Air compressor piping

[Ron Kaplan]

Does anyone have a good resource for figuring out what size copper pipes to use to plumb in air lines around my shop? I'm looking for pipe size versus airflow at various pressures.

Thanks,
Ron

[Ken Garlock]

Hello Ron, and welcome to the Creek. Do stop by often and splash around.

When I plumbed my shop with compressed air, I used 1/2" L-type copper. Depending on your compressor, you might want to consider putting it in separate room or building a closet for it. My 5hp Ingersoll Rand is one loud machine. It is located in an adjacent garage, and I carry a little over 100 psi on the lines. If you are looking for specs on copper tubing, the people at copper.org (http://www.copper.org/applications/plumbing/techref/cth/tables/cthindex_table.html) has all you need to know, and more. Regarding size, 1/2" is all I have ever needed, even with an impact wrench blasting away.

Hope this helps.:)

[Tony Cox]

I went with 3/4" copper from the compressor, around the perimeter of the shop and a run down the middle. Then I came down the wall with 1/2" copper to my outlets. I have a 5HP 60 gallon 2 stage Eaton compressor and this setup has never left me needing more air. I can run the sandblast cabinet and the compressor will pump up and shut off. I have used HVLP guns for finishing and painting, die grinders and impact guns and they have never been starved for air.

I don't know of anything to use for calculation. I just went with the information that I gathered from reading about air plumbing on several forums.

Good Luck!!

[Mitchell Andrus]

+1 for 1/2".... unless you're pressurizing the tanks on the space shuttle. Keep in mind you'll want to pitch the lines towards the outlets so moisture can escape.
.

[Josh Bowman]

Do as you want but I used 1/2" pvc in my basement shop for 15 years and absolutely no problems. I did the same in my new shop. I haven’t found anything that the 1/2" will not supply and I've used all but a commercial sandblaster. The air compressor is the least common denominator. I have up graded mine to a larger vertical. I did not do this in either installation, but suggest you use rigid piping for the drops so the coupling doesn't wiggle around so much

[Kent A Bathurst]

FWIW - extra $$ involved - the lines act as a tank/reservoir, so depending on what you use it for, the bigger the main line dia, the bigger your tank is.

As an extreme example, in a previous life in manufacturing, we ran 4" main pipes in a 300,000 sf factory that used a lot of air tools (drivers, wrenches, pneumatic clamps, etc). THAT was one heckuva tank. Of course, it was supplied by 5 x 50hp Hydrovane compressors - might be a little more than you will need :D but you get the point.

[Rick Lizek]

http://www.tptools.com/StaticText/airline-piping-diagram.pdf
A basic diagram to follow.
1/2" is fine for a home shop. If you want to use plastic there is ABS rated for airlines. PVC and pex aren't rated for air lines. Pitch your lines back to the tank and install your drops pointing up, then down. It keeps the water from collecting in your drops. A commonly overlooked practice. You cannot imagine how many shops have water issues because they don't understand the fundamentals of air line piping.

[Jim Becker]

Do as you want but I used 1/2" pvc in my basement shop for 15 years and absolutely no problems.

Please note that PVC should not be used for compressed air applications...it's not rated for that. And folks with shops that get cold run the risk of sudden shattering at low temperatures which can be deadly.

[David G Baker]

I have read a lot of posts about folks swearing by PVC pipe used for compressed air and have found that a lot of typing is wasted trying to get them to realize that they are playing Russian Roulette. Guess they will only be convinced after the the PVC shatters until they become believers.
I agree with using a larger pipe if you need more air storage if you can afford the larger size pipe.

[Randal Stevenson]

http://www.tptools.com/StaticText/airline-piping-diagram.pdf
A basic diagram to follow.
1/2" is fine for a home shop. If you want to use plastic there is ABS rated for airlines. PVC and pex aren't rated for air lines. Pitch your lines back to the tank and install your drops pointing up, then down. It keeps the water from collecting in your drops. A commonly overlooked practice. You cannot imagine how many shops have water issues because they don't understand the fundamentals of air line piping.


There is a pex that is rated for airlines. Price wise, it isn't a bargain, and doesn't have the cooling/condensing properties of metal piping though.

As said, pvc isn't rated for it, and if this is a commercial shop, you face the possibility of OSHA fines.

[Mitchell Andrus]

[/URL] Pitch your lines back to the tank....


