The compression (or expansion) of air can be thought of as taking place under isothermal or adiabatic or polytropic conditions.
Isothermal compression process takes place under constant temperature condition.
When a volume of air in a system is compressed or expanded instantly, there is no time to add or dissipate heat into or out of the system, and the process is said to be adiabatic.
These compression processes are considered to be theoretical and hence are presumed to be taking place under ideal conditions.
In actual practice, compression of air occurs between the two limits of compression. The polytropic compression process represents the actual compression process in compressors.
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