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Thermal Systems


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 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.



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:









Consider four similar houses except for the differences in the thickness of wall and roof insulation as shown in the following table.


Which house is the most economical to heat?


Thickness of exterior wall

Thickness of roof (attic)


5 cm

5 cm


10 cm

2 cm


10 cm

10 cm



10 cm





Heat Transfer