Fluids/Solids Handling
Optimize Mixing by
Using the Proper
Baffles
It is common knowledge that baffles
promote better flow in an agitated vessel,
but how to apply them and what kind to
use take some ingenuity.
Kevin J. Myers,
University of Dayton
Mark F. Reeder and
Julian B. Fasano,
Chemineer, Inc.
A
gitated vessels are used throughout the
chemical engineering industries (CEI) for
diverse applications including storing,
blending and reacting materials. Agitator
design requires specification of the motor, drive and
impeller system that will satisfy both process (1, 2,
3) and mechanical (4) requirements. In addition,
most agitated vessels are baffled, and the design of
the baffle system must also economically satisfy
process objectives.
Why use baffling?
During agitation of a low-viscosity liquid, the rotating impeller imparts tangential motion to the liquid. Without baffling, this swirling motion approximates solid-body rotation in which little mixing actually occurs. Think about stirring a cup of coffee or
a bowl of soup: The majority of the mixing occurs
when the spoon is stopped or the direction of stirring is reversed. The primary purpose of baffling is
to convert swirling motion into a preferred flow pattern to accomplish process objectives. The most
common flow patterns are axial flow, typically used
for blending and solids suspension, and radial flow,
used for dispersion. However, baffling also has
some other effects, such as suppressing vortex formation, increasing the power input and improving
mechanical stability.
A common agitation objective is suspending settling solids in a low-viscosity liquid, and Figure 1 il-
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lustrates the effect of baffling on this. In the unbaffled vessel on the left, the swirling flow field is ineffective at dispersing the solids that are grouped in a
rotating pile below the pitched-blade impeller. Also,
a large surface vortex is visible at the top of the
shaft. In the vessel on the right, the baffles are visible on the left and right sides of the vessel and as a
thin gray strip that bisects the impeller and shaft.
The presence of baffles produces axial flow, in
which the discharge flow produced by the impeller
impinges on the base of the vessel, flows radially to
the vessel wall, then up the wall, returning to the impeller from above. This flow pattern can be inferred
from the solids that are distributed rather uniformly
throughout the liquid. All parameters (impeller,
speed, solids, etc.) are the same in the two vessels in
■ Figure 1. Agitation in an unbaffled vessel (left) leads to swirling flow
with surface vortex formation and poor solids distribution, while
standard baffling (right) with a pitched-blade turbine promotes axial flow
that results in good solids distribution.
Why consider baffling alternatives?
As evidenced by Figure 1, in most applications, the
key to successful agitation is providing the proper flow
field to achieve process objectives. In some instances,
particularly the most challenging ones, selection of the
proper baffling system is critical to providing the optimal flow field.
Standard baffling
Many agitated vessels, including the one on the right
in Figure 1, use standard baffling, which consists of
four flat vertical plates, radially-directed (i.e., normal
to the vessel wall), spaced at 90 deg. around the vessel
periphery, and running the length of the vessel’s
straight side. Standard baffle width is 1/10 or 1/12 of
the vessel dia. (T/10 or T/12) (5). Sometimes, baffles
are flush with the vessel wall and base, but, more often,
gaps are left to permit the flow to clean the baffles.
Recommended gaps are equal to 1/72 of the vessel dia.
(T/72) between the baffles and the vessel wall, and 1/4
to one full baffle width between the bottom of the baffles and the vessel base.
Why use standard baffling?
The decision to use standard baffling is often an easy
one. First, standard baffling typically provides near-optimal performance, and because of the symmetric placement of the baffles around the vessel periphery, standard baffling provides a high degree of mechanical stability. In addition, in turbulent operation, many impeller
characteristics, such as the power and pumping numbers (Pgc/N3D5ρ and NQ = Q/ND3, respectively), are essentially independent of the impeller Reynolds number
when standard baffling is used. Fully turbulent agitation
occurs for impeller Reynolds numbers (NRe = ND2ρ/µ)
greater than about 10,000. In contrast, in under-baffled
vessels, the impeller power number continually decreases with increasing Reynolds number, introducing
an additional complication to the design process (5).
And last, but far from least, because of its widespread
use, the choice of standard baffling is supported by extensive design and scaleup data. It is the lack of such
data and the potential sacrifice in mechanical stability
that are the primary cautions when considering the use
of non-standard baffles.
Baffling effects
Figures 2 and 3 illustrate how baffling in turbulent
operation affects two primary agitator characteristics.
