Which gas is used to counteract freezing while evacuating a system with a large amount of moisture?

• A. Oxygen
• B. Helium
• C. Argon
• D. Nitrogen

Answer: (D)

When evacuating a system that has a lot of moisture, the danger is that water vapor can condense and freeze on cold surfaces or in the vacuum path as pressure drops, forming ice plugs that block the purge and hinder pumping. Using a dry, inert purge gas helps push moisture out and keep the system dry without introducing reactive species. Dry nitrogen fits perfectly: it’s inexpensive, readily available, and inert, so it won’t react with oils or refrigerants while displacing moist air. By lowering the water vapor in the system, dry nitrogen reduces the chance of freezing during evacuation and makes it easier to achieve a good vacuum. Other gases pose drawbacks—oxygen can cause oxidation and safety issues, while helium and argon are less practical for moisture purge due to cost or less effective drying.