Efficient Motor Modulation

[billjg]

I am a ChemE and am working on energy efficiency in pumping. 90% of the pumps out there are centrifugal with 60-70% best efficiency at one point on the pressure-flow curve. The design size is always larger than the operating size, so they actually operate at 50-60% efficiency or so. Worse, we put the design sized motor on and it actually only runs at 70% of rated load. Most pumps are also small (<25 hp) so at full load they are only 85-90% efficient. At 70% load they are probably 80% efficient. (The vast majority of motors are inductive.)

There is more... the flow needs to be throttled so in order to be efficient we use a VFD, but it only typically runs at 70% load (? efficiency). Add it all up and only 30-40% of electrical power is making into the pumped fluid.

Pumps use a large chunk of power in chemical plants -- is there a more efficient way to modulate <25 hp motors? What about different motor designs?

billjg

 

[itsmoked]

Hi billjg;

First, motors of that size are now always in the 92%+ range of efficiency. It would be very hard to improve on that.

Also your VFD number is WAY OFF as they are all in the 90++ percent efficient realm.

Improving on the pump will be your money maker.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MetalworkerMike]

It makes sense to use a bit more pump than you need because the piping builds up scale, and the impeller wears a bit and everything moves the pump down on its efficiency curve. Unless you really, really enjoy replacing mechanical seals it pays to use a big pump and don't work it hard.

Mike

 

[dpc]

Agree with itsmoked. Most of the losses are in the pump, not the motor or drive. From wires to pump shaft, you probably have efficiency of 80 to 90% depending on the operating point.

 

[MikeHalloran]

It makes sense to use a pump that's _a_little_ bigger than you really need.

My first experience with centrifugal pumps was measuring a building and making pipe drawings for a system to carry paper stock from a basement mixer, up three floors and deliver it gently into the bottom of a tank above a paper machine. The plant's engineer figured the pipe drop extra conservatively, up-sized the pipe for fun, and up-sized the pump and the motor. My boss, the consulting engineer, told him the pump was two sizes too big, but was shouted down, and shut up and took the money.

We got a call right after they started the pump; the paper stock shot from the bottom of the tank, and hit the ceiling, 20 feet above. Not our mess. They had to install a gate valve and screw it down until the pump squealed to get the flow right.



Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

My old pump books typically showed performance curves at 3550 RPM, 1750 RPM and sometimes for 1175 RPM. With VFDs you are no longer limited to fixed speeds.
For best efficiency you may consider extrapolating the performance curves to determine the combination of pressure, pumped volume, and pump speed that is most efficient and then use a VFD to run at the best speed.
respectfully

 

[itsmoked]

I wonder if since you have a VFD you couldn't use a positive placement pump running at exactly what flow you need at a much higher efficiency?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

Keith,
With a PD pump you would lose the affinity law benefits you have with VFDs on centrifugals. I have been told by pump people that the difference in efficiency does not make up for that.