I used to think this too. It's better to push the water out of the system with the flow to the main's drain drop and the driers at the end of each drop. The main should have a drain drop at its lowest point furthest from the tank and as stated drops should travel up, then down to encourage water to flow to the end of the line, away from the tank as shown on the plans below.

http://www.sharpe1.com/sharpe/sharpe.nsf/Page/Air+Piping+Layout (http://www.tptools.com/StaticText/airline-piping-diagram.pdf)

[url]http://www.oldsmobility.com/air-compressor-piping.htm

"Slope main lines at least four inches (10 cm) per 50 feet (12.7 m) of pipe away from air compressor so that condensate travels with the flow of air and away from the compressor. As warm air leaves the compressor, it cools and thereby condenses as it travels through the pipe. This water vapor, a problem in itself, can also cause scaling and rusting inside the piping. Install drop legs for condensate removal."
.

[Rick Lizek]

Duraplus and Chemaire are ABS plastic rated airlines. What is the pex for airlines? Is it ABS? When I contacted the Pex people they would not back it up for air and it degrades in sunlight.

We are putting up a new building and I'm checking out the Duraplus for potential airlines. My local plastic supplier carries it and I'm going to compare pricing with black pipe over the next week. It's certainly easier to work with. Whoever did our original piping in the main building certainly broke every air line plumbing rule.

[Chris Parks]

http://www.tptools.com/StaticText/airline-piping-diagram.pdf
A basic diagram to follow.
1/2" is fine for a home shop. If you want to use plastic there is ABS rated for airlines. PVC and pex aren't rated for air lines. Pitch your lines back to the tank and install your drops pointing up, then down. It keeps the water from collecting in your drops. A commonly overlooked practice. You cannot imagine how many shops have water issues because they don't understand the fundamentals of air line piping.

How does the water escape? The lines should be as large as possible, 1" at least if used in a commercial situation and even at home if a lot of air is used. This means the air moves slow enough for the water to drop out of suspension. Small lines mean high air speeds and suspended water vapour. The modern modular "plastic" systems are way better than metal pipe and install in a quarter of the time. They can even be pulled apart and changed if if needed.

[Mitchell Andrus]

How does the water escape? The lines should be as large as possible, 1" at least if used in a commercial situation and even at home if a lot of air is used. This means the air moves slow enough for the water to drop out of suspension. Small lines mean high air speeds and suspended water vapour. The modern modular "plastic" systems are way better than metal pipe and install in a quarter of the time. They can even be pulled apart and changed if if needed.

I looked at snap-connector type plastic pipe on a roll but found that in order to keep sags from forming over time and trapping condensation, I'd need to support it at such close intervals.... it was easier to use copper. Also, copper fitting will never be impossible to find in a hurry. My small system in copper with about 80 feet and 5 drops cost just about the same and only took 3 hours.
.

[Glenn Vaughn]

I am thinking of using 1/2" DOT Reinforced Airline in combination with PVC pipe to make a run of about 100 feet from the compressor in my shed to the garage. The PVC will be buried and the airline will run through it. Found a good price at
http://www.trainhornexpress.com/product_p/the-hose-03.htm

[Ron Kaplan]

It sounds a little like everyone is trying to rationalize the systems that they already installed. I appreciate the inputs, however I am still looking for some documentation on airflow versus pipe size and flow length.

Any experiences with that?

Ron

[Roger Newby]

Ron, google cfm air flow vs pipe diameter and you'll get all the information you need. The larger the pipe the more cfm you get and also less pressure drop. You can have 100 psi at the compressor but you can't push enough cfm thru the pipe to satisfy an air tool 30 feet away if the dia. is too small.
Before I retired from my Tool & Die Maker Job (last week!! YAY!:D) I often used smaller diameter tubing as a speed control. You get the psi you need but the restriction of flow will slow the action on a rotary actuator or air cylinder.
If you are running a nailer or similar tool you want all the cfm you can get, therefore, run the largest pipe you can afford.

[Aaron Wingert]

Sorry, but CFM isn't the issue here at all. Fact is that unless you're in an industrial shop with a MASSIVE compressor, 1/2" copper's capacity is far greater than the CFM output of the compressor.

Think of it this way...
Most rubber air hose is 3/8" and it'll run your air tools just fine and never starve any of them for air. It'll run your impact gun (unless you work on bulldozers), your nailers, die grinders, etc. Using the blow gun to clean the shop it'll empty your compressor at a rate that most compressors can barely keep up with, if at all. The air tools you use have a certain size oriface to allow a certain flow of air at a given recommended pressure, so they use a given amount of air per shot or per unit of time.

