How does an air-cycle system produce cool air?

An air-cycle system produces cool air primarily by cooling and expanding hot engine bleed air. In this system, hot bleed air is drawn from the engine and then passed through a heat exchanger, where it is cooled to a more acceptable temperature. After this initial cooling, the air is allowed to expand through a turbine, which further lowers its temperature as a result of the thermodynamic principles of expansion in gases.

This process is efficient because it leverages the high temperature and pressure of engine bleed air to create a cooling effect without the use of conventional refrigerants, making it a well-suited method for aircraft where weight and space are critical. The air that undergoes this process ends up being circulated into the cabin as cool air for passenger comfort.

The other options describe different processes: compressing humid air is not the primary method for producing cool air in an air-cycle system, burning fuel creates heat rather than cooling air, and using refrigerant liquid is characteristic of vapor-cycle systems rather than air-cycle systems. Hence, cooling and expanding hot engine bleed air is integral to the operation and effectiveness of air-cycle systems in aircraft environmental control.