Air compressor advice, please...

I've been using my friends compressor in the shop for about two years now, and while it's been very good, it's moving on to his new shop. It is a "5"
hp Ingersol type unit, vertical 60 gallon tank, and I measure it as a true 2.5 (maybe 3) hp and 8 cfm. I need to replace it with something equivalent. My old compresson, which is a Webster true 2 hp motor with twin cylinder head on a 10 gallon tank, actually measures out at about 7 cfm, which is close enough. It's problem is small tank size which causes frequent cycling when I keep the regulator pressure at 90 PSI or so for air tool use, AND it's at least 25 years old. It has been completely reliable, mind you, and it has held 80 psi for the two years that it's been stored under the bench.
So should I resusitate the old Webster updated with a 60 gallon tank for abut $300C and have a new/old compressor, or should I get the Princess Auto 5 hp unit here: http://www.princessauto.com/_osn.cfm?CTRY ÊN&output=OSN&SRCH=1&SRCHAREA=OSN&T1=5%20HP%20LIGHT%20%20INDUSTRIAL%20AIR%20COMPRESSOR
True 5 hp, 18.5 cfm etc. More compressor than I've needed so far, but then I've never decided to sandblast a car or anything either - knowing that I couldn't do it.
Thoughts on keeping the old faithful webster, basically. I'd hate to buy it a new tank and have it promptly blow up.
Thanks, brian
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Looks like you already have your answer.
Get something that will keep up with a sand blaster, you won't regret it.
-- Jack
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http://www.princessauto.com/_osn.cfm?CTRY ÊN&output=OSN&SRCH=1&SRCHAREA=OSN&T1=5%20HP%20LIGHT%20%20INDUSTRIAL%20AIR%20COMPRESSOR
This Princess Auto unit looks very nice. I would be very happy to own something like that. I am curious however, whether a 60 gallon tank at 18 CFM is sufficient for sandblasting (I have never done any sandblasting).
i
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Ignoramus10397 wrote:

I have. While you *can* do some limited blasting with 18 CFM, you really need a lot more. If I ever do sandblasting again (doubtful, I use a whole barrage of other metal cleaning tricks first) I will rent a big quiet compressor with at least 75 CFM.
GWE
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Thanks Grant. Also thank you for the trailer books, they are great.
i
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On Tue, 20 Dec 2005 17:06:46 GMT, Ignoramus10397

It's more than enough. OTOH, any high volume air work begs for a two-stage compressor. You get much more capacity at the same power rating.
Don't be confused, two cylinder does not equal two stage. A single stage compressor will have one or more cylinders, all the same size. A two stage compressor will have at least two but possibly more, cylinders. One will be about twice the size of the other. The large cylinder draws in air from the filter and pushes it into the smaller cylinder. The smaller cylinder pushes that air into the tank. This setup yields a higher pressure and higher cfm capacity using a similar size motor.
-- Jack
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Princes doesnt compare to say ingersoll in terms of quality BUT the "royal service"
Means that even if you can blow it up (doubtful) they will replace it if youre willing to deal with the PITA (Pain in the A$$) factor of hauling the compressor back in the them
The more compressor you have the less hard it works. the bigger the tank the less starts the motor does and the longer it lasts. same thing with a 2 stage versus a 1 stage more air held in the same 60 gallons so the runtime drops even further.
running a motor at its duty cycle is not what roasts it usually its the frequent start/stops
Brian wrote:

http://www.princessauto.com/_osn.cfm?CTRY ÊN&output=OSN&SRCH=1&SRCHAREA=OSN&T1=5%20HP%20LIGHT%20%20INDUSTRIAL%20AIR%20COMPRESSOR
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CDN$900 is a good price for that compressor. If you can pick it up locally, I would strongly consider it. Maybe you'll get 25 years out of it.

http://www.princessauto.com/_osn.cfm?CTRY ÊN&output=OSN&SRCH=1&SRCHAREA=OSN&T1=5%20HP%20LIGHT%20%20INDUSTRIAL%20AIR%20COMPRESSOR
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I'd just rework the old compressor, it's a good unit. Your frequent cycling is the result of the small tank, a bigger 30 to 60 gallon tank would work wonders. Given the death rate of the Sears Oil less compressors, I'm sure that you could find a suitable tank for free or trivial $$. I had a 30 gallon one here for repairs last summer, gave it back to the owner who promptly asked me how to dispose of it. Just put an ad in www.craigslist.com (or one of the others) asking for a dead compressor.
Brian wrote:

