Page 23 - Reciprocating Compressors Operation Maintenance
P. 23

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.
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