Page 140 - Root Cause Failure Analysis

128 Root Cause Failure Analysis

expressed for a steady-flow open system such as a compressor by the following
relationship:

Net energy added to Stored energy of mass Stored energy of mass
system as heat and work + entering system - leaving system = 0

Second Law of Thermodynamics
The second law of thermodynamics states that energy exists at various levels and is
available for use only if it can move from a higher level to a lower one. For example,
it is impossible for any device to operate in a cycle and produce work while exchang-
ing heat only with bodies at a single, fixed temperature. In thermodynamics, a mea-
sure of the unavailability of energy has been devised, known as enrropy. As a measure
of unavailability, entropy increases as a system loses heat, but remains constant when
there is no gain or loss of heat as in an adiabatic process. It is defined by the following
differential equation:

where Tis the temperature (Fahrenheit) and Q is the heat added (BTU).

PressureNolumeRemperature Relationship
Pressure (P), temperature (T), and volume (V) are properties of gases that are com-
pletely interrelated. Boyle's law and Charles' law may be combined into one equation
that is referred to as the ideal gas law. This equation is true always for ideal gases and
for real gases under certain conditions:

For air at room temperature, the error in this equation is less than 1 percent for pres-
sures as high as 400 psia (absolute psi). For air at one atmosphere of pressure, the
error is less than 1 percent for temperatures as low as -200" Fahrenheit. These error
factors will vary for different gases.

Pressu re/Compression
In a compressor, pressure is generated by pumping quantities of gas into a tank or
other pressure vessel. The pressure is increased by progressively increasing the
amount of gas in the confined or fixed-volume space. The effects of pressure exerted
by a confined gas result from the force acting on the container walls. This force is
caused by the rapid and repeated bombardment from the enormous number of mole-
cules present in a given quantity of gas.

Compression occurs when the space between the molecules is decreased. Less volume
means that each particle has a shorter distance to travel, thus proportionately more colli-