4 Evacuation and Dehydration Procedures
Learn how refrigerant recovery, leak testing, evacuation, and dehydration fit together, and how to set up and interpret model-specific vacuum procedures safely.
Purpose and procedure endpoints
Evacuation removes refrigerant, air, and other gases from a refrigeration circuit. removes water vapor and moisture. A deep vacuum can help dry a system, but it does not replace refrigerant recovery or a leak test.
and deep are separate procedures with different endpoints. For U.S. stationary appliances containing more than five pounds of refrigerant, EPA Section 608 specifies evacuation levels for refrigerant recovery when opening equipment. For a low-pressure appliance, the listed level is absolute. This regulatory recovery level is not the deep-vacuum target used to verify before charging.
Follow the chiller manufacturer’s procedure and applicable regulations. Pressure and vacuum limits vary by model, so generic targets and pressure-rise limits should not be applied to every chiller.
Safety and preparation
Use qualified service personnel and the equipment’s service manual. Isolate and lock out electrical power as required. Recover refrigerant with approved equipment before opening the circuit; do not vent refrigerant.
Do not use the compressor to evacuate the system, and never pressure-test with oxygen or compressed air. Use only with a regulator and within the manufacturer’s pressure limits. Ventilate the work area and prevent nitrogen from displacing breathable air.
Before evacuation, repair leaks and complete the required . Remove standing water from vessels using the manufacturer’s procedure; a vacuum pump is not a substitute for draining bulk liquid. Keep the system and exposed components clean and dry.
Vacuum equipment and setup
Use a high-capacity vacuum pump in good condition, clean pump oil, large-bore, short vacuum hoses, and a calibrated . A conventional manifold gauge is not accurate enough to verify deep vacuum.
Place the at the chiller or as close to the system as practical, not at the pump. Hose restriction can make the pump-side reading appear better than the actual system vacuum.
Connect to suitable service ports so the pump can evacuate both sides and relevant sections of the circuit. Open required service valves and solenoids according to the service manual, close all openings to atmosphere, and cap ports. Check the pump and connections for leaks before drawing down the chiller.
Evacuation and sequence
Follow the manufacturer’s staging instructions throughout the procedure. Some chiller procedures call for an initial evacuation and hold before proceeding to a deeper vacuum.
Pressure-test for leaks. Use the manufacturer-approved test method and pressure. Find and repair leaks before ; otherwise, a system leak can prevent a reliable vacuum and waste time.
Begin evacuation. Start the pump and monitor absolute pressure at the chiller with the . Follow the specified staging instructions rather than applying a generic target.
Manage moisture. Water boils at lower temperatures as pressure falls. If moisture is present, evaporation can cool it enough to freeze, temporarily slowing drying and making readings misleading. Where the manufacturer permits, maintain suitable vessel temperature and use repeated evacuation with a dry-nitrogen break between stages. Never exceed the equipment’s pressure limit.
Isolate for a . Close the valve between the system and pump so the chiller is isolated; then stop or isolate the pump as directed. Record the starting micron reading, time, and system temperature. Watch the pressure rise for the manufacturer’s specified hold period.
Interpret the trend. A pressure rise can indicate leakage, remaining moisture, or both. A rise that tends toward atmospheric pressure suggests a leak; a rise that slows or stabilizes below atmospheric pressure can be consistent with moisture boiling off. Temperature changes also affect pressure, so compare readings at similar temperatures. If results are uncertain, repeat the manufacturer’s staged test and perform further leak checks. A alone does not conclusively distinguish a leak from moisture.
Complete final evacuation and verify. After the pressure test and are satisfactory, evacuate to the model-specific final target and complete the specified standing-vacuum hold. Charge only after the system passes the required checks, following that chiller’s charging instructions.
Verifying results and documenting work
Leak tightness requires passing the manufacturer’s before evacuation and meeting the specified vacuum-hold criteria after evacuation. If pressure rises beyond the limit, investigate for leaks and/or moisture rather than assuming the system is ready.
Dryness is verified by reaching and holding the manufacturer’s final vacuum target. A micron reading at the pump while it is running is not proof of dryness. Use the isolated-system hold test and pressure trend; repeat if the trend indicates continuing moisture.
Record the refrigerant, test method and pressure, vacuum target, starting and ending micron readings, hold time, temperature, and any repairs or repeated evacuation cycles.
Example of model-specific criteria
One YORK centrifugal chiller procedure gives an example, not a universal requirement: an initial -micron hold test, followed by a final evacuation target in the -micron range and an -hour hold. It gives a -micron rise during the first minutes as an acceptance limit.
These values apply to that example procedure only. Always use the procedure for the specific chiller being serviced.