Rather than one very long single pull, a technician evacuates a contaminated system three times and breaks the vacuum with dry nitrogen between the pulls. Why does that approach clean the system more thoroughly?
Correct Answer
C) Each nitrogen sweep mixes with and carries away moisture and non-condensables, so the following pull starts from a cleaner system
A single long pull leaves you waiting for the last stubborn moisture to boil off on its own. Breaking the vacuum with dry nitrogen does two useful things: it raises the pressure so trapped moisture in oil and in dead-end passages can move, and the dry gas mixes with the water vapor and contaminants so the next pull carries them out together. Repeating the cycle reduces what remains by a fraction each time, so three moderate pulls with sweeps beat one very long pull on a wet or contaminated system.
Why This Is the Correct Answer
The sweep gas acts as a carrier. Dry nitrogen entering a system under vacuum dilutes whatever water vapor and non-condensables are present and reaches passages the pump alone drains slowly. Each pull then removes that mixture, so the residual contamination falls by a multiple with every cycle rather than crawling down a plateau.
Why the Other Options Are Wrong
Option A: Each nitrogen charge chemically neutralizes the acid that the burnout left behind in the oil and the tubing
Nitrogen is chemically inert and neutralizes nothing. Acid from a burnout is dealt with by removing the contaminated oil and refrigerant and by installing the appropriate filter driers, not by a gas charge.
Option B: The nitrogen raises system pressure enough to push the moisture out through the open service valves between one pull and the next one
The nitrogen is not blown out through the valves in this procedure; it is drawn out by the next evacuation. Purging a system that still held refrigerant out through open valves would also be a release.
Option D: Breaking the vacuum lets the pump cool between pulls, so each new pull reaches a deeper vacuum than the last
Pump temperature is not what limits the pull here, and a pump does not reach deeper simply for having rested. What improves between pulls is the state of the system, not the state of the pump.
Memory Technique
Nitrogen is a broom, not a chemical. Sweep, pull, sweep, pull.
Reference Hint
40 CFR 82.156 sets evacuation requirements. Triple evacuation with dry nitrogen sweeps is accepted industry practice for wet or contaminated systems.
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