Can you use the Ideal Gas Law to determine air compressor capability? 3

[chopengnr]

I need to determine the present cfm that we are running in our compressed air system. Can I use the following:

P1V1=m1RT1 ; P2V2=m2RT2

assuming m1=m2 and RT1=RT2 yields P1V1=P2V2

My company has a 10 hp air compressor rated at 33cfm at 175psi. We typically run around at 130 psi. Using the above you get:

(33cfm)(175psi)/(130psi) = 44cfm

this seems to follow the rule of thumb of 4-5cfm per HP (I read it on another website)

 

[TD2K]

First, you need to work in absolute pressures, not gauge pressures for the ideal gas law. 175 psig is about 190 psia (add 14.7 psi to your gauge pressure, technically the 14.7 applies to sea level but you are likely fine unless you are in Denver).

Secondly, is the 33 cfm at 175 psi or is it at standard conditions? Other than that, your approach is right.

130 psi will also heat up the air less than if the compressor is having to discharge it at 175 psi but I assume you have some sort of cooler so that's wash more or less, at least for what you are doing.

 

[chopengnr]

the 33cfm is at 175psi.

 

[SooCS]

For a quick estimate, your assumption for T1=T2 may be adequate but in reality, when you compress gas the temp will increase significantly. Eg compressing air from atm to 100 psig, the temp could increase to about 200 deg F depending on the efficiency of the compressor and the temperature change from 130 to 175 psi is about 80 deg.

I am not a compressor expert but shouldnt the capacity of the PD compressor be expressed in term of suction pressure? The suction capacity should not change regardless of the discharge pressure especially if the rpm remain the same. In your case the energy to compress the air to 130 psi is less than that to compress to 175 psi, but the net volume in term of the suction pressure remain the same.


Net capacity of a compressor in m3/h @ 101 kPa and suction pressure is as express in GPSA

m3/h = PD x VE x Ps/ (100x 101 x Zavg)

PD = piston displacement, m3/h
VE = Volumetric efficiency = 96-r-C{Zs/Zd[r^(1/k)-1]}
r = compression ratio
Zs = compressibility at suction
Zd= compressibility at discharge
Ps = Suction pressure
Zavg = average compressibility

 

[TD2K]

Chopengr, I think your 33 cfm is at standard conditions, not 33 ft3/min of compressed air at 175 psig. The compressor may be rated to deliver air at 175 psig but I bet its capacity is at standard conditions.

If that is correct, then you will have very little additional capacity running at 130 psig discharge pressure versus 175 psig since as SooCs pointed out, the capacity of a recip is essentially set by suction conditions, discharge has a MUCH smaller effect. Your clearance effects will be reduced at 130 psig giving you more air but it's not going to be much.

Just as a check, I did a very quick calc for 33 scfm of air being compressed from atmospheric presssure to 175 psig in a 2 stage compressor. Required Hp was about 7 Hp which matches with your 10 Hp compressor. 33 cfm of air at 175 psig would be nearly 425 cfm at standard conditions (I'm assuming here that the compressor discharge is cooled so I don't have to account for difference in the air's temperature between where you say you have 33 ft3/min versus standard conditions). 425 scfm would need a lot bigger compressor than 10 Hp.

 

[chopengnr]

Oh, I see. The supply side has to be at standard conditions in order to get 33cfm, regardless of what output pressure I run. So basically, I'm not going to get any more significant capacity by adjusting the pressure...that makes sense.

If that's the case, how can I get more capacity if I need it? Addtional compressors? I've got an unused screw compressor (rated 35cfm 125 psi) that I may can add to the system. It has a dryer that I may can use as well.

I know these are elementary questions, but this is the first time I've dealt with a compressed air system.

 

[zdas04]

As an aside, compressors just don't create the gas they are compressing. If you put 100 molecules of gas into a (itty bitty) compressor at atmospheric pressure, you will get 100 molecules of that gas out the back side at a higher pressure and a higher temperature (because PV really does equal NRT).

There is a lot of arithmetic out there to calculate the Hp input required to compress a given volume (in standard units) from a selected pressure up to a selected output pressure. From those calculations you'll see that discharge pressure has a much larger impact on Hp required than it does on throughput.

You get more capacity by changing either the compressor size/configuration or by changing the Hp or both.

 

[TD2K]

Another compressor is about your only solution.

You 'could' run your existing compressor faster IF that is possible (eg, you have a belt drive and change out pulley sizes) but you should talk to the vendor, increasing rpm is not something I'd want to decide on my own. Plus, even if the compressor will handle a higher speed, you might need to increase the motor depending how much faster you drive it.

If you've got another compressor available, I'd go that route.

 

[ccfowler]

Compressors are normally described by their scfm (standard cubic feet per minute--effectively the volumetric flow rate at the compressor's suction port) rating at a particular delivery pressure. Usually, the scfm rating decreases slightly as the delivery pressure increases depending upon the performance characteristics of the particular compressor.

If your compressor is rated as 33 scfm at a delivery pressure of 175 psig, it would probably have a modestly greater scfm rating for a delivery pressure of 130 psig.

Yes, the ideal gas relationships do apply to the performance of compressors, but it is important to recognize the performance characteristics of the particular compressor type. Compressors with multiple stages with intercooling will more nearly approximate isothermal compression (which minimizes power consumption).

So much for the easy part. As the system pressure increases, a greater portion of the ambient humidity that was part of the air drawn into the compressor will drop out as condensate either in inter-coolers, after-coolers, receiver tanks, and the piping system. If the compressor is operating in a warm, high humidity environment, the amount of condensate produced can be substantial. This condensation reduces the net volume of compressed gas (air) to serve the necessary functions of the system. Normally, this is not troublesome reduction in the delivered flow volume. This could be an issue if the compressor capacity is only marginally adequate with low ambient humidity conditions, and then the system must continue to function during high ambient humidity conditions.

Usually, all reasonable efforts should be undertaken to remove moisture from a compressed air system. It is a common source of erratic functioning of equipment and internal corrosion problems. In cold climates, freezing of the condensate can further complicate system problems.

From the equipment that you describe as being available to you, you may do well to consider using the screw compressor as your base load machine and using the other compressor (presumably a reciprocating compressor) to provide additional delivery needs.

Before getting into adding compressor capacity to your system, be sure to check your system to eliminate all leaks and unnecessary uses of compressed air. Very often, careful reduction in the compressed air leaks and uses can eliminate the need to add compressor capacity to a system.

If your most serious problem is supplying an intermittent load remote from the compressor, you may be able to install a receiver tank near that load rather than increasing compressor capacity (and operating costs).