Which of the following refrigerants is known to be not miscible with oil?

Ammonia (R-717) is known for its distinct property of being not miscible with oil, which is significant in refrigeration systems. Miscibility refers to the ability of two substances to mix in all proportions, forming a homogeneous solution. In refrigeration applications, it is crucial for the refrigerant to mix well with the oil to maintain adequate lubrication of the compressor and other moving parts.

Ammonia's non-miscibility with oil can lead to operational advantages and challenges. It reduces the risk of oil flooding the evaporator and allows for better heat transfer properties in the system since they do not mix. However, care must be taken to ensure that the compressor is appropriately lubricated, as ammonia's non-miscibility can complicate lubrication processes.

In contrast, other refrigerants like R-134a, R-22, and R-410A are typically miscible with oil. This property allows them to coagulate and work effectively alongside the lubrication required for compressors and other components, ensuring smooth operation and longevity of the system. Hence, the distinct characteristic of ammonia being not miscible with oil makes it the correct answer in this context.