Page 353 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Mixing of Liquids 321

Read closest motor hp = 20. However, the 0.5 hp differ- Baffles
ence between the 19.5 and 20 may not be sufficient to han-
dle the power loss in the gear box. Most industrial practice is Vertical side-wall baffles (Figures 5-23B and C and 5-
to take the closest standard motor hp to the 22.9 hp deter- 34) projecting about Xo to Yi2 of' the tank diameter into the
mined above, which is 25 hp. The gear box must have an out- vessel perform a helpful purpose in controlling vortex
put speed of 90 rpm and will use only the hp determined by action. The baffles are set off from the tank wall a few
the impeller shaft even if the motor is larger, that is, 25 hp. It inches to prevent build-up of particles.
will only put out the net hp required, that is, the sum of The important dimensional features and/ or ratios for
impeller shaft and losses through the gear box. a center-mounted mixer unit (vertical) are [29, 30]:
Referring to Table 5-3 for turbulent, baffled systems, if
power is held constant and the system has too large a 1. Flat bottom tank
shear characteristic and apparently too small a volume or a. Number of vertical baffles in a vertical vessel: four
flow, the impeller can be increased 20% and the new (more than four provides little, if any, benefit)
speed at constant Power, P, will be: b. Width of' baffles: Yio to Yi2 tank diameter, w
c. Distance baffles off wall, cl: 3 in. to 6 in.
( 5 - 65) d. Baffle spacing: on 90° around tank
e. Distance of baffles off flat vessel bottom: 4 in. to
6 in., b
N [ D ys13 f. Height of liquid in tank: Z
1 1 I (5- 66)
N2 = D2 ) g. Height of impeller off flat bottom: IC., equal to
impeller (turbine) diameter, or IC = D; some-
times IC = % D is suggested
( D2 )-5/3 ( l 2 ys13
N = N - = N l-"-J = 0. 738 (N ) (5- 67) h. Liquid depth over top of impeller or turbine: Z-C
2 I DI l 1.0 l shouid be 2D
i. Baffles extend above liquid level
(Use Table 5-4 for viscous systems.) 2. Dished bottom tank, center mounted mixer, 4 baffles
or the new speed will be 73.8% of the original, using the a. Essentially same criteria as for flat bottom tank
20% larger diameter impeller. This is true for geometri- b. Impeller distance off bottom: essentially same ref
cally similar systems. However, there is little power change erenced to vessel tangent line as for flat bottom vessel
over a wide ratio of impeller-to-tank diameter. 3. Cone bottom tank. Handle as for dished bottom
For a constant amount of power available to a system, the above.
flow and turbulence effects and ratios can be changed by 4. For fluids with viscosity up to 5000 cp, and even up
replacing one impeller by another dimensionally similar. Fig- to 30,000 cp for some situations, use standard baf-
ures 5-29 and 5-30 illustrate the type of studies which should fles described above. The baffle widths can be
be made in evaluating a system. If the density or viscosity of reduced as the viscosity increases from 5,000 cp Lo
a fluid changes during scale-up, then in the turbulent range 12,000 cp. [30], and may be eliminated completely
the horsepower is directly proportional to density, and thus for viscosities over 12,000 cp. There are exceptions,
viscosity has very little effect. In viscous flow the density has such as mixing wide range of fluids of low and high
no effect while the horsepower is proportional to viscosity viscosities.
[8]. The effect is small in the range from 1 to 1000 cen- 5. Special baffles. For certain mixing problems, various
tipoise, but amounts to a factor of 1.4 when changing from baffling arrangements have been found to be advan-
1000 to 10,000 centipoise. Above this point the change is tageous (see Reference [30]).
quite iarge and should not be handled by proportion. 6. Baffles can be omitted when propeller mixers are
Figure 5-28 summarizes the scale-up relationships for top mounted at an angular off-center position (see
many of the important and controlling functions, Figure 5-23D) and vortex swirling is prevented. This
depending upon the nature of the process equipment. is not recommended for large power systems on
The figure identifies which curves apply to turbulent (T) large tanks, due to shaft fatigue.
ano laminar (L) flow patterns in the fluids being subject-
ed to the mixing operation. Note that the Froude num- Baffles that extend from the liquid levei down, but not
ber, � Fr is a function. This scale-up chart applies to sys- to, the tank bottom allow heavy swirling action in the bot-
tems of similar geometry. When the geometry is different, tom of the tank, but no vortex at the top. W11en baffles
special and specific analysis of the system must be made, extend from the bottom up, but not to the liquid level,
as the chart will not apply. some vortex and swirling action will take place at the top.