I'm going to install approx 18 ft of black pipe for an air line extension in my garage. Mainly to have a good place to install a water separator. I just have a small portable compressor. It will be connected with std flexible air hose. What should the minimum pipe ID be? I know it has to be reduced down to 1/4" NPT at the ends to fit the hose.

Von,

I suggest 3/4".
Check out the info at tptools.com
http://www.tptools.com/statictext/airline-piping-diagram.pdf

The ideal setup is to use metal piping* (regular black metal piping), (1/2" ID up to 75ft., 3/4" ID over 75ft.), from your compressor to
various locations in your shop. Mount piping along the walls of your shop with separators and regulators fastened
to the wall and a valve installed at each outlet. By using metal piping of proper size, not only does the piping serve
as an "air reservoir", it also enables the moisture-laden air to cool so that the water separators can do their job. A
properly installed system of metal piping will eliminate most problems of moisture and will provide sufficient air
volume to the air-operated equipment. The most important feature is to use "Risers" (shown above). "Risers" allow
system to draw dry air, leaving moisture in the piping where it can be removed with water separators.
*Use regular black metal piping for installation. PVC or copper is not recommended.

Thanks.

Actually, copper is highly recommended for air compressor piping...just use type L copper, 3/4". I plumbed my compressor this way from the basement where the compressor lives to the garage about 60' away...very nice way to go. I did use a stronger 95/5 solder instead of the 50/50 tin lead solder.

Type M copper and regular lead free plumber's solder will work fine and handle much more pressure that an air compressor will ever put on it.

My garage is plumbed with 1" PVC, did it ten years ago, no problems at 175psi. Cheap and easy.

This subject comes up alot here and on the welding sites I visit. It may be controversial to some, but OSHA stronly warns against using PVC for compressed air delivery If it's hit with anything, it can crack and explode sending shrapnel everywhere and can severely injure or kill someone. A good buddy had both his body shop and home shop plumbed with schedule 40 PVC...which I feel is very risky.

Do what you want, but I personally woudl rather spend $140 to plumb mine (actual cost of the plumbing system I mentioned above) in copper than to save a few bucks and have that kinda risk.

Also, you need some sort of flex line connection between the compressor tank and the hard line. I had a piece of 3/4" hydraulic hose made up which works mint and is rated at 4000+ psi. The flex connection is needed to eliminate vibration between the tank & hard line as well as stopping any heat transfer from the tank to the hard line.

"Cheap and easy."
hey baddbob71 you knew my girlfriend too? :)

Like 10secBu said PVC splinters pretty badly, IF I was going to use plastic (which I wouldn't) to save a couple of bucks and a little time, I would at least use CPVC

For my short 18 ft run and small compressor I'm going to use 1/2" black pipe with a filter/water separator at the downstream end. The compressor will be connected with a 15 ft section of std 3/8" air hose and quick connectors.

I think you might be a little undersized using the 3/8 hose Von but one advantage is it will increase the velocity which helps carry the moisture through.

Yeah, I've heard about the pvc exploding long after mine was installed. Fortunately the piping is plumbed in the walls and above the ceiling of my garage so barely any of it is exposed. If I ever do it again I'm sure I'll use threaded pipe. But the pvc has held up just fine. I went over the rafters with mine and down the walls inside in 6 areas with drop legs to help collect moisture and it seems to work just fine.

Actually, companies do market a plastic / vinyl piping for compressed air. I did mine in 1/2" Galv. pipe. We just installed a system in a buddies garage and used 3/4" galv..

Bob, the bad part is it holds heat. Heat = condensation.

And with yours "hidden" that means it doesn't get good air movement around it to cool.

Hot air holds the condensation and then it gets to the trap the droplets of water are not big enough to be trapped.

It can make a HUGE difference in how you plumb the air lines.

There ain't much heat above the ceiling in my garage today, 20 degrees fahrenheit and won't reach any higher than 30 during the rest of the day. There is plenty of breathing room for the pipe up there.

We used 3/4" copper to each final run then downsized to 1/2". Works great!!

I wouldn't even think of runing pvc/cpvc given the small price diffrence.

Black or gal is great IMO if you have access to it and equip to thread it.