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Select the Right Impeller Julian B. Fasano Fasano Mixer Engineering Co.
The key to effective, efficient mixing is selecting the t he appropriate impeller.
F
ifty or 60 years ago, most industrial mixer manufacturers had only ve or six impellers in their arsenal. Over the years, research has shown that the impeller’s design plays a large role in mixing efciency. Today, mixer manufacturers typically offer 20 to 30 different impellers. Determining which is best for a particular mixing applicaapplica tion can be a challenge. The most common mixing functions include: • blending • heat transfer • solids suspension/solids drawdown • gas dispersion • immiscible liquid-liquid suspension. For each of these tasks, certain impellers tend to perform best. The optim optimal al type type of impeller impeller for for a partic particular ular applicati application on depends on whether the ow regime is turbulent, transitransi tional, or laminar. laminar. In immiscible liquid-liquid applications, the best impeller class depends on the desired droplet size. This article explains how an impeller functions, describes the various classes of impellers, and recomrecom mends which class of impeller to use for common mixing applications.
sure will reduce the pumping capacity of the impeller. impeller. Rotating impellers in turbulent ow form a trailing vor tex system on the back side of the blade (Figure 2), and these trailing vortices disrupt the back side pressure. Figure 3 shows another way to visualize a trailing vortex system with telltales (or streamers) on a pitched impeller blade in turbulent ow. The trailing vortex system on the back side of this impeller blade involves about one-half of the back-side area, as indicated by the tangled telltales on the left side of the blade. The size of the trailing vortex coming off an impeller blade determin determines es how much the back back side side of the impell impeller er can pump: The larger the vortex, the lower the pumping capability. capability. However, impellers can be designed to minimize the size of the vortex system on the back side of impeller blades blades in turbul turbulent ent ow ow. This This can be observe observed d in the the highhighefciency impeller in Figure 4 — the trailing vortex system is much smaller than the vortex system in Figure 3. The straight telltales on the left side of the impeller blade indicate a small trailing vortex system. Negative Pressure
Impeller basics A rotating impeller creates a pressure differential on a blade to achieve achieve mixing mixing and and pumping. pumping. An impelle impellerr blade blade rotating in turbulent or transitional ow develops a positive pressure pressure on the front front side, side, or or leading leading side, side, and and a negati negative ve pressure pressure on the back back side, side, or or trailing trailing side, of of the blade (Figure 1). The negative pressure on the back side of the blade is responsible for about two-thirds of the uid pumped. pumped. Therefo Therefore, re, anythin anything g that that raises raises this this back back side side pres pres30
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Positive Pressure
1. As the impeller rotates, the front side of the blade pushes the p Figure 1. As fluid, while the back side of the blade pulls the fluid to create pumping. The red arrows indicate the direction of flow created by the positive pressure on the front side of the blade and the negative pressure on the back side. The blue arrows indicate the rotational direction of the impeller.
Copyright © 2015 American Institute of Chemical Engineers (AIChE)
u Figure 2.
Rotating impellers in turbulent flow form a trailing vortex system off the edge of the blade’s back side. The red arrows indicate the direction of the vortex system's flow.
p Figure 3. This pitched impeller blade with telltales (black streamers)
An impeller’s Reynolds number, N Re, is dened as:
where ρ is the density, N is the impeller rotational speed, D is the impeller diameter, and μ is the uid viscosity. For a mixing impeller, Reynolds numbers above 2,500 correspond to turbulent ow. Transitional ow occurs at Reynolds numbers between 2,500 and 200. Reynolds numbers between 200 and 10 characterize near-laminar ow. Flow is laminar at Reynolds numbers less than 10. Trailing vortices are largest in turbulent ow, smaller in transitional ow, extremely small at Reynolds numbers between 200 and 500, and nonexistent at Reynolds numbers below 50.
shows the back side trailing vortex system. The tangled telltales on the left side of the blade indicate a strong trailing vortex system.
p Figure 4. The system of trailing vortices formed by this high-efficiency
axial-flow impeller is smaller than that of a typical pitched-blade impeller (such as the one in Figure 3).
