Air CFM issue - will this workaround work?

[SRT Mike]

I have a machine (not a cutting machine actually, it's a pick-and-place) that requires 8CFM of air at 80psi. My compressor is only rated 7.8CFM but at 120psi... the other problem is that the compressor is on the other end of my shop, and I am planning to run a copper pipe along the wall. Right now I just use an air-hose on the floor but the length of hose means I lose a ton of pressure and volume by the time it gets to the machine, so it's not really able to run other than intermittently.

The machine needs quite a bit of air in terms of CFM but only for a short time. So, I was thinking... if I got maybe a 60 gallon vertical tank and mount it at the machine, then put a one-way valve on the tank and connect it to my air supply so that the 60 gallon tank will charge up from my normal compressor, but it will not allow air to leave the tank (for example when my mill/lathe or other stuff is running), then that should provide plenty of capacity to offset the high-load/short-duration needs of this particular machine.

I have heard of putting a tank at a machine that uses short bursts of a lot of air but I wasn't sure if the one-way valve was a good idea or a bad one? Any other suggestions? If the machine uses 8CFM @ 80 psi is there an easy way to calculate how long I can run it on this 60-gallon tank before the pressure falls from 120psi (compressor pressure) down to below 80psi (minimum needed for the machine)????

[Mike Horne]

I think you are in luck. The cfm for a given compressor usually goes up when the pressure required goes down... so you may get 7.8 at 120, but 8 at 100, or 9 at 80. Your Mileage may vary. It depends on your machine, but check your manual. The volume of air in the pipes between your compressor and the machine also act as a reserve.

If you have an air tool that is close to the same consumption, then you can test to see if your machine will run the test tool continously from dead empty without pause (not likely). From the regulator before the tool at 80psi and how long/if the pressure drops (fairly likely). If these are good, then you may need no additional reserve.

60 gallons of additional air seems to me that you'd have no interruption in pressure. However, the extended duty time for your poor compressor motor may be extended, and you might also want to check into that. If your application is critical, then getting another compressor at the same time you get the tank may be the way to go. Simply because I think they charge a lot for the tank, and you might get a better deal on a combo. And you will be able to get many more cfm and instantaneous "umphf", terribly technical term I know



Luck

Mike Horne

[Geof]

Originally Posted by SRT Mike ...The machine needs quite a bit of air in terms of CFM but only for a short time. So, I was thinking... if I got maybe a 60 gallon vertical tank and mount it at the machine, then put a one-way valve on the tank and connect it to my air supply so that the 60 gallon tank will charge up from my normal compressor, but it will not allow air to leave the tank (for example when my mill/lathe or other stuff is running), then that should provide plenty of capacity to offset the high-load/short-duration needs of this particular machine. I have heard of putting a tank at a machine that uses short bursts of a lot of air but I wasn't sure if the one-way valve was a good idea or a bad one? Any other suggestions? If the machine uses 8CFM @ 80 psi is there an easy way to calculate how long I can run it on this 60-gallon tank before the pressure falls from 120psi (compressor pressure) down to below 80psi (minimum needed for the machine)????

Perfectly valid approach which I used to compensate for small diameter air lines to several machines using smaller tanks on each machine. If you have the non-return valve just make sure your remote tank has its own vent valve. Not that it is ever likely to get over-pressurized but you could see some increase if it was filled very cold and then came up 10 or 15 degrees.

You can maybe do a crude calculation for how much use is available between 120 psi and 80 psi. One atmosphere is about 15psi so at 120psi you have 8 atmospheres; in other words 480 gallons of air is crammed into the 60 gallon tank. At 80psi it is 5.3 atmospheres so you have about 320 gallons crammed in at this point making the difference about 160 gallons. One cubic foot is about 10 gallons, my gallons that is, your gallons it would be about 12.5 so your 160 gallon difference is about 12.8 cubic feet. At 8cfm you would have 1-1/2 minutes of flow?

Quite honestly I don't know if this calculation has any value.

[SRT Mike]

Good info - thanks!

The pick and place has an air cylinder that acts as a plunger, i think thats really the only air actuated thing. It has a theoretical max of 14400 parts/hour but realistically will be lucky to do a quarter of that, so about 3600 parts/hour. My circuit board has 17 parts on it and I do them in panels of 10, so 170 parts, so I guess it would take around 3 minutes to do a board. So as long as I can get 3 minutes I will be OK. I don't mind waiting a few minuted for the system to recharge before I load the next board if need be.

Now I don't need to run the machine *just* off the 60 gallon tank since my compressor will back it up and I have 3/4" copper pipe (about 100 feet of it from the compressor to where the new tank will be), and my compressor has a 35 gallon tank.

