Radial, Axial and Mixed Flow for Newbies

After plowing through a few small texts and online trainings, I’ve started in on Igor Karassik’s Pump Handbook. I’ve heard it referred to as the bible of pumps and it’s a fitting description! I couldn’t even turn three pages before getting stumped. It turns out that dynamic centrifugal pumps are a bucket category for a variety of flow options. First of all, radial and axial are words I had heard, but had never really brought into practical use. That was a problem! And anyone who knows me well enough knows the approach I took to solving it: Merriam-Webster Online. It has been my most frequently visited site for decades.

Where I got stumped was in visualizing this. Both radial and axial refer to force taking place outside of a circumference. If they’re both that, then what? Pump shafts are the circumference in this case, and the water flowing adjacent to them represent the force. It also matters that the direction of flow directly impacts the head-capacity curve and best efficiency point. And when we’re trying to understand that, head is a measurement of how far the pump can pump water from its suction end. There is more that factors in, involving suction lift, specific gravity, and net positive suction head, but those are topics for another post. Head is measured in feet rather than pressure pounds per square inch, though one can convert to the other. It seems useful to measure it as feet of head from a visualizing standpoint. Capacity is maximum volume flow rate through a pump in gallons per minute in our measurement standard.

In radial pumps, water flows at 90 degree right angles from the impeller. That means the water is flowing out to the casing radius, to each side. Higher centrifugal forces are providing higher head and proportionally lower flow. In axial pumps, water flows parallel to the shaft in a perpendicular vertical flow, resulting in lower head with higher flow. Think of it as the water flowing upward in a straight line adjacent to the impeller, where the axial impeller acts as a propeller. What makes that happen is the angle of the impeller vanes. Mixed pumps are somewhere in between, 45-80 degree flow. The head-capacity curve for mixed pumps rises in uniform toward shut off head. Shut off head is what it sounds like, and it’s the reason head-capacity curves are useful in striving for best efficiency, selecting and sizing pumps appropriately to avoid premature pump failure.

Are axial pumps actually centrifugal pumps, then? There a different opinions on this. For the most part, the consensus is to categorize them as a subset of centrifugal pumps. Yes, the flow is vertical rather than radial, but all else aligns in terms of being rotodynamic, which is just another way of saying this classification of pumps is designed for continuous flow rather than at interval displacement.

It is all well and good that directional flows have been defined within centrifugal pumps. This isn’t even close to capturing the full spectrum just within the centrifugal category. It’s a good running start in that direction. And applications for each type are useful to know. It’s a topic for another post. If this information seems like a lot to digest, it was for me, too. If this also helps anyone else, that’s awesome and all the better.

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