Type III - Low-Pressure Appliances
What makes an appliance low-pressure
Low-pressure appliances use refrigerants whose evaporator pressure sits below atmospheric, so the machine runs in a vacuum and air and moisture leak in rather than refrigerant leaking out. R-11, R-113, and R-123 centrifugal chillers are the standard examples. Because the shell is not built for positive pressure, a low-pressure chiller typically has a rupture disc set near 15 psig rather than a spring-loaded relief valve, and the vessel must never be pressurized with air or above its design pressure.
Air and moisture
A purge unit removes non-condensables and water that enter through seals and gaskets. Excess air raises condensing pressure and destroys efficiency, which is why high condensing pressure on a chiller points to a failing purge. Moisture can freeze inside the evaporator tubes. Never add heat with a torch to speed recovery; use warm water or a heat blanket and keep tube water above freezing.
Evacuation and recovery
The required evacuation level for low-pressure appliances is 25 mm Hg absolute, about 25,400 microns. It is absolute pressure, not inches of vacuum. A standard high-pressure recovery machine cannot pull a chiller below atmospheric, so a liquid pump or a machine rated for low-pressure refrigerant is used, often in a push-pull arrangement.
Charging and traps
Charge R-11 or R-123 as a liquid into the evaporator so water in the tubes does not freeze. Expect the exam to test units: 25 mm Hg absolute versus 25 inches of vacuum is the favorite distractor. Leak testing is done with a standing vacuum test or dry nitrogen plus a trace of refrigerant - never with oxygen or compressed air.