Does high altitude make my compressor run longer?

[38Chevy454]

I recently moved from approx 300 ft elevation in CA to 7200 ft in NM. I can't say I have done any scientific tests or measurements, but it sure seems that my air compressor takes longer to fill.

Is it my imagination or is this a true observation? I know the atmospheric pressure is lower and so in theory that would affect the pressure pushing air into the inlet. Just like your car loses power because thinner air, the compressor would take in less air to compress? The compressor still makes it to the same cutoff pressure per the gauge, but it just seems to take longer to reach that point.

Am I crazy or can someone confirm my theory?

 

[wreckercologist]

Yes, the higher your elevation, the lower the CFM.

 

[rsanter]

yes
there is a higher differential pressure that has to be met to trip your switch

bob

 

[Schrodingers Cat]

less density = more pumping for the same pressure
plus the inlet pressure is lower, so less intake volume per stroke

 

[M3Pilot]

Another yes. I took fisics in kollege a long time ago & there's somebody's Law of Something that explains this phenomenon.

 

[diggertodd]

I moved mine to the second story of my shop, didn't seem to affect it

 

[ddawg16]

Another yes. I took fisics in kollege a long time ago & there's somebody's Law of Something that explains this phenomenon.

Yea....some guy named Boyle....or was it Boil....Bowl....something like that.....

Actually, your pressure is almost 4.5# less at that altitude....or, otherwords....you have only abou 75% of the starting pressure as compared to sea level....so I would expect the compressor to take at least 25% longer to get to the same pressure....

 

[z28snksknr]

Ideal Gas Law:

P x V = n x R x T

Where:
P = pressure in psia (in this case, atmospheric pressure)
V = volume in ft^3 (volume of air per compressor stroke)
n = number of air molecules in "moles"
R = Gas Constant
T = temperature in degrees Rankine (Fahrenheit degrees + 460)

In this case, we can examine the effect of the change in atmospheric pressure will have on the amount (mass, not volume) of air moved by looking at a single stroke of the compressor motor. Since the volume of the motor (one stroke) doesn't change when you move to a higher elevation, the gas constant remains the same (its a constant after all) and assuming the same temperature, we can ignore them in the equation. The comparison of the two scenarios looks like this:

P1 / n1 = P2 / n2

Taking 300' elevation to be basically sea level, P1 = 14.7 psia
7200' elevation atmospheric pressure P2 = 11.34 psia

So for every 1 mass unit of air your compressor pulls in in one stroke, solving the equation for n2 = 0.771 or 77.1% of the air mass at your higher elevation.

In summary, your compressor needs to work 22.9% LONGER to achieve the same "charge" in the tank.

However, since you will have a slightly larger differential when the air tool uses the air, your tools will actually have a bit more power, so you get some of that energy back (the equivalent of raising your supply pressure up by 3.4 psi).

 

[M3Pilot]

Ideal Gas Law:

P x V = n x R x T

Where:
P = pressure in psia (in this case, atmospheric pressure)
V = volume in ft^3 (volume of air per compressor stroke)
n = number of air molecules in "moles"
R = Gas Constant
T = temperature in degrees Rankine (Fahrenheit degrees + 460)

In this case, we can examine the effect of the change in atmospheric pressure will have on the amount (mass, not volume) of air moved by looking at a single stroke of the compressor motor. Since the volume of the motor (one stroke) doesn't change when you move to a higher elevation, the gas constant remains the same (its a constant after all) and assuming the same temperature, we can ignore them in the equation. The comparison of the two scenarios looks like this:

P1 / n1 = P2 / n2

Taking 300' elevation to be basically sea level, P1 = 14.7 psia
7200' elevation atmospheric pressure P2 = 11.34 psia

So for every 1 mass unit of air your compressor pulls in in one stroke, solving the equation for n2 = 0.771 or 77.1% of the air mass at your higher elevation.

In summary, your compressor needs to work 22.9% LONGER to achieve the same "charge" in the tank.

