Buying a used air compressor for plasma cutting or any other shop air tool

[jimcolt]

Questions regarding air compressor sizes and capacities are often in my email inbox, as well as on this and many other forums. Since I recently had the need to replace my 30+ year old shop compressor I decided to write a post showing what I am doing to upgrade the air system in my shop. I am running on a backup compressor that I had...just in case my old one failed, so I am not in any particular hurry to get a new air system installed. This will be an ongoing thread as I slowly replace the air system in my shop!

If any of you follow my posts you will know I am a fan of buying used equipment. Craigslist and Ebay (and a few others) are great places to shop for good quality, used shop equipment of almost any type. Since my home shop is a hobby shop it is not necessary for me to buy everything new....so in this thread I will discuss comparisons between two compressors I just bought on Craigslist and Ebay respectively, as well as eventually describing two other compressors that I own, one a very small compressor designed for nail guns, another (the backup shop compressor I have been running on for a few weeks) is a used Craigslist compressor that I bought a few years ago with this scenario in mind (failure of my old shop compressor).

OK....the old shop compressor was one that I bought in the early 1980's, a "5 hp" Campbell Hausfield with a 60 gallon vertical tank. I remember paying around $450 for it brand new from Northern Hydraulics, now Northern Tool. It was a big by for me...in my early 20's. The most expensive tool in my shop. My limitation was the 100 amp single phase power entrance that had to be shared with lights, and all other tools (Welders, power tools, drill press). Yes, I worked for Hypertherm when I bought this compressor, but I did not have a plasma....as Hypertherm did not produce air plasma cutters until about 1984. My need was to power air tools for my stock car building, fabricating and racing hobby. More about the old compressor later.....it now is tired, rusty, and needs a new motor and maybe even a new pump. When I look closer I may condemn the tank as unsafe.....so it is up for replacement.

So, a quick shopping session on the New Hampshire Craigslist and I found a compressor that was very similar to my old one, it was a 60 gallon, "6HP" Devilbus branded compressor. The ad stated that the compressor pump was noisy, but otherwise it had low hours and looked good, and it would come with a new Harbor Freight replacement compressor pump. I contacted the seller for more details regarding the data tag on the motor as well as the specs on the compressor tank, it turned out it was 240 volts, single phase, a 3450 RPM motor, and had a flow rating of 10.2 cfm @ 90 psi. Similar to my old compressor. The seller bought the compressor used, wired it up and determined it was too noisy.....so he ordered the new pump ($139 from HF), then decided he did not know how to install it (different pulleys, belt styles made it look difficult). I bought it for $100 with the new pump.



I brought it home, pressure washed it, then wired it up for a test. The unit was no noisier than any similar compressor....so I suspect the buyer just was not familiar with this type of high speed compressor.....when you see a 3450 RPM motor on a "low cost" compressor of this style, expect it to be quite noisy. While cycle testing I noticed that the on cycle started at about 100 psi, the off was at around 135 psi. Looking up the specs online, as well as the tag on the receiver (tank), this unit was factory designed to operate from 90 to 125 psi......so I tried the adjusting screws on the pressure switch....and quickly determined the switch was junk. I keep a compressor switch in stock, although there are a ton of them available on Ebay for less than $25, so I replaced the switch and adjusted the compressor cycle to start at 90 psi and stop at 120 psi, just as my old compressor did. The belt on the compressor was in good shape....but the belt guard was missing, so I fired up the cnc plasma and fabricated a nuce sheet metal belt guard from galvanized sheet. Next...I noticed that the system had a few leaks....so I did a bit of replumbing, replacing the outlet quick disconnects and tightened loose connections between the pump and the receiver, as well as the unloader valve connections between the pump and the pressure switch. No leaks, my new (used) $100 compressor works perfect, and I still have the brand new replacement pump in the box for future use. I may use it to build a gas powered compressor....

A few more pics of this unit:



This compressor is ready.....but wait, a friend just tipped me off of an Ebay auction for a NH based industrial quality compressor.....

Jim Colt

[wb4rt]

Jim - good post. Good information, thanks.

