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  #1  
Old 09-22-2017, 02:46 PM
Dodgefran Dodgefran is offline
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Default Air compressor size info needed

After a few years, I'm actually going to start my patch panels on my 38 Dodge panel truck. My first enlightenment is that my 5 hp 25 gal Sears air compressor has only enough air to run my cut off wheel for about 30 sec. I don't think that my small planishing hammer is going to work eather. I would like your opinions on what I should do.

I have read that some have bought a second compressor of the same size and joined them. I am working at home and doing it outside. I hope that you all can help me with suggestions. Thanks for looking. Fran
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Old 09-22-2017, 03:30 PM
Gareth Davies Gareth Davies is offline
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5hp should easily run a 10-12CFM pump and your cutoff saw probably uses about 4CFM so something doesn't sound right. Is it an old compressor that is maybe worn out? It could just need stripping down and servicing to make sure the piston rings or valves aren't gummed. I'd start with the simple stuff before you think about replacing.
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Old 09-22-2017, 04:10 PM
StingRay StingRay is offline
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Quote:
Originally Posted by Gareth Davies View Post
5hp should easily run a 10-12CFM pump and your cutoff saw probably uses about 4CFM so something doesn't sound right. Is it an old compressor that is maybe worn out? It could just need stripping down and servicing to make sure the piston rings or valves aren't gummed. I'd start with the simple stuff before you think about replacing.
That would be an actual 5hp to get 10-12 cfm at 90psi. Typical consumer grade compressors are generally fictitiously rated in their horse power numbers. I have what was called a 5hp. It's 10 cfm at 90 and won't keep up with most air tools. It'll start and run all day on a 15A 220 breaker. Given that starting loads are higher and I have never tripped a breaker and motor efficiencies are about 80% mine is more like 3hp if I'm lucky. 5 real HP is 3.7 KW divided by .8 which is 4.625KW and will need to run at above 20 amps at 220 volts. It'll need at least a 30A or larger breaker.

Hp numbers are useless. CFM at pressure, motor current and required breaker,tank size, footprint and of course cost are the real criteria for choosing a compressor.
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Last edited by StingRay; 09-22-2017 at 04:21 PM.
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Old 09-22-2017, 05:16 PM
Oldnek Oldnek is offline
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It annoys me how they rate compressors. As Ray mentioned they like to over or incorrectly rate them,
The only real way is knowing is the free air rate, of the pump, because when the Air is consumed from the tank it needs to be replenished at a greater rate than any air tool can use.
The old theory to fit a larger or to piggy back another tank, is useless unless your pump can deliver the rate of air supply back to the tank again, once its initially filled, then yes you will have more air, but when it drops to or below 90psi your pump is going to run non stop to keep up and then take triple the time to fill to capacity again.
Compressor should be chosen on the free air delivery application and use not the size of the motor or tank.
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Old 09-22-2017, 07:13 PM
cliffrod cliffrod is offline
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If you're planning a long term upgrade, buy bigger- as big as you can. Figure your needed cfm in relation to pump speed. Slower compressor pump speeds are your friend, both for a more accurate reference of actual delivers air and longevity of the equipment. Google Champion and Quincy and read up on what they have for information. They're both trustworthy commercial-consumer crossover brands.

If you're buying for one project of higher cfm needs, you may find a gas-engined compressor keep up with air needs better. They make more noise and cost more to run all day, but can be cheaper than finding an electric compressor that will supply the same continuous amount of air.

Linking compressors- don't waste your time imho. Not saying it cannot be done, but it's challenging to perfectly synch the pumps? The tanks are just more storage. One pump will always come on first and run almost continuously. The other one may never run.

Whatever you get, keep the tank drained- make sure you have an easy to access manual drain with real ballcock valve (not a crappy petcock valve). Don't trust automatic drains.
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Old 09-23-2017, 04:05 PM
Dodgefran Dodgefran is offline
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Default Air compressor size info needed

Thank you all for your help. The Scottish in me says do it as cheap as I can, and the logical side of me says spend the money for the Big Boy and do it right. I'm retired and have to be as frugal as I can on this project. Thanks again.

Fran
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Old 09-24-2017, 05:51 PM
1960bus 1960bus is offline
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I think uk compressor power ratings are very different to the US. I have a couple of single phase 6hp compressors one cast iron one and one with just steel cylindersthat are the biggest you can run on a domestic supply here. They have two 3hp motors on a 270litre tank. They start at different times to avoid the overload on startup and the pressure switches are set differently. They do work as two together and put out fad around 20 21 cfm. They certainly aren't strained by my strained hvlp guns. Maybe you could rig up a tandem in to one tank?
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Old 09-25-2017, 08:25 AM
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Steve Hamilton Steve Hamilton is offline
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Horse power formulas have a factor of time in them, so if the motor is run at a faster RPM. The horse power rate will come out higher.
Read up on the pump CFM / SCFM ratings for comparison of brands.

Cheap air tools usually consume higher CFM and have less power!
Buy cut off blades that are 4 inch they cut better due to more surface feet / min..
Get the thinnest blades 1/32 which require less power to run due to less metal being removed.

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Old 09-25-2017, 07:49 PM
crystallographic crystallographic is offline
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I had a Sears 5HP 220V 1ph rolling compressor years ago that I did a lot of metal working with - paint gun, air saw, sander, drill motors, etc etc. The DA consumed enough that it would bog down after a few minutes and the pump just could not quite get it humming. For those first minutes I could really make some dust, though. Forget sandblasting. I bought another 3HP, same style, and ran them both together for a few happy years (8hp, total), until I got a 10hp 3ph unit put together. But running those two together made quite the racket when both came on and ran for 10 minutes while I sandblasted a small piece of something, like a door frame or hood frame.

Tools have been downgraded a lot since the 1990's, when the Asian herd really thundered into the marketplaces. I try to find old US-made gear that is still repairable because I know I can count on getting another 15 years out of my rebuild efforts ....
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Old 09-26-2017, 12:56 AM
John Buchtenkirch John Buchtenkirch is offline
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Here’s my take on compressors but I certainly don’t claim to be an absolute expert. Most manufactures have realized many people rate compressors by horsepower instead of “cubic feet per minute” plus at what pressure which likely confuses some people. So what they did was replace the 1725 RPM motors with smaller 3450 RPM motors which are still rated at 5 horse-power. Then with twice the RPM they run smaller cheaper pumps which in many cases are single stage. These higher RPM compressors are more reasonably priced for the weekend mechanic but never seem to last long in a professional shop (running 40+ hours a week) plus the single stage pumps may not have the pressure to do some jobs.

What I’d want in a compressor for myself. Certainly a 5 horsepower, 1725 RPM, 220 volt or 3 phase motor for longevity. A two stage cast iron air pump for longevity & higher pressure. At least a 60 gallon tank for reserve air pressure. Pipe threads on the intake port so you can run whatever intake filter or silencer you might like, the filter on Sears compressors comes to mind as being shamefully inadequate. Change the oil, keep the intake filter clean and drain the water from your tank regularly and you will have a compressor you can run for many years and give to your offspring . ~ John Buchtenkirch
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