air pressure... 90psi vs 110psi

[NSXRguy]

i have a IR 80g 7.5hp compressor rated at 24cfm @ 175psi

originally bought it to run a blast cabinet

now, the compressor is mostly used for a impact gun and occasional tire changer use

from my understanding, as the psi goes up, the cfm goes down

will dropping to 90psi which is the working pressure of the impact gun increase efficiency? or just stay at 110psi?

 

[Showkey]

Insignificant in your case........but be sure to have the required pressure at the device ( at the end of the piping and hose). Example: So set the line pressure at 120-150 to get 100psi at the end of a 50’ hose while the device is operating.

 

[Mamrak76]

I agree. I usually keep mine at 175 and by the time I get thew all my piping and 100 foot air hose it’s more like 160 at the end. At work we have a single stage compressor. It stops at 130 psi. I can never get my tires up to the required 130 (steers) because of the drop. It usually tops out at 115.

 

[matt_i]

I drop my tank pressure to ~130psi via the pressure switch.

I don't need the extra psi, the motor works harder to go from 75-150 than it does to go from 0-75, which is not a big deal if you use it rarely.

Also of concern is the increased oxygen pressure driving corrosion reactions faster. Rusting the tank if moisture is left in it.

 

[NSXRguy]

i dont have to worry about moisture building up in the tank or lines

i have an electric drain on the tank, and the air goes through an air filter, then air dryer, then dessicant system before it goes out into the circulation

decided to keep my line pressure to 110 to account for drop at the end of the hose

thanks guys

 

[zkling]

You will actually see a decrease in overall efficiency. You need to look into the duty cycle and thermal operating efficiency of the pump. Makers like Quincy offer different valving configs depending on the demand need. If that is a 30 series pump, IR should offer the same.

The bigger concern than "oxygen pressure" is running hot enough to get the moisture out of the valves. Which is very problematic on the quincy disc valve design. Especially on heavy intermittent use.

 

[ChrisLS8]

Run the gun at 175 to bust them nuts

 

[Showkey]

Sorry oxygen pressure does not even make the top 25 list on things to worry about.

Also not worried about wear and tear on an IR 7.5hp compressor.........that thing will last my life time and my kids lifetime. If I was concerned starting and stopping vs running up to the designed 175 psi likely is better in the long run. Similar to my neighbor running his lawn mower at half throttle to save the mower....but it takes twice as long to cut the lawn, not mention the thing bogging down.

 

[sberry]

Sorry oxygen pressure does not even make the top 25 list on things to worry about.

Also not worried about wear and tear on an IR 7.5hp compressor.........that thing will last my life time and my kids lifetime. If I was concerned starting and stopping vs running up to the designed 175 psi likely is better in the long run. Similar to my neighbor running his lawn mower at half throttle to save the mower....but it takes twice as long to cut the lawn, not mention the thing bogging down.

This,,, the people built it spent good money on engineering to design it. Why fiddle with it? Why buy it just to turn it down, a guy could have bought a pos to start with.

 

[NSXRguy]

Run the gun at 175 to bust them nuts

 

[vanapplebomb]

i have a IR 80g 7.5hp compressor rated at 24cfm @ 175psi

...

from my understanding, as the psi goes up, the cfm goes down

will dropping to 90psi which is the working pressure of the impact gun increase efficiency? or just stay at 110psi?

That compressor uses a two stage pump. And, I would be willing to bet that the pump internally compresses the air to around 100 psi before going on to the second stage. So, if you set your pressure switch to around 100psi, the second stage is doing absolutely noting. It’s just along for the ride not doing any compressing, and causing unneeded friction losses, heating the pump.

If you want a compressor less than 175psi, get a single stage. At lower pressures it will be more efficient.

