Dateline: May 29, 2007

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Technically Speaking

How can altitude affect the performance of a compressor?

"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 increase 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,

1. The actual capacity at intake (column 4) decreases only slightly;
2. the dense air delivered to the tools decreases materially (columns 5 and 6); and,
3. 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."

Milton Beychok via Cheresources.com Forums