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VFD Compressor actually save money?

TiTillIdie

Plastic
Joined
Aug 27, 2015
Our shop fluctuates a lot on our CFM needs. We currently have a 20HP 77 CFM Ingersoll rand compressor running modulation mode @ 90 PSI, about 22-24 amps. We have kicked around a few ideas like getting 2 10hp and running them one or two, but I really like the idea of a VFD. I cannot find anything that PROVES there is power savings by running the motor slower. We have an old machine at the shop that has a VFD to run the spindle. We turned it on at like 100 RPM, 6 amps, cranked it to 1000 rpm, 6 amps, cranked it to 3600 rpm and still 6 amps. I know its not under load, but I would have thought this would be proof that its using less amps to run. So if a 20HP VFD is running at 5000 rpm and slows down to 2500 RMP, is it really using less power? Has anyone replaced their screw compressor with a VFD and seen a difference in their power bill?
 

GregSY

Diamond
Joined
Jan 1, 2005
Location
Houston
When you slow down a compressor, the output drops off pretty dramatically. I don't recall the relationship on a screw compressor specifically. But the motor draws amps based on load (assuming voltage stays constant) so unless your process changes a lot, there wont be much savings with a VFD. By 'process' I mean whatever the compressor is producing. In this case, compressed air which is a pretty fixed variable....assuming you have a tank that acts as a reservoir. If you didn't have any tank it would be a somewhat different story. You still need, I assume, the same PSI no matter what the flow so that limits your change in process. So essentially....is there a savings in letting the compressor run for an hour at reduced output versus running for 15 minutes at full output? Yes...but not a whole lot. Plus you have to hear it running for an extra 45 minutes.

Also, slow down the compressor too much and you can run into lube issues.
 

TiTillIdie

Plastic
Joined
Aug 27, 2015
Oh I should add, our compressor set in on off mode runs from 90 to 125 PSI and takes approximately 3.5 minutes. The cool-down is set to 1 minute but the compressor its self will not run for less than a 10 minute cycle so it idles for 6.5 minutes, then shuts off for 30 seconds maybe 2 minutes and kicks back on. That's why we went to modulation mode. we have a 600 gal tank. If we were to run the 70/30 loaded unloaded, we would need like a 2000 gal tank!
 

Bill D

Diamond
Joined
Apr 1, 2004
Location
Modesto, CA USA
My swimming pool pump is variable speed. I do not have a flow meter but I do have pressure guage on the filter. Rpm I have set from 3000 -980. The watts go from about 1,100 to 100 under load. The 100 watt setting is about 800 RPM. Still flows 5-10 gallons a minute as guesstimate. pressure goes from 10Psi -0. So lower rpm definatley uses less power.
In your no load test all the load is friction and wind resistance which will not vary much at those speeds.
Bill D
 

Bill D

Diamond
Joined
Apr 1, 2004
Location
Modesto, CA USA
I know nothing of screw compressors but piston ones have a minimum RPM to maintain lubrication by spalsh or pump. Screw compressors need to run hot enough,often enough, to boil off moisture inside the pump.
Bill D
 

johansen

Stainless
Joined
Aug 16, 2014
Location
bainbridge island
irrigation pumps are not the same thing as air compressors.



the energy required to push water uphill is constant per gallon. -but when half of your losses are friction in the pump and the pipe, (and the over loaded 80% efficient induction motor driving the pump) running the pump at 2/3rds the rpm may still pump water at half the cost per gallon, because the friction drops off with the square or cube of the flow rate. (secondly if the vfd is sufficiently intelligent, the motor's efficiency will increase from 80% at full load to 90% at half load at 2/3rds rpm. this gets you an extra 8% savings or so)



this is not the case with air compressors.. there is little friction in the valves, and the slower speed may increase the volume of blow by past the piston rings on a per stoke basis because it takes longer for the piston to travel up the bore. the biggest benefit comes from lowering the speed causing the compressor to run longer at a lower average pressure.


If you want to try an experiment, get yourself two compressors, one twice the size of the other. run both from the same motor, pulleys set to drive both at the same rpm. pipe the larger compressor through a radiator to cool off the air, into the smaller compressor. Take detailed measurements of both setups (single stage as you had previously) and record your total power bill over the course of a month.
 

GregSY

Diamond
Joined
Jan 1, 2005
Location
Houston
Most electric motors (AC) will have their max efficiency at around 75% load. But the 100% load effcy is not much different, and their 1/2 load effcy is going to be worse but not by a huge amount.

For induction motors, what does get worse is their power factor....it only goes one direction - down - as you unload the motor. This is less important when a VFD is in the picture, as it can compensate.

Small motors (for example 50HP) are generally poor performers; they pull a lot of inrush current and have not-the-greatest efficiencies. But large motors....97% efficiency is pretty common and few motors have less than 95%.
 

BT Fabrication

Stainless
Joined
Nov 3, 2019
why not buy one smaller pump. have it run 100-120 psi to maintain it.
and when you really need it let the large one run 90-110 psi when its really using air.
most new screw compressors already have a ton of progamming for this type of thing.
 

Comatose

Titanium
Joined
Feb 25, 2005
Location
Akron, OH
My power bill dropped about 2 grand a month going from a 50hp modulating compressor to a 50hp variable speed compressor. Compressed air misters are part of our humidity control system, though, so we're pulling 20cfm or so constantly, plus the various machine and blaster needs. The payback was super fast.

Modulating compressors are the worst possible case, though.
 

huleo

Hot Rolled
Joined
Feb 12, 2014
Location
UT
My power bill dropped about 2 grand a month going from a 50hp modulating compressor to a 50hp variable speed compressor. Compressed air misters are part of our humidity control system, though, so we're pulling 20cfm or so constantly, plus the various machine and blaster needs. The payback was super fast.

Modulating compressors are the worst possible case, though.
IIRC, a modulating compressor actually relieves partially compressed air as to "meet demand" and keep it from cycling? I can't say I am familiar.
 








 
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