Compressed Air System Design

[jkesselr]

Hey guys,

I am working on designing my air compressor system and need some advice. I do various things in my shop, including paint and auto repair. I have paint at one end of the shop and auto repair at the other. I am looking at replacing my IR T30 80gal 7.5HP with a similar Emax unit in the 80-120 gallon range. I anticipate somewhere in the neighborhood of 600-800 feet of 3/4" pex-al-pex running around the shop. The compressor will be near the paint end of the shop and roughly 120 feet from the auto repair side of the shop. The question is: do I need a second reservoir tank at the auto repair end of the building? I am sure it would be ideal, I am just curious if it is necessary to keep tools fed with enough volume and pressure. If so, can I get away with just installing a second 60-80 gallon compressor tank (minus compressor)? Thoughts?

 

[DAWrench]

I added a small 20 gallon tank on the far end of my shop away from the compressor. Only about 50' from the compressor. I feel like it helps with my impact for shorts bursts but my air piping is very basic and probably undersized. I am sure someone smarter than me will have an answer but it wouldn't think a second big tank would necessary and might even be counter productive

 

[jkesselr]

I added a small 20 gallon tank on the far end of my shop away from the compressor. Only about 50' from the compressor. I feel like it helps with my impact for shorts bursts but my air piping is very basic and probably undersized. I am sure someone smarter than me will have an answer but it wouldn't think a second big tank would necessary and might even be counter productive

Thanks DA, I appreciate the feedback. Maybe a smaller second reservoir might be more helpful than an additional large tank.

 

[GeoBruin]

I just plugged your numbers into the Gates air pressure drop calculator. Assuming you are running unregulated at a max tank pressure of 175 PSI, using a tool that consumes 40 cfm, you'll only see 1.5 psi drop at the end of your 120 foot run. Obviously it will drop more with an air hose hooked up, but that would happen anyway.

Doesn't seem like much benefit to adding a tank to me. Even at a more typical shop pressure of 100 psi, you'll only see about a 2.5 psi drop.

 

[FullRaceMerc]

Instead of a trunk line down the center with several branches, I think it's better to run a loop around the shop with drops as needed. That way any point along the way is fed from both sides of the loop.

 

[mariom2019]

Honestly, with a 7.5HP Emax and an 80 to 120-gallon main tank, you’ve already got a massive amount of volume at the source.
With 3/4" PEX-AL-PEX, your main restriction over that 120-foot run to the auto side isn't going to be total volume, it's going to be friction loss during high-flow spikes (like running a heavy 3/4" or 1" impact or a sandblaster).
You definitely don't need another massive 80-gallon tank down there—that's just extra volume for the compressor to pump up initially and more places for condensation to trap. If you find your high-draw tools starving for a second during initial trigger pulls, just plumb in a small 10 to 20-gallon receiver tank right at the auto work station. It acts like a local capacitor, supplying that instant hit of air right where you need it while the main line catches up.
Keep it simple first. Run your 3/4" loop, see how the tools perform, and only add a small local tank if you notice a pressure drop when hitting the trigger.

 

[72Anthony]

If it's a single man shop, I think you'll be fine, unless you are running un-attended machines like a CNC plasma that needs continuous air, while doing other work.

As others said, run a loop to allow flow from both sides, and some valves to allow isolation to parts of the system.

All that being said, I'd install a tee with a cap or vavle at the far side of the shop in case you ever wanted to plumb in an additional tank or even a standby/secondary compressor as a back up. Some members here are able to pickup incredible deals on the secondhand market.

 

[FTG-05]

Used compressor tanks are cheap:



I got this 120 IR tank for $200 maybe.

Sharp eyes will note the 3/8" line plumbed in parallel to the 3/4" Rapidair line.

 

[Chipm]

Scaling this idea up a bit, If one were to run two compressors, how would you set that up? Would one have a higher kick-on point? Would they need check valves?

 

[FullRaceMerc]

We have 2 at the shop. Yes, we stagger their start pressures so only 1 runs unless the demand is high. We don't use additional check valves.

Most of the time only 1 is needed. Number 2 has been shut off for years. We originally had times where we ran both, but with the current equipment & crew size 1 is plenty. We're plumbed in a fashion where the 2nd tank can be used for added volume, or turned off so the single tank is all that has to be filled. The 2nd tank is almost always off.

Having the second compressor was handy when the primary bit the dust. Turn the valves, throw the right switches, & keep running while taking our time to determine if it was repair or replacement time.

 

[jkesselr]

I just plugged your numbers into the Gates air pressure drop calculator. Assuming you are running unregulated at a max tank pressure of 175 PSI, using a tool that consumes 40 cfm, you'll only see 1.5 psi drop at the end of your 120 foot run. Obviously it will drop more with an air hose hooked up, but that would happen anyway.