Unless you have numerous machines all utilizing air at the same time, such as you would in a production cabinet shop or millwork facility, there is no reason to upsize the main trunk of an air line from which smaller branch lines project. It is all based on demand. If you're in a home shop you can install 1/2" lines and never never never starve anything for air, and your compressor will not keep up with the potential flow/capacity of the pipe.

There is merit in using larger pipe for extra air storage space. But you're not really gaining that much, comparing 1/2" and 3/4" unless you have hundreds of feet of pipe.

As for needing larger pipes to catch water, I don't know. Might have merit, might not. I have 1/2" pipe throughout my shop and my drip legs do get water in them just fine. I bleed them off from time to time with the valve at the bottom of each one (just below the quick connectors for the flexible hose to the tools). It is wise to install a water separator between the hardpiped lines and the compressor anyway, but I haven't done that yet.

I installed type L copper, 1/2", throughout my shop and absolutely love the system.

As for PVC, my research led me to believe that it is a poor idea. 1/2" copper isn't that expensive that I'd risk it. PVC failures are known to blow shards of plastic in many directions...Doesn't sound fun.

[Mitchell Andrus]

It sounds a little like everyone is trying to rationalize the systems that they already installed. I appreciate the inputs, however I am still looking for some documentation on airflow versus pipe size and flow length.

Any experiences with that?

Ron

I think you're over-complicating this... but, Google "air flow pipe calculator" etc., and see if you can make heads or tails of it all. You'll likely do some math on your own.

If you don't understand fluid dynamics (yes, air behaves as a fluid in certain cases), and how to determine the difference between laminar flow and turbulent flow, everything else becomes guesswork. Go for 1/2" or 3/4" pipe because the parts are at Home Depot.

Over-simplified yes, but remember.... this is a woodworking forum.

http://www.newton.dep.anl.gov/askasci/eng99/eng99525.htm

http://www.pipeflowcalculations.com/


Question: What is the effect of tube length on air flow at a given
pressure. Example: If a tube of X length flows Y air at Z pressure.
What will the flow Y be if the tube length is 10 times X with the
same pressure.
---------------------------------------
As with so many things, the relationship depends on how precise you want
your answer to be.

If you are looking for a precise mathematical relationship to calculate air
flow in a pipe, things get very complex very quickly. There is no one single
X, Y, Z relationship that can be used universally. There is a thing called
'turbulence'. At low enough velocity, fluids (such as air or water) flow
uniformly (called 'laminar' flow -- 'in layers'). As the fluids speed up
though, they start to flow chaotically, forming vortexes and eddies as they
move down the pipe ('turbulent'). The equations for laminar flow are quite
different than turbulent flow; the relationships for X, Y, and Z depend on
what X, Y, and Z are. And, the flow can transition from one regime to the
other during flow, so you might have to make three calculations: one for
each regime, and one for the transition phase. Several practical factors
also affect your answer. The geometry of your pipe, the roughness of the
sides, the purity of the air (is it oil-free?) all affect your answer. Thus,
you not only need X, Y, and Z, but also A, B, C, D, and E (so to speak).
Want more? Air (unlike water) is compressible. At high pressures, and high
pressure drops, compressibility affects flow. Also, as air flows, compresses
or decompresses, it can change temperature, which affects pressure and
therefore flow.

In short, a precise, predictive calculation is actually quite challenging.
Fortunately, there are tons of resources on-line that can perform
calculations for you. If you want to do it yourself, you can find several
different equations (they are different because they rely on different
assumptions -- for example, one might assume constant temperature). Google
'air flow in a pipe' or 'air flow in pipe calculator' and you'll have
several options.

If you're looking for a *very* rough estimate, you can assume pressure drop
to be linear across a straight pipe. In other words, if your inlet pressure
is 10 atmospheres at the start, and 9 atmospheres at X, then it would be 8
atmospheres at 2X, 7 atmospheres at 3X, etc. This is a rough estimate only
-- there are many factors that do not scale linearly. This is a much better
estimate for water than it is for air.

