When air is compressed at high pressure and subsequently cooled down by expansion, heat transfer occurs due to two main mechanisms: conduction and convection.
During compression, air molecules are pushed closer together, increasing their kinetic energy and resulting in an increase in temperature. This rise in temperature occurs primarily due to adiabatic compression, where no heat exchange occurs with the surroundings. However, if the compression process is not perfectly adiabatic, there can be some heat transfer due to conduction.
Conduction is the process of heat transfer through direct molecular interaction. When air is compressed, the molecules in the compressed region collide more frequently, leading to a transfer of kinetic energy. This energy transfer occurs between neighboring air molecules and between the compressed air and the surrounding materials, such as the walls of the container or the compressor itself. As a result, heat is transferred from the compressed air to its surroundings, leading to an increase in the temperature of the surrounding materials.
After compression, when the air is allowed to expand, the opposite occurs. The expansion causes a decrease in pressure and temperature. As the air expands, it does work on the surrounding environment, and this work comes at the expense of the internal energy of the air. The air molecules move farther apart, reducing their kinetic energy and resulting in a decrease in temperature.
During expansion, heat transfer can also occur through conduction and convection. Conduction plays a role if there is contact between the expanding air and cooler surfaces, allowing for direct molecular interaction and heat exchange. Convection, on the other hand, involves the transfer of heat through the movement of a fluid. As the air expands, it can come into contact with cooler air or surfaces, and heat can be transferred via convection currents.
It's important to note that the efficiency of compression and expansion processes can be affected by factors such as the speed of compression/expansion, the insulation of the system, and the specific conditions and characteristics of the materials involved. Different compressors and expansion devices may have varying degrees of heat transfer due to differences in design and operating parameters.