Doesn't seem like much benefit to adding a tank to me. Even at a more typical shop pressure of 100 psi, you'll only see about a 2.5 psi drop.

Thank you for running the numbers. I agree, based on this, it doesn't seem significant enough to worry about. The compressor runs to a max tank pressure of 175. Even at the farthest reaches, if I rot really carried away, a particular connection point might be 210 feet, which would only be a 2.7 psi drop. Seems like I need to spend my time focused on other concerns.

Thanks!

 

[jkesselr]

Instead of a trunk line down the center with several branches, I think it's better to run a loop around the shop with drops as needed. That way any point along the way is fed from both sides of the loop.

I hadn't really thought about that, but I suppose it is a possibility.

 

[jkesselr]

Honestly, with a 7.5HP Emax and an 80 to 120-gallon main tank, you’ve already got a massive amount of volume at the source.
With 3/4" PEX-AL-PEX, your main restriction over that 120-foot run to the auto side isn't going to be total volume, it's going to be friction loss during high-flow spikes (like running a heavy 3/4" or 1" impact or a sandblaster).
You definitely don't need another massive 80-gallon tank down there—that's just extra volume for the compressor to pump up initially and more places for condensation to trap. If you find your high-draw tools starving for a second during initial trigger pulls, just plumb in a small 10 to 20-gallon receiver tank right at the auto work station. It acts like a local capacitor, supplying that instant hit of air right where you need it while the main line catches up.
Keep it simple first. Run your 3/4" loop, see how the tools perform, and only add a small local tank if you notice a pressure drop when hitting the trigger.

This makes a lot of sense to me. I am hoping to land the blaster closer to the compressor, maybe 40-50 feet or so. I am seeing a few suggestions for running a loop, instead of a trunk line. I guess I really need to think about that being a better option.

 

[jkesselr]

If it's a single man shop, I think you'll be fine, unless you are running un-attended machines like a CNC plasma that needs continuous air, while doing other work.

As others said, run a loop to allow flow from both sides, and some valves to allow isolation to parts of the system.

All that being said, I'd install a tee with a cap or vavle at the far side of the shop in case you ever wanted to plumb in an additional tank or even a standby/secondary compressor as a back up. Some members here are able to pickup incredible deals on the secondhand market.

Yeah, it's just me and a few buddies from time to time. I could imagine a time where one of us is using an impact, another using the plasma cutter and a third using a blast cabinet, but that would be a very rare occasion.

 

[Hohn]

I just plugged your numbers into the Gates air pressure drop calculator. Assuming you are running unregulated at a max tank pressure of 175 PSI, using a tool that consumes 40 cfm, you'll only see 1.5 psi drop at the end of your 120 foot run. Obviously it will drop more with an air hose hooked up, but that would happen anyway.

Doesn't seem like much benefit to adding a tank to me. Even at a more typical shop pressure of 100 psi, you'll only see about a 2.5 psi drop.

I think rather than running a buffer tank, most people would be better served running a service loop in their shop. This has two advantages that I see. First, it cuts the flow in half along any one run of plumbing, which halves line losses. Second, it makes every drop on the run balanced with every other other parallel drop.

IN other words, anything pulling air at any point on the loop drops pressure evenly across all other parallel drops.
Especially with many of the PEX/AL/PEX kits being so affordable now, I think it makes a lot more sense to run a loop than just a linear run with drops.

It might allow you to even drop down a pipe size and have an overall more cost effective system for the performance level, depending on the linear distance involved.

 

[Hohn]

Instead of a trunk line down the center with several branches, I think it's better to run a loop around the shop with drops as needed. That way any point along the way is fed from both sides of the loop.

This. I should have read more before posting essentially the same thing.

 

[Citation]

I hadn't really thought about that, but I suppose it is a possibility.

You might try running your for calculations with 1/2 pex pipe. It's just a lot easier to work with.

 

[Chipm]

This is a good thread. I am planning a long skinny building, so a trunk line would cost half of what a loop does.

What about dryers? At what point do we need one of those for typical auto repair use? For just me, I've gotten by with a vapor separator on the regulator, but if we step up to a handful of users does a dryer make sense?

 

[jkesselr]

You might try running your for calculations with 1/2 pex pipe. It's just a lot easier to work with.

I will do that. I just figured the size of the building would make sense at 3/4, but I will take a look at it.

 

[FullRaceMerc]

This. I should have read more before posting essentially the same thing.

Yeah, but you did a better job of explaining it.

