Buying a used air compressor

June 10, 2009 11:47 PM

I am thinking about buying a used one of these, and I need to get an air compressor for it. Where can I find used air compressors? What do I need to know about air compressors before buying?

I know this much so far: I need a 110v or 220v electric compressor, capable of 110 psi and 22cfm (probably around 7.5hp).

I have been told to get an 80+ gallon tank, and 2-stage instead of 1-stage.

Noise is a potential issue, are certain compressors louder?

Any help locating one of these is greatly appreciated! (Also any operating or maintenance tips, horror stories, etc.)

posted by Jsn7821 to grab bag (19 comments total)

Googled and found this:

"278-318VL Electric 7.5 hp, 32.5 cfm Air Compressor" from here. (I know nothing about this Company).
And yes, air compressors can be noisy.
posted by lungtaworld at 1:31 AM on June 11

Apologies, I see you need a used one...........
posted by lungtaworld at 1:33 AM on June 11

Noise is a potential issue, are certain compressors louder?

If by "certain compressors", you mean "nearly all of them", then yes. Not ear-plugs loud, but probably what-did-you-say-speak-up loud and definitely annoy-the-shit-out-of-adjoining-apartments loud. Some of them are actually ear-plugs loud, though. I've seen some quiet compressors (like you can still have a conversation around them), but it's an intentional design feature, and it radically increases the price. There may be some that are just quiet by happenstance, though, that don't have the accompanying price hike.

The other thing to know about air compressors is that many of them have fairly short lifespans. I've never owned one that lasted more than two or three years with regular use; but, those have all been consumer level single-stage compressors. It may be that a pro quality version will last longer, or at least be fixable instead of disposable.

For reliable 110psi, you do need a two-state compressor. I don't know about tank requirements or flow rates.

As for where to get it used: I'd just trawl craigslist until you find something that suits. Auctions, estate sales, maybe garage sales (with very questionable quality, probably).
posted by Netzapper at 2:02 AM on June 11

Okay, that's a big compressor. 22 CFM is roughly the output of a 6hp compressor running continuously; a 7.5 hp compressor is indeed a better choice.

One thing you should know is that compressor manufacturers lie about horsepower ratings. Much like they do with vacuum cleaners, cheap table saws and the like; they talk about "peak horsepower" or "developed horsepower" which is worthless as an indicator of what the equipment can actually handle. (This may not be as much of a problem with industrial-level equipment like you need as it is in consumer-grade stuff.) Expect a real 7.5 HP compressor to have a motor rated somewhere north of 30 amps at 220V single-phase. (And it will be 220V. Definitely.)

Note that you'll probably need a 40-amp 220V dedicated circuit for this.

Be careful not to buy 3-phase equipment unless you're setting this up in an industrial building that has 3-phase service.

Most compressors of this size will have oil-lubricated cast-iron pumps. Avoid anything aluminum or "oil-less" as these are hideously noisy. Beyond that, it's better to find a model that uses poppet valves (like those in a car's engine), because these are quieter than the cheaper and more-common reed valve types. That said, my 5HP reed-valve model is not much noisier than a washing machine on the spin cycle.

I don't see the need for a 2-stage compressor here. 110 PSI is well within the reach of single-stage compressors.

For reference, I have a 5-horse compressor that was about 35 years old when I bought it for $300. It had been abused and mostly forgotten, and needed some immediate repairs, but I could tell by looking that the problems weren't serious. I probably spent $150 or so on parts, and fixed it up myself. You need a bigger compressor, and you probably need it to be in better condition; I'd guess you're going to be spending over $1000 for anything worth having.
posted by jon1270 at 4:46 AM on June 11

On further reflection, a 2-stage compressor may be a good idea. If you need 110 PSI all the time, that's pushing the envelope for single-stage machines.
posted by jon1270 at 4:49 AM on June 11

General, not specific thoughts (after converting your specs into terms I know - cubic metres, litres/minute, etc):

Be aware that the delivery rate is often quoted as "F.A.D." (Free Air Delivery) - that is, without any backpressure. Depending on how the press is rated (does it require 22cfm@110psi or 22cfm F.A.D. equivalent?), you might need to go considerably higher.

