Air compressor question

[n74tg]

My shop compressor puts out about 6 cfm at 100 psi.  I need 10 cfm at 100 psi.  Would there be any problem with borrowing a second compressor (about the same size and capacity) and manifolding the outputs together?

I don't particularly want to have to rent ($75/day) a unit capable of what I need.

thanks

[n74tg]

Here is a link to a website that discusses using a manifold to combine output from multiple compressors and info about how to do it safely.

http://www.tpub.com/eqopbas/189.htm

So, my question now becomes, anybody know anybody that makes/sells them?

[glenn kangiser]

You can use pipe fittings - 1/2 inch tee and air couplers screwed into them - short hose is available at an auto parts store - NAPA carries Weatherhead hose and push on fittings.  Harbor Freight has the couplers cheap - get their steel ones - not the brass ones which break.  They also carry a multiple coupler but it is reversed and you would have to change the fittings.  Don't use their imported hose - it is pre-rotted.  Their Goodyear hose is good.

[NM_Shooter]

I want to tack a question on this as well.

My oil less compressor seems to be taking longer and longer to get fully charged, and runs longer when in use.  Do these things have a tendency to wear out over time?  Are they worth fixing?

[Onkeludo2]

Compressors are just two-stroke piston engines with an EXTERNAL power source.  With an oilless compressor it is really simple: Electric motor to crankshaft o connecting rod to piston.  If the piston rings wear out or the reed valve goes bad, you can often get a $15 rebuild kit unless the cylinder is scored...even then, you can usually get $15 dollars worth of use out of the rebuild kit.

First, make sure the air filter is clear and clean assuming you have one.  Also, make sure the regulator is not malfunctioning because the compressor will keep trying to pump up an already full tank.

Mike

[glenn kangiser]

As Mike said and using them heavy like I do I find I can melt them down to where they are pretty worthless in what seems like a pretty short time.  Last one the piston as well as rings was pretty well not worth messing with.

I went to the lower speed DeWalt after that -still high capacity though - just runs around 1800 rpm rather than 3600 or so.

If considering rebuilding it, I think sooner rather than later might make a better chance of repairing it.  Don't forget we have a link to electric repair parts as well as drawings from the non-spammer member with online parts.

[MountainDon]

http://countryplans.com/smf/index.php?topic=8480.msg110088;topicseen#msg110088

you are     

[n74tg]

My original compressor is a Craftsman (25 gal I think).  It has worked well for me.  Looking in the Sears store I find a 33 gal 1.6 hp on sale for $299 (down from $379).  The sales rep said to check out sears dot com for info.  I did and while there are 27 good review about their compressors, there are also 7 really bad reviews about them.

I've heard elsewhere that Craftsman isn't what it used to be.  Can anyone confirm this?

thanks

[Onkeludo2]

Most oilless compressors are of similar quality these days.  The exceptions are some of the low-speed compressors that Glenn mentioned.

If you must go oilless, the Harbor freight units tend to be as good as anything except for the noise and their return policy is stellar.  My personal preference for a stationary compressor (25-33 gal ain't goin' nowhere once installed in my garage) would be a used cast iron, belt driven oiled compressor.

Don't toss the old compressor either.  Remove the motor and compressor and turn it into a pony tank for additional storage.

Mike

[glenn kangiser]

Tony, my buddy used to buy all Craftsman.  He also says they are no longer the quality they used to be.

[n74tg]

Okay, got another question.  I borrowed a friends compressor (Craftsman 5.5 hp, 30 gallon) and manifolded it to my Craftsman 3.5 hp, 25 gallon), both compressors oil-less varieties.  Set both regulators on 110 psi and I'm seemingly not getting enough cfm to do my work (spray foam).  The airline hose (brand new) that I used to manifold them together is rated for 200 psi, but it is swelling when under pressure (diameter increasing by maybe 1/8" when compared to the hose ends where the air fittings are).

So my question is, is there any way these pressures are additive and I actually have 220psi as oppossed to 110 psi with increased cfm volume?  I don't think they are additive, but with all the problems I'm having, I'm beginning to question my own thinking.


thanks.

[MountainDon]

The pressures do not add.

