Page 329 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Mixing of Liquids 297

the system, in general being a function of viscosity,
impeller diameter and liquid depth in the tank. In gener-
al, dual impellers may be indicated for fluids of 45 cen-
tipoise and greater and where the fluid travels more than
four feet before being deflected.

The circulating capacity of 3-blade square pitch pro-
pellers is theoretically of the magnitude given in Figure 5-
9. The speed ranges indicated may be grouped as [2]:

high speed, 1750 rpm: for low viscosity fluids,
such as water
medium speed, 1150 rpm: for medium viscosity fluids,
such as light syrups and
varnishes
low speed, 420 rpm: for high viscosity fluids
such as oils, paints, or for
tender crystals or fibers: or
if foaming is a problem.
The mixing efficiency is generally higher ( 40%-60%)
for the slow 400 rpm speed and lower (25%-45%) for the
Figure 5-SA. Dual Impeller mixer and drive. Courtesy of Lightnin (for- 1750 and 1150 rpm speeds. This is given in Figure 5-10 for
mer1y Mixing Equipment Co.), a unit of General Signal.
general estimating use. Note that the turnover of tank
capacity is involved through the selected impeller diame-
ter and speed.

Mixing Concepts, Theory, Fundamentals

Slondord Make Righi Angle A mixer unit or impeller, regardless of its physical design
Motoreducer
features, is a pump of varying efficiency for pumping of flu-
ids. Generally speaking, all designs are low heads compared
Double Mechon,col Seals to a conventional centrifugal pump, because there is no
With All Ports Accessible
Without Removing the Drive defined confining casing for the mixing element.
The action of the impeller design produces flow of the
Manhole W,th Hinged Cover fluid, head on the fluid, or shear in the fluid, all to varying
degrees depending on the specific design. A general identi-
fication of these characteristics for several types of impellers
Semi-flexible Coupling
Protects Drive Equipment is given by [27]. (Note: Use consistent dimensions).
ond Pressure Seals
Type Impeller Flow Decreases Head (Shear)
Cooling and Heating Coils from Top Increases from Top
�����--�����
Rakes, Gates
Spirals, Anchors,
Removable Arm Radial
Propeller Agitator ,Giving Paddles
Excellent Shearing Action
ond Rapid Overturning Propellers
of fhe Liquor,Self-
cleaning in Operation Axial Flow Turbines
Flat Blade Turbine
Bar Turbine
Removable Bottom Guide Bladeless Impeller
Bearing
Close Clearance
Impeller and
Stator
Bottom Flush Valve Colloid Mill,
Homogenizer
Figure 5-88. Multiple impellers. Courtesy of Struthers-Wells Corp.