Compressed Air Systems

[Stoney83]

I know this topic comes up from time to time but I thought I would get some input for my specific application.

Basically I have 5 mills in the shop, a water jet, robot and some other stuff that that can suck a lot of air. The compressor room has 2 - 5hp piston compressors each with 80 gallon tanks. The one air compressor is my dads old one with a tag from 1969 the other is a fairly new Quincy. I was wanting to retire the old one but this summer switched it back to the main compressor because it doesn’t have any thermal overloads and just runs.

There is also a 350 gallon capacitor tank on the back of the shop that bought me a couple years on upgrading the compressors.

The “main” compressor never stops running and the other compressor runs alot of the time. Generally speaking 10hp of compressed air will keep up but not always.

Lately we have been running an air blast on one machine nearly all the time. This is by far the biggest air user but I don’t see any alternative to air blast.

Lately we have bypassed the air drier because it doesn’t keep up and is the weak link.

The water jet needs real dry air for the garnet hopper so the machine with air blast doesn’t run at the same time the water jet runs. Which is irritating.

Should I:

1. Upgrade the drier? And use 10hp of air?
2. Buy a better air system? If so what kind? New, used? Used a person can get 20-40hp and a drier fairly cheap from industrial surplus places but they are at least 10 years old.
3. Plum up one compressor to the airblast? I’m cutting plastic so it can be wet air.
4. Something else?

 

[Bill D]

Claim is a 7.5 hp screw compressor will give the same output as 10Hp piston pump. For your continuous use a screw compressor will save a lot of money. They do not do well in intermittent use where they do not get hot enough to dry out inside.
Bill D

 

[CarbideBob]

The “main” compressor never stops running and the other compressor runs alot of the time. Generally speaking 10hp of compressed air will keep up but not always.

Duty cycle (time on on and off) for pump #2? That gives us an estimate at CFM being sucked up.
Assume it set for a lower cut in? Guessing simple separate control on/off circuits?
Why is the air drier a bottleneck? Not rated for flow or can't dry the air or...?
You can keep your tanks and upgrade the pump and motor.
Piston pumps do not like full run time. 50/50 is the rule of thumb.
Even though the pumps I bought made in China I have had very good luck with Eaton /Polar air.
At this volume I may also consider those radiator/fan type coolers and drain before the air drier to help remove the load.

You do not want to hear this but plan on a $2500-$4000 upgrade min. cost plus time spent.
Bob

 

[Stoney83]

That’s a good idea on upgrading the motor. Maybe swap the pulleys at the same time.

The air dryer doesn’t keep up because it’s not rated for the flow. I bought it years ago with less machines and only one air compressor at that time. The drier is slightly oversized but it’s been really hot lately and seems to do better in the winter.

Tomorrow I can check the duty cycle of the second compressor. It’ll be a real high airflow day because two high use jobs will be running.

I’m ok with spending some money…. Better to spend it now than deal with wet garnet or a water damaged spindle.

This weekend I can apply some small bandages (couple small air leaks, plum the air-blast job directly). Longer term… it’s time for an upgrade. I suppose Bill is correct that a screw type will be more efficient and if it saves some money on electricity it’ll eventually pay for itself.

 

[Stoney83]

Bob, yeah separate control circuits and the secondary compressor kicks on/off at a slightly lower pressure. Seems if both compressors are running we gain pressure and if one is running we lose pressure.

Only other temporary bandaids I can think of is putting a Venturi on the air blast to pick up some “free” air. Or possibly putting the air blast job on a faster machine or have my second shift guy run those parts.

 

[Bill D]

Claim is a screw compressor is more efficient and saves money but I hear they cost a lot in required oil changes of very expensive oil. I would say a piston compressor is only compressing 1/2 the time while a screw is compressing 100% of the time it is running. I have heard around 5Hp is when it pays to go from single stage to two stage piston compressors. When to go to screws I have no idea.
Bill D

 

[MwTech Inc]

I also have had very good results with the Eaton pumps, have one of the 10Hp 2 stage 43 cfm units for over 12 years .
Is used to sandblast along with a 5hp pump.
They are $1300 each , you could pickup used 10 hp motors, and end up with 80 cfm.
You could make up your own flip/flop control, or just use one as a primary.....
I keep a rough idea of hours and only use Mobil Rarus 427 oil.........