http://www.princessauto.com/_osn.cfm?CTRY ÊN&output=OSN&SRCH=1&SRCHAREA=OSN&T1=5%20HP%20LIGHT%20%20INDUSTRIAL%20AIR%20COMPRESSOR
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Does anyone make a compressor with a 100% duty cycle? My buddies compressor has a large tank compared to the one on my dad's old compressor, but it ran a lot more than my dad's. I was always taught not to let the compressor's engine run too often because you could burn up the engine. Thoughts? I'll be in the market for one soon after I finish wiring the garage.
Mathius

http://www.princessauto.com/_osn.cfm?CTRY ÊN&output=OSN&SRCH=1&SRCHAREA=OSN&T1=5%20HP%20LIGHT%20%20INDUSTRIAL%20AIR%20COMPRESSOR
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Mathius wrote:

The idea is to size your largest demand in CFM, and then size the compressor so that when it's delivering that CFM you keep the air pump to 50% duty cycle or lower.
I personally haven't heard of an air compressor designed to run 100% of the time.
GWE
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On Mon, 26 Dec 2005 16:21:36 -0800, Grant Erwin

They are called "rotary" or "screw" compressors.
Gunner
"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.
Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner
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I do not know why a compressor's motor cannot be run at 100% duty cycle.
I had some periods when I had to run my compressor for at least an hour, continuously, with no problems (maybe longer). The motor in it is a 3 HP baldor motor designed for continuous running. The compressor did not seem to mind very much. Mine is a Curtis econoline compressor
http://igor.chudov.com/projects/CurtisCompressor/
100% duty cycle motors are not very rare.
Perhaps the pump could overheat of run too long, but it did not seem to hurt my compressor.
i
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On Mon, 26 Dec 2005 19:17:17 -0500, "Mathius"
Well...ummm...a compressor on a turbofan jet engine is a 100% duty cycle type...
In the shop, a piston type air compressor running 100% of the time would suggest that you are either very good at sizing your compressor very precisely to match your air demand or, most likely, your air demand is greater than what the compressor can supply.

A compressor undersized for the air demand and mismatched with a large tank will "run a lot".
You need to balance air demand versus tank size versus compressor size so that the machine runs about 50% of the time with AVERAGE air demand.

The issue is RESERVE CAPACITY. If you are running a tire shop and your compressor is already running 100% of the time with AVERAGE air demand, what happens when you have a one week tire sale and sales pick up during that week? "Ummm...we can't sell you any tires until midnight because we don't have enough air to inflate them" doesn't quite sound right...
The rule-of-thumb with small installations is that a properly sized machine should run 50% of the time with AVERAGE air demand. If it runs less than 50% of the time, you have purchased too much machine and wasted money that could have been used to purchase other equipment. If it runs more that 50% of the time with AVERAGE air demand, you don't have enough machine to provide you with sufficient RESERVE CAPACITY.
A lack of reserve capacity will limit your activities at the most inopportune time, especially with a small installation. For example, you are running a service station with one mechanic. During the busy season, you hire a second mechanic for a two week period. Two mechanics = two impact wrenches in operation = double the air demand. Can your compressor handle a 100% increase in air demand or, will your second mechanic be sitting around waiting for the air compressor? If you are so busy that you need to temporarily hire a second mechanic, do you think you will have time to go shopping for a larger compressor?
On the other hand, if you operate a garage with 20 mechanics, each with an air impact wrench, and during the busy season, you hire two extra mechanics. The two extra mechanics would represent only a 10% increase in air demand verus a 100% increase in air demand in the previous example where the service station goes from one mechanic to two mechanics. In a large installation, like in this latter example, you can size your compressor so that it runs 70-80% of the time with AVERAGE air demand and still be OK to handle a temporary 10% increase.
How efficient you can make your installation in terms of duty cycle depends upon what your AVERAGE air demand is (in cubic feet per minute), how much variation (measured in per cent) there is from average, and what the causes of the variation are (variation between tools versus variation in business activity). Air demand on a factory production line is fairly constant with very little variation; in an autobody shop, air demand varies considerable not only between air tools, but also seasonally. All factors must be taken into consideration when sizing your compressor.

You need to make sure you get an air compressor with enough RESERVE CAPACITY for your particular application.

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The Husky 60gal (ca. 3hp) sold at Home Depot in the U.S. for about $450 is rated for 100% duty cycle. It's made by Campbell Hausfeld, and they do recommend filling the pump with full-synthetic Mobil One if you are going to be using it at 100% duty. Perhaps some of the other bigger models are also rated at 100%, but I haven't noticed.
IIRC, the 60g is also rated for something like 5000 hours. If you need 100% duty very often, you would be far better off buying a larger compressor and not working it so hard.
sdb
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