Perhaps the hose diameter is insufficient for the CFM that is desired?   

http://www.plantservices.com/articles/2004/69.html

scroll down to the hose selection chart

[n74tg]

Thanks Don, I considered that my 3/8" hose diameter (about 100' length) might not be sufficient.  That is a question I posed to the foam supplier technical support people.  They say they have an Campbell Hausfield 80 gallon 5hp 220v oiled type compressor that they run a 100' x 3/8" hose to test spray each batch of their foam and it works fine.

[n74tg]

I happened in to the local Tool Source store today because I saw some big compressors inside.  One thing led to another and a conversation ensued about the subject, and more specifically manifolding compressors together.  I left learning a lot more than I already knew (which apparently wasn't much).

Come to find out, with higher end compressors they advertise two specs, cfm at the compressor and "delivered" cfm (down 100' of hose at some specific pressure)(all these specs were at 175 psi).  The delivered cfm is always about 20-25% less than the at the compressor cfm.  The guy said your home version compressors (Sears, Lowes, etc) never state the delivered cfm, only the compressor cfm.  He pulled out a manual for one of the higher end compressors and showed me how the better manufacturers provide much more data than the home version guys.

Additionally, we got into the subject of specifying cfm versus scfm.  Apparently only the home compressor guys state scfm, which is standard cfm, meaning cfm at standard conditions of 75 degf and 29.92" of mercury atmospheric pressure, ie at sea level elevation.  Stating cfm this way gives them lots of wiggle room if you are using the compressor at an altitude higher than sea level and not getting the performance you thought you should get. The higher end guys don't use scfm, they promise cfm period.  Buried somewhere in the fine print I'm sure there's a limit on elevation and temperature.

We also talked about manifolding compressors together to increase the cfm volume.  The guy said this doesn't work nearly as well as you hope it will.  While he couldn't show it to me in a manual, he said that from his experience if you manifold two equal cfm compressors together all you will increase the cfm by is about 20-25%.  He mentioned one guy who bought a cheaper ($500) 10 cfm unit from him that wasn't doing the job.  Instead of trading it in on a $1500 quality compressor that would do the job, this guy bought a second $500 unit and manifolded them together.  He was still very unhappy when the total cfm measured out at 12 cfm.

So, now I'm sure I will go ahead and spend the bucks to rent a 11 hp gas powered 25 cfm unit instead of trying to cheap out a workable solution.  Probably should have gone that way in the first place, but all it cost me was a little gas to go pick up my friends compressor and about $10 worth of airhose and fittings, which I will give to friend anyway (along with maybe a bottle of gin, his favorite poison).

I'll be interested in what you guys say about this conversation.

[MountainDon]

That is all super info. I knew, and forgot, about the SCFM specification.    Shame on me sitting at 5500 feet at home and 8800 in the mountains for forgetting that. I'm always all over people about elevation loss when sizing generators.

Also interesting to have it confirmed that the theory of adding CFM up doesn't come through in practice.

[glenn kangiser]

I also noticed that it didn't seem to produce as well as I thought it would when I put two together a couple times.

[n74tg]

The journey continues.  I go rent the 25 cfm, 11 hp, gas powered monster from the rental place and my spray gun still doesn't work right .  Major league frustration/anger/pssssd off is setting in quick, the kind where things without wings might learn to fly if something positive doesn't happen pretty soon.  I'm wondering if it's the compressor, or is this Soythane foam system a true POS.  Several phone calls to their tech support people (who do a good job by the way) and they even offer to overnite me another spray gun so I won't lose any more time.  I tell them I want to try to locate another compressor and try the gun out on that.  So where am I going to find another big compressor.  Turns out my neighbor's son just bought a trailer mounted soda blasting business.  I had just looked at it the day before, only to find out it's just basically a really big compressor (83 hp, 225 cfm at 175 psi).  We hook up my spray gun to it and it works perfectly.  Okay, so my rental compressor is inadequate.  

I call up the rental place telling them I want to bypass the governor and goose up the rpm's to get more pressure/volume.  They send me a mechanic, who looks at and pronounces it DOA.  They order replacement parts, it should be repaired by Monday 17th, but I'm considering other alternatives too.  Fooled me once, shame on you kind of thing.

So I ran into an engineer friend of mine who wanted to hear the whole story, particularly about my attempts to manifold two compressors together.  When I told him about that attempt being a miserable failure, he reminded me of some fluid hydraulic principles I had obviously forgotten over the years.