 

[NSXRguy]

maybe im misunderstood

the pressure on the compressor tank itself is unchanged from the 175psi

i was talking about the pressure first coming out of the 3/4" from the tank, into the pressure regulator set at 120psi, then entering the air filter, air dryer, and dessicant system before the general circulation

maybe ill buy a pressure regulator connected at the end of the hose just to see what the actual pressure is at the tool

 

[sberry]

maybe im misunderstood

the pressure on the compressor tank itself is unchanged from the 175psi

i was talking about the pressure first coming out of the 3/4" from the tank, into the pressure regulator set at 120psi, then entering the air filter, air dryer, and dessicant system before the general circulation

maybe ill buy a pressure regulator connected at the end of the hose just to see what the actual pressure is at the tool

You got it right. Tank pressure to regulator. You can check it at the tool but some guessing is good enough. 50 ft of 3/8 on a reel needs 120/130 to get 90 to a heavy 1/2 impact, most the rest of the tools a little less. Can run 2 6 inch da from that with a t and not even notice. Most tools a little drop doesn't hurt, it's not a Nascar race.
Over the years have tested most of it. Not worth any worry. My comp on 175 24/7 for 45 years.
We were working the other day and noticed we actually DID run 2 air ratchets at the same time,,, so rare I commented on it, a few seconds overlap, couldn't tell except for the sound.

 

[matt_i]

So here's another relevant question...Anyone's air prep stuff (filtration) rated for 175psi? What I'm getting at is a lot of industrial stuff is rated at 0.9 MPa ~~=~~ 130psig.

I'm just assembling some filters this week (Master Pneumatic - Detroit) and those are 150psi rated. They are going to run on the tank pressure/line pressure so that is important to me.

I had to dig quite hard in my component pile to find gages that are 150psi rated for showing the 130psi tank-pressure. I have some very nice Ashcroft gages but those are only 100 psi so they have to be used past a regulator.

I've worked in large industrial plants for ~25 years now and none of them ran over 110psi. My thought on it is: leaks. If you leave your air compressor "on" (energized and waiting for the pressure switch to drop) its eventually going to cycle in the middle of the night or when you don't expect it. Its excellent to chase leaks in the name of saving on your electrical bill but those pesky quick connector bodies are the worst. You can of course disable the compressor motor to allow just 1 tank of air to be lost.

I have to take technical issue with the thought that the compressor is more efficient by running it up to the 175psi limit. A good mechanical analogy is a tractor pull where the tongue weight slides forward (denoting back pressure/tank pressure rise) and increasing the HP needed to continue work. The last few feet of tractor pulling isn’t an efficient process. To pull the same linear distance, it would be more efficient to stop halfway, reset the tongue weight, and continue on, which is a back to the analogy of lowering the tank pressure and running the motor on 2 cycles instead of one.

The compressor is built for this, for sure, and a few hundred cycles isn’t going to kill it…but at the end fo the day I’m the person out at the end of the rope paying for everything involved, the electrical energy, the initial cost of the compressor, etc. My take is to check all of the boxes in trying to get what I need by minimize my long-term costs.

Some people are very threatened by the difficulty of adjusting the pressure switch, but its hardly a thing. One removes power, opens the cover, takes a hollow shaft nut driver or deep socket, and loosens the nut a few turns to relax the spring, then restore power and run the compressor a cycle and see where it stops. As long as power is off while adjusting and the over-pressure relief valve(s) are working then its hard to make a mistake.

 

[sberry]

Air isn't effeciency anyway, not why it's used. It's used to get work done, used to multiply effort. This isn't an industrial plant either. As for gages, got them in 100#, 200, 400, 3000# and maybe a 4k one kicking around. I use some 200 on water, they actually cost the same, come from the same dealer. I keep them in stock.

 

[dylanmitchell]

Why does SCFM drop with higher PSI? 90 PSI ratings always have lower CFM than 40 PSI ratings but it seems counterintuitive that more pressure equals less air flow.

 

[sberry]

It takes a little more work but in the end hi pressure is more stored energy and is more effeciency to transmit. Let's smaller pipe and hoses to be used to get the same volume. Pressure differential is what makes air flow. Low pressure takes larger pipe and while you could dump volume the tool would simply come to a stall as the work load went up.

 

[TRWham]

Why does SCFM drop with higher PSI? 90 PSI ratings always have lower CFM than 40 PSI ratings but it seems counterintuitive that more pressure equals less air flow.

