PURPOSE: To raise a temperature on a refrigerant side in an evaporator and prevent failure of a cooling pipe caused due to freezing by directly detecting temperature and pressure of a refrigerant in the evaporator to raise pressures in the evaporator as well as to raise the temperature of the refrigerant or effect emergency shut down of a compressor before freezing on the chilled water side.

CONSTITUTION: Temperature and pressure of a refrigerant in an evaporator 4 are detected by a temperature sensor 11 and a pressure sensor 12. When temperature and pressure in the evaporator 4 vary, they are directly detected by the refrigerant temperature sensor 11 and the pressure sensor 12, so that before a freezing condition of a chilled water set by a controller 7 is met, a solenoid valve 13 is opened to permit gas of high temperature and high pressure to be supplied to the evaporator 4 from a condenser 2, thereby raising internal temperature and pressure in the evaporator 4 for prevention of temperature drop of the chilled water. In this manner, accidents caused by freezing in a cooling pipe 10 can be prevented by directly detecting temperature and pressure of the refrigerant in flooded cooler and supplying a gas of high temperature and high pressure to the evaporator 4.