Figure 2 shows that the impeller power number increas5
Relative Power Number
Figure 1. The only difference is the presence of baffles.
However, note that baffles do lead to a difference in
power input. This point will be discussed later.
4
3
2
Radial Impellers
Mixed Impellers
Axial Impellers
1
0
0
1
3
2
4
Number of Standard Baffles
■ Figure 2. Increased baffling increases the power draw of an agitator.
Nomenclature
D =
gc =
N =
NP =
NQ =
NRe=
P =
Q =
S =
T =
impeller dia., m
force conversion factor, kg-m/s2/N
impeller rotational speed, s-1 (rev/s)
impeller power number (Pgc/N3D5ρ), dimensionless
impeller pumping number (Q/ND3), dimensionless
impeller Reynolds number (ND2ρ/µ), dimensionless
impeller power draw, W
impeller pumping rate, m3/s
impeller submergence (distance below surface), m
vessel dia., m
Greek letters
µ = viscosity, Pa-s(kg/m•s)
ν = kinematic viscosity (µ/ρ), m2/s
ρ = density, kg/m3
Relative Blend Time
1.0
Radial Impellers
Mixed Impellers
Axial Impellers
0.8
0.6
0.4
0.2
0.0
0
1
3
2
4
Number of Standard Baffles
■ Figure 3. Baffling also reduces blend time.
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February 2002
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Fluids/Solids Handling
es as the number of standard width baffles (T/12) is increased. Data are presented for three impeller styles: radial-flow impellers, such as straight-blade and Rushton
turbines, mixed-flow impellers, such as pitched-blade
turbines, and axial-flow impellers, such as high-efficiency impellers. All data in this figure are normalized
with respect to the unbaffled condition, with each impeller style being normalized individually, rather than
with respect to a common reference. “Normalized”
means that, if the unbaffled radial-flow power number
is 2.5 and the unbaffled high-efficiency impeller number is 0.2, then all of the radial-flow impeller data are
divided by (normalized) 2.5 and all of the high-efficiency impeller data are divided by 0.2.
The power number of radial-flow impellers is most
strongly influenced by the extent of baffling, continually increasing with the number of baffles. The mixedflow and axial-flow impeller power numbers are affected to a lesser extent, approximately doubling for a single baffle compared to an unbaffled system, but increasing only marginally as the number of baffles is
further increased.
there are situations in which no baffling is used. Baffles
are rarely used with side-entering agitators or with
close-clearance impellers, such as gates, anchors and
helical ribbons, for which the impeller-to-tank dia. ratio
is typically greater than 90% (D/T > 0.90). Baffles are
also generally not used in rectangular or square tanks
that prevent swirl by providing some natural baffling in
their sharp corners as illustrated in Figure 4. The flow
field in the unbaffled square vessel in this figure is
quite similar to that of the baffled cylindrical vessel in
Figure 1 (all conditions, such as speed, impeller and
solids, are the same in Figures 1 and 4: only the vessel
has been changed).
For impeller Reynolds numbers less than about 50, the
viscous action of the liquid at the vessel wall causes a
natural baffling effect, eliminating the nearly solid-body
rotation that can occur during agitation of low-viscosity
liquids in unbaffled vessels. Thus, no baffles or narrow
baffles might be used (6). Simply for convenience, small
agitated vessels, less than a few hundred gallons, also
may not be baffled. In these systems, angled and/or offcenter mounting of the agitator can be used to eliminate
Flat-plate baffles are the norm because of their ease of manufacture
and installation and the associated economy.
It may seem counterintuitive to want to increase the
power input to an agitated vessel. Why not operate at a
lower power input in an unbaffled vessel? The reason
for using baffles is that the higher power input is often
necessary to achieve process objectives. Figure 3 illustrates that at equal power input with surface addition of
the material to be incorporated, the turbulent blend
time of an unbaffled vessel is substantially greater than
that of a baffled system. In this instance, all impeller
styles are affected in a similar manner, with the addition of a single baffle significantly decreasing the
blend time, but the addition
of further baffles having
minimal effect. Again, all
data have been normalized
with respect to the unbaffled condition for each particular impeller style.
■ Figure 4. Even when
unbaffled, the flow in square
and rectangular vessels is
similar to that in fully-baffled
cylindrical vessels.
