Chemical Engineering January 2018 - 44

consumption involves the motor efficiency
(Em), which can be obtained
from the vendor or estimated from the
BHP using the Peter and Timmerhaus
correlation, Equation (15) [6].
(15)
(13)
(14)
For an effective cost analysis, estimate
the operating hours for an entire
year to obtain an electricity cost
for one year. Then estimate the lifetime
of the pump, how often it needs
to be repaired or replaced, and the
associated costs. Also, engineers
need to contact the pump vendor
and ask for a quote on the pump to
get an initial cost. This information
can be used to perform a simple lifecycle
analysis. Consult Example 1 to
see how to do this analysis.
Suction specific speed
The specific speed is a useful index
to help get a general idea of the type
of pump to be chosen. All pumps
can be broadly classified with a " dimensional "
number, as shown in
Equation (16).
(16)
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In Equation (16), N (rpm) is the
actual pump rotating speed, Q (gal/
min) is the pumping capacity, and
H (ft) is the total head at the best
efficiency point, corresponding to
speed N and capacity Q. Suction
specific speed (Ns, rpm) values -
obtained by substituting NPSHa
for H - of less than 8,500 rpm are
typical for commercially available
pumps [7]. Specific speed values
between 8,500 and 12,000 rpm
would likely have to be specially ordered
from a pump manufacturer,
and values greater than 12,000 rpm
are typically not available at all [7].
As defined here, the specific
speed represents the pump rotating
speed (in rpm) at which a theoretical
pump that is geometrically similar
to the actual pump would run at its
best efficiency to deliver a proportional
flowrate.
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Automated Excel spreadsheet
The main goal in developing the Excel
spreadsheet is to find the total dynamic
head to use in reading a pump
curve. It can be downloaded at the following
URL: http://design.che.vt.edu.
The Excel file is broken up into several
sheets to allow the user to tackle the
sizing of their pump in a series of logical
steps. Sheet 1 consists of user inputs
and the main outputs of the Excel
program (Figure 6). Sheet 2 provides
a quick method to calculate the vapor
pressure of a fluid using the Antoine
equation. In Sheet 2, several variables
for the Antoine equation are included
for convenience, but the parameters
for other fluids not included in the
JANUARY 2018
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Chemical Engineering January 2018

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