High PSI vs high Volume compressors

[Dropstone]

I’d like to get a compressor for both construction type use(portable, small, single hookup) but also a garage car maintenance compressor(110v, high volume, impacts, air ratchets, maybe a polisher). My requirements seem to be around 5 or 6 SCFM.

My question.. what’s the difference between a small compressor that’s at say 200 psi, 4 gallon with an SCFM of 5 and a large compressor at 120psi, 21 gallon and also 5 SCFM?

If the two have the same performance, why not save my garage space and get the smaller one that I can also use for construction stuff.

 

[Aquamoose]

Volume & recovery time is directly related to capacity. Sprayers, nailers can deliver high psi but don’t need volume. Impact wrench & sanders use LOTS of air and require volume.

https://www.garagetooladvisor.com/air-tools/what-size-air-compressor/


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[BillK]

The difference is going to be if you need to use any air tools that require a lot of cfm for long periods. Examples are a DA sander, sand blasting, spray painting etc. If all you are going to use it for is short burst type tools like an impact wrench and maybe an air drill then the smaller one will be fine.

 

[Dropstone]

Can high PSI compensate for high volume?

4 gallon, 250psi, 5 SCFM

VS

20 gallon, 120psi, 5 SCFM

If tool requirement is only 90 PSI@5 SCFM, do either of these compressors work the same?

 

[The Cobbler]

Im not sure where you're finding 4 gal, 250 psi compressor?

 

[Dropstone]

I just made that compressor up to help state my question but Home Depot has some pretty high psi compressors like.... Model# OF45200SS. It’s a Rigid that’s 4.5 gallon and 200psi that does 5.1 SCFM. There are other ones that high different but similar specs.

 

[99LeCouch]

The CFM is how much the pump can supply once the tank drops to the point the pump kicks on. It has nothing to do with capacity. A compressor can often deliver far more CFM at 90 psi than the pump is rated for. The capacity determines how long it can deliver the required amount of air before it falls behind.

So a 20 gallon compressor will be able to deliver more air than a 4 gallon. The higher psi will help some, but nothing makes up for sheer capacity when the difference in capacity is large.

Impacts and ratchets typically aren't used for that long. A smaller compressor should run them fine, if you are okay with the occasional wait. Make sure the pump and motor are rated for higher duty cycles if you go smaller.

 

[Dropstone]

The CFM is how much the pump can supply once the tank drops to the point the pump kicks on. It has nothing to do with capacity. A compressor can often deliver far more CFM at 90 psi than the pump is rated for. The capacity determines how long it can deliver the required amount of air before it falls behind.

So a 20 gallon compressor will be able to deliver more air than a 4 gallon. The higher psi will help some, but nothing makes up for sheer capacity when the difference in capacity is large.

Impacts and ratchets typically aren't used for that long. A smaller compressor should run them fine, if you are okay with the occasional wait. Make sure the pump and motor are rated for higher duty cycles if you go smaller.

Ohhhhhhhhhh..... now I get it. Thanks for that. I just couldn’t understand the concept untill you said CFM is how much the pump can supply after the tank is empty. I thought SCFM was a combined effort between pump and tank. Thanks!

 

[jkesselr]

One other consideration is pump speed. Some compressors are high speed to create their numbers. The high speed units tend to generate more noise and more water, in my experience.

 

[Citation]

OP,
You got it on what the pump can supply. In very general terms a bigger tank or higher pressure in the tank result in basically the same thing, more stored air. In very general terms, a 5 gallon 200 psi tank has the same amount of air as a 10 gallon 100 psi tank.

That is a very simplistic way to look at it and not actually correct but probably good enough as a rule of thumb. In reality there are other considerations. When you compress air it heats up. When my compressor shuts off I see perhaps a 5-10 psi loss in tank pressure as the air in the tank cools to ambient temperature. So even though my compressor says it runs to 120 psi, if I let it sit overnight I have just 110 psi the next day (this isn't a leak as it will hold that 110 psi for weeks on end). The higher the pressure the more you heat the air and thus the bigger the pressure drop if you let the air cool inside the tank prior to use. So in theory it's better to have a bigger tank that is kept at lower pressure. Of course that has a practical limit.

