Page 27 - Plant design and economics for chemical engineers

1 0 PLANT DESIGN AND ECONOMICS FOR CHEMICAL ENGINEERS

of 3.43 in. Instead, the engineer would choose a standard pipe size which could
be purchased at regular market prices. In this case, the recommended pipe size
would probably be a standard 3$in.-diameter pipe having an inside diameter of
3.55 in. (9.02 cm).
If the engineer happened to be very conscientious about getting an
adequate return on all investments, he or she might say, “A standard 3-in.-
diameter pipe would require less investment and would probably only increase
the total cost slightly; therefore, I think we should compare the costs with a 3-in.
pipe to the costs with the 3$-in. pipe before making a final decision.” Theoreti-
cally, the conscientious engineer is correct in this case. Suppose the total cost of
the installed 3$in. pipe is $5000 and the total cost of the installed 3-in. pipe is
$4500. If the total yearly savings on power and fixed charges, using the 3$-in.
pipe instead of the 3-in. pipe, were $25, the yearly percent return on the extra
$500 investment would be only 5 percent. Since it should be possible to invest
the extra $500 elsewhere to give more than a 5 percent return, it would appear
that the 3-in.-diameter pipe would be preferred over the 3$in.-diameter pipe.
The logic presented in the preceding example is perfectly sound. It is a
typical example of investment comparison and should be understood by all
chemical engineers. Even though the optimum economic diameter was 3.43 in.,
the good engineer knows that this diameter is only an exact mathematical
number and may vary from month to month as prices or operating conditions
change. Therefore, all one expects to obtain from this particular optimum
economic calculation is a good estimation as to the best diameter, and invest-
ment comparisons may not be necessary.
The practical engineer understands the physical problems which are
involved in the final operation and maintenance of the designed equipment. In
developing the plant layout, crucial control valves must be placed where they
are easily accessible to the operators. Sufficient space must be available for
maintenance personnel to check, take apart, and repair equipment. The engi-
neer should realize that cleaning operations are simplified if a scale-forming
fluid is passed through the inside of the tubes rather than on the shell side of a
tube-and-shell heat exchanger. Obviously, then, sufficient plant-layout space
should be made available so that the maintenance workers can remove the head
of the installed exchanger and force cleaning worms or brushes through the
inside of the tubes or remove the entire tube bundle when necessary.
The theoretical design of a distillation unit may indicate that the feed
should be introduced on one particular tray in the tower. Instead of specifying a
tower with only one feed inlet on the calculated tray, the practical engineer will
include inlets on several trays above and below the calculated feed point since
the actual operating conditions for the tower will vary and the assumptions
included in the calculations make it impossible to guarantee absolute accuracy.
The preceding examples typify the type of practical problems the chemical
engineer encounters. In design work, theoretical and economic principles must
be combined with an understanding of the common practical problems that will
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