Impeller Size vs Efficiency 1

[wrd2032]

We're trying to squeeze some electricity out of our oversized pumps, but they already have VFDs. Typically they run no higher than 75%. I heard you can improve efficiency by making sure the impellers are maximum size. Is that so? Any other improvements available to a pump on a VFD already?

Thanks.

 

[Softedge]

wrd2032

any kind of throttling or recirculation of flow to match pump performance wastes energy. also, most vfd's are efficient down to about 60% of 60Hz, i.e. 36Hz. to maximize the efficiency of your existing pumps trim the impellers so that the vfd operates between 50% and 100%. the best option is to know the 'mode' flow rate, this is the flow rate where the pump operates most of the time. the impeller should be trimmed so the pump has maximum efficiency at this flow rate. after you have established this point check the low and high extremes for pump and vfd performance.

for the range that the vfd is operating it appears the pumps are too large for the service. to maximize efficiency and performance, the pumps should be changed to a size where the vfd can operate between 60% and 100%. in most cases changing the pumps to the right size will give you a +ive NPV within 5 to 10 years.

these comments are based on standard pumps and vfd's, there are of course always special designs and cases.

softedge