A 16 m line, a 9 m lift and no nitrogen on site
A 2.0 TR (7.0 kW) R-32 inverter split is being installed in a first-floor office. The outdoor unit goes on the terrace 9 m above the indoor unit, and the line run is 16 m with three 90 degree bends. The manufacturer's label gives a factory charge of 1.10 kg for 5 m of pipe and an additional 20 g per metre of liquid line beyond that. There is no nitrogen cylinder on site. The installer plans to braze the joints with the lines open to air, then 'purge' the system with refrigerant from the cylinder to push out the air before opening the service valves, and to skip evacuation because 'the factory charge is already inside'. The condensate drain runs 6 m at a fall of 1:50 to a soak pit. The installation must be finished in one day because the office reopens next morning.
1. Calculate the total refrigerant charge and state exactly how and when it is put into the system.
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Line length beyond the factory allowance: 16 - 5 = 11 m.
Additional charge: 11 x 0.020 kg/m = 0.22 kg.
Total system charge: 1.10 + 0.22 = 1.32 kg.
Method: the factory 1.10 kg is already sealed inside the outdoor unit, so it must not be used to purge anything. Complete the pipework, pressure test, evacuate the entire system including the indoor unit and lines to below 500 microns (66 Pa), and confirm the vacuum holds. Only then open the liquid and gas service valves to release the factory charge, and add the extra 0.22 kg as liquid, by weight, using a charging scale and the liquid service port, with the unit running in cooling and the refrigerant cylinder upright. R-32 is a single-component refrigerant so there is no glide to worry about, but it must be charged as a liquid to avoid drawing cylinder additives or, in a blended refrigerant, fractionating.
Finally, record the total charge, line length and the date on the unit label and in the service record. The 1.32 kg figure is what any future service technician will need.
2. Explain why brazing without nitrogen and purging with refrigerant are both unacceptable.
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Brazing copper in air oxidises the inside of the tube and leaves black copper oxide scale. That scale later breaks loose, travels to the filter drier, capillary tube or electronic expansion valve, and blocks or erodes them; fine oxide also contaminates the compressor oil. Dry nitrogen, flowing slowly through the pipe while it is brazed, displaces oxygen and keeps the bore bright. The cost of a nitrogen regulator and a small cylinder is trivial against a blocked expansion valve or a seized compressor.
Purging with refrigerant does not remove air; it merely dilutes and pushes some of it along, and it leaves the rest, plus whatever moisture was in the pipe, inside a sealed system. Non-condensables such as air and nitrogen sit in the condenser, raise discharge pressure and discharge temperature, and reduce capacity. Moisture with POE oil forms acid and sludge, and at the expansion device it freezes and blocks the flow.
Purging also vents refrigerant to atmosphere, which is both illegal and damaging: R-32 has a GWP of about 675, so every kilogramme released equals the warming effect of well over half a tonne of carbon dioxide.
Evacuation is the only method that removes air and boils off moisture; a triple evacuation with nitrogen breaks between stages is the practical way to reach a stable 500 microns.
3. Comment on the vertical lift, the drain and the one-day deadline. What do you insist on?
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Vertical lift: a 9 m rise with the outdoor unit above the indoor unit is beyond most manufacturers' no-trap limit. Oil leaving the compressor must be carried back up the suction riser against gravity, so a U-trap (oil trap) at the bottom of the riser is normally required, and some manufacturers also call for an intermediate trap above about 7.5 m. The installation manual for the exact model must be followed, and the line sizes must be checked against the allowable lengths and elevation limits; if the lift exceeds the limit, the outdoor unit must be relocated lower. Refrigerant charge, capacity and oil return all depend on this being right.
Drain: 1:50 over 6 m gives 120 mm of fall, which is acceptable, but the run needs insulation to stop condensation, a continuous slope with no sag, and a trap or a suitable termination at the soak pit so air and insects cannot enter. Confirm the indoor unit can drain by pouring water through before the ceiling is closed.
Deadline: do not trade procedure for time. Insist on a nitrogen cylinder and a vacuum pump before brazing starts; if they cannot be arranged today, complete the mechanical work and return to pressure test, evacuate and commission properly. Handing over a system that was never evacuated means a callback within weeks, at the customer's expense and your reputation.