Sadi Carnot
introduced the concept of reversibility and laid the foundations of
the second law. A reversible process, also called an ideal process,
can reverse itself exactly by following the same path it took in the
first place. Thus, it restores to the system or the surroundings the
same heat and work previously exchanged.
In reality, there
are no ideal (reversible) processes. Real processes are irreversible.
However, the
degree of irreversibility varies between processes. There are many
sources of irreversibility in nature.
The most important
ones are friction, heat transfer, throttling, and mixing. Mechanical
friction is one in which mechanical work is dissipated into a heating
effect.
One example would
be a shaft rotating in a bearing. It is not possible to add the same
heat to the bearing to cause rotation of the shaft.
An example of
fluid friction is when the fluid expands through the turbine,
undergoing internal friction. This friction results in the
dissipation of part of its energy into heating itself at the expense
of useful work.
The fluid then
does less work and exhausts at a higher temperature. The more
irreversible the process, the more heating effect and the less the
work.
Heat transfer in
any form cannot reverse itself. Heat transfer causes a loss of
availability because no work is done between the high- and
low-temperature bodies.
EXTERNAL AND INTERNAL
IRREVERSIBILITIES
External
irreversibilities are those that occur across the boundaries of the
system. The primary source of external irreversibility in power
systems is heat transfer both at the highand low-temperature ends.
Internal
irreversibilities are those that occur within the boundaries of the
system. The primary source of internal irreversibilities in power
systems is fluid friction in rotary machines, such as turbines,
compressors, and pumps.
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