Adding a tank to an air compressor?

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I have a 30 gallon, 6 HP 150 PSI Craftsman air compressor, and I was planning on painting a car or two with a HVLP gun.  It's been recommended that this compressor is too small, that a 50-60 gallon one would be much better, I wouldn't need to stop and wait for the compressor to catch up.  Can I add another tank to the compressor?  How about external ones?  2 of the HF 10 gallon tanks would only set me back $75 + plumbing, and give me 50 gallons capacity.  I think the compressor has enough power to do so, but I don't exactly know the mechanics/physics involved in power vs. larger tanks.It would also be nice to be able to run my cutoff wheel wide open for longer than it lasts now, as well as a bunch of my other tools.
Reply:That is a common idea. However, even with a larger reserve of air, the overall pressure is still going to drop below the preset cut-in pressure, which turns the compressor on and pumping. Now, instead of filling up a 30 gallon tank it has to fill a 60 gallon tank, it is going to take longer to reach the cut-out pressure and shut off. Sounds almost ok, but now your compressor is running longer than it was designed for and may, I stress may, overheat and fail on you. At the very least, it is running longer and reducing it's available cooling time and reducing it's longevity.So you will get more spray time in the beginning, but maybe not after you start to lose pressure in the tanks.Lincoln Power Mig 210MP MIGLincoln Power Mig 350MP - MIG and Push-PullLincoln TIG 300-300Lincoln Hobby-Weld 110v  Thanks JLAMESCK TIG TORCH, gas diffuser, pyrex cupThermal Dynamics Cutmaster 101My brain
Reply:What is the CFM requirement of your spray gun, and what is the CFM output of your compressor? If supply is more than demand then you don't need to do anything in order to spray indefinitly.
Reply:Thanks, Joker11, that makes sense.  I understand that the compressor would kick back on even with the larger reserve of air, the way it was explained to me was that the larger reserve lets you get a whole coat done before the pressure drops below what the paint gun needs.  Apparantly (and, again, I haven't done this, I'm just using info that was given to me), a HVLP gun will drain the compressor even with it running, so it's a matter of having a large reserve to drain at first, once the compressor kicks in you will still be losing pressure, but at a slower rate.  I'm guessing the compressor motor will be OK, since I've had it charge from 0-150PSI, then ran tools that had it trying to recharge for 5 minutes straight a bunch of times with no issues/problems.  Of course, that might have just weakened it enough to be almost broken now, we'll see.
Reply:I've thought about doing the same thing in my small shop for my little Coleman 30 gallon compressor. This is the first time I've heard talk of it and was suprised to see this post. John -  fabricator extraordinaire, car nut!-  bleeding Miller blue! http://www.weldfabzone.com
Reply:Originally Posted by maxyedorWhat is the CFM requirement of your spray gun, and what is the CFM output of your compressor? If supply is more than demand then you don't need to do anything in order to spray indefinitly.
Reply:Hey guys- Have 2 5hp compressors connected together each has its own tank they will start and stop together. For a while I was using a lot of air, as you know when the compressors runs a lot you get heat, and with heat you get water. Did not have the money to spend on a dryer, so I ran the air from the 2 compressors across my shop ( 50 feet ) into another tank. came out of that tank to supply air to the shop. by doing this it gave the air time to cooland I trapped the water in the supply tank. So, if you do add another tank to your system you might want to put the other tank away from the compresser.It helps when you are painting, or running air tools.Thanks,MikeToolmaker  www.rolxtools.com
Reply:Originally Posted by PatrickJSATA NR2000 gun :10.3 cfm @ 29 PSISharpe Cobalt gun: 13.9 cfm @ 50 PSIOther two guns: UnknownCompressor: 6.4 cfm @ 90 PSI  or 8.6 cfm @ 40 PSII don't know exactly how to calculate cfm @29 PSI for the compressor.  If cfm vs. PSI is a straight line graph, it would be just over 9 CFM, but I'm pretty sure it would curve, and don't have a good way to estimate it.  I'm guessing I have just enough compressor for the SATA without extra tanks, but not enough for the Sharpe.
