Across the line vs Frequency Drive

Good Morning ,

Our energy supplier has a rebate program. We would like to install a frequency drive for our air compressor.This is what we have at the moment.....

150 HP , 1785 RPM , 1.15 service factor
Starting Operation- Across the line Starter
( No wye/delta start just across the line )

Our plan is to put a frequency drive on this motor.

I need to justify this purchase and also convince the power company to pay 50% of this project.

On our bill, the first demand block is....
Billing units is 500 and the Charge rate is 1.72 for $860.00

Second demand block is......
Billing units is 437 and the charge rate is 1.69 for $738.00

By installing a frequency drive on this 150 hp motor , would that reduce these demand blocks. Could you also share some of the math that I would need to justify this with both parties.

Thanks so much in advance.

For the power company, there should be an Energy Efficiency department with engineer on staff to help you with qualfication requirement. They might also help you with justification to your own company, YMMV.

With air-compressors, the retrofit with a VFD is little more complicated than in HVAC application. You will probably need to modify recycle/surge logic as well and you might need to bring in the compressor manufacture for a quote.

This is such a common application that, as Harry says, your utility rebate program probably already has a canned rebate or guidelines, plus some evaluation asistance.

Here are some wites that might help:
http://www.compressedairchallenge.org/
http://www.motorsmatter.org/resources/asds.html
http://www.efficiencyvermont.com/stella/filelib/CompressedAir_FactSheet_FINAL.pdf
http://www.cee1.org/ind/mot-sys/mtr-ms-main.php3

In general terms compressor power for a compressor is a function of flow rate and pressure. If this is a shop air system, you probably can't cut the pressure much. A VFD will only save you on demand charges if you can re-fill your reservoir at a lower flow rate over a longer period of time. If the compressor duty cycle is on 25% off 75% then cutting the flow rate in half will require half the power demand, but the same total amount of energy. Note that efficiency falls and discharge temperature rises as you slow down most compressors, so you don't want to go too far down in speed. That's where a detailed analysis or help from the utility or compressor supplier may come in.

Rob send a pm with your email, i got an excel file with that calculations done, so you know exactly when you recover your inversion based on energy savings.

Hold on, fellas! First, a key piece of information, the type of compressor (piston, turbine, or screw) matters a whole lot.

A piston compressor is a hard start even with the air pressure unloaded due to the high torque required to get the first piston over top center.

Most screw compressors and turbine compressors cannot be slowed down without running into lubrication problems. Your compressor manufacturer is your best source for protection on this. (if variable speed is possible, they will likely press hard to sell you their own solution)

It may be that your energy costs are largely due to the peak demand charges encountered during starts. You may well find that resetting your system with fewer starts (a larger accumulator tank, for example) or a soft starter will give you as much savings as the more expensive VFD. Incidently, the softstarter will not save you energy but it may reduce your energy costs due to peak demand charges.

Bottom line: better study this carefully and avoid plugging in what looks like a quick fix. It might not be a fix at all.

And, all that from a "drive guy"!!!

Thanks for the links. I have some more reading to do.

Rob S. said:

Thanks for the links. I have some more reading to do.

Click to expand...

Rob
FIRST reread what Tom Jenkins and Dick DV said.

Next re demand charge - often this will be the largest load started in 15 minute period. This varies with utility so you really need to know how they are calculating demand charge.

IF demand charge is really bugging you guys the cheapest solution is to start your large loads every 15 minute. Again talk with the utility.

Is the 150 HP compressor your largest load? You really need to know this AND how often and WHEN you are starting the larger loads.

Dan Bentler

leitmotif said:

demand charge - often this will be the largest load started in 15 minute period. This varies with utility so you really need to know how they are calculating demand charge.

Click to expand...

A couple of years ago I was tasked to trim the utility bill at a lumber mill. My experience with the demand charge was similar but slightly different than Dan's. The demand charges were about half of the bill. They were based on the highest KW usage in a fifteen minute period. There was a demand charge during peak usage hours and a demand charge during off peak. The utility was trying very aggressively to lower their peak usage. So they offered a $0.10 off peak demand and a $14.05 on peak demand fee.

The facility had some 24/7 processes and some 5 day a week loads. The 24/7 loads were mostly air moving fans for drying lumber in kilns. Some research yielded some facts about drying lumber. The air speed the fans delivered was a compromise between high performance and economy. The best quality drying demanded high air flow after a temperature setpoint rise, but after several hours the air speed can be reduced with no degrade. With air moving fans when you decrease the speed by 20% the horsepower used is cut in half.

The solution was to make temperature setpoints and corresponding air speed increases as soon as the peak demand period was over.

I would suggest that a 'Walk' through the factory during a shutdown period - Air compressor running only - would be a good Idea.
as has been stated - soft start or VFD - But if your compressor is fully loaded due to air leaks - which is often the case - you need to fix that first.
Can you drop your peek air by 5 PSI
Can you increase your differential by 5 psi
these will all make a huge difference.

I have experiance with 2 x 600cfm screw compressors running 32 'A' class presses - these did not run fully loaded - plenty of air receivers in the plant.
And the engine plant ran 6 of these compressors @ 100% load
with no air storage othe than 6" pipes
this was not a good cenario at all.
Either of these factories are amazing when you walk through an hear the air leaks - Big Shock for all.
the Press Plant - is monitored regularily
the engine plant - NMP - not my problem hehehe