Air Compressor Terminology 385r4

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AIR COMPRESSORS Measuring compressed air: Compressed air equipment is measured in pressure– pounds per square inch gauge (psig), and flow-cubic feet per minute (cfm). Depending on the type of product and the manufacturer, the cfm rating may be displacement, actual, free, standard, or inlet.The differences are:

Displacement cfm: (DCFM) Displacement Cubic Feet per Minute. Generally quoted in reference to reciprocating air compressors.This is the volume of air displaced by the piston. It is not the amount of air actually produced. It is a measurement of the air prior to considering inefficiencies and internal losses. Actual cfm: (ACFM) Actual Cubic Feet per Minute.This is the actual amount of air produced by an air compressor. However, this can be measured at various points of the air compressor package, including prior to considering internal losses that occur before the air exits the package. Free cfm: (FAD) Free Air Delivered.This is the actual amount of air produced by an air compressor, and measured at the discharge of the package. It is measured after all internal losses are considered. Standard cfm: (SCFM) Standard Cubic Feet per Minute. This is the amount of air measured at a stated standard set of conditions.The generally accepted standard conditions are 14.696 psia, 60°F, and 0% relative humidity. Inlet cfm: (ICFM) Inlet Cubic Feet per Minute.This is the amount of air measured at the inlet to the air compressor package. It is a measurement of the air prior to considering inefficiencies and internal losses. How can you measure a compressor’s output? Pump up time [in minutes]: Receiver tank volume [gal.] x [final tank pressure–initial tank pressure] 7.48 x atmospheric pressure [psia] x compressor delivery [cfm]

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Reciprocating Air Compressors: • For proper installation, vibration pads should be utilized between the baseplate (or tank feet s) and a level floor. • A high temperature rated steel braided, vibration isolation flex hose should be installed between the compressor discharge [or vacuum inlet] and any permanent plant piping. • Dual control means both automatic start & stop and constant speed control. Rotary screw air compressors: • Rotary screw compressors can, and “like” to be run 100% loaded. • Always install an air receiver tank with a rotary screw air compressor. [Minimum of 1 gallon capacity per each cfm that the compressor is rated for.] • Generally speaking, 125 psig rated oil flooded rotary screw air compressors produce approximately 4 cfm per horsepower. Filters: • Typical, standard compressed air filters have a 3–10 psi drop from inlet to outlet, depending on cleanliness. • Specify a pressure differential indicator on filters to alert of excessive pressure drop and needed element change. Dryers: Refrigerated air dryers: • Will dry to approximately a 33–39°F pressure dewpoint when properly sized. • Will remove approximately 96% of the moisture from the compressed air system. • Typically have a 3–5 psi drop from inlet to outlet. Desiccant air dryers: • Can be sized to dry to a –40°F or a –100°F pressure dewpoint. • Will remove approximately 99.6% of the moisture from the compressed air system. • Require a coalescing (oil-removal) prefilter to be installed in front of them. • Always install a receiver in front of a desiccant dryer, so that the purge flow does not send a false signal to the air compressor to load up.This would cause the air compressor to short cycle.

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