Physics! Tandem air compressors.

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I have two 5 gal. twin stack air compressors. Each puts out 5.1 cfm. If I run an airline off of each one, into a T, will the output from the single line be 10.2 cfm? If each unit is providing 90 psi, what will be the pressure at the tool? Obviously it will require two electrical circuits. Any physics gurus in the house? Thanks.
Reply:In theory; pressure will be the same, 90psi. Volume will double to 10.2 cfm or total of the two compressors capacity. In reality; Subtract losses due to restrictions in undersize plumbing.Good LuckLast edited by denrep; 01-24-2008 at 11:10 AM.
Reply:The output will be = to the pressure of the compresser that's set the highest.  I doubt that they would kick of and on at the same time unless they are both controlled by the same cutout switch.  If you tandem the outputs only one compressor will run till it falls far enough behind for the pressure in the secondary to fall enough to kick the second unit on.  Hope this makes sense.When you quit using air each unit will cut off when it reaches it's own cutout pressure.When youtandem the comps the pressure is not addative but you will add to volume of air that can be supplied.DennisThermal Arc 185-TSWMillermatic Challenger 172VictorO/AAtlas Craftsman 12 by 24 LatheEsab PCM-875Wholesalem Tool Mill-Drill
Reply:Not 100% sure but its possible that when the two outlet lines tee they will form a balance tube between the receivers and if the pressure differential is not set exactly the same on the two compressors , one pump may be kicking in earlier and doing a lot more than another.A good guess is better than a bad measurement
Reply:Originally Posted by denrepIn theory; pressure will be the same, 90psi. Volume will double to 10.2 cfm or total of the two compressors capacity. In reality; Subtract losses due to restrictions in undersize plumbing.Good Luck
Reply:Originally Posted by denrepIn theory; pressure will be the same, 90psi. Volume will double to 10.2 cfm or total of the two compressors capacity. In reality; Subtract losses due to restrictions in undersize plumbing.Good Luck
Reply:We use to use 2, and sometimes 3, compressors on the same air system at work before we got the new monster compressor that does it all by itself.   So, yes, the idea does work, not only in theory but in real life too.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:I would recommend checking the current rating on the pressure switches and if possible wire both motors to one switch, that way the one with a higher cut in pressure won't end up being overworked, it'll even out the workload, if the current is not high enough to run both motors, consider a higher current pressure switch and install in on the T connector between the two tanks.
Reply:Another way would be to use one pressure switch tooperate a high current relay that would run the actual motors.  Better yet let the pressure switch run two seperate relays and make one of the relays a time delay.  Start current of two motors might be enough to trip the breaker.  Even if the pressure is big enough it might be a good idea to use a time delay.DennisThermal Arc 185-TSWMillermatic Challenger 172VictorO/AAtlas Craftsman 12 by 24 LatheEsab PCM-875Wholesalem Tool Mill-Drill
Reply:At high(er) CFM draws, the pressure switches in the units or the tank sizes don't matter at all.Because at that point, all you are doing is running the pumps and motors continously.  So all you are getting is the airflow from the pumps.For lower CFM draws, where the pumps fill the tanks and then cycle on and off, tanks sizes and pressure switch cut-ins and cut-outs play a little bit.But like Denrep originally said, your airflow capacity will be roughly the sum of the airflow capacities of each compressor.  Minus plumbing losses.  And the plumbing losses for trying to flow twice as much air through little hoses and fittings can be noticable.
Reply:i'm sure most of the above is saying the same thing (in different fashions) but the bottom line is:the max. pressure will be that of the machine set the highest.the flow (CFM), dissallowing restrictions, will be that of the machine(s) running. i.e. two  5.1 CFM machines running at the same time is 10.2, but as soon as one machine kicks out/never starts the flow is 5.1 cfm.
Reply:We didn't do anything fancy with cut-out switches when running them altogether.  Each was hooked up to an inlet to the main line on it's own.  We do have some big arse air plumbing in the shop though, starting about 2" dia and working down from there to smaller dia's at the ends of the 'fingers' of each run. These compressors had 3-phase motors and 2-stage, 2- to 4-cyl, 175 psi pumps with large tanks (smallest was 80 gal, largest was about 250 gal), feeding a line that's regulated to only 100 psi.  The one we have now has a rotary or turbine pump that runs continuously and never goes over 125 psi but provides a huge amount of air flow, upwards of 100 cfm.  The new compressor blows away any piston pump compressor I've ever worked with.  The air supply is seemingly endless.Last edited by DesertRider33; 01-25-2008 at 04:48 PM.MM350P/Python/Q300MM175/Q300DialarcHFHTP MIG200PowCon300SMHypertherm380ThermalArc185Purox oaF350CrewCab4x4LoadNGo utilitybedBobcat250XMT304/Optima/SpoolmaticSuitcase12RC/Q300Suitcase8RC/Q400Passport/Q300Smith op
Reply:Wow! Thanks. That clears that up and adds some great notions that I had not considered. It's just for home use, so I don't need a 30 cfm unit. Since I already have two portables, this setup will accommodate a little more duty from a die grinder or paint gun etc. They have a 75% duty cycle, so it should work in a hobby setting. A great community for information, based on professional experience. May your bottles never go empty.