Page 357 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

You are given an assignment that involves writing a report that is to be completed and presented
in two weeks. You work diligently and feel confident you have come up with a respectable
solution. You present the written report personally to your director (boss), who asks you to
summarize only your final conclusions. Immediately after providing this information, your boss
declares that “your results must be wrong” and returns your report unopened and unread.

You return to your desk angry. Your comprehensive and well-written report was not even opened
and read. Your boss did not tell you what was wrong, and you did not receive any “partial credit”
for all your work. After a while, you cool off and review your report. You find that you had made a
“simple” error, causing your answer to be off by an order of magnitude. You correct the error
and turn in a revised report.

What remains is the nagging question, “How could your boss know you made an error without
having reviewed your report or asking any questions?”

The answer to this nagging question is probably a direct result of your director’s experience with a
similar problem or knowledge of some guideline that contradicted your answer. The ability of your boss
to transfer personal experience to new situations is one reason why he or she was promoted to that
position.

It is important to be able to apply knowledge gained through experience to future problems.

11.1.1 Introduction to Technical Heuristics and Shortcut Methods

A heuristic is a statement concerning equipment sizes, operating conditions, and equipment performance
that reduces the need for calculations. A shortcut method replaces the need for extensive calculations in

order to evaluate equipment sizes, operating conditions, and equipment performance. These are referred
to as “back-of-the-envelope calculations.” In this text, we refer to both of these experience-based tools as
guidelines or heuristics.

The guidelines provided in this chapter are limited to materials specifically covered in this text (including
problems at the end of the chapters). All such material is likely to be familiar to final year B.S. chemical
engineering students and new graduates as a result of their education. Upon entering the work force,
engineers will develop guidelines that apply specifically to their area of responsibility.

Guidelines and heuristics must be applied with an understanding of their limitations. In most cases, a
novice chemical engineer should have sufficient background to apply the rules provided in this text.

The narrative started earlier is now revisited. The assignment remains the same; however, the approach to
solving the problem changes.

Before submitting your report, you apply a heuristic that highlights an inconsistency in your

initial results. You then review your calculations, find the error, and make corrections before