Important metrics when buying a compressor?

[larryq]

I don't need a compressor yet (though a small one that fills tires and blows shop air to clean dust and dries wet parts would be nice) so I'm sticking to research at the moment.

My goodness, the compressor industry is a tricky one sometimes. Reminds me of the Sprint-MCI days when they made it nearly impossible to comparison shop different long distance plans, they all had unique twists to them.

My understanding is that HP is easily fiddled and not a great indicator of compressor value. CFM is king from what I'm told. Are there ways to game that too in the spec sheet?

One question I have is regarding those CFM @ PSI numbers you always see. For instance, "16.1 CFM at 40 psi and 14 CFM at 90 psi." May as well ask and get it over with-- Why is it that CFM decreases at higher psi? Wouldn't it increase with more pressure?

I know that oil lubricated motors are preferred to oilless, they tend to be quieter and last longer. Cast iron cylinders are preferred too.

I'm not an electrical guy by any means so I'm not sure what the 208/240 power requirements mean for the larger compressors. I've seen 208, 230 and 240 numbers before. I'm used to 120v stuff, what does a laundry/dryer usually run off, and are one of those 200+ numbers more common in the home than the other two? Thanks everyone.

 

[Outlawmws]

The higher the pressure is, the more back pressure you get, and therefor fewer CFM. remember you are COMPRESSING the air...

The higher the pressure the less of a cubic foot the compressor actually delivers.

 

[TW95Z]

The key question is what do you want to do with it? If you need to carry it from place to place or move it around to run a nailer lets say, then you want something portable, doesn't need much storage capacity, and will definitely run on 110 volts. If you are building an automotive shop for 6 mechanics you'd be looking at the other extreme. I picked a IR Garage Mate. It was 20 gal, oil lubricated, rated at 5 something cfm at 90 psi, had wheels so I can move it around (not up and down stairs) and 110 volts. It runs my impact and ratchet with no problem, fills tires, and doesn't cycle all that frequently. Could I use it in a paint booth, probably not. Whatever you pick it will be a compromise, so figure out what will keep you from saying "I wish it were bigger/smaller" to frequently.

 

[olytdi]

Work backward. What do you envision doing that would require compressed air? Once you know this, you can size your compressor to the need at hand. Just make sure to oversize!

Sandblasting can take a lot of air so deciding what you will and will not be using it for is key. Once you know your delivery requirements, the questions you're asking will have better context.

 

[kythri]

I don't claim to be an expert, just commenting based on what I've learned.

For CFM, look at the 90 psi rating. I'm not sure what you would use that would require setting the regulator at 40 psi, but just about all of our air tools are operated at 90.

40 psi could be something legitimate, or it could simply be marketing fluff.

As far as voltage goes, I think 208V specifically refers to 3-phase, whereas 230V is a motor voltage and 240V is single-phase line voltage.

That said, a lot of this stuff gets used interchangeably to the marketplace, it seems.

If you find a compressor that says it's 208V, but it's definitely a single-phase motor, you should be OK.

It's kinda like the whole 110V/115V/120V **** for single-phase.

It'd be nice if things were clear and consistent...

That said, a good estimation of how much air you're going to need would be to look at the requirements of some of the tools you're looking to use, and size from there.

 

[RKA]

I'm far from an expert, but here is a synopsis...
1. On a 110v 15 amp line (which is common around your house), the max capacity you'll see is 5.5-6.0 cfm @ 90 psi. That is sufficient to run impact guns, ratchets and other general purpose stuff. It can work for short spurts on die grinders and cut off tools, but you'll constantly be waiting for the pump to refill the tank. Paint and sandblasting is a no go for something like this.

2. Next step up is a 240v compressor. 240v is what an electric dryer would use. If you have access to your electrical panel and can easily run a new branch circuit to the garage, then this would be an option. You're looking at 10 cfms and up with these, typically with 60-80 gallon tanks.

3. 208v is for 3 phase power. Typically your larger motors will need this. A residential home is not typically supplied with 3 phase power from the utility company, so if you're planning to put this in a house, you don't want this. You'll typically see a compressor like this used in a shop or other commercial setting that is supplied with 3 phase power.