So I am figuring a 60 gallon tank will be more than enough extra "oomph" to cover the machine. It runs OK now with the air line on the floor from the compressor but I also just have been doing demos on the new machine, I haven't run it full tilt for a whole set of 10 boards yet. So my half-ass math tells me 60 gallons should be OK

I would love to get a new compressor but I'd have to get a quiet one and a rotary screw would be an easy $3k or more - way out of the budget for now. I can get a new tank for $300 maybe less for a used one.

Good idea on the pressure release valve - I will make sure it has a pressure release on it also in addition to the anti-back flow valve.

Thanks for confirming my thoughts that this should work OK (key word being "should")

[Geof]

Originally Posted by SRT Mike .....I would love to get a new compressor but I'd have to get a quiet one and a rotary screw would be an easy $3k or more....

Where are you able to find a rotary screw at this price? I bought a small one from Chicago Pneumatic, 5cfm and it was over $4k. My most recent Kaeser which is around 30cfm was close to $12k with a dryer.

[SRT Mike]

Originally Posted by Geof Where are you able to find a rotary screw at this price? I bought a small one from Chicago Pneumatic, 5cfm and it was over $4k. My most recent Kaeser which is around 30cfm was close to $12k with a dryer.

It may be more than $3k.. while searching for air tanks I found these guys

http://www.compressorworld.com/rotar...e_mounted.aspx

7.5hp 28cfm@125psi is $4250 and they have a tank mounted 7.5hp rotary screw for $3900 or a 5hp tank mounted for $3500. Thats just the first place I came across... I didn't need 28cfm but I figured a smaller one would be a bit less.

I honestly dont know if they are quality or not? I know that you can get a new 5hp ingersoll for $4500 and less if you buy one at a show like Eastec.

That's without a dryer of course, though, which if you need will surely add more!

[Geof]

Where were you with that link about a year ago when I bought my small one????

The price cuts shown are either a lot of hooey or someone at one time was taking a rather large mark-up.

[SRT Mike]

Originally Posted by Geof Where were you with that link about a year ago when I bought my small one???? The price cuts shown are either a lot of hooey or someone at one time was taking a rather large mark-up.

I know those guys at that site are legit and honest - I have gotten a few things from them over the years without issue. There's another one that's even cheaper but I can't remember the name, something like allcompressors.com or something. They were even less as I recall.

On the other hand, if I call up Ingersoll Rand and ask for their local rep, I bet I could easily pay $10k for that $4,500 compressor. Many big companies don't really seem to bother shopping around so they happily pay 2x or more what cheapos like me will pay!

I find that buying at the shows can be good too. It ain't cheap for them to ship that stuff back and if you buy from the manufacturer they can cut you a great deal and they still make more than if they sell it to a dealer. I almost bought a Lincoln Precision Tigrunner a couple months back. MSRP is around $5.5k as I recall. The super duper cheap online price is $4,500 + shipping (which adds a few hundie). They were willing to sell me the one at the show for $3,600 and pay shipping. In retrospect I should have done it!

[Geof]

Originally Posted by SRT Mike ...... Many big companies don't really seem to bother shopping around so they happily pay 2x or more what cheapos like me will pay! ......

I am not sure how I should take the implication I can read into this.

I paid twice, or maybe more, those prices for my Kaeser rotary compressors so I guess I can conclude I am not a cheapo. But does that make me a big company?

I also bought a used Atlas Copco; originally rated at 60 cfm now only capable of 45 cfm but it cost me $1000. Have a redeemed a small amount of 'cheapo' status?

Incidentally I think you should also get a dryer. I have learnt my lesson about trying to make do with water traps and been burnt by a solenoid valve sticking because of water contamination at the wrong time.

[SRT Mike]

Originally Posted by Geof I am not sure how I should take the implication I can read into this. I paid twice, or maybe more, those prices for my Kaeser rotary compressors so I guess I can conclude I am not a cheapo. But does that make me a big company? I also bought a used Atlas Copco; originally rated at 60 cfm now only capable of 45 cfm but it cost me $1000. Have a redeemed a small amount of 'cheapo' status? Incidentally I think you should also get a dryer. I have learnt my lesson about trying to make do with water traps and been burnt by a solenoid valve sticking because of water contamination at the wrong time.

Sorry, I didn't mean any negative implication.

I used to work for an aircraft repair facility and it was a huge place. They bought new machines every X years regardless of how work the old ones were. They had a few suppliers they bought from and if one of those suppliers said "A rotary compressor is $15k" then they would just pay $15k. Other folks like me can't afford that so I always shop everything around. I don't mean that you are foolish for not shopping around or anything, it sounds like you got a great machine there! I just mean that I think the variance in price is related to some larger customers who will pay any price and some smaller guys who won't - so you get a range from low to high

[Geof]

I wasn't offended, just taking a little dig.

But I wish you well in getting to my position someday so you can just spend to save time looking and haggling. I went through that stage of not having enough capital and I sincerely hope I never go back. It is difficult but can be worth every stressful minute of it.