Page 22 - Reciprocating Compressors Operation Maintenance
P. 22
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