When an ammonia compression system is pumped down below atmospheric pressure, what precaution must be taken?

When an ammonia compression system is pumped down below atmospheric pressure, it is crucial to ensure that no air is drawn into the system. This is vital because the presence of air can lead to several problems, including the formation of corrosive acids when moisture from the air reacts with ammonia. Additionally, air can adversely affect the efficiency of the refrigeration cycle, leading to potential failures or decreased performance.

Maintaining a vacuum below atmospheric pressure helps prevent any outside air from entering the system. By ensuring that no air is drawn in, the integrity of the refrigeration system is preserved, and the risk of contamination and resultant operational issues is minimized. This practice is critical for the longevity and efficiency of the refrigeration system, especially in systems using ammonia, which is hygroscopic and can easily absorb moisture if air is introduced.

The other options, while important considerations in general operations, do not directly address the immediate risk associated with pumping down below atmospheric pressure.