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Old 11-21-2011, 11:00 AM   #1
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Air compressor build....help, opinions?


Hello all, great forum...I'm more of an automotive buff, but building cool cars eventually leads to needing a cool shop...in my brief browsing of DIY Chat, it seems I've come to the right place.

A little backstory:

On a recent trip to the salvage yard, I picked up an 80gal compressor tank and pump for scrap price.

The pump - Unknown specs, but I was assured I can sell it back for scrap price if I don't use it. It's a single head unit, looks rather old.

The tank - in great shape inside and out, so I at least have that part covered.

I have an unused 220v line in my shop, but I rent, so I'd like a motor that can use 110v if needed.

I need to know where to go from here.

I'd like to be able to use: body saw, angle grinder, air shears, air drill and my blast cabinet for now....and an HVLP gun and plasma cutter when my lottery numbers hit.

The motor - http://www.harborfreight.com/2-horse...tor-67842.html , this is the one I was considering, they make a 3hp version, but it is 220v only. Will this be enough?

The pressure switch - http://www.ebay.com/itm/Air-Compress...ht_1180wt_1396

What I need is wiring/plumbing info and opinions on ways to improve my set-up.

Any help is greatly appreciated, thank you very much.
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Old 11-21-2011, 11:21 AM   #2
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Air compressor build....help, opinions?


without going to extremes, any motor can be used for any pump. Its the size of the pulleys you use. If you use too small of motor, it will have to spin the pump too slow and your tank will never fill up. If your pump is too small for the tank, you will burn up the pump because it will have to work too hard. That being said, most 80gallon compressors have at minimum 3 hp motors, but more commonly 5hp and up.


Don't plan on ever using this on a 110v 20 amp circuit, 80 gallons is just too much air for that.


I know you already have stuff youd like to use, but in all honesty, youd probably be better off finding a good used setup. I paid 200 each for the two compressors in my shop. Both are 3hp, 60 gallon, twin cylinder setups
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Old 11-21-2011, 11:43 AM   #3
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Air compressor build....help, opinions?


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Originally Posted by jimmy21 View Post
without going to extremes, any motor can be used for any pump. Its the size of the pulleys you use. If you use too small of motor, it will have to spin the pump too slow and your tank will never fill up. If your pump is too small for the tank, you will burn up the pump because it will have to work too hard. That being said, most 80gallon compressors have at minimum 3 hp motors, but more commonly 5hp and up.


Don't plan on ever using this on a 110v 20 amp circuit, 80 gallons is just too much air for that.


I know you already have stuff youd like to use, but in all honesty, youd probably be better off finding a good used setup. I paid 200 each for the two compressors in my shop. Both are 3hp, 60 gallon, twin cylinder setups
I've had my eye out for a deal like that, if I find one, I'll happily pick it up, but I haven't had much luck. I also prefer this tank because it's horizontal, it fits well in my shop.

If using this tank on 110v is out of the question, I can live with the price jump to 5hp.

Is there a "formula" or some way of figuring the appropriate pulley size? The pump was attached to the tank so that leads me to think on e of two things.

The pump is too small and burnt out, hence the motor was removed and the rest scraped.

or

I worked, but wasn't enough for the previous owner's needs.

The marks and stuff on the pump and tank lead me to believe the pump has been bolted to the tank for a long time.

I appreciate your advice and will keep looking for a deal, but I haven't had much success.

PS- I also have a 5.5hp Emglo wheelbarrow compressor I guess I could use to take some of the load off the pump if I was gonna use the compressor a lot.

Last edited by TurboSE; 11-21-2011 at 11:46 AM.
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Old 11-21-2011, 11:51 AM   #4
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Air compressor build....help, opinions?


