Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Centrifugal compressor impeller tip speed.

Centrifugal compressor impeller tip speed.

Centrifugal compressor impeller tip speed.

Why impeller tip speed should normally range between 650 and 900 ft/sec (198 and 274 m/sec) ?
What is the consequences going over range or under range ?

RE: Centrifugal compressor impeller tip speed.

Going beyond 800-900fps brings the gas at the impeller closer to Mach 1.0 at low pressures / low specific volume - see the expression for sonic velocity under theoretical 100% isentropic conditions. Approaching or exceeding Mach 1.0 makes for unstable flow and creates shock waves at the impeller tip. Industry working limit for max developed polytropic head per impeller ( which is dependent on tip speed) is approx 10e3ft.lbf/lb mass of gas. And max no of impellers you can cram into a single casing is about 10, which translates to approx 100e3 ft.lbf/lb mass per casing. See the chapter on refrigeration cycles, subsection on centrifugal compressors in Perry Chem Engg Handbook, 6th edition.

Would guess polytropic / isentropic eff drops off rapidly as you decrease speed and becomes nonoptimal from a selection - design point of view.

RE: Centrifugal compressor impeller tip speed.

You can easily operate at 300 m/s impeller peripheral speed and beyond while still achieving low peripheral mach numbers.
Example: Hydrogen gas.

So the real reason to answer OP question is :

On the one hand, decreasing the number of impellers makes the bearing span shorter and improve rotordynamics; subsquently casing costs less, marginally but still it is less. But putting less impellers increase the peripheral speed. Having higher peripheral speeds means higher stress. If the gas is corrosive, then the yield strenght performed for a given material is also reduced which is even more constrainful. Ultimately the limit is dictated by the yield stress utilized vs. peformed for the selected impeller material. On a side note: open impellers tend to have better aptitude to tolerate higher stress ;some integrally geared compressors for example could spin at much higher speeds (+400 m/s for titanium material / open type); spining faster in this particular case - considering the impeller diameter fixed - would increase the flow coefficient thus the efficiency.

RE: Centrifugal compressor impeller tip speed.

The expression for sonic velocity is also a function of pressure at the upstream side of the impeller. i.e vel = (k.P.v)^0.5, [vel=m/sec, P=N/m2, v=m3/kg] so at high suction pressure, yes, you could be operating at higher tip speeds to keep within 0.8-0.9Mach max. The k value for irreversible compression processes is somewhat higher than ϒ, but may be safer not to take credit for this.

Taking credit for higher suction pressure in max permissible tip velocity is yet another safety critical reason to install a PSLL on the suction side of the compressor.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close