Container refrigeration system

1. A container refrigeration system, comprising:

a refrigerant circuit configured to perform a refrigeration cycle and including

a main circuit in which a compressor, a condenser, an expansion valve, and an evaporator are connected together in this order, and

a hot gas bypass circuit in which compressed refrigerant of the compressor is supplied to the evaporator by bypassing the condenser and the expansion valve;

a compressor control section configured to, in a defrosting operation in which the compressed refrigerant of the compressor returns to the compressor through the hot gas bypass circuit and the evaporator and the evaporator is defrosted, control an operating speed of the compressor such that a pressure of the compressed refrigerant of the compressor reaches a target value; and

a refrigerant amount control section configured to, depending on a superheating degree SH of the compressed refrigerant of the compressor in the defrosting operation, perform

a refrigerant discharge operation in which part of the compressed refrigerant of the compressor flows into a high-pressure liquid pipe of the refrigerant circuit in which the condenser is provided, and is stored in the high-pressure liquid pipe, and

a refrigerant supply operation in which the refrigerant of the high-pressure liquid pipe is supplied to the compressor along a path bypassing the evaporator,

wherein in a cooling mode in which refrigerant circulates through the main circuit to perform the refrigeration cycle and an inside of a container is cooled,

when a temperature inside the container is in an out-of-range state, the defrosting operation is started when a predetermined time has elapsed from a start of the cooling mode, the elapsing of the predetermined time being the only condition for starting the defrosting cycle while the temperature inside the container is in the out-of-range state, and

when the temperature inside the container is in an in-range state, the defrosting operation is started after both the predetermined time has elapsed from the start of the cooling mode, and a calculated amount of the evaporator's cooling capacity actually being used is reduced to equal to or less than a predetermined value, wherein the amount of cooling capacity actually being used is calculated based on the cooling capacity of the evaporator, a measured inlet air temperature of the evaporator, a measured outlet air temperature of the evaporator, and a pressure equivalent saturation temperature of refrigerant flowing into the compressor.

2. The container refrigeration system of claim 1 , further comprising:

a supercooling heat exchanger provided in the high-pressure liquid pipe; and

a supercooling branch pipe through which refrigerant branched from the high-pressure liquid pipe supercools liquid refrigerant of the high-pressure liquid pipe in the supercooling heat exchanger and then flows into a low pressure gas pipe of the refrigerant circuit or a compression chamber of the compressor in an intermediate pressure state,

wherein, in the refrigerant supply operation, the refrigerant amount control section allows the refrigerant of the high-pressure liquid pipe to flow into the compressor through the supercooling branch pipe.

3. The container refrigeration system of claim 1 or 2 , further comprising:

a fan control section configured to, in the refrigerant discharge operation by the refrigerant amount control section, operate a condenser fan of the condenser.

4. The container refrigeration system of claim 1 or 2 , wherein

in the defrosting operation, when an outlet refrigerant temperature of the evaporator reaches equal to or higher than a predetermined value, the target value for the pressure of the compressed refrigerant of the compressor is changed to a higher value.