Here's a question for the pro's.

I have a small oil-less compressor. It's a craftsman, 6gal, 2hp, rated at 150psi. I only use it for airing tires and a small finish nailer so I know it's more than adequate. My question is this. I read somewhere that the smaller tanks and higher psi can be compared to a smaller psi, larger tank unit. I *think* the comparison was that a 150psi 6gal compressor is comparable to a 20 gal, 100psi compressor due to the fact that roughly equal amounts of air are in both tanks given the difference in psi. (ie, the same air, just more compressed if that makes any sense). I am not an expert and am not really in need of more capacity, hence the small light duty compressor. I was just curious what some of the pros out there thought, especially since you can find "contractor" sized units that put out much higher psi yet only have 4 gallon tanks. Maybe this will be a good discussion, which is my primary goal.

Thanks

 

oh gee this is going to be fun.
i'll keep this simple.

atospheric pressure = 1BAR
1BAR = 14.7 psi
150 psi = 10.2 BAR
100 psi = 6.8 BAR
85 psi = 5.7 Bar
6 gal X 10.2 = 61.2 gal* @ 1 BAR
20 gal X 6.8 = 135 gal* @ 1 BAR

But you need atleast 85 psi to use air tools.

10.2-5.7=4.5 BAR of useable air pressure in the 6 gal
6.8- 5.7=1.1 BAR of useable air pressure in the 20 gal
6 gal X 4.5 = 27 gal*
20 gal X 1.1 = 22 gal*
20 gal @ 125psi = 56 gal

* assumeing the temp of the air remains the same.
does this help?


P.S. a machinery's handbook a is very helpful.

 

The only thing I don't get is the last calculation of 56 gal.

 

FYI- One thing that you lose with higher pressure when it is regulated down to the 70-90 psi that most air tools call for is energy you waste :

To compress air with a single stage compressor takes 0.218 HP at 150 PSI for each SCFM, and 0.170 HP at 90 PSI. You waste the difference or 0.048 HP for each SCFM that passes thru a regulator. It is not a lot for home use but it matters to big industrial air users.

 

Tkd:

Excellent thread! However, I to don't get the "56 gal." calculation. I'm not sure where the 125 psi came in.

F150d: the calculations are beautiful to me, b/c I sucked at math in school, yet what of what moves me is in the worlds of science and technology. Thank you for making a mathmatical analysis come alive for me and mean something--that doesn't happen every day.

Is converting everything to a common denominator, measured in Bars (which I knew of already--concepts I get, unless they're math concepts--lol) the way it's done by professionals, when calculating needs ands capacities?

C'mon, you guys. I love it when actual experts teach me stuff that most people just shrugged their shoulders when asked about. Let's keep it goin'

Peter

 

Being a scuba diver, I completely understand the concept of "bars" and pressure as it is vital for figuring effects of water pressure and the impact on nitrogen loading, but I'm still just not getting the 56 gallon measurement.

Since I'm not the only one, bear with us and enlighten us.

Thanks,

 

125 psi = 8.5 BAR
8.5-5.7= 2.8 BAR of useable air pressure
2.8 BAR X 20 gal= 56 gal*
20 gal tank of air @ 125psi has 56 gal (or 7.48 cubic feet)* of useable air.
i commonly see 20 gal air tanks with 125 psi, thats why I put that in there.

7.48 gal = 1 cubic foot (1 ft3)

* assumeing the temp of the air remains the same.

 

I have a good article "some where" that talks about a 175 psi compressor. You drop the cut off psi but increase the rpm to the same amps the motor would draw at 175 psi. This results in a more efficient operation.

 

correct me if i am wrong here but i just red this and sounds like the first statement was asking about tanks -- i do have a 125 psi rated tank with a 175psi wayne pump on it i run it about 120 -- just out of fear of a big BOOM -- i did hydro test it and it was fine at 175 -- i also use a little pony tank from a gas grill and that too gets pumped up to 120 but that is rated at least 225 -- itsnt there more to it than if the tank is large or small on what pressure it can handle i have seen some 500- 2000 gal tanks take pressure of over 1200 psi --

from what i get out of the first statement -- is generally yes i larger tank cant handle the pressure of a smaller one -- but in your case the time that it would take to pump up a 30 gal tank well could take a while

yes mike your talking about the amp curve of a motor an electric motor uses a curve that lets you know what the best performace is ---on startup is the most waste and then you useally get around 80% of full load amps is is most efficient over that you start too loose the efficiency -- that is an tool to have if your using alot of motors for a company -- for me i dont use mine everyday and it is only a (TRUE) 5hp not compressor rated so it isnt very big -- if it was running all the time then i would try to set it up little better but its good enough for me

 

Got it now, Thanks

 

got to think about it a little bit if your wanting more vol you could always add more tanks -- i have been thinking about piping mine to my attic and up there have propane tanks it goes to that is easy and cheap you can get the tanks just about anywhere - there easy to move not like one huge tank there out of the way - and the pressure is well high enough you cant ever go ove burt pressure -- and you can go all out put in a quick disconnect so you can just keep your same tank and commpressor - and just plug the hose in and it will then drain to all the tanks you added and represser - if you put on a block valve or a check valve you can disconnect it and you will keep them aired up -- alot you could do -- good luck

 

Good idea kmrs, should I find myself needing more air, that would make sense.

 

kmrs,

We use the large 100lb propane tanks as portable air tanks. They would work to plump in for extra volume if you need it. The problem I see though is your compressor probably isn't rated for high CFM so it would take a long time to fill the tanks. To me, prolonged running of a compressor isn't very good for it. I'm not an expert, just my thoughts. You'd probably get away with it for airing up tires and only use a couple small tanks, but trying to run air tools might not work to well because you would have to wait longer to build up the pressure needed, even though you would have to extra capacity when you first started.

 

i dont know much about oil-less compressors i have always had oiled ones -- not sure is there damage done if prolong run time -- i just thought it was more or less run forever with no or very little damage

as far as the rebuild of pressure it would be quit a while and if you need air now you could be waiting for a while i would think but i would be willing to try 2-3 small tanks but then again does that give you enough to really help if you have to sit and wait for it to rebuild

i guess you gain something but then loose something else

 

F150d:

Thanks for the 56 gal. explanation. I get it now too.

Re: the attic tanks (sorry, forgot who this was): Would you not have a water drainage PITA? Or would you:

a) have an effective water trap/manifold design at your compressor, such that no water would go up in the attic, or:

b) since it's just cheap tanks, you wouldn't worry about it? I'm assuming you're contemplating the little (are the 20lb?) gas grill tanks?

Thanks.