Page 23 - Reciprocating Compressors Operation Maintenance
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1 O Reciprocating Compressors: Operation and Maintenance
Unless otherwise specified, the second definition is generally used in
reference to reciprocating compressors.
Actual capacity is a term that is sometimes applied to the capacity of a
compressor at intake conditions. It is commonly expressed by either the
term ICFM (intake cubic feet per minute), or ACFM (actual cubic feet
per minute).
Volumetric efficiency is the ratio of the actual capacity of the compres-
sor to displacement and is expressed as a percentage.
Compression efficiency is the ratio of the theoretical horsepower to the
actual indicated horsepower required to compress a definite amount of gas.
Mechanical efficiency is the ratio of the indicated horsepower in the
compressing cylinders to the brake horsepower delivered to the shaft. It
is expressed as a percentage.
Overall efficiency is the product of the compression efficiency and the
mechanical efficiency.
Adiabatic compression occurs when no heat is transferred to or from
k
the gas during compression (PV = C).
Isothermal compression occurs when the temperature of the gas
remains constant during compression (PV = C).
Polytropic compression occurs when heat is transferred to or from the
gas at a precise rating and where the compression and expansion lines
follow the general law PV" = C.
Frame load is the amount of load or force the compressor frame and
running gear (i.e., the connecting rod, bolts, crosshead, crosshead pin,
piston rod, connecting rod bearings, and crankshaft) can safely carry in
tension and compression, expressed in pounds. This is a design factor,
and any changes in cylinder bore size, suction or discharge pressure, or
the type of gas handled may adversely affect component life. Also,
mechanical failure of the parts could result.
Compression ratio is the ratio of the absolute discharge pressure (psia)
and the absolute inlet pressure (14.696). Thus, a compressor operating at
sea level on plant air service with a 100 psi discharge pressure would
have a compression ratio of 7.8:
(10Qpsig+ 14.7)* 14.7 = 7.8
Piston displacement is the net volume displaced by the piston at rated
compressor speed. On double-acting cylinders, it is the total of both head
and crank end of the stroke. It is expressed in cubic feet per minute, or CFM.