Following are some clarifying definitions of terms used, and often misused, to define capacity or flow in centrifugal compressors.
Capacity is the quantity of air at which the compressor will operate at a specific discharge pressure. For all industrial compressors, capacity is rated at the conditions of pressure, temperature and moisture content existing at the compressor inlet flange. The basic reference for all discussion on air flow relates back to mass flow. All definitions and conversions are directly related to conservation of mass flow through the compressor.
Mass flow (lb/min or kg/hr) is a specific value independent of the air inlet conditions. The compressor functions on how much mass flow (lb/min) flows through the machine. For this reason it is the best means of comparing one flow value to another. The problem with using it all the time is that compressor manufacturers size compressors on actual inlet air volume flow. This is done because the compressor size is a function of the actual inlet volume it can hold.
- lb/min wet is the mass of air including the water vapor in the air.
- lb/min dry is the mass of the air without the water content i.e. dry air (0% relative humidity)
CFM (cubic feet per minute) (M3/min) is a volumetric measurement not dependent on inlet conditions such as temperature, pressure and humidity. When working with centrifugal compressors, this should be clarified (when working with all compressor types, this should also be clarified).
ACFM (actual cubic feet per minute) (M3/min) is the actual inlet air delivered at the discharge flange. ACFM represents useful air and is independent of the seal losses through the machine. The commonly used value for seal losses with carbon seals is about 1%.
ICFM (inlet cubic feet per minute) (or M3/hr) is a measure of the air entering the compressor referred back to inlet conditions. ICFM is the most common method of determining centrifugal compressor selection. CAGI and PNEUROP often rate centrifugal compressors in icfm. The only difference between icfm and acfm is that acfm is measured at the compressor discharge flange, whereas icfm is measured at the compressor inlet flange or at the discharge flange and corrected for seal losses. Some centrifugal compressors may have other air losses between the inlet and discharge flanges.
Unlike positive displacement type compressors where icfm is almost always significantly higher than acfm and with the exception of such small items as potential seal leakage, icfm and acfm are often used interchangeably with regard to capacity flow ratings.