Impeller classifications The rst step in selecting an impeller for a specic job is understanding the characteristics of each impeller class. Impellers are generally classied as axial-ow ( e.g., highefciency impellers), mixed-ow (e.g., pitched-blade impellers), or radial-ow (e.g., 90-deg. paddle-style impellers). A radial-ow impeller pumps radially at both high and low Reynolds numbers. As an axial-ow or mixed-ow impeller is subjected to lower Reynolds numbers, it tends to pump more radially, as shown in the computational uid dynamics (CFD) vector plots in Figure 5 (1). Every impeller has a power number, N , dened as: p
Chord Angle and Attack Angle
T
he chord line of an impeller blade is an imaginary straight line that connects the leading and trailing tips of the blade. The chord angle is the angle between the chord line and the plane of rotation. This angle can vary from hub to tip of the blade (if the blade is twisted) or it can be constant. The attack angle is the angle between the vector of the incoming fluid and the chord line. The attack angle is typically smaller than the chord angle.
Wide-Blade Hydrofoil Impeller
where P is the power. At Reynolds numbers above 10,000, the power number is constant and is referred to as the turbulent-range power number. In applications such as blending and solids suspension, an impeller with a low power number tends to be more efcient than one with a high power number. However, in applications that require gas dispersion, impellers with low power numbers are not as efcient as impellers with high
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Chord Angle
Attack Angle
C h o r d
L in e I nc o m in g F l ui d Plane of Rotation
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Turbulent, Axial Flow
Transitional, Mixed Flow
Laminar, Radial Flow
p Figure 5. A high-efficiency axial-flow impeller will pump more radially as it is subjected to lower Reynolds numbers. The impeller produces an axial-flow
pattern in turbulent conditions (left), a mixed-flow pattern under transitional conditions (center), and a radial-flow pattern at laminar conditions (right).
a.
b.
p Figure 6. A pitched-blade impeller typically has two to eight flat blades,
a width-to-diameter ratio of 0.15–0.30, and a 45-deg. chord angle.
power numbers. So, the application will determine whether or not the impeller’s optimal power number is high or low. Pitched-blade impellers. Pitched-blade impellers (Figure 6) are often referred to as mixed-ow impellers. The mean ow angle out of the impeller, measured from vertical, is typically 30–60 deg. The term mixed-ow refers to the fact that the mean ow is neither vertical nor radial. Pitched-blade impellers typically have two to eight at blades. The blades have a width-to-diameter ratio of 0.15–0.30, and the chord angles at the blade tip are generally between 25 and 60 deg., but most commonly 45 deg. These impellers are characterized by moderate turbulent-range power numbers, typically 0.80–2.00, but most commonly about 1.00–1.50. Paddle impellers. Paddle impellers are also referred to as radial-ow impellers because the mean ow angle out of the impeller is mostly radial (i.e., 0 deg. relative to the impeller rotational plane). There are two types of paddle impellers: those that use a hub to support the blades (Figure 7a) and those that use a disc to support the blades (Figure 7b). 32
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p Figure 7. Paddle impellers can have either hub-mounted (a) or disc-
mounted (b) blades.
