What type of air system operates using hot compressor bleed air on some turbine aircraft?

The type of air system that operates using hot compressor bleed air on some turbine aircraft is the ram air cycle cooling unit. This system utilizes hot bleed air from the engine, which is then cooled through a series of components before being circulated into the cabin. The cooling process occurs as the hot air passes through a heat exchanger, where it is cooled by external ambient air—often referred to as "ram air"—that enters through inlets on the aircraft.

This method leverages the aircraft's velocity and external air to facilitate temperature regulation inside the cabin and is particularly efficient because it uses existing engine bleed air, which would otherwise be wasted. The ram air cycle cooling unit operates independently of the engine's power settings and helps maintain comfortable cabin conditions during flight.

On the other hand, the other options refer to different systems: a cold air supply system typically relies on external air without needing to be heated, a chemical oxygen generator system provides emergency oxygen for crew and passengers but does not deal with cabin temperature control, while a vapor-cycle cooling unit employs refrigerants in a closed-loop system to cool air, rather than using direct hot bleed air from the compressor.