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Non-standard baffling
applications
Despite the popularity
of standard baffling, there
are many instances in which
non-standard baffling is
commonly used. In fact,
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excessive swirling. Mechanical complications and the associated costs generally preclude the use of angled and
off-center mounting with larger agitators.
The flow patterns produced by off-center mounting
in unbaffled vessels are shown in Figure 5. In the vessel
on the left, the impeller is mounted vertically, but midway between the vessel centerline and wall, rather than
on the vessel centerline. This reduces, but does not
eliminate swirl. Although the solids are somewhat dispersed through the liquid, they are still grouped in a
loose swirling pile at the center of the vessel base.
In the vessel to the right, in addition to mounting the
■ Figure 5. Off-center agitator mounting (left) reduces swirl in unbaffled
vessels, while angled, off-center mounting (right) approximates the flow in
fully-baffled vessels.
1.00
1.0
0.86
Relative Power Number
impeller off the vessel centerline, the agitator is angled
at approximately 10 deg. to
the vertical. This combination in an unbaffled vessel
approximates the flow field
produced in a fully-baffled
tank. When using angled
mounting with an axial-flow
■ Figure 6. A single baffle,
impeller, the discharge flow
often used in glass-lined
produced by the impeller
vessels, reduces, but does
not eliminate, swirl and
should oppose the swirling
surface vortexing.
motion produced by the
impeller’s rotation (5).
Baffles might not be used
in vessels that require sterility or in which material
hang-up during draining is problematic. Although
choosing not to use baffles makes vessel cleaning easier, it can make optimal agitator design difficult. Some
agitated vessels do not use baffles per se, but contain
internals such as heat-exchanger tube-bundles that
provide sufficient baffling to accomplish process objectives. In fact, some reactors that are used to carry
out highly exothermic or endothermic reactions contain so many heat exchanger tubes that the vessel is
over-baffled, making it difficult for the agitator to promote sufficient flow.
Flat-plate baffles are the norm because of their ease of
manufacture and installation and the associated economy. A potential problem with them is that material can
hang up or become trapped in stagnant regions near
them, particularly in more viscous or non-Newtonian liquids, or in the presence of filamentous materials. This
leads to the use of profiled baffles, often triangular or
semicircular in shape, attached flush to the vessel wall
0.80
0.8
0.67
0.6
0.4
0.2
0.0
One
Beavertail
One
Concave
Two
Concave
Four
Standard
Baffle Configuration
■ Figure 8. The concave baffle increases power input in systems that use a
limited number of baffles.
that eliminate stagnant regions. The use of profiled baffles is limited to critical applications such as polymerization reactors and clean-in-place reactors, which are commonly used in the pharmaceutical industry. Another option is using baffles that are not mounted normal to the
vessel wall, but that are angled away from the direction
of impeller rotation.
Glass-lined vessels
Use of a limited number of baffles, one or two, is
usually avoided because it does not provide adequate
mechanical stability. However, there is one notable ex-
Relative Drawdown Power
20
16
S = 0.5D
S=D
12
8
4
2
None
One
T/12
Two
T/12
Four
T/12
Four
Half
Four
T/40
Baffle Configuration
■ Figure 7. Common baffle styles for glass-lined vessels are the beavertail
■ Figure 9. The power required to draw down floating solids is affected by
baffle (left) and the concave baffle.
the baffle system and the impeller submergence, S.
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Fluids/Solids Handling
■ Figure 11. Partial lower
■ Figure 10. Drawdown of floating solids in an unbaffled tank (left) relies
upon a large surface vortex that reaches into the impeller, while narrow
baffles (right) can promote drawdown and distribution of floating solids with
a reduced vortex.
ception. To ensure the integrity of the vessel lining,
rather than being mounted on the vessel wall, baffles in
glass-lined vessels hang from flanges in the vessel’s top
head. Typically, due to the limited space in this head,
no more than two baffles are used in glass-lined tanks.
Figure 6 demonstrates that use of a single baffle is
baffles can be used to
satisfy concurrent process
objectives such as drawdown of floating solids
(white) and suspension of
settling solids (black).
form distribution of settling
or floating solids and enhanced gas dispersion. An additional benefit of the concave
baffle is that it prevents surface vortex formation, and is
therefore more effective at
avoiding gas entrainment at
high power inputs in underbaffled vessels.
