R-32 VRF tightness test: is that half a bar a leak?
You are commissioning a new R-32 VRF system in a converted office: one outdoor unit, six indoor units, 38 m of 12.7 mm liquid line and 42 m of 15.88 mm gas line. The manufacturer specifies a design pressure of 4.15 MPa (41.5 bar) and a nitrogen tightness test at 4.0 MPa for 24 hours, followed by evacuation. You fill the pipework with oxygen-free nitrogen through a regulator at 17:00 on Tuesday; the gauge reads 40.0 bar and the site thermometer reads 22 C. At 17:00 on Wednesday the same gauge reads 39.4 bar and the thermometer reads 15 C. The site clerk says the system has 'dropped half a bar' and asks whether to book a second visit to find a leak.
1. Using the readings, decide whether a leak is indicated. Show the temperature-compensated calculation and state what you would do next.
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Method
- Work in absolute pressure and absolute temperature; a sealed volume obeys p1/T1 = p2/T2.
- p1 = 40.0 + 1.013 = 41.013 bar abs; T1 = 22 + 273.15 = 295.15 K.
- Gradient = 41.013 / 295.15 = 0.1390 bar per K.
- T2 = 15 + 273.15 = 288.15 K, so predicted p2 = 41.013 x (288.15 / 295.15) = 40.04 bar abs, i.e. 39.03 bar gauge.
Result
- The gauge reads 39.4 bar gauge, which is slightly above the temperature-corrected prediction. The 0.6 bar fall is fully explained by the 7 K overnight cooling. No leak is indicated.
Next steps
- Record the start and finish pressures, both temperatures, the time and the corrected calculation on the commissioning sheet.
- Repeat or extend the test if the margin is small relative to gauge and thermometer accuracy, and confirm no part of the circuit was isolated or valved off during the test (a trapped section invalidates the whole-circuit result).
- Only then release the nitrogen and start evacuation.
2. Explain why oxygen-free nitrogen is used, the controls needed while the system is under test, and why the test pressure of 4.0 MPa is acceptable here.
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Why nitrogen
- It is dry, inert and does not support combustion or react with the oil. Oxygen or compressed air in a refrigeration circuit can cause oil oxidation, and with any residual refrigerant present, a flammable or explosive mixture at pressure.
Controls during test
- Use a pressure regulator – never connect a cylinder directly to the system; test pressure must not exceed the design pressure of the lowest-rated component.
- 4.0 MPa (40 bar) is acceptable because it is below the 41.5 bar design pressure of the equipment and is the manufacturer's stated test pressure; if any component (e.g. a low-pressure rated valve or a relief device) is not rated for it, isolate or remove that component.
- Pressurise in stages, check joints at each stage, keep people clear of the ends of pipework, and do not use a naked flame or soap that attacks the pipework.
- Nitrogen is an asphyxiant: ventilate the plant room, never discharge a full cylinder into a small enclosed space, and label the system clearly as pressurised and under test.
Records
- Test pressure, holding time, ambient temperature at start and finish, and who witnessed it.
3. Evacuation stalls at about 1200 microns even after a long pull-down. Explain how you would decide between a leak and moisture, and describe the evacuation you would carry out.
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Diagnosing the stall
- Isolate the pump and watch the vacuum: a rise that continues steadily points to a leak (or a leak in the gauge line or pump connection). A rise that climbs and then plateaus at a value consistent with the vapour pressure of water in the system points to moisture, which needs further drying, not leak hunting.
- Rule out the cheap causes first: a leaking gauge hose, a worn pump, a vacuum measured at the pump rather than at the far end of the circuit, and liquid trapped in a trap or an oil-logged compressor.
Evacuation procedure
- Use a two-stage pump and a gauge connected directly to the system, not to the pump manifold; use large-bore, short hoses.
- Triple evacuation: pull down to the target (typically 500 microns / 0.67 mbar or the manufacturer's figure), break the vacuum with oxygen-free nitrogen to about 1 bar, repeat, and finish by holding the final vacuum for the specified period with the pump isolated.
- Never use the system compressor as a vacuum pump: the motor relies on refrigerant for cooling and the oil will not lubricate correctly at low pressure.
- Record the final vacuum and the standing time before charging.