5 replies to this topic

[Sadananda Konchady]

For a given compressed air flow in terms of Nm3/h or scfm, will more power be required for an air compressor at an altitude of 1500 m. compared to sea level?

Thanks,

Escape

[Art Montemayor]

The altitude at which a compressor is installed must always be given consideration. As altitude above sea level increases, the weight of the earth's atmosphere is less. This is reflected in the barometer and absolute intake pressure which decrease with altitude. This fact is well understood and allowed for with process compressors. It is not always considered with standard air compressors that are usually used for 100 psig service to operate pneumatic tools and rock drills. These are, at times, moved from one site to another without thought as to original design conditions.

The horsepower formulae show that with decreased values of the absolute initial pressure, P1, the work will decrease. The value of the Compression Ratio increases if the discharge gauge pressure is kept the same as at lower altitude, but this has a smaller effect on the power than the decreased initial absolute pressure, so that at altitudes above sea level, the horsepower of compressing a given volume of air at atmospheric pressure decreases.

A given compressor's ability to operate tools (or instruments) varies with the installation elevation. The effect of moving a 2-stage compressor (designed for sea level operation) to 10,000 feet altitude is shown in the table attached.

It will be noted that as the altitude increases,

• The actual capacity at intake (column 4) decreases only slightly;
• The dense air delivered to the tools decreases materially (columns 5 and 6); and,
• The Brake Horsepower (Bhp) decreases materially. See the comments below.

Column 6 is the measure of the number of tools operable with this compressor as compared with sea level operation.

The example is somewhat extreme since commercial sea level rated 2-stage reciprocating compressors are sold for operation only to 5,000 feet altitude. At higher altitudes, the low-pressure cylinder size has to be increased to provide greater inlet capacity and to bring the Bhp on the frame and running gear closer to normal values.

Single-stage reciprocating and other positive-displacement compressors are limited somewhat by the allowable compression ratio and discharge temperature. They frequently must be derated materially for altitude operation. The manufacturer should always be consulted when applying positive displacement compressors at altitudes far above sea level.

Although the power required by a given compressor falls as the altitude increases, the ability of engines and electric motors to safely develop this power usually falls even more rapidly. Therefore, the question of the suitability of a compressor and its driver for other than design altitude applications should be carefully discussed and consulted with the manufacturer.

 Recip_Air_Compressor_at_Altitude.xls   16.5KB   458 downloads

[Sadananda Konchady]

Thank you very much Art for clarifying the effect of altitude on air compressors.

I am aware that the altitude effect on combustion air fans in boiler applications is significant where the mass flow of air is the desired parameter. Our application was for the purpose of meeting instrument air in the plant.

[Dineshpoduval]

I have recently measured power input of a few GA 110 model of atlas copco screw compressors at 3000 ft above msl and 11000 ft. above msl.

There were 3 nos compressors each with the above rating. The power input to compressor at 3000 ft ( and temperature of 20 deg.C) was 104 kW. At 11000 ft( and temperature of 6 deg.C), the power was 84 kW.

Rotational speed of the compressor being the same, I suspect the air delivery in Nm3/h could be less at high altitude.

[katmar]

The original question was quite specific in stating that it concerned a given flow rate in Nm³/h (which is effectively a fixed mass flow rate), but it did not state whether the delivery pressure was to be the same in gauge or absolute terms. Let's put some example numbers to the question to see what impact this has.

Assuming that the compressor suction is at atmospheric pressure and the delivery pressure is 10 bar then:

1. If the delivery pressure of 10 bar is 10 bar gauge then at sea level the suction pressure is 1 bar absolute. This makes the compression ratio (10+1)/1 = 11. At an altitude of 1500m the atmospheric pressure would be about 0.88 bar absolute. To achieve a delivery pressure of 10 bar gauge again the compression ratio is (10 + 0.88)/0.88 = 12.36

2. If the delivery pressure of 10 bar is 10 bar absolute then at sea level the suction pressure is 1 bar absolute. This makes the compression ratio 10/1 = 10. At an altitude of 1500m the atmospheric pressure would be about 0.88 bar absolute. To achieve a delivery pressure of 10 bar absolute again the compression ratio is 10/0.88 = 11.36

These examples show that it does not matter whether you are trying to achieve a fixed delivery pressure in gauge or absolute terms. In both cases the compression ratio increases as the altitude increases. If the mass flow rate is constant, and the compression ratio increases, then more work is being done and the power used has to increase.

The example given by Dinsehpoduval where the power used by a specific model of compressor decreased as the altitude increased is true to what happens, but as he suspects the flow rate in mass terms will definitely decrease and this example does not represent the original question. In order to achieve what the original question asked either a different compressor, or a different speed with the original compressor, would have to be used. This muddies the situation a bit because the compressor efficiencies would probably be different as well and this would have an impact on the power used. But if we make the assumption of constant compressor efficiency then we can say that to achieve a fixed delivery pressure and a fixed mass flow rate then you will use more power at a higher altitude.