lowest possible pump speed for VFD

[hvacers]

till how low can i reduce the speeds of my condenser water pumps? is there a limit that the pump cannot be set as low as 30hz. we are 60 hz here in our country.

the application of the pump is because i really have a low delta T with my heat exchanger, the pump flow is 2000gpm and i am planning to reduce it to around 1000gpm by installing a VFD is is possible? or there will be problem with the pump (cavitation or whatsoever.)

please share some insights

 

[toughguy1]

Check your pump curve first regarding pressure etc., but my answer on the speed is that you can't go lower than the minimum speed to remove heat from the pump. A pump can burn up if you don't flow enough water through it. A conservative rule of thumb is 30% minimum speed (18 Hz) for long term usage. Your numbers sound fine.

 

[QualityTime]

I don’t know much about condenser pumps but I typically work with municipal centrifugal pumps or positive displacement pumps:

1. The electronics in a VFD can typically give you a 10:1 turndown (i.e. 60 Hz down to 6 Hz) but the turndown on a motor is an entirely different thing
2. For VFD service the pump motor has to rated for inverter duty service.
3. I typically do not run a pump at less than 30 Hz because the motor will overheat. When a motor is rated for VFD service the motor casing size is typically increased so that you have more surface area to dissipate the heat
4. I suppose you could ask a motor manufacturer what the allowable turndown is. If you want very low speeds you may have to go to things such as water cooled motors

 

[QualityTime]

If it is a centrifugal pump why don't you just trim the impeller and forget about the VFD

 

[hvacers]

hmm.. i dont know about triming down the impeller because if you trim it down the flow will definitely be lowered permanently unlike if you use VFD it can react to the actual change of load. where that is the main concern that i want to happen, i want my pumps only supplying the exact flow depending on the building demand.

what you guys think?

 

[Artisi]

Is motor overheating a real problem in this situation - halving the speed reduces the power required by a factor of 8 - (30/60)^3. Suggest you check with the motor manufacturers.

 

[Ziggypump]

If you go with a VFD for the pump, estimate that the pump will only be able to get down to 30% of the rated flow (2000 gpm *.3 = 600 gpm).

Think about swapping out the pump motor with a premium efficient motor. These are almost always called for when adding a VFD to water loops.

 

[sshah84]

If you know that your demand may get change lets say from 1000 gpm to 2000 gpm, then better stay away from trimming. Moreover, if you intend to bring down flow from 2000 to 1000 (50%) by trimming pump impeller, that may not be a good idea in terms of pump efficiency.

I would recommend first get pump curve for different speeds (4 or 5 curves). If you know your pump supplier, he/she should be able to get this for you (I did same way!!!). This is very crucial since either you trim impeller or use VFD, along with the flow rate pump is bound to loose it's head capacity. Make sure @ 1000 gpm your pump is able to generate pressure more than your system pressure otherwise after reducing speed/trimming impeller you may find your pump is running at dead head!

To my knowledge, you should run VFD at least ~ 10% (6 Hz), this will ensure motor (if it is air-cooled) doesn't overheat. I am not sure about existance of such limit on pump side (as long as you don't run it on extreme side of curve).

 

[CH5OH]

you can run the VFD to 0 rpm, the problem of overheating is often dealt with by removing the cooling fan from the motor shaft and cool the motor with a separate fan motor.