Page 331 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Mixing ot Liquids 299

T ...... / 420 Viscous flow, NRe less than 10 to 300 is expressed:
@....-
60 -;;,"
\ 20 .,., / K
10 / p = _ 2 µ(N.)2 (D)3 (5-4)
/
\ ...... g
\ ,,., ......
\ 50 K 2 = from Table 5-1
100 ,� 15 P = power, not power number, P 0
/
200
300
\ \+ Fully developed turbulent flow, NRe over 10,000, in a tank
r�o 40 10% of the tank diameter:
400 containing four equally spaced baffles having a width of

,.,ooi p = _ 3 p (N )3 (D )5 (5-5)
K
g s
l K3 = from Table 5-1
t 30
..
2,000! "" ... 5 1150 where g = conversion factor, 32.2 lb mass-ft/lb force/
..
(sec) (sec)
e
j.,oo . a: - H = total potential head during flow, ft
c
c.
..
..
...
P = power, ft-lb/sec
• ...
.. � '#, l <I) W = impeller blade width, ft
µ = viscosity, lb/ft-sec
j,.oo,f ;:. • ... p = density, pounds/ cu ft
�
c
.!!
i
e
� ilJ 20 0.. 1750 N, = revolutions/sec
o,
Example: Horsepower, (HP) = P /550 (5-6)
Fiuld Viscosity : IC C P Construct Lines I and 2
Propeller Diameter = 10" Reod Propeller Mixing
Propeller Speed : 420 Rpm Efficiency of 54 %
NN 3 D5Sg·
Figure 5-10. Propeller circulation efficiency. This is used with theo- 1 HP p m I T b' (5-6A)
retical propeller capacity to determine actual capacity. By permis- mp. = 1.523 31013 ur me
sion, Fluid Agitation Handbook, Chemineer, Inc. (Symbols below)
[29]
Power, P; Power Number, P 0; and Reynolds Number,
NR,, Table 5-1 shows that in a cylinder tank, four baffles,
each ).{2 tank diameter above flat bottom, liquid depth is
Power equal to tank diameter, impeller shaft is vertical and at
centerline of tank.
Power is the external 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- (5-7)
tion, fluid density and viscosity, vessel dimensions and
internal attachments, and position of the impeller in this or, [29],
enclosed system.
The power requirements cannot always be calculated NRc = (10.754· NmD;2 Sg)/(µ') (5- 8)
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) Nm = impeller speed, rpm