Compressor's efficiency

Hmmmmm I know that an air compressor works best above sea level...

If its below, water gets into it....

If you mean sea level atmospheric pressure, then yes there will be an adverse effect of operating a compressor at low air pressure levels, as if its a positive displacement, the mass of air that can be drawn in, per stroke is less at low air pressure, so efficiency will be reduced...

John.

Yes, if air compressor, various sea levels impact various pressures at suction, hence various efficiencies possible.

But, a gas compressor taking from a source which is not influenced by atmospheric pressure should not have a effect of efficiency as such.

If you are compressing a gas in a closed low pressure system to another closed high pressure system then atmospheric pressure means nothing.

If you are sucking in air to compress it then the higher the atmospheric pressure the less the work needed to compress the air.

So if you are at 14.7 PSI at seal level and your compressor is 10:1 = 147 PSI out. fly that in a plane at 10,000 feet and the imput might be 7.5 PSI and so your compressor will create air at 75 PSI on a 10:1 ratio

Both Aurizon and Yesyen are correct. In a closed system where the inlet and discharge gas temperatures and pressures are constant using absolute temperatures and pressures, changes in elevation have no effect on the efficiency of the compression unit, since the gas density, mass flow rate and pressure ratios are identical.

If by "compressor" you mean the entire package including the driving unit or "prime mover" and the prime mover is a gas-powered engine drawing air in from the atmosphere, then elevation will affect the efficiency of the entire package by affecting the effectiveness of the prime mover.

Through my involvement in the gas compression business, I have dealt with both definitions regularly, hence the caution.

I think we can safely say that the original question would not relate to a closed system as a closed system is exactly that.

However, as I stated, with a normal ambient air pressure to output compressed air system the efficiency would be severely effected by a low ambient air pressure...

In the worst case if the air pressure was zero then the motor would drive the compressor with no output and hence the efficiency would be 0%!!!

For a constant pressure output and with a fixed compression ratio the efficiency would improve with increasing ambient air pressure up to the point that the compressor is designed to work and above this point the efficiency would fall again, do to overloading of the drive motor.

John.

With regard to most air compression systems, where ambient air is drawn into the compressor inlet and the discharge pressure is controlled via a regulator which references the ambient conditions in the vicinity of the air compressor, Electroman is correct.

With regard to process gas systems which are typically where high pressure gas compressors are found, the gas system either is a closed loop system or closely approximates a closed loop system since the inlet and discharge pressures are generally well controlled. In this case, the closed loop assumption stands as described by Yesyen and Aurizon.

I believe our only disagreement is whether Asim is using an open loop (standard air compression package), a well regulated open loop (continuous process gas system), or a closed loop. At this point, Asim should be able to tell us which type system he is working on. Either way, I hope we've helped you Asim.

I presume you mean what is the effect if the compressor is taken to altitudes above sea level? If so, I can explain I used to sell turbochargers in Spain to people in Madrid (Which is at a very high altitude) and Barcelona,(Which is at

Sea level on the Med.). The customer bought Turbochargers designed to run at Sea Level and when these were operated in Madrid, the Diesel engine had higher smoke levels owing to the lower density at Madrid's altitude. So, despite the compressor speed increasing at high altitude, the performance and efficiency was worse at altitude. De-rating the fuelling of the engine in Madrid was one solution.