Compressed air piping size?

Picked up a new-to-me air compressor - 6.5hp, two-stage, 80 gallon tank. 17.4 SCFM at 40 psi, 16.9 SCFM at 90 psi.  Massive overkill for my needs, but the price was right, and overkill is the best kind of kill.  I'm getting ready to hook it up and pipe the shop for air, and have read every thread I can find on GRM regarding the Rapidair Maxline kit.  For my compressor size/hp, Rapidair is recommending 3/4" tubing, which is fine and not much more than the 1/2" kit.  I checked the tank output fitting on the compressor and it only has a 1/2" NPT out (which the prior owner had necked down to 3/8").  Given that I have a 1/2" outlet, is the 3/4" kit still the way to go?  Seems like with a restriction on the front end might minimize the benefit of the larger tubing past that?  I know it would still provide extra air capacity over the 1/2" stuff.

For reference, the only two big air consumers will be a tire machine at some point in the near future (hobby use only) and a bead blast cabinet down the road.  I don't plan on doing any painting (that I know of) or hours and hours of DA sanding.  :)

Thoughts/experience/feedback?

Thanks!

Jason

The nice thing about large tubing is it adds considerably to the effective tank size you have.

On super high volume things, small pipe cuts flow noticeably.

1/2" NPT isn't going to be a big restriction on 3/4 tubing. The ID of the tubing will be less than 3/4, possibly around 5/8 and the ID of a 1/2NPT fitting is close to that. 

I built in a pressure drop to mine to help combat moisture in the line. 3/8 line is plenty for my supply (air tools and even porting with an air grinder) but I was getting lots of moisture after a few minutes. After doing some research and deciding that a dryer was way overkill I went with a pressure drop setup. 3/8 line from tank into 1/2" pipe. The pipe make a U on the wall with a drain at the bottom and then goes to my separator/oiler after that and back to 3/8 line. That 10ft or so of 1/2" pipe over doubled the continuous use time before getting moisture at the tool. The 1/2" steel pipe also acts as a cooler which further helps the situation. If you do keep the smaller exit from the tank to much larger system line it will have the same affect.

Thanks all!  I should've mentioned...the Maxline tubing is measured ID, and apparently specs at 0.80".  That said, after some more InterGoogling, here's what Rapidair had to say on the topic (popular enough for its own FAQ):

My tank outlet size is smaller than the recommended pipe diameter.

The flow rate calculator says to use 3/4″ for my pipe size, but my air compressor only has a 1/2″ outlet.

The thread size of the port on your air compressor tank does not determine the pipe size for your piping system. It is calculated with the length of your piping (building size) and air compressor CFM output. Even though the port on your tank may be small, the air is only traveling through a short distance, so the restriction is minimal. Air is only restricted over a long distance with small piping.

 So....settle down, Francis, and just run the 3/4" kit.  

I've got to sit down and figure out the extra fittings needed (which add up quick), but am hoping to get this installed this month.  Should be fun!  I like that U-shaped pressure drop/drain idea...may think about incorporating something like that into the design.

Thanks again...

Jason

In reply to Rotaryracer :

Exactly... it is one thing to accelerate the air through a small orifice like, say, the quick connect on your air tool, but it is quite another if the air had to travel that fast over 20-40' of pipe.  Lots of friction losses there.

I really like that stuff.  One piece of advice when installing - make sure all the line is secured to the wall before you put any air into it.  Whilst wrestling a line into place, I put one fitting on hand-tight only while I got all the line in the spot where I wanted it.  Guess I forgot to go back and check that one before I powered up the compressor.  Compressor on > cool > (5 sec) > POP.  Had the line not been secured, it could have gotten real ugly.