I'm sure someone near you will give you $100 for that compressor if you decide to go for the industrial one! I know I would.

[jimcolt]

Lets talk about specifications on this compressor.....what do they mean? The big joke on many compressors is the horsepower rating, and as you can see from my original post regarding this used compressor....the tag on the front says "6HP, 60 gallon". Now look at the motor nameplate, specifically at the Amperage....it says 15 Amps. Now if we do a little math we can multiply the Amperage x the Voltage.....which will give us a rough indication of the Wattage or "power" that this unit uses. so 240 (volts) x 15 (amps) = 3600 watts. Horsepower and Watts are both measures of relative power......and there are 745 Watts per horsepower. So, if we do a little more math....we take the calculated Watts, 3600 and divide by 745, we get 4.83 horsepower. In reality, with losses from friction and heat this is probably about a 4 hp compressor.

Whats the point? Don't compare horsepower between compressors.....some manufacturers use different ways to calculate horsepower so they can use a bigger number in their sales brochure. Use the motor amperage draw, multiply it by the rated voltage, then compare the resulting number (which is wattage) as a fair power to power comparison.

I saw a compressor advertised in Home Depot the other day.....it claimed to be 6 HP, and it ran on 120 volts with a 13.5 amp motor amperage. 120 x 13.5 = 1620 watts, divide that by 745 (watts per horsepower) and you get 2.17 HP. Keep in mind that any compressor that operates on a 120 volt household circuit (20 amps) will produce less than 3 Horsepower as a standard 120 volt circuit can only produce around 2400 watts maximum.

Jim

[Slob]

Jim, I've been partial to Quincy compressors for over 40 years and the QR-25 series will serve you well. My primary compressor in the shop is over 37 years old, (purchased new) being rebuilt once in that timeframe working six guys full time in an autobody shop. The basic design has been around since 1935 using disc valves. This is a 900rpm compressor pump using a 5hp single phase drive and they are very quiet, and run cool in operation. These are also positive displacement oil pump pressurized lubrication systems. Spin on oil filters are an option of which all three of mine utilize. Everything, and I mean literally everything is supported, (parts wise) for these compressors. The air delivery at the pressures indicated on the placard of your photo are near double.

As you are probably aware, the best thing you can do after filtering the compressor discharge air is to run it through a refrigerated air dry prior to end point use by a tool no matter what that tool is.

I've seen and read a lot of your postings on other websites through the years. I too have a plasma setup not erected and I'm certain we will talk more later.

Thanks,

[jimcolt]

OK......so I looked at the Ebay ad for a "Kellogg- American" 5 HP compressor. It said it had some filters and a "heater". It was on auction and there was only 45 minutes left...the compressor was located about an hour from my shop. I tried contacting the seller to ask some important questions....Is the motor single phase or 3 phase (I have single phase power as most US residences do). What is the receiver (tank) size? Does it run?, etc. I got no response. The Ebay ad had very little info and a couple of fuzzy pictures. I do know that Kellogg makes very nice (read very expensive) 100% duty cycle industrial compressors, and I know that most of them are powered with 3 phase motors. You can replace a 3 phase motor with a single phase, however these units use 100% duty cycle low rpm (1725 rpm) motors.....which are next to impossible to find used, and rather expensive new.

A little research showed that a new 5 HP rated Kellogg compressor would be priced at about $2400. I searched other used ones on Ebay....used units of the same size would be in the $1200 to $1600 range.....with single phase motors. I took a chance, as the vast majority of people in NH are brutally honest and trustworthy... and I followed the bidding right to the end.....and I bought the Kellogg compressor sight unseen for $237.50. I jumped in my truck and drove to pick it up.

I arrived at the sellers location and he informed me that he was disabled and he could not help me load the compressor (over 600 lbs), but he would show me where it was in his cluttered shop. The compressor appeared in excellent shape, it had a single phase motor, it had a set of new particulate and dessicant dryer filters, unused, and the big bonus....what he called a "heater" was a refrigerated air drying system that appeared in excellent condition. The seller said he had bought it all (used) a few years prior...and never even powered it up. Before he could think about it...I had everything loaded and strapped in my truck (yes, a 600 lb compressor was loaded by 58 year old Jim, by hand with a rope and some planks), and I was on the road quickly.

