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Low pressure compressed air supply.

Low pressure compressed air supply.

I have a facility full of CNC machining centers and CNC lathes that uses a great deal of air. They have a 25HP screw compressor that runs 24/365 at about an 85% duty cycle during operation hours and about 70% during non-op hours. By op hours I mean periods when humans are around, lots of these machines run unattended for days on end fabbing one part.

Investigating a little I found that lots of the air they use is for keeping motor shaft seals pressurized. They all continuously leak air, hssssssssssssssssssss. I found that they regulate their 121PSI plant air down to 15PSI for this sealing service. Seems to me that it would be much more efficient -lower operating costs- to re-plumb to these machines that use purge air, and run a low pressure compressor that provides something like 20PSI to this one specialized air task.

If this efficiency premise is true it brings up a few questions.

1) Who makes low pressure compressors for this type application? And, can anyone recommend a supplier?

2) What to do for estimating what the current high-to-low pressure situation is costing them as wasted energy so some ROI can be calculated?

3) Any other suggestions you folks have about this situation.

Keith Cress
kcress -

RE: Low pressure compressed air supply.

A Roots blower can be a good solution.
... but be warned, they are incredibly noisy.
You will want to locate it outside, in a concrete enclosure, and put a muffler in the discharge pipe, and some sort of noise suppression in the ventilation ports in the enclosure.

Mike Halloran
Pembroke Pines, FL, USA

RE: Low pressure compressed air supply.

Compressed air is wonderfully expensive. I was trained years ago to recognize that any time one hears a hissssss, that is costing the company $0.50 per minute in electricity costs. I suspect that cost is much higher, on the order of dollars per minute. So, my training indicated one should eliminate all leaks. Using compressed air as a broom for sweeping is stupid, too. As is blowing off chips at higher than 30 psi (which, BTW, OSHA says is a bad idea).

Eliminate all leaks & wasteful usage, reduce consumption, reduce capacity needs, reduce costs.

We had Ingersoll compressors at my plant. I worked with Ingersoll Applications Engineering as a paid consultant to analyze my plant's compressed air requirements. I suspect most of the other big players (Sullair, Atlas-Copco, etc.) may still provide this type of service and give good guidance. Solutions for your situation may be strategically-placed accumulator tanks & a leak-elimination campaign instead of new compressors.

When I did my analysis, I found on the Department of Energy website somewhere a link for calculating dollar-values for compressed air consumption & usage for our two compressors. Working with the Ingersoll team, this analysis lead me to replace the high-pressure/low-volume screw on one compressor with a low-pressure/high-volume screw. And turning off the 2nd compressor. Turning the 2nd compressor off saved me $75K per year. The Operations Manager thought I a miracle worker.

Another plant needed a new compressor. After some analysis, I chose to go with one of Ingersoll's compressors with the Variable Frequency Drive system. Cost extra money up front, but the compressor ran only enough to meet the plant's demand. Big money savings for my situation.

Many plants go through expansions over time. The original compressed air piping layout gets modified over & over to accommodate expansions. A thorough analysis will show if the piping is sufficient, has piping choke points that reduce flow/pressure, needs accumulators, etc. If these problems exist, it usually doesn't cost too much to do a little retrofitting (replace small diameter pipe with larger pipe, etc.) that will restore the balance and capacity.

Blue Technik LLC
Virtuoso Robotics Engineering

RE: Low pressure compressed air supply.

One plant that I worked in realized that there was equipment much like yours that used lower pressure, and it needed to be clean. We put in a separate system in all stainless steel piping (we used Victalic pressfit sch5) and then ran a blower to produce the air that we needed. This air was dehumidified and the blower was oil free. Our air costs only went down a little, but maintenance costs went down to nearly zero. We realized later that we could have supplied these off of our high purity bulk nitrogen tanks for about the same cost.

In your case the equipment manuals should tell you what pressure and flow is required for this part of the service. Start by making sure that air isn't just being dumped. A survey is the place to start.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Low pressure compressed air supply.

At a previous employer we used multi-stage centrifugal blowers to get up to about 30 psi or so. Cheaper and much quieter than a Roots type blower and more tolerant from a (lack of) maintenance perspective.

Do these machines regulate the seal air pressure on-board? If you drop the supply pressure to the machine expect to increase the size of the distribution pipework considerably. If you also need the 8 Bar air for other purposes then I guess you're installing a new distribution system anyway.

RE: Low pressure compressed air supply.

Tyger: I'm thinking you mean 50 cents/day not minute? That's a 1/3 of a million dollars. But I take your point. It could be a process change would pay back much better. These seal flows probably don't really need to run even when the machine is standing idle as the machines often are. I'm thinking maybe an off-delay timer where they continue the purge for, say, 60 minutes after the machine is idled so no drippy water capillarys up the shaft, but then after that stopping the flow entirely.

