Why Is Carnot Cycle Not Possible In Heat Pump And Refrigerator?

Read: COP of air refrigerator working on reversed carnot cycle with PV and Ts diagram

In working of refrigerator, the term Carnot cycle represents the reversed Carnot cycle. As we know that, between the fixed temperature limits, a heat engine operating on Carnot cycle give highest possible efficiency. Similarly, a refrigerator working on reversed Carnot cycle give the highest possible coefficient of performance. Even though no heat engine and refrigerator used Carnot cycle. This is because that the isentropic process of Carnot cycle requires a high speed of air while the isothermal process of Carnot cycle requires extremely slower speed. This large variation in speed of air is simply not possible.

Two Isentropic process - Process must be extremely fast and it requires perfect insulation and no friction

Two Isothermal process - Heat transfer at constant temperature difference is extremely slow process (Quasistatic)

- What are the temperature limitations for refrigeration cycle (Reversed Carnot)?