Which substance is classified as a non-condensable in refrigeration systems?

In refrigeration systems, non-condensables are substances that do not change from gas to liquid at the system's operating temperatures and pressures. Nitrogen is classified as a non-condensable because it remains in a gaseous state and does not liquefy under typical refrigeration conditions. Its presence in a refrigeration system can negatively affect system efficiency and performance, as it does not participate in the heat exchange process like refrigerants do.

In contrast, Freon and R-134a are refrigerants specifically designed to undergo phase changes from gas to liquid, facilitating efficient heat transfer in refrigeration cycles. Carbon dioxide, although it can behave as a refrigerant under certain conditions, is primarily treated as a refrigerant that can condense. Thus, nitrogen stands out clearly as a non-condensable substance in this context.