Level 1, Unit 2, Section 2 – 2.4 Evacuation & Dehydration Procedures

2.4.1 — Purpose of Evacuation & Dehydration

Evacuation is performed after a successful pressure and leak test to remove all air, moisture, and non-condensable gases from the internal surfaces of the refrigeration system before refrigerant is introduced. Failure to achieve a proper deep vacuum is one of the most common causes of premature system failure, reduced efficiency, and compressor damage.

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Moisture

Water vapour remaining in the system reacts with refrigerant and oil to form acids and sludge, corroding metal components and blocking the expansion device.

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Air & Non-Condensables

Air and other non-condensable gases raise condensing pressure, reduce system capacity, and cause overheating — they cannot be condensed by the refrigeration cycle.

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Contamination

Residual test gases and atmospheric contamination introduced during service must be fully removed to protect the compressor and maintain refrigerant purity.

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Micron Level

Vacuum depth is measured in microns (µm Hg). The lower the micron reading, the deeper the vacuum. Target levels are set by codes and manufacturers — typically 300–500 microns or lower.

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Dehydration = Boiling Off Moisture

At very low absolute pressures, the boiling point of water drops dramatically. A deep vacuum lowers the pressure enough that any residual moisture in the system vaporizes and is drawn out by the vacuum pump — this is the dehydration effect. The deeper and longer the vacuum, the more thoroughly the system is dehydrated.

2.4.2 — Equipment Required

Achieving a proper deep vacuum requires the right equipment in good condition. Undersized pumps, worn pump oil, small-diameter hoses, or leaking connections will prevent the system from reaching target vacuum levels.

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Vacuum Pump

Critical

Must be a two-stage high-capacity vacuum pump sized appropriately for the system volume — a pump that is too small will not achieve the required vacuum depth in a reasonable time.
Oil must be clean and at the correct level. Contaminated or moisture-saturated pump oil severely reduces pump performance. Change the oil before each job and again during long evacuations if it becomes cloudy.
Verify the pump's blank-off pressure (the deepest vacuum it can achieve) before use — this should be well below the target system vacuum level.

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Micron Gauge (Electronic Vacuum Gauge)

A compound gauge on a manifold set is not accurate enough for evacuation — it cannot distinguish between 1,000 microns and 10 microns. A dedicated electronic micron gauge is required.

Connect the micron gauge directly to the system — not to the pump side of the hose — so you are reading actual system pressure, not pump pressure.
Allows accurate monitoring of vacuum depth and the standing vacuum test after pump isolation.

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Large-Diameter Hoses & Core Removal Tools

Hose diameter is one of the most overlooked factors in evacuation speed and final vacuum depth.

Use large-diameter (3/8" or larger) vacuum-rated hoses — standard 1/4" manifold hoses create significant flow restriction and prevent deep vacuums.
Schrader core removal tools allow evacuation through the full bore of the service valve rather than through the small Schrader pin — dramatically increasing flow rate and reducing evacuation time.
Minimize hose length where possible — longer hoses increase restriction and add volume that must also be evacuated.

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Manifold Set & Isolation Valves

A manifold set connects the vacuum pump to both the high and low sides of the system simultaneously for a full-system evacuation.
Isolation valves on the manifold allow the pump to be closed off from the system when performing the standing vacuum test — without disconnecting any hoses.

2.4.3 — Deep Vacuum Evacuation

Deep vacuum evacuation is the primary method used to remove air and moisture from the internal surfaces of refrigeration and air conditioning systems. It is performed after a successful pressure and leak test, once all test gas has been safely handled.

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Deep Vacuum Evacuation Procedure

Follow In Order

Handle test gas safely. After passing the pressure test, all test gas must be safely vented (for inert gases such as dry nitrogen) or recovered if the test gas was mixed with refrigerant. Never vent refrigerant-containing mixtures to atmosphere.
Connect the vacuum assembly. Open the system only enough to connect the vacuum pump hoses and micron gauge. Use large-diameter hoses and install core removal tools on Schrader valve ports where possible. Connect to both the high and low side of the system.
Start the vacuum pump and open the manifold valves. Confirm the pump is running properly — listen for normal pump operation and observe the micron gauge begin to drop.
Evacuate to target micron level. Continue evacuation until the system reaches the target vacuum depth specified by codes or the equipment manufacturer — typically 300–500 microns or lower. Do not rush; deep dehydration requires sustained low pressure.
Isolate the pump and perform a standing vacuum test. Close the manifold valve between the pump and the system. Monitor the micron gauge for a minimum of 15–30 minutes (or as required by procedure). A stable or slowly rising reading that levels off indicates a clean, dry, leak-free system. A rapid or continuous rise indicates a leak or residual moisture.

Standing Vacuum Test — Pass

Micron level holds steady or rises very slowly and levels off
Indicates system is leak-free and sufficiently dry
Proceed to charging

Standing Vacuum Test — Fail

Rapid continuous rise — likely a leak; recheck all connections and re-leak test
Rise that levels off at a higher micron level — likely residual moisture; continue evacuation or consider triple evacuation
Do not charge a system that fails the standing vacuum test

2.4.4 — Triple Evacuation

Triple evacuation is a procedure used when systems are suspected of high moisture content, have been open to atmosphere for an extended period, or have experienced severe contamination. It uses dry nitrogen breaks between evacuation cycles to assist in removing moisture that a single deep vacuum may not fully extract.

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Triple Evacuation Procedure

High Moisture Situations

First evacuation. Evacuate the system to a specified vacuum level — typically 1,500–2,000 microns for the first stage. This initial pull removes the bulk of the air and begins to lower the moisture content.
Break the vacuum with dry nitrogen. Introduce dry nitrogen to bring the system back up to a low positive pressure (e.g., 2–5 psig). The nitrogen breaks the vacuum and helps to absorb and sweep residual moisture from internal surfaces and oil.
Second evacuation. Evacuate again to the specified vacuum level. Each nitrogen break and re-evacuation cycle removes additional moisture that was not captured in the previous pull.
Break with dry nitrogen again. Repeat the nitrogen break as in step 2.
Third (final) evacuation. Perform the final deep vacuum evacuation to the target micron level required by code or the manufacturer. This final pull should achieve a deeper vacuum than the first two cycles as most contamination has been removed.
Verify with micron gauge and standing vacuum test. Confirm the final vacuum level with the micron gauge and perform the standing vacuum test before proceeding to charge.

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Why Three Cycles?

Each evacuation and nitrogen break cycle operates on the principle of dilution and displacement. The first pull removes the majority of air and moisture vapour. The nitrogen break re-saturates remaining moisture sites and allows them to migrate into the gas phase. The subsequent evacuations then remove what the first cycle could not. Three cycles is the standard minimum — more cycles may be used in severely contaminated systems.

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Use Only Dry Nitrogen for Vacuum Breaks

Only dry nitrogen (with a dew point of −40°C or lower) is acceptable for breaking the vacuum between evacuation cycles. Using compressed air or any other gas defeats the purpose of the procedure — air reintroduces moisture and oxygen, and other gases may contaminate the system. Always use a regulator when introducing nitrogen and never exceed the system's design pressure.