4. CFM is simply a measure of how much air it can deliver at a given pressure. What they don't tell you is how long it can sustain that air delivery. This is related to the tank size. Larger tanks can hold more air which allows them to deliver that air for a longer period of time. The downside is it can take the pump a lot longer to refill that tank, so cycle times can be long, esp. with a 110v compressor. That's on reason why you see 110v compressor's top out around 30 gallons. Any larger and the pump would be running for an abnormally long period of time.

5. 110v compressors are decidely light duty, as such they are built to a price point and you don't always see quality pumps like you might find in the better 240v pumps. The garagemate mentioned above is one of the better one's imho (I have one). Still, the pump looks like it was made in India or China to me, and in operation it doesn't have the smoothness one would expect in a high quality pump. It's not bad, but it's probably as good as you'll find in a new 110v compressor.

 

[Davefr]

One question I have is regarding those CFM @ PSI numbers you always see. For instance, "16.1 CFM at 40 psi and 14 CFM at 90 psi." May as well ask and get it over with-- Why is it that CFM decreases at higher psi? Wouldn't it increase with more pressure?

At 90 PSI there are more air molecules squeezed into that cubic foot then at 40 PSI.

I know that oil lubricated motors are preferred to oilless, they tend to be quieter and last longer. Cast iron cylinders are preferred too.

No it's the pump that you want to be cast iron, belt driven and oil lubricated.

I'm not an electrical guy by any means so I'm not sure what the 208/240 power requirements mean for the larger compressors. I've seen 208, 230 and 240 numbers before. I'm used to 120v stuff, what does a laundry/dryer usually run off, and are one of those 200+ numbers more common in the home than the other two? Thanks everyone.

240 VAC is the common standard in the US

You also need to "rightsize" the compressor by calculating the air consumption of your high duty cycle air tool needs. You also need to make sure you forecast your future needs.

You'd be foolish to spend the $'s and waste the floor space on a 80 gallon/5 HP unit if all your needs boil down to low air consumption tasks. (filling tires, etc).

However if you plan to do air sanding, sandblasting, grinding, etc then these tools can require a huge volume of air AND at a high duty cycle. Impact wrenches use lots of air but usually at a low duty cycle so the reserve capacity of the tank can buffer those needs and you can get by with a smaller compressor.

(ex: I have a little 5" Sioux DA Sander that requires 15 CFM at 90 PSI. It keeps my 5 HP/60 gallon IR compressor running at nearly a 100% duty cycle when sanding.)

You also need to decide if you want portability.

Picking a compressor is all about planning.

 

[larry_g]

I'm far from an expert, but here is a synopsis...


4. CFM is simply a measure of how much air it can deliver at a given pressure. What they don't tell you is how long it can sustain that air delivery. This is related to the tank size. Larger tanks can hold more air which allows them to deliver that air for a longer period of time. The downside is it can take the pump a lot longer to refill that tank, so cycle times can be long, esp. with a 110v compressor. That's on reason why you see 110v compressor's top out around 30 gallons. Any larger and the pump would be running for an abnormally long period of time.

5.

I disagree with your #4. CFM rating is what the pump/motor can deliver on a sustained basis. Tank size has nothing to do with it. That said a lot of the consumer advertising will skew the numbers but on a commercial pump ratings have nothing to do with a tank size. To measure CFM's you adjust the flow from the tank till it reaches and stabilizes at a pressure you want to test at and then read the flow that the system is delivering. This is what the pump will deliver and keep delivering at the test pressure. Once the system stabilizes the accumulative properties of the tank are removed and the tank essentially becomes a big pipe.

lg
no neat sig line

 

[RKA]

If that's true, someone is lying (*shocked*). My compressor is rated for 5.5 cfm @ 90 psi, my cut off tool is rated for 5.5 cfm @ 90 psi. And yet after 15 seconds of sustained use (starting from a full tank), the compressor is running out of wind, the pump is running at 100% duty cycle and the pressure has dropped enough that I need to stop and wait for the compressor to catch up.

