Chemical Engineering November 2012 - 46

Feature Report
ations cannot be justified for typical
new applications because the engineer
or manager would be facing something
like an economic version of Heisenberg's
uncertainty principle. Attempting
to model many cases, to account for
the costs of minor items, or to estimate
the costs to high accuracy can cost
enough, in itself, to alter the results of
the evaluation. The methods used and
the items considered must be limited to
those that are appropriate to the size of
the investment being considered.
For example, two 37 kW (50 hp) VFDs
would cost approximately $10,000.
Allow credits of approximately $2,000
for the starters and $4,000 for the
control valve station. If there were
no other costs to consider, the net investment
would be about $4,000. It
would obviously be unwise to spend
$4,000, or any significant fraction of
it, doing an evaluation. It would be
better to spend the money on the VFD.
A lot of time cannot be justified to
evaluate such small net investments.
Generally, the pump offerings of all
vendors will be similar and will also
be similar to the engineer's preliminary
pump selection because everyone
would be thinking alike about the
selection. An outlying offering would
probably be eliminated in a bid tabulation.
However, it is possible that a
significantly different offering could
be viable or that a usable, surplus
pump could be available that is not
close to what would be selected new.
Shukla and others [6] give an example
of evaluating multiple options. Such
possibilities might require evaluating
more than one case, but it would still
be necessary to limit the time spent on
the evaluation.
Like the cases mentioned above,
retrofits may require a more extensive
analysis. More items may be involved
because items, such as motors
or cables may have to be replaced. The
associated costs would not offset for
retrofits; therefore the evaluations are
more complex. This article can serve
as a preliminary checklist of items
that might contribute cost. The investment
is likely to be higher for retrofits,
thereby justifying more time for evaluation,
but the evaluation must still
be limited. Only items that contribute
enough cost to affect results should be
selected, and they should be evaluated
without going into great detail.
Medium-voltage VFDs require a
more thorough evaluation. The need
to spend very little time in the evaluation,
as discussed for low-voltage
VFDs, does not apply to mediumvoltage
VFDs. Also, the approximate
evaluation method does not apply to
them, although it may be a starting
point. Unlike their low-voltage counterparts,
medium-voltage VFDs are
expensive and will justify a more thorough
evaluation.
Although there are the exceptions
mentioned, little time can be
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Chemical Engineering November 2012

Table of Contents for the Digital Edition of Chemical Engineering November 2012
