Adiabatic walls are theoretical concepts since, if they exist, they would be a perfect thermal insulator. Nowadays, any thermal insulation, no matter how good it is, always allows some heat energy transfer.
Examples of adiabatic walls:
- The insulation of a house. Good insulation in a home is a good approximation to an adiabatic wall. A perfect insulation would not allow heat to enter the home or heat to leave the home in winter.
- The air layers of a solar collector. In solar collectors, between the dark surface where solar radiation is captured and outside, there is a layer of air. Air, when still, is a very good insulator. The objective of creating this air layer is to obtain the approximation of an adiabatic wall that allows no thermal energy transfer from the fluid of the solar module to the outside.
- Double glazed windows. The goal of double glazing windows is actually to create a layer of air between them. This air layer, being insulating, acts as an adiabatic wall.
What is an adiabatic process?
An adiabatic process is a process in which the system does not exchange heat with its environment. Adiabatic processes can also be isentropic, in which case, the process is also reversible.
The term adiabatic refers to elements that prevent heat transfer with the environment. An isolated wall is quite close to an adiabatic limit.
In air conditioning, the humidification processes (contribution of water vapor) are adiabatic, since there is no heat transfer, despite the fact that the air temperature and its relative humidity can be varied. Another example is the adiabatic flame temperature, which is the temperature a flame could reach if there were no heat loss to the environment.
Adiabatic heating and cooling are processes that commonly occur due to the change in pressure of a gas. This can be quantified using the ideal gas law.
What difference is there between an adiabatic process and an isothermal process?
A process is adiabatic if heat is not exchanged with the environment. In contrast, an isothermal process is the opposite case: in an isothermal process the maximum heat transfer takes place.
When an exothermic process is performed adiabatically, the system temperature increases because the system retains the generated heat.
On the contrary, when an endothermic process is performed adiabatically, the system temperature decreases, because the required heat is not supplied from the outside. An example of this is the operation of a refrigerator in which a compressed gas expands through a constriction. The gas temperature thus decreases.
As is clear with the refrigerator, it is sometimes not difficult to isolate a system from the outside world. When a process takes place very quickly, there is no time to exchange heat with the environment, and the process in the first approach is adiabatic. For example, the working stroke in a combustion engine can be perfectly described adiabatically.
The opposite of an adiabatic process is one in which heat exchange with the environment ensures that the temperature does not change during the process. This is called an isothermal process. Many cyclical processes used in the art contain both adiabatic and isothermal components.
In practice, the state changes are neither completely adiabatic nor completely isothermal, but the state changes lie between the two. This is called a polytropic state change.