First off - more than one place has told me you can manifold compressors together and they can increase your cfm flow nicely.  What they don't tell you is, you need a larger diameter airhose and fittings - and you need to limit the length of your air hose.  Said another way, while one compressor putting out 6 cfm at 100 psi will push adequate volume through your air hose, manifolding two of them together and trying to push twice that volume through that same airhose increases the air friction so much that it robs you of the extra flow you were seeking.  Add in that I am using 100' of 3/8 ID air hose and you get LOT's of friction and lots of volume/pressure reduction.  

Now, the guy at the Tool Source store showed me a table in one of his books that said a 1/4 ID air hose can carry something like 17 cfm at 100 psi (if memory serves).  Of course I didn't write it down, so this morning I google "3/8 air hose carrying capacity" and get the link at bottom  where they talk about how much reduction (they call it psi reduction) you get over various lengths of hose.  They also talk about some other very interesting things about compressor use that can kill your compressor if done wrong, but insure a long happy life if done right (so take the time to read the article).  One example:  if your compressor runs more than 2/3 of the time, you're killing it (overheat).  Example2:  if it's an oiled compressor, absolutely do NOT use automobile motor oil.  

So, long story short, today I think I'm gonna build a new larger diameter (3/4 ID) manifold and see what I can do to increase the hose diameter for most of my hose length.  If it works I don't have to rent the $215 per week compressor.  If it doesn't, well maybe they will get it repaired by tomorrow as promised.

It's like the old Navy TV commercial:  "It's not a job, it's an adventure".

http://www.tptools.com/StaticText/AirConsum.aspx

Glenn:  Way back at the beginning of this post you said, use 1/2" tee's, etc.  Sure do wish I'd paid attention back then.  

Oh well, that which doesn't kill us...only makes us stronger, smarter, more tired.

[n74tg]

That engineer friend said why not try regular larger diameter garden hose as a cheap alternative.  He figures it's probably rated for 60 psi normal use and with engineering safety factors included (3 to 1), it may very well work for the project I need to do. 

I'm thinking since it's garden hose, 3 to 1 is probably a little unrealistic, but how about 2 to 1 or even 1.5 to 1.

What do you think?  I expect 50' of 3/4" ID pressure hose won't be cheap, nor will the fittings be cheap.

[diyfrank]

I bought a hose that was rated for water/air. I don't remember where,maybe Lowe's. Ive used it as air hose without problems. It wasn't the cheap plastic garden hose tho. It was a good rubber one.

[MountainDon]

Our city water pressure at the house is about 120 PSI. Hoses would not immediately burst left with the faucet open and the nozzle shut. Laying in the hot sun like that would eventually create a burst hose.  Hoses should MTL be good to 80 PSI as that is the usual maximum that water pressure should reach.

[glenn kangiser]

The better rubber garden hoses may work.  Our cheap plastic ones blow up to about an inch sitting in the sun with 34 lbs pressure in the summer .  We replace some annually.

Possibly you could rent jack hammer hose from a rental company then adapt from the chicago fittings to your sprayer.  I own quite a bit of jackhammer hose and use it when running the big impact or jack hammer for higher cfm needs.

[n74tg]

Well, after making all my air hose 5/8" I.D, including the manifold, I thought I might have some luck.  But, it apparently just isn't meant to be.  The manifolded compressors just can't maintain 10 cfm over the long run.  

So, I guess I'm back to the rental compressor, if they got it fixed today.

Oh well, had to try it.

P.S.  Sure enough, the medium duty, nylon reinforced 5/8 garden hose did hold 100 psi just fine.

[glenn kangiser]

That is great to know, Tony.  Thanks.

[n74tg]

All right, I'm tired of farting around with compressors.  After looking at about 100 compressors for sale on Craigslist, bought this one today.  I asked the guy why it was gray, Quincy's were suppossed to be blue.  He said it needed a paint job, sure enough I can still smell the fresh paint.

Now, all I've got to do is figure how I'm going to get it unloaded from the back of my pickup (weighs about 500#).  But, that's a project for tomorrow.

http://dallas.craigslist.org/dal/tls/1748667304.html

[glenn kangiser]

I have a similar 5 hp Quincy gas powered I bought new around 1986.  I don't use it often but it is a top of the line compressor.  I used to use it every day.