Surface incorporation
In some applications, it is
actually critical that the impeller draw in material — gas,
floating liquid or solids —
from the surface. In these instances, standard baffling
may not be the best approach. Partial lower baffling is
often used for drawdown of material from the vessel
headspace. In these instances, four baffles of standard
width are used, but they extend only about half way up
the vessel’s straight side, leaving the upper portion un-
Sometimes, when it is critical that the impeller draw in material,
standard baffling may not be the best approach.
an improvement over an unbaffled system, but the flow
is still highly tangential (note the small surface vortex
at the top of the impeller shaft). The flow field in a vessel equipped with two standard baffles very closely approximates that in a fully-baffled vessel.
There are two primary challenges for baffling in
glass-lined vessels. First, the surface of the baffle must
be contoured because sharp corners cannot be coated
with glass. As a result, the most common type of baffle
used in glass-lined vessels consists of a pipe flattened
to yield an elliptical cross section. This type of baffle is
commonly referred to as a beavertail (Figure 7, left).
The second challenge is that glass-lined vessels are
under-baffled, and it can be difficult to provide sufficient power input to achieve process objectives. To
overcome both of these challenges, a patented concave
baffle has been developed (7, 8) (photo at the right of
Figure 7, shown without the glass coating).
The data of Figure 8, taken with a retreat-curve impeller, the most commonly used impeller in glass-lined
vessels, illustrates that the concave baffle increases
power input relative to the beavertail baffle and that
two concave baffles approach the power input of four
standard baffles. Studies with the concave baffle confirm that the higher power input associated with this design leads to process improvements, such as more uni-
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baffled. An impeller, often a pitched-blade turbine,
placed near the liquid surface, will generate a controlled vortex that aids in the incorporation of material
into the liquid.
Figure 9 compares the power requirements to draw
down floating solids with a single down-pumping
pitched-blade turbine and various baffle systems. Data
are presented for impeller submergences below the liquid surface equal to one-half of the impeller diameter (S
= 0.5 D) and the impeller diameter (S = D). All data in
Figure 9 are normalized with respect to a common reference. The power requirements of the unbaffled system are the lowest, but as shown on the left of Figure
10, the unbaffled system does a poor job distributing
the solids throughout the liquid. Further, this system
has a large vortex, reaching into the impeller and leading to undesired air entrainment and the potential for
mechanical instability.
As the extent of baffling in the system is increased,
from unbaffled to one, two and four standard baffles,
the power requirement continually increases, particularly for the larger impeller submergence. The power requirement of the system consisting of four lower-half
baffles is substantially lower than that of the fully-baffled system, being comparable to that of the two-baffle
system. The advantage of the half-baffle system com-
pared to the two-baffle one is greater mechanical stability and better mixing in the lower portion of the vessel.
The right-most data set of Figure 9 is for a narrow
set of four baffles that is recommended for solids drawdown (9). This system uses four baffles that run the
length of the vessel’s straight side, but the baffles are
narrow, having a width equal to approximately 2% of
the vessel diameter (typically T/50 to T/40, rather than
the standard of T/10 or T/12). This system provides
symmetry, and an associated degree of mechanical stability, as well as very low drawdown power requirements, good solids distribution throughout the liquid,
and limited surface vortexing.
Additionally, the drawdown power requirement of
the narrow baffle system is relatively unaffected by impeller submergence, a distinct advantage for processes
in which the liquid level varies. The superior solids
drawdown performance of the narrow baffle system is
shown on the right of Figure 10.
In some instances, it is necessary to simultaneously
satisfy a number of process objectives. Figure 11 shows
how a dual-impeller system in a vessel equipped with
Literature Cited
1. Fasano, J. B., et al., “Advanced Impeller Geometry Boosts Liquid
Agitation,” Chem. Eng., 101, pp. 110–116 (Aug. 1994).
2. Corpstein, R. R., et al., “The High-Efficiency Road to Liquid-Solid
Agitation,” Chem. Eng., 101, pp. 138–144 (Oct. 1994).
3. Bakker, A., et al., “How to Disperse Gases in Liquids,” Chem. Eng.,
101, pp. 98–104 (Dec. 1994).
4. Fasano, J. B., et al., “Consider Mechanical Design of Agitators,”
Chem. Eng. Progress, 91 (8), pp. 60–71 (Aug. 1995).
5. Bates, R. L., et al., “Impeller Characteristics and Power,” Chapter 3
in “Mixing: Theory and Practice,” V. W. Uhl and J. B. Gray, eds.,
Academic Press, New York (1966).
