8 Type III Certification Practice

Recognizing leaks in low-pressure systems

Low-pressure chillers can operate below atmospheric pressure, so a leak may draw air and moisture into the system rather than allowing refrigerant to escape. A Purge unit removes noncondensables; excessive purge operation can signal a leak. High head pressure may indicate air in the system, and persistent moisture collected by the purge can point to a leak in chiller or condenser tubes.

Raising pressure and locating leaks

For leak testing, controlled heating is the preferred way to raise pressure in a low-pressure system. Hot water or a built-in heating or pressurization device can be used; nitrogen is another method where permitted. A low-pressure centrifugal chiller must not be pressure-tested above $10\ \text{psig}$.

Use a suitable leak detector or bubble solution to locate the leak. A pressure-decay test can show pressure loss, but it cannot pinpoint the leak's location.

Recovering refrigerant safely

Recover liquid refrigerant first, then vapor. Removing liquid first speeds recovery, but continue recovering vapor because substantial refrigerant can remain after the liquid is removed.

During evacuation, circulate or remove chiller water as needed to prevent it from freezing. Before removing oil, heat it to $130^\circ\text{F}$ to reduce refrigerant release.

After reaching the required recovery vacuum, wait briefly and check the pressure again. A pressure rise may mean refrigerant remains in the system or is boiling out of the oil; continue recovery as needed. Recovery equipment used with low-pressure appliances commonly has a $10$-psig high-pressure cutout.

Normal evacuation endpoint

For normal opening or disposal, the required evacuation endpoint for a low-pressure appliance is 25 mm Hg absolute. This endpoint applies whether the recovery equipment was manufactured before or on or after November 15, 1993. It is an absolute pressure, not $25\ \text{mm Hg}$ of vacuum. The $200$-pound charge distinction that applies to some other appliance types does not change the low-pressure endpoint.

Alternative condition before opening

A limited alternative applies to a non-major repair when the appliance will not be evacuated to the atmosphere after the repair. Before opening, pressurize the low-pressure appliance to no higher than $0\ \text{psig}$, using a method allowed for that refrigerant.

For refrigerants with boiling points at or below $85^\circ\text{F}$ at standard atmospheric pressure, do not use a method such as nitrogen that requires subsequent purging. For higher-boiling-point refrigerants, use heat as much as possible; nitrogen may raise pressure from the level achieved by heating to atmospheric pressure.

Major repairs and leaks that prevent evacuation

A major repair includes removing a compressor, condenser, evaporator, or auxiliary heat-exchange coil. It also includes leaving an opening with more than $4\ \text{in}^2$ of flow area uncovered for more than $15$ minutes.

If a leak prevents the normal evacuation level from being reached, isolate leaking from non-leaking components where possible. Evacuate non-leaking components to the normal required level. Evacuate the leaking component to the lowest level attainable without substantially contaminating recovered refrigerant, but not above $0\ \text{psig}$.