EPA Section 608 Universal Certification ExamType III (Low-Pressure Appliances)Medium
When evacuating a low-pressure chiller, why is it common practice to add a small amount of nitrogen to the system to break the vacuum before removing the vacuum pump?
- ATo dilute any remaining refrigerant vapor before charging.
- BTo prevent air from being drawn into the system when the vacuum pump is disconnected.
- CTo assist in the complete removal of any residual moisture from the system.
- DTo pressurize the system for a leak test immediately after evacuation.
Show answer & explanationAnswer & explanation
Correct answer: B. To prevent air from being drawn into the system when the vacuum pump is disconnected.
Breaking the vacuum with nitrogen before disconnecting the vacuum pump ensures that the system pressure is slightly above atmospheric, preventing ambient air (and its moisture) from rushing into the system when the hose is disconnected.
Why the other options are wrong
- A. Nitrogen is a non-condensable and would need to be removed before charging, so this is not a beneficial step for dilution.
- C. Nitrogen itself does not assist in moisture removal; the vacuum pump does that. Introducing nitrogen *ends* the dehydration process.
- D. While nitrogen can be used for leak testing, that's not the primary reason for breaking the vacuum *before* disconnecting the pump.
Breaking Vacuum with Nitrogen
After achieving a deep vacuum in a refrigeration system, it is standard practice to introduce a small amount of dry nitrogen to slightly raise the system pressure before disconnecting the vacuum pump.
- Prevents air/moisture ingress.
- Ensures positive pressure.
- Nitrogen is dry and inert.
Memory trick: Nitrogen in, air stays out, no vacuum doubt!