Reply:Jamlit,  are you sure about the specs on your compressor.  I have a similar one from Sears that is 4hp 30 gallon in addition to an 80 gallon Ingersoll Rand, 5hp two stage unit that barely beats those specs.  My little sears model doesn't put out near the specs your mentioning. 18 and 15 CFM is a ton of air for a oiless model to maintain.  What you have to watch for on combining tanks to get the extra air capacity is that unless the compressor itself is rating for continuous duty you will wear it out in no-time (speaking from experience on 2 different units)  I finally picked up the IR/80 gallon unit with a continuous duty motor because it was cheaper in the long run.  Those Oiless motors are great for small work but if you run the compressor in a continuous mode even for 30 or 45 minutes it will begin to deteriorate.  Just my experience.   Also, Jamlit do you have a part number for the Sears unit you have.  If it indeed has that CFM rating I may need to find one as a spare for portable applications.  Its not feasible to lug my 80 gallon unit around.   Most of my applications are media blasting and running chipping hammers.Michael
Reply:As mentioned, the two problems are ability of the compressor to make up air, and the rated duty cycle of the compressor, which is dictated by cooling.You can generally get away with doubling the reservoir size for any compressor safely, but with a limited duty machine, like most smaller compressors, you need to keep the AVERAGE draw within the machine rating. The 6.4CFM is (probably) the rate at which the pump makes air, but isn't the continuous rating... if you draw air at that rate for a long time, the pump will run constantly and burn out. The main reason to increase reservoir size would be for intermittant high draw, like an impact driver, where the greater reserve gives you a longer rattle. Secondary is the added cooling the air gets, which lets it dry out a bit more.Looking at the 10.3CFM gun: 10.3CFM is about 80gal per minute. This rating is based on FREE AIR (uncompressed) so it is about 15gal per minute of air compressed to 60PSIG, or 10 gal per minute of air compressed to 105PSIG. If you charge a 50 gal reservoir to 105PSIG, you could run this gun for via a regulator at 29PSI for (roughly) 2.5 minutes before the resevoir pressure dropped to 30PSI and you would lose your regulator. This would be stretched to maybe 4 or 5 minutes, maybe a bit longer,  with the compressor running. Of course, the compressor would fail fairly quickly if you do this on a regular basis. But for a one shot deal, painting in stages with a good fan blowing cooling air at the pump and motor, you can probably get away with it. If it was my only compressor and I had other needs for it, I probably wouldn't.
Reply:A couple of points.First, there ain't no such thing as a free lunch, aka TANSTAAFL.  Sustained operations of an air tool are typically limited by the CFM output of the pump, because once the tank is drawn down below the kick-in setting the air supply for an air-hungy tool is limited by the output of the pump.For continous usage, you need a pump (and motor to drive that pump) that can supply more CFM than the tool(s) are drawing.But, you can partially compensate for a lower CFM output pump and motor for short term tool usage by adding more storage tank capacity and/or by storing the air at a higher pressure in the tank(s).  Then you can intermittently use air-hog tools for a longer 'burst' period than you could with just the smaller tank capacity.  Just remember that TANSTAAFL, and you will run your pump/motor longer to refill the larger capacity tank(s).And as mentioned above by others, not all pump/motor units are rated for continous usage.  If the unit is not rated for continous usage, then you may burn it out faster by adding tank capacity beyond the original tank.Next, the HP rating on some/many compressors, especially the 'consumer' ones like Sears units, is a LIE.  The makers/retailers were sued about that recently, and they lost and acknowledged the fact that the numbers were a lie.  What they did was measure the PEAK current draw as the motor stalled or started and used that to convert to a HP rating.  In the real world, an electric motor needs about 10 amps at 120 volts to make one HP.  On 240V, a motor would use 5 amps to make one HP.  So a typical consumer 120V electric tool of any type running off of a standard wall outlet is only going to be a MAX of 1 to 1.5 HP.  That's it.Rule of thumb, is that you get about 4 CFM of air per real HP from the motor.  Give or take.So it's not very likely that a 120V compressor is really going to supply enough air to feed the OP spray gun needs on a continous basis.  For 'short' bursts of usage, the spray gun or othe air-hungry tool will draw off the tank's air and will run OK.  