Page 339 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Mixing of Liquids 307

larly true in continuous chemical processes where the
general nature of the fluid remains constant but the vis-
cosity, density or solid particle content may change as the
plant progress from 'just erected' to steady production
and even on to new and different products. This transi-
tion may take from a few months to several years and
should be economically evaluated. The gear mechanism is
not a place to reduce costs for this equipment, since
improper application can create costly maintenance.

Usual practice, particularly for good estimating is to
assume that the gear drive requires 5% of the impeller
horsepower and that system "surging or variations"
require a minimum of 10% of this impeller horsepower.
Thus

Actual motor hp = impeller required hp/0.85
(minimum) (5-41)

When the actual maximum gear box horsepower is known
from the manufacturer, it should be used as long as it is
equal to or greater than the 5% allowance noted above.
The impeller/fluid horsepower allowable variation
should still be 10% or greater. For example, if the calcu-
lated required motor drive (or turbine drive) hp = 23,
(i.e., 19.55/0.85), the next standard motor is 25 hp, so use
this, never less than the 23 indicated above because 23 hp
is non-standard, and no such motor hp exists.
Figure 5-20 illustrates a vertical propeller mixer assem-
bly, with vertical mounting with gear box and motor. Fig-
ure 5-21 is a typical right angle, vertical impeller shaft with
horizontal gear and motor drive.

The mixer manufacturer should always be consulted
for proper mechanical features design and strength char-
acteristics, such as horsepower, gear rating AGA, shaft Figure 5-20. Portable VektorCTM) vertical propeller mixer assembly.
diameter, shaft deflection, critical speeds, bottom steady By permission, Lightnin (formerly Mixing Equipment Co.), a unit of
bearing, and side shaft bearings. General Signal.
Materials of Construction
Steady Bearings
In general,just about any material that can be worked
into the impeller design is available, including steel,
The installation of mixers on long shafts in tall tanks stainless alloys, copper alloys, nickel and alloys, hard rub-
may become a problem if "whip" of the shaft develops. To ber, and lead, rubber and plastic coatings on impellers
reduce this possibility, a bearing support in the bottom of and shafts.
the tank will hold the shaft steady. Lubrication is by the
tank fluid. Therefore this has limited application if abra- Design
sive particles are present. Normally the manufacturers'
designs avoid this extra bearing. Sometimes a guide bear- Normally the proper impeller selections and horse-
ing is installed about midway in the tanks to steady the power requirements are handled in a cooperative manner
shaft at this point. Again it is preferable to avoid this, if with the manufacturer of this equipment in order to
possible, and the manufacturer should make recommen- obtain the best analysis of a given application. There is no
dations for the installation. substitute for performing the proper test runs to evaluate