Paddle impellers typically have two to eight at blades with width-to-diameter ratios of 0.15–0.30 and an attack angle of 90 deg. from horizontal. They have high turbulentrange power numbers, typically 2.00–5.50, and have a pumping capability about 20–40% less than a pitched-blade impeller of the same horsepower at the same speed. High-efciency axial-ow impellers. Figure 8 presents examples of high-efciency axial-ow impellers. These impellers are characterized by a relatively small number of blades (typically three) and narrow blades with width-todiameter ratios of 0.15–0.20. They have low attack angles, and their chord angles at the blade tip are generally 15–25 deg. Copyright © 2015 American Institute of Chemical Engineers (AIChE)
High-efciency axial-ow impellers have low turbulentrange power numbers, typically 0.20–0.35. The blades have complex shapes, which makes these impellers more expensive than at-blade impellers. They typically have a pumping capability about 70% greater than a pitched-blade impeller of the same horsepower at the same speed. Wide-blade hydrofoil impellers. Wide-blade hydrofoil impellers (Figure 9) are characterized by a relatively small number of blades, typically three or four; wide blades, typically with width-to-diameter ratios of 0.25–0.35; and chord angles at the blade tip of 30–40 deg. These impellers have moderate turbulent-range power numbers, typically 0.60–1.10. Their blades have complex shapes, which translates into higher capital costs than those of paddle impellers and pitched-blade impellers. They have a pumping capability about 20–40% greater than a pitched blade impeller of the same horsepower at the same speed. High-efciency radial-ow impellers. Paddle-style impellers create large vortices when operated in turbulent ow. High-efciency radial-ow impellers (Figure 10) generate dramatically smaller trailing vortex systems, or, in some cases, no vortices. High-efciency radial-ow impellers most commonly have six blades, but may have as few as four or as many as eight. The blades are concave on the forward side, and blade heights are typically about 20–30% of the impeller diameter. These impellers produce near-perfect radial ow and have high turbulent-range power numbers of 2.4–3.5. Their pumping capability is about 50–70% greater than a paddle impeller of the same horsepower at the same speed.
a.
b.
p Figure 9. Wide-blade hydrofoil impellers have width-to-diameter ratios
of 0.25–0.35 and typically have only three or four blades. Examples include the Chemineer Maxflo W (a) and the Lightnin A315 (b).
a.
a.
b. b.
p Figure 8. High-efficiency axial-flow impellers have only a few blades,
typically three. The blades have complex shapes, which make them more expensive than paddle impellers or pitched-blade impellers. Examples include the Chemineer HE-3 (a) and the Lightnin A310 (b).
p Figure 10. High-efficiency radial-flow impellers have concave blades
that are designed to minimize the trailing vortex system. Examples include the Ekato Phasejet (a) and the Chemineer BT-6 (b). Article continues on next page
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a.
c.
b.
p Figure 11. High-shear impellers trade off pumping for shear. There are three types of high-shear impellers: open (a), disc (b), and rotor-stator (c).
a.
b.
Auger
p Figure 12. Helical-ribbon impellers are a type of laminar-flow impeller that can be manu-
factured with or without a center auger. The single helical-ribbon on the left (a) has a center auger, while the double helical-ribbon on the right (b) does not. q Figure 13. The anchor impeller is a type
of laminar-flow impeller that provides high process-side heat tr ansfer.
p Figure 14. An auger impeller is a type
of laminar-flow impeller that provides efficient blending of viscous fluids in a draft tube.
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High-shear impellers. High-shear impellers trade off pumping for shear. Operations that require high shear typically require a high power density as well. There are three styles of highshear impellers: the open impeller (Figure 11a), the disc-style impeller (Figure 11b), and the rotor-stator impeller (Figure 11c). High-shear impellers operate at high speeds of 350–5,000 rpm, depending on size. They operate at high power densities, sometimes as high as 0.3 hp/gal. They have turbulent-range power numbers from about 0.1 to 0.6. Laminar-ow impellers. Most highviscosity uids are rheologically complex and require special impellers for effective mixing. These impellers are often referred to as closeclearance impellers, because they operate with a small gap between the impeller and the vessel wall. One type of laminar-ow impeller is the helical-ribbon impeller (Figure 12). Some helical-ribbon impellers have a center auger (or center screw), as shown in Figure 12a. If the auger is too small, it will not provide as much pumping as the ribbons, and the circulation rate will decrease (2). To avoid this complexity, most helical-ribbon impellers are manufactured without a center auger (Figure 12b). The anchor impeller (Figure 13), although it is the least expensive, does not provide good top to bottom uid movement. As a consequence, the blend time for anchor impellers is longer than for helical-ribbon impellers of the same horsepower at the same speed. When heat transfer through the vessel wall is important,
Copyright © 2015 American Institute of Chemical Engineers (AIChE)
Table 1. Recommended impellers for miscible fluid blending. Fluid Regime
Turbulent
Reynolds No.
