Final answer:
In an operating evaporator, oil would stay on top of ammonia due to ammonia's lower density compared to the density of oil. The evaporator's role is to convert liquid refrigerant into a gas, while the condenser then changes it back to liquid. Performance of refrigeration systems is measured by the coefficient of performance.
Step-by-step explanation:
In the context of refrigeration systems, the operating evaporator deals with the phase change of the refrigerant as it absorbs heat and converts from a liquid to a gas. The separation of oil from refrigerant in such systems can occur due to differences in density, where oil being less dense than the refrigerant, may rise to the top. Of the refrigerants listed, oil would stay on top of ammonia (NH3) due to its higher density relative to oil. The other refrigerants, like R-134a and R-22 (hydrochlorofluorocarbons), are heavier than ammonia and would typically see oil settling to the bottom.
Correct operation of the evaporator is crucial, as it is responsible for converting liquid refrigerant into gas in the cooling cycle. The condenser part of the system then converts this gas back into a liquid form. Refrigerants like carbon tetrachloride, methyl chloroform, and hydrochlorofluorocarbons were commonly used, but many are being phased out due to environmental concerns. Current refrigerants are selected for their thermodynamic properties as well as their environmental impact. It's also important to note that the performance of a refrigerator is evaluated based on its coefficient of performance (COP), which measures the heat removed from the refrigerated space against the work input.