Non-condensables — air/nitrogen entering the system, diagnosis and removal

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Diagnosing non-condensables — identifying by pressure vs temperature. Check: Principle: air, nitrogen, and other non-condensables introduced to a system don't liquefy in the condenser and accumulate as gas at the top of the condenser → discharge pressure rises abnormally above the PT-chart value. Procedure: ① Stop the system and let it equalize to ambient (≥2 hours typically, longer for big systems). ② At equilibrium, read high-side pressure on the manifold. ③ Compare with PT-chart saturation pressure for ambient — reading >0.5 bar above PT chart = non-condensables suspected. ④ Estimate non-condensable fraction from the pressure gap; 1 bar gap ≈ 5–10% by volume (varies with system size). Additional signs: discharge line very hot, high compression ratio, condenser top hotter than usual. What to do: Verify with parallel checks: ① Liquid line sight glass — clear when healthy. With non-condensables at the top of the condenser, fine bubbles can show in the liquid line. ② Take a gas sample to the identifier — 'Air' channel ≥3% = non-condensables. ③ Inspect the recovery tank at the same time — gas can accumulate at the top after recovery work. Entry paths: ① Insufficient evacuation (residual air left in system). ② Running after a leak (low side falls below atmospheric and pulls air in). ③ Wrong charging sequence (nitrogen pressurized then weak vacuum, or air ingested from a can). Removing non-condensables and preventing recurrence. Check: Removal procedure: ① Recover the system → evacuate ≤500 micron + 30-min decay (also a leak check). ② On pass, recharge fresh refrigerant (also replace the filter-drier). ③ Non-condensables in a recovery tank — let the tank stand upright for 24 hours, then slowly bleed only the gas through the top valve (small amount). Only do this where regulations allow (check EPA 608 etc.). Prevention: ① Standardize evacuation — micron gauge, ≤500 micron + 30-min decay pass. ② Annual leak check. ③ At charging, after vacuum charge as liquid through the liquid line immediately (avoid nitrogen residue or air ingress). What to do: Reference — manifold pressure at rest equilibrium by ambient. R-22: 25°C ≈ 9.5 bar, 30°C ≈ 11.0 bar, 35°C ≈ 12.7 bar. R-410A: 25°C ≈ 15.5 bar, 30°C ≈ 18.0 bar, 35°C ≈ 20.7 bar. R-32: 25°C ≈ 16.0 bar, 30°C ≈ 18.5 bar, 35°C ≈ 21.3 bar. R-134a: 25°C ≈ 5.6 bar, 30°C ≈ 6.7 bar, 35°C ≈ 7.9 bar. Reading within ±0.3 bar of the table is normal (gauge tolerance included). Reading >0.5 bar above the table = suspect non-condensables. With insufficient rest time (<2 h), residual liquid refrigerant in the system can keep pressure high — that distorts diagnosis. Adequate rest time is the key to accuracy.

Non-condensables — air/nitrogen entering the system, diagnosis and removal

Likely causes

Diagnosing non-condensables — identifying by pressure vs temperature

Removing non-condensables and preventing recurrence

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