H/T
> 2,500
≤ 1.50
High-efficiency axial-flow
< 0.50
High-efficiency axial-flow
Impeller Class
Table 2. Recommended impellers for solids suspension. Fluid Regime
Turbulent
Reynolds No.
H/T
Impeller Class
> 2,500
≤ 1.00
High-efficiency axial-flow
< 0.33
High-efficiency axial-flow
High-efficiency radial-flow Transitional
500–2,500
< 0.75
High-efficiency axial-flow
High-efficiency radial-flow Transitional
500–2,500
< 0.50
Pitched-blade < 0.33
10–500
< 0.50
Pitched-blade
High-efficiency axial-flow
< 0.33
High-efficiency radial-flow
Pitched-blade
Pitched-blade
Paddle
Paddle
Pitched-blade
10–500
< 0.50
< 10
< 0.50
Pitched-blade
< 0.33
Pitched-blade Paddle
Anchor Helical ribbon
≥ 0.50
Pitched-blade Wide-blade hydrofoil
Paddle Laminar
High-efficiency axial-flow
High-efficiency radial-flow
Wide-blade hydrofoil < 0.33
High-efficiency axial-flow
Helical ribbon*
*An auger impeller could also be used if little or no heat transfer through the vessel wall is required.
helical-ribbon and anchor impellers provide high processside heat-transfer coefcients. The auger impeller (Figure 14) has been used effectively when the ratio of the impeller diameter to the tank diameter is in the vicinity of 0.5, but it does not produce good velocities at the vessel wall. An auger impeller in a draft tube is one of the most efcient methods for blending highly viscous uids (3).
Select the right impeller for blending miscible fluids The most common mixing task is the blending of miscible uids. The right impeller for this depends on the uid regime (i.e., turbulent, transitional, or laminar). Table 1 recommends the most effective impeller for miscible uid blending based on the uid regime, the Reynolds number, and the ratio of the liquid control height to the tank diameter ( H/T ) per impeller. The liquid control height is the height that can be effectively controlled with a single impeller. For applications with higher H/T values, use multiple impellers of the same type.
Select the right impeller for suspending solids Many applications require the suspension of solids to a specic level of uniformity, ranging from off-bottom to 100% uniformity. In applications with viscous uids, this blending requirement will determine the most appropriate impeller. The blending requirement may also dominate impeller selection in applications that require the creation of
a slurry (i.e., applications in which solids must be suspended and the contents blended to a uniform slurry consistency within a specied period of time), when the level of agitation or the mean velocity required for blending is more than six times than that required for suspending solids. Table 2 recommends impellers for use in solids suspension. Solid-suspension-dominated applications in laminar ow are very uncommon. Solids-settling velocities in laminar ow are typically very low, and blending is the controlling factor — the particles, in essence, follow along with the ow. When the application also involves the drawdown of oating or hard-to-wet solids, the optimal impeller may or may not coincide with the recommended impeller classes shown in Table 2. In those instances, refer to the mixer manufacturer’s suggestion.
Selecting the right impeller for dispersing gases Gas dispersion is especially important in the pharmaceutical industry, where it is used in fermenters, and in hydrogenation applications. Table 3 recommends impellers for dispersing gases in liquids. In gas dispersion, a key parameter is the aeration num ber, N A, which is dened as:
where Q g is the volumetric owrate of gas at the impeller, N is the impeller rotational speed, and D is the impeller diameter. High-efciency radial-ow impellers can handle large gas owrates in turbulent ow — with aeration numbers as high as 2.5. In contrast, paddle impellers can typically handle aeration numbers up to only about 0.4. Article continues on next page
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Table 3. Recommended impellers for gas dispersion. Fluid Regime
Reynolds No.