6. Bakker, A., and L. E. Gates, “Properly Choose Mechanical Agitators for Viscous Liquids,” Chem. Eng. Progress, 91 (12), pp. 25–34
(Dec. 1995).
7. Hairston, D., “Mixing Powders into Liquids,” Chem. Eng., 107 (5),
pp. 29–35 (May 2000).
8. Reeder, M. F., and C. J. Ramsey, “Concave Baffle,” U.S. Patent
6,059,448 (May 9, 2000).
9. Hemrajani, R. R., et al., “Suspending Floating Solids in Stirred
Tanks — Mixer Design, Scale-Up and Optimization,” Proc. Sixth
European Conference on Mixing, pp. 259–265, Pavia, Italy (May
24–26, 1988).
10. Drewer, G. R., et al., “Suspension of High Concentration Solids in
Mechanically Stirred Vessels,” Eighth European Conference on Mixing, (IChemE Symposium Series No. 136), pp. 41–48, Cambridge,
U.K. (Sept. 21–23, 1994).
11. Joosten, G. E. H., et al., “The Suspension of Floating Solids in
Stirred Vessels,” Trans. of the Institution of Chem. Eng., 55, pp.
220–222 (1977).
12. Siddiqui, H., “Mixing Technology for Buoyant Solids in a Nonstandard Vessel,” AIChE J., 39, pp. 505–509 (1993).
partial lower baffles can be used to simultaneously
draw down floating solids and suspend settling solids.
The lack of baffles in the upper portion of the tank permits sufficient swirl to incorporate the floating solids,
while the baffles in the lower portion promote axial
flow that is effective at suspending the settling solids.
For some applications, partial upper baffling is the
preferred approach. In pulp-and-paper agitation, the
baffles may not extend below the impeller to prevent
material hang-up and stagnant regions. A second example is high-solids-loading slurries that can be difficult
to agitate, particularly if settled solids must be resuspended. Full baffling can cause the impeller and solids
to bind, while removing the lower portion of the baffles
allows tangential motion that can improve solids suspension performance (10). Partial upper baffles have
also been shown to be somewhat effective for drawdown of floating solids (11, 12).
Concluding remarks
When agitating low-viscosity liquids, standard baffling typically provides near-optimal process performance and good mechanical stability. In addition, standard baffling is backed up by extensive design and
scaleup data. However, as described here, there are situations in which standard baffling may not be the best
choice. The guidelines presented here are intended to
identify the baffle system modifications that can be used
to improve performance and the situations in which
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these modifications should be considered.
KEVIN J. MYERS is a professor in the Dept. of Chemical and Materials
Engineering, Univ. of Dayton (Dayton, OH 45469-0246; Phone: (937) 2292627; Fax: (937) 229-3433; E-mail: kevin.myers@notes.udayton.edu). He
has conducted extensive agitation research, particularly in multiphase
systems. He received his bachelor’s degree from the Univ. of Dayton and
his DSc in chemical engineering from Washington Univ. in St. Louis. Myers
is a member of AIChE and the American Society for Engineering Education.
MARK F. REEDER is a principal research engineer with Chemineer, Inc. (P.O.
Box 1123, 5870 Poe Ave., Dayton, OH 45401; Phone: (937) 454-3346; Fax:
(937) 454-3395; E-mail: M.Reeder@chemineer.com). He works in the R&D
laboratory and is involved in development work for a wide range of mixing
products, including agitators, static mixers and rotor/stator mixers. He
received his bachelor’s degree from West Virginia Univ. and his master’s
and PhD degrees in mechanical engineering from Ohio State Univ. Reeder’s
experience also includes a post-doctoral appointment at NASA Glenn, for
optical diagnostics used in jet mixing; he is also a professional engineer.
JULIAN B. FASANO is director, parts and field service, for Chemineer, Inc.
(Phone: (937) 454-3263; Fax: (937) 454-3379; E-mail: J.Fasano@
chemineer.com). He has been with Chemineer for 30 years, with primary
emphasis on R&D and custom equipment design. From 1984 to 1995, he
served as technical director. He holds a BSc in chemical engineering from
the Univ. of Dayton, an MSc in chemical engineering from Lehigh Univ., and
a PhD in materials engineering, also from the Univ. of Dayton. Fasano is a
PE in Ohio and a member of AIChE.
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