Low-Pressure System Servicing and Refrigerant Handling Practices
Type III servicing questions focus on low-pressure chiller work: charging, purging non-condensables, preventing air and moisture ingress, and handling refrigerant safely. Low-pressure chillers often operate below atmospheric pressure, so a small leak draws air and moisture in rather than pushing refrigerant out.
Core rules
- Use a manifold gauge set and hoses rated for the low-pressure range. Do not use high-pressure gauges as a substitute; they may not resolve the vacuum-side readings you need.
- Charge by weight with a scale. Pressure alone is unreliable because low-pressure refrigerant saturation pressure changes with temperature and non-condensables raise pressure.
- Charge liquid refrigerant into the liquid line or through a charging valve; do not dump liquid into the suction of a running compressor. Throttle to avoid slugging.
- Remove non-condensables with the purge unit. A high-efficiency purge minimizes refrigerant loss; never use a manual “vent and watch” purging practice.
- Keep the system clean and dry. Replace filter-driers, use good vacuum practices, and avoid leaving the system open. Moisture in a low-pressure chiller can form acid and damage motor windings.
- Follow the refrigerant SDS for exposure limits and PPE. Low-pressure refrigerants can displace oxygen in confined spaces; ventilate and use a gas monitor where required.
- Never use oxygen or compressed air for pressure testing. Use dry nitrogen and respect the low-side design pressure.
Worked example
An R-123 chiller is short of capacity. At 70°F the technician reads 5 psig on the low side and adds vapor until the gauge reaches 10 psig. Why is this wrong? R-123 has a normal boiling point above 70°F, so a properly charged chiller at 70°F should sit in a vacuum, not at positive pressure. Positive pressure suggests air or another non-condensable is in the shell. Adding refrigerant masks the problem and overcharges the machine. The correct response is to find and fix the air ingress, purge the non-condensables, and then charge by weight to the nameplate charge.
Traps that cost marks
- Charging by pressure instead of weight.
- Adding refrigerant to raise pressure when the real problem is air in the chiller.
- Using a vacuum pump as a recovery machine.
- Ignoring purge-unit performance and refrigerant loss.
- Treating low-pressure refrigerant as harmless because it is not a high-pressure gas.