H/T
N A
> 2,500
≤ 0.70
< 2.5
High-efficiency radial-flow lower impeller to provide dispersion, combined with wide-blade hydrofoil upper impeller(s) to provide recirculation.
≤ 0.05
Wide-blade hydrofoil
Turbulent
Impeller Class
High-efficiency radial-flow lower impeller to provide dispersion, combined with wide-blade hydrofoil upper impeller(s) to provide recirculation Transitional
500–2,500
< 0.33
< 1.25
High-efficiency radial-flow or wide-blade hydrofoil lower impeller to provide dispersion, combined with wide-blade hydrofoil upper impeller(s) to provide recirculation
10–500
< 0.33
< 0.05
Paddle Wide-blade hydrofoil
Laminar*
< 10
< 0.33
< 0.05
Paddle Wide-blade hydrofoil
*This application area is extremely uncommon.
Select the right impeller for blending immiscible liquid-liquid suspensions A suspension is a mixture in which the dispersed particles have Sauter mean diameters generally larger than 1 μm. The Sauter mean diameter is dened as the diameter of a sphere that has the same volume-to-surface-area ratio as the particle of interest. Suspensions should not be confused with emulsions, which contain colloidal particles that are typically 1 to 1,000 nm in diameter. The particle size will depend on power density (i.e., horsepower per unit volume), Table 4. Recommended impellers for immiscible liquid-liquid suspensions in turbulent flow. Sauter Mean Dia., µm
1–40
Impeller Class
High-shear
40–100
Pitched-blade Paddle
> 100
High-efficiency axial-flow High-efficiency radial-flow
the viscosity of each phase, and the surface tension between the phases. Chapter 12 in the Handbook of Industrial Mixing (4) is a good source for more detail on immiscible liquidliquid suspensions. Table 4 lists recommended impellers for turbulent-ow and upper-transition-ow applications.
Care in impeller selection The information presented here should be used only as a guide and a starting point for impeller selection. At times, process requirements may dictate the use of impellers that do not coincide with the recommendations in this article. Seek CEP the advice of the mixer manufacturer.
JULIAN B. FASANO is President of Mixer Engineering Co. (Troy, OH; Email:
[email protected]). He has 44 years of experience in solving mixing problems for both process and mechanical design. He worked for Chemineer, Inc., for 34 years, where he held positions as Technical Director and Director of Engineering and Development. He has authored over 95 technical papers on mixing. Fasano has a BS in chemical engineering from the Univ. of Dayton, an MS in chemical engineering from Lehigh Univ., a PhD in materials engineering from the Univ. of Dayton, and an MBA from the Univ. of Dayton. He is a registered Professional Engineer in the state of Ohio, and is a member of AIChE and the North American Mixing Forum (NAMF).
Nomenclature D H N N A N p N Re P Q g T
= the impeller diameter = liquid control height = impeller rotational speed = aeration number = the impeller's power number = the impeller's Reynolds number = power = volumetric owrate of gas at the impeller = tank diameter
Greek Letters ρ = uid density μ = uid viscosity
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Literature Cited 1.
Dickey, D. S., and J. B. Fasano, “How Geometry and Viscosity Inuence Mixing,” Chemical Engineering, 111 (2), pp. 42–46 (Feb. 2004).
2.
Nagata, S., et al., “Mixing of Highly Viscous Non-Newtonian Liquids,” International Chemical Engineering, 12 (1), pp. 175–182 (Jan. 1972).
3.
Nagata, S., “Mixing of High Viscosity Liquids,” Kodansha Ltd., Tokyo, and John Wiley & Sons, New York, NY (1975).
4.
Leng, D., and R. Calabrese, “Immiscible Liquid-Liquid Systems,” in Paul, E. L., et al., eds., “Handbook of Industrial Mixing: Science and Practice,” John Wiley & Sons, Hoboken, NJ (2004).
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