3 Evacuating Low-Pressure Systems

Distinguish recovery from dehydration

Low-pressure chiller work uses the word evacuation for two different processes. Refrigerant recovery removes refrigerant before a system is opened; dehydration evacuation removes air and moisture after the system is sealed and leak-tested. Their pressure targets are not interchangeable: reaching a recovery endpoint does not establish that dehydration is complete.

Follow the chiller manufacturer’s procedure and applicable requirements. This work should be performed by qualified technicians using appropriate recovery equipment and safety procedures.

Why dehydration matters

Low-pressure chillers can operate below atmospheric pressure, so leaks may draw air and moisture into the system. Moisture can freeze, corrode components, or contribute to acid and sludge formation. A vacuum lowers water’s boiling point so that moisture can vaporize and be removed.

Drying can be slow when liquid water, cold surfaces, oil, or moisture trapped in materials are present. Evacuation alone may not remove pooled water effectively.

Prepare the system and equipment

Use a vacuum pump, clean pump oil, vacuum-rated large-diameter hoses, suitable isolation valves or a vacuum manifold, and an electronic micron gauge. Check connections for leaks and confirm that the pump and gauge operate correctly. Arrange hoses to reach the system through appropriate service connections while minimizing hose length and restrictions.

Put the micron gauge at the chiller, or as close to the equipment as practical, rather than at the pump. Pressure drop through hoses can make the pump reading appear better than the vacuum actually achieved in the system. A conventional compound gauge cannot reliably measure deep vacuum.

Before dehydration, recover the refrigerant and complete the required leak test. Set valves as directed by the manufacturer so that every section requiring evacuation is connected to the pump; a closed valve can leave part of the system undried. Protect the chiller and its water circuits as the manufacturer directs. During refrigerant removal, maintain or remove water as specified to prevent freezing in heat-exchanger tubes.

Choose an evacuation method

For a straightforward, dry system, evacuate through low-restriction connections and monitor pressure with the micron gauge at the system. For a system exposed to moisture or opened for substantial repair, triple evacuation can help remove moisture: evacuate to a specified intermediate vacuum, isolate the pump, break the vacuum with dry nitrogen, and repeat at progressively lower pressures.

One example sequence has stages of $1{,}500$ microns, then $1{,}000$ microns, then a final $500$-micron vacuum, followed by a standing-vacuum check. This is an example, not a universal chiller specification; use the applicable manufacturer’s instructions and requirements.

Introduce dry nitrogen safely and in accordance with equipment instructions when breaking a vacuum. Never use oxygen or compressed air for this purpose. Do not exceed the system’s allowable pressure during pressurization or a leak test.

Measure and verify the vacuum

A micron, written as $\mu\text{m Hg}$, is $10^{-6}$ of a meter of mercury pressure. Lower absolute pressure means a deeper vacuum. A micron gauge measures absolute pressure, unlike a compound gauge, which indicates pressure relative to the atmosphere.

For a standing-vacuum test, keep the gauge connected to the system side of the isolation valve so it measures the chiller, not the pump. Use this sequence to verify evacuation:

1. Reach the specified final vacuum while the pump is operating.

2. Close the isolation valve between the chiller and the pump, leaving the gauge sensing the chiller.

3. Record the pressure and observe its rise over the manufacturer- or standard-specified hold period.

4. Compare the result with the specified endpoint and allowable pressure rise, not merely with the lowest reading reached while pumping.

A rapid rise toward atmospheric pressure suggests a leak or an isolation or connection problem. A slower continuing rise may indicate moisture outgassing, refrigerant remaining in oil, or a leak. If the reading fails the required hold test, investigate and correct the cause, then repeat evacuation and verification. A standing-vacuum test indicates system tightness and dryness behavior, but it does not, by itself, locate a leak.

Keep recovery and dehydration targets separate

For EPA Section 608 recovery requirements, the listed evacuation level for low-pressure appliances is $25\,\text{mm Hg absolute}$, equivalent to $25{,}000$ microns:

This is a refrigerant-recovery requirement, not a deep-vacuum dehydration target. Service dehydration commonly requires a much deeper vacuum, but the precise endpoint and hold criteria depend on the applicable standard and chiller manufacturer.

After reaching a recovery endpoint, allow pressure to stabilize and check for rebound as required. A pressure rise can indicate refrigerant still boiling out of liquid or oil, so recovery may need to continue. Meeting a recovery endpoint does not prove that the system has met its dehydration target.