Here it is setting next to the compressor we have already discussed...does it look more industrial? The quick math on horsepower and wattage based on the pictured motor nameplate: 23 amps x 240 volts = 5520 watts. 5520 divided by 745 =7.4 HP. The nameplate says 6 HP......so the manufacture likely has (honestly) rated this unit as a 6 HP compressor after friction and heat related losses. This is real 6HP, the other one is not!



No pics yet of the filters and air dryer....I plugged in the dryer (120 volt) and it powered up and the internal compressor quickly cooled down the output line....I suspect it will work fine. It is equipped with an auto drain for accumulated moisture.

More pics of the compressor:



I wired up the Kellogg, checked the oil level (looks clean and full) and powered it up. Very quiet....does not make nearly the noise as compared to the Devilbus. This is due to the heavy duty two stage pump (Kelloggs are one of the best) and low RPM (motor RPM is 1/2 that of the Devilbus) and much lower pump speed. The ratings for this unit (found online from the stock number) are 20 cfm 90 psi, pretty much twice the air capacity of the Devilbus. Do I need that much air? No. But....I like the low speed and the quiet operation. The decision is made, Kellogg stays in Jim's shop, the Devilbus goes up for sale on Craigslist.

Devibus sold to the first responder for $325. I still have the new replacement pump that came with it. So, I now have a big industrial compressor, and air dryer (I already have one, so one will go on the block), some filtration, and a spare pump for future use.....and I have over $50 bucks more in my pocket than I started with. I love Craigslist and Ebay.

In the next post I will compare the specs (HP, Wattage, CFM, Pressure, etc) between the devilbus unit and the Kellogg. I think the Kellogg may cost more to run, however it may run less......which means it may be more economical.....we'll see!

Jim

[jimcolt]

Thought I would show the "backup" compressor that I have been using in my shop since my big old one failed a few weeks back:




I bought this unit on Craigslist 3 or so years ago for $75. It appeared to have little use however it had a 3 phase motor.....I had a perfect 2 HP (real 2HP!) single phase motor in my shop so I bought it, refurbished the plumbing and installed the single phase motor. This is an old industrial grade compressor from Graingers (SpeedAire is their compressor brand)....and is a low speed two stage compressor. It provides adequate air for my Powermax85 cnc plasma...but runs continuously when I cut. It can be wired for 120 volt (draws about 17.5 amps) or 240 volt.....which is what I am running it on. This size compressor is more than adequate for many shops with one man, oops, person working. It will not handle sandblasting, or a lot of air powered sanding as it is only about a 6.5 cfm @90 psi flow rating.

Again, in future posts I will cover the specs in regards to pressure and flow.

Jim

[rlitman]

Holy crap, that deserves a huge you suck! The Baldor motor is easily worth twice what you paid for that compressor. Congrats on the find!
Just a critique on your math, you also need to multiply the amps by the motor power factor to get to true watts. Otherwise you only get VA, which is not a measure of power (either on your electric meter, or delivered through torque).

Cost to run is probably not worth considering in a home shop, but if you really want to compare, just look at the NEMA efficiency on the motor plate.
The upgrade from single to double stage also helps a little in efficiency.

The real electrical cost in running an air compressor in a home shop is due to air losses. Even a tiny leak can easily become 99% of your air use if you're only doing weekend work. It's like a dripping faucet, but not as easy to see. I have good quick connects that do not leak. If they show signs of leaking, I replace them.
I've got ball valves sectioning off my plumbing, so I never have to drain all pressure from the tank, and ball valves in front of a few plumbed in thinks that are known to leak (if I leave the valve to my pneumatic chain hoist open, it will cycle the compressor about every 10 minutes).

20CFM seems like a lot, until you use a DA sander that needs 16-17 to run continuously (which is something you do a lot with a sander).

