EPA 608 Technician Certification Low-Pressure Chillers, Purge Units, and Safety Summary

Air and moisture leak into low-pressure chillers. The purge unit removes these noncondensables while returning as much refrigerant as possible to the system. High-efficiency purge units release very little refrigerant.

Wrong choices

• Adds refrigerant automatically: The purge unit does not charge refrigerant.
• Cools the compressor oil: It does not cool the oil.
• Relieves pressure if the chiller gets too hot: Pressure relief is the job of the rupture disc or relief valve.

R-123, an HCFC, became the common replacement for R-11, a CFC, in low-pressure centrifugal chillers. Both are low-pressure refrigerants.

Wrong choices

• R-12: R-12 is a medium-pressure CFC used in other equipment.
• R-502: R-502 is a high-pressure blend.
• R-22: R-22 is a high-pressure HCFC.

Low-pressure refrigerants such as R-123 boil at low temperatures only under vacuum, so the evaporator runs below atmospheric pressure. This is why leaks let air in.

Wrong choices

• About 300 psig: 300 psig is typical of high-pressure systems, not low-pressure chillers.
• Exactly 0 psig at all times: The pressure is normally below 0 psig while running.
• About 100 psig: 100 psig is far too high for a low-pressure evaporator.

Air and other noncondensables are trapped at the top of the condenser because they do not condense. The purge unit draws from there to remove them.

Wrong choices

• The oil sump: The oil sump is not where noncondensables collect.
• The water box: The water box holds water, not refrigerant.
• The bottom of the evaporator: The bottom of the evaporator holds liquid refrigerant, not trapped air.

The rupture disc protects the low-pressure chiller from overpressure and is usually set at 15 psig. That is why leak test pressure is kept at 10 psig or less.

Wrong choices

• 10 psig: 10 psig is the usual maximum leak test pressure, kept below the disc setting.
• 50 psig: 50 psig is far above the usual setting.
• 150 psig: 150 psig is a high-pressure range.

EPA's Type III test topics state that ASHRAE Standard 15 requires an equipment room refrigerant sensor for R-123, along with the oxygen-deprivation protection that applies to all refrigerants.

Wrong choices

• Only a first aid kit: A first aid kit alone does not meet this rule.
• Only a fire extinguisher: A fire extinguisher alone does not meet this rule.
• A window that stays open: An open window does not replace a refrigerant sensor.

Refrigerant vapor is heavier than air and can displace oxygen. Do not enter without checking the alarm and using a self-contained breathing apparatus (SCBA) if needed. Ventilate the room.

Wrong choices

• Electric shock from the refrigerant: Refrigerant does not cause electric shock.
• Radiation exposure: Refrigerant is not radioactive.
• Hearing damage from the leak noise: Noise is not the main danger.

R-123 boils at about 82°F at atmospheric pressure. At 75°F, its saturation pressure is a little below atmospheric, so a shut-down chiller shows a slight vacuum. Use the pressure-temperature relationship of low-pressure refrigerants to judge readings.

Wrong choices

• About 200 psig: 200 psig is a high-pressure range.
• About 100 psig: 100 psig is far too high for R-123 at room temperature.
• Exactly 15 psig: 15 psig would be near the rupture disc setting.

R-134a is listed as a medium-pressure refrigerant (covered by Type II). R-11, R-123, and R-113 are low-pressure refrigerants (Type III).

Wrong choices

• R-113: R-113 is a low-pressure refrigerant.
• R-11: R-11 is a low-pressure refrigerant.
• R-123: R-123 is a low-pressure refrigerant.