 

[kartracer55]

If that's true, someone is lying (*shocked*). My compressor is rated for 5.5 cfm @ 90 psi, my cut off tool is rated for 5.5 cfm @ 90 psi. And yet after 15 seconds of sustained use (starting from a full tank), the compressor is running out of wind, the pump is running at 100% duty cycle and the pressure has dropped enough that I need to stop and wait for the compressor to catch up.

Your cut off tool is rated for 5.5cgm@90psi AVERAGE. In a nut shell, the average is assuming 15 seconds of use over a 60 second interval. CFM at load will be significantly higher. The reason the average rating is used is to account for the accumulative properties of the tank. It's all BS though.

Read the specs on this (CFM vs CFM@ Load)
http://www.northerntool.com/shop/tools/product_31666_31666

 

[kythri]

Yup. Marketing hype at it's finest!

 

[RKA]

LOL! So I was right initially...but it's not the compressor that can't sustain the airflow, it's the tool that exceeds the demand thanks to bogus specs. That explains it...so I'm not kicking my compressor to the curb, but if anyone wants some free tools...

Okay, not really, but thanks for clearing that up!

 

[trboxman]

The higher the pressure is, the more back pressure you get, and therefor fewer CFM. remember you are COMPRESSING the air...

The higher the pressure the less of a cubic foot the compressor actually delivers.

Huh?

 

[bcradio]

I disagree with your #4. CFM rating is what the pump/motor can deliver on a sustained basis. Tank size has nothing to do with it. That said a lot of the consumer advertising will skew the numbers but on a commercial pump ratings have nothing to do with a tank size. To measure CFM's you adjust the flow from the tank till it reaches and stabilizes at a pressure you want to test at and then read the flow that the system is delivering. This is what the pump will deliver and keep delivering at the test pressure. Once the system stabilizes the accumulative properties of the tank are removed and the tank essentially becomes a big pipe.

lg
no neat sig line

Ok so this explains why there are fewer CFM's at higher psi. The compressor has to work harder to keep the tank at 90 psi, so fewer cfm's can pass before the pressure drops. Got it

 

[Outlawmws]

SNIP

One question I have is regarding those CFM @ PSI numbers you always see. For instance,

"16.1 CFM at 40 psi and

14 CFM at 90 psi."

May as well ask and get it over with-- Why is it that CFM decreases at higher psi? Wouldn't it increase with more pressure?

SNIP.

The higher the pressure is, the more back pressure you get, and therefor fewer CFM. remember you are COMPRESSING the air...

The higher the pressure the less of a cubic foot the compressor actually delivers.

Huh?

Efficiency loss?

As the pressure of the the tank increases, more of the compressed air is left behind each stroke in the area between the valve and the top of the piston.

The higher the pressure, the higher the parasitic loss. bump that up to a higher pressure, and it will drop off again. How much is dependent on the volume of space when the valve fully closes.

 

[shampoop]

1. OIL LUBE!!! - quieter, longer lasting

2. Belt drive - smoother and quieter

3. 120V vs 240V - Depends on your needs and what you have at your house. 240V has much higher power potential, but 120v outlets are EVERYWHERE

4. HP is mostly useless, what to look for is CFM @ 90PSI and/or amperage. 2 different compressors both rated at 20A at 120V are both both going to be very similar.

Something like this can handle most mechanical work pretty easily, is portable, and runs off of common 120v outlets.
http://www.garagejournal.com/forum/showthread.php?t=109314

It won't be large enough to do any serious painting projects, sand blasting, or power basically any spinning air tool for prolonged periods of time

 

[winnipegtibook]

I just saw this thread after posting a new one! Sorry GJ members. So, if one was planning to change tires, fill tires, and maybe do the occasional brake job, what would be recommended. I am just realizing my little P-Cable pancake (which survived a garage build and two decks) isn't up to the task. I am in a pickle, with 30 cm's of snow expected in the next 72 hours!