Adding storage tank capacity can lengthen the 'bursts', but at the possible expense of burning up/out the under-capacity pump/motor.  TANSTAAFL.Next, WalMart has some 7 gallon air storage tanks for $20.  Less expensive price per gallon than the HF ones, unless you have a HF super coupon or such.Also, watch out for the pressure rating on those portable air-storage tanks.  I've seen 125 psi MAX on them, and many more recent compressors are running at 150-155 or 175 psi.  So you'd have to run a regulator to drop the max air psi out of the compressor and original tank down to 125 psi to fill your auxiliary tank(s) safely.  Also, the not-so fine print on the portable air tanks is a 5-7 year life.  They aren't supposed to be used after that.  I suspect it's because they are thinner walled than a 'real' air tank and also because they don't have a drain to get water out easily.  You're supposed to pick up the tank and turn it upside down and then blow the water out that way.Adding tank capacity and a line of hose or metal pipe (no plastic pipe used for air lines!!!!) between tank(s) and the compressor and the end use tool(s) is a good plan, precisely because it does give the air a chance to cool down out of the compressor and thus give the water in the air a chance to condense out.Can the OP run an HVLP gun off his original compressor?  Sort of, but not on a long-use or even close to continous basis.  Adding some air storage would not hurt the tool usage and would lengthen the time that air-hungry tools could be used flat-out, until the compressor kicks on and then it's back to waiting until the tank(s) are full again before continuing to work.  TANSTAAFL.Oh, and air-hungry tools aren't going to be fully fed off of little 1/4 inch fittings or air hose.  And some filters, regulators, or other 'things' in the air path might or might not be able to flow the full amount of needed air as well.  You might have to check the flow ratings on them to make sure you don't have a choke point that is starving your tool of its air demands.Unless the scale of something in jamlit's pic is off, there is NO way that a 120V compressor can supply 15-18 CFM of air.  Unless it running at 120V-50A, which would probably need at least a 60A breaker to handle the start-up current surge.  Possible, but not likely.  The -tank- may be able to supply 15-18 CFM of air for a few seconds, but the unit can't supply that much air continously.  To supply that much air would typically take at least a  240V 5HP (a real 5HP, not Sears or HF horsepower) compressor.TANSTAAFL.
Reply:Originally Posted by 383bigblockJamlit,  are you sure about the specs on your compressor.  I have a similar one from Sears that is 4hp 30 gallon in addition to an 80 gallon Ingersoll Rand, 5hp two stage unit that barely beats those specs.  My little sears model doesn't put out near the specs your mentioning. 18 and 15 CFM is a ton of air for a oiless model to maintain.  What you have to watch for on combining tanks to get the extra air capacity is that unless the compressor itself is rating for continuous duty you will wear it out in no-time (speaking from experience on 2 different units)  I finally picked up the IR/80 gallon unit with a continuous duty motor because it was cheaper in the long run.  Those Oiless motors are great for small work but if you run the compressor in a continuous mode even for 30 or 45 minutes it will begin to deteriorate.  Just my experience.   Also, Jamlit do you have a part number for the Sears unit you have.  If it indeed has that CFM rating I may need to find one as a spare for portable applications.  Its not feasible to lug my 80 gallon unit around.   Most of my applications are media blasting and running chipping hammers.Michael
Reply:Jamlit,See if it still has a model# or part# on it.  If those specs are right I would be interested in searching the proverbial web to find an older unit as my portable. My current portable Sears puts out 1/4 those specs on a good day.  Now my IR/80 Gallon really pushes air and I love the 175 psi which comes in handy every now and then.Michael
Reply:I will look tomorrow. I know I still have the manual in my shop cabnet which had all the specs in it. I will scan the model and specs sheets and post them tomorrow night.Miller DVI2Lincoln Precision Tig 225Thermodynamics Cutmaster 38Everything else needed.
Reply:Thanks Jamlit,  you can send me a PM if that is easiest instead of prolonging the thread.Michael
Reply:I PM you 383bigblock after getting the manual and finding I was a little off I thought I better post my findings. Sears craftmanmodel no. 919.726330V twin cylinder33 gal tank125 psi cut out15 CFM @ 40 PSI11.5 CFM @ 90 PSIthats it, works great for me, for now anyway.Miller DVI2Lincoln Precision Tig 225Thermodynamics Cutmaster 38Everything else needed.
