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!
  • Students Click Here

*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


affinity laws, axial fan

affinity laws, axial fan

affinity laws, axial fan

Dear all,
Hope somebody can help me to clairfy this topic.
I have seen some threads about this topic, but none of them is answering my next question.
I have measured the air flow, rpm and absorbed power of a three phase axial fan at two different fan speed – delta and star connection.
I will call 1 and 2 this two conditions..
What puzzles me is, if I consider affinity laws
- equation (air flow 1/air flow 2) = (n1/n2) is respected. The two ratios are practically identical.
- equation (absorbed power 1/ absorbed power 2) =  (n1/n2)^3 is not respected. (the power is the total absorbed power)
There is a large difference between the two ratios.

Please, is there something I am missing or is it due to a wrong measurement ?
Many thanks

RE: affinity laws, axial fan

So, you are changing the voltage, which changes the slip, which slightly changes the speed, and without changing the system, you are observing changes in flow rate and input electric power, correct?

Some things to consider:

1 - The pump/fan laws DON'T tell us what happens to the operating point with change in sped. They tell us what happens to the pump curve.  To convert that knowledge of change in pump curve to knowledge of change in operating point, we would need to know sometihng about the SYSTEM characteristic curves (the operating point is the intersection).  As it turns out, ONLY if the system characteristic curve obeys DP~flow^2 do these relations apply directly to the operating point.
2 - affinity laws apply to incompressible fluid.  For a fan, they are pretty close if the dp is far less than the absolute pressure (so that pressure and density don't change by a large factor). They are not good if the dp is a sizeable fraction of the absolute pressure, such that density is not even close to constant.
3 - affinity laws apply to bhp, but you are measuring input power I think.  If there is significant change in efficiency then you have an error.

(2B)+(2B)'  ?

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!


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