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The Greek prefix
therm means heat (causes and effects, generation and
usage), and Latin prefix temper means mixed
(originally used for temperatura caeli, the sky
combination). A thermal system is a multipart assembly
of coupled components (some of them thermal), showing a
common structured behaviour; e.g. a refrigerator is a
combination of pipes, compressor, electric motor, heat
exchangers, valves, insulation, casing, doors, lamp,
etc., interacting to a common goal of cold production
within. Therefore, a refrigerator is a thermal system,
whereas the refrigerant fluid and the interior space are
thermodynamic systems.
Thermodynamics
Thermodynamics
is the study of the laws that govern the conversion of
energy from one form to another, the direction in which
heat will flow, and the availability of energy to do
work. Thermodynamics is governed by three laws.
The laws of thermodynamics, discovered in the 19th
century through painstaking experimentation, govern the
nature of all thermodynamic processes and place limits
on them. In
simplest terms, the laws of thermodynamics dictate the
specifics for the movement of heat and work.
Laws of Thermodynamics
Basically, the
first law of thermodynamics is a statement of the
conservation of energy. The second law is a
statement about the direction of that conservation.
While the third law is a statement about reaching
absolute zero (0° K). However, since their conception,
these laws have become some of the most important laws
of all science and are often associated with concepts
far beyond what is directly stated in the wording.
Thermal systems
usually require some services as electrical supply (for
power or for control), water intake and exit, air intake
and exit, fuel supply and flue stack, etc.
Temperature
One of the
central principles of thermodynamics is temperature.
Temperature is a measure of
the warmth of an object. It is determined by the average
random kinetic energy of the object’s atoms. In order to
define a temperature scale, we need to make certain
assumptions. The first one is that there will be a
linear relationship between how hot an object is and its
temperature. This allows us to specify two points on the
scale and divide the rest of the scale evenly.
Temperature Scales
In general, there are two main scales
today. The first one is the Celsius, or
Centigrade, scale. It is the main temperature scale
used in most of the world, and is the main temperature
scale used in technological work in the U.S. In this
scale the freezing point of water is defined to be at 0
degrees, and the boiling point is defined to be 100
degrees. The other main scale is the Fahrenheit
scale which is commonly used in the United States. It
takes the freezing point of water and associates the
number 32 with it, and it associates the number 212 with
the boiling point of water. Thus, there are 180 degrees
between the freezing and boiling points of water. Notice
however, that neither of these scales actually refers to
any physical properties of water. Instead, two arbitrary
points for an arbitrary material were chosen and used to
define the scale. We can define a temperature scale that
uses the physical properties of matter, the absolute
temperature scale.
The Celsius version of the absolute scale
is the Kelvin scale (symbol K, without a degree
sign). Similarly, the absolute version of the Fahrenheit
scale is the Rankine scale. All of these scales
can be related to one another by the following
relationships:
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