Hope this helps,
Burr Zimmerman
================================================== ==================
What you are asking is: "What is Poiseuille's Law?" If you do a Google (or other
search engine) search, you will find numerous sites, deriving and explaining P's
Law at any level of mathematical sophistication you care to use. Two points worth
making is that the resultant equation is different for "slow" and "fast" flow,
for turbulent flow, and for bends in the tube. So there is not a "universal"
equation. A couple of sites to get you started are:

http://en.wikipedia.org/wiki/Poiseuille's_law (http://en.wikipedia.org/wiki/Poiseuille%27s_law)

http://en.wikipedia.org/wiki/Fluid_dynamics

One has to be careful about whether the fluid is compressible (air) or
incompressible (water), and whether viscosity is important. See: Compressible
vs incompressible flow

http://en.wikipedia.org/wiki/Fluid_dynamics

A fluid problem is called compressible if the pressure variation in the
flowfield are large enough to effect substantial changes in the density
of the fluid. Flows of liquids with pressure variations much smaller than
those required to cause phase change (cavitation), or flows of gases
involving speeds much lower than the isentropic sound speed are termed
incompressible.

For flow of gases, to determine whether to use compressible or incompressible
fluid dynamics, the Mach number of the problem is evaluated. As a rough guide,
compressible effects can be ignored at Mach numbers below approximately 0.3.
For liquids, whether the incompressible assumption is valid depends on the fluid
properties (specifically the critical pressure and temperature of the fluid) and
the flow conditions (how close to the critical pressure the actual flow pressure
becomes). Acoustic problems require allowing compressibility, since sound waves
can only be found from the fluid equations which include compressible effects.

The incompressible Navier-Stokes equations can be used to solve incompressible
problems. They are simplifications of the Navier-Stokes equations in which the
density has been assumed to be constant

Vince Calder
================================================== ==================
The formulas for fluid flow follow pretty much the same math as the formulas for
electrical current. Remember Ohm's law, V = IR, where V is voltage across a
component, I is the current through the component, and R is the resistance of
the component? Same for fluid flow, except use flow rate (say, gallons per
minute) instead of current and pressure instead of voltage. Resistance follows
the same rules; a 20-foot pipe has twice the resistance of a 10-foot pipe of the
same diameter.

So in your case, you are comparing pressure drop Z, flow rate Y, length X to
pressure drop Z, flow rate y, length 10X. We can say Z = (flow rate)
(resistance) in both cases, and we want to find the flow rate y. Let's
say the resistance in case 1 is R; the resistance of a 10-times longer
pipe will be 10R. So we set up for equal pressure drops:

Z = Z
YR = y (10R)

Now we need to solve for y:

y = YR/(10R) = Y/10

So not surprisingly the flow will be 1/10 as much for the same pressure
drop.

Richard Barrans
Department of Physics and Astronomy
..
University of Wyoming

.

[Chris Parks]

There are a lot of experts here aren't there, none of which have done more than use 3/8" line flexible line and 1/2" copper pipe. I have done the comparisons running air tools and yes Virginia there are big differences in both the tools performance and the systems performance. This is my experience backed up by a fair knowledge of how air behaves.

I have both 3/8" and 1'2" flexible air lines in my system running off a 3/4' main line and using a die grinder there is a very large speed and power difference at the grinder when supplied by 3/8". At the same time there is a large amount of water pulled through the system when this tool is in operation either on the 3/8" or 1/2" but more so on the 1/2" and this water gets dumped into an automatic water trap.

Ah! you say, he doesn't drain his system, sorry but I do. It is drained on by an automatic drainer at the compressor, which BTW is 17CFM and brand new, and this drainer operates every 15 minutes and also every time the compressor is turned on so draining is not the issue. Then it runs from the compressor through a 3/4" pipe vertically for about 20' and at the bottom of this vertical run there is another auto drainer to catch the condensate that should drop out of the water during the vertical journey. It then enters the workshop and runs through another water trap/reg/oiler into the system within the workshop.

At the time of installation I was told that all my lines up to the flexible take offs should be one inch to overcome the water suspension issue I would encounter, this by a guy who had been doing industrial installations for many years. I reasoned the vertical rise would cool the air well enough to cause the water to drop out and fall into the auto drainer at the bottom, more on that in a minute. He explained all about water suspension and air speed and just about pleaded with me to put in 1'' aluminium modular but as it was essentially only a single run in a home workshop and not very complicated I decided not to on the grounds that I literally could not be bothered and it was all a lot simpler to do it in 3/4'' copper. yes it will flow the air but it leads to water issues as I was to find out, BIG MISTAKE. BTW all the air equipment, driers, etc in the system are NORGREN, top stuff, look it up if you don't believe me.

