# On a centrifugal compressor impeller what is the difference between the radial vanes and rotating guide vanes?

I thought the radial vanes were a machined part of the impeller that just force the airstream radially? And that another name was rotating guide vanes as they are rotating with the impeller disc.. But the picture from a textbook I have is referring to them as 2 different things? Or have i misunderstood something?

Thanks

The radial vanes are the main compression channels, where air gets squeezed into 5 - 10 bar and gets slung out of the tip traveling at 500 m/s.

The rotating guide vanes scoop the incoming air into the impeller vanes, kind of like a pre-whirl tower. They’re at the inside diameter and travel at much lower linear speed, subject to lower centrifugal force.

This picture was used in an earlier answer, and illustrates the different roles of the channels and vanes. As the pressure ratio on the horizontal axis increases, we can observe the following:

• Efficiency decreases: losses are higher for higher single stage pressure ratios, which is the main reason that commercial aircraft turbine engines use axial compressors.
• Impeller height increases. The curved shape allows for a more gradual change of direction of the airflow: inflow is in axial direction, outflow in radial direction. A large potential source for efficiency loss.
• Inside vane shape points more and more in radial direction: it scoops the airflow into the impeller, and pre-whirls into the radial direction that will be imposed by the impeller.

The right-most impeller combines an axial stage with a centrifugal stage. In an axial compressor the rotor is followed by a stator, in our axial-centrifugal combination there must be no stator because we want whirling flow now.

With advances in production technology, this has resulted in impeller shapes with double curved inlet vanes, such as above. The inner bits of the impeller are subject to much lower stresses and may be shaped much more funkily than the impeller outer diameter.

• I'm still confused.. If the radial vanes are the machined part of the impeller, where are the rotating guide vanes mounted? And how do they rotate at a lower speed than the impeller? Do you have an image that shows them both clearly?
– Jono
Commented Jun 20, 2019 at 15:28
• Oh wait is the rotating guide vane and radial guide vane and impeller just one assembly? Impeller being the disc or hub, rotating guide vanes being the vanes you can see mounted on the impeller or machined part of.. and the radial vanes are the channels beneath the vanes that the air travels up towards the diffuser stage?
– Jono
Commented Jun 20, 2019 at 15:44
• Yes exactly. The guide vanes rotate at the same speed, the linear speed is lower. Commented Jun 20, 2019 at 18:44
• The "guide vanes" may be the taller ones closer to the center of rotation. The next sentence after the OP highlighted one tells more. Commented Nov 20, 2019 at 16:09