Remember that air compressors are rated based on the volume of air they take in at atmospheric pressure, not the amount they discharge. For piston compressors there is a small amount of air which cannot be expelled from the cylinder before the discharge valve closes. It is known as clearance or dead volume and is the space within the discharge port and between the piston and cylinder. Some pressure differential is required to open the valve, so this air re-expands from slightly above discharge pressure on the next intake stroke and will not allow the piston to draw in any air until the cylinder pressure drops below the intake or inter-stage pressure as the piston moves back down. The higher the discharge pressure the more stroke is wasted before the pressure equalizes. Disc valves may have less clearance volume than reeds so they lose less capacity to re-expansion. Screws and scrolls have no re-expansion so you will see they lose no capacity with higher pressure.

 

[sberry]

Good splainin.

 

[jkesselr]

I have the same compressor as the OP and am just now getting it hooked up. Is there any issue with running the straight 175 psi direct to my impact, DA, air saw, ratchet, etc?

 

[ClappedOutBport]

I have the same compressor as the OP and am just now getting it hooked up. Is there any issue with running the straight 175 psi direct to my impact, DA, air saw, ratchet, etc?

Uh yeah. If you want your tools to last you're gonna need a regulator.

 

[ddawg16]

i have a IR 80g 7.5hp compressor rated at 24cfm @ 175psi

originally bought it to run a blast cabinet

now, the compressor is mostly used for a impact gun and occasional tire changer use

from my understanding, as the psi goes up, the cfm goes down

will dropping to 90psi which is the working pressure of the impact gun increase efficiency? or just stay at 110psi?

Don't confuse efficiency with performance.

Not sure where you heard "as the psi goes up, the cfm goes down".....not true.

CFM has a lot of variables.....If we are talking about the CFM straight out of the compressor pump....then, yes....that would be true. But no one runs an impact wrench straight off a compressor...it's run off the tank.

The spec for a compressor is going to be X CFM at Y Pressure. It's kinda like the V=IR. As you change one variable, one of the other values changes.

 

[cvairwerks]

From the Ideal Gas Law, P1*V1=P2*V2 Change any one variable and the others change accordingly.

 

[ddawg16]

From the Ideal Gas Law, P1*V1=P2*V2 Change any one variable and the others change accordingly.

That applies to pressure and volume....CFM is a different matter.

 

[ClappedOutBport]

That applies to pressure and volume....CFM is a different matter.

Uh. You might want to reconsider that. A cubic foot, is after all, a unit of volume. Adding per minute just gives it a rate basis. In other words, P1Vdot1=P2Vdot2

 

[sberry]

I was told there wouldn't be any math. But cfm is the volume.

 

[TRWham]

...

Not sure where you heard "as the psi goes up, the cfm goes down".....not true.

...

Probably from all the air compressor manufacturers who publish lower capacities at higher discharge pressures.

 

[ddawg16]

We need to get back to basics and the original context of the OP's post.

i have a IR 80g 7.5hp compressor rated at 24cfm @ 175psi

originally bought it to run a blast cabinet

now, the compressor is mostly used for a impact gun and occasional tire changer use

from my understanding, as the psi goes up, the cfm goes down

will dropping to 90psi which is the working pressure of the impact gun increase efficiency? or just stay at 110psi?

As I understood it, the OP was asking about the effect on the impact gun. Ideal gas law does not really fit here. Unless he is running the impact gun faster than the actual compressor can keep up with, the determining factor is the volume and pressure of the tank and pressure drop at the gun due to hose sizes.

Uh. You might want to reconsider that. A cubic foot, is after all, a unit of volume. Adding per minute just gives it a rate basis. In other words, P1Vdot1=P2Vdot2

Yes....but that is under static conditions. In the OP's case, we are talking about flow.

Probably from all the air compressor manufacturers who publish lower capacities at higher discharge pressures.

On just the compressor side....if we use the spec the OP gave us, 24 CFM @ 175 PSI, then we assume it puts out 24 CFM at that pressure. If the pressure is lower....say 100 PSI, then the CFM would be higher.

But that is not the determining factor for the impact gun. The OP is not driving the impact gun directly off the compressor....he is running it off the tank. The compressor fills the tank.

As the pressure goes up, the efficiency goes down (mainly due to heat losses). The impact gun will be more efficient at higher pressures.