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Old 05-11-2021, 12:20 PM   #1
FAKKY
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Air Compressors - mainsplain

So ...... need someone to man"splain" me 2 thigns with compressors .....


1) Why does HP matter to CFM ....... meaning generally speaking you get a 110v/2HP compressor that peaks out at 5cfm ........ or you step up to 220v/5HP and get 12cfm.

Would have thought that was completely a displacement and RPM thing ...... nothing to do with HP as such .....



2) How come this compressor can do 6.0CFM at 3.0HP (peak) ...... and 99% others are < 4cfm.

https://www.makitatools.com/products/details/MAC5200

I'm basically toying up with buying it ..... but wondering if it can keep up with my low CFM sander.

https://www.harborfreight.com/6-in-p...der-64416.html

Listed at 4CFM at 90psi.

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Old 05-11-2021, 04:56 PM   #2
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Re: Air Compressors - mainsplain

I'm not any kind of expert but I believe if your air tool consumes 4 cfm and the air compressor can supply 4 cfm once you pulled down the tank your air compressor will run constantly depending how long you run the air tool trying to keep up.
If you can at all afford it buy a 220 volt two stage air compressor. You won't regret it.
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Old 05-11-2021, 05:20 PM   #3
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Re: Air Compressors - mainsplain

Horsepower matters because something has to drive that displacement. The more horsepower, the more air the compressor is capable of displacing in a given time (cubic feet per minute). If you have a displacement that is too large for a given horsepower, the motor either won't turn or it will burn up really quickly.

I don't know the answer to your second question.
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Old 05-11-2021, 05:57 PM   #4
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Re: Air Compressors - mainsplain

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Originally Posted by LS short box View Post
I'm not any kind of expert but I believe if your air tool consumes 4 cfm and the air compressor can supply 4 cfm once you pulled down the tank your air compressor will run constantly depending how long you run the air tool trying to keep up.
If you can at all afford it buy a 220 volt two stage air compressor. You won't regret it.
This doc sort of helps a little with regard to dutiy cycle.

https://www.northerntool.com/shop/to...ir-compressors

I would do a 220v if I was planning to be in the house more than 2 years ......
and I didnt have to put in a subpanel and run 220v outside
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Old 05-11-2021, 05:58 PM   #5
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Re: Air Compressors - mainsplain

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Originally Posted by truckster View Post
Horsepower matters because something has to drive that displacement. The more horsepower, the more air the compressor is capable of displacing in a given time (cubic feet per minute). If you have a displacement that is too large for a given horsepower, the motor either won't turn or it will burn up really quickly.
SO are you talking rpm ?

I mean the piston size isnt changing ? OR is it
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Old 05-11-2021, 06:59 PM   #6
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Re: Air Compressors - mainsplain

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Originally Posted by FAKKY View Post
SO are you talking rpm ?

I mean the piston size isnt changing ? OR is it
Cylinder displacement times RPM equals CFM.
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Old 05-11-2021, 07:32 PM   #7
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Re: Air Compressors - mainsplain

Quick way to consider air compressors:
The tank is the battery. A 60 gallon tank holds twice the air as a 30 gallon.
Tank pressure is the voltage. A 150 psi tank holds more air then the same sized one at 100 psi.
The pump/motor is the charger. The more real HP, the faster the tank gets refilled.
Note that the hard equation is 1 HP = 750 watts assuming 100% efficiency on the motor. A lot of stores will advertise 'Peak HP' in excess of 5HP on a 120V compressor. Peak HP is mostly meaningless for a compressor. It doesn't really matter if the motor generates 6HP for that fraction of a second it is starting to spin up. What matters is the running HP. Be wary of any compressor claiming over 2HP on a 120V plug. 2HP = 1500 watts which is close to the max rating for most 120V outlets. Plus most motors are not 100% efficient.

CFM. Be sure to compare the CFM at the pressure your tool wants. Some low end compressors will list a decent CFM number at 40psi and a much smaller number at 90psi. Most tools want 90psi and most paint guns want 40psi.

A decent rule of thumb is 4CFM @ 100psi per HP. https://www.quincycompressor.com/air...r-horse-power/

Avoid the 'oil less' compressors. They tend to be under performing and overly noisy.
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Old 05-11-2021, 07:39 PM   #8
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Re: Air Compressors - mainsplain

Also consider the tools you are going to use. A lot of them are rated at a certain cfm, or scfm at a specific pressure. Here's an example of an impact wrench, https://www.toolbarn.com/ingersollrand-231ha.html/ Notice the "average CFM at load." This is something else to consider when you purchase an air compressor.
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Old 05-12-2021, 01:33 PM   #9
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Re: Air Compressors - mainsplain

Quote:
Originally Posted by Dead Parrot View Post
Quick way to consider air compressors:
The tank is the battery. A 60 gallon tank holds twice the air as a 30 gallon.
Tank pressure is the voltage. A 150 psi tank holds more air then the same sized one at 100 psi.
The pump/motor is the charger. The more real HP, the faster the tank gets refilled.
Note that the hard equation is 1 HP = 750 watts assuming 100% efficiency on the motor. A lot of stores will advertise 'Peak HP' in excess of 5HP on a 120V compressor. Peak HP is mostly meaningless for a compressor. It doesn't really matter if the motor generates 6HP for that fraction of a second it is starting to spin up. What matters is the running HP. Be wary of any compressor claiming over 2HP on a 120V plug. 2HP = 1500 watts which is close to the max rating for most 120V outlets. Plus most motors are not 100% efficient.