Dang, Ed. That's a little disappointing that reducing the compressor pressure by switching to a blower paid back poorly based on the cost of the energy. I'll look into these shaft purge requirements some more. Seems pretty wasteful to me as compared to just keeping a positive pressure in the motor. Why would actual flow really be needed if you want water to stay out of the motors. I'd think a foot of water column air pressure would do the job. Perhaps there are regulators out there that would allow us to set a foot of water column and keep it there instead of 15PSI of straight blow-by.

Keith Cress
kcress -

RE: Low pressure compressed air supply.

Don't get me wrong, down a little was $5k/mo. Many plants spend over $25k/mo on compressed air.
But running a lower pressure system will require large diameter tubing, but it can be very thin wall.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Low pressure compressed air supply.

At 24hr/ 365 / 100% I calculate your using abut 155,000 kWh power year. Don't know what your elec cost is but you've got quite a small compressor. Low pressure air will need bigger pipes as said before.

So divide annual figure by volume to get price per cubic foot per year.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Low pressure compressed air supply.

What is the environment inside the machine tool work area in regards to coolant?

What is the lubrication method used for the machine tool spindles?

RE: Low pressure compressed air supply.

Thanks Little, I also think the compressor will spit out it's kwhrs which can contribute to the puzzle too.

Tmoose; There is water based coolant dribbling on the grinding faces but since the grinders spin at about 1kRPM there is pretty significant mist in the machine/spindle space. But when the machine goes into standby they open the door and the machine will sit there for minutes, hours, or days. The machining space will totally dry out in about 3 or 4 hours I'd say.

Keith Cress
kcress -

RE: Low pressure compressed air supply.

Our CNCs also use 80PSI air during the tool changes to blow out the spindle from inside and one of them has an air cylinder counterbalance that needs 80PSI, too. If your any of your machines need even a little high pressure air, I think you're SOL.

RE: Low pressure compressed air supply.

Simplification: You trade pressure for volume, so there really isn't much savings.

Some possibly useful calculators:

Idea about using tanked N2 is interesting, and warrants investigation.

RE: Low pressure compressed air supply.

John25; The grinding tools are add-on the standard machines so the 'purge' air while currently derived from the machine's 'tool' air can be pretty easily separated out.

It's sounding like making 120psi air then regulating it back down to 15psi isn't much of a penalty verses making the 20PSI and regulating it down to 15. That surprises me as all that heat being generated at the compressor then the reverse 'cold' appearing at the regulator during expansion.

Thanks Mint! I'll dig around on that nice calculator page and see if I can't square this better with my seat-of-the-pants possibly incorrect understanding.

Keith Cress
kcress -

RE: Low pressure compressed air supply.

Take this for what it's worth: I work at a factory with a bunch of pneumatic equipment and a toolroom with 3 Machining centers. We run the CNCs around the clock maybe half the time. We leave our smallest VFD screw compressor (50HP) running. I just looked at it and it's running at 28% elec power (13 KW), according to the controller. In my neck of the woods that's about $0.75/hr. That includes the CNCs and all the other myriad leaks in the building. I tried a small (5HP) recip a long time ago and it would work if I shut off the connection to the rest of the plant and chased down the leaks in the toolroom. The problems with that were making sure the valve got switched and, more importantly, keeping on top of the leaks. If the recip couldn't keep up then the NCs shut down and we lost a night or a weekend's production. It didn't take long to say "the heck with it" and just run the bigger compressor. True, it's easy to make the argument for saving electricity, but for me it's just not worth it.
Good Luck,

RE: Low pressure compressed air supply.

Thanks John, I love hearing data like that, it helps glue my understanding together.

This plant has 3 compressors. The 25hp a 15hp and a 10hp. They're staged so they come on in that order. If something drops the 25 like a recent oil leak where a pin-hole manifested in the spin-on oil filter, the other two come on in seconds and people start running around like a hatch opened on a submarine. They shut down any air users that are tolerant, like cleaning lines.

That leak sprayed oil into the intercooler air-stream where it coated EVERYTHING making it extremely hard to find. LOL

Keith Cress
kcress -

RE: Low pressure compressed air supply.

I have often used a rough rule of thumb that 100 psi air requires 0.25 hp per cfm. 750 watts per hp, and 10 to 15 cents per kWhr. Continuous air leaks are very expensive and very few people are aware of it or pay attention.

The most efficient compressor for low pressure high volume purge air is usually a regenerative blower.

These are relatively inexpensive and low maintenance so placing small units near the point of use may be more practical than long piping runs from a central source.

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