Page 328 - Applied Process Design for Chemical and Petrochemical Plants Volume I

Mixing OF Liquids 299

’420 Viscous flow, NRe less than 10 to 300 is expressed:
0 0,
K2
P = ---p(N,)* (D)3 (5-4)
g
K2 = from Table 5-1
P = power, not power number, Po

Fully developed turbulent flowj NRe over 10,000, in a tank
containing four equally spaced baffles having a width of
10% of the tank diameter:

(5-5)

K3 = from Table 5-1

1150 where g = conversion factor, 32.2 Ib mass-ft/lb force/
(sec) (sec)
H = total potential head during flow, ft
P = power, ft-lb/sec
W = impeller blade width, ft
p = viscosity, lb/ft-sec
p = density, pounds/cu €t
N, = revolutions/sec
1750
:
~x~rn~~e Horsepower, (HP) = P/350 (5-6)
Fluid Viscosity f IO CP Construct Lines I and 2
Propeller Diameter : IO” Read Propeller Mixing
Propeller Speed :: 420 Rpm IZfficiency of 54%
N,N;D; sg
-10. Propeller circulation efficiency. This is used with theo- Turbine (5-6A)
retical propeller capacity to determine actual capacity. By permis- Imp. HP = 1.523 3
sion, Fluid Agitation Handbook, Chemineer, Inc. (Symbols below)
~291
5 Power Number, o; and Reynolds Number,
Table 5-1 shows that in a cylinder tank, four baffles,
NRe
each ?G tank diameter above flat bottom, liquid depth is
equal to tank diameter, impeller shaft is vertical and at
centerline of tank.
Power is the extlernal measure of the mixer perfor- The Reynolds number NRe for mixing is: (dimension-
mance. The power put into the system must be absorbed less)
through friction in viscous and turbulent shear stresses
and dissipated as heat. The power requirement of a sys-
tem is a function of the impeller shape, size, speed of rota- (3-7)
tion, fluid density and viscosity, vessel dimensions and
internal attachment,s, and position of the impeller in this
enclosed system.
The power requirements cannot always be calculated
for any system with a great degree of reliability. However,
for those systems and/or configurations with known data, where for these units, above equation
good correlation is the result. The relations are [21]:
D = impeller diameter, ft
P = QpH (5-3) N, = impeller speed, rpm