At which component in an air-cycle cooling system does air undergo a pressure and temperature drop?

In an air-cycle cooling system, the expansion turbine is the component where air experiences a critical drop in both pressure and temperature. As the air flows through the expansion turbine, it undergoes an expansion process. During this process, the air pressure reduces significantly, which leads to a corresponding drop in temperature due to the principles of thermodynamics, specifically the Joule-Thomson effect. This cooling effect is essential for the system's function, allowing the air to be conditioned effectively for cabin comfort.

In contrast, other components in the air-cycle system serve different functions. The compressor, for instance, is responsible for increasing the pressure of the air, which actually raises its temperature before it undergoes further processing. The heat exchanger function is to transfer heat between the incoming and outgoing air streams, aiming to moderate temperatures but not to cause an expansion. The condenser also has a different role, primarily involving the rejection of heat, rather than achieving a pressure and temperature drop through expansion. Thus, the expansion turbine is uniquely positioned in the cycle to facilitate the necessary decrease in both parameters, making it the correct choice.