 

[Shoreline_]

Your cfm lowers at higher pressure for given hp. A 40 cfm compressor which is about 10hp @ 90 psi (I think it's actually 38 cfm). At 175 psi it's less. Just keep that in mind. If you're gonna sandblast I highly recommend an air dryer at those cfm.

 

[jkesselr]

Your cfm lowers at higher pressure for given hp. A 40 cfm compressor which is about 10hp @ 90 psi (I think it's actually 38 cfm). At 175 psi it's less. Just keep that in mind. If you're gonna sandblast I highly recommend an air dryer at those cfm.

Thank you for the heads up. The compressors I am looking at are 31 or 38 CFM at 100 psi. How does that impact the discussion above?

Also, I have a large refrigerated air dryer.

 

[Shoreline_]

Thank you for the heads up. The compressors I am looking at are 31 or 38 CFM at 100 psi. How does that impact the discussion above?

Also, I have a large refrigerated air dryer.

It only affects if you plan on not having a little extra capacity in your pump. Some customers size their stuff right on the edge of their needed volume. In the real world 7.5hp should be about 28 cfm at 90 pounds. So with sandblasting go for the biggest hp your circuit can handle because storage won't make a difference.

 

[jkesselr]

It only affects if you plan on not having a little extra capacity in your pump. Some customers size their stuff right on the edge of their needed volume. In the real world 7.5hp should be about 28 cfm at 90 pounds. So with sandblasting go for the biggest hp your circuit can handle because storage won't make a difference.

Okay, good to know. So, if I understand you correctly, if I am blasting I won't see any material difference between a 7.5 hp 80 gallon unit and a 7.5 hp 120 gallon unit. Conversely, I would be better off with a 10hp 80 gallon unit, than a 7.5 hp 120 gallon unit?

 

[Shoreline_]

Yes because it will run non stop anyways so why pay for extra storage. At a certain point you will hit equilibrium where the air leaving the tank is as much as the air being pumped into it and at that point it doesn't even matter the size of the tank. Our sandblasting shops customers have small reservoirs compared to the size of the machine. They are basically to smoothen out the vfd command. Anyways If you can find a pump with continuous run option that would be even better. It'll have head unloaders.

 

[jkesselr]

Yes because it will run non stop anyways so why pay for extra storage. At a certain point you will hit equilibrium where the air leaving the tank is as much as the air being pumped into it and at that point it doesn't even matter the size of the tank. Our sandblasting shops customers have small reservoirs compared to the size of the machine. They are basically to smoothen out the vfd command. Anyways If you can find a pump with continuous run option that would be even better. It'll have head unloaders.

It sounds like you know your ****. Given that this is a 1 man weekend hobby shop (maybe 3 man when have a couple buddies over), can you recommend a compressor of those I am looking at? I presently have an old (1980s) IR T-30 80 gallon with a 7.5hp motor. I am looking to replace it with one of the Eaton Emax units in either 80 or 120 gallon and with a 7.5 hp motor or more. I only have single-phase power and am only replacing the IR due to its age (I am scared of an old tank) and noise level.

 

[Shoreline_]

Hey thanks Ive only been around this stuff professionally for 2 years but I have a great teacher and hes been doing it for a job since 1992 (Its my dad lol)

Those 80s T-30s are great units as long as you keep oil in them. If you're not sandblasting what is your average air consumption? Like what are you using the air for?

 

[jkesselr]

Sounds like you've been paying attention! I play with cars, so it could be blasting, plasma cutter, impacts, painting full cars, tire machine, various other air tools, airing up tires, etc.

 

[Shoreline_]

Does the IR keep up most of the time?

 

[Hohn]

Okay, good to know. So, if I understand you correctly, if I am blasting I won't see any material difference between a 7.5 hp 80 gallon unit and a 7.5 hp 120 gallon unit. Conversely, I would be better off with a 10hp 80 gallon unit, than a 7.5 hp 120 gallon unit?

You really need to think about 1) transient peaks, and 2) how long they last. In other words, don't think about flow rate in terms of CFM, you need to think in terms of total volume of air-- cubic feet or SCF.

It comes down to how much your air demand exceeds the compressors output. That determines the "storage burden." IN theory, you could make a 2hp compressor work for a 50SCFM load if you had a big enough tank and enough time to recharge it because the 50SCFM demand was short lived.

This is why you can run a big air impact off a little pancake compressor if you are only busting a couple lug nuts loose and not in a hurry. Heck, I have a 1hp 1 gallon compressor I keep in the house for just blowing things off with a blow gun or powering nailers and such. A short blast with a blow gun drops it from 90 psi to 40psi, but the job is done and it works just fine.

Most of the time our home shop air systems are using tank size to try to compensate for a shortage of compressor output. In the real world a 10hp 60 gallon unit would crush a 5hp 80 gallon compressor.

