Page 139 - Root Cause Failure Analysis

Compressors 127

FIRsfsIAGE FIRST-STAGE CONDENSATE
DIFFU!ER INTERCOOLER SEPARATOR

BULLGEAR

FOURTH-SAGE
ROTOR

AGE

DISCHARGE
Figure 10-4 Bullgear centri@gal compressor (Gibbs 1971).

tive-displacement compressors, especially reciprocating compressors. The standing-
wave pulses created by many positive-displacement compressors create enough varia-
tion in the discharge manifold to cause potentially serious instability.

In addition, the large helical gear used for the bullgear creates an axial oscillation or
thrusting that contributes to instability within the compressor. This axial movement is
transmitted throughout the machine train.

Performance

Compressed-air systems and compressors are governed by the physical laws of ther-
modynamics, which define their efficiency and system dynamics. This section dis-
cusses the first and second laws of thermodynamics, which apply to all compressors
and compressed-air systems. Also applying to these systems are the ideal gas law and
the concepts of pressure and compression.

First Law of Thermodynamics
This law states that energy cannot be created or destroyed during a process, such as
compression and delivery of air or gas, although it may change from one form of
energy to another. In other words, whenever a quantity of one kind of energy disap-
pears, an exactly equivalent total of other kinds of energy must be produced. This is