An air-conditioning apparatus directly detects leakage of a plurality of types of refrigerant by computing refrigerant concentrations, and ensure safety. On the basis of calibration curve information, a computing device computes the concentration of heat source side refrigerant in a heat medium relay unit from detected information received from a concentration detecting device.

A multi-temperature air conditioning system, a control method thereof and a transport refrigeration vehicle. The multi-temperature air conditioning system includes an outdoor unit; a first indoor unit; and a second indoor unit; a number of a first four-way valves corresponds to the number of the first indoor units, and a number of a second four-way valves corresponds to the number of the second indoor units; and a section flow path which could be connected or disconnected is further included, which connects the first indoor unit between the first throttling element and the first on-off valve, and connects the second indoor unit between the second throttling element and the second on-off valve.

Power consumption is reduced in a refrigeration cycle system including a cascade heat exchanger that causes heat exchange between a CO.sub.2 refrigerant and a refrigerant other than CO.sub.2, the refrigeration cycle system implementing a first refrigeration cycle using the CO.sub.2 refrigerant and a second refrigeration cycle using the refrigerant other than CO.sub.2. A controller controls a first compressor and a second compressor such that an intermediate temperature Tm between an evaporation temperature of a first refrigerant and a condensation temperature of a second refrigerant in a cascade heat exchanger satisfies T2+(T1T2)0.1TVT2+(T1T2)0.4, where T1 C. represents a pressure equivalent saturation temperature of a refrigerant discharged from the first compressor, and T2 C. represents a pressure equivalent saturation temperature of a refrigerant sucked into the second compressor.

To provide a refrigeration cycle device that can suppress a disproportionation reaction of a working medium without impairing comfort for users.

A refrigeration cycle device 1 according to the present disclosure is a refrigeration cycle device that includes an outdoor unit 30 including a compressor 31, and that includes a plurality of indoor units 10 each including an expansion valve 15, wherein a working medium containing an ethylene-based fluoroolefin is used as refrigerant, the refrigeration cycle device includes a control unit 70, and when indoor units 10 of the plurality of indoor units 10 are in an operating state, and when a discharge temperature T from the compressor 31 is a predetermined temperature T1 or higher, the control unit 70 opens the expansion valve 15 of an indoor unit 10 of indoor units 10 in an operation stop state.