Industrial Utility Efficiency

The Impact of Altitude on Air Compressor and Tool Sizing


Two things happen to the air at higher altitude:

  1. The air pressure is reduced. Air pressure is a factor of the weight of the air above; as altitude increases there  is less air above . . . so less pressure. For Example: At Sea Level the air pressure is 14.69 pounds per square inch (psi), while at 5,000 ft. above sea level it is only 12.22 psi. This decreased air pressure causes something else:
  2. The air is lighter or less dense. Another way to say this is that there are less molecules of air (or less weight) in a given volume; i.e.: One cubic foot of air weighs less— or has fewer air molecules in it at 5,000 ft. above sea level than at sea level.

The purpose of this article is to analyze what effect this has on the air compressor and using tools or air motors. Keep in mind the following definitions:

  1. FAD / ACFM - This is the actual air delivery of the compressor in cubic feet per minute rated at inlet conditions of the compressor. ACFM is also called “cubic feet of free air per minute.” Most air compressors are rated in ACFM.
  2. SCFM = “Standard” cubic feet per minute is ACFM  / FAD  converted to “Standard” intake conditions - (60F., 60% RH & 14.7 psig usually) for rating using equipment. To size for other than standard conditions, i.e. Altitude or Hot Weather “corrections” must be made.

Correction Factors (Multiplier)
For Estimating Performance at Altitudes (Single Stage)


Altitude—Feet Above Sea Level






Pressure PSI

Act. Cap.



Act. Cap.



Act. Cap.



Act. Cap.



Act. Cap.















































The Effect of Altitude on Air Compressors

Reciprocating compressors - Single Stage

Because of the increased compression ratios and consequent increased effects of reexpansion when operating at higher altitudes, single stage reciprocating compressors lose volumetric efficiency considerably more than injected-cooled rota- ries. We have included a table that can be used for correcting the capacity (and horsepower) of an air system from a single stage reciprocating compressor. These multipliers are good for estimating only . . and for exact details the compressor manufacturer should be contacted.

Reciprocating Compressors - Two Stage

Two stage reciprocating compressors, although not losing capacity as much as single stage, also will lose more than injected-cooled rotaries. Some manufacturers offer special altitude units. We have included a table that can be used for correcting the capacity (and horsepower) of a two stage reciprocating system. Again, these multipliers are only for estimating and for exact details the compressor manufacturer should be consulted.

Rotary Compressors – Injected Cooled - Vane or Screw

Non-lube rotaries will have about the same losses as a comparable single/two-stage reciprocating and other similar positive displacement units.

The air intake pressure is reduced so the compression ratio  is increased to obtain the same discharge pressure. For ex- ample: At Sea Level we have a compression ratio for 100 psig of 114.69/14.69 = 7/8:1 where at a 10,000 ft. elevation we have an intake pressure of 10.10 psia and a    corresponding compression ratio of: 110.10 psia/10.10 psia = 10.9:1.
1.    This increased compression ratio causes a loss in volumetric efficiency (VE) because the increased pressure differential causes greater leakage back to the intake and greater    reexpansion effect.

The following chart shows the actual loss in capacity of an injected-cooled single-stage rotary compressor is only slightly more than 3% at 10,000 ft.

Example: Using this chart, what acfm will we get from an injected-cooled rotary at 10,000 ft. from a compressor delivering  600 acfm at sea level? The chart shows a 96.8% of rated capacity at 10,000 ft. altitude; therefore, .968 x 600 = 581 acfm air available from acfm lubricated rotary compressor  at 10,000 ft. elevation.

2.    You would expect the horsepower requirement of the compressor to be increased because of the increased com- pression ratio, and it is . . . but this increase is offset by the fact that the volume of air being handled is less dense and lighter - meaning that even though the compressor is handling almost the same volume of inlet air it is a lower “mass flow” or “weight” of air. The net result is that the horse- power draw of a lubricated rotary compressor remains almost constant as altitude increases.


Effect of Altitude on Air Tools 

To visualize what happens in an air tool with regard to air flow, think of the tool as an orifice that will pass a given amount of air by weight with a given pressure differential. For example: a given air tool (orifice) will pass 600 acfm of   air at sea level with a 100 psi pressure differential. If we go to a higher altitude with the same tool and the same pressure differential we are now using a lighter, less dense air by weight and consequently the tool (orifice) offers less resistance and will be able to pass a higher volume. In other words . . . a greater volume of less dense (lighter weight) air is required to establish the same working pressure against the orifice (or tool).



In order for a compressed air system to be designed properly, altitude must be taken into consideration. In order to calculate the demand profile, tool air consumption must also be adjusted for altitude.


For more information contact Hank van Ormer, Technical Director, or Don van Ormer, Senior Auditor, Air Power USA at tel: 740.862.4112, email:

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