pumps with different kw rating

[Thuba]

l have 2 pumps one is 2.2 kw and another 5.5 kw. l have 2 questions regarding these.
1. The higher the kw rating doesnt imply higher flowrate, as one with 2.2kw has a higher capacity than the one with 5.5kw?
2. Can l replace the motors with one with the same kw rating but differene rpm? lnstead of 2900rpm(2 poles) with the 1500rpm(4 poles).

 

[TenPenny]

The kW rating is a measure of what power is required for the flow and head as specified on the datasheet.

Notice that the impeller diameters are different. So, these two pumps are selected for different operating points, so therefore the power requirement is different.

 

[Thuba]

The kW rating is a measure of what power is required for the flow and head as specified on the datasheet.

Notice that the impeller diameters are different. So, these two pumps are selected for different operating points, so therefore the power requirement is different.

also the one with a smaller impeller diameter has a higher capacity of 10m3/h

 

[Snickster]

The capacity of the pumps are about the same 9/10 m3/hr but the head of the smaller motor pump 2.2 kW is 20 m and the larger motor pump 5.5 kW is 40 m. This makes sense - you have twice the horsepower for twice the head at approx. same flow.

Therefore pumps require different motor sizes as indicated. Installing motors of different RPM will change the head versus flow performance of the pump. Pumps would produce lower head at lower flowrates and no longer have required capacities.

 

[LittleInch]

1) As pointed out above, the 2.2 kW pump is pumping at half the head/pressure of the second pump.

Also I suspect the first pump is more efficient than the second pump and hence needs less power.

2) Yes you can replace them, but your flow will be half and your head / pressure will be 1/4 of what it was before. Look up pump affinity laws. Doesn't seem like a good idea to me.

 

[Thuba]

1) As pointed out above, the 2.2 kW pump is pumping at half the head/pressure of the second pump.

Also I suspect the first pump is more efficient than the second pump and hence needs less power.

2) Yes you can replace them, but your flow will be half and your head / pressure will be 1/4 of what it was before. Look up pump affinity laws. Doesn't seem like a good idea to me.

Thanks, l am replacing 2 pole motor with a 4 pole motor, hence l was wondering l should half the rpm too?

 

[Thuba]

So for the 2 motors, if l place a flowmeter and a pressure gauge downstrean the pump; volumetric flowrate will be the same, but gauge pressure different. More for a pump with higher horsepower?

 

[Snickster]

Yes, for existing motors/pumps flow will be about same 9 vs. 10 m3/hr but twice head for higher horsepower motor. That is if the actual system pressure drop versus flow is what is shown on the motor nameplates.

 

[Artisi]

what are you trying to achieve, other than running around in circles - you are not making much sense at this point.

As littleInch has already said, changing to 4 pole motors will halve the flow rate and give you 1/4 of the head, the first pump would now be rated -- 5 metre head @ 5m3h, second one - 10metre head @ 4.5 m3h.

it is never a problem if you give more detail on what you are trying to achieve - we don't have crystal balls nor are we mind readers.

 

[pierreick]

Hi,
Share with us pumps curves and data sheets.
I believe this thread is going nowhere.
Why these questions?
Pierre

 

[LittleInch]

Thanks, l am replacing 2 pole motor with a 4 pole motor, hence l was wondering l should half the rpm too?

You don't have a choice. If you replace a 2 pole motor with a 4 pole motor it will run at half the speed.

Half the speed for a centrifugal pump gives you half the flow and a quarter of the head.

So not the same thing.

 

[Thuba]

Ok thank you all for your valuable information, and l was just moving around the plant and l made an observation on the pumps. And l dont want to change anything, l was just observing that almost all motors are 2 pole (3000rpm) and just out of curiosity l was wondering what will happen if l use a 4 pole motor. From your answers, l got my answer that l should still maintain 3000rpm regardless if its 2pole/4 pole. Thank you all!

 

[Artisi]

Unbelievable comment
"that I should maintain 3000rpm regardless if it's 2pole / 4pole."

2 pole 50 htz can only be approx 3000rpm whereas 4 pole can only be approx 1500rpm.

 

[Thuba]

Unbelievable comment
"that I should maintain 3000rpm regardless if it's 2pole / 4pole."

2 pole 50 htz can only be approx 3000rpm whereas 4 pole can only be approx 1500rpm.

oh unbelievable indeed, l replied with confusion, my bad.
rpm= frequency×120/number of poles,
technically 4 poles cant have 3000rpm. l guess some pumps are meant to be run on 2 poles only motors.

 

[TenPenny]

Thanks, l am replacing 2 pole motor with a 4 pole motor, hence l was wondering l should half the rpm too?

It's not a question of 'should you' half the rpm; if you go from a 2 pole to a 4 pole, you WILL half the rpm, that's what the number of poles on a motor means.

In a 50Hz system, 2 pole is 3000 rpm; 4 pole is 1500 rpm; 6 pole is 1000 rpm, etc etc
In a 60 Hz system, 2 pole is 3600 rpm; 4 pole is 1800 rpm; 6 pole is 1200 rpm, etc etc.

From there, the motor speed is reduced based on the efficiency of the motor design, so you'll often see 2900 rpm, etc etc, as the nameplate speed of the motor.