Basically, the air cycle cooling system is supplied from air bled from the main engine compressor. The air is expanded through an air turbine that drives a compressor or fan, thus converting heat energy into useful work.
For low speed flight, integrated cooling and pressurization requirements for cabin cooling can be met advantageously by air cycle systems. As a result, most propeller driven and many jet transports use air cycle systems for cooling and pressurization.
During high speed flight at high altitudes, maximum cooling requirements are coincident with maximum pressurization requirements.
For any application involving a high heat load, the amount of air flow required for cooling is considerable, requiring large diameter ducts and a large ram air drag penalty. If bleed air from a jet engine is used, the high mass flow necessary would result in high fuel penalties. For these reasons the air cycle system has its limitations.
Another limitation of the air cycle system is that frosting in the downstream air ducts can occur if the turbine outlet temperature falls to the freezing point.
