Minimum flow for a VFD pump running in close loop 4

[cholopa]

Hi:

I've been searching on this topic a lot and i can't find a reason why a pump can't operate at 1 Hz in the following conitions:

A system in a big AHU(cooling coil) with one VFD pump and no three way or two way valves.

Let's say at a given point a need 10% of the cooling power of that ahu. A tipical coil for water entering at 44ºF "would need" around 5% of the flow.

As the circuit is a close loop, the head needed would be extremlly low, so the power of the pump would be around 1 or 2%. I think i would probably need the VFD to run the pump at 5Hz or less.

I know the efficency would be horrible, but even then , the power consumption would be much much less that a system running with three or two way valve.

All the cons i have read (overheating, Radial hydraulic thrust, Flow re-circulation in the pump impeller...), i think are nonsense, as the speed of the pump is so slow that they are not of a consern. Take into account that the slip of the motor at those Hz is so high, that the real rpm is extremlly low (maybe 60 r.p.m), or in other words, just like if you were turning the pump with your hand at a rate of one rev per second.

What do you guys think of this?

Thanks for your time and help,

cholopa.

 

[Pumpsonly]

I had actually posted a long response to birm. There was no personal attack. Only facts.But don't know why is not published.

 

[BigInch]

It must have "cavitated".

We will design everything from now on using only S.I. units ... except for the pipe diameter. Unk. British engineer

 

[Artisi]

Pumpsonly, I saw you post and only had time for a brief read - and as you say it has vanished - may have been red-flagged for some reason - agree from memory it was only facts - but then it was a quick read.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[bimr]

The above post states that manufacturers set minimum and maximum speeds for the pump/motor/VFD systems for various reasons and quotes several of them.

Then the post says to get your information directly from the manufacturer of the installed equipment for the best project outcome.

pumpsonly pointed out in his deleted post that several of the quoted manufacturers have set different minimum and maximum speeds.

pumpsonly also posted something about different motor windings having different speeds.

pumpsonly also posted something about running a fan at over 60 Hz.

I then posted that these points was irrelevant because of paragraph 2 above.

cholopa then followed up in a reply that he contacted the actual equipment manufacturer (Grundfos) and was told that 8% (from my memory) was the minimum motor speed because the motor would overheat if run slower.

 

[Pumpsonly]

My Original post was wrongly quoted or wrong comprehension by some one.
What I actually posted were

1) This thread is about the practicality of a pump operating at very much reduced speed. It is not about VFD as some claimed.

2) VFD vendors only talk about output in HZ. Not speed or RPM as is a function of both the motor winding and Hz.

3) That the Danfos article about pump should be have minimum speed of 18Hz has been contradicted by articles from the Franklin Electric and Goulds Pump at 30Hz minimum post by the same person.

4)The Franklin Electric and Goulds Pump articles are about their ESP pumps with VFD drives limited to minimum 30Hz because the cooling of the motor is by the flow of the water over the motor casing which is directly proportional to the motor/pump speed.

5) That the Danfos articles has a few contradicting points itself. Specified 18 Hz minimum for pump and 6 Hz for fan. Both are driven by motors, why there is lubrication problem at 18Hz for pump motor and not for 6Hz for fan motor?

6) The Danfos article also stated the maximum speed of pumps and fans should not be exceed 60Hz due to excessive power consumption. Ask around and you will find many pumps operation at more than 60Hz as long as the motor and VFD are sized for it. Danfos itself offered VFD at more than 100Hz out put.


I am not sure if this re-post will be deleted again but main purpose is to set the issue straight that any statement made by a manufacturer's article should be used for reference only and should not be quoted as norm. The limiting value mentioned could be due to their product limitation or for a particular application of equipment matching.

The upper and lower limits depends on the individual application and equipment vendors.

 

[TenPenny]

I do not understand how someone can make a statement that the minimum speed of a pump is 18Hz.

18Hz is not a rotational speed, and therefore the statement is pretty much meaningless.

 

[bimr]

Manufacturers do not always use the same speed reference for the pump curves. Some show synchronous speed (3600, 1800, or 1200 RPM) while others use the slip speed of the motor used during testing. Depending upon the manufacturer and motor design, the actual operating speed will be 2 – 5% lower than that of synchronous speed.

When reviewing a pump curve under VFD control, one doesn't need to be concerned about these differences. The operable range of speed is typically expressed as a percentage and VFD manufacturers find it is aimple to grasp 18hz/60hz (3/10) rather than 525 rpm/1750 rpm.

 

[marcoh]

Advice I have had from motor manufacturers that for a pump/motor load curve the minimum recommended speed is 10% to maintain adequate cooling. There used to be a paper on the TECO website with detailed information but I can't find it anymore.

 

[BigInch]

That would be correct for most pumps. Every time I do a thermal anlysis it turns out to be true, but that 10% flow does not consider motor or VFD cooling requirements. VFDs should be sleeping by the time they reach 20%; they're usually not doing useful work after that anyway ... and certainly not saving money.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky