Why is nitrogen used to purge the system before using the flushing solvent?

• A. Nitrogen is cheaper
• B. Nitrogen is more reactive
• C. Nitrogen absorbs moisture
• D. Nitrogen is heavier than air

Answer: (A)

When you purge a system before flushing solvent, you want to replace the air (which brings in moisture and oxygen) with a gas that won’t react with residual refrigerants or solvents. Nitrogen is chosen mainly because it’s inexpensive and readily available, so you can purge large volumes without adding cost burden. Its inert nature means it won’t react with remaining refrigerant, oil, or the flushing solvent, reducing the risk of unwanted chemical reactions during the purge. It’s also dry, helping to keep moisture levels down while displacing air. The idea isn’t that nitrogen absorbs moisture or is heavier than air; those aren’t the reasons it’s used. The cost-effectiveness of nitrogen makes it the practical choice for routine purging operations.