In thermodynamics, an adiabatic wall is a wall that does not allow the transfer of heat from one side to the other. An adiabatic wall does not let heat in or out of a thermodynamic system.
Adiabatic walls are theoretical concepts since, if they exist, they would be a perfect thermal insulator. Despite being excellent, any thermal insulator always allows some heat energy transfer.
The difference with a diathermal wall is that the diathermal wall energy transfer is possible, but the mass transfer is not.
Adiabatic wall examples
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The insulation of a house - in case of being perfect - does not allow thermal interaction with the outside of the house.
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The air layers of a solar collector. In solar collectors, between the dark surface where solar radiation is captured and the outside, there is a layer of air whose objective is to prevent heat flows.
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Double glazed windows aim to create a layer of air between them and prevent heat flow.
What is an adiabatic process?
An adiabatic process is a process in which the system does not exchange heat with its surroundings. These processes can also be isentropic, in which case the process is also reversible.
This term refers to elements that prevent heat transfer to the environment. An isolated wall is quite close to this concept.
In air conditioning, the humidification processes are adiabatic since there is no heat transfer. Another example is the adiabatic flame temperature, which is the temperature that a flame could reach if there were no heat loss to the surroundings.
Adiabatic heating and cooling are processes that commonly occur due to the change in pressure of a gas. It can be quantified using the ideal gas law.
Adiabatic processes cannot occur in open systems.
Difference between an adiabatic process and an isothermal process
A process is adiabatic if no heat is exchanged with the environment. On the contrary, an isothermal process is the opposite case: the maximum heat transfer takes place in an isothermal process.
When an exothermic process is performed adiabatically, the system's temperature increases because the system retains the generated heat.
In contrast, when an endothermic process is performed adiabatically, the system's temperature decreases because the required heat is not supplied from 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 evident with the refrigerator, it is sometimes not difficult to isolate a system from the outside world. However, when a process takes place very fast, there is no time to exchange heat with the environment, and the process in the first approach is adiabatic.