×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

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

Jobs

VFD efficiency pump application

VFD efficiency pump application

VFD efficiency pump application

(OP)
For a given flow rate is it more efficient to run one VFD controlled pump at 90 percent or two VFD controlled pumps at 45 percent?

RE: VFD efficiency pump application

Quote (KRAkid)

For a given flow rate is it more efficient to run one VFD controlled pump at 90 percent or two VFD controlled pumps at 45 percent?
90% or 45% of what? Flow? Speed? Power?

Assuming that you most likely mean speed, then probably run one motor at 90% speed. To really know the answer you would have to plot the pump efficiency curves against the motor / drive efficiency curves, but my but feeling says that single pump will win. The pump likely was sized for optimum efficiency closer to the top of it's curve rather than the bottom (most pump curves dont go below some point anyway, so 45% is closer to the bottom, not the middle). Then the added motor losses from having to energize a second machine, plus the added mechanical friction, windage and copper losses in that machine will push it way over the edge of being lower efficiency than just one pump.

But to this, you have to factor in redundancy. One pump may be more efficient, but if it fails and it was all by itself, your efficiency goes to hell in a hand basket doesn't it?!

"Will work for (the memory of) salami"

RE: VFD efficiency pump application

(OP)
I will check the pump curves. Thanks

RE: VFD efficiency pump application

Dear Jraef, it must be flow as 90% of a 110kW pump will be far more than 2 x 45% of 55kW pumps at the same suction head. Smaller pipe sizes add to the loses. Large motors are more efficent and have better power factors. As you say the key is in the pump curves. I have tried for many years to get a pump supplier to give me a pump curve with a fixed suction head, variable speed against volume. Never suceeded yet. There must be some clever people out there that can do it.

RE: VFD efficiency pump application

jraef is right from a very simple fact that pump head is not additive in a parallel operation. This means, for example, if the head requirement is 30 meters at 90% flowrate, two pumps at 45% flow rate should deliver a head of 30 meters each. Here, the power consumption will not follow affinity laws. With constant head, whether 90% or 2x45% is one and same. The other losses are already mentioned above.

squeeky, with variable flow pumping, the operating point generally follows system curve and not pump curve. So, that data is redundant in most cases.

RE: VFD efficiency pump application

I have just found this. It is specific for filing requirements and not like a manifold that requires a constant pressure like a multiple pivot installation. It states that in a filling application on long to medium feeds where friction head is the major concern the stop start method of filling is more efficent than VSD. Can some please let me know if the up load works. Thnaks.

RE: VFD efficiency pump application

(OP)
Squeeky,
The link worked for me. We are not using VFDs for filling applications, but good to know why.
KRAkid

RE: VFD efficiency pump application

Interesting paper, only SLIGHTLY biased in that Aucom does not make VFDs...

The crux of their argument lies in this statement:

Quote (Aucom paper)

However, there is increasing evidence that for many pumping applications where real-time flow control is not necessary, VSDs are not the most energy efficient solution.

VFDs are for flow control ergo their premise, as applied to the smaller subset of pumping applications that do NOT involve real-time flow control, is accurate. But "increasing evidence"? What the heck does that mean?

Also, this line of "marketing rationalization" is like that which decries how much more energy efficient trains are compared to trucks, as measured by energy consumed per unit of payload. Perfectly good argument, but only as long as your destination has train tracks going to it...

"Will work for (the memory of) salami"

RE: VFD efficiency pump application

I high doubt that 90% and 45% refer to the pump speed.

Squeeky - the pumps would be the same size, not 1 large pump vs 2 half-sized pumps.

I would also expect that running 1 pump near full flow capacity is the more efficient way to operate.

As for the paper. There are many VFD manufacturers who give the example of changing from cyclic filling to continuous filling as a way to save energy.

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!


Resources