Final answer:
Oil stays on top of the refrigerant in an evaporator because of the difference in density, where this separation process is part of the refrigeration cycle involving condensation and expansion.
Step-by-step explanation:
In an operating evaporator, oil tends to stay on top of the refrigerant due to differences in density between oil and refrigerants. This separation occurs during the refrigeration cycle when the refrigerant condenses from a gas to a liquid. The liquid refrigerant then flows through an expansion valve to the outdoor evaporator coils, where it is cooled and expanded. This cooling and expansion process is part of an evaporator operation in various systems, including air conditioning and refrigeration cycles. For example, in household refrigerators, a coolant with a low boiling point absorbs heat at the evaporator, causing it to vaporize and subsequently condense, repeating the cycle. The effectiveness of such systems, like the coefficient of performance, depends on how well this cycle is maintained.