Performance Characteristics of Industrial Mixer Impellers (Part 1 of 3)
No industrial mixer topic is subject to more misunderstanding, misinformation and misapplication than impeller pumping capacity. This may surprise some, since it appears to be THE simple quantitative way to compare different mixers designed for the same application.
The good news is impeller pumping capacity can be a quantitative means to compare industrial mixers. However, there are caveats, and it should not be the ONLY comparative measure you use for the following reasons:
- It is subject to interpretation
The impeller pumping capacity is subject to interpretation, and it is practically impossible to verify without specialized equipment. With a mixer or pump, you can easily verify actual power by putting an amp meter on the motor. With the pump you can also verify flow performance by putting a flow meter in the line or running a timed transfer between two vessels. But how can you check mixer flow? You can’t.
- The pumping capacity value has no direct relationship to any specific process requirement in mixing engineering
Impeller pumping capacity is hard to compare on a one to one basis and is subject to “inflation”. The impeller pumped flow has no direct relationship to any specific process requirement in the realm of mixing engineering. You need to evaluate mixer performance, and while some analogies do apply, a mixer is not a pump (discussed in part 2 of this blog series on Performance Characteristics of Industrial Mixer Impellers).
Continue reading for more information on how ProQuip addresses these problems to effectively use impeller pumping capacity as one of the means to compare industrial mixers. Read the next two blog posts in this series on impeller characteristics for information on several additional measures ProQuip uses to comprehensively and accurately compare mixers.
Defining Mixer Pumping Capacity
Mixing impeller pumping capacity is often equated to centrifugal pump capacity and used to evaluate/compare performance of various agitators. A mixing impeller does have a reportable flow capacity, but it is not directly related to the performance of the mixer in practical applications. To understand this, we first need to look at what mixer pumping capacity is and how it is determined.
At first glance, determining mixer pumping capacity looks pretty simple – at least for turbulent flow. The pumping capacity of a given impeller geometry equals Flow Number (NQ) times impeller speed (N) times impeller diameter cubed (D3) or
This is a dimensionally consistent expression where NQ is a dimensionless constant. If, for example, you take N in RPM and D in feet, you get flow Q in cubic feet per minute. Any other set of dimensions can be used consistently (e.g., RPS and meters for cubic meters per second, etc.).
How do we get a value for NQ? It has to be measured, and there are a number of ways to do so. Since NQ depends on impeller geometry, experiment with a sample impeller on a test stand to determine flow capacity.
Measuring Pumping Capacity
There is no practical way to measure impeller flow directly. The standard procedure for an axially pumping impeller is to measure a velocity profile across the face of the impeller and integrate for the resulting flow. Then divide the flow by the test impeller’s ND3 to get NQ. There isn’t enough space in a blog for additional details, but you might use laser-doppler anemometry, particle tracking or a number of other techniques that are described in the mixing literature to determine velocity profile. The classical method uses a small propeller velocitometer (which is preferred by ProQuip), but this requires the use of a full-scale test impeller to get good results. We typically make numerous runs at different speeds and with different impeller diameters to ensure consistent results.
Regardless of how you measure pumping capacity, there are problems in interpreting the data:
- The flow readings are unstable. In turbulent flow, you must take data at each test point long enough to get a valid average.
- Vortices are being shed off the blade tips. At about 80% of the impeller radius, the flow direction can totally reverse for a few seconds and then go back to “normal.”
- There is a downward flow beyond the blade tips. This is liquid entrained into the direct flow of the impeller.
You must decide on a method to average your velocity data and an outer bound for your integral. Most of the flow developed by the impeller is generated around the outer portion of the blade. This part of the impeller both moves the fastest and sweeps out the largest area. But this is also the region where the data shows the most scatter and is hardest to interpret.
ProQuip addresses the problem of data scatter and flow boundary by measuring axial thrust and velocity at the same time. Since the density of water is known, we can integrate the velocity profile to get the thrust. If the profile fit to the data is reasonable, then the calculated thrust will agree with the measurements.
Locating the Measurement Reference Plane
Another problem is the measurement reference plane. It’s supposed to be at the plane of the impeller, but it’s not practical to get measurements there. You can’t stick an instrument into a rotating impeller, and it’s hard to measure velocity between the blades even indirectly. You have to pick a measurement plane at some distance below the impeller. However, the measured velocity profile varies with distance from the impeller. Because of momentum transfer, the velocity goes down but the profile widens as you move away from the impeller. This results in an increase in measured flow. So if you want an impeller to show a higher or lower flow number, relocate the measurement plane.
ProQuip takes measurements at a number of planes and extrapolates back to the face of the impeller to get reference velocities. We typically find the flow numbers of industrial mixers from other vendors (yes, we test their impellers, too), as well as many commodity-design impeller flow numbers reported in the literature, are anywhere from 10 to 50 percent too high. These are also “consistently inconsistent” with their measured axial thrust.
The increase in flow at different reference planes is real and often reported as “entrained flow” as opposed to “direct flow.” When reporting flow, some mixer vendors will start with direct flow and then multiply by an entrainment factor anywhere for 1.5 to 2.5. A reported flow should state if the basis is “direct” or “entrained.” You should be cautious when using entrainment factors.
What if I Have Multiple Impellers?
If you have two impellers in the vessel, should you add their flows to get a total flow for the mixer? The answer: maybe. An axial flow impeller in a vessel is somewhat analogous to an axial flow pump without a casing.
The question then becomes, “Do I have two pumps in series or in parallel?” If the impellers are in parallel, you can add the flows. If the impellers are in series, the flow is not at all additive and is a strong function of the particular installation.
The real case for your mixer can only be determined from the actual layout of the impellers in the vessel. The impellers-in-series model is closer to reality. The addition of a second impeller has a moderate effect on total flow (but can have a profound effect on the overall flow field in the vessel – discussed in the next post in this series). However, it is common practice to add impeller flows if a pumped flow rate is requested.
For More Information
As mentioned above, read our next two blog posts on impeller performance characteristics for information on additional measures you should you use to compare industrial mixers. For additional information on pumping capacity or help determining the right industrial mixer for your application, contact ProQuip at 330-468-1850 or firstname.lastname@example.org.
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