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Variable speed pumps vs Constant speed pumps

Variable speed pumps vs Constant speed pumps

Variable speed pumps vs Constant speed pumps

I need to get some guidance for use of variable speed pumps vs constant speed pumps in a domestic water pressure boosting situations.

I've found that for buildings that have a smaller footprint and a relatively large static head requirement, that constant speed pumps are probably better.  And for the larger footprint, less static head, variable speed pumps are better.

What I am trying to do is come up with some rules of thumb that would give guidance to what to do in the middle of these scenarios.

RE: Variable speed pumps vs Constant speed pumps

You are absolutely right.

Danfoss developed a software to check this situation. I think you can get it from their website for free. You have to compare the reduction in energy bill to the investment on VFDs.

You get better savings if you are running your pump at BEP at all speeds. I don't feel you should depend upon thumb rules for this.

However, details of a specific case will help you and us in analysing the problem in a better way.


RE: Variable speed pumps vs Constant speed pumps

You are correct that high static head requirements may eliminate VFDs from consideration, lower static heads can make excellent VFD applications.

Pump selection is a key consideration however to coordinate properly with the controls.

In these applications the pump often runs unavoidably for long periods well off BEP, so you must select a pump that can do this well.  I see incorrect pumps used very often in these applications, you know, low bid wins usually and many engineers do not know much about pumps so how would they know what pump is best or even correct for the job?

Although not a comprehensive list, nor a list that would help much in the hands of inexperienced person, but the following list will give you some idea of the pump considerations:

1 - NPSHr
2 - Ns
3 - Nss
4 - Diffuser or no diffuser
5 - Multistate or single stage
6 - Stainless or Cast Iron or ...?
7 - End suction or double suction
8 - Steep or Flat curve geometry
9 - Power to flow curve relationship
and lots more.

How will you prioritize those above items?


RE: Variable speed pumps vs Constant speed pumps

I fully agree with quark's and PUMPDESIGNER's comments.  Each application must be thoroughly considered on its own merits.  Any attempt to make choices on the basis of rules of thumb is an invitation to much trouble and excessive costs.

In addition to quark's and PUMPDESIGNER's comments, I recommend that serious consideration be given to the system pressures.  Very commonly, such systems are designed and operated at needlessly high pressures.  This results in more pump and energy costs being wasted through needless throttling losses.  Paying due attention to the actual pressures needed can result in significant savings in both initial and operating costs.  It may be practical and economically attractive to operate the system at varying pressures depending upon actual pressure needs.

When you study the actual vs. "standard" pressure needs model of the subject system, don't be surprised by seemingly dramatic pump and energy needs differences.  I've had experiences where reducing the pressure of an existing system by about 10% resulted in being able to operate very satisfactorily while using about half as many of the pumps most of the time.  While the energy savings were quite obvious, the real savings came from nearly eliminating pump repair costs.  Staggering pump maintenance costs were what prompted the review of the actual system pressure needs.

Don't be surprised to find that a VFD pump in such an application will spend much of its operating time well away from its BEP, but even so, the use of a VFD may still be justified.

RE: Variable speed pumps vs Constant speed pumps

I am glad that ccfowler agreed with me, not that I need it, nor does ccfowler need my agreement to know that he is correct,

But I am glad because this needs to be heard more.

All the discussion goes towards controls, little or nothing towards the pumps.

The main reason for this is that low Ns specific speed) pumps are extremely flexible and tolerate much abuse, and when they do have problems the problems are long range and subtle.  Sooo, during job start up and for the first year, a pump that is very wrong appears to be fine, at least to unkowledgeable persons.

No complaints, everything must be fine.
When complaints do come, they are 1-2 years down the road, something must have changed, poor maintenance, something other than wrong pump.

Also, as ccfowler points out, power use is very high, but how often is that noticed?  This is another long term, subtle, never noticed item that does not cause concern.  VFDs are not ever questioned, people assume they are saving power when in fact many VFD systems are out there using more power than a constant speed pump system would, but who is there to notice this or make mention of it, and worse yet, the engineer specifying the VFD would look bad if it was brought to light?

Thus, we seen people treat not only the pumps, but also the controls with a casualness (rules of thumb and automatic assumptions about VFDs) that is oversimplification.


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