Vacuum Rise After a Filter-Drier Replacement
After replacing a liquid-line filter drier and repairing a pinhole in the suction header, a technician commissions a -25 °C R448A blast freezer. The system charge is 45 kg. The technician triple-evacuates to 500 microns (0.67 mbar) and isolates the vacuum pump. The micron gauge is connected at the suction service valve. After 10 minutes the reading rises to 1,800 microns and then remains almost steady for 30 minutes. The pipework is cold and the ambient is 16 °C. The site needs the freezer back tonight. A colleague suggests charging R448A now because “the vacuum held.” The technician has dry nitrogen, a leak detector and a heat gun available.
1. How would you interpret the rise from 500 to 1,800 microns followed by a near-steady reading?
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A rise that slows and then plateaus is not the classic signature of a large leak, which would normally continue rising towards atmospheric pressure. It is more consistent with moisture or outgassing from the system, especially after a repair that opened the circuit. However, a small leak can also produce a slow rise, so the result is not conclusive. The reading is also above the usual target for a deep evacuation. The fact that the pipework is cold makes moisture more likely to remain in the oil or on surfaces. Charging now would carry that moisture into the refrigerant circuit, where it can form acids, freeze at the expansion valve and damage the compressor. The vacuum must be investigated, not accepted.
2. Describe a test sequence to distinguish a leak from moisture, using the nitrogen and vacuum equipment available.
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First break the vacuum with dry nitrogen and pressure-test the repaired section and the whole system at a safe test pressure, typically 10–15 bar depending on component ratings. Isolate the nitrogen source and record the pressure; a stable pressure over a suitable period indicates no significant leak. If pressure falls, use leak-detection fluid or an electronic leak detector at the repaired joints. If the nitrogen test holds, re-evacuate with a new filter drier, apply gentle heat to the low points and oil to drive off moisture, and use dry-nitrogen sweeps. Evacuate again to below 500 microns and isolate. A standing vacuum rise of only a few hundred microns over 10–30 minutes is acceptable; a continuing rise indicates a leak or insufficient evacuation. Repeat until the result is stable.
3. Would you charge the system tonight? Justify your decision and state what you would tell the site.
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No. The freezer should not be charged until the vacuum test is stable and the moisture question is resolved. A 1,800-micron plateau with cold pipework is a warning, not a pass. Charging 45 kg of R448A into a system with residual moisture risks ice blockage at the expansion valve, acid formation, oil breakdown and compressor failure. It would also make later diagnosis much harder. I would tell the site that the repair is complete but commissioning is not, and that the system needs a pressure test and further evacuation with heat and a fresh drier. If the nitrogen test holds and the vacuum falls below 500 microns and stays stable, charging by weight can proceed the same night. If not, the freezer stays off until the fault is corrected.