An aircraft’s environment control system provides the proper environment for its safe and stable flying. The refrigeration and air-conditioning systems are the core parts of an aircraft. The functional requirements

thermodynamic principles and system processes of refrigeration and air-conditioning systems are introduced. Air compression refrigeration cycles (such as basic cycle

two-wheel booster refrigeration cycle

three-wheel booster refrigeration cycle

four-wheel booster refrigeration cycle

and reverse booster refrigeration cycle)

vapor compression refrigeration cycles

and combined refrigeration cycles are compared. The advantages and disadvantages of high-pressure water separator and low-pressure water separator are also discussed. Advanced refrigeration and air-conditioning technologies

such as thermos-electric refrigeration system

pneumatic control

humidifying technology

loop heat pipe

etc. are also introduced. Research directions and key technologies for the future are summarized.