[jimcolt]

Thanks for the info regarding power factor. Many compressor motors do not even show it....so you have to "dead reckon" a bit using just the wattage.....often close enough for comparison! I do feel as though I stole these compressors.....although it was all legal!

Jim

Holy crap, that deserves a huge you suck! The Baldor motor is easily worth twice what you paid for that compressor. Congrats on the find!
Just a critique on your math, you also need to multiply the amps by the motor power factor to get to true watts. Otherwise you only get VA, which is not a measure of power (either on your electric meter, or delivered through torque).

Cost to run is probably not worth considering in a home shop, but if you really want to compare, just look at the NEMA efficiency on the motor plate.
The upgrade from single to double stage also helps a little in efficiency.

20CFM seems like a lot, until you use a DA sander that needs 16-17 to run continuously (which is something you do a lot with a sander).

[cope]

Your two stage unit will put out drier air than a single stage. I decided to save ±$150 in 1990 and bought a 60 gallon single stage compressor instead of an 80 gallon two stage. I have regretted the decision ever since, but I am 66 years old and cannot justify upgrading unless I did it like you did.

[rlitman]

True enough. A good compressor such as yours will have a tank, a pump, and a motor, which all carry their own ratings (ASME for the tank, NEMA motor, etc).
Most of the oilless garbage being sold is made like a vacuum cleaner. Yeah, it has the same components as above, but it isn't made to buy a commodity motor and stick it on. Like the granola bar that comes in the box, with each bar marked "not labeled for individual sale", the parts in cheap compressors may not have ratings plates.

Anyway, you're completely right about horsepower if you look at it this way: Motor amperage sets an upper limit on true horsepower. The actual power output will be lower than what you calculated, but will never be equal or higher.

The other thing to consider is that while CFM ratings are an easier point of comparison between compressors, not all compressors are designed for continuous duty. The GE and Baldor motors pictured above both are, but lesser machines would probably not be. The GE motor is rated for continuous duty at the nameplate current, but no more. The Baldor is rated to run at 1.15 times that, so it is designed to accommodate being overloaded. Little details like that can add up to failures when you're pushing your equipment to the limits.

Anyway, the best part of your find is that it is single phase. Real 5HP single phase motors in that size are pricey, and hard to find. Single phase tooling sells fast, because it's what home shop users need.

[rlitman]

Your two stage unit will put out drier air than a single stage. I decided to save ±$150 in 1990 and bought a 60 gallon single stage compressor instead of an 80 gallon two stage. I have regretted the decision ever since, but I am 66 years old and cannot justify upgrading unless I did it like you did.

Possibly. That's a difference I had not really considered, but there is probably some condensation happening in the intercooler tube. I have an 80 gallon two stage now, and have not had moisture issues. But I don't recall getting more or less water out of the tank than when I had a 60 gallon single stage.

[tnjind]

Moisture in the air is the same no matter what type compressor compresses it.

Jimcolt- nice score

[rlitman]

Moisture in the air is the same no matter what type compressor compresses it.

Not so fast. Read this:
http://cascousa.com/compressed-air-1...e-compressors/

In a two stage piston compressor, the air leaving the first stage is hot. As it travels through the intercooler, it drops in temperature, which can condense out water. Liquid water can then pass through the second stage, and drop to the bottom of the tank, leaving the air in the tank ever so slightly dryer. Of course, it would work much better with a water separator stage (with a drain) in the intercooler, and I'm not sure this will get anything that an after cooler would get (which is also an effective method of removing water), but most compressors do not come with aftercoolers, while most two stage compressors do come with intercoolers.

How much it will help though is a matter for debate.

[jimcolt]

I think if the compressor has cooling before the receiver (tank) then some of the water condenses into larger, heavier particles and pools in the bottom of the tank, which leaves a bit less suspended in the air....which comes out of the receiver near the top. As I work on my new shop air system I will address all of this including cooling, filtering, receiver draining, etc.

Bottom line...cooling the air does little unless trapped moisture is removed from the system with some type of draining device!