Cheap compressors for home & trade use are often ... um, 'optimistically' ... rated. Kinda like the "200W PMPO" ghetto blaster that's got 2 x 5watt amps in it. Also, that 'optimistic' rating is often for short-term delivery with a near-100% pump duty cycle - it can deliver something that it alleges may occasionally come close to 22cfm, but only for 30 seconds at a time, and the compressor then takes 2 minutes continuous running to top up the tank. I suspect something like that press is likely to want 22cfm@110psi most of the time it's running.

That's actually a fair-sized compressor you're after - not "comes on its own trailer" sized, but definitely shop-sized. Not the sort of thing you buy from Home Depot for $100 to pump up your tyres and run the odd spray gun, or that tradies cart around from job to job in the back of the ute to power their air tools. It's "I'm a small panel shop that needs to feed a couple of sprayguns &/or air spanners, most of the day" sized.

I'd seriously suggest getting the full specs on the requirements of that press (continuous rate, duty cycle, etc) - then going to a proper industrial supplier and talking to them, looking at a few on the shop floor, etc. Even if you don't end up buying from them, you'll have a much better idea of what you're looking for in the used market. It's likely to be a 2 stage or screw compressor (screw compressors are quieter, btw), with a delivery rate more like 50% higher than you're expecting (e.g. 34cfm or more), and not particularly transportable.

Oh, and I dunno exactly how such things go with US 110V/220V power, but here in 240V land a compressor that can genuinely put out 22cfm@110psi continuously, without the pump running all the time, is bordering on requiring 3-phase power.
posted by Pinback at 5:14 AM on June 11

Industrial compressors do last an awful lot longer than consumer ones. However, unless you buy an incredibly expensive one, it will be almost certainly be well within that which you would consider noisy.

If noise is an issue, you could cheaply build some sort of acoustic damping box, as long as you were aware of the heat dissipation requirements and air flow needs of the unit, but if noise is a major problem, you need a screw-type compressor. These are fantastic, and don't make much more noise than either a loud hum (or that of a car idling at absolute worst, to try and hit a comparison).

Second hand/quiet/lots of pressure and CFM requirements like this tend to get a little bit mutually exclusive if you really do want to keep costs down. In addition, good compressors are an investment - the more you spend, the more it'll look after you.
posted by Brockles at 5:18 AM on June 11

Yes, compressors are (usually) quite noisy. In most shops this is dealt with by putting the compressor outdoors, or in another room, or by building a little closet (with adequate ventilation) for it. If you are in an apartment building, think carefully about whether you can run such a large piece of equipment (not to mention the textile printer) without angering your neighbors.
posted by Forktine at 6:04 AM on June 11

A few thoughts:

• You should get a SCFM requirement from the manufacturer as CFMs are semi meaningless. Buy at least a bit more than that one piece of equipment requires as once you have a ready source of air you'll find all sorts of uses for it. Just don't use it to blow stuff off.
• Don't know your setup but if at all possible you'd want this equipment in it's own room or outside as the noise will be objectionable.
• Plumb your connections with Type L copper or black iron pipe. Do Not use PVC as there is a risk of breakage that will send sharp shards of pvc in all directions. OSHA won't allow it in shops they regulate.
• Any industrial supplier that sells new equipment will often have trade ins. I'd just open the phone book and make a few inquires. This compressor and tank is going to be heavy enough that you'll want to shop locally if you can.
• Depending on the age of the tank you might want to consider having it tested. Or ar least pull out one of the larger bungs and make sure it isn't heavily rusted or pitted inside.
• A vertical tank will take up less room than a horizontal tank but requires more work to secure. You'll probably be limited to whats available but if you have a choice that is the trade off.
• Even if you have to put the compressor inside routing your intake hose to the outside (make sure you have a filter at the free end of the intake) will reduce inside noise levels inside.
• You'll need serious 220V outlet for this compressor.
• Change the oil when you get the compressor and then at regular intervals. I generally change mine every year but that is under hobby conditions. If you are running this unit continuously you'll want a more frequent change interval.
• Don't buy more comprssor than you need, you'll just be wasting electricity. You say you need 110PSI so get a compressor that produces ~125psi. Avoid a ~175PSI compressor. IE: minimize the amount you need to regulate.
• Find out from your equipment manufacturer how dry the air needs to be. At a minimum you'll want to run a two stage filter/separator. A good practise is to construct a radiator immediately up stream of your water separator. A simple radiator is just lengths of pipe zigzagging back and forth on the wall next to the compressor. 50-100' of copper is adequate. The radiator allows water in the air to condense out before it gets to the separator.
• If the air has to be dryer than that you'll need to look at desiccant systems or refrigerated dryers.

posted by Mitheral at 8:01 AM on June 11

Thanks, all of this is very good information so far.

