Here is the basic knowledge about pressure and air volume that can be useful to learn about compressors.
Gauge pressure and absolute pressure
The atmosphere that surrounds us actually also has pressure. Air is actually compressed by its own weight and generates force. This is referred to as the “atmospheric pressure." The indication of pressure that does not take into consideration this atmospheric pressure is referred to as the “gauge pressure," while the indication of pressure that does take this into consideration is referred to as the “absolute pressure” (these may also be indicated with “G” or “abs” appended after respective unit symbol). Ordinarily pressure described in documents relating to air compressors is gauge pressure, unless indicated otherwise. All pressure indications on our catalogs are given in gauge pressure.
Air volume
The air flow rate discharged by a compressor is indicated by the volume of air. The most typical unit used to indicate the amount of air is m3/min., which is read “cubic meters per minute."
The air flow rate discharged by a compressor is indicated by the volume of air. The most typical unit used to indicate the amount of air is m3/min., which is read “cubic meters per minute."
It is also important to remember that the amount of discharged air is indicated in terms of suctioned air volume. It may be confusing in that the amount “discharged” is expressed in terms of suctioned air volume, but this is a common rule that applies to the compressor industry. More specifically, the amount of air discharged as featured on catalogs are values of discharged air volume converted to suction conditions. The suction conditions stipulate temperature and relative humidity of suctioned air and the value varies depending on manufacturers and standards.
The unit referred to as Nm3 is also used as a unit of air volume. This is referred to as “normal cubic meter." This is a unit of measure for air at 0°C, with relative humidity of 0% and under atmospheric pressure, which called Normal conditions. The suction conditions of values featured on catalogs are stipulated by standards and manufactures as described above, and air volumes described on catalogs are ordinarily not expressed in Normal conditions. Since ambient conditions for catalog air volume and Normal conditions are different, a conversion is necessary to compare figures expressed in one with the other. This becomes slightly specialized, but the formula described below may be used to convert the amount of air under certain conditions to Normal conditions.
[Reference: Method for conversion of air volume to Normal conditions]
As shown by the result of the calculation, the 10m3 of air with conditions of [temperature: 30°C, humidity: 75% and atmospheric pressure] converted to Normal conditions is about 8.7 Nm3.
Careful attention is required when selecting a compressor when the amount of air required by the facility is given in Normal conditions, for instance, since the value for the amount of air changes depending on ambient conditions as described above.