However, since you will have a slightly larger differential when the air tool uses the air, your tools will actually have a bit more power, so you get some of that energy back (the equivalent of raising your supply pressure up by 3.4 psi).

Show Off!!! He,He

 

[ddawg16]

Show Off!!! He,He

Yea....but I was damned close just doing the estimates in my head.....

 

[z28snksknr]

Show Off!!! He,He

With the amount of knowledge around here, I relish the rare times I can chime in with something useful.

Score Update:
Garage Board Members: 58,923,854,095
z28snksknr: 1

 

[38Chevy454]

Thanks for the answers and confirming my suspicions. I should have known to just use the gas law. I never really calculated the significant effect the altitude has.

Maybe this is a good excuse to get a bigger compressor..........but wait I need to build that detached workshop garage first! It really ***** going from 4.5 car garage to 2 car garage. Should be only temporary problem, I plan to build a nice 26 x 40 (or more) detached garage once I get more settled in and get serious next springtime.

 

[Falcon67]

My wife is from Albuquerque (which is how come I know how to spell it), so you picked a darn pretty place to put a shop. I imagine your ears are still popping from the altitude change.

 

[uhcrandy]

Wow, good Physics review. But as I understand it, the tank mesures relative pressure not absolute pressure. In otherwords, if I set my compressor @ 150 PSI, isnt this 150 PSI above the ambient air pressure? So I live at 6000 ft and the air pressue is much less than sea level, but the air inside my take is still at 150 above the air pressure at my elevation. Now how long it take to fill you all are right on, due to less dense air. Am I correct?

 

[ddawg16]

You are correct....the pressure switch is vented....so the relative pressure is the same....

Say that tank had 100 PSI in it at 7200'. Seal the tank and take it down to sea level and it will now read about 95.5 PSI....it has the same 'amount' of air, but the relative pressuer is lower.

Something to add to your 'so what' list...

When they test aircraft fuselages, they will pressureize them up to about 13.5 PSI....a couple of more PSI and it would be the same as being in space....

 

[z28snksknr]

The air inside the tank has zero connection to the air outside the tank walls. However, since the pressure gauge on the tank reads "gauge pressure", which is the pressure above (ambient) atmospheric pressure, the gauge reading on a sealed tank will change with elevation changes but the pressure inside the tank will not.

In other words, your system will shift down 3.36 psia (not psig) on low pressure activation and high pressure cut-off due to the lower ambient air pressure. Since the DIFFERENTIAL remains the same, the potential energy of the air remains constant.

However, since the density of the air being compressed is lower (and increasing the mass of air in a given volume is what generates pressure), per my calc above it will take longer to reach that high pressure cutoff.

 

[porschedude996TT]

Just like you or your car. The perormance drops and starts gasping for air...

 

[z28snksknr]

Just like you or your car. The perormance drops and starts gasping for air...

And for me, liquor tolerance as well

 

[dmw56]

Hey, I moved from 90ft in CA to Edgewood, NM in 2002!

 

[ddawg16]

In most cases, when PSI is used, it implies PSIG.

PSIG is PSI Gauge....or relative to ambient atomsphere pressure.
PSIA is PSI Absolute...or relative to Vacuum...hence, sea level is aprox 14.696 PSIA.

PSIG is sometimes refered to as a vented gauge...i.e., it's vented to outside air...hence it is making a relative measurement.

So....if we take that air tank and pump it up to 150 PSIG at sea level, it will have 150 PSIG in it, or 164.696 PSIA.

If we take that same tank without letting out any air up to 7200'....it will still have 164.696 PSIA in it...no change in total volume. But if we measure the pressure using a PSIG gauge (vented) it will measure about 153.5 PSIG.

Same amount of air....
Or....if you want options....
150 PSIG = 10.34Bar = 305.4 InHg = 4156.13 In Water = 1.034 MP = 7757.24 Torr

 

[Steevo]

I don't have any fancy mathematical formulas to share, but congrats on escaping CA.

NM>CA