On a centrifugal system, from where does the purge unit take its suction?

The purge unit on a centrifugal system is designed to remove non-condensable gases from the refrigerant cycle, which helps maintain the system's efficiency and performance. The correct answer indicates that the purge unit takes its suction from the top of the condenser. This position is critical because non-condensable gases typically rise to the top of the condenser due to their lower density compared to the refrigerant vapor.

By taking suction from the top of the condenser, the purge unit can effectively capture these unwanted gases and vent them outside the system. This prevents the accumulation of non-condensables that can lead to reduced heat transfer efficiency, increased pressures, and potential operational issues within the system.

The other options do not align with the proper functioning of a purge unit in a centrifugal system. For instance, taking suction from the bottom of the evaporator or the exit of the expansion valve would not efficiently remove non-condensables as these locations primarily contain refrigerant in its operational state. Similarly, the middle of the compressor is not an ideal location for the purge unit, as it would not selectively target the non-condensables that accumulate at the top of the condenser.