Type III - Low-Pressure Appliances
What this domain covers
Type III covers low-pressure appliances, almost always centrifugal chillers using R-11, R-113, or R-123. These refrigerants boil above ambient at atmospheric pressure, which means the machine normally runs in a vacuum: a leak pulls air and moisture in instead of pushing refrigerant out. That single fact drives nearly every rule in this domain.
Rules you must be able to state
- Recovery target is 25 mm Hg absolute (roughly 1 in. Hg absolute, about 29 in. Hg of vacuum on a compound gauge). Standard high-pressure recovery machines generally cannot reach it, so equipment must be rated for low-pressure refrigerants.
- Never use the chiller's own compressor to pull the system below 0 psig. Use a recovery machine.
- Never open a low-pressure system to the atmosphere. If it must be opened, keep it at positive pressure with dry nitrogen; otherwise moisture-laden air rushes in, and water freezes at the metering device and forms acids in the oil.
- Purge units remove air that leaks in. Excessive purging is a leak indicator, and purged refrigerant must be recovered, not vented.
- Rupture disc protects the chiller from overpressure; if it bursts, the entire charge is lost and the release is reportable.
- Leak testing is done by pressurizing with dry nitrogen - you cannot leak-check a machine that is sitting in a vacuum.
Traps that show up in questions
- Confusing 25 mm Hg absolute with 25 in. Hg vacuum; they are wildly different targets.
- Assuming a low-pressure chiller cannot leak because it is under vacuum - it leaks inward, which is why purge logs matter.
- Charging liquid refrigerant into the compressor instead of following the manufacturer's charging point.
Rule of thumb
If the gauge is a compound gauge reading inches of vacuum, convert before comparing: 25 mm Hg absolute is about 0.98 in. Hg absolute, essentially a full vacuum, not a 25-inch pull.