While we all would love to have a full 7.5hp or 10hp air system for our shops, the reality of residential power sets in and single phase 7.5hp or 10hp is a big hassle and super inefficient on single phase. The inrushes are massive and start stop usage is not a good idea.

IMO what makes more sense is a pair of 5hp compressors with one of them set for continuous run. Flip it on in the morning when you start your shop day, shut it off at the end of the day. I'm really loving the idea of remote tanks and baseplate/skidmount compressors these days.



This little Rolair is super compact (only #310) and very reasonable at $2400. If you want a constant run 5hp two stage, this is a really great option. Is it a Jenny or Champion? No. But in constant-run, you aren't likely to wear this out any time soon at only 8-15 hours per weekend and not every weekend. Use a good synthetic oil and it'll last awhile.

With one of these tankless 5hp units in constant run and loading at 120psi or so, you can then add additional compressor output and tanking as it makes sense to do so.

Like someone else said, used air tanks in the form of old compressors are a dime a dozen. No point in paying another $1000 or more for a shiny new air tank when you really need more pump and motor and not more tank.

Let's say you have a small 2hp 120v compressor and want more air. Add a 5hp tankless or wheelbarrow compressor in parallel to that 30 gallon unit and you'll have lot of useful compressor output with faster pump up time while still keeping the smaller unit running in start/stop when necessary. You'll need to set the cut in/out points intelligently (have your start stop cut in at a lower pressure and cut out at a higher pressure than your continuous unit unloader settings; this will prevent excessive starts on the start stop unit).

With some clever architecture, you can get 7hp worth of air on a combination 2hp/5hp setup, a legit 25CFM at 90psi in most cases, and you can do it in a more compact setup (no massive tank) that costs less and comes up to pressure much faster.

 

[Shoreline_]

You really need to think about 1) transient peaks, and 2) how long they last. In other words, don't think about flow rate in terms of CFM, you need to think in terms of total volume of air-- cubic feet or SCF.

It comes down to how much your air demand exceeds the compressors output. That determines the "storage burden." IN theory, you could make a 2hp compressor work for a 50SCFM load if you had a big enough tank and enough time to recharge it because the 50SCFM demand was short lived.

This is why you can run a big air impact off a little pancake compressor if you are only busting a couple lug nuts loose and not in a hurry. Heck, I have a 1hp 1 gallon compressor I keep in the house for just blowing things off with a blow gun or powering nailers and such. A short blast with a blow gun drops it from 90 psi to 40psi, but the job is done and it works just fine.

Most of the time our home shop air systems are using tank size to try to compensate for a shortage of compressor output. In the real world a 10hp 60 gallon unit would crush a 5hp 80 gallon compressor.

While we all would love to have a full 7.5hp or 10hp air system for our shops, the reality of residential power sets in and single phase 7.5hp or 10hp is a big hassle and super inefficient on single phase. The inrushes are massive and start stop usage is not a good idea.

IMO what makes more sense is a pair of 5hp compressors with one of them set for continuous run. Flip it on in the morning when you start your shop day, shut it off at the end of the day. I'm really loving the idea of remote tanks and baseplate/skidmount compressors these days.



This little Rolair is super compact (only #310) and very reasonable at $2400. If you want a constant run 5hp two stage, this is a really great option. Is it a Jenny or Champion? No. But in constant-run, you aren't likely to wear this out any time soon at only 8-15 hours per weekend and not every weekend. Use a good synthetic oil and it'll last awhile.

With one of these tankless 5hp units in constant run and loading at 120psi or so, you can then add additional compressor output and tanking as it makes sense to

This is a very long winded reply of just saying split your demand between two units. It's very common. Also The rolair is junk. I'd never recommend it unless it was free.

 

[Hohn]

ANother advantage of a divorced tank setup: you can plumb your setup with a high-cracking-pressure check valve on the air tank inlet and let the system piping come up to pressure BEFORE you fill the tank.

If you let your system sit for days at a time, and it has any slow leaks (and they always seem to) then it's really nice to have the compressor kick on and pressurize the piping FIRST and only then fill the tank.

 

[Hohn]

This is a very long winded reply of just saying split your demand between two units. It's very common. Also The rolair is junk. I'd never recommend it unless it was free.

I have no experience with Rolair personally, I'd have picked a different example if I could find some other 5hp electric wheelbarrow to illustrate the idea.

Baseplate/skids IMO have a lot to recommend them.

I used to play combo amps for electric guitar, but then I switched to separate heads and cabinets and man I was so happy I did. A skid compressor is sort of the shop air version of that epiphany.
And me? Long winded? Why, I never! (edit effectively).