When an air grinder or cutter is running I can see the water separating at the water trap in the workshop, yes it does separate but by this time it has come out of the compressor which is auto drained and thus dry, through the vertical run which has a water trap at the bottom but not through that water trap and into the one at the top of that vertical. Now for water to travel 20' vertically in copper with all its huge cooling capacity which it is known for was a bit of an eye opener. It turns out that yes, the air travels so quickly the water carried cannot drop out through all that copper pipe and yes I should have used 1" and yes the guy who told me to was right and I was wrong.

I am now going to duplicate the vertical rise using 3/4" pipe in an effort to reduce the issue but if I had done it properly in the first place then it might not have arisen. As for plastic modular systems, they are quick to install but do not shed the latent heat in the air. Aluminium is far better as it must be by its nature and using one of those modular systems is a breeze. plan it and the supplier will do all the work of piercing the pipe where needed if you don't want to buy the tooling. In these systems it takes 10 secs to do a joint and it can be undone in the same time and changed if need be.

What got me about my system was I knew the issue of water suspension v's speed and I was sure that using 3/4'' copper would overcome it as in my previous system I only had 1/2'' all the way through. It turns out not to be the case, even 3/4'' is not big enough using high demand air tools of which I am guilty. If in doubt consult an expert and listen, I consulted but did not listen.

[Robert Malone]

I bought the "Complete Garage Air Kit" (item #66747) from HF to make it easy (hopefully)
to do a complete makeover of my air system. I have had both oil and dry-air lines for
over 10 years now.

[Randal Stevenson]

Duraplus and Chemaire are ABS plastic rated airlines. What is the pex for airlines? Is it ABS? When I contacted the Pex people they would not back it up for air and it degrades in sunlight.

We are putting up a new building and I'm checking out the Duraplus for potential airlines. My local plastic supplier carries it and I'm going to compare pricing with black pipe over the next week. It's certainly easier to work with. Whoever did our original piping in the main building certainly broke every air line plumbing rule.

Personally, I would still prefer the black or copper pipe. I thought Duraplus (EXPENSIVE) had gone oob? (or stopped making it).

The pex for airlines, is Pex-al-Pex. My understanding is there are different ways to make it (which is probably why I have only seem some brands say gas/propane/compressed air), in relation to the aluminum center. It is meant as an in wall pipe (not exposed to uv).

[Steve Milito]

Here's a link that I found.
http://www.engineersedge.com/pipe_flow_capacity.htm

This implies that 100' of 1/2 copper tubing @ 100 PSI will have a ~10% pressure drop and an air flow of 54 CFM.

3/4" will have a 5% pressure drop and 80 CFM, I'm guessing a 15% price penalty.

Can't speak much about the velocity / water / cooling issue. Most of the time the air velocity is determined by the tool you are using. Otherwise, when the system is wide open and you potentially constrained by pipe, you are actually constrained by the compressor in a very short time period (assuming a home shop).

My impression is that 1/2" is more than fine for under 100', and 3/4 is more than fine for 100-200' with any non-industrial compressor. In either case, have a cooling zig - zag run of 30'-40' seems like a good idea.

[Kent A Bathurst]

MItchell - thanks. That clears a lot of stuff up. I was applying Poiseuille's Law to laminar flow, but using the Navier-Stokes equations for incompressible fluid dynamics - which was clearly a mistake.

Back to the drawing board, eh?

[Steve Milito]

Here another set of data.

http://www.engineeringtoolbox.com/compressed-air-pipe-pressure-d_36.html

or
http://www.pipeflowcalculations.com/airflow/index.htm
click on airflow and then
'click here for calculator'.
You can plug in your airflow in CFM, and pipe size, and see the velocities and pressure drops.

[Jim Beachler]

In my shop, I run 3/4 lines from the compressor for 40 feet in a upward back and forth line. This gives the air time to cool before it hits the water separator. If the temp is too high, the moisture will not separate efficiently. At each drop I have another small water separator. Also use an automatic tank drainer which also goes off every 15 minutes. Even using all of that, I still get moisture in the drip legs which are drained every month. Not much but some in several of them.

As much as people are worried about volume, I would be more concerned with moisture. Moisture can kill your tools.

