Starting to plan out a 41 x 25 ft 3 bay garage renovation, thus currently lurking around several air line threads; this one got me interested enough to comment.
The 1/2" vs. 3/4" line diameter (ID) conversation (run length influenced) is as common as the debate about choice of materials in virtually every thread.
My own meagre 13x20 ft 'shop' (and another 10 ft run outside) are 1/2" black iron pipe (all premade lengths/fittings) for several reasons:
1) welding and grinding on occasion and don't want to risk bursting a permanent pipe, it's bad enough having the flexible/accordion hoses
2) iron pipe will cool the compressed air and has 5 drops that act as water traps with ball valves
3) have found that inexpensive tools (like $10 die grinders) and the stand up HF blasting cabinet are all huge air hogs, along with the IR and Blue Point 1/2" impact guns. Thus sending the most air volume at the lowest pressure differential (lots of Tee's and elbows) calls for the largest practical diameter. And, err on the side of overbuilding instead of underbuilding........
Have a 60 gall 5 hp comp cobbled together with a HF two cyl pump that specs at app 15 cfm @ 90 psi, relief valve set to 145 psi.
Have around a 50 ft run outside to feed the car shelter and driveway beyond, mostly for tire inflation using a Goodyear 3/8" rubber hose.
For the 3 bay project, thinking of 3/4" feed from the compressor (remote location likely) to individual 1/2" drops or a central 3/4" distribution manifold /multi branch setup terminating into 1/2" drops. Each bay and each work surface (probably 3 or 4) will receive one coiled overhead line (likely 1/4") as well as several wall mount quick disconnects for 3/8 rubber hoses. For sure there will be one or two primary (3/4") water drains, as well as additional ones, likely on each drop.
Wrestling with the in wall vs on wall issue since the ceiling still needs to be insulated, there is a bit of electrical, HVAC, plumbing, etc. work to still be done prior to drywall/sheet rock enclosing everything. This is going to require some more planning and decision making.
I like the iron pipe also because it's easy to neck up/down with standard reducers and the union style fittings in the appropriate places allow for easy servicing / section removal in case of system change etc. My controller/pressure switch at the compressor is plumbed this way in case it or the tank/pump ever need to be pulled out.
Will likely start a separate thread on this project once we get some of the big decisions made. Acquired a big pipe threader setup a while ago, will need to investigate if it works and that the proper dies are included, that would allow for the 14" HF metal chop saw to get a work out while saving some $$ and time to make the install nice and tidy.
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