NEED AN ANSWER 1

[bara]

If I have 5 centrifugal pumps maintaining the same head 165 feet and 2800gpm and the pumps are VFD driven pumps,it is a closed loop pcw(process cooling water system), is it more efficient to have three pumps running the same condition or 5 pumps running?
I did some calculation and found the motor efficiency and it showed that with three pumps running, each pump will pump about 900 gpm and more flow increases the efficiency of the motor and the pump.
Please explain it to me if you see it differently and tell me why?

 

[TD2K]

You need to look at your pump curve to determine if 3, 4 or 5 pumps are the most efficient.

Each pump curve has a BEP (best efficiency point) and that is where your pump has the highest efficiency. It wouldn't be unexpected if you were running 3 pumps, instead of 5, at a constant total flow rate that you could find yourself closer to the BEP and thus, your total Hp would be reduced compared to 5 pump operation (with 5 pumps, they would all be 'throttled back' for flow rate and possibly have a lower efficiency. Normally, when you throttle back a pump, you also increase the head and thus the Hp requirements though in your case, you are maintaining a constant discharge pressure I believe with the VFD).

 

[MortenA]

TD2K is right

Either you throttle or you vary the pump speed.

A common set-up would be four pumps with on-off control and one with VFD. When capacity requires it (i.e. when the VFD controlled pump is nearing max speed) then one more pump is turned on and the speed of the VFD is reduced accordingly or the other way around if demand is reduced.

Best Regards

Morten

 

[bara]

The efficiency will increase when I change that to three pumps since the RPM of the motor increases and the capacity for each pump will increase. But I have some people who disagree with me based on measuring the I(current), and V(volts). Based on your experience could I miss something in my calculation that I see a better overall efficiency when I change 5 pumps to three pumps?

 

[TD2K]

You can email me your pump curve at testdog2000@yahoo.com or fax it to me at 508-448-8536 and I'll take a look at it. It's something you have to review your pump curves to be certain, it's not an automatic conclusion that fewer pumps will be more efficient since at some point, you'll start to move past the BEP.

 

[Bala0610]

Hi

There have been excellent responses to this query from pump point of view. I also understand that the motor also has an efficiency which peaks at about 85% or so of the load. This the manufacturer of motor will be able t confirm. Hence, when switching to 3 pumps, you may be increaseing the motor efficiency too.... As TD2K says, needs a closer look, but is definately worth the effort...

Rgds

 

[quark]

BARA!

M/S Danfoss has a Pump Controller which is a PLC into which your pump characteristics are fed. When the pumps are performing below par either the pumps are swithced off or swithced on to keep the system to operate in a better efficiency zone. If your requirement is varying it is a better option.

Regards,