The radiator of a steam heating system has a volume of 35 L and is filled with superheated water vapor at 200 kPa and 200°C. At this moment, both the inlet and the exit valves to the radiator are closed. After a while it is observed that the temperature of steam drops to 80°C as a result of heat transfer to the room air, which is at 21°C. Assume the surroundings to be at 0°C.
Determine the entropy change of the steam during this process.