Air compressor motors are sized by the pump specifications. Without knowing the pump manufacturer and model, motor size is only a guess. Just an opinion, but based on the number of DIYer's that have air compressors, there is a good demand for good used ones, so the fact that somebody scrapped it, rather than selling it as an air compressor, even if the motor were bad, would cause me to believe that the pump and/or tank are bad. And, old tanks, by the way, which may have been neglected, meaning not drained regularly, can be dangerous.
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Old 11-21-2011, 11:55 AM   #5
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Originally Posted by DexterII View Post
Air compressor motors are sized by the pump specifications. Without knowing the pump manufacturer and model, motor size is only a guess. Just an opinion, but based on the number of DIYer's that have air compressors, there is a good demand for good used ones, so the fact that somebody scrapped it, rather than selling it as an air compressor, even if the motor were bad, would cause me to believe that the pump and/or tank are bad. And, old tanks, by the way, which may have been neglected, meaning not drained regularly, can be dangerous.
Thats why I'm assuming the pump is bad. The tank has two large (3.5") plugs on the front that I removed and looked in with a flashlight and felt around...there was no sign of rust and very little surface rust on the outside.

Are there any ill effects to be had if I use a 5hp motor with a smaller pump?
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Old 11-21-2011, 12:01 PM   #6
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Air compressor build....help, opinions?


Probably not, other than electic use. What RPM are you going to run it at?
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Old 11-21-2011, 12:09 PM   #7
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Are there any ill effects to be had if I use a 5hp motor with a smaller pump?
no. id get a 5hp. That way it leaves room to get a bigger pump

i have no idea on figuring out pully size. Other than maybe figuring out the rpm of the motor, and the rpm the pump should be operating at and then calculate the difference in size of the pulleys from there
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Old 11-21-2011, 12:09 PM   #8
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Probably not, other than electic use. What RPM are you going to run it at?
The 5hp I'm looking at is rated at 3460RPM
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Old 11-21-2011, 12:11 PM   #9
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Air compressor build....help, opinions?


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no. id get a 5hp. That way it leaves room to get a bigger pump

i have no idea on figuring out pully size. Other than maybe figuring out the rpm of the motor, and the rpm the pump should be operating at and then calculate the difference in size of the pulleys from there
I'll give the pump another look when I get home, maybe I missed some info somewhere.
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Old 11-21-2011, 12:14 PM   #10
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Air compressor build....help, opinions?


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And, old tanks, by the way, which may have been neglected, meaning not drained regularly, can be dangerous.
i looked this up because i figured they would just start leaking first. It seems that tanks failing catastrophically does happen
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Old 11-21-2011, 12:26 PM   #11
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Air compressor build....help, opinions?


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i looked this up because i figured they would just start leaking first. It seems that tanks failing catastrophically does happen
I've heard the horror stories, but i think you'd run this risk with any used tank.

I am considering building a small outdoor "shed" just to be on the safe side.
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Old 11-21-2011, 12:33 PM   #12
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Air compressor build....help, opinions?


The most rust will accumulate nearest the bottom drain. Also, be sure to install a properly sized pressure relief valve
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Old 11-21-2011, 12:48 PM   #13
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The most rust will accumulate nearest the bottom drain. Also, be sure to install a properly sized pressure relief valve
I specifically checked that area...smooth as can be and didn't even leave any rust on my fingers.

Should the pressure valve be threaded right into the pressure switch?
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Old 11-21-2011, 12:59 PM   #14
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Should the pressure valve be threaded right into the pressure switch?
Probably separately, and removed from any air flow. The less chance of getting gummed up by water or entrained oil, the better.

I can't remember the formula, but it should pop off at around 110-20 pounds. It should be sized appreciably bigger than the design CFM of the pump
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Old 11-21-2011, 02:10 PM   #15
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Probably separately, and removed from any air flow. The less chance of getting gummed up by water or entrained oil, the better.

I can't remember the formula, but it should pop off at around 110-20 pounds. It should be sized appreciably bigger than the design CFM of the pump

110psi would be too low. Most compressors shut off higher than that. itd be more like 150psi
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