Reply:I think the compressor in the beginning of this thread will have no trouble running a sata jet @ 30-40 psi enough to do the project he described. I have painted many with a similar one.Bill
Reply:Thanks guys, a lot of great info in this thread for what I want to do.  Thanks to enlpck and Moonrise for the details, and everyone else for everything else.I have a really small car I want to paint, a 71 Fiat 850 Spider (12' long, 4'10" wide, less than 3' tall at the fender), so I'm not looking for a free lunch; an inexpensive, not very filling lunch will do.  I'm guessing less than 10 minutes would be needed to spray one coat (I think I could use a 1" brush and do a coat in 10 minutes), I plan on taking the doors, hood, and engine cover off and painting them separately.  I haven't done this before, so if I'm grossly underestimating the time needed, please let me know!  Based on everybody's info above, I'll probably just get one additional 10 gallon tank, two 50' long 3/8" hoses (one for cooling/condensing between compressor and tank), a high flow filter, charge everything up to 150 PSI (compressor max) put a fan on the compressor and attempt a primer coat.  If it lets me do one coat, I will immediately shut it off, let it cool for 5 minutes, switch it on to charge (~5min) and let it cool for another 5 minutes, which should be match the flash time between coats that I can start again.  If that doesn't give me enough time to complete a coat (or barely enough time), I'll get another tank only if it was really close.  If it isn't close to enough, I've since talked to a buddy of mine who also has a 30 gal. Craftsman compressor he will let me borrow if it's needed.Here's the monkey wrench in the works - the gun with the larger (1.6) tip size for primer is of unknown CFM requirements.  Does anyone know where to get SATA tip sets?  The only ones I find when I search are for guns with the tip sets.  I need a 1.4 for the candy paint I'm going to use.  I got these guns for dirt cheap, and the SATA came with only a 1.0 tip, which I'll probably never use.  Anyone want to swap?
Reply:pssst, I saw a reference to a Sata NR-2000 someplace else and your air specs don't match.Sata NR-2000 : 15CFM at 29 psiReview said it was a -real- nice spray gun, and has a price to match.A quick web check found a seller offering a 1.3 tip for the NR-2000.  I have no recommendation or info for or against the seller.  http://www.amazon.com/dp/B00069YH2M/...M&linkCode=asnI don't think your compressor will let you do the job right.  I think you'll be waiting at least a few times for the tank to refill as you try to paint even that little Fiat.  Even 30 gallons of air, or 50 gallons, will go FAST with the air-hungry HVLP gun.Maybe check around for a BIG tank from a fried compressor.  craigslist in my area recently had someone listing a tank that was supposed to be fine but the compressor was fried and tossed.If you want to do it right, I really think you will need more air than you have.  Also look into maybe running some lengths of metal pipe around the shop/garage with drip legs to give the air more of a chance to cool and condense the moisture out.  Run your 2 30-gal compressors through some metal pipe with drip legs and then into a T and then into a large holding tank (60-80 gallon) and then through a filter and maybe a dryer and then into your Sata gun.  Or get a big 60-80 gallon 240V 5 or 6 HP (that's real horsepower, not phony inflated Sears or other maker HP), run the metal pipe with drip legs and then filter-dryer-spray gun.Have heard that Eaton makes good compressors.  www.eatoncompressor.comLast edited by MoonRise; 03-27-2007 at 05:45 PM.
Reply:When I'm out in the shop tonight, I'm going to time my compressor and see how long it runs. It doesn't seem to run longer than a minute or two from low startup to high shutoff. Adding another tank, slightly smaller, would only make it run 2 minutes or 4 minutes...not bad IMO.John -  fabricator extraordinaire, car nut!-  bleeding Miller blue! http://www.weldfabzone.com
Reply:If you apply more than one coat of paint, the engine may not be able to move the extra weight.Former Fiat 850 Spider ownerTeddCoHTP Microcut 400 & Invertig 160DC  Smith Dual Guard MD-510 OA Rig  Lincoln SP135+  Hobart Stickmate LX 235/160
Reply:Originally Posted by TeddcoIf you apply more than one coat of paint, the engine may not be able to move the extra weight.Former Fiat 850 Spider owner