Jim

Not so fast. Read this:
http://cascousa.com/compressed-air-1...e-compressors/

In a two stage piston compressor, the air leaving the first stage is hot. As it travels through the intercooler, it drops in temperature, which can condense out water. Liquid water can then pass through the second stage, and drop to the bottom of the tank, leaving the air in the tank ever so slightly dryer. Of course, it would work much better with a water separator stage (with a drain) in the intercooler, and I'm not sure this will get anything that an after cooler would get (which is also an effective method of removing water), but most compressors do not come with aftercoolers, while most two stage compressors do come with intercoolers.

How much it will help though is a matter for debate.

[cope]

As I work on my new shop air system I will address all of this including cooling, filtering, receiver draining, etc.

Bottom line...cooling the air does little unless trapped moisture is removed from the system with some type of draining device!

Jim

I bought a Harbor Freight auto drain, and it has quit working in less than 6 months. I am going to get one of these.

http://www.ecompressedair.com/drain-...ain-valve.aspx

[tnjind]

Hot moist air does indeed need to cool to condense the water. Refrigerated dryer is often used.

[killdozerd11]

After looking at the deals you got i have determined that you double suk Jim

[MinnesotaDave]

After looking at the deals you got i have determined that you double suk Jim

Triple suck

nice compressors Jim - and interesting write ups on both

[12345678910]

Good topic and post, but you must be the only guy doing well witih classified ads.

[jimcolt]

The key to buying good used equipment on Craigslist and other forums is to know what you are looking for....and to have patience. Don't buy without research, and wait for the deal to come to you.

Jim

Good topic and post, but you must be the only guy doing well witih classified ads.

[Pipeliner]

....and to have patience.

Hmm...........if I could only buy a 40 yard roll off dumpster load of that I would be good to go!

I want I want I want NOW NOW NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOW!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[GlenC]

Jim, nice find!
You mentioned setting the pressure switch to 90psi... You really don't want to do that. Most tools are designed to run at 90psi, if you are using a sander or any tool and the compressor kicks in at 90psi, you will most likely drop below 90psi during use, reducing performance.
For a shop system, 100/135-145 for single stage and 110/165-175 for two stage should work well. Two stage installations usually have lower cfm and can use more room to stay ahead of usage.

Funny thing, I have a Atlas Copco 5HP two stage direct drive 175psi and have been thinking I should have gotten a higher volume single stage....

[storeman]

http://weldingweb.com/vbb/showthread.php...f-CL-yesterday

Jim, I got lucky on CL yesterday. Made in USA mid-1980s Ingersoll Rand 60 gallon compressor, 2-stage, 18 amp 230v true 5hp Ingersoll Rand single phase motor. Pressure cut of switch was non-operational but it ran perfectly when the switch was jumped. A cheap tank pressure gauge was also non-op. Replaced the pressure gauge with an oil-filled one I had and ordered a new cut off switch last night. Brand decal is missing except for the "Energair 1". All paint is original. No rust in the tank moisture. $250.00

I've found reference to it's model number 3B30E6VB but can't find an owner's manual or parts list. I pressurized it to 150 lbs, backed it down to about 125 and it held overnight with no pressure loss. No SCFM data listed but I think it will do 10-12 CFM, which will be a huge improvement over my 10 year old Craftsman 25 gal 110 unit.
Jerry

[DougAustinTX]

Man, I have NEVER gotten a deal like that... and I have all of Texas to scrounge. I think you guys who live up north of the Mason-Dixon have way better access to stuff than we do down here. I have been looking for a nice Hit-Miss engine for years and all the ones I find are way up north of me. Same goes for finding a decent anvil. The advantage we have here though is you can buy a 40 year old car with no rust...heh.

[storeman]

Man, I have NEVER gotten a deal like that... and I have all of Texas to scrounge. I think you guys who live up north of the Mason-Dixon have way better access to stuff than we do down here. I have been looking for a nice Hit-Miss engine for years and all the ones I find are way up north of me. Same goes for finding a decent anvil. The advantage we have here though is you can buy a 40 year old car with no rust...heh.

A decent anvil is high on my list to complete the shop.
Jerry