Do any of these look like good deals?

http://seattle.craigslist.org/skc/for/1214891571.html

http://boise.craigslist.org/tls/1200159243.html

http://sfbay.craigslist.org/pen/tls/1207473155.html

http://seattle.craigslist.org/tac/tls/1210467617.html

On the last one, when it's "17.2 cfm 175 psi", would that work for 22cfm 110psi or no? In other words if I run it at a lower PSI do I get more CFM out of it?
posted by Jsn7821 at 1:56 PM on June 11

You can't trade pressure for flow. Higher pressure in your tank will let you draw excess CFMs from the tank for short periods but your machine has a continuous draw.

The first compressor is 25hp which is 125As at 240V but undoubtedly has a 3 phase motor.

The twistair is a scroll which is better and quieter than a piston for continuous duty. Still 10hp so you'd need 50As at 240V. It also likely has a 3 phase motor.
posted by Mitheral at 4:12 PM on June 11

On the last one, when it's "17.2 cfm 175 psi", would that work for 22cfm 110psi or no?

No. Running it at lower pressure you might get a bit more volume out of it, but not that much. For example, my 5HP compressor is rated for 16.5 CFM @ 175 PSI, or 19.1 CFM @125 PSI. Furthermore, getting that extra couple of CFM requires changing a pulley and belts, not just flipping a switch.

Careful about the Porter Cable model - it's the only one here likely to have a cacophonous aluminum pump and inflated horsepower claims.
posted by jon1270 at 5:19 PM on June 11

I bet that PC model is just like this one. Notice that this "7 HP" compressor can't even manage 10 CFM @ 90 PSI. Worthless for your purposes (and mine).
posted by jon1270 at 5:30 PM on June 11

If that Sullair wasn't out of your price range, this might be a possibility.
posted by jon1270 at 5:39 PM on June 11

You have not mentioned an air dryer anywhere. This is CRITICAL. You may also need a sepreate oil coalescer / air filter before the dryer. The Tuf. vendor needs to tell you a couple of things
1. Air pressure / flow in scfm.
2. Air Quality: ppm oil, dew point, max ppm of any other particulates. With this information you will need to go and get a filter/dryer
(Line about the dryer here: http://www.tufproducts.com/parts/service/m1.cfm)
posted by defcom1 at 8:09 PM on June 11

Whoops, I meant to include max size of particulates. Memail me if you need help with the details, I've designed (industrial) instrument and plant air systems.
posted by defcom1 at 8:22 PM on June 11

I believe the automatic that I am looking at has a chiller included with it, so I am hoping that will suit the presses needs. I will call Tuf tomorrow and ask them for details, thanks for the heads up.

The Snap On 80 gallon looks great, and it is very close by which is a big plus. It's on the very high end of my budget though, I was hoping to spend closer to $1,000. Does anyone know what the noise of this compressor compares to others? For $2,400 should I keep looking for an older screw compressor, or is the fact that this one is so new worth the extra money?

Will I even be able to find a 1-phase screw compressor that fits my power requirements? All of them seem to be 3-phase.
posted by Jsn7821 at 9:37 PM on June 11

They do exist, but they're not common. I'd also expect used ones to be priced higher than similar 3-phase units. If you're hoping to find a single-phase rotary compressor, in good condition, for ~$1000, I think you're likely to be disappointed.

Before you get stuck on the idea of rotary, you should make the trip to hear a few of these things run. A good iron piston-type pump may be fine.
posted by jon1270 at 3:59 AM on June 12

I went with the Snap On 80 gallon, 7hp, 175 psi, etc. The owner just came and dropped it off this evening. It's practically brand new and I didn't break the bank too bad (it was about twice the price I was hoping for though, but I feel good about it). I just plugged it in and it's not much louder than my conveyor dryer with it's 2hp fans!

Thank you Jon1270 for bringing that to my attention. I missed it since I capped the search-price at 2,000 on Craigslist!
posted by Jsn7821 at 3:26 AM on June 13