One component in a particular automotive air-conditioning system is a "receiver", a small vertical cylindrical tank that contains a pool of liquid refrigerant, HFC-134a, with vapor above it. The receiver stores extra refrigerant for the system and helps to regulate the pressure. The receiver is at equilibrium with surroundings at 330 K. A 5 mm diameter, spherical thermistor inside the receiver monitors the liquid level. The thermistor is a temperature-sensing resistor driven by a small electric current; it dissipates a power of 0.1 W.
When the system is fully charged with refrigerant, the thermistor sits below the liquid surface. When refrigerant leaks from the system, the liquid level drops and the thermistor eventually sits in vapor.
The thermistor is small compared to the receiver, and its power is too low to affect the bulk temperature in the receiver.
a. If the system is fully charged, determine the temperature of the thermistor.
b. If enough refrigerant has leaked that the thermistor sits in vapor, find the thermistor's temperature. Neglect thermal radiation.