As far as volume goes, calculate your heaviest usage and build to twice what you think you will need.

As far as plastic, why go cheap?? If there is a chance it will blow up, do you want to take that chance??

[Chris Parks]

As much as people are worried about volume, I would be more concerned with moisture. Moisture can kill your tools.

As far as volume goes, calculate your heaviest usage and build to twice what you think you will need.



Volume, air speed and moisture are part of an equation in the design. Slow air speed means time for the moisture to drop out, pretty simple really. What is the problem with using modular aluminium......

http://www.transairaluminumpipe.com/

If I was looking at installing a commercial system there is no contest, aluminium wins every time. Speed of construction, design help, removable if there is the need and can be re-installed in another building etc. Surely, there could be no other choice.

If the system is designed to supply air with enough idle time for the compressor and a slow enough air speed for the main lines you won't have an issue. If the compressor keeps working the problem starts there as the air id not stalling in the main reservoir long enough to cool but being dragged out as soon as it is coming out of the compressor head. We all look at air systems and think it is easy to sling up a few pipes and get air, it is but doing it properly is another issue entirely and that is not easy.

[Glenn Vaughn]

I bought the "Complete Garage Air Kit" (item #66747) from HF to make it easy (hopefully)
to do a complete makeover of my air system. I have had both oil and dry-air lines for
over 10 years now.

This is on sale for $79.99 - order from catalog link - #66747-1AGH

[Terry Ryshak]

Hi Guys,
Just want to throw my 2 cents in, if I may?

I agree, the larger the airline the better as it does act as an accumulator, and gives you more storage, the debate of Black steel lines is good providing the individual is capable of making good airtight joints, as it is truely an artform?! LOL
Plastic is great for a "TEMPORARY" line, but not for a permanent installation, due to climatic variables. Copper is still cheaper than aluminum lines, and does dissipate heat faster than aluminum, for cooling of the air in the lines prior to the tool in use. I have heard of you using Water separators, with Auto drains set at 15 min. intervals, which is a great Idea, "BUT" I did not hear of anyone using a Air dryer in their system? This is quite perplexing, you will always have moisture in your air unless you install an Air Dryer, either desiccant or refrigerant dryer in your system. This unit is installed after your Accumulator and preferably just as you enter into your line distribution system. Desiccant dryers are very cost effective, and passive units, and very highly recommended if you want clean dry air.
Again, just my 2 Cents?

TER
:D

[Michael Flores]

http://www.rapidairproducts.com/index.asp

[Chris Parks]

As far as plastic, why go cheap?? If there is a chance it will blow up, do you want to take that chance??

There is plastic and plastic, see the link above. It is not a good idea to label all plastic air lines as cheap, meaning dangerous, it is false and misleading.

[Jim Beachler]

Hi Guys,
? This is quite perplexing, you will always have moisture in your air unless you install an Air Dryer, either desiccant or refrigerant dryer in your system. This unit is installed after your Accumulator and preferably just as you enter into your line distribution system. Desiccant dryers are very cost effective, and passive units, and very highly recommended if you want clean dry air.
Again, just my 2 Cents?

TER
:D

For my spray booth, I use another water separator and 2 desiccant dryers.
When spraying paint and lacquer, I do not want any contamination.

For my tools, I oil them every 2 weeks to deal with the occasional moisture that make it in.

[Steve Milito]

Looking at all the options, it’s hard to sort it all out, but the following is my opinion.
From my reading, you want to keep the air velocity below 20 ft/sec. Looking at 5 HP compressors it seems a max CFM of 15 is reasonable, and it’s hard to justify anything more than 18 CFM without jumping to a 7.5 HP compressor. Thus, for a continuous duty 5 HP dual stage compressor with less than a 150’ run, it really makes ¾” pipe ideal. Granted, with 1” pipe, the Duratec system becomes attractive, but only for long runs with few drops. Additionally, in a small shop environment you’d like the drops to be ½” or so, and you need to buy Duratec in 100’ spools. Thus, mixed sizes are cost inefficient for small installs. Additionally, the fittings are expensive especially large fittings. Thus, even though copper pipe is ~25% more expensive than Duratec the fittings are way less expensive. Bottom line, the least expensive route in small or medium sized shop is using copper pipe. Although soldering the vast majority of the joints makes it cost effective, you still can use an occasional Sharkbite fitting in tight spots or for ease of assembly and still come out cheaper than using an AL based system. Additionally, if you are willing to use soft copper for the drops, you can use all solderless fittings and still come in cheaper than Duratec.
In industrial environments with really high volume systems, long runs, and many drops, the Al based systems become cost effective.

