9.2 Purge Units and Non-Condensable Gas Management
Key Takeaways
- Non-condensable gases (mostly air) collect at the TOP of the condenser
- NCGs increase condenser pressure, waste energy, and carry moisture that causes acid/corrosion
- Purge units draw NCG/refrigerant mixture, condense the refrigerant (return it), and vent only air
- High-efficiency purge units are 99%+ efficient at recovering refrigerant
- Frequent purge operation indicates air leaks that need repair
9.2 Purge Units and Non-Condensable Gas Management
Since low-pressure systems operate in vacuum, air and other non-condensable gases inevitably infiltrate through leaks. Managing these gases is one of the most important aspects of Type III system maintenance.
What Are Non-Condensable Gases (NCGs)?
Non-condensable gases are gases that do not condense into liquid at the temperatures and pressures found in a refrigeration system. The primary NCG is air (nitrogen and oxygen), which enters through leaks in the vacuum portion of the system.
Effects of NCGs in Low-Pressure Systems
Non-condensable gases cause serious problems:
| Effect | Impact |
|---|---|
| Increased condenser pressure | NCGs collect in the condenser and raise pressure, reducing efficiency |
| Increased energy consumption | Higher condenser pressure means the compressor works harder (up to 5-10% per 1 psi increase) |
| Reduced cooling capacity | Less effective heat rejection |
| Moisture contamination | Air carries moisture, which causes acid formation and corrosion |
| Chemical breakdown | Moisture + heat + refrigerant = acid, sludge, and copper plating |
How Purge Units Work
A purge unit (also called a purge recovery unit) continuously monitors the chiller and removes non-condensable gases while recovering and returning any refrigerant that would otherwise be vented with the air:
Basic purge unit operation:
- A mixture of refrigerant vapor and NCGs is drawn from the top of the condenser (where NCGs accumulate)
- The mixture passes through a cooling coil that condenses the refrigerant vapor back to liquid
- The liquid refrigerant is returned to the chiller
- The remaining NCGs (mostly air) are vented to the atmosphere
- The process repeats continuously during system operation
High-Efficiency vs. Low-Efficiency Purge Units
| Feature | Low-Efficiency Purge | High-Efficiency Purge |
|---|---|---|
| Refrigerant loss | Higher (older design) | Very low (<0.1 lb per lb of air removed) |
| Efficiency | 85-90% | 99%+ |
| Monitoring | Basic | Electronic with data logging |
| Regulation | Must be maintained per specs | Must be maintained per specs |
| Cost | Lower initial | Higher initial, lower operating |
High-efficiency purge units are now required on all new low-pressure chillers and should be retrofitted on older units. They recover nearly all refrigerant before venting the air, minimizing losses.
Purge Unit Monitoring
Technicians should monitor purge unit activity as an indicator of system health:
- Frequent purge runs = the system has significant air leaks that need to be found and repaired
- Infrequent purge runs = the system is well-sealed with minimal air infiltration
- Excessive purge activity without an increase in system load suggests a developing leak
- Purge unit runtime logs should be reviewed during regular maintenance
Purge Unit Releases
Releases from a properly operating high-efficiency purge unit are considered de minimis (minimal, permitted) under Section 608. However:
- The purge unit must be maintained and operating per manufacturer specifications
- Deliberate venting of refrigerant (bypassing the purge unit) is always illegal
- If the purge unit is not functioning properly, it must be repaired promptly
For the Exam: NCGs collect at the TOP of the condenser. Purge units draw the gas/refrigerant mixture from the condenser, condense the refrigerant (return it), and vent only the air. High-efficiency purge units are 99%+ efficient at recovering refrigerant. Frequent purge operation indicates significant air leaks.
Where do non-condensable gases accumulate in a low-pressure chiller?
What does frequent purge unit operation indicate about a low-pressure chiller?
What efficiency level do high-efficiency purge units achieve in recovering refrigerant?