Note..they do make big 25 hp pumps = 100cfm...but i would prefer two as then the chances of "no air" are pretty slim.

 

[fmari --MariTool-]

Mills are the worst. From the factory they set the air purge for the spindles like compressed air is free. Crazy!
I've always turned them down, never had any issues and over the coarse of decades has probably saved me tens of thousands of dollars in electricity and compressor wear. Same for my Doosan MX - multitasking machines. B-axis spindle has an air purge, and yes I turned it down. Spindle still running smooth after 15 years.

 

[kustomizer]

I found lowering the pressure and increasing the storage was a big savings in the price of compressed air. I can easily run 3 VMC's on a single 3hp Quincy recip, though I have a bank of 4 in a rotation coming on at 80psi and off at 100psi as compared to the 175 psi factory setting. They have to work hard to make double the pressure as opposed to twice the volume at a lower pressure. Mine are continuous duty and one will run about 45 minutes out of every hour to keep up which makes minimal starts. I have 180 gallons of stored dry air. Lower pressure = less water and heat.

 

[CarbideBob]

Air purge on the spindle front in a horizontal seems logical.
A customer had 20 mid size Mazaks. 24/7 operation and they were eating front spindle bearings due to coolant infiltration.
After much engineering work they turned off the air purge and the bearing life doubled or tripled.
Who would of thought?
Air flow across these internal surfaces is a funny thing. Sometimes it makes little circles on it's way out.

 

[surplusjohn]

A couple of suggestions. Air amplifiers like from exair should be used for air blast, big air savings. They trade pressure for volumn.
Or consider using a blower instead. Ie think leaf blower, I think a high pressure blower would be more energy efficient and would remove that load from your compressor. Not exactly the same thing but I once had a set up where we had to turn wooden parts that made a mess throwing off shavings. So I had a shop vac sucking the dust with the output side blowing on the part, so a regenerative suck n blow. Worked well.
Then storage, even small surge tanks at the point of use can make a function more effective, ie the function wont be held back by lack of flow.
Intercooling. Increasing the cooling between your cylinders will save electricity because the second cylinder isn't trying to compress hot air. More coils, or a air to water heat exchanger. I once put a coil of pex in a barrel of water. That fed the toilets and had a float valve to refill it. It worked well but the landlord made me remove it. If you have free well water or a pond, this might be worthwhile. I sold some machinery from a shoe factory that had it's own dam and generators. The compressors were electric but they had air to water heat exchangers on them using water piped directly from the dam. So free cold water. Originally I was told there were compressors that ran off their own turbines.

 

[Stoney83]

Air usage today was pretty high but the secondary wasn’t kicking on a lot…. Maybe 20% or so. Also had one call off so the big Haas wasn’t running and we didn’t fire up the water jet until the afternoon but could have.

Frank I’ll have to look at the purges tomorrow. The brothers machines don’t seem to use a lot except for the M140. Ordered a handful of Mari hydraulics earlier this week! Thanks for making them! The haas machines seem to use a bit more especially when using the Midaco.

John I’ll have to look at the exair amplifiers. That air blast job isn’t going away and it sucks a ton of air.

On second shift I shut off air to certain machines and that seemed to make a big difference. One compressor was easily keeping up with 3 machines for the time I stayed over.

Thinking our low use case (2nd shift) is maybe 2-3 hp and high use case on 1st shift is right around 10hp. Guess I gotta conserve air where I can and and need to upgrade the drier. Leaning towards a drier rated around 15-20 hp just so I’m good for any future needs.

 

[Stoney83]

Figured I’d give the thread some closure.

Got a drier rated for 20hp which is double the amount of air we can use and should future proof the drier for a while.

An air nozzle with an amplifier for the air blast job. Should use about 3 cfm of compressed air instead of 10 or so and plumbed it directly to bypass the drier. Another fan for the compressor room to keep it cooler and some minor stuff here and there.

Gonna take the shop air more seriously in the future and probably get a small screw compressor down the road