Also, most tools need some minimum pressure to operate. To make the math easy I'm going to assume 50 in the tank is my minimum needed pressure for some tool I have. So a 10 gallon tank at 100 psi actually gives about the same volume of usable air as a 5 gallon tank at 150 psi. The total air in the 10 gallon tank is 10 gal * 100 psi = 1000 gal*psi. For the 5 gallon I have 5 gal* 150 psi = 750 gal*psi. So you would think the 10 gallon tank has more usable air. However, if you calculate how many standard gallons (that 1 gal @ 14.7psi) I can pull from each tank before the tank pressure is at 50 psi the math changes.
10gal*(100-50 psi) = 500 gal*psi.
5 gal* (150-50 psi) = 500 gal*psi.
But if my minimum is 80 then the 10 gallon is worse (200 gal*psi vs 350). If my minimum is lower (say 30 psi) then it goes the other way.

Sorry, I'm sure I over complected things but you didn't have to pay for it.

 

[driz]

If you are worried about space then you can locate the larger size compressor in another room or even out doors in a tiny shed. That way you don’t lose any space at all. The quietness will be greatly appreciated as well when you’re working.


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[Dropstone]

Citation.... Out F’n standing

 

[MattT]

If tool requirement is only 90 PSI@5 SCFM, do either of these compressors work the same?

Your "tool requirement" SCFM will be average air consumption. Using an IR 231 impact as an example it's rated 4.2 SCFM average and 22 SCFM working. So the gun will outpace either compressor pump and the storage determines how long the tool will run in between breaks. The flip side is that more storage also extends the length of the breaks.

A 20+ gallon normal pressure compressor is gonna have more useable storage than a high pressure framing nailer compressor. Personally I prefer the longer runtime from more storage. And the longer breaks

 

[Showkey]

Real world

I have a 200 psi, rated 5 cfm @90 psi, 4.5 gallon tank. Works fine for nail guns, impact, blowing off work, tire inflation. Does not work well for even a small spray gun, cabinet sand blasting even for small items, poor on air die grinder and small air sander. It fails to keep up. Even when the tools are regulated down to 90 psi the 4.5 gallon gets used up quick.

I have not tried connecting to larger tank. This came up prior in posts , the oilless 200 psi unit is rated for continuous use for up to 60 minutes. 70% duty after one hour. Life expectancy would drop like a rock working the unit long and hard. Piston ring and cylinder is less than $20. Mine is 5 years old never serviced. Used in hobby wood working mainly for nail guns.

Large air consumption tools like cabinet blasting or painting generally need a 220v 10-20 cfm at 90 psi with a large tank. Not everyone needs a 80 gallon tank two stage unit. My bench top blast cabinet runs on 10cfm @90 compressor, 240v, 20 gallon with aux 15 gallon (when painting) but it does run continuously. It’s over 20 years old with no down time......but it’s a hobby shop not a business.

Large tank makes for cooler air and cool air allows for the water to trapped and contained in the tank or at the point of use or both.

My case........The 200 psi unit was purchased because it’s portable, 120v and was used in wood flooring, interior finish work and roof installs.

 

[geartow]

5 cfm in way small your tool will run about 30 seconds and you will be out of air.

 

[md21722]

There is really no reason for an air ratchet anymore. Just get a cordless. Even for impacts, many/most can be well served by a cordless.

For the folks that truly need air, and lots of it, for constant use of die grinders, angle grinders, sanders, and blasting, then you typically need a 220V 5-7.5 HP air compressor.

A small pancake compressor is fine for a nailer, but completely unsuitable for a die grinder where a large air supply is needed.

Tools are rated in CFM which is an industrial rating, but really use SCFM. CFM is about 25% of SCFM. Your typically die grinder will around 20 SCFM.

As far as pressure is concerned, tools are generally rated to be used at 90 PSI. This means you have to set the regulator around 110-120+ or so a gauge at the tool says 90 when it is working. This is the difference between static and dynamic pressure. It's also the reason the mechanics who run without regulators on a dual stage at 175 PSI aren't really running nearly 2x the pressure the tool needs. After a 100 foot hose reel, swivels, and whatever, they are actually still running somewhat close to 90 PSI.