CFM. Be sure to compare the CFM at the pressure your tool wants. Some low end compressors will list a decent CFM number at 40psi and a much smaller number at 90psi. Most tools want 90psi and most paint guns want 40psi.

A decent rule of thumb is 4CFM @ 100psi per HP. https://www.quincycompressor.com/air...r-horse-power/

Avoid the 'oil less' compressors. They tend to be under performing and overly noisy.
Thanks - yeah I was trying to see if I could get away with a compressor that puts out 6cfm on 110v. But thinking about it - I really dont need it right now as the only tool I have that really needs the CFM is the orbital sander - and I have a decent electric one also.

If it was my long term house - I would run a subpanel and the 220v ..... but just not worth it here.

So think I will give in and get me a 2.5cfm pancake for $120 to run plasma cutter and pump up tires so on ...... then when I get more permanent house in retirement then look to a full sized compressor.

Quote:
Originally Posted by Rickysnickers View Post
Also consider the tools you are going to use. A lot of them are rated at a certain cfm, or scfm at a specific pressure. Here's an example of an impact wrench, https://www.toolbarn.com/ingersollrand-231ha.html/ Notice the "average CFM at load." This is something else to consider when you purchase an air compressor.

thanks yes noticed that - appreciated.
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Old 05-13-2021, 12:41 AM   #10
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Re: Air Compressors - mainsplain

I bought the 1nnf7 and couldn't be happier

220v, 10.2 CFM free, 9.0 at 135psi

https://www.grainger.com/product/SPEEDAIRE-230VAC-1NNF7
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Old 05-13-2021, 08:23 AM   #11
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Re: Air Compressors - mainsplain

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Originally Posted by Warrens69GMC View Post
I bought the 1nnf7 and couldn't be happier

220v, 10.2 CFM free, 9.0 at 135psi

https://www.grainger.com/product/SPEEDAIRE-230VAC-1NNF7
damn.
thats actually a nice unit overall. thanks for tip.
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Old 05-14-2021, 03:29 PM   #12
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Re: Air Compressors - mainsplain

Quote:
Originally Posted by FAKKY View Post
Thanks - yeah I was trying to see if I could get away with a compressor that puts out 6cfm on 110v.

you will want to shoot yourself in the face with an elephant gun the 30th time you hear that high cfm buzz box (direct drive) 110 compressor come on


AAANNNNANANNNNNNNNNNNNNNNNNNNNNNNG.

your pancake will come on 5x as often. invest in muffs.


110 direct compressors are motor->crank. no intermediate gearing or intermediate stage. 220 compressors usually have a belt/pulley to drive a twin stage crank, which will compress a little, then compress a lot. the 220 gives them double the voltage (two 110v legs) and more hp

to your original question hp is important because if you tried to use a cordless drill to turn the crank on any compressor it could not do it even though it can (probably) meet the (unloaded) RPM requirements and piston displacement is still the same. so could a ceiling fan motor, but that wouldnt work either.

I run a plasma table and my compressor runs a lot, its not bad to hear the gulpgulpgulpgulp of the 2 stage 220v compressor, when it was broken I had to use a 110 20gal BAAAAAAANNNNNNNNNNNNN machine. it was torture, eventually I put it outside and let the neighbors deal with it. haha.
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Old 05-14-2021, 04:33 PM   #13
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Re: Air Compressors - mainsplain

First two things I wired into my shop after setting the sub panel were a 20 amp duplex outlet and my 220V, 60 gallon compressor. I must admit though, I got the compressor for free when my company moved warehouses and they didn't need it any longer. I asked if they wanted to sell it and the office manager said if I could make it disappear that day I could have it.

220V motors run lower amperage than 110 at the same horsepower but some folks just aren't set up for it.
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Old 05-16-2021, 12:54 AM   #14
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Re: Air Compressors - mainsplain

HP and CFM aren't related. HP has to do with time to fill (and refill) the tank at to pressure, and CFM is related to air delivery from the tank. HP is just a BS selling point. There's an awful lot of BS in compressor marketing so be careful.

One thing that is related to HP is motor RPM. Since HP is a function of speed, or RPM in this case a 5HP motor spinning at 3450RPM is half the size (and cost) of a motor spinning at 1725RPM. A fast motor is a sign of a cheap system.

CFM is meaningless unless the measurement is provided with an at PSI value. A box fan will provide a huge CFM, but no pressure.

Of equal importance is the air system, meaning anything past the compressor tank. That's the flex line leading to hard lines, coolers, regulators, hoses and air tools. Tools don't live long running on hot wet air.

My compressor: VT-PL-735-80 17CFM@175PSI
https://www.saylor-beall.com/tank-mounted-vertical/
Bought used off the internet...

Last edited by franken; 05-16-2021 at 01:11 AM.
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