[Jim Hoti]

I just made all my drops 1 foot longer than the location of the outlet. Put a quarter turn ball valve at the end of the drops. After use, open the ball valves and any accumulated moisture drips out. Didn't have to figure slope or pitching back or anything......

[Callan Campbell]

Sorry, but CFM isn't the issue here at all. Fact is that unless you're in an industrial shop with a MASSIVE compressor, 1/2" copper's capacity is far greater than the CFM output of the compressor.

Think of it this way...
Most rubber air hose is 3/8" and it'll run your air tools just fine and never starve any of them for air. It'll run your impact gun (unless you work on bulldozers), your nailers, die grinders, etc. Using the blow gun to clean the shop it'll empty your compressor at a rate that most compressors can barely keep up with, if at all. The air tools you use have a certain size oriface to allow a certain flow of air at a given recommended pressure, so they use a given amount of air per shot or per unit of time.

Unless you have numerous machines all utilizing air at the same time, such as you would in a production cabinet shop or millwork facility, there is no reason to upsize the main trunk of an air line from which smaller branch lines project. It is all based on demand. If you're in a home shop you can install 1/2" lines and never never never starve anything for air, and your compressor will not keep up with the potential flow/capacity of the pipe.

There is merit in using larger pipe for extra air storage space. But you're not really gaining that much, comparing 1/2" and 3/4" unless you have hundreds of feet of pipe.

As for needing larger pipes to catch water, I don't know. Might have merit, might not. I have 1/2" pipe throughout my shop and my drip legs do get water in them just fine. I bleed them off from time to time with the valve at the bottom of each one (just below the quick connectors for the flexible hose to the tools). It is wise to install a water separator between the hardpiped lines and the compressor anyway, but I haven't done that yet.

I installed type L copper, 1/2", throughout my shop and absolutely love the system.

As for PVC, my research led me to believe that it is a poor idea. 1/2" copper isn't that expensive that I'd risk it. PVC failures are known to blow shards of plastic in many directions...Doesn't sound fun.
Here's how I solved my air piping and compressor issues:
It's not always the pipe diameter that usually gets you, it's the restrictions from the 90 degree fittings that really slows things down. It's hard to use just 45s for most applications, we tend to need a 90 fitting somewhere. When you look at the difference between a copper 90 and a black pipe 90, you see how air/your gas can get restricted, causing supply/delivery issues downstream where you need it. Intermittent usages of air won't show the issue for many tools, but a heavy, constant consumer like a grinder or a sander will. The other issue is long runs with a single stage compressor. Since you don't have the higher pressure output of a two stage unit, you can't easily "turn" the max output of the compressor higher to compensate since it may not give any higher pressure to help you stop the low end point pressure at a long run. I ran into this with my underground 1/2" pipe run to my garage from my basement mounted single stage compressor. At the end of the run, I barely could manage 110psi at a static reading with the air line regulator maxed out. Once a large tool started running, the pressure went down to close to 90 to 100 psi until the the air in the tank got low, then it might lower to 80-90 before the compressor was kicking in to raise the system pressure back up. I wasn't running any sanders, just 1/2" or 3/8" impact guns and some intermittent 1/4 die grinder usage. But the grinder of course, was the heaviest consumer of air.
All this changed with my getting a two stage compressor. Higher output meant higher delivered pressure to the garage as well.[I don't run it above 150psi] More differential pressure to play with. I also changed all the shop pipe to 3/4" to help flow air better before it heads out the wall and back into the 1/2" underground line that feeds my garage. Really changed things around, and I was going from a 5HP single stage unit to a 3HP industrial 2 stage unit, but also from a 60 gallon tank to a standard 80 gallon tank. Having more stored air helps too:D

[Steve Milito]

Not to keep kick a dead horse, but . . .

I've been toying with one of the online calculators, and other basic design issues.
One thing I looked at is the storage capacity of lines of various diameters. Bottom line, unless you go with massive sized tubing, there is no storage advantage. One interesting thing from the math is that 80 gals is just under 11 Cubic feet.
Here are charts that show pressure drops and terminal velocities of compressed air at a few volume rates assuming 90 PSI and 100 linear feet of tubing.