bins and hoppers, and bagging machines.

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