Page 233 - The Mechatronics Handbook

as shown by the following relationship between the Celsius temperature and the Kelvin temperature:

T ˚C ) = T K() 273.15 (12.1)
(
–
Two other temperature scales are commonly used in engineering in the U.S. By definition, the Rankine
scale, the unit of which is the degree rankine (˚R), is proportional to the Kelvin temperature according to

(
T ˚R ) = 1.8T K() (12.2)

The Rankine scale is also an absolute thermodynamic scale with an absolute zero that coincides with the
absolute zero of the Kelvin scale. In thermodynamic relationships, temperature is always in terms of the
Kelvin or Rankine scale unless specifically stated otherwise.
A degree of the same size as that on the Rankine scale is used in the Fahrenheit scale, but the zero
point is shifted according to the relation

T ˚F ) = T ˚R ) 459.67 (12.3)
(
(
–
Substituting Eqs. (12.1) and (12.2) into Eq. (12.3) gives

(
T ˚F ) = 1.8T ˚C ) + 32 (12.4)
(
This equation shows that the Fahrenheit temperature of the ice point (0˚C) is 32˚F and of the steam
point (100˚C) is 212˚F. The 100 Celsius or Kelvin degrees between the ice point and steam point
corresponds to 180 Fahrenheit or Rankine degrees.
To provide a standard for temperature measurement taking into account both theoretical and practical
considerations, the International Temperature Scale of 1990 (ITS-90) is defined in such a way that the
temperature measured on it conforms with the thermodynamic temperature, the unit of which is the
kelvin, to within the limits of accuracy of measurement obtainable in 1990. Further discussion of ITS-90
is provided by Preston-Thomas (1990).

Irreversibilities
A process is said to be reversible if it is possible for its effects to be eradicated in the sense that there is
some way by which both the system and its surroundings can be exactly restored to their respective initial
states. A process is irreversible if both the system and surroundings cannot be restored to their initial states.
There are many effects whose presence during a process renders it irreversible. These include, but are
not limited to, the following: heat transfer through a finite temperature difference; unrestrained expansion
of a gas or liquid to a lower pressure; spontaneous chemical reaction; mixing of matter at different
compositions or states; friction (sliding friction as well as friction in the flow of fluids); electric current
flow through a resistance; magnetization or polarization with hysteresis; and inelastic deformation.
The term irreversibility is used to identify effects such as these.
Irreversibilities can be divided into two classes, internal and external. Internal irreversibilities are those
that occur within the system, while external irreversibilities are those that occur within the surroundings,
normally the immediate surroundings. As this division depends on the location of the boundary there
is some arbitrariness in the classification (by locating the boundary to take in the immediate surroundings,
all irreversibilities are internal). Nonetheless, valuable insights can result when this distinction between
irreversibilities is made. When internal irreversibilities are absent during a process, the process is said to
be internally reversible. At every intermediate state of an internally reversible process of a closed system,
all intensive properties are uniform throughout each phase present: the temperature, pressure, specific
volume, and other intensive properties do not vary with position.

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