Air Compressor

[ray66v]

Compressors are like tractors.
It's only too big, if:

1. You can't afford it.
-or-
2. You don't have room for it.

No matter how big the tank, you're basically still running off the pump, when you use a tool that draws a lot of air volume.

You can convert pressure to volume. When you are using a tool that is not using the full pressure and volume of the pump, whatever the pump is creating over what you are using, is going into the tank. So, you will have more air volume, if your compressor is a two stage, pumping at 175 psi, vs a single stage at 125 psi, or 150 psi, of the same size. Two stage is a good thing. Bonus, if you have a bolt that will not budge, you can get a lot more power out of an impact, with higher pressure.

Never buy an air compressor, or an air tool without hearing it run. Unless you can return it easily when you do, and find out it's loud. See what your friends, and neighbors have, and how loud they are.

Some are so loud, they will cause hearing damage. While it seems impressive, to cavemen. The sound level in no way equates to power. The only thing it proves, is that you didn't shop around, and buy one that was quiet, they are out there. And, not that hard to find.

I have good tools, and some cheap tools, that are all quiet. They all perform very well.

My compressor, is from the 80's, and it's no louder than a window air conditioner on low.

 

[SPYDERLK]

Compressors are like tractors.
It's only too big, if:

1. You can't afford it.
-or-
2. You don't have room for it.

No matter how big the tank, you're basically still running off the pump, when you use a tool that draws a lot of air volume.

You can convert pressure to volume. When you are using a tool that is not using the full pressure and volume of the pump, whatever the pump is creating over what you are using, is going into the tank. So, you will have more air volume, if your compressor is a two stage, pumping at 175 psi, vs a single stage at 125 psi, or 150 psi, of the same size. Two stage is a good thing. Bonus, if you have a bolt that will not budge, you can get a lot more power out of an impact, with higher pressure.

Never buy an air compressor, or an air tool without hearing it run. Unless you can return it easily when you do, and find out it's loud. See what your friends, and neighbors have, and how loud they are.

Some are so loud, they will cause hearing damage. While it seems impressive, to cavemen. The sound level in no way equates to power. The only thing it proves, is that you didn't shop around, and buy one that was quiet, they are out there. And, not that hard to find.

I have good tools, and some cheap tools, that are all quiet. They all perform very well.

My compressor, is from the 80's, and it's no louder than a window air conditioner on low.

The issue is that 2 stage hold their rated CFM to much higher pressure than 1 stage. ... 2 stage have a compression ration that is very high in relation to that of a 1 stage.

 

[murphy1244]

The issue is that 2 stage hold their rated CFM to much higher pressure than 1 stage. ... 2 stage have a compression ration that is very high in relation to that of a 1 stage.

They also produce more heat.

 

[SPYDERLK]

They also produce more heat.

Yeah. ... Equal pressure for pressure - but more pressure capability. The thing is, around 170 a single stage is only making heat - no air, whereas a 2 stage has barely fallen off its rated CFM output.

 

[Redneck in training]

2 stage is much more efficient at usable system pressure. If you can do with 80 or 90 supply, OK. ... If you need more to get decent tool pressure at flow the 2 stage will handle it w/o cfm derating.

I disagree with the statement above. Power required by the compressor is head (pressure) times mass flow. That implies that to get higher pressure with the same power you have to decrease volume. If your tool requires 120 psig then single stage will cope better with volume because it will have larger displacement.
If you need higher pressure then single stage compressor will run way too hot. Therefore you split the compression to two steps and put intercooler between the stages. If you want to maintain the same volume as single stage then you will need bigger motor.
If you have two stage with intercooler generating lower pressure then I would agree that you could maintain slightly higher volume with the same power.

 

[SPYDERLK]

I disagree with the statement above. Power required by the compressor is head (pressure) times mass flow. That implies that to get higher pressure with the same power you have to decrease volume. If your tool requires 120 psig then single stage will cope better with volume because it will have larger displacement.
If you need higher pressure then single stage compressor will run way too hot. Therefore you split the compression to two steps and put intercooler between the stages. If you want to maintain the same volume as single stage then you will need bigger motor.

No. ... And yes. - Due low compression ratio of single stage a larger amount of the charge remains in the cyl to inhibit the new intake stroke. This is pure waste. It is the reason most single stage CFM is rated in the 40 - 90 range and you see it dropping all thru the range. -- True. The single stage power does not rise as fast with PSI because as pressure rises it does less and less whereas the 2 stage continues to pump unchanged volume to much higher pressures.

 

[Redneck in training]

No. ... And yes. - Due low compression ratio of single stage a larger amount of the charge remains in the cyl to inhibit the new intake stroke. This is pure waste. It is the reason most single stage CFM is rated in the 40 - 90 range and you see it dropping all thru the range. -- True. The single stage power does not rise as fast with PSI because as pressure rises it does less and less whereas the 2 stage continues to pump unchanged volume to much higher pressures.

Isn't it true that the cylinder of the compressor is designed with the smallest dead volume possible regardless of the final pressure? Even single stage compressor can develop high discharge pressure. The discharge pressure is given by back pressure and the head temperature is controlled by the pressure switch to about 130 psig max.

 

[SPYDERLK]

Isn't it true that the cylinder of the compressor is designed with the smallest dead volume possible regardless of the final pressure? Even single stage compressor can develop high discharge pressure. The discharge pressure is given by back pressure and the head temperature is controlled by the pressure switch to about 130 psig max.

Yes, but there are realistic limits imposed by flow issues at top dead center. ... A 2 stage has these too in each cylinder of the 2 stage pair. But when you consider that the 1st cyl supercharges the 2nd to about 60PSI you see some relaxation in the need for the very high CR requirement needed to output good pressure in a single stage. 130 PSI is well past the point of efficient single stage compression.
,,,,, My 5 HP Kellogg American is rated 20CFM @ 100, 18 @ 150, and 15 @250. These are not at the same RPM, but optimized for use of rated motor power at the subject delivery pressure. Throughout the range the compressor exhibits about 75% volumetric efficiency between displacement and delivery.

 

[murphy1244]

My Polar air does 17 cfm at 150 psi. Single stage 5 hp. Lower RPM than most and much quieter.

 

[SPYDERLK]

My Polar air does 17 cfm at 150 psi. Single stage 5 hp. Lower RPM than most and much quieter.

According to its specs that CFM delivery requires the max rated compressor rpm and its takes a 7.5 HP motor to do it. Volumetric efficiency is about 53%.