Page 22 - Reciprocating Compressors Operation Maintenance
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Reciprocating  Compressors and Their Applications  9


           * Dynamic  compressors   are  rotary continuous-flow  machines  in
             which  the  rapidly  rotating  element  accelerates  the  gas  as  it  passes
             through the element, converting the velocity head into pressure. This
             occurs partially in the rotating element  and partially in stationary dif-
             fusers  or blades.
           * Centrifugal  compressors  are  dynamic machines in  which one  or
             more rotating impellers, usually shrouded on the sides,  accelerate the
             gas.  Main gas flow is radial.
           * Axial compressors are dynamic machines  in which gas  acceleration
             is obtained by the action of the bladed rotor. Main gas flow is axial
           •Mixed   flow  compressors  are  dynamic machines  with an  impeller
             form  combining some  characteristics  of  both the  centrifugal and
             axial types.

         __                 COMPRESSOR DEFINITIONS                    __


           Gauge pressure (psig) is the pressure, in pounds per square inch, above
         local atmospheric pressure.
           Absolute  pressure  (psia)  is  the  existing  gauge  pressure  plus  local
         atmospheric  or  barometric  pressure.  At  sea  level,  absolute  pressure  is
         gauge  pressure  plus  14.7  psi.  At  elevations  above  sea  level,  the  atmos-
         pheric  pressure  or barometric  pressure  becomes  less.  For  example,  it  is
         approximately  12.2 psia at 5,000 feet  elevation.
            Displacement  of  a compressor  is  the  volume swept through the  first-
         stage  cylinder or  cylinders  and  is  usually  expressed  in  cubic  feet  per
         minute.
            Free  air is air at normal atmospheric  conditions.  Because the  altitude,
         barometric  pressure,  and  temperature  vary  at  different  localities  and  at
         different  times, it follows that this term does not mean air under identical
         conditions.
            Standard air unfortunately does not mean the same to everyone.

            1. ASME power test code defines  air at:
              68°F;14.7psia;RHof36%
            2. Compressed Air Institute defines air at:
              60°F;  14.7 psia and dry
            3. Natural gas pipeline industry defines air at:
              14.4 psia; @ suction temperature
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