Two Stage Screw Air Compressor

Two stage air compressors work in a very similar manner with the primary difference being that they compress the air in 2 steps or stages. During the first step or stage, air is drawn in and compressed to an intermediate pressure. After being compressed in the first stage, the air is piped, usually through an intercooler where the air is allowed to cool, to be compressed in the final or second stage. Two stage compressors are normally good for pressures up to 200psi. Two stage pumps are more efficient at higher pressures because the air is cooled between the stages.

DCFM, SCFM and ACFM

            Displaced CFM (DCFM) is a mathematical formula that calculates the bore, stroke and rpm into a CFM figure(Bore x stroke x rpm/2200=DCFM). This figure will always be the highest CFM because this formula does not take into account variables like temperature, atmospheric pressure, humidity, friction and heat dissipation.

Another CFM term often used is Standard CFM (SCFM).  It defined as the measured flow of free air and converted to a standard set of reference conditions (14.5 PSIA, 68 Degrees F, and 0% relative humidity).

Yet another CFM term is Actual CFM (ACFM). AFCM can be determined in a number of different ways. The most common methods include measuring the volume of air that is moved through an orphic plate or measuring pump up times on large compressor tanks and running through a simple calculation. This CFM number takes in effect all the variables and will give the true output of the pump at the current working conditions (i.e. temperature, altitude, humidity, …).

Often times, CFM numbers are also shown at various pressures.  These numbers can be very useful to help determine if a compressor produces enough CFM for the desired application, but can be confusing when comparing differing pressures or volumes or different compressors.

The best way of comparing compressors is through SCFM. Since all the measurements are calculated back to a set of reference standard conditions, it levels the playing field among the multitude of different manufacturers.

Electric Motors and Horsepower

         Compressors are often rated by Horsepower(HP). As simple as this sounds, there are different variations of HP.  Some compressor manufacturers rate their air compressors by peak horsepower, also known as brake horsepower. Peak horsepower is the maximum output that a motor can produce while the motor has the start windings engaged. Peak HP can be as much as 5-7 times the rated or running HP. Under normal operating conditions, the start windings are only engaged for a small fraction of a second. Therefore, using peak horsepower as a comparison tool can be somewhat misleading since under normal operating conditions the motor only develops this horsepower during startup.  If  a motor drive system causes the start winding to remain engaged for a long period of time, the motor will either overheat if it has thermal protection or fail prematurely.

Most electric motor manufacturers rate their motors by the horsepower developed after the motor has come up to its designed operating rpm’s and disengaged the start windings. This is often referred to as running or rated horsepower and is a true indication of the HP a motor can sustain over a long period of time.

Some other factors to consider when looking at motors is Duty Cycle and Service Factor.  Duty cycle is normally rated as either intermittent or continuous and is defined as the time rating under full load.   In other words can the motor run at full load horsepower continuously or only intermittently. The other of the two noteworthy factors is the Service Factor (S.F.) rating of the motor. It is defined as the percentage of rated horsepower at which the motor can safely operate (i.e. 1.15 SF = 115% of rated HP).  Higher service factors allow motors to handle more varied conditions without causing motor overheating or premature motor failure.  Examples that could cause a motor to run within its Service Factor could be caused by low voltage, higher ambient temperatures, higher startup load …etc.

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